From Idea to Prototype: Chassis
CAD
While designing the drive train, I also started drafting the chassis in a CAD program. Using a 3D CAD tool helped determine the fit of each drive train component within the set boundaries of the chassis. I highly recommend taking the time to draw your robot in a CAD program before ordering parts. Pairing the v-belt for a pulley of the right diameter with the bore size to fit the bearings and shaft requires a precision computers are simply better at.
I used Google Sketchup for the CAD program. SketchUp is a free, entry level CAD program that runs on both Windows and Mac OS X. Of course I would like to use Solidworks or AutoDesk Inventor, but the cost is too high for a hobbyist. Plus, SketchUp meets all my needs at this stage.
Build
After I was happy with the design and ordered the parts for the drive train, I set out to create a physical chassis. For some, the steps may be obvious, but coming from a computer science background, I have found the transition from idea on paper to tangible object non-trivial.
The overall layout was simple enough to make out of wood, but I wanted a more precise chassis. I needed the holes bored in the correct place and each component to line up. And really, a fabricated chassis would just look cooler. I decided on the routes of 3D printing or CNC milling. 3D printing essentially prints plastic along three axes. The cost of a low-end 3D printer has dropped significantly thanks to projects like MakerBot and RepRap. Milling is a subtractive process, taking a block of plastic and cutting to form.
Most companies that do stereolithography (3D printing) or milling require an STL file. An STL file represents the surfaces of the 3D drawing. SketchUp does not export to the STL format natively, but CADspan has a free plugin to do just that. An excellent tutorial can be found here.
I contacted a few rapid prototype companies in Austin for quotes on printing the chassis. I had no idea how much to expect. The first quote back was for a “Stereolithography Apparatus Prototype” and cost $1,989. Whoa. Roughly $2,000 was far more than I expected to spend.
Another response from a Mr. Allison at ATi recommended a local milling company to try. As milling carves from a solid block of plastic, it would be sturdier and the chassis design is simple enough for the operation. Hopeful now, I contacted the milling company and got a quote for $1,155 ($480.00 set-up + $675.00 per part). The contact explained the cost of the 3” ABS plastic was $175.00 per sq.
Needless to say, I had to reevaluate my options. With no access to a 3D printer like the RepRap (though a new Austin HackerSpace may change that soon) and professional services proving too expensive, I decided to go the cheapest, simplest route: wood. It’s an initial prototype after all.
I know, the result is even simpler than the CAD drawings. Let’s call it a prototype of the prototype. I wanted to move on to the brains of the robot instead of focusing too long on the drive train for this iteration of the prototype.
For those curious, I attached the inline skate wheel to the motor hub by drilling the same plank of wood to both the wheel and the hub. It worked surprisingly well.
