Page 214 - Build Your Own Combat Robot

Welding, Joining, and Fastening Chapter 9: Robot Material and Construction Techniques 195
We’re not about to tell you all there is to know about fasteners in these few pages
or give you a course in Fasteners 101. The McMaster Carr industrial supply cata-
log has more than 250 pages of fasteners for sale. We cannot even tell you which
particular fastener is best for your particular robot project because so many vari-
eties of robot designs are built for so many purposes. We will attempt to list and
describe those fasteners that have proven useful in robot projects we’ve been in-
volved with or that have had positive feedback.

Structural Design for Fastener Placement
Before even laying out the design and figuring out where you need fasteners, you
need to have an idea of the load paths that are present in the robot’s normal opera-
tions, as we discussed earlier for structural members. You determine a load path
by examining every possible location where a load may be placed, and then determine
just what pieces of structure might transfer that load.
As your robot sits on a workbench or shop floor, it must bear very little weight;
but once a robot begins to operate in and out of the arena, stresses build up, especially
in a combat robot. You don’t need complex finite element analysis or fail-
ure-mode analysis software to determine load paths and stress analysis. You can
imagine that the robot was made of sticks and cardboard and held together with
thumb tacks and consider this: “What would happen if I pressed here or struck it
here?” You might want to construct a model made of balsa wood and cardboard
to determine where you might want to place welded fillets or support brackets.
Some of the failures of a combat robot occur as a result of a failed structural
design. The robot’s skin is peeled off because the designer did not contemplate all
of the potential stress areas. A weld breaks, a screw is sheared in half, or a weapon
comes loose and flies across the arena only to have the robot disabled due to an unbal-
anced condition. A designer sees his robot flattened by a weapon because an internal
member was fastened with cheap pop rivets, and $2000 worth of electronics is
fried in the resulting short.
Once you’ve got your robot’s design all worked out, you can start to think
about the best ways to assemble it. If you’re building a combat robot, words like
strong, tough, resilient, and similar phrases come to mind. Your creation will
leave your workshop and enter an unfriendly battlefield where every opponent is
trying to smash it to bits, not to mention the actual arena itself with its many hazards.
Your machine has to stand up to a lot of abuse.
If you look at heavy off-road equipment, you see that its sturdiness comes not
from fasteners, but from heavy steel construction. Large machines weigh many
tons, far above even the heaviest robot. Heavy steel forgings and castings are welded
together or connected by huge bolts and pins. Battle robots contain heavy batteries,
weapons, and motors and have a minimal amount of mass left to apply to structural
needs. Careful design using strong but light fastening methods is important.