Page 165 - Robot Builder's Bonanza

134 ASSEMBLY TECHNIQUES

Which type of lock washer should you use? Split washers provide the highest locking power. But
to do their job, you have to tighten the fastener enough that the split in the washer is
G compressed. That means they’re best suited for metal.
Toothed lock washers are best used with softer materials like wood, plastic, or even
aluminum (the teeth dig into the material to hold it), and for those applications when you must
use nylon fasteners or can’t apply a lot of force when tightening.

FASTENER MATERIALS

The most common metal fastener is steel plated with zinc, and these are the kind you’ll use
for almost everything. But so you know, other fastener materials may include brass (typically
used for decorative purposes), stainless steel, and nylon. Nylon fasteners are lighter than steel
fasteners, but can’t be used where high strength is required.

SHOPPING FOR FASTENERS

You can save considerable money by purchasing your fasteners in quantity. If you think you’ll
make heavy use if a certain size of fastener in your robots, invest in the bigger box and pocket
the savings. Of course, buy in bulk only when it’s warranted. As you build your robots, you’ll
discover which sizes you use the most. These should be the ones you purchase in bulk.
Robot builders gravitate toward favorite materials, and fasteners are no exception. I can’t
tell you which sizes of fasteners to buy, because your design choices may be different from
mine. But I can tell you what is used the most in my robot workshop. Perhaps that’ll give you
a starting point if you’re just now stocking up.
For small tabletop robots I try to use 4-40 screws and nuts whenever possible, because
they’re about half the weight of 6-32 screws and, of course, they’re smaller. I use 4-40 1/2″
screws and nuts to mount servos on brackets, and 4-40 3/4″ screws for mounting small
motors. Larger motors (up to about a few pounds) may be fastened using 6-32 or 8-32 hard-
ware.
I try to keep a few dozen of the following fasteners in stock at all times:

• 4-40 steel machine screws in lengths 1/2″, 3/4″, and 1″
• 6-32 steel machine screws in lengths of 1/2″ and 3/4″
• 8-32 steel machine screws in lengths 1/2″ and 1″
• #6 1/2″ and #6 3/4″ wood screws, for fastening together panels of rigid expanded
PVC
And, of course, I keep around corresponding stock of nuts, plus some flat washers and lock
washers in #4, #6, and #8 sizes. For all other sizes I buy them when needed.

TAPPING THREADS
You don’t absolutely need a nut to hold a machine screw in place. If the materials you’re
assembling are thick enough, you might be able to use self- tapping machine screws or tap the
hole with threads so that the screw will firmly anchor inside it.
Self- tapping machine screws look a lot like regular screws, except they’re pointed at the
very end. The threads are much coarser, as well. They’re best used in soft plastics and metal
(like aluminum). To use, you drill a hole just smaller than the diameter of the screw. You then
tighten the screw into the hole, making threads as it goes.

13-chapter-13.indd 134 4/21/11 11:47 AM