AC 2007-2428: APPLICATION OF CADD/CAM TO ENGINEERINGTECHNOLOGY COURSES AND SOME REAL-LIFE PROJECTS

B. Sridhara, Middle Tennessee State UniversityDr. B. S. Sridhara is a professor in the Department of Engineering Technology and IndustrialStudies at Middle Tennessee State University. He received his B.S.M.E. and M.S.M.E. degreesfrom Bangalore University and Indian Institute of Science, Bangalore, India. He received hisM.S.M.E. and Ph. D. degrees from Stevens Institute of Technology, Hoboken, New Jersey, andAuburn University, Alabama. Dr. Sridhara has published several peer-reviewed articles in theareas of Acoustics, Vibration, finite element methods, and Engineering Education.

Rick Taylor, Middle Tennessee State UniversityMr. Taylor is the director of the machine tool technology in the Department of EngineeringTechnology and Industrial Studies at Middle Tennessee State University. He received his B. S.degree in Industrial Studies and M. S. degree in Engineering Technology and Industrial Studiesfrom Middle Tennessee State University. Mr. Taylor teaches basic and advanced machine tooltechnology classes and helps the undergraduate students with their experimental vehicles project.He is well versed with the operation and maintenance of CNC and rapid prototyping machines.

© American Society for Engineering Education, 2007

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Application of CADD/CAM to Engineering Technology

Courses and Some Real Life Projects

Abstract:

Computer-assisted drafting/design (CADD) and computer-assisted machining (CAM) are

very powerful tools both in engineering education and industry. At Middle Tennessee State

University (MTSU) we have a strong CAD/CAM curriculum. Our Engineering Technology

students learn two-dimensional CADD and solid modeling in our three CADD classes using

software such as AutoCAD, Inventor and SolidWorks. The two Machine Tool Technology

classes help students learn the basics of CNC machining, rapid prototyping and laser engraving

while incorporating their CADD abilities. In the advanced class the students design and build an

air motor from scratch learning how CADD and CAM are directly linked. They design the parts

using CADD and the parts are created using a Fadal vertical machining center (VMC) CNC unit,

a CNC lathe, and a StrataSys fused deposition modeling (FDM) rapid prototyping machine that

extrudes molten ABS plastic layer by layer to create a 3-dimensional part. The air motor capable

of rotating a 14x6 propeller at over 2200 rpm has six cylinders with a rotary configuration

(cylinders move radially) while receiving air supply through ports. Power is transmitted from

the motor shaft to a propeller through a 6 to 1 (geared up) planetary gear unit. The advantage of

the planetary gears is that they are compact and still produce the desired speed ratio. The air

motor is used in our Fluid Power class to demonstrate the working of a pneumatic motor. The

planetary gear unit is used in Dynamics to illustrate the rotation of a rigid body about a fixed axis

and calculation of physical quantities such as angular velocity, acceleration and momentum.

Spur or helical gears with the involute tooth profile are used in the planetary unit. The involute

is generated in two dimensions using AutoCAD and imported to the Inventor three-dimensional

modeling software for generating 3D models. In our Engineering Technology and Industrial

Studies (ETIS) department we compete in several national contests such as the moon buggy,

mini-Baja, solar bike and solar boat. Size, strength-to-weight ratio and performance are key

factor in all these vehicles. A planetary gear unit is used in some of these projects for speed

reduction or increasing between the motor shaft and the drive shaft because of its compact size

and inherent high torque capabilities. The planetary components on the 6-cylinder rotary engine

are ABS plastic and the Moon-buggy’s and the Mini-Baja’s components are machined from

aluminum and steel.

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Introduction:

Our Engineering Technology (ET) program has an enrollment of approximately three

hundred students. We have computer, electro-mechanical, and mechanical engineering

technology concentrations. The electro-mechanical and mechanical majors are required to take

Computer-Assisted Drafting/Design (CADD I – 2D), CADD II (3D) and Machine Tool

Technology courses besides several other courses. The Mechanical Engineering Technology

(MET) majors are required to take Advanced CAD and choose Advanced Machine Tool

Technology as an elective. Almost every student takes the elective because they get several

opportunities apply their CADD skills while learning how to build prototypes directly from the

3D models and use the state-of-the-art CNC machine. They also take Design of Machine

Elements and Fluid Power. There are several student projects such as the Moon Buggy, Mini

Baja, Formula SAE, Solar Bike and Solar Boat. These projects provide our students a unique

opportunity of designing, fabricating, testing of these alternative energy vehicles (AEV) and

compete at the national and international levels. Additional information on our AEV projects

can be found at http://etis.web.mtsu.edu/studentprojects.htm.

An Overview of Our CADD/CAM Instruction

In CADD I we primarily teach AutoCAD and students learn to create orthographic views

of mechanical components using CADD as a tool. They also learn to construct electrical circuit

diagrams, and draw house plans and plant layouts. In CADD II they learn to create mechanical

parts and assemblies using AutoDesk Inventor and SolidWorks. They learn to create

presentations and make animation movies. In Advanced CAD student learn advanced topics and

create more complex assembly projects using Inventor and SolidWorks. In both classes students

working on the alternative energy vehicle projects are encouraged to substitute equivalent part

and assembly projects for the regularly assigned ones. While assembling components the

students learn to import standard parts such as bolts, nuts, bushings, washers, shafts and keys

from the content center. However, they do not find every part that is required for a given

assignment and particularly for a student project. For example, spur gears of different sizes and

number of teeth are available in the Inventor library but it is often difficult to find a custom spur

gear or the helical type to suit a particular application. In such cases we have to design gears by

generating the involute gear tooth profile and helical path. We will discuss the details of this

aspect is given in the next section.

In the Advanced Machine Tool Technology class our students get plenty of opportunities

to apply their 2D and 3D CADD skills in designing an air motor (Fig. 1) that runs using

compressed air. They will design and build/machine each part of this device with help and

guidance from the instructor. This unit, made of aluminum and ABS parts, can be easily carried

in one hand and fits in a 15x15x4 inch box. The planetary gear parts including the ring, planets,

and driver and driven gears were designed using AutoCAD and Inventor. Fig. 2 shows the three-

dimensional Inventor model of a typical planetary gear assembly. The stereo lithography (STL)

files of the part models were transmitted to the workstation where the 3D models were sliced into

0.010-inch layers. This file is transferred to the StrataSys FDM rapid prototyping machine

which built the parts in 0.010-inch layers utilizing molten ABS extruded plastic. The process

took several hours and we could run the machine overnight and obtain the required parts on time.

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Acrylic parts of the rotary engine were created on a Universal brand 30-watt laser engraving

machine that can read information directly from AutoCAD drawing (dwg) files. Other parts

including the pistons, cylinders, connecting rods, and the air inlet and outlet ports were machined

on the Fadal CNC machine. The Fadal is a state of the art machine that has a twenty-one-tool

station and can be programmed using mastercam toolpath software to perform machining

operations such as drilling, milling, honing, reaming, boring and tapping in the desired sequence.

At the end of the semester each students takes home his/her air motor.

Fig. 1. Air motor designed and built by Engineering Technology students.

In the Design of Machine Elements course our students learn how design components

such as shafts, pulleys, gears and springs using different theories of failure. Many of these

students participate actively in the AEV projects and design/build many parts including the main

frame, axles, hubs and gear train. Computer-generated hub and gear train assembly, and the

actual unit built using aluminum and steel are shown in Figs. 3 and 4. In these figures the

computer model and physical unit are rotated through 180o with reference to one another.

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Fig. 2. Computer-generated assembly of the planetary gear.

Fig. 3. AutoDesk Inventor-generated assembly of the hub and gear train.

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Standard parts such as bolts, nuts, washers, bushings and bearings are purchased and

other parts are custom built. Some parts are made of ABS plastic and the other parts are

machined out of steel or aluminum. But to ensure proper matching of parts in assemblies, ABS

plastic prototypes are produced and tested before making the parts out of the actual material. In

the case of gear design many a time it is difficult to find standard gears of the desired size and

number of teeth. Therefore, the students should know how to construct a two-dimensional gear

tooth profile and sweep this profile along a defined straight or helical path (or use other 3D

modeling techniques) to create the three-dimensional tooth that can be arrayed to obtain the

desired number of teeth on a wheel. As there are no books or documentation currently available

in this regard we developed our own CADD method to design gear tooth profile.1

Fig. 4. Actual hub and gear assembly used in the Mini Baja vehicle.

Involute Gear Tooth Design

The section refers to involute gear design using AutoCAD but the profile can be

generated using Inventor also. Optionally we can create the two-dimensional profile using

AutoCAD and import it to Inventor for 3D modeling. The Fadal CNC machine that we have can

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read the information directly from two-dimensional AutoCAD drawings and machine the parts.

This is a useful feature when we have to custom-build spur gears. However, in the case of

helical gears we have to generate the 3D model using Inventor and send the STL file to the

StrataSys machine for building the part out of ABS plastic. We hope the following procedure

for generating an involute gear profile will be helpful to those who have a particular interest in

CADD/CAM.

The objective here is to design an enclosed reducing planetary gear train. We have used

this type of gear train in our 2006 Mini-Baja vehicle. We choose for our demonstration a

planetary gear with a 6 to 1 reduction ratio. The sun gear as well as the meshing planets has the

same number of teeth and the internal (ring) gear has 45 teeth. We have found that this choice

facilitates less design and machining time. A typical system and close-up view of the meshing

gear teeth are shown in Fig. 5.

Fig. 5. A typical planetary gear system and a close-up view of the meshing gear teeth.

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Standard gear nomenclature and formulas listed below can be found in any Machinery

Handbook, Machine Design textbook or Mechanical Engineers’ Handbook.2, 3, 4

• Diametral Pitch (p): The ratio of the number of teeth of a gear to the number of inches

of pitch diameter. System arranged to provide a series of standard tooth sizes.

p = N/Dp, where N is the number of gear teeth.

• Addendum (A): Radial height of the tooth above the pitch circle on an external gear

and below the pitch circle of an internal gear.

A = 1/p

• Dedendum (D): Radial height of the tooth below the pitch circle external gear and

above the pitch circle of an internal gear.

D = 1.25/p

• Pitch Diameter (Dp): Diameter of the pitch circle

• Pressure Angle (Pangle): The angle between a tooth profile and a radial line at the pitch

point of mating gears.

• Base Circle Diameter (Db ): The circle from which an involute tooth curve is

generated. The circle by which an imaginary line is unwound such that the end of that

line would describe an involute curve.

Db = cos Pangle*Dp

We will now begin to design our planetary based upon the following criteria:

Diametrical pitch 5

Pitch Diameter of sun and planet gears 3 inches

Number of teeth on sun and planets 15

Number of teeth on the ring (internal) gear 45

Pressure Angle 30o

1. First lets set-up our Autocad software to have at least five different layers/colors for a

better contrast between the many lines required in close proximity to develop the involute

of our gears.

2. Begin by drawing a red circle representing the 3 in. pitch diameter. Offset this circle to

the outside by 0.200 in. (Addendum, A = 1/5).

3. Now, offset the pitch circle by 0.250 in. (Dedendum, D = 1.25/5). Assign the cyan color

to the two offset circles.

4. Calculate the base circle diameter Db from the given equation and draw a circle of this

diameter that is concentric with the other three circles.

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5. Draw a green line from the center of the circles to the upper (90-degree) quadrant of the

base circle, and then drag it horizontally to the left a couple inches past the cyan

addendum circle.

6. Array (or create a circular pattern of) the vertical and horizontal lines sixty times that is

two lines each for each tooth and tooth space multiplied by the number of teeth which is

fifteen in our case. If the above steps are followed properly your sketch should resemble

that shown in Fig. 6.

Fig. 6 – Pitch, addendum, dedendum and base circles, and radial and tangent lines for

generating gear tooth profile.

7. Now, zoom in if necessary and delete the base circle.

8. Calculate the base circle diameter from the above given formula, multiply the result by pi

and divide by 60 to obtain the arc length for each gear tooth and the spacing between two

consecutive teeth.

9. Therefore, Db = (cos 30o)(3) = 2.59808 in. and (2.590808)(3.15029)/60 = 0.13603 in. is

the radius of the first circle for trimming the first arrayed line. We will also use this value

as the amount to offset successive circles to trim some of the other arrayed lines as

follows:

10. When a 0.13603-inch-radius circle is drawn and this circle and each subsequent circle is

offset by 0.13603 in. These circles are shown in Fig. 7.

11. In order to locate sufficient number of points on the involute we need to move the centers

of approximately ten circles to the end points of the radial lines drawn from the center of

the base circle (Fig. 8). A close up view of this modification is shown in Fig. 9.

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Fig. 7. The 0.13603-inch-radius circle and other circles offset by 0.13603 inch.

Fig. 8. The offset circles corresponding to step 11. Page 12.245.10

Fig. 9. A close up view of the modification corresponding to step 11.

12. Trim the tangential lines of the base circle that are intersecting the offset circles (of the

above step) so that only the extending part of each line is removed (Fig. 10). When this

step is followed properly and the offset circles are deleted we should see the tangential

lines as shown in Fig. 10.

Fig. 10. Trimmed tangential lines corresponding to step 12.

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Fig 11. A spline fitted through the end points of the tangential lines shown in Fig. 10.

13. Now we can fit a spline through the end point of the tangential lines to obtain the involute

for one side of the gear tooth (Fig. 11). Hereafter it is the question of trimming and

mirroring this profile making sure proper tooth width is maintained.

14. Extend the radial line to the pitch circle. Create a .004in. radius circle at the intersection

of the pitch circle and the extended radial line. Draw a line from the intersection of where

the .004 radius circle and the pitch circle intersect to the center of the pitch circle and

continue to where the involute curve intersects with the pitch circle.

15. Dimension the angle between these two lines and rotate the involute components that to

obtain the sketch in Fig. 12. The final shape of the gear tooth profile is shown in Fig. 12.

Figs. 12. Rotated involute profile using the .004 radius circle to determine backlash.

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Fig. 13. Final shape of the gear tooth and finished15-tooth sprocket.

Procedure for designing a 45-tooth ring (internal) gear

The following steps are for designing a of a 9-inch-diameter, 45-tooth ring (internal).

Any 2D or 3D software can be used to generate the ring gear and here we describe how

AutoCAD can be used in the design.

1. Draw a 9-inch diameter circle representing the pitch circle of the internal gear. Offset this

circle 0.200 in. and 0.250 in. to represent the addendum and dedendum, respectively, just

as done with the previous sketches. The difference is that the addendum is offset inside

and the dedendum outside the pitch circle.

2. Now create a 30-degree (pressure angle) construction line so that it passes through the

90-degree quadrant of the pitch circle. Draw another circle from center tangential to this

30 degree line. In fact, this is the base circle of diameter equal to 9*cos 30.

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3. Draw a vertical radial line from the center to the base circle and continue this line

tangentially (to the left) past the dedendum circle. Since the internal gear has 45 teeth we

must array these lines 180 times (2 lines for the tooth and 2 for the space) as shown in

Fig. 14.

Fig. 14. Addendum, Dedendum, Pitch circles, and arrayed lines.

4. Just as it is done on the previously sketched 15-tooth gear, create a 0.13603 in. radius

circle at the intersection (point A) of the first radial line and base circle. Offset this circle

by the same distance and place each offset circle in such a way that its center is at the

intersection of the base circle and a successive radial line until you have enough circles to

trim the tangent lines outside the dedendum circle (about 21 circles). See Figure 15.

Fig. 15. Results of steps 1 thru 4.

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5. Do the necessary deletions to make your sketch appear as Figure 16 below.

Fig. 16. Trimmed radial lines whose end points mark the involute curve.

6. Fit a spline (as done previously) through the end points of the tangential lines shown in

Fig. 16.

7. Extend the radial line of Fig. 16 to the pitch circle and draw a 0.004-inch-radius circle at

that intersection.

8. Draw a line from the intersection of the pitch circle and spline, to the center, and continue

the line to the left side of the intersection of the 0.004-inch-radius circle and the pitch

circle. Now, dimension the angle between these 2 lines (Line-1 and Line-2 shown in Fig.

17). Dimension the line from the left side of the .004 radius circle as we are creating a

tooth space instead of a tooth.

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Fig. 17. Lines and angle corresponding to step 8.

9. Rotate the involute profile the angle shown in Fig. 18 about the center. Mirror the

involute profile about the line passing through the 90-degree quadrant pitch circle and the

dedendum circle and array (circular pattern) both profiles 45 times. Do the necessary

trimmings and deletions to make your sketch should appear as Fig.18.

Fig. 18. Segment of finished ring gear sketch.

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Conclusions:

Our CADD/CAM courses and facilities have been very useful in our instruction and student

projects such as the Moon Buggy, Solar Bike, Solar Boat, Mini Baja and Formula SAE. We use

the 3D models built on the StrataSys rapid prototyping machine and the Fadal CNC machine in

our CADD I and CADD II classes. These models help our students understand the principles of

orthographic views of mechanical components and create three-dimensional parts and

assemblies. We use the air motor that is designed and built by students in our Fluid Power class

to illustrate the working of a pneumatic actuator. In the design of planetary gears we decided to

use a 300 pressure angle because a thicker tooth-base can be obtained with a larger angle

although the conventional values have been 14- ½ 0 and 20

0.

The pressure angle is a very

important because it is, in simple terms, the inclination of the line of action of the pressure

between a pair of meshing teeth.3 So far our large pressure angle has not shown any adverse

effect on the Mini Baja drive train as the ring gear of the planetary system was mounted on the

frame which could handle the additional lateral force. However, we want to investigate this

further and plan to introduce a project in our Machine Design course giving our students an

opportunity to do design and analysis for real life projects. To the best our knowledge this is the

first time a graphical method of involute gear design has been developed for CADD/CAM

applications. We have successfully applied this in our classroom as well as the alternate energy

vehicle student projects. We are using the planetary gear in our Dynamics class as an example

for calculation of angular displacement, velocity, acceleration and momentum.

Bibliography:

1. Rick Taylor, “Instruction manual prepared for the Advanced Machine Tool Technology Students,” Middle

Tennessee State University, Murfreesboro, TN (2006).

2. Machinery’s Handbook, 27th

Edition, Industrial Press, Inc., New York (2004)

3. M. F. Spotts and T. E. Shoup, Design of Machine Elements, Seventh Edition, Prentice Hall, New Jersey

(1998)

4. Marks’ Handbook for Mechanical Engineers, Eighth Edition, McGraw Hill Book Company, New York

(1978)

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