Build Your Own Combat Robot

Chapter 14: Real-Life Robots: Lessons from Veteran Builders 317
we decided our first step should be getting the motors; we could design the robot
around them.
We decided to use cordless drill motors in our bot. My friend Larry Barello, a
FIRST competition mentor, recommended that we use Bosch or Dewalt drill motors.
After some searching, we found a Bosch 18-volt cordless drill that had a stall
torque of 430 in.-lb., and a no-load speed of 500 RPM. Some quick calculations
showed that with 8-inch diameter wheels, our robot would top out at 12 MPH,
which is pretty quick for a robot.
After spending $400 on the first two drills, we decided to get the rest of them
from a local Bosch repair facility. We now had the replacement part numbers and
all we needed was the electric motors and gearboxes. Why spend the extra money
on the case, batteries, and the drill body and chuck since we were not using them?
Step 3: Adding Wheels
Next we had to figure out how to drive the wheels. I originally wanted to use timing
belts to drive the wheels, but I decided to go with regular chains and sprockets
because they were cheaper. From the Grainger catalog, we could see that a No. 40
chain had a maximum load rating of 1,000 pounds. With a service factor of 2 for
intermittent and shock loading, this would equate to a load rating of 500 pounds.
Since this was greater than the stall torque of the motors, we decided that this
chain should work fine.
At this point, we ordered a whole mess of parts from Grainger: sprockets,
chains, spherical pillow blocks for the four axles, and four flange mount pillow
blocks for the motor mounts.
Another friend, Robert Niblock, told me about a local machine shop that
builds custom racing go-karts and suggested that they might sell me some used
parts. So Dave and I ran over to the machine shop to see what we could haggle
over. Ken Frankel showed us his high-speed, state-of-the-art racing go-karts. They
looked just like miniature Lemans or Indy racing cars. We talked for a few hours,
and he sold us some of his used aluminum wheels and a dozen used racing tires,
along with a set of four mounting hubs. The used racing tires were great because
they were already gummed up from racing, so they provided lots of extra traction.
Step 4: Adding Motor Housings and Controllers
The next step was to build the motor housings. Cordless drill motors are not designed
to be used as regular motors, so there really isn’t any good mounting
points on the motor and gearbox. I used a pair of calipers and reverse engineered
the exterior geometry of the motors and gearboxes. Figure 14-7 shows a layout of
the components used to make the mounts for the gearbox, and Figure 14-8 shows
a photograph of the assembled gearboxes. The parts were machined using an
abrasive waterjet. Yup, water and sand was used to cut all these metal parts.
When water is pressurized to 55,000 psi and a little sand is added to it, it can cut
any material known to man. One of the nice things about an abrasive waterjet is
that it can cut some rather intricate features without difficulty.