Experiment 32: A Little Robot Cart
274 Chapter 5
Figure 5-88 Figure 5-89. Figures 5-86 through 5-89
illustrate four steps to join two pieces of
ABS using #4 sheet-metal screws. Cut
7/64-inch holes on a line 1/8 inch from the
edge of the first piece, then mark through
the holes to the edge of the second piece.
Drill 3/32-inch holes that are precisely
centered in the edge, then screw the
pieces together.Figure 5-90. Three #4 screws driven into
the edge of ABS, using a 1/16-inch guide
hole, a 5/64-inch guide hole, and a 3/32-
inch guide hole. respectively Because the
first two guides holes were too small, the
plastic swelled around the screw (but did
not break).Framing Your Cart
For reasons that will soon be apparent, I’ve chosen an unusual diamond-
shaped configuration of wheels. In the rendering shown in Figure 5-91, the
front wheel (at the far end of the cart) applies power, the rear wheel (at the
near end of the cart) steers the cart when backing up, and the side wheels
prevent it from falling over.
Depending on the type of motor that you buy, you’ll have to improvise a way
to mount it in the front section of the cart. Don’t be afraid to use kludges such
as cable ties, duct tape, or even rubber bands to attach the motor to the frame.
We’re making a rough prototype, here, not a thing of beauty (although if you
decide you like the cart, you can always rebuild it beautifully later).
The plan in Figure 5-92 shows the pieces that you will need. Part A is the body
of the cart. If you’re going to bend it from ABS, you should drill half-inch holes,
with a forstner bit, at the four inside corners, so that these corners have round-
ed edges. If you simply saw the plastic to make sharp 90° corners, the plastic
may develop fissures at the corners when you bend it. If you don’t have a plas-
tic bender and don’t feel inclined to buy one, you can make Part A from three
separate rectangles and then screw them together.Figure 5-91. If you have 3D rendering
software, it can be a great way to test the
feasibility of a construction project before
you start cutting materials and trying to
fit pieces together. This rendering was a
proof-of-concept for the Little Robot Cart.