Di g i ta l e Fa b r i cat i e

SPiF

Gert-Willem Van GompelMaster in de ingenieurswetenschappen: architectuur

Promotor: Vande Moere Andrew Corneel Cannaerts (ir. arch.) Marc Lambaerts (FabLab, dep. Werktuigkunde) Matthias Mattelaer (DMOA, ir. arch.) Hans Vanhove (dep. Werktuigkunde)22 oktober 2015

Ontwerp prOductie

waarOm digitale fabricatie?

Ontwerp prOductie

waarOm digitale fabricatie?

missing linktussen ontwerp en productie

Ontwerp prOductie

waarOm digitale fabricatie?

doel thesis: volledig digitaal proces in een bepaalde testcase doorlopen zowel digitaal, parametrisch ontwerp en optimalisatie als digitale fabricatie

onderzoeksvraag: Hoe kan Single Point Incremental Forming geïntegreerd worden binnen de context van een digitaal ontwerp- en productieproces?

wat is spif?

Figure 17: Single Point Incremental Forming of a cone.

Figure 18: FLDo for different step sizes for AA 1050-0, with upper and lower bounds, with a 12 mm diameter tool [59].

Minor strain

Maj

or s

train

-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4

1.2

1.0

0.8

0.6

0.4

0.2

0

1.4

1.6

1.8

Scattering band for mild steel (t=1.5 mm)

2.0

6 mm tool

10 mm tool

30 mm tool

31

42

2

3

4

1 x

x

x

x

Minor strain

Maj

or s

train

-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4

1.2

1.0

0.8

0.6

0.4

0.2

0

1.4

1.6

1.8

Scattering band for mild steel (t=1.5 mm)

2.0

6 mm tool

10 mm tool

30 mm tool

31

42

2

3

4

1 x

x

x

x2

3

4

1 x

x

x

x

Figure 19: FLD for to = 1.5 mm DC04; influence of forming

tool size upon forming limits [60]. Graph points x1 to x4 correspond to positions on the sheet marked by x.

Kim & Park [70] focused their attention on the influence of anisotropy on formability. For this purpose, a set of measurements of the major and the minor strains were carried out both along the rolling direction (RD) and the transverse one (TD). The tests were developed for pyramid specimens with a varying tool diameter. The material was the aluminium alloy 1050-O, with E = 70GPa, σο = 33MPa, R0 = 0.51, R45 = 0.75, R90 = 0.48. They concluded that formability along the transverse direction is greater when small diameter tools are utilized, while along the rolling direction it is larger with large diameter tools. In order to fully understand the increase in formability with AISF, a simple FEM was developed by Micari et al. [59] and Bambach et al. [69]. They found that with decreasing step size, ∆z, the strain increments imposed at each loop decrease and any point is overlapped other ‘active loops’, while strains increase with increasing wall angle. Also, from a stress point of view, a negative mean stress distribution is observed under the tool and in the nearby elements; in this way, the tool action postpones ductile fractures during the process, until the tool is out of contact with the sheet. Finally, at decreasing ∆z the stress value along the wall decreases too, so that a higher deformation can be imposed without tears occurring. Nontraditional Forming Limit Diagrams Forming limit diagrams usually have the dashed V-shape shown as FLC in conventional forming in Figure 20. However, extensive research has shown that not only are much higher strains achieved in this process, but the Forming Limit Curve (FLC) in SPIF has a negative slope as shown in Figure 20 [32, 62]. In one case, the maximum forming strains [32] were actually much higher because the AA 1050-0 sheet used in the experiments had been etched with a grid causing stress concentrations and hence premature failure to occur. Young and Jeswiet [62] used more than one shape to develop a composite FLD for 1.21 mm thick AA 3003-0. They used five different shapes with varying angles, and convex and concave curves. The shapes were: a dome, a cone, a hyperbola, a pyramid and a shape with both compressive and tensile stresses. These all have their own FLD, and when all placed in one, common FLD, they represent most combinations of shape found in a part. An example of their work is shown Figure 21. The boundary for safe forming, without any tearing, is shown by the dashed red line. It can be seen that both Filice [32] and Young [62] have the same slope for the boundary and that very high strains were achieved. It can be seen that the non-traditional FLD’s being developed [32, 62] provide the designer with an additional method of judging if a design can be made in one pass with AISF.

εmin

-0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6

Maj

or s

train

,εm

ax

1.00.90.80.70.60.50.40.30.20.1 FLC conventional forming

FLC incremental forming

εmin

-0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6-0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6

Maj

or s

train

,εm

ax

1.00.90.80.70.60.50.40.30.20.1 FLC conventional forming

FLC incremental forming

Figure 20: FLC for 1.21 mm thick AA 1050-0 forming of

pyramids [32]. Both traditional and non-traditional curves.

UUppppeerr lliimmiitt

LLoowweerr lliimmiitt

Εmax, FLDo 3.5

Depth of step, ∆z, mm

0.5

1.5

2.5

0

1

2

3

0.00 1.00 2.00 3.00 4.00

Jeswiet, J., Micari, F., Hirt, G., Bramley, A., Duflou, J., & Allwood, J. (2005). Asymmetric single point incremental forming of sheet metal. Cirp Annals-Manufacturing Technology, 54(2), 623-649.

points to generate new, virtual target geometry. This virtual part geometry forms the basis for the determination of an improved toolpath. Using a scale factor of 0.7 was found to provide optimal results for part made of DC04, 1.5 mm.

V shaped tub

Ambrogio et al. [106] use an in-process measurement system that allows the determination of deviation between the anticipated intermediate part geometry and the actually realized intermediate shape. Per layer (incremental toolpath contour) the observed deviations are measured to correct the toolpath geometry for the next contour.

Cone Cross Hexagon

The proposed system has been tested with a discrete point contact measurement system, used interactively, thus simulating the availability of real in-process measurement equipment. The toolpath optimization algorithm has been tested with pyramid part geometry. The author claims significant accuracy improvements. No quantitative output is however available to evaluate the achievable dimensional accuracy.

HyperbolaDome 5 lobe shape

5 EXAMPLES OF APPLICATIONS The major advantage of asymmetric incremental forming is it can be used to make asymmetric parts, quickly and economically, without using expensive dies. Shapes used to demonstrate the abilities of the process are shown in Table 8. Some of the shapes illustrated have been used to conduct springback experiments, and in determining the maximum draw angle φ, others are just for demonstration of process abilities. The asymmetric single and two point incremental forming processes are still in their infancy. Much research work remains to be done and to do this appropriate shapes are needed to develop: FLD’s, springback models and models to test accuracy. Standard shapes are used to determine what the draw angles should be for different materials and thickness. Standard shapes are needed to compare experimental results and for testing the process for maximum speeds of deformation. When testing materials for the maximum draw angle φ a truncated cone as shown in Figure 15 was agreed upon [59]. This shape has also been used by others [31, 32, 33, 59, 64, 66]. A pyramid shape has also been used on many occasions to demonstrate the asymmetric abilities of the new process plus it has become useful in determining the springback that occurs for different material thickness’ and for conducting pre and post process material tests [59, 64, 65, 67, 77]. Jadhav [100] has also used the pyramid to conduct studies of twisting in a shape once forming is completed. There is not agreement on which shape or shapes should be used to develop FLD’s for the new processes. Filice [32] and Hirt [78] have used a pyramid. Young is a proponent of using several shapes each of which has elements contained in most part shapes [62]. 5.1 Rapid Prototype Examples Making Rapid Prototypes, with sheet metal as the base material, giving a part that can be used directly in the function for which it is intended, is one of the major advantages of using this Incremental Forming process. The parts shown in table 9 are for designs that were made as Rapid Prototypes for the automotive industry. They are for the reflective surface of prototype headlights, for the first two cases. The third case is for a heat/noise shield, which is used over exhaust manifolds. Table 10 shows examples of rapid prototypes made for non-automotive applications. The first example is of a solar

Table 8: Shapes used to demonstrate the viability of the process and for experiments.

oven cavity for use in developing country applications. The last two are for the same manufacturer of custom motorbikes; the first part is for a motorbike seat and the second is part of a gas tank. 5.2 Custom manufacture of a solar oven The SPIF process has made it possible to manufacture an aluminum solar oven cavity, economically without dies. The ability to make a sheet metal cavity, inexpensively, has allowed designers of the solar oven to redesign other parts of a product, thereby reducing the cost and labour in making a solar oven. A solar oven has been designed for export to developing countries. Originally the oven cavity was made from fibreglass, and painted black. The major drawback, was the fibreglass wall was 7 mm thick, heavy, time consuming to build, and labour intensive. The possibility of using dies to form sheet metal into solar cooking cavities was investigated and found to be very expensive. This lead to considering SPIF of sheet metal as a method of manufacture. Figure 49 shows how the assembled solar oven works. The red shape is the part which is formed by SPIF. A three dimensional model was made in a Unigraphics environment and Figure 50 shows the model and increment details. The total depth of the model is 74 mm. The downward step for each pass was set up differently for the two sections shown; ∆z = 0.4 mm from A to B, and ∆z =0.3 mm from B to C. The method of programming included a z-level profile following part contour, which is available in Unigraphics, was used. Details of the process set-up in Unigraphics are:

Maximum drawing angle: set to 65o, everywhere. Forming tool: 25.4 mm diameter, highly polished. Forming speed (feedrate): 1125 mm/min. Additional note: the forming tool rotates freely, and is not matched to the machine feedrate. 1.3 mm thick AA 8008-0 Aluminum.

The final, successful result is shown in table 10. Although the customer was unconcerned by springback, the cross-section was measured to find out how much springback actually occurred; see Figures 47 and 51.

Truncated pyramid

Faceted cone

Multi-shaped surface

wat is spif?

Fig. 5 Top DSIF Method A with a forming tool and a support tool. Bottom DSIF Method B withtwo forming tools

40 A. Kalo and M. J. Newsum

Fig. 9 A component with performative textures and features

44 A. Kalo and M. J. Newsum

Kalo, A., & Newsum, M. J. (2014). An Investigation of Robotic Incremental Sheet Metal Forming as a Method for Prototyping Parametric Architectural Skins. Robotic Fabrication in Architecture, Art and Design 2014, 33-49.

wat is spif?

Kalo, A., & Newsum, M. J. (2014). An Investigation of Robotic Incremental Sheet Metal Forming as a Method for Prototyping Parametric Architectural Skins. Robotic Fabrication in Architecture, Art and Design 2014, 33-49.

Fig. 10 Comparison of overall geometric improvements with the ‘ribbing’ system

Fig. 11 Prototype of a textured performative water shedding surface

An Investigation of Robotic Incremental Sheet Metal Forming 45

regarding part geometries and dimensions. Furthermore, the process can be applied on conventional CNC milling machines with minor modifications [8]. In the ISF process the sheet is fixed in a sheet clamping system. A universal CNC-controlled forming tool generates the part by local forming along a tool path. The tool path is usually following the contour-lines of the part geometry layer-by-layer. After finishing one horizontal layer, the tool performs a vertical step down and the forming continuous in the next layer. The most common variants of the ISF process (see Fig. 2) are the single-point incremental forming (SPIF) and two-point incremental forming (TPIF).

Figure 2: Single Point Incremental Forming (left) and Two Point Incremental Forming (right)

The high formability in ISF allows for an effective generation of high cones to increase the distance of the layers. Furthermore, the material volume stays constant in forming processes and thus with a constant mass, the load bearing performance in relation to the self-weight can be increased significantly.

With the process combination of stretch forming and ISF the forming of global shape and local features can be performed in an integrated procedure (see Fig. 3).

Figure 3: a) Clamping the sheet blank; b) Generating a preform with stretch forming; c)

Forming details and features with ISF

Compared to pure ISF, the process combination allows for a shorter process time as well as improved geometrical accuracy and sheet thickness distribution [9]. Compared to pure stretch forming, the process combination enables the realization of changes in curvature within one panel geometry and the generation of features, such as the described cones.

Production Routine and Tooling Concept The tooling concept for the described production approach is a combined die for the stretch

forming and the incremental forming process. Due to the low forming forces in ISF, the use of very cheap tool material for a die is possible. This way a bonded block of medium density fiberboard (MDF) has been prepared for the milling of the customized dies. However, the MDF block deforms under the high pressure induced by the stretch forming, but due to the homogenous structure of MDF, these deformations are uniform and can easily be taken into account in preliminary simulation of the process.

The tooling concept is illustrated in Fig. 4 and the steps of the production routine for the freeform panels are the following:

1) Milling of the die (based on a prepared MDF block) 2) Stretch forming of the first outer layer 3) Trimming of the first outer layer 4) For an optional second outer layer, the steps 2 and 3 will be conducted twice

44 Sheet Metal 2015

Figure 5: Manufacturing steps for the structural layer of freeform panels: a) Milling of the

global die shape; b) Milling of the cones; c) Clamping the sheet; d) Stretch forming of the global part shape; e) Incremental forming of the cones; f) Trimming of the panel contour

All panels have been formed from aluminum sheets of grade AA5182, with a thickness of 1.5 mm. Certainly, the process is also transferable to other aluminum alloys as well as mild steel and even stainless steel grades, mainly depending on the dimensioning of the stretch forming units of the machine in terms of applicable drag forces. An example of a produced panel is shown from both sides in Fig. 6.

Figure 6: Produced and joined freeform panel: Smooth outer layer (left); Structural layer (right)

The entire assembled prototype structure is presented in Fig. 7 and serves as proof for the producibility as well as the mountability of the panels.

Figure 7: Assembled prototype structure with 8 panels (4 different panel shapes)

46 Sheet Metal 2015

mal gebruikt om dubbele kromming te bekomen nadeel: elk nieuwe geometrie, nieuwe mal nodig

Case Study In order to assess the applicability of the proposed panel system for large-scale applications, a

further case study has been investigated by means of design and structural analysis. The developed case study is a shell spanning over four foundation points, which build a square of 8.15 m by 8.15 m. The maximum height at the apex is 4.80 m.

Figure 8: Case study design for a large-scale freeform structure

The construction results in 100 panels with a constant effective thickness of 150 mm. To produce all panels, a die block with a height of only 650 mm is necessary. Due to the double symmetric geometry of the shell, 25 different typologies of panels are to be formed, and consequently the die has to be adjusted 25 times to the panel shape by milling.

For the same symmetry reason it was possible to conduct static simulations with a finite element model representing a fourth of the whole shell. A uniformly distributed live load of 1 kN/m2 acting on the vertical projection of the shell surface has been assumed. With this simulation, the necessary thickness of steel-sheets of standard steel grade S235 was dimensioned to 1.2 mm. For the whole structure a maximum vertical displacement of 1.7 mm was observed in the simulation, with maximum tensile stress of 183 MPa. The overall weight of the structure results in 2600 kg for an average value of 24 kg/m2.

In a first dimensioning the distribution of the stiffening cones over the surface followed the maximum density possible with the fixed die configuration. The amount of cones per panel was 25-27, depending on the panel geometry, whereas the total amount for the structure was 668.The drawback of this generous distribution of cones is a higher production time, because each cone requires a forming time of several minutes. Therefore, a further optimization was conducted to reduce the number of cones and thus the production time, but maintaining the static performance. Starting from the maximum number of cones, an iterative static analysis has been performed. In each iteration step the cones with a lower level of stress (defined as overall sum of von Mises stress acting on all finite elements of the cone) are removed. For the next iteration step the static analysis is based on the reduced configuration. Only the removal of the four cones at the corner of each panel is prohibited. The process stops, when the static limitations on maximum tensions and maximum deflections are no longer respected. For the given sheet thickness and boundary conditions the last statically reliable configuration is achieved after removing of 20% of the cones. As expected, the result shows a higher density of cones in areas of stress concentrations.

Conclusion and Outlook The paper demonstrates the successful interdisciplinary development of a production-oriented

design and construction concept for freeform objects. The developed panel system can provide truly double-curved surfaces and is stiffened as multi-layered assembly in order to build self-supporting structures. The production oriented design of the panels and the combination of stretch forming and ISF, together with a smart tooling, concept allows for a widely automated manufacturing. It can be

Key Engineering Materials Vol. 639 47

wat is spif?

Bailly, D., Bambach, M., Hirt, G., Pofahl, T., Della Puppa, G., & Trautz, M. (2015). Flexible manufacturing of double-curved sheet metal panels for the realization of self-supporting freeform structures. Key Engineering Materials, 639, 41-48.

tOepassingen?

Project 2XmT by Christopher Romero, Nicholas Bruscia (self-structuring and lightweight architectural screens from sheet metal)http://blog.archpaper.com/2013/07/reinventing-the-facade-skin-competition-names-four-first-stage-finalists/#.VifKmxDhCAx

tOepassingen?

Eventueel toepassing binnen huidig of toekomstig project van DMOA

tOepassingen?

© Otto Wöhr GmbH, Friolzheimhttp://brunier-ernst.com/Design-Carport

beschrijving van het toolpath eventueel integraal in Grasshopper genereren (alles binnen één softwarepakket)

tOOlpath

output: lijst met coördinaten en richtingsvectoren

tOOlpath

Definiëren van verschillende test cases met als doel de verschillende productiestappen te tonen.

1. Parametrisch ontwerp structurele optimalisatie uitvoeren op bepaalde geometrie spanningen/doorbuigingen minimaliseren

2. Vertalen 3D model vertalen naar robot-instructies toolpath laten genereren

3. Digitale productie element laten fabriceren door KUKA-robot

4. Assembleren elementen aan elkaar schakelen

Andrew Vande MoereCorneel CannaertsMatthias Mattelaer

Hans VanhoveMarc Lambaerts

Andrew Vande MoereCorneel CannaertsMatthias Mattelaer

test cases

planning

Januari Februari Maart April MeiNovember December

Case Study In order to assess the applicability of the proposed panel system for large-scale applications, a

further case study has been investigated by means of design and structural analysis. The developed case study is a shell spanning over four foundation points, which build a square of 8.15 m by 8.15 m. The maximum height at the apex is 4.80 m.

Figure 8: Case study design for a large-scale freeform structure

The construction results in 100 panels with a constant effective thickness of 150 mm. To produce all panels, a die block with a height of only 650 mm is necessary. Due to the double symmetric geometry of the shell, 25 different typologies of panels are to be formed, and consequently the die has to be adjusted 25 times to the panel shape by milling.

For the same symmetry reason it was possible to conduct static simulations with a finite element model representing a fourth of the whole shell. A uniformly distributed live load of 1 kN/m2 acting on the vertical projection of the shell surface has been assumed. With this simulation, the necessary thickness of steel-sheets of standard steel grade S235 was dimensioned to 1.2 mm. For the whole structure a maximum vertical displacement of 1.7 mm was observed in the simulation, with maximum tensile stress of 183 MPa. The overall weight of the structure results in 2600 kg for an average value of 24 kg/m2.

In a first dimensioning the distribution of the stiffening cones over the surface followed the maximum density possible with the fixed die configuration. The amount of cones per panel was 25-27, depending on the panel geometry, whereas the total amount for the structure was 668.The drawback of this generous distribution of cones is a higher production time, because each cone requires a forming time of several minutes. Therefore, a further optimization was conducted to reduce the number of cones and thus the production time, but maintaining the static performance. Starting from the maximum number of cones, an iterative static analysis has been performed. In each iteration step the cones with a lower level of stress (defined as overall sum of von Mises stress acting on all finite elements of the cone) are removed. For the next iteration step the static analysis is based on the reduced configuration. Only the removal of the four cones at the corner of each panel is prohibited. The process stops, when the static limitations on maximum tensions and maximum deflections are no longer respected. For the given sheet thickness and boundary conditions the last statically reliable configuration is achieved after removing of 20% of the cones. As expected, the result shows a higher density of cones in areas of stress concentrations.

Conclusion and Outlook The paper demonstrates the successful interdisciplinary development of a production-oriented

design and construction concept for freeform objects. The developed panel system can provide truly double-curved surfaces and is stiffened as multi-layered assembly in order to build self-supporting structures. The production oriented design of the panels and the combination of stretch forming and ISF, together with a smart tooling, concept allows for a widely automated manufacturing. It can be

Key Engineering Materials Vol. 639 47

Test Case 1 Test Case 2 Test Case 3

Eventueel verschillende (2 à 3) test cases, telkens op grotere schaal...

Figure 5: Manufacturing steps for the structural layer of freeform panels: a) Milling of the

global die shape; b) Milling of the cones; c) Clamping the sheet; d) Stretch forming of the global part shape; e) Incremental forming of the cones; f) Trimming of the panel contour

All panels have been formed from aluminum sheets of grade AA5182, with a thickness of 1.5 mm. Certainly, the process is also transferable to other aluminum alloys as well as mild steel and even stainless steel grades, mainly depending on the dimensioning of the stretch forming units of the machine in terms of applicable drag forces. An example of a produced panel is shown from both sides in Fig. 6.

Figure 6: Produced and joined freeform panel: Smooth outer layer (left); Structural layer (right)

The entire assembled prototype structure is presented in Fig. 7 and serves as proof for the producibility as well as the mountability of the panels.

Figure 7: Assembled prototype structure with 8 panels (4 different panel shapes)

46 Sheet Metal 2015

•structurele optimalisatie: evolutionair algorithme (Galapagos), topologische optimalisatie ?

•naast structuur, eventueel ook integreren van technieken: HVAC, verlichting,... ?

•context: gevelpanelen (VM Zinc?), dakconstructie (dakbedekking?),... ?

•toolpath in Grasshopper genereren

•SPIF zonder backing plate mogelijk?

•in twee fasen omvormen?

•niet loodrecht omvormen?

???

Unikabeton selected for Fabricate 2011 publicationhttp://www.digitalcrafting.dk/?p=1688

tOpOlOgische Optimalisatie

cassette structuur

willekeurig rib-patroon

eerste tests

mOdeleren van structurele ribben