Computer Aided Design

Using software for better designs

Rob Stehlik, Crescent School (Team 610)rstehlik@crescentschool.org

Agenda• Which CAD program is best?• More than pretty pictures• The value of 2D• 3D parts and assemblies• Design tips• Demo: Gearbox design• Resources

Which CAD program is best?• The short answer: Solidworks! • Solidworks is fairly easy to use, and quite powerful. It’s

also very common in industry.• But really it doesn’t matter, just use what you have• Learn what your mentors use• Both Autodesk Inventor and Solidworks are available

free for students involved in FRC

More than pretty pictures• CAD is a tool for creating designs, not rendering super

realistic images

The value of 2D• You can accomplish a lot with simple 2D sketches

-robot layout

-mechanism geometry

-size constraints• Don’t rush to 3D• Examples:

kicker designbump traverse

3D parts• Basic process: make a 2D sketch, then turn it into a 3D

part

• Combining simple features, you can make very complex parts

• Demo: make a wheel

3D assemblies• Once you have a set of parts, you can put them together

in an assembly• Assemblies help to make sure everything fits together,

keep track of weight, create a bill of materials for ordering components

• The more detailed your assembly, the smootheryour build

3D assemblies• Keep track of your weight• Assign materials to the parts• Example: wheel assembly

Coyobot VII, team 610 in 2006

Build weight: 142 lbs

Regulation weight: 10,000 holes later

3D assemblies• Subassemblies help keep things

organized, and make changes easier

Design Tips

• Write out all of your requirements• Start with a standard material size

(check mcmaster carr)• Work within your manufacturing

capabilities• Talk to your machinists!• Download 3D models, but if

possible measure the real thing

Gearbox DesignRequirements

-single speed gearbox-driven by one CIM motor-50:1 gear ratio-use AndyMark gears

Gearbox Design

Some terminology and background info:• Gears are specified by the pitch,

which is the number of teeth per inch• The pitch circle is an imaginary circle

within the profiles of the teeth of a gear, such that it rotates against a similar circle on a meshing gear

• So how do we determine the correct centre distance?

Gearbox Design

• Pitch diameter can be calculated by D = N/p

• Some common AndyMark gear sizes are shown here

• Let’s choose our gears and design the gearbox!

pitch # of teeth pitch diameter20 12 0.6

14 0.715 0.7528 1.440 250 2.556 2.8

ResourcesFIRST CAD libraryhttp://www.firstcadlibrary.com/

MadTown robotics CAD libraryhttp://team1323.com/cad/index.html

MadTown CAD tutorialshttp://www.team1323.com/pages/cad_tutorials.php

FRC Designshttp://www.frc-designs.com/index.html