Build Your Own Combat Robot

198 Build Your Own Combat Robot
The oval-head screw is a combination of the flat head, in that it is counter-sunk,
and a pan head that is not flush. These screws usually are of the most common
slotted-head or Phillips variety, with many available with hexagonal sockets for
Allen wrenches. Many other types of screws can be used for security and other
purposes, which we won’t cover here.
Unless you have access to aerospace-quality fasteners, when you need to select
machine screws for robot construction purposes, your best sources are your local
larger hardware store or maybe a surplus store. Quite often, you will find that
round-head screws are not of the highest quality. Their steel may be of lower quality
and the screws tend to break easily. They are also not the best fasteners for attaching
the robots “skin” to the internal structure, as they protrude outside the skin
and can be struck by a swinging weapon.
Flat-head machine screws that can be countersunk into a robot’s protective
skin usually prove to be the best. They are made of a higher quality steel, usually
18-8 stainless steel or other steel alloys, and the better varieties are of the Phillips
type. Drill the center hole and then counter-bore the hole to accept the recessed
head of the screw. Drilling to the correct depth takes a bit of practice, and the use
of a drill press is recommended because most have adjustable stops to keep the operator
from making the hole too deep.
The countersink usually used for flat-head screws is 82 degrees, and you can
buy drill/countersink combinations at larger tool supply places and from mail-order
catalogs. Most experimenters find that a three- or four-flute countersink with
a half-inch diameter works well with aluminum. One bad feature with using
flat-head screws with countersunk holes is the chance of going a bit too deep and
ruining that location for fastening. Another bad feature is that countersunk
flat-head machine screws provide the least “holding power” due to the weak rim
of the countersunk hole. Nevertheless, when properly machined, these screws
seem to be the best for external robot skin applications.
Most cap screws are also one of the strongest types of screw. They are about the
same strength as “grade 8” hardware. Flat-head cap screws rather than flat-head
machine screws may be used when the protruding screw head is not an issue. The
hexagonal drive type for cap screws is the most common variety because an Allen
wrench can use a lot of torque for tightening. You won’t find a wide variety of cap
screws in a small hardware store, but larger suppliers will have a good selection
for your project.
The pan-head machine screw seems to be the best for internal structural assembly.
Most of the better varieties are made of 18-8 stainless steel and are of the Phillips
type. This screw has excellent holding power due to the large head and larger flat
area touching the metal. The pan-head machine screw, as well as the round-head,
can use a washer to increase the holding area and, therefore, the tensile strength
(the ability of the screw to prevent itself from being stretched apart or being pulled
out of the hole).
All of the screw types mentioned here have either threads that are along the
whole length of the shank or partially near the end. Either type will normally work
fine for most robot applications.