Experiment 32: A Little Robot Cart
272 Chapter 5
Making Plans
I like to use drawing software to create plans, and I try to print them at actual
scale. I tape them to the smooth side of a piece of white or natural-color ABS,
then use an awl to prick through the plan into the soft surface beneath. I re-
move the paper and connect the awl marks by drawing onto the plastic using
a pencil or a fine-point water-soluble pen. Its lines can be wiped away later
with a damp cloth. Don’t use a permanent marker, as the solvents that you will
need to clean it may dissolve the plastic.
Because ABS will tend to open a fissure when you bend it at any inside corner
where you don’t have a smooth radius, you need to drill holes at these loca-
tions, as shown in the cart plans in Figure 5-92 on page 275.
A regular half-inch drill bit is too aggressive; it will tend to jam itself into the
plastic within one turn of the drill. Use Forstner bits (shown in Figures 5-83 and
5-84) to cut nice smooth circles.
Note that the heat from bending will tend to make any marks on the plastic
permanent.Bending It
A big advantage of plastic over wood is that you can make complex shapes by
bending them, instead of cutting separate pieces and joining them with nails,
screws, or glue. Unfortunately, bending does require an appropriate bender: an
electric heating element mounted in a long, thin metal enclosure that you place
on your workbench. The bender I use is made by FTM, a company that offers all
kinds of neat gadgets for working with plastic. Their cheapest bender, shown in
Figure 5-85, is just over $200 with a 2-foot element. You can get a 4-foot model
for about $50 more. Check them out at http://thefabricatorssource.com.Avoid Burns While Bending
A plastic bender will inflict serious burns if you happen to rest your hand on it ac-
cidentally, and because it has no warning light, you can easily forget that you have
left it plugged in. Use gloves!To bend plastic, lay it over the hot element of a plastic bender for a brief time
(25 to 30 seconds for 1/8-inch ABS, 40 to 45 seconds for 3/16-inch, and up to a
minute for 1/4-inch). If you overheat the plastic, you’ll smell it, and when you
turn it over you’ll find it looks like brown melted cheese. Naturally you should
learn to intervene before the plastic reaches that point.
ABS is ready to bend when it yields to gentle pressure. Take it off the bender
and bend it away from the side that you heated. If you bend it toward the hot
side, the softened plastic will bunch up inside the bend, which doesn’t look
nice.Figure 5-83. A Forstner drill bit makes
clean, precise holes; a large regular drill bit
will chew up ABS plastic and make a mess.
Figure 5-84. By drilling holes at any
location where two bends intersect, you
reduce the risk of the plastic fissuring.
Figure 5-85. Making clean, precise bends
in ABS is simply a matter of resting the
plastic over a bender that consists of an
electric heating element.