Page 216 - Build Your Own Combat Robot

Robot Material and Construction Techniques
Chapter 9:
place, or some liquid may have entered the crack in the weld, and the resulting re- 197
pair will be poor, at best. Unless you have a large mobile van filled with welders
and tools on site, manned by a team of mechanics, your better bet is to use some
type of removable fasteners to attach your bot together. Welds, when properly
made, are quite often the best, and sometimes the only way to attach two pieces of
metal; but home experimenters should concentrate on nuts, bolts, and screws.

Screws, Bolts, and Other Fasteners

Fasteners such as screws, bolts, and rivets have the ability to give a bit when stressed
and still retain their fastening strength. This may seem like a weakness, when, in
fact, it is a strength. Of course, the ability to easily remove a fastener to disassemble
a part of your robot for repairs or replacement is priceless in the field of battle.
A rule of thumb for bolts and machine screws is that the thickness of the mate-
rial that has the threads tapped into it must be at least four times the thickness of
the thread pitch (or the length of four threads). All the loads in a machine screw or
bolt are supported by the first four threads. The rest of the threads do not support
the loads until the fastener starts to stretch. When using screws in thin materials,
the machine screw or bolt diameter should be selected based on the thickness of
the material they are being screwed into—not just the diameter of the fastener.
Most fasteners that we commonly think of in robot construction are screws,
bolts, and rivets, with the needed nuts and washers. Many other types of fasteners
and many varieties of the above-mentioned fasteners, such as cotter pins, blind or
“pop” rivets, nails, threaded rod stock, set screws, retaining rings, and so on, are
also important. These are all important mechanical construction fasteners, but
we’ll focus on bolts and machine and self-tapping screws for our robot building.
If you look in industrial supply catalogs, you’ll see items sometimes listed as
bolts, and other times called screws. For argument’s sake, we’ll called the threaded
items that usually require a screwdriver or an Allen wrench to install screws and
the other items that generally require a wrench to install a bolts. Generally, screws
are of the smaller variety from 4 to 40 and even smaller, to about 1/4 to 20 in size.
Bolts are larger. (More about these sizes a little later.)Two types of screws are used
in robot construction that involves fastening to metal: the sheet metal or self-tap-
ping screw that looks something like a wood screw, and the machine screw that
normally uses a nut to complete the fastening. Of course, you can drill and tap a
hole in a piece of metal and insert the type of screw that normally uses a nut to fasten
pieces of metal together.
The machine screw is available in numerous configurations; some are so similar
that most people can’t tell them apart. The round-head machine screw is probably
the most common and has a partially spherical head that fits entirely on top of the
piece of metal it’s fastened to. The pan-head machine screw is a common variation
that is similar to the round head but slightly flattened. The flat-head screw re-
quires a counter-sunk hole and the round head screw head is sunk into the metal
with the top flush to the metal.