Bachelor of Engineering Thesis Feasibility study on single ...

Bachelor of Engineering Thesis

Delft University of Technology Faculty of Aerospace Engineering

Department of Aerospace Structures and Materials Remko Kuitert

11 October 2016

Feasibility study on single point incremental forming A study of die-less preforming of double curved aluminum sheets for GLARE laminates

Master of Science thesis

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Remko Kuitert I | P a g e

Feasibility study on single point incremental forming A study of die-less preforming of double curved aluminum sheets for GLARE laminates

By

Ing. R. Kuitert

In partial fulfilment of the requirements for the degree of Master of Science in Aerospace Structures and Materials

At the Delft University of Technology Faculty of Aeronautical Engineering

Department of Aerospace Materials and Structures

To be defended on Friday, 28th of October, 2016 at 9:00 AM.

Supervisor: Ir. Jos Sinke Thesis committee: Ir. Jos Sinke, TU Delft

Dr. Ir. Rene Alderliesten, TU Delft Dr. Ir. Sonell Shroff, TU Delft


II | P a g e Remko Kuitert

Copyright © R. Kuitert All rights reserved


Remko Kuitert III | P a g e

Delft University of Technology Author: ________________________________ Ing. R. Kuitert Delft University of Technology Supervisor: ________________________________ Ir. Jos Sinke

This Master thesis investigates the feasibility of single point incremental forming as a method to die-less preform double curved aluminum sheets used in GLARE laminates. A small scale experimental setup was used to see if this method could achieve the required speed and accuracy. The results of these experiments were translated to a large scale production line, such as the Airbus A320. However it was concluded that single point incremental forming is not a feasible process for large scale production. Even though the dimensional accuracy is rather good, the process speed to achieve this accuracy is far too low.

Keywords: single point incremental forming, CNC machining, GLARE laminates, production speed, dimensional accuracy.


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Preface

Remko Kuitert V | P a g e

Preface In the last year of the Master Aerospace Engineering at Delft University of Technology in Delft, the student will get the opportunity to perform a thesis project. During this graduation period a thesis has to be written. This report represents my thesis for the Master of Aerospace Structures and Materials at the faculty of Aerospace Engineering. I had the opportunity to perform my graduation project at the Delft Aerospace Structures and Material Laboratory of Delft University of Technology in Delft. The main focus of this project was to perform a feasibility study on the subject of incremental forming for double curved aluminum sheets used in GLARE laminates. This thesis would not have been possible without the guidance of several people who supported me throughout the graduation period. Their support, feedback and critical eye contributed to the quality of this thesis report. In particular I would like to thank my girlfriend, Wouke van Veldhuizen, for standing by my side despite all the setbacks during this thesis. I would also like to give a special thanks to my supervisor, Jos Sinke, for his support during the entire project and valuable feedback. Without him the experimental setup would have never been possible. I would also like to thank Gertjan Mulder and Micha Huizinga for all their help during the design and construction phase of the experimental setup. Finally, I would like to say thank you to my mother, my brother, my friends and fellow students for their support during this Master thesis.

Remko Kuitert Aalsmeer, The Netherlands

October 28, 2016

Summary

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Summary Although GLARE is now applied on the A380 and considered as a material for future aircraft, several cost and production issues still have to be solved for future applications of this material. GLARE panels are currently being produced by a hand lay-up and self-forming method, which is a suitable production method for the low volume production line of e.g. the Airbus A380. But even with this method the production of double curved GLARE panels remained a problem as wrinkles occur if the aluminum sheets become too large. In the aircraft industry double curved metal sheets with large radius, for e.g. fuselage application, are currently made by stretch forming. This process includes the use of a die which is product specific. But for the production of GLARE the use of dedicated dies becomes too expensive. At the moment several processes exist, but none of these processes meet the standards for accuracy and/or cycle time yet. Therefore this research focused on a new type of process, the incremental sheet forming process, in order to determine its feasibility for the application of preforming metal sheets. As a start an analysis was made what kind of production rates and accuracy are to be expected. It turned out that the production rate is in the range of 72-129 m

2/hour and the required accuracy should be at least 90%

of the forming depth. Thus for the process to be feasible a combination of a feed rate of 600 m/min and a step size of 0.5 mm/pass is needed at a single machine to meet these requirements. In order to verify if this is feasible an experimental approach was chosen, due to a lack of accurate computer models. This started by designing and constructing an incremental forming machine, which was required for the experiments. This machine was capable of operating at feed rates of 0-3000 mm/min and step sizes of at least 0.1 mm/pass or larger. The workable area was limited to 100x100 mm by both the machine and the jig, which was used to clamp the specimens. The specimens used in the Thesis were made of aluminum 2024-T3 and were 0.4 mm thick, which is the same as the aluminum layers used in GLARE. The experiments were split up in four categories. The first category was the geometrical experiments. These experiments showed that despite the change of geometry, the repeatability of the process was in the range of 0.25 mm. The second category of experiments was related to the feed rate. These experiments provided information of the effects of higher feed rates. It was shown that if the feed rates increase the dimensional accuracy decreased. This implies that the feed rate could not be increased as easily as thought. The third set of experiments was related to the step size experiments. These experiments showed that a small step size gives a higher accuracy and vice versa. The fourth and last set of experiments was the geometric correction experiments, which were used to increase the accuracy of the process by iteration. It was shown that after only a single iteration the specimens achieved 97.2% of the input model forming depth, while only 65% was achieved without iteration. Even though relative high accuracies were obtained the conclusion of this thesis is that it is not feasible for single point incremental forming to be applied for large scale industrial applications, like the Airbus A320 production line, in this form. It is therefore recommended that future research should focus on different strategies for incremental forming, such as an origami approach or the use of multiple forming tools simultaneously. Also it is suggested that more research should be done for a better prediction model in order to avoid the iteration step. Last, but not least, it is also recommended that the current experiment setup should be replaced by a more professional setup in order to improve the control of the spindle speed for example.

Table of Contents

Remko Kuitert 1 | P a g e

Table of Contents

CHAPTER 1 INTRODUCTION ........................................................................................... 6

1.1 BACKGROUND ............................................................................................................................ 6 1.2 PROBLEM DEFINITION .................................................................................................................. 7 1.3 THESIS OBJECTIVE ........................................................................................................................ 7 1.4 RESEARCH QUESTION ................................................................................................................... 8 1.5 RESEARCH APPROACH .................................................................................................................. 9

CHAPTER 2 LITERATURE REVIEW .................................................................................. 12

2.1 GLARE, A FIBER METAL LAMINATE ............................................................................................... 12 2.2 INCREMENTAL SHEET FORMING ................................................................................................... 18

CHAPTER 3 REQUIREMENTS ......................................................................................... 22

3.1 APPLICATIONS OF GLARE ........................................................................................................... 22 3.2 INDUSTRY REQUIREMENTS .......................................................................................................... 23 3.3 THEORETICAL ANALYSIS .............................................................................................................. 25

CHAPTER 4 EXPERIMENTAL SETUP ............................................................................... 28

4.1 MACHINE SELECTION ................................................................................................................. 28 4.2 MACHINE DESIGN ...................................................................................................................... 30 4.3 JIG AND SPECIMENS ................................................................................................................... 39 4.4 MEASURING ............................................................................................................................. 40

CHAPTER 5 EXPERIMENTS ............................................................................................ 41

5.1 CALIBRATION ............................................................................................................................ 41 5.2 TEST PROTOCOL ........................................................................................................................ 43 5.3 GEOMETRY EXPERIMENTS ........................................................................................................... 46 5.4 FEED RATE EXPERIMENTS ............................................................................................................ 49 5.5 STEP SIZE EXPERIMENTS .............................................................................................................. 50 5.6 GEOMETRY SPRING BACK CORRECTION .......................................................................................... 52 5.7 EXPERIMENTAL RESULTS ............................................................................................................. 53

Table of Contents

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CHAPTER 6 DISCUSSION ............................................................................................... 57

6.1 LARGE SCALE APPLICATIONS ........................................................................................................ 57 6.2 LIMITATIONS ............................................................................................................................ 59 6.3 IMPROVEMENTS........................................................................................................................ 59

CHAPTER 7 CONCLUSIONS ........................................................................................... 60

7.1 CONSOLIDATION OF THE WORK.................................................................................................... 60 7.2 CONCLUSION ............................................................................................................................ 61 7.3 RECOMMENDATIONS ................................................................................................................. 61

REFERENCES ................................................................................................................. 62

APPENDICES ................................................................................................................. 65

APPENDIX A – MATLAB SCRIPTS.................................................................................... 66

A.1 3D SKETCH OF A SINGLE CURVED FUSELAGE PANEL .......................................................................... 66 A.2 SCAN GRID GENERATOR SCRIPT ................................................................................................... 67 A.3 G-CODE GENERATOR SCRIPT ...................................................................................................... 68

APPENDIX B – REPETIER FIRMWARE CHANGES .............................................................. 70

B.1 DISABLE ALL EXTRUDERS ............................................................................................................. 70 B.2 CHOOSING THE CORRECT MOTHERBOARD ...................................................................................... 70 B.3 DISABLE ALL END STOPS .............................................................................................................. 70

List of Figures


List of Figures Figure 2-1 GLARE Layup [8] ................................................................................................................. 12 Figure 2-2 Different GLARE Grades with their fiber orientation and their specific constituents [16] 14 Figure 2-3 Post stretching effect with Stress Strain curve [20] .......................................................... 15 Figure 2-4 SPIF setup (left) and TPIF setup (right) .............................................................................. 18 Figure 2-5 Back pressure SPIF (left), partial die SPIF (center) and full die SPIF (right) ....................... 19 Figure 2-6 Incremental Stretch Forming with a die ............................................................................ 19 Figure 2-7 Single Point Incremental Forming Inaccuracy [5] .............................................................. 20 Figure 3-1 GLARE on an A380 [34] ...................................................................................................... 22 Figure 3-2 3D sketch of a A320 Fuselage panel generated by the Matlab script in Appendix A-1 ..... 25 Figure 3-3 3D Sketch of the double curved panel ............................................................................... 26 Figure 3-4 Step size vs. Toolpath length for large panels ................................................................... 26 Figure 3-5 Feed rate vs. Production time for large panels ................................................................. 26 Figure 3-6 required step size vs. the feed rate for minimum and maximum production rates ......... 27 Figure 4-1 A 3-axis router machine (left) vs. 3-axis knee milling machine (right) [37] [38] [39] ........ 28 Figure 4-2 HBM BF16 Drilling/Milling machine [39] ........................................................................... 30 Figure 4-3 Conventional Y-axis close up [38] ...................................................................................... 30 Figure 4-4 Catia sketch of the Y-axis stepper motor fitting ................................................................ 31 Figure 4-5 Complete CNC Y-axis of the HBM BF16 ............................................................................. 31 Figure 4-6 Conventional X-axis Close up ............................................................................................. 32 Figure 4-7 Additional Coupling Piece X-axis ........................................................................................ 32 Figure 4-8 Complete CNC X-axis of the HBM BF16 ............................................................................. 32 Figure 4-9 Conventional Z-axis close up [38] ...................................................................................... 33 Figure 4-10 Complete CNC Z-axis of the HBM BF16 ........................................................................... 33 Figure 4-11 Complete Conversion of the HBM BF16 .......................................................................... 33 Figure 4-12 Stepper motor Nema 23 – 3 Nm [43] .............................................................................. 34 Figure 4-13 Leadshine DM556 - 2 Phase Digital Stepper Drive [42] ................................................... 35 Figure 4-14 PSU 48VDC 6,7A [41] ....................................................................................................... 35 Figure 4-15 Arduino Mega 2560 [38] .................................................................................................. 36 Figure 4-16 Ramps 1.4 Shield [39] ...................................................................................................... 36 Figure 4-17 1SL02 IP66 Wall Box [40] ................................................................................................. 36 Figure 4-18 3D printer wiring scheme [46] ......................................................................................... 37 Figure 4-19 Custom design wiring scheme ......................................................................................... 37 Figure 4-20 Complete Electrical Control Box ...................................................................................... 37 Figure 4-21 Repetier Host interface .................................................................................................... 38 Figure 4-22 The jig ............................................................................................................................... 39 Figure 4-23 Example of an aluminum 2024-T3 0.4 specimen ............................................................. 39

List of Figures


Figure 5-1 Calibration specimen measurement .................................................................................. 41 Figure 5-2 Calibration Specimen ......................................................................................................... 41 Figure 5-3 Standard deviation between calibration specimens including sheet bending .................. 41 Figure 5-4 Differences between individual measurement cycles in mm ............................................ 42 Figure 5-5 Deviations within a single measurement in mm ............................................................... 42 Figure 5-6 Machine used for specimen production ............................................................................ 43 Figure 5-7 Clamping of the specimen with convex side upwards....................................................... 43 Figure 5-8 The jig on the XY Table ....................................................................................................... 43 Figure 5-9 Starting point determination X and Y ................................................................................ 44 Figure 5-10 Starting point determination Z ........................................................................................ 44 Figure 5-11 Z-axis with (Left) and without power (Right) ................................................................... 44 Figure 5-12 Test protocol flowchart ................................................................................................... 45 Figure 5-13 Geometry Experiment 1: Average obtained geometry ................................................... 46 Figure 5-14 Geometry Experiment 1: Deviation between the specimens.......................................... 46 Figure 5-15 Geometry Experiment 1: Specimen (red) vs. Model (blue) ............................................. 46 Figure 5-16 Geometry Experiment 2: Average obtained geometry ................................................... 47 Figure 5-17 Geometry Experiment 2: Deviation between the specimens.......................................... 47 Figure 5-18 Geometry Experiment 2: Specimen (red) vs. Model (blue) ............................................. 47 Figure 5-19 Geometry Experiment 3: Average obtained geometry ................................................... 48 Figure 5-20 Geometry Experiment 3: Specimen Deviation ................................................................ 48 Figure 5-21 Geometry Experiment 3: Specimen vs. Model ................................................................ 48 Figure 5-22 Feed rate experiment 2: Differences 2000 mm/min vs. 1000 mm/min .......................... 49 Figure 5-23 Feed rate experiment 3: Differences 3000 mm/min vs. 1000 mm/min .......................... 50 Figure 5-24 Step size experiment 2: Absolute differences vs. 0.2 mm/pass ...................................... 51 Figure 5-25 Step size experiment 2: Specimen shape differences between 0.1 mm/pass (red) vs. 0.2 mm/pass (green) ................................................................................................................................. 51 Figure 5-26 Step size experiment 3: Absolute differences vs. 0.2 mm/pass ...................................... 51 Figure 5-27 Step size experiment 3: Specimen shape differences between 0.3 mm/pass (red) vs. 0.2 mm/pass (green) ................................................................................................................................. 51 Figure 5-28 Specimen 1 without correction ....................................................................................... 52 Figure 5-29 Specimen correction 1 ..................................................................................................... 52 Figure 5-30 Specimen correction 2 ..................................................................................................... 52 Figure 5-31 Overview of the contribution of each effect on the accuracy mismatch ........................ 52 Figure 5-32 Correction 1 vs. CAD Model (left), difference iteration 1 (middle) and difference iteration 2 (right) ................................................................................................................................. 52 Figure 5-33 Overview of the contribution of each effect on the accuracy mismatch ........................ 53 Figure 5-34 Achieved forming depth as a % of the computer model depth ...................................... 53 Figure 5-35 Pillow effect as a % of the achieved specimen depth ..................................................... 54 Figure 5-36 Pillow effect in mm for the geometry tests ..................................................................... 54 Figure 5-37 Sheet bending effect in mm for the geometry tests ....................................................... 54 Figure 5-38 Spring back effect in mm for the geometry tests ............................................................ 55 Figure 5-39 Spring back in % for the geometry tests compared to the computer model depth ....... 55 Figure 5-40 Overview of the contribution of each effect on the depth mismatch............................. 56

Abbreviations


Abbreviations

Term Description

2024-T3 Aluminum 2024 with T3 temper

3D Three Dimensional

ARALL Aramid Reinforced Aluminum Laminate

CAD Computer Aided Design

CARALL Carbon Reinforced Aluminum Laminate

CNC Computer Numerical Controlled

DASML Delft Aerospace Structures and Material Laboratory

FML Fiber Metal Laminate

G-Code General Code

GLARE Glass Laminated Reinforced Epoxy

KU Katholieke Universiteit (Catholic University)

LVDT Linear Voltage Displacement Transducer

M6/M8/M12 Metric bolt with 6/8/12 mm diameter

PSU Power Supply Unit

R Radius

Rev Revolution

SLC Structure Laminate Company

SPIF Single Point Incremental Forming

Sym Symbol

TPIF Two Point Incremental Forming

TU Technical University

UV Ultra Violet

Introduction


Introduction Chapter 1

Background 1.1Since the application of aluminum in aircraft structures around 1930, sheet metals were used extensively within civil aircraft. The advantages of having high specific properties provided the material to be economically very attractive. Even today the amount of aluminum used in modern aircraft is still significant compared to other materials. However in the 1950s several accidents occurred due to metal fatigue. This resulted in the development of new aluminum alloys that provided slightly better properties for corrosion resistance (7000 series) and fracture toughness (2000 series). But these changes did not lead to any significant improvement of the fatigue life of the material [1]. The real breakthrough for metal fatigue came from Rob Schliekelmann. After experimenting for a few years at De Havilland with metal bonding, he switched to Fokker. He introduced the metal bonding technique in the Fokker F-27 project. During fatigue tests with the F-27 wings it was discovered that the metal bonded parts had favorable resistance to fatigue crack growth, simply because the crack only grew in a single layer. This effect considerably slowed down the crack growth in the complete structure [1] [2]. This discovery of the improved fatigue properties provided the basis for Schijve and Vogelesang, at Delft University of Technology, to perform research in the field of bonded structures. In the 1970s they started experimenting with adding Aramid fibers between the aluminum sheets, which provided even better properties in terms of fatigue. But the ARALL material had very poor compressive properties. Therefore an alternative had to be found to make the Fiber Metal Laminate concept successful in the aircraft industry [1]. As an alternative Delft University of Technology developed a new type of FML called GLARE, which was a combination between glass fibers and aluminum. This material provided to be very promising for fatigue sensitive aircraft applications; however costs became an issue on applying the material into aircraft industry. These were partially solved by the weight savings that were possible and the developed spliced method. This provided the breakthrough for the application of GLARE on a large aircraft, the A380-800 [1]. Although GLARE is now applied on the A380 and considered as a material for future aircraft several cost and production issues still have to be solved for future applications of this material. Delft University of Technology has been involved in the development of GLARE from the start of FML laminates. Due to their involvement the TU Delft noticed that current production methods and standards are no longer sufficient in terms of speed and accuracy and thus a new method of producing GLARE has to be found.

Introduction


Problem definition 1.2GLARE panels are currently being produced by a hand lay-up method, which is a suitable production method for the low volume production line of e.g. the Airbus A380. In 1999 Broest and Sinke have provided the method to fabricate curved GLARE panels out of non-deformed flat sheets using a self-forming technique [3] [4]. This self-forming technique, initially invented a few years earlier by Structures Laminate Company, is now the standard for manufacturing of curved GLARE panels. Several studies have shown that GLARE can offer significant weight savings in other types of aircraft [1]. As a result, the demand for GLARE panels is most likely to increase. The current hand lay-up method is not suitable for dealing with high quantities of production and thus an alternative has to be found. Other options for forming GLARE focus on deforming post-cured GLARE panels, such as brake forming, roll forming, stretch forming, laser bending and shot peening. The issue with this method is that only limited out-of-plane formability is possible as the glass fiber layers significantly limit the post-cured formability of GLARE panels, especially in thicker laminates. Another major challenge with GLARE panels is the production of double curved panels. These days double curved GLARE panels are made of flat sheets with the earlier mentioned hand lay-up method. These flat sheets are then forced into a double curved mold to their final shape. Yet this method is limited to certain curvatures as the sheet edges will start to wrinkle due to the enclosed residual stresses. Some of these wrinkles can disappear and can be translated into internal sheet stresses during vacuum bagging and autoclave cycles. However, if the curvature is too large or the sheet is too wide, wrinkles will remain within the product. In practice the supplier of these sheets can deliver up to 1500 mm in width. But to avoid wrinkles the applied width is usually smaller. This leads to a large number of splices in large panels, which is unfavorable for both weight and simplicity. One way of avoiding wrinkles is by preforming the aluminum sheets to obtain a certain curvature. In theory this preforming should allow the production of larger panels with larger curvatures and less splices. In the aircraft industry double curved metal sheets with large radius, for e.g. fuselage application, are currently made by stretch forming. This process includes the use of a die which is product specific. But for the production of GLARE the use of dedicated dies becomes too expensive. At the moment several processes exist, but none of these processes meet the standards for accuracy and/or cycle time yet. Therefore this research will focus on a new type of process, the incremental sheet forming process, in order to determine its feasibility for the application of preforming metal sheets.

Thesis objective 1.3The main goal of this thesis is to determine the feasibility of incremental forming in terms of dimensional accuracy and process speed to preform the aluminum layers used in GLARE laminates. This starts with a literature study on GLARE material and incremental forming. The literature study is the basis of the theoretical analysis in which the process requirements are determined. This is followed by an experimental phase, which consists of obtaining an experimental setup and performing several experiments. The results of these experiments are combined with the theoretical analysis to determine to what extend the process is feasible. The thesis objective is defined as:

“To perform a study on the combination of GLARE material and incremental forming to define and perform an experimental setup and test plan to determine to what extend incremental forming is feasible as a method to preform the aluminum sheets used in GLARE.”

Introduction


Research question 1.4The main problem is to figure out to what extend incremental forming can be used to preform aluminum layers within GLARE laminates. Therefore, the main research question is:

“Can die-less incremental forming be a feasible process for preforming the aluminum layers used within GLARE laminates?”

In order to be able to answer the main research question, several sub-questions are formulated. These are listed below: 1. What is GLARE made of?

1.1. How is GLARE defined? 1.2. How is GLARE produced? 1.3. What are the material properties?

2. What is die-less incremental sheet forming?

2.1. How is incremental sheet forming defined? 2.2. What are the recent developments?

3. What are the process conditions?

3.1. What is the aim for production speed? 3.2. What is the aim for dimensional accuracy?

4. What is the experimental outcome?

4.1. Is the process repeatable, predictable and controllable? 4.2. What are the effects of different process settings on accuracy? 4.3. What are the effects of different process settings on process speed?

5. How can incremental forming be implemented for large scale manufacturing?

5.1. Which settings provided valid results for the process conditions? 5.2. What possibilities are there to improve in terms of dimensional accuracy? 5.3. What possibilities are there to improve in terms of production speed? 5.4. Which limitations does the process have?

Introduction


Research approach 1.5To accomplish the thesis objective and to answer the research questions, the research will be divided in several steps.

1.5.1 Literature study To obtain more knowledge about the material and the incremental sheet forming process a literature study will be done on these subjects. The knowledge obtained in this literature study can be used for the rest of the research. This literature study will also provide the answers to the first three sub-research questions and provide the basis of this research.

1.5.2 Experimental setup This part will focus on getting familiar with the experimental setup. It will describe the experimental setup, which includes the sheet material, the forming tool, the choice for the custom designed small size milling machine and clamping system being used. The material selected for the specimens is aluminum 2024-T3 with a thickness of 0.4 mm. The specimen sheet size is 120 x 120 mm. The forming tool being used is a straight steel tool with a 16 mm diameter. This forming tool is originally from an impact tower setup, but is sufficient for this research. This forming tool has sufficient wear resistance to neglect any effects of tool wear during the experiments. The small size milling machine is capable of moving along the specimen. The movement in X and Y axis are at least 100 mm in order to cover the area to be processed of the specimens. The movement in Z axis can be limited as the parts only require small curvature, which limits the depth of the part. In order to clamp the material, a jig is used to hold the material in place during the process. This jig is already available in the Delft Aerospace Structures and Material Laboratory at Delft University of Technology as it is also part of the impact tower setup. In order to work with this setup, a thorough knowledge of this setup is required. In the first place such a machine is not available in the Delft Aerospace Structures and Material Laboratory (DASML), thus a custom designed machine has to be designed and constructed. After which familiarization with the software is required to provide the input of the process. Therefore an extensive study is done for this special purpose setup.

Introduction


1.5.3 Experiments

Geometry experiments Each of the radius experiments will contain a specific pre-determined shape with a 100 mm or 200 mm longitudinal diameter and a 100 mm or 200 mm diameter in transverse direction. These diameters are much smaller compared to the diameters encountered in aircraft industry. This downscaling is required in order to obtain measurable dimensions during the experiments on small scale. During these experiments the other process parameters are kept constant at 1000 mm/min feed rate and a vertical step size of 0.2 mm per pass. After these specimens are formed they are measured by means of linear voltage displacement transducer before being released from the jig. This provides data about the local spring back during the process. These radius experiments are performed several times for each combination to provide information about the scatter of the results. In the first option, which consists of a 100 mm diameter in both directions, the experiments are conducted five times. If the results obtained provide no significant deviations the following experiments are conducted three times instead of five times. The reason to perform these experiments three times is to see if any large deviations occur during these tests. If these three experiments have a significant deviation to one another, the specific experiment is conducted again to obtain statistical data of the scatter.

Feed rate and step size experiments In order to see the effects of speeding up the process, the feed rate and vertical step size are adjusted. During these experiments the geometry is fixed at 100 mm diameter in both directions and only one variable is changed per experiment. The feed rate is varied from 1000 mm/min to 3000 mm/min in steps of 1000 mm/min, where the step size is fixed at 0.2 mm/pass. The step size is varied afterwards from 0.1 mm to 0.3 mm in steps of 0.1 mm, where the feed rate is fixed at 1000 mm/min. This results in a total of two additional experiments for the feed rate and two additional experiments for the step size. At last there will be a geometrical correction experiment according to Micari’s method of over bending / optimized trajectories [5]. In this experiment two iterations will be made to verify to what extend this method improves the geometrical accuracy. During these test it may be that the vertical step size is variable depending on the trajectory, while the feed rate remains at 1000 mm/min.

Number Step Size / Feed Rate Number Step Size / Feed Rate

1 0.2 mm / 1000 mm 5 0.2 mm / 3000 mm

2 0.2 mm / 1000 mm 6 0.1 mm / 1000 mm

3 0.2 mm / 1000 mm 7 0.3 mm / 1000 mm

4 0.2 mm / 2000 mm 8 Variable / 1000 mm Table 1 Test matrix of the geometry, feed rate and step size experiments

Introduction


1.5.4 Results, outcome and relevance

Data During the eight different experiments data is generated. This data has to be stored and processed. Depending on the amount of scatter during the experiments this implies that a minimum of 25 specimens (1x5, 6x3 and 1x2) is used up to a maximum of 37 specimens (7x5 and 1x2). In order to collect the geometrical data each specimen is measured four times at first and each point will be measured twice. If the measurements are proven to be reliable the following specimens will only be measured once, even though each point is still scanned twice. This is done with the aid of a linear voltage displacement transducer or LVDT. The data obtained from these measurements can be stored in an excel sheet containing the following information:

- Process settings - Intended geometry from the computer model - Achieved geometry from the actual specimen - Measured production time by machine

Outcome The goal of this Master thesis is to determine the feasibility of die-less forming of double curved aluminum sheets using single point incremental forming. The experiments performed on the different geometries will show which range of diameters can be achieved and at which accuracy these can be achieved. From these experiments it is expected that small diameters will provide more accurate results as the ratio between elastic and plastic deformations becomes smaller. Regarding the experiments performed with the different settings, different results are expected. It is expected that lowering the step size provides higher dimensional accuracy with longer production times. Considering the feed rate, it is expected that increasing the feed rate will decrease the process time, without affecting the dimensional accuracy.

Relevance The results obtained from the experiments can be used to link the different settings with the different process requirements. For example the geometric experiments can be used to obtain a relation between the intended geometry and the achieved geometry and thus the dimensional accuracy. These experiments also provide insight in the differences in production times between different forming depths as it requires more passes to form deeper products. Another important step is that the results of these small scale experiments have to be translated to large scale production in order to determine the feasibility.

1.5.5 Management During the research, management is required to keep track of the data, outcomes and the relevance to the research question. Management is also required for the deliverables required during the research, such as the interim report, Master thesis and the presentation of the research performed.

Literature Review


Literature Review Chapter 2

GLARE, a fiber metal laminate 2.1Fiber metal laminates are a group of materials with a metal and a fiber reinforced polymer component. This allows FMLs to have their properties tailored by changing the direction of the fibers for specific orientations and applications. GLARE is a so called Fiber Metal Laminate.

2.1.1 Patents Since the introduction of Fiber Metal Laminates in the 1970s many patents have been filed. The original patent on FMLs was filed by Delft University of Technology with patent NL8100087 and NL8100088 [6] [7]. The first patent contains the information of the constituents, while the second patent contains the information regarding the manufacturing process. These patents were based on ARALL, but they also covered the application of carbon (CARALL) and glass fibers (GLARE). Nevertheless GLARE has been patented separately with patent number US5039571 [8]. This patent focusses on the aluminum layers combined with glass fiber reinforced layers, instead of the earlier mentioned Aramid fiber. This patent also mentions specifically that the laminate consists of 3 to 25 layers with a thickness between 0.3 and 0.7 mm per layer. The aluminum layers alternate with glass fiber reinforced plastic layers. Therefore the laminate will always consist of N+1 layers of aluminum vs. N layers of glass fiber reinforced plastic as can be seen in Figure 2-1 [9]. Though these patents are used as a basis for GLARE in general, several other patents have been filed with slight deviations to the original patent.

Fiber layer deviations In 1996 Roebroeks and Mattousch filed a patent that includes a concept with fibers in perpendicular directions between the metallic layers [10]. They specified this concept as an impact resistant material. A deviation of this concept is made in 2003, when Roebroeks filed a patent that contains cross ply fiber layers between the aluminum layers, which consisted of two different fibers instead of one as mentioned in the previous patent [11].

Metal layer deviations The first patent that was filed with a change in the metallic layer was in 1989 by Vogelesang, Paalvast and Verbruggen [12]. This patent included ceramic layers at the impact side instead of metallic layers to improve the impact resistance of the material for armor plate applications. In 1990 Gunnink filed a similar patent to improve impact resistance; but instead of ceramic layers he filed a patent for a thick aluminum layer at the impact side [13]. This patent also included a change in bonding thick tapered aluminum layers to a FML with a prepreg.

Figure 2-1 GLARE Layup [8]

Literature Review


2.1.2 GLARE grades As GLARE developed, the material got standardized with a specific notation: GLARE AA-B/C-D. The AA indicates the Grade of GLARE and thus the orientation of the fibers and the aluminum alloy being used. The B/C part indicated the amount of metal / prepreg layers being used in the laminate. The D is used to define the metal layer thickness. As can be seen from Figure 2-2, on the next page, the aluminum material is either a 7475- or a 2024-T3 aluminum alloy [8]. The prepreg used is S2 Glass fiber prepreg with an epoxy resin FM94 for aluminum 2024-T3 or FM906 for aluminum 7475-T761 [1] [14]. Grade 1 has been developed initially as a proof of concept in the research phase of GLARE and was a direct spinoff of ARALL. This is the only Grade that uses aluminum 7475-T761 and the corresponding FM906 resin. This material has the highest ultimate tensile stress of the different GLARE Grades; but the material has a rather low formability. As a result Grade 2 has been developed with aluminum 2024-T3 and FM94 resin. This material provides a slight decrease in ultimate tensile stresses, but offers higher strain limits which makes the material easier to handle in part manufacturing. Both Grade 1 and Grade 2 are mainly used in unidirectional loaded structures and patching purposes [15]. Nevertheless the unidirectional Grade 1 and 2 weren’t very useful for the intended applications of fuselage material and Grade 3 and Grade 4 were developed. Grade 3 is mainly used on top of the fuselage where the ratio between circumferential stresses and longitudinal stresses is close to one. Grade 4 is used on the sides of the fuselage, where the loads in one direction are about twice as high compared to the other direction. Later Grade 5 was developed as an impact resistant material, which proved to be useful for impact sensitive areas such as lower wing skins, lower fuselage sections and leading edges. Grade 6 is a special Grade designed for shear areas and is the only Grade that has the fibers in a +/-45 degrees direction. The applications for Grade 6 are limited to shear webs or door corner doublers.

Literature Review


Figure 2-2 Different GLARE Grades with their fiber orientation and their specific constituents [16]

Literature Review


2.1.3 GLARE production

GLARE Laminates The aluminum layers in GLARE usually have a surface treatment before being laminated into a GLARE panel. The pretreatment of these layers consist of first anodizing with either chromic acid or phosphoric acid. Then the layers are primed with BR-127 corrosion inhibiting bond primer [17]. The fibers are usually delivered in the form of a prepreg with FM94 for all Grades except GLARE Grade 1 due to its different constituents. The aluminum layers are bonded together with the prepreg layers inside an autoclave, at temperatures up to 120 degrees Celsius and a maximum pressure of 6 bars [18]. At these temperatures the materials expand and cure together. Due to the differences in thermal expansion coefficients between the layers, the aluminum layers want to shrink more compared to the prepreg layers after cooling down. This implies that the aluminum layers will have tensile residual stresses, while the prepreg layers have compressive residual stresses. This has to be taken into account when the material is used. If the material is used at elevated temperatures the residual stresses will become lower; while at lower temperatures the residual stresses increase.

Post Stretching A solution to change the unfavorable stress state of GLARE after curing is post stretching of the material. During this post stretching process one reverses the stress state within the material by straining the metallic layers plastically. These layers will be elongated permanently, while the elastic fibers remain elastic. As a result, the residual stresses within the material will be reversed as can be seen in Figure 2-3, which is favorable for the fatigue life of the material [19].

Figure 2-3 Post stretching effect with Stress Strain curve [20]

But post stretching isn’t used in GLARE skin structures, due to the additional and difficult process step. The benefit of post stretching is that the crack initiation is extended; but the crack initiation is only a minor part of the fatigue life. Therefore the costs are much higher than the benefits of this post stretching method.

Literature Review


2.1.4 GLARE material properties

Tensile properties Tensile properties of GLARE have been measured by standardized tensile tests. In Table 2.1 a summary is made of some typical values found in literature for the most common GLARE Grades and the aluminum alloys used in these materials [15] [21] [22]. The tensile properties of GLARE are depending on its constituents: the glass fiber orientation, the aluminum layer and the adhesive or resin. The yield strength of GLARE is dictated by the aluminum alloy, which is the constituent with plastic behavior. Despite the other parameters, the ultimate strength is dictated by the failure of the glass fibers. The limit strain depends on the orientations of the fibers. If there are fibers in the strain direction, the limit strain will be dictated by the fibers. But if there are no fibers in the strain direction, the strain is dictated by a combination of the epoxy limit strain, the aluminum limit strain or a consistency failure in the material. This is only applicable to GLARE Grade 1 or 2. Another important feature of GLARE is the stress strain relation after the yield point has been reached. The strain hardening beyond the yield stress is almost linear. This is caused by the elastic deformations of the fibers; their contribution to the stress is significant after yielding. As a result the stress strain relation becomes a straight line, resulting in a bilinear stress strain curve. What can be seen in Table 2.1 is that Grade 1 and 2 have high ultimate tensile stresses compared to the other GLARE Grades and aluminum. This has to do with the fiber orientation in these Grades, which is unidirectional. This leads to superior properties in the main direction (L) over the aluminum alloys; in the secondary direction (LT) however, the material is inferior. The properties of Grade 3 and Grade 5, in both L and LT direction, are roughly equal to each other. This can be expected as the fibers are orientated in both directions in equal amounts and the aluminum itself is isotropic, which should result in a material that is more or less similar in both directions. Grade 4 on the other hand has twice as many fibers in one direction opposed to the other direction. This result in higher properties in one direction compared to the other direction. Though the difference between the two directions is not as large as with Grade 1 and 2. Therefore Grade 4 has in superior properties in both L and LT directions compared to aluminum. Table 2.1 Mechanical properties for GLARE laminates and aluminum alloys [15] [21] [22]

Property Sym Dir Dim GLARE 1 GLARE 2 GLARE 3 GLARE 4 GLARE 5 2024-T3 7475-T6

Young’s Modulus

E L GPa 65 66 58 57 59 72.5 71

LT 50 50 58 50 59 72.5 71

Yield stress

σy L MPa 545 360 305 352 297 324 483

LT 333 228 283 255 275 290 469

Ultimate tensile stress

σult L MPa 1282 1214 717 1027 683 440 538

LT 352 317 716 607 681 435 538

Failure Strain

ε L % 4.2 4.7 4.7 4.7 4.7 19 8

LT 7.7 10.8 4.7 4.7 4.7 19 8

Literature Review


Fatigue resistance The most important property of GLARE is its high fatigue resistance. Typically for aluminum alloys the majority of the fatigue life is required for crack initiation. This implies that once a crack is initiated the crack grows relatively fast. For GLARE this is the opposite; the minor part is used for crack initiation and the majority is used for crack growth. And since the amount of cycles to initiate a fatigue crack in both aluminum and GLARE is almost identical, GLARE has a longer fatigue life at higher stress levels. The crack growth goes slowly in GLARE, because of the so called fiber bridging. The crack typically initiates in the metal layers, keeping the fiber layers intact. This means that the load is being bypassed by the fibers, which reduces the stress concentrations at the crack tips in the metallic layers. Due to the ability to bypass the load in the fibers instead of the crack tip, the crack grows slower in GLARE compared to monolithic aluminum, which results in the superior fatigue properties.

Impact resistance Another good property of GLARE is the impact behavior. Vlot [23] studied the effects of impact on Fiber Metal Laminates. This research showed that several Fiber Metal Laminates provided smaller post-impact damage zones compared to fiber reinforced plastic materials due to the metallic layers within FMLs. An impact damage zone in a monolithic metallic material will leave small local damage in which the region is plastically deformed absorbing most of the energy. The fibers in a fully composite structure do not have the ability to deform plastically and absorb the loads. As a result impact on a composite structure will provide a large impact zone as the energy is spread through the different layers, possibly causing large spread delamination. In the case of an FML impact will cause the metallic layers to absorb most of the energy as they deform plastically, which prevents the impact from spreading as much as in a pure composite. This provides a smaller post-impact damage zone, which results in a higher residual strength after impact damage has occurred [21].

Fire resistance FMLs also excel in fire resistance. When a monolithic metal alloy is exposed to a heat source, the heat will penetrate the entire sheet. In GLARE the heat penetrates the outer metallic layer first. This layer will distribute the heat and possibly melt away, exposing the glass fiber layer. This glass fiber layer doesn’t conduct heat as well as the metallic layer, resulting in a smaller heat affected area at the next layer. Therefore this layer will act like insulation for the second metallic layer. Due to this process the second layer of metal is exposed to less heat and as a result it takes significant longer for heat to penetrate the entire material [21].

Corrosion resistance The corrosion resistance is another advantage of FMLs, which is due to the composite layers in between the metal layers. The composite layers between the metal layers will prevent corrosion to penetrate to the next metallic layer and thus the corrosion is limited to the outer layer of the FML in general. The composite layers deteriorate under influence of UV radiation and moisture, but these layers are shielded from these influences by the metal layers. So the deterioration of the composite layers is limited, which improves the general corrosion resistance behavior of the entire laminate [21].

Literature Review


Incremental sheet forming 2.2The single point incremental forming process is part of a group of processes, which are called incremental sheet forming processes. Basically, an incremental sheet forming process is a process in which a forming tool follows a specific tool path and while following this tool path the forming tool presses down on the sheet. As the forming tool presses down on the sheet, the material will locally deform due to the applied pressure. As the forming tool continuous to follow the tool path eventually the entire sheet will be deformed, piece by piece. This is where the incremental forming processes are different from conventional processes, as conventional processes apply the deformation to the whole sheet at once instead of locally.

2.2.1 Origin and classification Incremental sheet forming finds its origins all the way back in 1967 were Leszak patented an Apparatus and Process for Incremental Die-less Forming [24]. At that time the incremental sheet forming machine was way ahead of its time as computers weren’t advanced enough to make this process competitive. It took until the 90s until computers were advanced enough to allow incremental sheet forming to become a popular topic for researchers. In 2005 Jeswiet stated in one of the key note papers on this topic that the process has potential and there are many future applications [25]. But until today the incremental sheet forming is not yet widely used in industry, except for rapid prototyping. The main arguments are the lack of process speed and/or the lack of accuracy of the process, which outweigh the flexibility of the process.

Classification Incremental sheet forming has its origins in stretch forming processes and metal spinning processes. It’s a relative new technique for deforming sheet materials by applying a stepwise increment to the deforming tool. The main process consists of four elements, which are the sheet material, a blank holder, a single point forming tool and a CNC machine. Within incremental sheet forming there are two main categories; the conventional incremental sheet forming techniques and the hybrid incremental sheet forming techniques. Conventional incremental sheet forming With the conventional technique the tool moves over the surface of the sheet and determines the shape. There are no other tools or pressures applied for deforming the sheet into the desired shape. There are two different options for the conventional technique. The first option is negative die-less forming or single-point incremental forming (SPIF) and is the most commonly found process and also the process of choice for this research topic. In this process only one tool moves over the surface. The second option is two point incremental forming (TPIF) or positive die-less forming, where two tools move over the surface. One of the tools is used to apply the deformation and another tool to support the surface, which can be used to counter sheet bending. The two different processes are shown in Figure 2-4.

Figure 2-4 SPIF setup (left) and TPIF setup (right)

Literature Review


Hybrid incremental sheet forming The hybrid incremental sheet forming techniques are modified forms of the conventional techniques. In these processes the material is supported by a pressurized hydraulic fluid, partial die or full die on one side. Therefore three categories can be made within these hybrid techniques, these being the single point incremental forming with back pressure, two point incremental forming with a partial die and single point incremental forming with a full die. The single point incremental forming technique with back pressure is usually in the form of a fluid cell. In the partial die technique a solid partial die is used to provide support for the sheet material. In the case of a full die the deformation tool moves over one side of the surface and forms the material according to the shape of the die. The three different processes are shown in Figure 2-5.

Figure 2-5 Back pressure SPIF (left), partial die SPIF (center) and full die SPIF (right)

2.2.2 Incremental stretch forming During an investigation into a new hybrid forming process Araghi decided to combine stretch forming with incremental sheet forming [26]. He specifically mentioned that, in order to find a wider industrial use for incremental sheet forming, several limitations have to be solved, for example the long process time, the limited geometrical accuracy and the lack of dedicated process planning and modelling tools. In his research he tries to solve the limitations in process speed and geometrical accuracy with a full die, as can be seen in Figure 2-6. In this process the die is first pushed into the sheet material and afterwards the incremental forming tool is used to create the concave parts of the product. This does significantly reduce the process time as the surface for incremental sheet forming is reduced and the use of a full die improves the accuracy compared to die-less forming techniques. Nevertheless the reduction in production time is only applicable if the incremental sheet forming area is identified in advance, such that only those parts are processed. For these specimens the production time was reduced from 35 minutes for incremental forming to 20 minutes for the combined process. Another advantage of this method is that the whole specimen is more uniformly deformed as the stretch forming process provides global deformations, this result in a more uniform thickness distribution. The down side of this process is that it still requires the use of full dies, where industry is looking for die-less solutions. The setup for this process does provide a suitable basis for the die-less incremental forming setup. The main difference between the setup used by Araghi and the setup used in this research is that the die will be removed.

Figure 2-6 Incremental Stretch Forming with a die

Literature Review


2.2.3 Deformation principle The deformation pattern in incremental sheet forming was long assumed to be rather similar to that of a spinning process, which implies that shear deformations are the dominant deformations. In 2000 Kim and Yang [27] performed a research to assess and improve the formability in this forming method. They suggested a double-forming technique, assuming that only shear deformation occurs in the material. Later on in 2002 Kim and Park [28] showed that several factors play an important role in improving formability. They stated that to improve formability one can use lower feed rates, ball tools instead of hemispherical tools and lower friction between the tools and materials by means of improved lubrication. Till then it was assumed that shear deformation was the only deformation occurring. However, Filice [29] noticed that if the tool moves straight on a horizontal plane, the deformation is mostly plane-strain stretching. Except at the beginning and ending of the tool path the deformation is mostly biaxial stretching. Therefore if the radii of a tool path increase, the deformation mode is more biaxial stretching instead of plane-strain. Formability in metal is optimal under plane-strain stretching as the minor strain is zero in this case, providing the largest major strain limit; this implies that small radii of the tool path reduce the formability of the sheet material. Hussain [30] proved the findings of Filice after conducting several experiments for formability with different radius of curvatures in which the formability increased as the curvature decreased.

2.2.4 Accuracy Cerro [31] did several experiments measuring the geometric accuracy with the process model as compared to the actual product made in the CNC machine, which showed that the computer model provided very close resemblances with the actual product. However, these researches were done using a die or partial die within the process. In the case of die-less incremental forming there is no tool on the other side of the sheet. This method of single point incremental forming provided to be less accurate as shown by Micari [5]. The inaccuracy can be split up into three main effects: the sheet bending effect, pillow effect and sheet spring back effect as can be seen in Figure 2-7.

Figure 2-7 Single Point Incremental Forming Inaccuracy [5]

These inaccuracies are influenced by process parameters, material parameters and design parameters. In the case of a process parameter the step size, the distance between two successive loops, influences the accuracy. This implies that a large step size results in less accurate parts. In the case of material parameters strain hardening and anisotropy influence the accuracy. While in the case of design parameters the blank thickness and geometry of the part are influencing the accuracy. Nevertheless the shape and dimensional errors occurring during die-less forming can be reduced using optimized trajectories as mentioned by Micari [5]. The concept of optimized trajectories is very similar to that of over bending: the idea is to apply more deformations during the process in order to compensate for the relaxation. However Micari noticed that the main issue with this method is that there is to-date no completely accurate model in order to predict the material behavior in single point incremental forming.

Literature Review


2.2.5 Process speed Typically incremental forming is done at feed rates in the order of 500 mm/min or lower. This forming speed is usually seen as a negative factor, especially in industry where production rates are important. One of the first researches on using high speeds for incremental forming was done by Hamilton and Jeswiet [32]. Their research aimed at speeds from 5080 mm/min up to 8090 mm/min and focused on the effects the different speeds had on material quality. During this research they also varied the rotational speed and step size to see the effects of those parameters in combination with high feed rates. The step size influences both the process time and the surface roughness. A large step size results in a less repetitive pattern over the surface; this reduced the total length of the toolpath; but it increases the surface roughness. Considering the surface roughness they noticed that the combination between rotational speed and feed rates have a major effect. The higher the rotational speed over the feed rate, the smoother the surface. For the thickness distribution there are no significant changes if the feed rate is increased. Finally, they investigated the effects on the microstructure of the material; they noticed that the step size is the dominant factor on the grain size. A smaller step size results in smaller grains. Although they noticed that a higher spindle speed or feed rate also reduces grain size. In 2013 Ambrogio continued the study of Hamilton and Jeswiet by expanding the field of research to different materials and even higher speeds up to 600 m/min [33]. They also considered the microstructure of the alloys used during their research and stated that the microstructure is very similar to the as received state. However in the case of 600 m/min feed rate the grain size is slightly larger compared to a feed rate of 60 m/min. This contradicts the findings of Hamilton and Jeswiet earlier as they stated that increased feed rates result in reduced grain sizes. On the other hand the data from Ambrogio shows that at feed rates up to 60 m/min the statements of Hamilton and Jeswiet are valid. Nevertheless both researches state that the increased feed rates do not affect the material microstructure, meaning that the increased mechanical work done on the sheet material is insufficient to cause any changes. The material roughness is affected by the increased feed rates, but Hamilton and Jeswiet stated that with a proper choice of step size and spindle speed one can obtain the desired surface roughness. This implies that the process speed can be increased significantly solving one of the major disadvantage of this process.

Requirements


Requirements Chapter 3

Applications of GLARE 3.1

3.1.1 Current applications The current main application of GLARE is on the fuselage of the A380. Due to the superior damage tolerance properties of GLARE most of the upper fuselage is covered with GLARE, resulting in 27 panels with a total area of 470 square meters per aircraft. In addition GLARE, due to its good impact behavior, also got applied in impact sensitive areas, such as the leading edges of the empennage. In Figure 3-1 the application of GLARE on an A380 is visualized [34].

Figure 3-1 GLARE on an A380 [34]

But the A380 isn’t the only application of GLARE. The Learjet 45 has a bulkhead made of GLARE and Comtek Advanced Structures offers a GLARE based cargo liner interior solution for regional jet aircraft. Also Galaxy Aviation Security developed a GLARE LD3 luggage container, which is capable of containing blasts of twice the magnitude of the bomb used in the Lockerbie disaster in December 1988 [1] [34].

3.1.2 Future applications GLARE and other FMLs are seriously being considered in new aircraft designs due to their unique properties. A study performed in 1990 showed that application of GLARE on the A320 fuselage could result in a 25.9% weight reduction at a price of 280 dollar per kilogram saved weight. A study of SLC and Aerospatiale on the A330 fuselage also showed that a 20% weight reduction was possible with GLARE and a follow up study on the A340 showed a reduction of 14-17% was possible for this aircraft [1]. So even though the weight savings were possible, each of these studies were declined due to economics or politics. Considering economics GLARE was 5 to 10 times more expensive per kilogram compared to the aluminum alloys being used in those aircraft, the weight saving benefits were outnumbered by the added costs in materials and manufacturing. Considering politics in the 90s Boeing decided to make their new aircraft, the Dreamliner, out of carbon. As a result Airbus decided that their new aircraft, the A350, also should contain as much carbon as possible and thus GLARE got neglected. The main reason why GLARE was applied in the A380 was due to required weight savings in order to make it fly and costs were not as important [1]. The cost of GLARE limited the possibilities for future applications of this material. But several advances in manufacturing methods of GLARE, such as the splicing concept and integrating stiffeners into the panel design, have made the application of the material already much cheaper. These ongoing costs reduction might result in future applications of GLARE in redesigned fuselage panels of A320s, A330s, A340s and A350s [1].

Requirements


Industry requirements 3.2

3.2.1 Curvature requirements Most of the GLARE on the Airbus A380 is located at its fuselage, as can be seen in Figure 3-1. Even though this fuselage of the A380 has a rather large radius compared to the much smaller Airbus A320s for example, these radii are not critical in the manufacturing of GLARE as the strains are rather low. The critical components with respect to curvature for GLARE production will be the double curved applications at the empennage or leading edges. These parts may have radii in one or both directions of about 100-200 mm and therefore during the production higher strains will occur to achieve these radii. Therefore the process should at least be able to form parts with a radius of 100 mm, which occur in leading edges, up to 2000 mm, which occur in fuselages.

3.2.2 Speed requirements

Production assumptions In contrast to the production numbers of the Airbus A380, which is currently at around 3 per month, the Airbus A320, for example, has a production rate of 46 aircraft per month as mentioned by Williams [35]. This means that a large production volume is required for future applications of GLARE in aircraft as these. In the current Airbus A380 the fuselage is made of 470 square meters GLARE material, as shown in Figure 3-1. The cabin length is 49.9 meters with an average diameter of 8 meters, meaning roughly one third of the entire cabin fuselage is made of GLARE. If kept in mind that the production and manufacturing costs of GLARE can be lowered successfully the percentage of GLARE used in the fuselage could range between 50% and 90%. If this 50% fuselage material is applied on the smaller Airbus A320, which measures 27.5m cabin length and 4m cabin width, the total square meters of GLARE could range from 172 m

2 (50%) to 311 m

2 (90%).

An important notification has to be made that the Airbus A380 is most likely to have 4/3 GLARE lay-up, as the loads are significant higher compared to the Airbus A320, where a 3/2 GLARE lay-up is most likely. This has implications for the laminating or preforming to shape method, as fewer layers have to be used. In Table 3.1 an overview is given of the current production numbers of the Airbus A380 vs. the production numbers for the A320 with either 50% or 90% GLARE Fuselage.

Current (A380) Future (A320 50% GLARE) Future (A320 90% GLARE)

Aircraft Per Year [-] 30 552 552 GLARE Lay-up [-] 4/3 3/2 3/2 GLARE / Aircraft [m

2] 470 172 311

Production / Year [m2] 14100 94994 171662

Production / Hour [m2] 7 24 43

Aluminum / Hour [m2] 28 72 129

Glass Fiber / Hour [m2] 21 48 86

Table 3.1: Production Assumptions Overview

Process speed assumptions As can be seen from Table 3.1 the production of GLARE at this moment is roughly 7 m

2 per hour, where it is

assumed that the production goes on 50 weeks a year and 40 hours a week. This has to be ramped up to 24 to 43 m

2 per hour for the A320 production, where it is assumed that the production runs 50 weeks a year and

80 hours a week. Where hand lay-up is now common practice in the production line of the Airbus A380 for the application of GLARE, for the Airbus A320 this is no longer viable as the numbers are simply too high. Therefore the process has to be capable of producing 24 -43 m

2 of GLARE 3/2 lay-up per hour. This implies processing 72-129 m

2 of

aluminum and 48-86 m2 of glass fiber prepreg per hour.

Requirements


3.2.3 Accuracy requirements The accuracy requirements are depending on the forming method being used. Basically there are two forming methods being used for FML panels, which have different requirements for the preforming of the aluminum panels. These methods are the forming to shape method and the laminating to shape method.

Forming to shape accuracy The forming to shape method uses flat laminates, which are then formed into the final product. As a result, this method requires a high degree of dimensional accuracy; though preforming of the aluminum layers is no longer possible for this production method. Besides this method of producing GLARE parts will result in high residual stresses enclosed within the part as the glass layers remain elastic during the deformations.

Laminating to shape accuracy The laminating to shape method is based on lay-up of the individual layers, which can be preformed in advance. After the lay-up has been complete the materials are placed inside an autoclave, which cures the laminate by applying temperature and pressure. During this phase the material obtains its final form. Using this method the layers can be preformed before the laminate is cured. Also the required accuracy doesn’t have to match the exact product dimensions during the preforming of the aluminum layers. The reason is that during the autoclave process the pressure will correct some of the dimensional inaccuracies before the product achieves its final shape. Because of this the dimensional accuracy, before the autoclave process, can be in the order of 80 to 90% of the forming depth.

Spring back The application of a die-less stretch forming process on slightly double curved panels implies that the elastic energy within the material is significant, as the deformations are minimal. This implies that the spring back could be a serious issue regarding the accuracy. Spring back can occur locally, during incremental forming processes, as the deformations are locally applied. After the forming tool moves on, spring back occurs on these places locally. On the other hand after the part has been formed and removed from the clamps global spring back occurs, which implies that the residual stresses left over after deformation are partially unloaded. This might affect the entire dimensional accuracy, instead of only the local dimensional accuracy. The easiest way to compensate spring back is to apply slightly larger deformations to the product. This can be done after a product has been formed and measured in order to determine the amount of over deformation required to compensate for the spring back.

Requirements


Theoretical analysis 3.3

3.3.1 Forming parameters As mentioned before, the parts required to be produced have a large range of radii, ranging from 100 mm to 2000 mm for the Airbus A320 and can be single or double curved. In order to be feasible, the incremental forming process should be capable of producing these parts with an accuracy of 80 to 90% or higher at a speed of 72-129 m

2 per hour.

This doesn’t say anything yet of what kind of feed rate or step size must be reached to achieve these production volumes with this kind of accuracy. In order to determine the minimum required feed rate or step size an analysis is required of what kind of parts can be expected and what forming depths are to be expected. The speed during incremental sheet forming mainly depends on the step size and the feed rate, where the accuracy mainly depends on the step size. Therefore, one could for example increase the step size to achieve higher production rates, at the cost of some accuracy. But one could also increase the feed rate instead of the step size to achieve the same higher production rates, without sacrificing for accuracy. Therefore if possible increasing the feed rate is favorable over increasing the step size. But increasing the feed rate is not always possible, as friction might become an issue during the forming process or the feed rate is already at its maximum. As a result a higher step size is required in order to increase production rates.

3.3.2 Forming depth As mentioned before, the forming depth largely determines the process time as for deeper parts more passes are required to achieve the forming depth. Also the size of the panels to be produced directly influences the forming depth. If a large panel is made with a similar radius as the smaller panel, the forming depth increases and the forming area increases as well. Same goes for a panel of similar size, but now the radius is decreased. In this case the forming area remains the same, but the forming depth increases. In both cases the process time will significantly increase.

Single curved panels For fuselage panels it’s preferred to have as few splices as possible, thus the size of the panels should be as large as possible. For a typical Airbus A320, as used before in this analysis, the fuselage has a radius of about 2000 mm. In the current production process the sheet width is limited to a maximum of 1500 mm, which means that to cover an entire circumferential of an Airbus A320 a minimum of 9 panels and 9 splices is required. This implies that in order for the incremental forming process to be a feasible process it should be able to deform aluminum panels of at least 1500 mm in width direction to not add any more splices. As a fuselage panel has almost no double curvature, it is assumed that the panels are single curved panels for further analysis. A panel with a width of 1500 mm with a 2000 mm radius has a forming depth of 139 mm, as can be seen in Figure 3-2. This implies that with a step size of 0.1 mm/pass a total of 1390 passes are required to achieve this depth. These 1390 passes do not include any spring back compensation as well, which results in even more passes that are actually required to obtain the preferred shape.

Figure 3-2 3D sketch of a A320 Fuselage panel generated by the Matlab script in Appendix A-1

Requirements


Double curved panels GLARE can also be applied in leading edges and empennages. These panels have a double curved shape. This double curved shape, as explained in the introduction, is usually the problematic part for GLARE production as wrinkles occur in large double curved panels. Also the forming depth of these double curved parts is higher than those of single curved parts with equal radii. For example we take a random panel, which is required to have a 400 mm radius in one direction and a 2500 mm radius in another direction. This panel has a width of 500 mm and a length of 2000 mm. As a result, the depth of the panel, as shown in Figure 3-3, is already 273 mm. This is already double the depth of the fuselage panel and thus requires 2730 passes of 0.1 mm step size to obtain this depth. The major benefit of this part is that the area to be processed is significantly less and therefore a short tool path is required in order to produce this part. As a result, these parts will not require high feed rates or step sizes, as compared to the fuselage panels. Instead it is assumed that the production rate of these panels is 1/10

th of that of the fuselage panels.

However the spring back effects and residual stresses in double curved panels is hard to predict in advance, as the whole material is strained in multiple directions. This increases the inaccuracies seen in creating double curved panels with die-less methods. As a result, the creation of double curved panels provides to be more difficult compared to the single curved panels.

3.3.3 Forming requirements The particular panel being shown in Figure 3-2 is a panel of 1500 mm and 5000 mm length, covering a total of 7.5 m

2 of

material. In order to achieve the required production speed, mentioned earlier in 3.2.2, one has to create 9.6 to 17.5 of these panels per hour or one every 3.5 to 6.25 minutes; assuming only one machine is being used. The toolpath length depends on the step size, which implies that larger step sizes will reduce the tool path length. In Figure 3-4, on the right, a plot is made of the step size vs. the tool path length. As can be seen, the relationship between the two closely resembles an inverse relation. Yet due to the curvature present within the panel the location of the coordinate’s changes slightly, making the relation not entirely inverse. The feed rate, on the other hand, has no effect on the distance, but it does have an almost inverse relation with the production time. Due to some accelerating and deaccelerating effects in the stepper motors of the machines the relationship between the feed rate and the process time is not entirely an inverse relation. Though in practice one may assume that if the feed rate is doubled, the production time is halved. In Figure 3-5 a plot is made which relates the production time in minutes versus the feed rate.

Figure 3-3 3D Sketch of the double curved panel

Step size [mm] Figure 3-4 Step size vs. Toolpath length for large panels

Too

lpat

h [

m]

Figure 3-5 Feed rate vs. Production time for large panels

Feed rate [mm/min]

Pro

du

ctio

n t

ime

[m

in]

Requirements


In order to get the production time down to 3.5 to 6.25 minutes per panel, the process either requires a high feed rate or a high step size or a combination of both. In Figure 3-6 one can see that in order to achieve a production time of 6.25 min / panel (Red line) one could use much lower feed rate compared to the 3.5 min / panel production rate (Blue line). However one can also see that a combination of high feed rate and high step size are required in order to actually create these panels at the desired production rate, whether this is 3.5 minutes/panel or 6.25 minutes/panel. It has already been proven that feed rates up to 600 m/min are achievable with special purpose machines [33]. This still requires a step size of about 0.5 mm/pass in order to create these panels at this feed rate. Nevertheless these panels should not be an issue for incremental sheet forming in terms of their high volume production, as long as the parts can be made with reasonable accuracy. The only possible issue with these large radii panels is that the amount of elastic deformations is relative large compared to the plastic deformation, which implies that a lot of spring back is achieved. But the effects of spring back in single curved panels are well known and can be compensated. The small double curved panels might prove more difficult to manufacture. As any small deviation within the process settings might cause relative large inaccuracies and the complex spring back mechanism within these double curved shapes also contributes to larger inaccuracies. Therefore it is expected that these small double curved parts are critical for the feasibility of the incremental sheet forming process. The above shows that it is important that during the experimental phase of this research the specimens should resemble this category as much as possible. As the specimen size during the experimental phase is relative small, the parameters should be compensated for such. This implies that radii of 100 to 300 mm have to be tested with step sizes up to 0.3 mm and feed rate up to 3000 mm / min.

Figure 3-6 required step size vs. the feed rate for minimum and maximum production rates

Fee

d r

ate

[m

/min

] Minimum production (red)

Maximum production (blue)

Step size [mm]

Experimental Setup


Experimental Setup Chapter 4

Machine selection 4.1

4.1.1 Background Incremental forming is usually performed at a modified computer numerical controlled (CNC) milling machine. The reason why a modified CNC milling machine is required is that during incremental forming the forces on the forming tool are much higher compared to the traditional milling process. Typically milling operations are performed at very high cutting speeds, which imply that each cut only removes a very small portion of the material. Therefore the cutting force remains typically below 10 N in any direction, despite the high feed rates. During incremental forming the tool presses on top of the material and is then moved in the XY plane afterwards. In order to deform the material the tool is first pressed into the material with a certain predefined tool path, in which the forces are highly depending on the tool path. Duflou et al. [36] have done research in the field of forming forces during incremental forming and they suggested the following equation:

𝐹𝑍 = 0.0716 𝑅𝑚 𝑡1.57 𝑑𝑡0.41 ∆ℎ0.09 𝛼 cos (𝛼)

In this formula the force in Z direction thus depends on the tensile strength(𝑅𝑚), the thickness of the plate(𝑡), the diameter of the tool(𝑑𝑡), the step height(∆ℎ) and the forming angle between the forming tool and the material in degrees (𝛼). In their experiments they used aluminum 2024 of 0.4 mm thickness, which is the same type of material being used in this study. The forces they measured during their experiments were in the order of 200 N in the Z direction for the 0.4 mm thickness and at angles of 20 degrees, which is similar to what can be expected during this study. Even though the forces in X and Y directions are significantly lower compared to the Z direction during incremental forming, these forces are still much higher compared to the milling forces that a typical CNC milling machine have to endure.

4.1.2 Delft structures and material laboratory The DASML has several CNC milling machines, but none of these machines were able to incremental form due to the forming forces. Most of the CNC machines within the laboratory were so called router CNC machines, see Figure 4-1. These CNC routers are simple, accurate machines and are perfect for 3D printing, engraving or milling operations. However as one can see in Figure 4-1 on the left side, if the Z-axis is exposed to the forces during incremental forming, the Y axis will bend as it has a rather low stiffness. Due to the low stiffness within the Y-axis the CNC routers in the laboratory were only capable of achieving forces in the order of 20 N and below to prevent any deformations within the Y-axis.

Figure 4-1 A 3-axis router machine (left) vs. 3-axis knee milling machine (right) [37] [38] [39]

Experimental Setup


There is also a different type of machine used for CNC milling. These are the so called knee milling machines, as can be seen in Figure 4-1 on the right side. These machines have their Z-axis separated from their other axis. The main advantage of these machines is therefore their rigidity. The X-axis and Y-axis are usually controlled with a solid XY Table, which can hold much higher forces compared to the rails system of the routers. The Z-axis is capable of delivering more force as the axis is fitted in a rigid top and fully separated of the other two axes, preventing any deformation to those axes. Therefore these types of machines are also capable of performing processes which include higher forces, such as incremental forming. The DASML has one CNC knee milling machine, the Proxxon Micro Frees MF 70. This is one of the smallest CNC knee milling machines available and comes with low end stepper motors. As a result this machine was only able to deliver forces up to 50 N according to the specifications before the stepper engines stop working. Another problem with this machine was its range, which was limited to 134x46 mm in the XY plane.

4.1.3 Catholic University of Leuven Because of the absence of a working machine at the DASML another option had to be considered for this project. Basically two options were available. The first was constructing a larger knee milling machine from scratch, however this would be highly time consuming and would cause a serious delay to the project. An alternative was to find a different university that had such machines available. One of these universities was the Catholic University of Leuven, which is one of the leading universities in the topic of incremental forming. After several contacts it turned out that the machine they usually used for incremental forming was damaged beyond repair. But they had other machines, which were available, but after sending more detailed information about the research there was no more contact and eventually the option of performing the experiments was dismissed.

4.1.4 Custom design As timed passed by and no progress was made with other companies or universities to perform the required experiments at one of their machines, the decision was made to design and construct a custom CNC knee milling machine within the DASML. This was done to prevent any additional delays to the research; however this did imply that a lot of information had to be obtained about how these machines are made and how they are controlled.

Experimental Setup


Machine design 4.2As explained before the decision was made to design and construct a CNC milling machine capable of incremental forming within DASML. Therefore the machine should be able to operate at feed rates of 3000 mm/min. The step size in Z-direction should be controllable up to 0.1 mm and the workable area should at least be 100x100 mm in order to fit the specimens.

4.2.1 Mechanical design Within the laboratory a manual controlled HBM BF 16 drilling/milling machine was available to be converted for the incremental forming experiments, as can be seen in Figure 4-2. This simple, yet rigid machine provided an excellent starting point for the mechanical design. However, it was required that the machine could be converted back to its original state after this research has been completed. This provided some challenges as any permanent changes to the machine itself were prohibited, while still making sure that the machine could be fitted with stepper motors on each axis. As a result, the mechanical design and assembly basically consists of three steps. The first step is the Y-axis assembly, which allows back and forward movement. This is followed by the X-axis assembly, which allows sideways movement, and finally the Z-axis assembly, which allows up and downward movement.

Figure 4-2 HBM BF16 Drilling/Milling machine [39]

Step 1: Y-axis modification The Y-axis allows the XY Table to move back and forth. The Y-axis in this machine consisted of a 15 mm rod with a thread pitch of 5 mm on the inside of the machine. This rod was connected to the machine by a ball bearing on the outside, which fed the rod towards the black handle visible in Figure 4-3. As the axis itself was still in good shape, it was decided to keep the axis as is. The only modification done to the axis itself is the removal of the black handle to allow a shaft coupler on the axis in order to close the gap between the different diameters of the axis itself and the axis of the stepper motor.

Figure 4-3 Conventional Y-axis close up [38]

Experimental Setup


Also the current axis was secured by two bolts just behind the black handle; one of them can be seen in Figure 4-3. These bolts will be replaced by longer bolts to secure an additional aluminum backing plate at the same position. This aluminum backing plate will function as a connection point for the stepper motor in order to avoid any permanent modifications on the machine. The design of the stepper motor fitting, as can be seen in Figure 4-4, is kept rather simple. The backing plate (green) will be fixed with two bolts, as mentioned earlier, on the machine itself. In this drawing, the location of the holes of these bolts are not shown as they were specified later on during the assembly. A total of four rods (red) with a diameter of 12 mm are used to go from the backing plate of the machine towards the stepper motor, while in the center the shaft coupler (yellow) is positioned in place to connect the two different axes to each other. Both the backing plate of the machine (green) and the stepper motor backing plate (blue) are fixed to the rods (red) by M4 screws.

Figure 4-4 Catia sketch of the Y-axis stepper motor fitting

The shaft couplers, Oldham D25 L30 D1-8 D2-12, were eventually ordered by Hardware CNC and were rather easy to install. But due to wear the end of the Y-axis of the machine wasn’t exactly 12 mm, this resulted in some slipping between the shaft coupler and the axis itself. In order to solve this issue two-component metal glue has been applied, which locks the two parts together. Eventually, this simple design resulted in a fully functioning Y-axis, as can be seen in Figure 4-5, in which the stepper motor is directly connected to the axis with the use of the shaft coupler.

Figure 4-5 Complete CNC Y-axis of the HBM BF16

Experimental Setup


Step 2: X-axis modification The X-axis is very similar to the Y-axis, except that this axis has handles on both sides. This means that, in theory, the stepper motor could be fitted on both sides. However if one keeps in mind that it’s the easiest to keep all the wiring on the same side of the machine, it has been chosen to attach the stepper motor to the left side of the machine. As the Z-axis stepper motor will be added to the left side as well. Just like for the Y-axis, the axis itself was still in good shape and there was no need to replace it with a new axis. Again the axis consisted of a 15 mm rod with a thread pitch of 5 mm on the inside of the machine and an exterior rod of 12 mm, on which the black handles were attached. Therefore the black handle, on the left side, was removed in order to make room for a shaft coupler. After a close inspection of the connection between the shaft coupler and the axis itself, it was noted that due to some wear on the axis the shaft coupler couldn’t provide sufficient grip on the axis itself. Therefore an additional coupling piece was required that was able to fit on the axis shaft (11.8 mm diameter) and then also provide a connection point for the shaft coupler (12 mm diameter) as can be seen in Figure 4-7. But if some torque is applied to this coupling piece it will loosen itself. The solution for this problem was relative easy; in order to secure the black handle a small M4 rod was used in a similar sized hole through the axis. This same hole was used to secure the coupling piece with a M4 bolt to the axis. As a result, the coupling piece was secured to the axis and couldn’t be loosened without the removal of the screw first, which resulted in a fully functioning X-axis as can be seen in Figure 4-8.

Figure 4-8 Complete CNC X-axis of the HBM BF16

Figure 4-6 Conventional X-axis Close up

Figure 4-7 Additional Coupling Piece X-axis

Experimental Setup


Step 3: Z-axis modification In the mechanical design the Z-axis provided the most difficulty as, unlike the X and Y axis, the Z axis lacked any ball bearing supports. However on the left side of the machine a safety shield was attached with two screws on the black mounting, as can be seen in Figure 4-9. Underneath the mounting of the safety shield were two M6 bolts located, which provided an excellent mounting point for the Z-axis assembly. Therefore the decision was made to remove the safety shield. In order to connect the shaft coupler to the axis itself the metallic round cap had to be removed, as can also be seen in Figure 4-9. If this cap is removed a spring will be visible. This spring prevents the Z-axis from falling down in the conventional state, but also allows the Z-axis going up automatically when the machine is not being operated.

Removing this spring is a critical step; once the spring is removed the entire Z-axis will fall downwards, which might damage the Z-axis or drill head. During this project the machine was first lowered to its lowest position and the handlebar with the red ends was secured with tape. This prevented the Z-axis from coming down as soon as the spring was removed. After removing the spring, the Z-axis was clearly visible, in good shape and easily accessible. With the additional two M6 bolts above as mounting point, the Z-axis could be fitted in the same way as the X and Y axes basically. Once again the same basis is being used as the one that can be seen in Figure 4-4. But some changes were required in order for this design to fit. One of the major changes was the size of the backing plate, which is much larger for the Z-axis. The reason for this larger plate is that the attachment points, the two M6 bolts, were located slightly further than those of the X or Y axis. Another change was the position of the shaft coupler as this one was not fitted in front of the backing plate, but partially behind the backing plate. As a result the length of the four rods is significantly shorter. In the end this method provided a fully functioning CNC Z-axis, as can be seen in Figure 4-10. The completion of the Z-axis concluded the mechanical conversion of the HBM BF16 from a conventional machine towards a CNC machine with displacement control for the three main axes. In Figure 4-11 the entire mechanical design is shown, which includes the conversion of the X, Y and Z-axis.

Figure 4-9 Conventional Z-axis close up [38]

Figure 4-10 Complete CNC Z-axis of the HBM BF16

Figure 4-11 Complete Conversion of the HBM BF16

Experimental Setup


4.2.2 Electrical design The electrical part of the experimental setup consists of a control box, which is used to translate the given computer input to the stepper motors. Typically, a 3-axis CNC milling machine consists of three separate motors, which are driven by three separate drivers. These drivers and motors are usually powered by a power supply unit and are connected to the computer via an interface card. The choice of which kind of motors are being used on the machine determines most of the electrical design. Therefore this part starts with the selection of which kind of motor will be used.

Stepper motors There are several types of stepper motors available for CNC purposes, such as servo motors, unipolar stepper motors and bipolar stepper motors. Servo motors have an advantage over uni- or bipolar stepper motors as they are capable of delivering a feedback to the system of their actual position. However this comes at a cost, which makes servo motors much more expensive compared to uni- or bipolar stepper motors. Even though uni- and bipolar stepper motors do not provide any feedback to the control system of their current position, they usually do not require it as long as no steps are lost during the process. Loss of steps occurs when the stepper motor has to deliver more torque than it could. In that case the software will send a signal to the motor that it should rotate, while physically the motor can’t deliver enough torque to actually rotate. As a result, the software assumed the motor has rotated, while in reality it stood still and only provided a clicking noise. Loss of steps could thus easily be avoided, if the process itself does not approach the torque limit of the stepper motor and thus make the feedback option of a servo motor redundant. This leaves the choice between uni- and bipolar motors. A bipolar motor works basically the same as a unipolar motor; however it has an additional set of magnets/poles. As a result the bipolar motors are more efficient and deliver more torque compared to a similar sized unipolar motor. Therefore bipolar motors are usually preferred for CNC milling operations. Now the choice has been made to use bipolar stepper motors, the next step is to determine the amount of torque required for the incremental forming process. As mentioned before Duflou et al. [36] performed similar experiments on the same type of material to determine a relation between the forming force and several parameters, with forces of about 200 N in the Z direction. For the Z-axis a worm drive gear arrangement is being used to transfer the torque of the stepper motor to the forming tool. It’s measured from the mechanical design that for every full rotation of the Z-axis the forming tool moves 25 mm. This implies that if the forming tool has to press down with 200 N, the torque on the Z-axis has to be around 0.8 Nm. However this is true for a static situation with 100% efficiency. The stepper motor will be constantly accelerating and deaccelerating to its correct position and at the same time the transmission will have some friction. Thus a higher torque is required to achieve this 200 N in reality. Because of the higher torque needed one might assume that the bigger the stepper motor, the better. But this is not always true as bigger stepper motors have larger moments of inertia. A bigger moment of inertia implies that it will require more torque to accelerate the motor itself, resulting in a less efficient motor. With the above points in mind the choice has been made to use Nema 23 – 3 Nm stepper motors from Hardware CNC, which can be seen in Figure 4-12.

Figure 4-12 Stepper motor Nema 23 – 3 Nm [43]

Experimental Setup


Stepper drivers In order to control the stepper motors, a stepper drive is usually required. For this research a certain degree of precision is required. Typically, a stepper motor has 200 steps per revolution, which implies that for the Z-axis (25 mm movement per revolution) each step accounts for 0.125 mm and for the other axes (5 mm movement per revolution) each step accounts for 0.025 mm. To obtain movement accuracies of 0.01 mm or below the stepper driver should be able to use so called microsteps. Microstepping is a method to control a stepper motor in such a way that each step on its own is divided into multiple steps. For example 1/8 microstepping allows a stepper motor 1600 steps/revolution instead of 200, which increases the precision of the stepper motor by a factor of 8. In order to achieve accuracies of 0.01 mm and below the Z-axis requires at least 2500 steps / rotation and the other two axes require at least 500 steps / rotation. As microstepping occurs in 2, 4, 8, 16, 32 or 64 microsteps per full step the Z-axis is required to have at least 16 microsteps / rotation and the other two axes require at least 4 microsteps / rotation. Another important factor is the voltage and amperage that the stepper drive could handle. In order to fully utilize the Nema 23 – 3 Nm stepper motors 48 Volts and 4 Amperes is required. This requirement of voltage and amperage, in combination with the microstepping requirement, resulted in the choice for the Leadshine DM556 – 2 Phase Digital Stepper Drive from Hardware CNC. This Leadshine DM556 stepper driver is able to provide up to 128 microsteps per revolution, which is more than required 16 microsteps for the Z-axis. Also this stepper drive can withstand a maximum of 50 Volts and 5.6 Amperes, which is more than the stepper motors require.

Power supply unit The power supply unit will be used to power the stepper drivers and the stepper motors separately. The standard 230 Volts can’t be used on the stepper motors or drivers, as the stepper driver has a maximum of 50 Volts and the stepper motors have a maximum of 48 Volts. Therefore a power supply unit of 48 Volts is the maximum one could use. Another important factor to keep in mind during the selection of a power supply is the current it provides. The stepper motors are using 4 Ampere, if they are to run at maximum speed / power. In order to power all three stepper motors at maximum speed / power, the system should require 12 Amperes. But the three stepper motors don’t run at maximum speed or power all at the same time. Therefore one could take a less powerful power supply unit. A rule of thumb, generally used and advised, is to take a little over 1.5x the maximum rated current of a single stepper motor for the power supply unit. In this case that would come down to a power supply unit, which has at most 48 Volts and minimum of 6 Amperes. The closest available match, who had more than 6 Amperes, was a 48 Volts 6.7 Ampere power supply unit offered by Hardware CNC, as can be seen in Figure 4-14.

Figure 4-13 Leadshine DM556 - 2 Phase Digital Stepper Drive [42]

Figure 4-14 PSU 48VDC 6,7A [41]

Experimental Setup


Interface card Even though the stepper motors, drivers and the power supply unit can work on their own; this system has to be connected to a computer in order to have a computer numerical controlled application. Modern stepper drivers have the option to directly connect the drivers to a computer. But since this system requires an external power supply, this would provide 48 Volt 6.7 Ampere towards the computer. This would ruin the computer and thus some kind of separation is required between the computer system and the machine system. This separation is usually done with an interface card. An interface card is able to separate the two electrical systems, while still transferring information between these systems. One end the interface card will be connected to the computer, while the other end will be connected to the stepper drivers. A commonly used interface card for several applications and also being used on this machine is; the Arduino Mega 2560 in combination with a Ramps 1.4 Shield, both from 123-3D. The Arduino Mega 2560, see Figure 4-15, can be programmed into whatever the programmers basically wants, which is in this case an interface card for CNC application. The Ramps 1.4 Shield, see Figure 4-16, can be used to connect the wires coming from the stepper drivers to the circuit in the right positions. The benefit of this shield, over any other shields, is that this one is made for CNC milling operations, including clear instructions which axis and wires should be positioned where on the board. This makes the assembly of the system much clearer.

Enclosure box Another important part of the electrical system is the enclosure box or control box, in which the electrical system is mounted. Basically, the enclosure box prevents the user from touching any of the electrical components in order to avoid damage to the components or the user from being shocked. As this system will run on 48 Volts and 6.7 Amperes an electrical shock from the system could injure the user. Thus the enclosure box used in this project is required to be made out of a non-conducting material and should be large enough to fit all the individual components. The choice was made to go for the 1SL02 IP66 Wall Box of RS components, as can be seen in Figure 4-17. This box was fully made out of thermoplastic material, which does not conduct electricity. Also the interior dimensions are sufficient to mount all the electrical components inside, while they can still be modified and/or accessed by opening the door.

Figure 4-15 Arduino Mega 2560 [38]

Figure 4-16 Ramps 1.4 Shield [39]

Figure 4-17 1SL02 IP66 Wall Box [40]

Experimental Setup


Wiring scheme Now that all components are selected for the electrical design, they still have to be properly connected in order to get a working system and also fitted inside the enclosure box. Considering the wiring, a wiring scheme is setup in order to successfully connect the individual components to each other for a working system. The wiring scheme is based on a 3D printer wiring scheme found online, as shown in Figure 4-18. In this wiring scheme a Ramps 1.4 card is shown in the center and shows how to connect the stepper drivers towards the Ramps 1.4 shield. The difference comes in the power supply units and the amount of stepper motors. The CNC machine used in this project only has three stepper motors and one PSU, instead of four motors and two PSUs. Also the system will not use any extruders, end stops, PSU for the Ramps 1.4 or any fans to cool the system. If this wiring scheme is adjusted with the right components, all that remains can be seen in Figure 4-19. However this only provides a direct system, without any emergency stops or any on/off switches. It is required for any machine to have an emergency stop and an on/off switch, before it may be used within the DASML. The on/off switch should stop the entire power supply to the whole electrical system and the same for the emergency button. As the PSU is the only component being powered from an external power source, it is most logical to place the on/off switch and emergency stop directly at this connection. Even though this works perfectly fine and the machine will stop when the switch is set to off or the emergency button is pressed, the entire system itself will still be partially fed. This is due to the Dutch power supply system; one can insert an electricity plug in two ways, which means that the “flow” of the electricity can enter a system on two sides. If the emergency stop and on/off switch are at the beginning of this “flow” the system will be entirely without power. If these switches are at the end of the “flow” the system is still connected and any contact with the electrical components will give the user a shock. As a result, a relay is used as an additional safety measure. This relay is active as long as the electricity is flowing. But as soon as the emergency stop is pressed or the on/off switch is set to off, the relay is deactivated and the power will be cut on both sides of the electrical system. Now it doesn’t matter which side is the positive and which side is the negative side within the system, as the power is cut off at both the beginning and the end. Therefore the entire system in between will be without any electricity and the user can’t be shocked by accident any longer. This resulted in the complete and safe electrical control unit, as can be seen in Figure 4-20, which can be used to control the machine.

Figure 4-18 3D printer wiring scheme [46]

Figure 4-19 Custom design wiring scheme

Figure 4-20 Complete Electrical Control Box

Experimental Setup


4.2.3 Software Regarding the software being used in this project, the DASML has been working with Repetier in the past for 3D printers. This software is also compatible for CNC milling operations and can thus be converted towards an incremental forming process. In order to convert the software for incremental forming it has to be understood that the software package comes with two separate units; the Repetier firmware for the Arduino board and the Repetier host software for the computer itself.

Repetier firmware The firmware is basically the program that is copied onto the Arduino Mega 2560 card. This software contains all the information to convert the computer signal, received by the USB cable, into an electronical signal that can be sent towards the correct pins at the Arduino card. This Arduino card can then transfer the signal to the Ramps 1.4 card and this Ramps 1.4 card can then transfer the signal towards the stepper drivers. The first change to this firmware is to switch all extruders off, as the machine will not have any. After this is done the correct motherboard has to be chosen from a list, included within the Repetier firmware. Last but not least, all the end stops should be turned off in order for the machine to actually work. All these changes in the firmware have to be made in the “Configuration.h” file. The full list of all changed lines of code can be found in Appendix B – Repetier Firmware changes. If all these changes are done correctly the firmware is ready to be uploaded to the Arduino Mega 2560 and can be used for any incremental forming operation or other 3-axis CNC milling operations without end stops / limit switches.

Repetier host On the other side of the USB cable of the Arduino Mega 2560 is a computer with the Repetier host software installed. This software package is able to communicate with the Repetier firmware located on the Arduino. However, this software package also requires some different settings compared to the usual settings in order to perform incremental forming operations, instead of 3D printing operations. If the Repetier host software is successfully connected the printer settings could be selected in the top right corner, as shown in Figure 4-21. Normally these settings are set for a Cartesian 3D printer, which implies that the printer has an X, Y and Z axis. Therefore, the amount of changes in the printer settings are limited as only the extruder options have to be disabled. In the interface, as seen in Figure 4-21, several sections are seen. In the middle there is the 3D sketch of the specimen to be produced. If a G-code is loaded, the shape that will be created will be visualized in this screen. In the lower part of the screen the data log is shown. This shows any errors obtained during the process, but also the position of the machine at a specific point in time. It has to be notified that Repetier host loads twelve points in advance. So in order to match the time with a location, one has to read twelve lines of G-code back to match that specific time. This becomes important during the calibration of the measurement grid later on. Besides these windows, there are also the control tabs. These are used to load objects, write G-code, slice objects or move the machine manually to a specific point. During this research the manual movement tab is used to maneuver the machine into the correct starting position. Also the print preview tab is used to load the generated G-code into Repetier-Host and to preview and verify the code to see if any flaws are made within the G-code.

Figure 4-21 Repetier Host interface

Experimental Setup


Jig and specimens 4.3As the machine design has been completed, the specimens and jigs have to be chosen in such a way that the machine is able to work with them. For example, the machine has a limited range in X and Y direction, if the specimens are too large or the jig is too large the machine couldn’t process the entire specimen.

4.3.1 The jig The jig chosen for this project is usually used for the impact tower setup, which is present in the DASML. The main reason to choose this jig is that it actually fits on the machine without any adjustments. Also the jig provides an easy way to clamp a specimen of 120x120 mm on four sides, with a process area of 100x100 mm. As can be seen in Figure 4-22, the jig consists of three parts, a foot and two clamping plates, which can be removed. The advantage of this setup is that the foot can be secured to the machine, while the clamping plates can be removed to change a specimen without adjusting the foot location. Therefore, the machine doesn’t have to be adjusted to the right position every time a specimen is changed. The clamping plates being used for these experiments are 150x150x10 mm and contain nine M12 bolts to secure them and the specimen in between to the foot. The reason to use two clamping plates is in order to make sure that the specimen has the same clamping conditions on the lower side as on the upper side, since the foot only contains nine M12 holes, but not a decent clean clamping surface.

4.3.2 Specimens The specimens used in this project are made of 2024-T3 aluminum, which is the aluminum most frequently used in GLARE laminates. Only the GLARE Grade 1 laminates use a different type of aluminum, but this type of GLARE is not used in aircraft industry in general. The thickness of these aluminum layers is usually 0.3 mm or 0.4 mm. For incremental forming a thicker plate is usually more critical than a thinner plate, therefore the choice was made to go for 2024-T3 0.4 aluminum instead of 2024-T3 0.3 aluminum. The length and width of the specimens is determined by the maximum allowable size of the jig, which is 120x120 mm. As a result the specimens for each test are made of 2024-T3 aluminum and should have a size of 120x120x0.4 mm. In Figure 4-23 an example of such a specimen is given.

Figure 4-22 The jig

Figure 4-23 Example of an aluminum 2024-T3 0.4 specimen

Experimental Setup


Measuring 4.4In order to measure the specimens at first a laser scanning setup was preferred. However this system was not available during the research. Therefore an alternative had to be found to measure the shape of the produced specimens to compare them with the input model and the other specimens within the same set of experiments. This resulted in a measurement setup with a linear voltage displacement transducer instead of the laser.

4.4.1 Linear voltage displacement transducer A linear voltage displacement transducer is an electrical transformer, which is frequently used to measure displacements or positions. Inside an LVDT is a variable resistance, which is used to determine the displacement in the end. Basically, if the LVDT is placed on a surface at the deepest position and afterwards moved towards a higher position, the LVDT is partially compressed. This results in a resistance change within the LVDT itself. The change in resistance can then be used to determine the displacement in the Z-direction and this is a linear relationship.

4.4.1 Measurement grid In principle the LVDT is capable of measuring continuously. But the data logging system had a delay of 1 to 1.5 seconds per measurement. Therefore, the results of the LVDT can’t be logged continuously, but it was chosen to take the measurement every two seconds. This did imply that a measurement grid had to be created to make sure that the same points were measured every time. Also to verify that the LVDT measures correctly, each data point was measured twice. Keeping in mind that scanning of a specimen shouldn’t be too coarse, but also not too slow a compromise had to be made. The specimens deformed area is 70x70 mm, which means that if the LVDT measures every 5 mm a total of 225 data points are created. The measurement of these 225 data points takes a total of 15 minutes, which is a reasonable scanning time, and still provides a dense enough grid to extract sufficient detail of the surface of the specimen. The Matlab script to generate the measurement grid can be found in Appendix A-2.

4.4.2 Calibration

Linear Voltage Displacement Transducer In this experimental setup two different LVDTs were used. The first had a range of 0-8 mm with an accuracy of 0.01 mm, where 0.05 mm or below is required, and is sufficient for most experiments. For the later geometry experiments it is possible that the depth is larger than 8 mm, thus a 0-100 mm LVDT is used. But this large LVDT doesn’t have a complete linear range due to a defect on the inside. But in a range of 32-100 mm this LVDT does provide a linear relation, which implies that an offset of 32 mm is required for this LVDT. Yet a calibration is still required, as the LVDT provides an output in Volts and not in mm. It was found that for the small range LVDT one Volt was equal to a change of 4.64252 mm, with an accuracy of +/- 0.01 mm in the range of 0-8 mm. For the large LVDT it was found that one Volt was equal to 10.03886 mm, with an accuracy of +/- 0.03 mm in the range of 32-100 mm. It had to be kept in mind that if this LVDT was used, the initial displacement position was already at 32 mm or higher, to ensure that this LVDT was in its linear range.

Measurement Grid Besides the LVDT the measurement grid also had to be calibrated as the movement of the machine and the measurement points had to correspond to each other. Therefore, four dents were made, in every specimen, at the corner points. These dents were large enough for the LVDT to detect and therefore the location of the LVDT could be matched with that of the machine coordinates. Although the dents are detectable, it only provides a correspondence with the first point of the measurement grid and the machine coordinates. If the LVDT is supposed to be measuring a different point every four seconds, the machine should change position of the specimen every four seconds. However, due to the non-linear acceleration of the stepper motors, the time it takes to displace a certain distance is not easily analyzed. It was experimentally determined that it takes the machine 0.54 seconds to displace 5 mm. This implies that at every point the machine was paused for 3.56 seconds. This provided a match between the measurement grid and the corresponding machine locations, which could be used for the measurements.

Experiments


Experiments Chapter 5 This chapter will cover the experiments performed with the earlier described experimental setup. In this chapter all the different experiments, as mentioned in the research approach, are described and analyzed. The results of these experiments are found in the end of this chapter.

Calibration 5.1Because it is yet unknown what kind of repeatability either the machine or the measurement setup has, a calibration is required. This set of experiments will be done with 100/100/0.2/1000 specimens. This type of specimens implies that R1 and R2 equals 100 mm, the step size is 0.2 mm/pass and the feed rate is 1000 mm/min.

5.1.1 Machine repeatability In order to determine the repeatability of the process five specimens are tested. Each of these specimens was tested under the same conditions and with the same tool path. Therefore the geometric differences between these specimens can be used to determine the repeatability of the machine. The average result of the measurements is shown in Figure 5-1. In Figure 5-2 a photograph is shown of the actual specimen. What can be seen from visual inspection is that the measurements are in agreement with the actual deformation. Although the average results of the measurements doesn’t tell much about the repeatability of the machine itself, it does verify that the machine is capable of reproducing the tool path given by the computer. But if one looks at the deviation between the four specimens, as can be seen in Figure 5-3, it becomes clearer that the differences between the specimens are small at just 0.25 mm at its maximum. However, as said in the literature review, there are also various effects which cause differences between parts being produced. These effects are the spring back, the sheet bending and the pillow effect. Since the specimens are produced with the same shape, the effect of spring back should ideally be the same and should not contribute much towards the deviations seen here. As can be seen, the deviations are relative constant in most of the specimen. While calibrating the forming tool, the tool itself is calibrated as soon as it contacts the surface of the specimen. However the specimens are slightly curved, which causes some deviations with this calibration method. Therefore the specimens are sometimes already under strained, before the process starts. This causes the deviations seen in Figure 5-3. A solution to reduce the deviations is not to calibrate the depth of the forming tool at the specimen itself, but at a solid point such as the jig. By doing so the deviations are reduced, as will be seen in later experiments.

Figure 5-2 Calibration Specimen

Figure 5-1 Calibration specimen measurement

Figure 5-3 Standard deviation between calibration specimens including sheet bending

Experiments


5.1.2 Measurement repeatability In order to determine the repeatability of the measurement and the reliability of the measurement system each specimen made during the previous part was measured four times and every time each data point was measured twice. As a result one can see the differences between two measurements at the same point within the same measurement, but also the differences between the different measurement cycles. At first the differences between the individual measurement cycles are analyzed. The results can be seen in Figure 5-4. The first thing that has to be notified is that the maximum deviation between these four measurements is in the order of 0.02 mm, which is significant less than seen in the deviations made by the machine or other effects. However the second thing that can be seen is that the deviations are located at the right side of the specimen. The scanning pattern starts in the right bottom and then moves up. It then moves 5 mm towards the left and scans another line from top to bottom, moves another 5 mm towards the left and scans again bottom to top etc. These small deviations might be caused due to the Y-axis. At the right side of the Y-axis there is a small gap, which allows the machine to rotate a little around the X-axis. This might be the reason for the deviations within the scanning of the specimens. But the deviations are small enough to be neglected. However, this only shows the differences between the measurement cycles, but doesn’t show any deviations occurring within the same measurement cycle. As each point is measured twice during each measurement cycle, the fluctuations within the measurement cycle can also be analyzed. This is done in Figure 5-5 Again the first thing that has to be noticed is that the deviations at almost all points are close to zero and could be neglected. However, three points show some deviations during the measurement. It has been noted that a vibration, for example from other students performing tests on the other side of the wall, could already cause such deviations within a measurement point. This could be the reason for these deviations. Nevertheless these deviations can still be neglected as they are in the order of 0.01 mm or less.

Figure 5-4 Differences between individual measurement cycles in mm

Figure 5-5 Deviations within a single measurement in mm

Experiments


Test protocol 5.2Before any further tests are conducted it is important to have a standardized testing protocol to avoid unnecessary deviations between the specimens, which might lead to faulty conclusions.

5.2.1 Specimens production In order to avoid any deviations within material properties the specimens are standardized. At first the specimens are all cut from the same batch of 2024-T3 0.4 mm aluminum to a size of 120x120 mm. This is done with a hydraulic cutting/shearing machine as shown in Figure 5-6. During this cutting it was noticed that the specimens were slightly curved. This happened because the aluminum was transported on a coil at a certain stage and even while the plate was straightened a small curvature remained in the plate.

5.2.2 Specimen clamping The slight curvature is important to keep in mind, during the clamping of the specimens, as the small specimens now have a concave and convex side. In order to avoid large deviations between the specimens all the specimens are positioned with the convex side upwards. This can be seen in Figure 5-7, where the slight curvature is visible due in the reflection of the light. Afterwards the specimen is securely clamped in the jig with eight M8 bolts as can also be seen in Figure 5-7. This ensures that the specimen will not move during the process. This phase is repeated every time a new specimen is used.

5.2.3 The jig The jig itself also has to be secured to the machine in order to avoid any unnecessary movement. The machine itself has an XY table with M12 slots for T-nuts. As the DASML didn’t have any M12 T-nuts or bolts and to avoid any further delays in the thesis, an improvised method has been developed as can be seen in Figure 5-8. This method consisted of four M12 bolts turned upside down in the T-nut slots. These bolts were combined with eight M12 nuts and four M12 washers. The washers were used to grip the jig, while the nuts were used to tighten the bolts securely in the T-nut slots. This step is only required to secure the jig onto the XY table. Therefore it doesn’t have to be repeated for every new specimen, unless the jig was removed for some reason.

Figure 5-7 Clamping of the specimen with convex side upwards

Figure 5-8 The jig on the XY Table

Figure 5-6 Machine used for specimen production

Experiments


5.2.4 Starting point As the jig is symmetrical, the center of this jig is being used as a starting point for the process. However in order to determine the center one could use a caliper to determine the forming tool distance to each side. The jig itself is 159 mm long and wide, thus the center of the jig is located at 79.5 mm from each side. However once a specimen is clamped, it’s not practical to determine the center based on this 79.5 mm. Since the forming tool also has to be centered at the same position. Instead of measuring and indicating the center of the plate, one could also include the forming tool during this step, as seen in Figure 5-9. It is known that the diameter of the forming tool is 16 mm. Therefore the distance from the left side of the forming tool towards the right side of the jig becomes 87.5 mm in total, and the same for any other sides. As a result the forming tool is positioned at the center of the plate and jig. This has to be repeated for each specimen. For the Z-axis a different method is used. As explained in 5.1.1, the sheet bending effect had a large impact on the deviations between the specimens. Therefore, the starting point of the Z-axis has to be taken at a solid fixed point instead of the specimen itself. As a result, a specific point (X60 Y20) was taken as a reference point, which is on top of the clamping plates. The thickness of one of the clamping plates is 10 mm, which means this point is, in theory, 10 mm above the center of the specimen if the specimens were perfectly flat. In practice it is already known that the specimens aren’t perfectly flat. However if one calibrates the Z-axis at the center of the specimen, the Z-coordinates for every specimen will be slightly different. This causes deviations as seen in 5.1.1. With the method described above, by calibrating the Z-coordinate at the jig instead of the specimen, the Z-coordinates of every experiment will be the same. Another issue with the starting point of the Z-axis is that as soon as the power is removed from the entire machine, the Z-axis automatically moves to its lowest position, as can be seen in Figure 5-11 with the red arrow. As without power the stepper motor provides less resistance, the weight of the Z-axis moves the forming tool downwards over time. Therefore, each time the power is removed from the system the Z-axis has to be calibrated with the method mentioned above. This doesn’t apply to the X and Y position, as these remain in place with or without power.

Figure 5-9 Starting point determination X and Y

Figure 5-10 Starting point determination Z

Figure 5-11 Z-axis with (Left) and without power (Right)

Experiments


5.2.5 G-code coordinates The specimen coordinates, or G-code coordinates for the software package, are automatically produced by a simple Matlab script. This Matlab script can be found in Appendix A-3. This script basically takes the size of the specimen, the two radii of the specimen, the required step size and the required feed rate. The script first computes the forming depth of the specimen based on the input radii and the size of the specimen to be created. Afterwards the first X, Y and Z coordinates are generated and the Z coordinate is checked with the initially compute forming depth. As long as the Z-coordinate hasn’t reached the forming depth, the loop has to continue to the next set of coordinates. In order to determine the next set of coordinates the distance between these coordinates and the previous coordinates is set to 1 mm, to keep the points evenly distributed over the length of the tool path. As a result the script computes the next set of coordinates with very small increments. In the end, this list of generated coordinates is stored with a G-code G1 representations within a text file, in order to run the coordinates in Repetier host, and is also shown in a figure for a visual inspection. Therefore, for every new specimen with different geometry or machine settings this script is used to generate a G-code list.

5.2.6 Test protocol flowchart The above described test protocol can be written in a flow chart, as has been done in Figure 5-12. This flowchart was used during each experiment in order to avoid mistakes or unnecessary deviations.

Figure 5-12 Test protocol flowchart

Generate G-Code

Specimen Produced

Secure the jig

Create Specimen

Yes

No

Jig Secured

Clamp Specimen

No

Yes

Specimen Clamped No

Starting position

Yes

Correct X position

Correct Y position

Correct Z position No

No

No

Yes

Yes

Yes

Correct G-code

Run Experiment

Yes

No

Start

Calibrate X

Calibrate Y

Calibrate Z

Experiments


Geometry experiments 5.3The double curved panels, as explained in the theoretical analysis, are usually found in the leading edges and empennages instead of within the fuselage sections of an aircraft. The radii of these parts are significantly smaller compared to that of a fuselage panel radius. As this research focusses on the feasibility of incremental sheet forming as a die-less solution for preforming aluminum layers within GLARE laminates, it should focus on those parts with a double curved profile and therefore the chosen geometries to be tested are as followed: 1. One set of specimens with 100x100 radii 2. One set of specimens with 100x200 radii 3. One set of specimens with 200x200 radii All of these specimens will have the same feed rate (1000 mm/min), same step size (0.2 mm/pass), same size (120x120x0.4 mm) and material (Aluminum 2024-T3). After these tests the specimens are scanned with the linear voltage displacement transducer in order to obtain a 3D image of the formed part. This 3D image is than compared to the input model, which was used for the process.

5.3.1 Geometry experiment 1: 100x100 Radii In Figure 5-13 the average obtained geometry of these experiments is shown and already a few things can be seen within these results. First of all the top of the obtained geometry has a little dent, this is an indication of the pillow effect. Also the area surrounding the deformed area show some small deformations as well, this is caused by the sheet bending effect. Besides that, the shape itself seems relative accurate at first sight. In the next image, Figure 5-14, the deviations between the different specimens of the geometry experiments 1 can be seen. What is most important here is that using the jig as a reference point for the Z-axis already provides fewer deviations, compared to Figure 5-3. Nevertheless the deviations near the center of the specimen are still in the order of 0.2 mm. This could be explained due to already present residual stresses within the material. However the deviations are small compared to the maximum forming depth. In Figure 5-15 the computer input model is compared to that of the created specimen. With this figure it becomes clear that the sheet bending (+/- 0.15 mm) and pillow effect (+/- 0.34 mm) are only causing small deviations, when compared to the spring back (+/- 2.26 mm). However the difference seems to be related with the forming depth, which implies that if over bending is applied, the result should be more accurate. Nevertheless the result of this specific test is that the forming depth of the specimen is about 50% off compared to the input model.

Figure 5-13 Geometry Experiment 1: Average obtained geometry

Figure 5-14 Geometry Experiment 1: Deviation between the specimens

Figure 5-15 Geometry Experiment 1: Specimen (red) vs. Model (blue)

Experiments


5.3.2 Geometry experiment 2: 100x200 Radii In the 100x200 radii experiments the radius of the Y-axis is changed to a 200 mm radius compared to the previous set of experiments. These specimens give insight in which results can be expected with single point incremental forming for a shape with two different radii. In Figure 5-16 the average obtained geometry of these experiments is shown. The sheet bending at the edges of the specimens and the pillow effect in the center are visible, just like with the 100x100 mm experiment. Also the shape of the specimen seems to be in line with the input model. In the next image, Figure 5-17, the deviations between the specimens are plotted. Even though the deviations are quite different compared to the previous set of experiments, the scale of these deviations is still in the order of 0.2 mm or less. In Figure 5-18 the computer input model is compared to that of the created specimen. In this figure it becomes clear that the obtained shape does not match the computer model. However just like the previous experiment it also becomes clear that the sheet bending (+/- 0.25 mm) and pillow effects (+/- 0.15 mm) are only causing small deviations, when compared to the spring back (+/- 1.77 mm). Even though the pillow effect, the effect which causes the center to pop back up, only plays a minor role in the total deviations of this experiment, it does seem to increase for the large radii. This can be clearly seen in Figure 5-16 and Figure 5-18 by the flat top. A reason could be that residual stresses are more easily relieved. Nevertheless the difference seems to be related again with the forming depth. Therefore it may be expected that the deviations with the computer model could be reduced if over bending is applied.


Figure 5-17 Geometry Experiment 2: Deviation between the specimens

Figure 5-18 Geometry Experiment 2: Specimen (red) vs. Model (blue)

Experiments


5.3.3 Geometry experiment 3: 200x200 Radii In the previous two geometry tests it was noticed that at first sight the shape of the specimen seems to match the geometry. Again for this test the same applies, as can be seen in Figure 5-19. This first thing that has to be noticed is that the obtained specimen is actually slightly deeper than the previous experiment. Even though it was expected to be shallower as the radius in both directions is increased. However the difference is in the same order of magnitude as the deviations typically seen, which might explain why the differences in achieved forming depth are observed. Next the deviations between the specimens are determined, as shown in Figure 5-20. Again the deviations are almost all below 0.2 mm. Although this specimen has a small peak at 0.2019 mm, which means that the scale of the axis is slightly different compared to the previous two experiments. For now it seems that the deviations between the specimens remain the same, no matter the geometry. Now all that remains is to compare the obtained geometry with the computer model input, to see if the differences can be explained. This is done in Figure 5-21. The pillow effect is slightly increased again, as expected from the results of the previous two experiments. In the first experiments with 100 mm radii the pillow effect was slightly visible. In the next set of experiments the pillow effect already became clearer. However, in this set of experiments the pillow effect (+/- 0.3 mm) is clearly visible as can be seen in Figure 5-19 by the dent in the center. This indicates that the residual stresses are less enclosed in the sides and thus popping the center of the specimen back up during unloading of the specimen. The sheet bending effect on the other hand increases and is about 0.25 mm in this specific set of experiments. Comparing this with the previous two sets of experiments, it seems that the sheet bending effect becomes larger if the radii are increased. However, just like with the previous two sets of experiments, the spring back (+/-1.55 mm) accounts for most of the differences between the specimen and the computer model. Therefore it may be expected that if a spring back correction is applied the deviations between the computer model and the specimen could be reduced.


Figure 5-20 Geometry Experiment 3: Specimen Deviation

Figure 5-21 Geometry Experiment 3: Specimen vs. Model

Experiments


Feed rate experiments 5.4In the theoretical analysis it was stated that increasing the feed rate shouldn’t have an effect on the dimensional accuracy. However in order to verify this statement a series of tests is conducted to see if that actually holds for double curved parts. Therefore the feed rate is increased from 1000 mm/min to 3000 mm/min in steps of 1000 mm/min. If the dimensional accuracy remains more or less the same it may be assumed that increasing the feed rate any further should have no effect on the accuracy of the process.

5.4.1 Feed rate experiment 1: 1000 mm/min This experiment is basically the same as Geometry experiment 1: 100x100 Radii, therefore the data measured at that experiment is used here as a basis for the feed rate experiments. As can be seen the results didn’t provided to be very accurate with respect to the computer model due to various reasons, such as sheet bending, pillow effect and spring back. However in order to determine the effect of the feed rate the geometrical accuracy compared to the computer model is not important. Instead the differences between the obtained geometries with different feed rate provide the required information. Therefore the specimens of the following feed rate experiments are compared to the data of the first geometry experiment to determine if the feed rate affects the geometrical accuracy in any way.

5.4.2 Feed rate experiment 2: 2000 mm/min In this experiment all the parameters are kept the same, except the feed rate. This is increased to 2000 mm/min to see if any effects occur if the feed rate is increased. As mentioned before, in order to determine this effect the results have to be compared with the previous experiment set at the regular 1000 mm/min feed rate. In Figure 5-22 the differences in shape are shown between the experiments performed at 2000 mm/min feed rate and at 1000 mm/min feed rate. What is shown is that there are small deviations between the two different feed rates, where the 2000 mm/min achieves less forming depth compared to the 1000 mm/min experiments. However it was earlier mentioned that the process itself has deviations in the order of 0.2 mm, as can be seen in Figure 5-14. Therefore it is assumed that the differences are a result of the process deviations. This implies that the feed rate itself has no significant effect on the geometry.

Figure 5-22 Feed rate experiment 2: Differences 2000 mm/min vs. 1000 mm/min

Experiments


5.4.3 Feed rate experiment 3: 3000 mm/min Just like the previous experiments all parameters have remained the same, except the feed rate. The feed rate is increased with another 1000 mm/min to 3000 mm/min. Once again the results are compared to the initial experiment with 1000 mm/min to see if any differences occur in the obtained geometry. In Figure 5-23 the differences in obtained geometry are shown between the experiments performed at 3000 mm/min and those at 1000 mm/min feed rate. What is most interesting is that there are large differences between these different feed rates. Unlike the previous comparison with the 2000 mm/min and the initial 1000 mm/min, the differences are now much larger than the deviations within a set of experiments, as seen earlier in Figure 5-14. Most of these differences can be found near the center of the specimen. At first it was assumed that local heating, due to the friction between the tool and the specimen, allowed the material to have a higher spring back. However the actual reason is that the Z-axis was skipping several steps. It is most likely that the increase in feed rate increased the force on the Z-axis during the process, which resulted in the loss of steps. As the process continuous the amount of steps lost accumulates near the center, causing larger deviations. It was measured that the Z-axis had lost between 0.9 and 1 mm in steps after an experiment at 3000 mm/min. This explains the majority of the differences seen within Figure 5-23. The remainder of the differences is most likely caused by either the friction or process deviations.

Step size experiments 5.5In the theoretical analysis it was mentioned that if the step size is increased, the dimensional accuracy of the process decreases. This is because the step size influences the tool path location; larger increments in step size imply shorter toolpaths and thus less local deformations. The benefit of a shorter toolpath is that this reduces the production time per part. To determine if and how much the dimensional accuracy is reduced by an increase in step size, experiments are conducted that will compare the achieved forming depths between different step sizes. The step size in these experiments is alternated from 0.1 mm per pass to 0.3 mm per pass in steps of 0.1 mm, all with a feed rate of 1000 mm/min. This provides three data points, which can be used to see the relation between the step size and the geometrical accuracy.

5.5.1 Step size experiment 1: 0.2 mm This experiment is the same as the first geometry experiment; therefore the data measured at that experiment is used here as a basis for the step size experiments, just like with the feed rate experiments. In the first geometry experiment section the results of these settings didn’t provided to be very accurate with respect to the computer model due to various reasons, such as sheet bending, pillow effect and spring back. However to determine the effect of the step size the geometrical accuracy compared to the computer model is not important. Instead the differences between the obtained geometries with different step size provide the required information. Therefore, just like the feed rate experiments, the specimens of the following step size experiments are compared to the data of the first geometry experiments in order to determine to effect of the step size on the geometrical accuracy.

Figure 5-23 Feed rate experiment 3: Differences 3000 mm/min vs. 1000 mm/min

Experiments


5.5.2 Step size experiment 2: 0.1 mm In this experiment all the parameters are kept the same, except the step size. This is halved to 0.1 mm per pass to see if the geometrical accuracy increases if the step size is decreased. As mentioned before, in order to determine this effect the results have to be compared with the first geometry experiments performed at 0.2 mm/pass step size. In Figure 5-24 the differences in shape are shown between the experiments performed at 0.1 mm/pass step size and at 0.2 mm/pass step size. What is shown is that there are deviations between the two different step sizes, which are significantly larger than the expected deviation of the process, as seen in Figure 5-14. This implies that the decrease in step size actually has an effect on the accuracy. However Figure 5-24 doesn’t provide any information whether the accuracy is increased or decreased. To determine if the accuracy is increased, the data obtained from this set (red) is plotted against the data of the first geometry experiment (green), as can be seen in Figure 5-25. In this figure it can be seen that the red figure is deeper compared to the green shape. This implies that the 0.1 mm/pass data set is slightly more accurate compared to the 0.2 mm/pass data set.

5.5.3 Step size experiment 3: 0.3 mm In this experiment all the parameters are kept the same, except the step size. This is increased to 0.3 mm per pass to see if the geometrical accuracy decreases if the step size is increased. This could be expected after the results of previous step size experiment. As mentioned before, to determine the effect the results have to be compared with the first geometry experiments. In Figure 5-26 the differences in shape are shown between the experiments performed at 0.2 mm/pass and at 0.3 mm/pass step size. In this figure it is shown that there are again deviations between the two different step sizes, which are significantly larger than the expected deviation of the process, as shown in Figure 5-14. These deviations are in similar size as the deviations seen in the previous step size experiment, where 0.1 mm/pass and 0.2 mm/pass were compared. This implies that the increase in step size also has an effect on the accuracy. However Figure 5-26 once again doesn’t provide any information whether the accuracy is increased or decreased. Therefore Figure 5-27 is used, which plots the specimen coordinates of the 0.3 mm/pass (red) against those of the 0.2 mm/pass (green). In this figure it can be seen that the green shape is slightly deeper compared to the red shape. This implies that the 0.3 mm/pass data set is slightly less accurate compared to the 0.2 mm/pass data set. This is in line with what could have been expected after step size experiment 2.

Figure 5-24 Step size experiment 2: Absolute differences vs. 0.2 mm/pass

Figure 5-25 Step size experiment 2: Specimen shape differences between 0.1 mm/pass (red) vs. 0.2 mm/pass (green)

Figure 5-27 Step size experiment 3: Specimen shape differences between 0.3 mm/pass (red) vs. 0.2 mm/pass (green)

Figure 5-26 Step size experiment 3: Absolute differences vs. 0.2 mm/pass

Experiments


Geometry spring back correction 5.6In order to verify the earlier claim that the accuracy could be increased by adding the difference, between the specimen and the input model, to the computer model as a method of over deforming a part; a series of tests has been conducted. In this particular case the experiments of 5.3.1 were taken as a reference to verify the method Micari proposed earlier in his paper [5]. This data set will be iterated with his method to obtain the first correction specimen. This correction basically implies that a specimen is first formed with the normal input model. Then the difference between the obtained geometry and the input model will be added to the input model and this new “corrected” input model will be used on the same, already formed, specimen. Afterwards this is done again for the second correction to see if any improvements are made with an additional iteration. The results have been made visual in Figure 5-28, Figure 5-29 and Figure 5-30. The first thing that has to be noted is that after only a single correction the forming depth is significantly increased, compared to the initial specimen. The initial specimen has about 3-4 mm forming depth, where the first correction already has 6-8 mm forming depth. For the second correction hardly any visual changes are directly seen compared to the first correction. Also just like the geometry experiments the different effects, which contribute to the inaccuracies can be analyzed for these samples. This is done in Figure 5-31 and shows a shift in the contributing factors. In the first experiments the spring back was always the dominant factor in explaining the differences. However after only a single iteration, as proposed by Micari, the effects of this iteration already become clear. The second correction doesn’t add much more in terms of accuracy, yet some small improvements are still made. Even though the spring back effect is actually increased, the correction compensates. This results in a more accurate result with respect to the forming depth. At the same time the sheet bending effect is slightly increased due to this correct method, while the pillow effect is slightly decreased. In order to provide insight in the overall accuracy improvement Figure 5-32 has been made. This figure clearly shows in the left image that the first correction specimen is rather accurate compared to the initial input model. The forming depth is matched up to 97.2% in fact. But this figure also that the differences between first (middle) and second (right) iteration are very minimal (97.2% vs. 98.3%). Therefore it is probably best to apply the correction only once, as the results from a second pass correction are minimal.

Figure 5-32 Correction 1 vs. CAD Model (left), difference iteration 1 (middle) and difference iteration 2 (right)

Figure 5-29 Specimen correction 1

Figure 5-30 Specimen correction 2

Figure 5-28 Specimen 1 without correction

Figure 5-31 Overview of the contribution of each effect on the accuracy mismatch

Experiments


Experimental results 5.7

5.7.1 Geometry experiments As has been shown during the geometry experiment the differences between the specimens and the computer models got less if the radius was increased in absolute values. In which the spring back is the most dominant factor in the difference between the computer model and the actual specimen, while the sheet bending and pillow effect only play a minor role. This is visualized in Figure 5-33, where geometry experiments 1, 2 and 3 are shown on the X-axis and the different effects in mm are shown on the Y-axis. However if the radius is increased the total forming depth is decreased. Therefore it may be logical that the difference between the computer model and the actual geometry actually becomes less. Instead it might be better to compare the relative error between the computer model and the actual obtained geometry. This provides a certain percentage, which can be used to compare the effect of different radii with respect to the forming depth. In order to provide such comparison the forming depth of the specimen is divided by the forming depth of the actual mode, to see what kind of percentage of the depth is achieved. The results can be seen in Figure 5-34. The numbers on the X-axis represent the number of the experiments, thus number 1 corresponds with geometry experiments 1. From this data it becomes clear that specimens with larger radii actually have quite a bit less accuracy in forming depth. However whether a specimen has two different radii or two of the same hardly influences the forming depth accuracy, as can be seen in the small difference between experiment 2 and 3. This implies that the amount of spring back is mostly influenced by the largest radii within the specimen, where a larger radius usually allows more relaxation afterwards. Therefore the achieved depth is usually less in larger radii parts.

Figure 5-33 Overview of the contribution of each effect on the accuracy mismatch

Figure 5-34 Achieved forming depth as a % of the computer model depth

Experiments


The pillow effect As mentioned earlier several reasons exist for the difference between the computer model and the actual specimen. One of those is the pillow effect, which causes the center of the specimen to pop back up. It was stated earlier that this pillow effect would increase with higher radii in both directions, which is also shown by the data in Figure 5-35. In this figure the pillow effect is divided by the achieved depth of the specimen. One can clearly see that the pillow effect increases if the radius in one direction is increased already. But also that if both radii are increased the effect increases. An interesting fact is that it also seems that the pillow effect does have a relative constant value if the radii are equal. For example in the first geometry experiments the pillow effect was 0.34 mm and in the geometry 3 experiments the pillow effect was 0.3 mm, as can be seen in Figure 5-36. But if the radii are unequal the pillow effect is limited to 0.12 mm. Therefore the pillow effect may have a constant value if the radii are equal. This is due to the residual stresses, which are enclosed inside the geometry if a specimen has two different radii instead of two identical radii. Since the specimen with equal radii are symmetrical it requires less force to pop back the center of the plate. Therefore the pillow effect tends to increase within these specimens, as the residual stresses are released easier. However the pillow effect remains small compared to the spring back effect for example.

The sheet bending effect The sheet bending effect may also play a role in the deviations seen between the computer model and the obtained geometry. During the geometry tests it was stated that the sheet bending effect seems to increase if the forming depth was decreased, which is unexpected. One would assume that if the forming depth is increased, the sheet bending would increase as well. Nevertheless the opposite seems true as can be seen in Figure 5-37. However the sheet bending effect is small and in the same order of magnitude as the deviations seen between specimens in the same set. This might be the cause of the unexpected trend seen in Figure 5-37. Yet the order of magnitude of the sheet bending effect remains small; especially compared to the differences between the computer model and the obtained specimens.

Figure 5-35 Pillow effect as a % of the achieved specimen depth

Figure 5-36 Pillow effect in mm for the geometry tests

Figure 5-37 Sheet bending effect in mm for the geometry tests

Experiments


The spring back effect The spring back, as mentioned earlier, is the main reason for the large deviations between the specimens and the computer model. While the previously mentioned pillow effect and sheet bending effect account for 0.15-0.35 mm deviations, the spring back accounts for 1.55-2.26 mm deviations. The spring back is visualized in Figure 5-38. It is important to mention that the spring back during each of these experiments is measured, with the specimen still clamped inside the jig. If the specimen is unclamped, the spring back will be even higher. However it becomes impossible to measure the shape of the specimen after being released from the jig with a certain degree of accuracy. It seems that the spring back effect reduces, if the radii of the specimens are increased. However this is only true for the absolute displacement caused by the spring back. The relative spring back with respect to the computer model depth is therefore shown in Figure 5-39. In this figure it becomes obvious that for the first geometry experiment the absolute spring back might have been higher, although the relative spring back is much lower. It also becomes clear that spring back might be more of an issue with double curved specimens with two different radii compared to specimens with equal radii. This can be seen between data point 2 and 3. The computer model depth is in these cases equal; but the spring back within experiment 2 is almost 10% more. This might be caused by the 100 mm radius being present, which causes more internal stresses, which in turn causes the specimen to have more spring back in the end.

5.7.2 Feed rate experiments From the first two sets of experiments it seems that the feed rate doesn’t have any influence on the geometrical accuracy as earlier predicted. But the third experiment clearly shows that the feed rate has an influence on the geometrical accuracy. The main reason for this deviation was that the machine itself had problems running at these feed rates, which caused a deviation of 0.9 to 1 mm in total. But this only partially explains the differences seen between the different feed rates. Another reason might be that the friction between the forming tool and the material causes the material to become warm. An increase in temperature usually increases the ductility of the material, which results in less plastic deformations. If one has the ability to either freely rotate this forming tool or control the spindle speed, the friction between the forming tool and the material could be reduced. In order to minimize friction the following formula could be if the spindle speed can be controlled:

𝜔𝑠𝑝𝑖𝑛𝑑𝑙𝑒 =𝑣𝑓𝑒𝑒𝑑

𝜋 𝑟𝑡𝑜𝑜𝑙 ∗ √12

(1 − cos (2𝛼𝑓𝑜𝑟𝑚𝑖𝑛𝑔)

Nevertheless the current results do imply that the feed rate has a negative effect on the dimensional accuracy and is thus can’t be increased over and over to achieve higher production speeds achieving the same dimensional accuracy as initially assumed.

Figure 5-39 Spring back in % for the geometry tests compared to the computer model depth

Figure 5-38 Spring back effect in mm for the geometry tests

Experiments


5.7.3 Step size experiments In order to determine the effect of the step size, the same parameters are compared as with the geometrical experiments. Therefore the total maximum difference between the obtained specimen and the computer model is analyzed and then split up into the pillow effect, the sheet bending effect and the spring back effect.

Total difference The total difference is once again the maximum difference between the obtained specimen and the computer model and is the summation of the pillow effect, the sheet bending effect and the spring back effect. In Figure 5-40 this is visualized.

Pillow effect Most surprisingly what can be seen is that if the step size is decreased sufficiently the pillow effect seems to play a rather large role in the inaccuracies, approaching the spring back effect. However as soon as the step size is increased, the pillow effect fades away.

Sheet bending effect From the same figure it can also be seen that the sheet bending effect hardly changes with a different step size and could be assumed independent of the step size. As a result the inaccuracies caused by the sheet bending effect can’t be changed by alternating the step size, but have to be counter with other process settings.

Spring back The spring back on the other hand increases with each increment of the step size, resulting in higher inaccuracies at higher step sizes. This is in line with the earlier mentioned expectations of what could happen when the step size is increased. However it seems the behavior between the spring back and the increase in step size is not linear and actually might have a horizontal asymptote. This implies that after a certain step size has been reached, the inaccuracies remain constant. Nevertheless further research is required in order to determine whether this holds, as only limited results are shown here.

Figure 5-40 Overview of the contribution of each effect on the depth mismatch

Discussion


Discussion Chapter 6 This chapter will cover the discussion regarding the results obtained in Chapter 5 and how these relate to the large scale application as mentioned in Chapter 3.

Large scale applications 6.1

6.1.1 Process repeatability At first the repeatability of the process is discussed. In Chapter 5, during the calibration tests, a series of tests were conducted to establish the repeatability of both the machine and the measurements. In this calibration process it became clear that the process has a deviation of approximately 0.25 mm for a 100x100 mm specimen. After the calibration phase, the deviations seen were still in the same range of 0.20-0.25 mm as can be seen in Figure 5-14, Figure 5-17 and Figure 5-20. However the average deviation was decreased significantly. The magnitude of these deviations is similar to the data found by Singh and Agrawi during their experiments at similar sized specimens with different geometry [40]. For the large scale application it is assumed that the deviations, seen at the repeatability of the experiments, are linear with respect to the size of the specimens. This implies that the deviations become in the order of 3 to 4 mm for 1500x1500 mm panels. This size of deviations is not uncommon for aircraft industry in fuselage panels. Therefore the repeatability shouldn’t be an issue for large scale products as long as the assumption that the deviations behave linear holds.

6.1.2 Process predictability The predictability of the process is basically implies to what extend the specimens resemble the input file / goal. This is different compared to the repeatability, as in the repeatability one wants the specimens close to each other, but not necessarily close to the goal. The main problem with the predictability for incremental sheet forming is that there are no accurate computer models that can predict the material behavior correctly. Nevertheless after a specimen has been formed, the repeatability of the process allows a rather accurate prediction of the next specimen. With the help of the correction method one could come much closer to the actual aim, which increases the predictability. Therefore the initial predictability is low based on computer models, but within a series production one could achieve predictable results after a first sample has been made and corrected. The problem arises if one wants to produce large panels in small series. As the process lacks a good initial prediction, the production of the first panel will be very difficult. However after the first panel has been successfully formed within its tolerances, the second panel can be made the same way. But the predictability might become an issue in large scale manufacturing, due to its lack of a good initial prediction by an accurate computer model.

6.1.3 Process controllability The controllability of the process is related to how well the process can be controlled, either by hand or by computer. In this case the controllability of the process is done with a CNC method. As long as the G-code of the CNC controller is written correctly, one should have no unwanted movements or flaws in the controllability of the process. The only possible deviation in the controllability arises from the backlash in the axes of the machine itself. The backlash occurs when the axes turns in the opposite direction. A small gap between the bearing and the threaded rod allows some backlash. However the backlash in most of these machines is limited to 0.01 mm or less. Therefore the controllability should not be an issue in large scale manufacturing.

Discussion


6.1.4 Dimensional accuracy One of the research questions was related the effect the process settings had on the dimensional accuracy. In the experimental phase the feed rate and the step size have been varied in order to provide some insight. Initially it was stated that the step size had an influence on the dimensional accuracy. This statement has been verified by the results of the step size experiments. There is a clear trend that if the step size is decreased, the dimensional accuracy is increased and vice versa. It was also stated that the feed rate has no influence on the dimensional accuracy. This statement has been proven wrong. The experimental results showed that, with increasing feed rates, the dimensional accuracy went down. Where the 3000 mm/min feed rate experiment shows this best. Nevertheless the overall dimensional accuracy at each of these experiments was still low and did not even approach the input model given. This was solved with the correction method experiment. This experiment used an iteration to correct the spring back in a specimen. As a result the forming depth approached the depth of the input model up to 97.2%. But at certain places the difference between the input model and the obtained specimen was still +/- 0.8 mm. These differences do provide an issue for large scale implementations. In these experiments the best case scenario was achieved with a single iteration, but still provided +/- 0.8 mm variation at certain places in the specimen. For the large scale implementation, larger sheets will be formed at a time and therefore the variations will most likely increase. As a result the dimensional accuracy of these plates may have such large variations that the process itself is not accurate enough. But if the predictability of the process becomes better, one could predict the material behavior of these large plates during the process. This could provide much higher dimensional accuracies and solve this problem. This does require more research on the different effects that cause these variations, which could result in an improved computer model for the process.

6.1.5 Process speed The process speed was another crucial factor being tested as for the large scale application a high process speed is required. Earlier in Figure 3-4 it was shown that the toolpath for a 1500x2000 mm panel is several km in length. Initially it was stated that the feed rate had a minimum effect on the dimensional accuracy and that one could increase the speed to the required speed. During the step size experiments it was shown that increasing the step size affects the dimensional accuracy. Therefore there are clear limitations towards the step sizes in order to stay within the dimensional accuracy limitations, which limit the maximum process speed. Regarding the feed rate experiments it was expected that doubling or even tripling the feed rate should have a minimum effect or even no effect to the dimensional accuracy and thus the feed rate could be increased up to the required feed rate. However the feed rate does influence the dimensional accuracy, as seen in the experiments. A higher feed rate cause more friction, which heats up the material and allows more relaxation and therefore less dimensional accuracy. The specimens in these experiments were processed between 1.5 and 5 minutes. For the large scale implementation a much higher process speed is required, which implies a much higher step size and feed rate. The experiments have shown that in order to reach these step sizes or feed rates, the dimensional accuracy gets affected. Therefore it’s not possible to obtain the required feed rate and step size. In order to obtain the required process speed a different approach is required. For example the process could use an origami method to fold or bend the material at certain locations, which requires much less toolpath length. Or multiple forming tools can be used at once by the same machine, which reduced the toolpath length per forming tool. Another solution would be to use multiple machines. But with the current limitations in feed rate and step size the amount of machines required would be tremendous in order to produce enough panels per hour.

Discussion


Limitations 6.2So far there have been several limitations with respect to the large scale implementation of incremental sheet forming. The typical arguments against applying single point incremental forming in large industrial applications are that the process is too slow and inaccurate. In order to produce the required 72-129 m

2 aluminum per hour, the feed rate should be in the order of 600

m/min and the step size should be in the order of 0.5 mm per pass. Even though machines exist that are capable of performing at these feed rates and step sizes, its most unlikely that the results are accurate. As seen in the experiments a step size of 0.5 mm per pass would be feasible, but would decrease the dimensional accuracy quite a bit. The main issue is the feed rate of 600 m/min. These feed rates cause a lot of friction, which decrease the overall accuracy. The friction can be reduced, by allowing the forming tool to freely rotate or have a controlled rotation. But even if this is done, the feed rate is usually limited to 10 to 30 m/min to avoid friction/heating problems or material surface damage. These results severely limit the feasibility of incremental forming for large scale applications like the one proposed in this research. But it might be possible that in the near future a better prediction model is developed. This prediction model can then be used to predict the material behavior at higher feed rates to compensate the input model in advance.

Improvements 6.3There are several improvements or ideas that can be used to make incremental forming more feasible for large scale applications, like the one suggested in this research.

Improved Prediction model The first major improvement that should be made is the understanding of how incremental forming influences the material. This could result in a better computer model, which could predict the material behavior during the process. This would also result in higher dimensional accuracies and possibly in much higher feed rates. Therefore an improved prediction model could solve the issues that avoid incremental forming from being used in industry.

Origami method A second improvement can be made, as already mentioned on the previous page, by changing the toolpath philosophy. Right now the toolpath is a spiraling toolpath that goes from the outside in, with equal steps in vertical direction. But one might want to investigate the option to use an origami folding method to reduce the toolpath tremendously. The main issue with this method is that it does require a thorough knowledge of the process and material behavior. But it could significantly reduce the required feed rate.

Multi-point incremental forming A third improvement can be made by using multiple forming tools, multi-point incremental forming, that simultaneously cover the toolpath. This could significantly reduce the distance each forming tool has to cover and thus reduce the feed rates. The problem with this solution is that it requires a complicated setup and steering system to ensure that the forming tools do not interfere with each other.

Back tool incremental forming Regarding the dimensional accuracy, one could increase the dimensional accuracy by using a second forming tool on the other side of the sheet. This second tool can then be used to shear the material between the forming tools. This could result in a much higher accuracy as the sheet bending effects becomes much less. Even though this would not solve the problem with the process speed, it can be used in combination with the multi-point forming method to improve both process speed and accuracy.

Iteration Method Even though the iteration method is used in this research on an existing formed specimen, one might be able to use the new toolpath obtained by the iteration method and apply it to a new flat specimen directly to see if the dimensional accuracy is also increased. If successful, this method could be used, while an improved prediction model is being developed.

Conclusions


Conclusions Chapter 7

Consolidation of the work 7.1The main goal of this thesis was to determine the feasibility of incremental forming in terms of dimensional accuracy and process speed for preforming aluminum sheets used in GLARE laminates. This was done with a literature study on GLARE and incremental forming. After a theoretical analysis was done in which the process requirements were determined. This was followed by an experimental phase, which consists of obtaining an experimental setup and performing several experiments. The results of these experiments were than combined with the theoretical analysis to determine to which extend the process was feasible or not. The thesis objective was defined as follows:

“To perform a study on the combination of GLARE material and incremental forming to define and perform an experimental setup and test plan to determine to what extend incremental forming is feasible as a method to preform the aluminum sheets used in GLARE.”

During the literature study, the definition of GLARE was described with the different Grades and their specific applications. Also the current production of GLARE, the hand lay-up method, was analyzed and in combination with the material properties it became clear why this current method was not feasible for double curved panels as described in this research. It was also shown that the material most likely to be used for these applications was either GLARE Grade 3 or 4. In the second part of the literature study the die-less incremental forming sheet is described and the most recent developments in the field of single point incremental forming were mentioned. It was shown that feed rates up to 600 m/min were achievable. But it was also shown that the accuracy depends on the sheet bending effect, the pillow effect and the spring back, and that there are no accurate computer models. Afterwards the literature study was used as a basis for the process conditions, where it was found that, in theory, the production speed of large fuselage panels shouldn’t be a problem as long as the combination between feed rate and step size doesn’t affect the accuracy. However for the small double curved panels, with radii between 100-2000 mm, the accuracy might become an issue, therefore this research was focused at these smaller, more curved panels. The experimental setup in combination with the experiments has proven that the process itself is repeatable, predictable and controllable, as seen during the calibration experiments. However, at first the dimensional accuracy of the process was around 60% with respect to the forming depth. The correction method, suggested by Micari, was proven to be useful to increase the accuracy of the process with a single iteration to 97.2% of the forming depth. It was also shown that the feed rate had a negative effect on the accuracy and that increasing the step size also had a negative effect on the accuracy. This implied that the step size could be increased to a certain limit, before the accuracy becomes too low. The same applies to the feed rate, where higher feed rates cause a decrease in dimensional accuracy. Therefore the feed rate could only be increased to a certain limit. The experimental results were used to analyze the feasibility of incremental sheet for large scale manufacturing, such as the Airbus A320 production line. According to the results the accuracy drops if either the feed rate or step size is increased and therefore limited. Therefore it is suggested to stay below a feed rate of 30 m/min and a step size of 0.3 mm/pass, to stay within the required dimensional accuracy. For large scale industrial applications, e.g. the production line of the Airbus A320, the above mentioned combination is not fast enough with respect to the required production rates. Especially if the splices are supposed to be reduced, the forming depth increases and the production time per panel increases. This makes the process, at this moment, not feasible to be used for the Airbus A320 production line. But for the smaller, lower volume, double curved parts the process might be feasible if multiple machines are used.

Conclusions


Conclusion 7.2The conclusion of this report is the answer to the main research question stated in Chapter 1, and repeated below, which is:

“Can die-less incremental forming be a feasible process for preforming the aluminum layers used within GLARE laminates?”

In order to be feasible the process should be able to produce parts with an accuracy of 90% or higher and at rates fast enough to keep up with the production rate of the Airbus A320. The experiments have shown that the accuracy can be achieved with the correction method suggested by Micari. But the main issue lies within the process speed, which requires a combination of a high feed rate and step size. As a result the incremental sheet forming process is not feasible for the fuselage panel production of the Airbus A320. Yet it is feasible to produce the smaller, highly double curved parts of the leading edges and empennage with this process, due to the lower required production volumes of these parts.

Recommendations 7.3As could be read in the conclusions the incremental forming process as used in this thesis is not a feasible method to be applied for large scale industrial applications, such as the Airbus A320 fuselage. However the author would like to provide a few recommendations for future research.

Toolpath variation Various methods exist to decrease the toolpath length, as mentioned in section 6.3. One of these solutions is to use the art of origami to bend thin aluminum sheets into certain shapes. This could significantly reduce the required toolpath length. So far, no research exists with incremental forming being applied in such a way and could be the key solution to preform aluminum sheets.

Forming tools variation For now it has been shown that the forming tool used in this research is capable of delivering relative accurate parts, but at a low rate. Future research could include the options of using multiple forming tools working simultaneously at the same product, as commonly seen in robotic welding operations in car industry. This decreases the amount of toolpath per forming tool, which decreases the overall production time. Another possibility is the use of a backing plate or tool, as seen earlier in Figure 2-4. This doesn’t decrease the process speed, but it could increase the process accuracy. At this moment such as machine is used for research at Ford, namely the F3T. Even though it seems promising the application has not be used for series production yet [41].

Prediction and iteration It is also recommended that the prediction models are improved for incremental forming processes. Right now the prediction models are too inaccurate to be useful. But if these prediction models are increased, it allows less errors and/or deviations when it is applied for large scale industry. The iteration method can also be used in the meantime to improve the dimensional accuracy of the products after a single pass. But eventually, the prediction model should become advanced enough to avoid any iteration passes in order to save valuable production time.

Machine selection Last, but certainly not least, the machine used in this research was handmade by the student itself. This implies that the machine doesn’t have the same specifications as professional machines, as seen in Leuven and Ford. It is therefore recommended that, if future research is done at the DASML, an investment should be made in a more advanced machine. For example, the machine used in this research was not able to control the spindle speed, which increased the friction between the forming tool and the specimen. A dedicated professional machine is usually capable of controlling the spindle speed.

References


References Picture used on the front-page of the report: http://www.lamieranews.it/files/2015/06/fig.2.jpg

[1] A. Vlot and J. W. G. , Fibre Metal Laminates: An Introduction, Dodrecht: Kluwer Academic Publisher, 2001.

[2] R. v. d. Veen, "Sprekende Machines," Delft Integraal, 4 2005. [Online]. Available: http://www.tudelft.nl/fileadmin/UD/MenC/Support/Internet/TU_Website/TU_Delft_portal/Actueel/Magazines/Delft_Integraal/archief/2005_DI/2005-4/doc/DI05-4-10sprekendemachines.pdf. [Accessed 8 September 2015].

[3] P. Broest, Assembling 3d-fml panels of undeformed aluminium sheets. estimation of internal stresses., Delft: Delft University of Technology, 1999.

[4] J. Sinke, "The production of 3d glare panels," Delft University of Technology, Delft, 1999.

[5] L. Filice, G. Ambrogio and F. Micari, "Shape and dimensional accuracy in Single Point Incremental," Journal of Materials Processing Technology, vol. 191, p. 390–395, 2007.

[6] T. H. D. A. L.-. E. RUIMTEVAARTTECHNIEK, "Laminate of aluminium sheet material and aramid fibres.". Nederland Patent NL8100087, 2 Augustus 1982.

[7] T. H. D. A. L.-. E. RUIMTEVAARTTECHNIEK, "Laminate of metal sheet material and threads bonded thereto, as well as processes for the manufacture thereof.". Netherlands Patent NL8100088, 2 August 1982.

[8] L. B. Vogelesang and G. H. Roebroeks, "Metal-resin laminate reinforced with S2-glass fibres". United States of America Patent US5039571 A, 11 Oktober 1988.

[9] TU Delft, [Online]. Available: http://www.lr.tudelft.nl/fileadmin/Faculteit/LR/Organisatie/Afdelingen_en_Leerstoelen/Afdeling_AMM/Structural_Integrity/Research/Ph.D_Projects__old_/Impact_behaviour_of_Fiber_Metal_Laminates;_Impact_and_dent_damage_Resistance/img/lay-up.jpg. [Accessed 7 September 2015].

[10] G. Roebroeks and A. Mattousch, "Impact resistant laminate". Patent US5547735, 20 August 1996.

[11] G. Roebroeks, "Laminaat uit metaalplaten en elkaar kruisende dradenlagen uit verschillende materialen in kunststof.". Patent NL1022706, 17 February 2003.

[12] L. Vogelesang, L. Verbruggen and C. Paalvast, "Armour plate composite with ceramic impact layer". Patent US4836084, 6 June 1989.

[13] J. Gunnink, "Composite laminate of metal sheets and continuous filaments-reinforced synthetic layers". Patent US4935291, 19 June 1990.

[14] R. Alderliesten, "On the Development of Hybrid Material Concepts for Aircraft Structures," Recent Patents on Engineering, vol. 3, pp. 25-38, 2009.

[15] W. Guocai and J. Yang, "The Mechancial Behaviour of GLARE laminates for Aircraft Structures," in Failure in Structural Materials, JOM, 2005.

http://www.lamieranews.it/files/2015/06/fig.2.jpg

References


[16] Fiber Metal Laminate Company, [Online]. Available: http://www.fmlc.nl/research-development/results-cases/. [Accessed 7 September 2015].

[17] Cytec Industries Inc., "BR®127 Corrosion Inhibiting Primer Datasheet," Cytec Industries Inc., 2003.

[18] E. Kroon, "Influence of general quality on Glare material performance," FMLC, The Netherlands, 2002.

[19] C. Pegels, "A study on the Residual Stress of GLARE 1," Delft University of Technology, Delft, 1995.

[20] R. Alderliesten, "Fatigue Crack Propagation and Delamination Growth in Glare," Delft University Press, Delft, 2005.

[21] J. Sinke, Forming technology for composite/metal hybrids, Abington: Woodhead Publishing Limited, 2007.

[22] J. Schijve, "Development of fibre-metal laminates, ARALL and GLARE new fatigue resistant materials," TU Delft, Delft, 1993.

[23] A. Vlot, "Impact loading of fiber-metal laminates," Journal of Impact Engineering, vol. 18, pp. 291-307, 1996.

[24] E. Leszak, "Aparatus and Process for Incremental Dieless Forming". Patent US3342051A1, 19 September 1967.

[25] J. Jeswiet, F. Micari, G. Hirt, A. Bramely, J. Duflou and J. Allwood, "Asymmetric Single Point Incremental Forming of Sheet Metal," CIRP Annals - Manufacturing Technology, vol. 54, no. 2, pp. 88-114, 2005.

[26] B. Araghi, G. Manco, M. Bambach and G. Hirt, "Investigation into a new hybrid forming process: Incremenal sheet forming combined with stretch forming," CIRP Annals - Manufacturing Technology, vol. 58, pp. 225-228, 2009.

[27] T. Kim and D. Yang, "Improvement of formability for the incremental sheet metal forming," Int J Mech Sci, vol. 42, pp. 1271-1286, 2000.

[28] Y. Kim and J. Park, "Effect of process parameters on formability in incremental forming of," J Mat Process Technol, vol. 130, pp. 42-46, 2002.

[29] L. Filice, L. Fratini and F. Micari, "Analysis of material formability in incremental forming.," CIRP Annual Manufacturing Technology, vol. 51, pp. 199-202, 2002.

[30] G. Hussain, L. Gao, N. Hayat and L. Qijian, "The effect of variation in the curvature of part on the," Int J Mach Tools Manuf, vol. 47, pp. 2177-2181, 2007.

[31] I. Cerro, E. Maidagan, J. Arana, A. Rivero and P. Rodriguez, "Theoretical and experimental analysis of the dieless incremental sheet forming process.," Material Process Technology, vol. 177, pp. 404-408, 2006.

[32] K. Hamilton and J. Jeswiet, "Single point incremental forming at high feed rates and rotational speeds: Surface and structural consequences," CIRP Annals - Manufacturing Technology, vol. 59, pp. 311-314, 2010.

[33] G. Ambrogio, F. Gagliardi, S. Bruschi and L. Filice, "On the high-speed Single Point Incremental Forming of Titanium Alloys," CIRP Annals - Manufacturing Technology, vol. 62, pp. 243-246, 2013.

References


[34] R. Hallet and P. Camanho, Composite Joints and Connections: Principles, Modelling and Testing, Philadelphia: Woodhead Publishing Limited, 2011.

[35] Airbus, " Airbus to raise A320 Family production to 46 a month by Q2 2016," Airbus, 26 February 2014. [Online]. Available: http://www.airbus.com/presscentre/pressreleases/press-release-detail/detail/airbus-to-raise-a320-family-production-to-46-a-month-by-q2-2016/. [Accessed 5 November 2015].

[36] J. Duflou, R. Aerens, P. Eyckens and A. Van Bael, "Force prediction for single point incremental forming," The International Journal of Advanced Manufacturing Technology, vol. 46, no. 9, pp. 969-982, 2009.

[37] "Wood Cnc Router Table Machine Kit Plans PDF Plans," [Online]. Available: http://pcria.org/MjI0Yjc1-cnc-router-table-machine-kit-plans.html. [Accessed 8 June 2016].

[38] Trochilidesign, "Building your own CNC router/milling machine," [Online]. Available: http://www.instructables.com/id/Building-your-own-CNC-milling-machine/. [Accessed 8 June 2016].

[39] "HBM 16 Boormachine / Freesmachine," [Online]. Available: https://www.hbm-machines.com/producten/metaalbewerking/metaalfreesmachines/metaalfreesmachinesssub/hbm-16-boor-freesmachine. [Accessed 8 June 2016].

[40] A. Singh and A. Anupam , "Comparison of Dimensional Repeatability and Accuracy for Deformation Machining Stretching Mode with Sheet Metal Componen," in 5th International & 26th All India Manufacturing Technology, Rupnagar, 2014.

[41] P. Ponticel, "Ford sheet-metal-forming technology on journey from lab to application," SAE International, 14 Augusts 2013. [Online]. Available: http://articles.sae.org/12385/. [Accessed 30 June 2016].

[42] "Arduino Mega 2560," Riecktron Embedded Solutions, [Online]. Available: https://www.riecktron.co.za/en/product/211. [Accessed 20 June 2016].

[43] "Ramps 1.4 Shield," 3D printers, [Online]. Available: http://3d-printers.co.za/products-page/electronics/ramps-1-4-shield/. [Accessed 20 June 2016].

[44] "1SL02 IP66 Wall Box," RS-Components, [Online]. Available: http://nl.rs-online.com/web/p/wall-boxes/4939930/. [Accessed 20 June 2016].

[45] "Voedingen: Voeding 48VDC 6,7A," Hardware-CNC, [Online]. Available: https://www.hardware-cnc.nl/nl/winkel/voedingen/voeding-48vdc-6,7a-detail. [Accessed 20 June 2016].

[46] "Driver Stappenmotoren: Leadshine DM556 - 2 Phase Digital Stepper Drive; Max 50 VDC / 5.6A," [Online]. Available: https://www.hardware-cnc.nl/nl/winkel/drivers-stappenmotor/leadshine-dm556-2-phase-digital-stepper-drive-max-50-vdc-56a-detail. [Accessed 20 June 2016].

[47] "Stappenmotoren: Stappen Motor Nema 23 - 3Nm," [Online]. Available: https://www.hardware-cnc.nl/nl/winkel/stappenmotoren/stappenmotoren-1/stappen-motor-nema-23-3nm-detail. [Accessed 20 June 2016].

[48] "Arduino-Controlled-CNC-3D-Printer," Instructables, [Online]. Available: http://www.instructables.com/id/Arduino-Controlled-CNC-3D-Printer/step12/Wiring-the-Electronics/. [Accessed 22 June 2016].

Appendices


Appendices

Appendix A – Matlab Scripts



A.1 3D Sketch of a single curved fuselage panel %% 3D Sketch of a single curved fuselage panel PanelWidth = 1500; %mm PanelLength = 5000; %mm

RadiusX = 2000; %mm RadiusY = 99999999999999; %mm, can be assumed straight

ThethaX = ((PanelWidth/2) / (2 * pi()* RadiusX))*360; %Angle in degrees ThethaY = ((PanelLength/2) / (2 * pi()* RadiusY))*360; %Angle in degrees

%Computes the effect panel length, width ProcessWidth = RadiusX*(sind(ThethaX)); %The multiplier is being used as

the panel is symmetrical ProcessLength = RadiusY*(sind(ThethaY)); %The multiplier is being used as

the panel is symmetrical

%Generates a grid of data points in X, Y of the panel XCoords = linspace(-ProcessWidth,ProcessWidth,100); YCoords = linspace(-ProcessLength,ProcessLength,100); [X,Y] = meshgrid(XCoords,YCoords);

%Now compute the correct depth at each coordinate ProcessDepthMax=(RadiusX*(1-sind(acosd(-

ProcessWidth/RadiusX)))+RadiusY*(1-sind(acosd(-ProcessLength/RadiusY)))); Z = ProcessDepthMax-(RadiusX*(1-sind(acosd(X./RadiusX)))+RadiusY*(1-

sind(acosd(Y./RadiusY)))); surf(X,Y,Z)

zlim([0 140])



A.2 Scan Grid Generator Script clc clear

%Scan Area XDir = 35; %mm half side YDir = 35; %mm half side Step = 5; %mm per step

Coords=[]; Round=1; for XCoords=-XDir:Step:XDir; Round=Round+1; if Round/2==round(Round/2); for YCoords=YDir:-Step:-YDir; Coords=[Coords;XCoords,YCoords]; end else for YCoords=-YDir:Step:YDir; Coords=[Coords;XCoords,YCoords]; end end end

%Graphical interpretation of the shape plot(Coords(:,1),Coords(:,2)); title(['The process time is: ',num2str(length(Coords)),' seconds']);

%Now just reverse all coords and write them to file! fid=fopen('Scancoords.txt','wt'); fwrite(fid,['G0 F600', char(10)]); for i=length(Coords):-1:1; XCoord = mat2str(Coords(i,1)); YCoord = mat2str(Coords(i,2));

%fwrite(fid,['G1', ' X', XCoord, ' Y', YCoord, char(10)]); fwrite(fid,['G1', ' X', XCoord, ' Y', YCoord, char(10), 'G4 P3456',

char(10)]); end

fclose(fid);



A.3 G-Code Generator Script clc clear

%Diameter definition: XRadius = 100; %mm (Radius of curvature X 200 300 or 400mm) YRadius = 100; %mm (Radius of curvature Y 200 300 or 400mm)

XStart = 35; %mm (Start position tool) YStart = 35; %mm (Start position tool)

StepSize = 0.2; %mm (Stepsize) DistanceOriginal =1; %mm (Distance between coords in G-Code)

%Forming Depth XDepth = XRadius-(XRadius^2-XStart^2)^0.5; YDepth = YRadius-(YRadius^2-YStart^2)^0.5; Depth = min(XDepth,YDepth);

%Initial XYZ Coords Step = 1; t=0; Z=0; Coords=[Step 0 0 -Depth DistanceOriginal];

while (Z<Depth); Distance=0; %Calculate the first location X = -((2*XRadius*StepSize*t-StepSize*StepSize*t*t)^0.5)*cos(2*pi()*t); Y = -((2*YRadius*StepSize*t-StepSize*StepSize*t*t)^0.5)*sin(2*pi()*t); Z = StepSize*t;

while Distance<DistanceOriginal; %1mm %Now move to the next part of the curve t=t+0.0001; %Determine the accuracy of the Distance

%Calculate the new position XNew = -((2*XRadius*StepSize*t-

StepSize*StepSize*t*t)^0.5)*cos(2*pi()*t); YNew = -((2*YRadius*StepSize*t-

StepSize*StepSize*t*t)^0.5)*sin(2*pi()*t); ZNew = StepSize*t;

%Calculates the distance from the previous position Distance = ((X-XNew)^2+(Y-YNew)^2+(Z-ZNew)^2)^0.5; end %Defines which step within the loop this is Step = Step+1;

%To prevent any coords being addedon the list above the limited depth if (Z<Depth); %If Z is still below the allowed depth %Add the step, x coord, y coord, z coord and the distance to the %previous point into the list Coords=[Coords;Step XNew YNew -(Depth-ZNew) Distance]; end end



%Graphical interpretation of the shape plot3(Coords(:,2),Coords(:,3),Coords(:,4));

%In order to view the specimen in scale enable the following limits xlim([-50 50]) ylim([-50 50]) zlim([-10 10])

%Now just reverse all coords and write them to file! fid=fopen('Specimen.txt','wt'); fwrite(fid,['G0 F1500', char(10)]);

%First point move the head towards zero fwrite(fid,['G1 Z5', char(10)]); fwrite(fid,['G1 X0 Y0 Z0', char(10)]);

%Adds all the coordinates of the shape for i=length(Coords):-1:1; XCoord = mat2str(round(Coords(i,2)*10^3)/(10^3)); YCoord = mat2str(round(Coords(i,3)*10^3)/(10^3)); ZCoord = mat2str(round(Coords(i,4)*10^3)/(10^3));

%Adds coords to the script fwrite(fid,['G1', ' X', XCoord, ' Y', YCoord, ' Z', ZCoord,

char(10)]); end

%Reference points to be added for measuring fwrite(fid,['G1 X0 Y0 Z5', char(10)]);

%Reference point 1 X35 Y35 fwrite(fid,['G1 X35 Y35 Z5', char(10)]); fwrite(fid,['G1 X35 Y35 Z-3', char(10)]); fwrite(fid,['G1 X35 Y35 Z5', char(10)]);

%Reference point 2 X35 Y-35 fwrite(fid,['G1 X35 Y-35 Z5', char(10)]); fwrite(fid,['G1 X35 Y-35 Z-3', char(10)]); fwrite(fid,['G1 X35 Y-35 Z5', char(10)]);

%Reference point 3 X-35 Y-35 fwrite(fid,['G1 X-35 Y-35 Z5', char(10)]); fwrite(fid,['G1 X-35 Y-35 Z-3', char(10)]); fwrite(fid,['G1 X-35 Y-35 Z5', char(10)]);

%Reference point 4 X-35 Y35 fwrite(fid,['G1 X-35 Y35 Z5', char(10)]); fwrite(fid,['G1 X-35 Y35 Z-3', char(10)]); fwrite(fid,['G1 X-35 Y35 Z5', char(10)]);

%Moves head back to the zero position fwrite(fid,['G1 X0 Y0 Z0',char(10)]);

%Close the script fclose(fid);

Appendix B – Repetier Firmware changes


Appendix B – Repetier Firmware changes

B.1 Disable all extruders #define NUM_EXTRUDER 0 #define HAVE_HEATED_BED false

B.2 Choosing the correct motherboard #define MOTHERBOARD 33

B.3 Disable all end stops #define ENDSTOP_PULLUP_X_MIN false #define ENDSTOP_PULLUP_Y_MIN false #define ENDSTOP_PULLUP_Z_MIN false #define ENDSTOP_PULLUP_X_MAX false #define ENDSTOP_PULLUP_Y_MAX false #define ENDSTOP_PULLUP_Z_MAX false #define ENDSTOP_X_MIN_INVERTING false #define ENDSTOP_Y_MIN_INVERTING false #define ENDSTOP_Z_MIN_INVERTING false #define ENDSTOP_X_MAX_INVERTING false #define ENDSTOP_Y_MAX_INVERTING false #define ENDSTOP_Z_MAX_INVERTING false #define MIN_HARDWARE_ENDSTOP_X false #define MIN_HARDWARE_ENDSTOP_Y false #define MIN_HARDWARE_ENDSTOP_Z false #define MAX_HARDWARE_ENDSTOP_X false #define MAX_HARDWARE_ENDSTOP_Y false #define MAX_HARDWARE_ENDSTOP_Z false #define max_software_endstop_x false #define max_software_endstop_y false #define max_software_endstop_z false #define ENDSTOP_X_BACK_MOVE 0 #define ENDSTOP_Y_BACK_MOVE 0 #define ENDSTOP_Z_BACK_MOVE 0 #define ENDSTOP_Y_RETEST_REDUCTION_FACTOR 0 #define ENDSTOP_Z_RETEST_REDUCTION_FACTOR 0 #define ALWAYS_CHECK_ENDSTOPS false

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