Build Your Own Combat Robot

Chapter 9: Robot Material and Construction Techniques 199
Generally, most of the screws used in experimental robot construction are
6-32, 8-32, 10-32, and 1/4-20. Here’s what these numbers mean: The 6-32 means
screw size number 6, or 0.138-inch diameter with 32 threads per inch. This is a
coarse thread for this size screw; likewise for a number 8 screw, but a fine thread is
used on a number 10 screw. In the 1/4-inch sizes, 1/4-20 is coarse, and 1/4-28 is
fine. Screws get much smaller, such as an 0-80, which is 0.060 inches in diameter
with 80 threads per inch—or even as small as 000 size, or 0.034-inch in diameter.
If you’re going through a surplus house and find a good buy on screws and
bolts, make sure you locate the proper nuts for them because, for example, a
1/4-20 nut will not fit on a 1/4-28 bolt or screw. Bolts are generally larger and
range from 1/4-20 or 28 to 1/2 inch or larger. Metric screws and bolts are becoming
increasingly popular, especially on automobiles, and are designated in millimeters
or fractions thereof; be careful not to mix the two types, though, as one will not fit
on the other.
We mentioned tensile strength earlier as the ability of the screw to withstand
stretching before breaking, but shear strength is probably the most important
quality of a machine screw in most robot mechanical applications. High shear
strength is the ability of the screw’s shank to withstand shearing action—not the
ability of the screw to be pinched in half or bent until it breaks. Hand-held
crimpers for wire terminal lugs often contain screw cutters that allow a person to
screw in a 4-40 to 10-32 screw and then shear it off to a desired length.
In a typical combat robot match, a robot can be struck repeatedly by an opponent’s
weapon(s) until its internal members literally start to shear the fastening
screws in half. Many mild steel screws purchased in small plastic packages at hardware
stores can easily fail the shear-strength test. You need to pay close attention to
the type of steel used in the screws. You will certainly pay more for 18-8 stainless
steel screws, or the even more expensive alloy steel screws; but large robot construction,
especially combat robots, requires the extra strength.
Now that you’ve got a good idea of what fastener you’re using on what parts of
your robot, take care to install them correctly. If you’re boring several holes in several
pieces of metal that use multiple fasteners to hold them together, clamp the
metal pieces together and bore the first hole through all the metal pieces. Insert
your fastener through the hole and tighten a nut on it. Do this with each new hole.
This way, the pieces of metal will have accurately matched sets of holes.
Don’t hesitate to use washers on each side of the nut/bolt or nut/screw combination
to spread the load, especially with softer metals such as aluminum and
brass. Use a lock washer, where applicable, such as a typical split washer, rather
than the lighter duty inside or outside washers. A fender washer that has a wider rim
than a standard washer is useful to bind objects together, such as a pulley attached
to the body of your bot.
In areas of your robot where vibration may be a severe problem, such as a combat
robot, the use of a lock nut is preferred. These types of nuts offer resistance to
screwing when tightening, but they also offer resistance to coming unscrewed during
vibration. Some lock nuts derive their binding resistance from being slightly