Make Electronics

What Next? 273

Experiment 32: A Little Robot Cart

You can work with it for about half a minute, and when you have it the way
you want it, spray or sponge water onto it to make it set quickly. Alternatively,
if you need more time, you can reheat it. The amount of force necessary to
bend the sheet increases in proportion with the length of the bend, so a long
bend can be difficult, and I usually insert it into a loose vise, push it a bit, move
it along to the next spot, and push it again.

Because plastic bending is very similar to making shapes in origami, it’s a good
idea to model your projects in paper before you commit yourself to ABS.

If you decide that you don’t want to spend money on a bender, don’t aban-
don plastic just yet—you can use screws to assemble separate sections with
greater ease and convenience than if you were working with wood.

Making 90-Degree Joints

Driving screws into the edge of a piece of plywood will almost always separate
its layers, but ABS has no layers (or grain, either), and never splits or shatters.
This means that you can easily join two pieces at 90° using small screws (#4
size, 5/8-inch long).

Figures 5-86 through 5-90 show the procedure for joining 1/8-inch (or thicker)
ABS to 1/4-inch ABS, which I regard as the minimum thickness when you’re
inserting screws into its edge:

1. Mark a guideline on the thinner piece of plastic, 1/8 inch from its edge.
   For #4 screws, drill holes using a 7/64-inch bit. If you’re using flat-headed
   screws, countersink the holes very gently.

Figure 5-86

2. Hold or clamp the pieces in place and poke a pen or pencil through the
   holes to mark the edge of the 1/4-inch plastic beneath.


3. Remove the thin plastic, clamp the 1/4-inch plastic in a vise, and drill
   guide holes for the screws at each mark, centered within the thickness of
   the plastic. Because ABS does not compress like wood, the holes must be
   larger than you may expect; otherwise, the plastic will swell around the
   screw. A 3/32-inch bit is just right for a #4 screw.


4. Assemble the parts. Be careful not to overtighten the screws; it’s easy to
   strip the threads that they cut in the soft plastic. Figure 5-87