CHAPTER 24 ■ BODY BUILDING
Place the motor shaft against the inner edge of the side lip to get the motor shaft as low to the ground
as possible, without damaging the container lip or obstructing the lid. This is important as it determines the
minimum diameter of wheels that you can install. If the motor shaft were higher in the body, larger wheels
would be required, causing the robot to move faster (possibly too fast).
The location of the motors must provide enough space in the front for the line-following circuit board
and enough space in the rear for switches and a 9 V battery.
- After attaching the stickers, manually position the motors, breadboard, and
9 V battery in the container. This is to make sure everything fits with the location
you’ve chosen for the motor holes.
Making Pilot Holes and Drilling Screw Holes
- Insert and remove a pushpin into the center of each circle on the sticker (again,
see Figure 24-7). This creates tiny pilot holes that guide the drill into the centers.
■ Note The plastic container is slippery. Without a pilot hole, the drill slides around, marring the surface and
causing the hole to be drilled in an inaccurate location.
- Remove the circuit board, motors, pushpin, and any other parts from the
container. Leave the stickers in place. - Insert a 2.6 mm (or^7 / 64 -inch) drill bit into a variable-speed rotary tool.
- Using the pilot holes and stickers as guides, drill the screw holes and the center
of the motor shaft circle. Even though the motor shaft circle will eventually be
made larger, it helps to start small.
The stickers may come off at some point during drilling, but that isn’t a problem since you have already
indicated the centers of the holes with the pushpins.
The plastic bends easily, making it difficult to drill. It also tends to grab and twist rather than drill out
cleanly. If necessary, place a piece of scrap wood behind the container wall to firm up the location for drilling.
- Drill the holes for the circuit board that you marked way back in step 2.
I use #4-40 screws to hold the circuit board, so at step 12, I swap down to a^1 / 8 -inch drill bit.
You can roughly determine the desired drill-bit size for the screws you’ve chosen by holding different
bits against the screws and comparing diameters by eye. If you want the screws to twist in and hold firmly
against the plastic, use a slightly smaller drill bit (my preference). If you want the screws to drop straight in
for easier insertion, use a bit the same diameter as the screw plus threads.
Introducing the Grinding Stone Accessory
A tiny drill bit rotating at high speed produces acceptable holes on plastic. However, larger bits walk all over
the piece and become entangled in the hole. Fortunately, there’s an easier way.