PARTS IS PARTS:
M unting Weapons
Blades and Pulleys
● by Pete Smith
Alot of builders dream of creating a bot with an active
weapon but are put off by the
apparent complexity. How do you
attach a blade to the axle? How
do you know if the axle will be
strong enough? How do you drive
the axle, and finally, what kind of
bearings should you use?
I faced all these questions
when I developed my first Blade
Spinner Surgical Strike (Figure 1).
The design needed to be simple so
that it would be something I could
build and repair with my limited
skills and budget. I also wanted to
design something that could be
the basis for a kit for more
advanced builders.
I lighten the top half of the
shafts by drilling out the first
couple of inches with a 0.3” drill in
my lathe (Figure 4). The soft core
drills quite easily and the pulley
end of the shaft does not have to
be as strong as that required to
attach the blade.
Figure 2.
Figure 4.
ground, case hardened, steel shaft
from
www.mcmaster.com. These
shafts have the very hard surface
and smooth finish required for a
bearing surface combined with a
softer, tougher core to resist
fracture. I made the mistake of
using 0.5” fully hardened shafts
on the first version of my robot
but moved to 0.625” case
hardened (McMaster part#
6061K111) after a couple of
them failed during fights.
I grind the bottom end of the
shaft into a dome shape (Figure 3)
so that the spinning of the shaft
has less effect on directional
stability. A flat end on the shaft
will tend to pull the bot to one
side or the other. The dome also
helps the axle ride over any
irregularities in the floor.
Figure 5.
Figure 3.
Figure 1.
The solution I came up with
uses mostly off-the-shelf parts to
make a tough, reliable, and very
successful weapon system.
The main component is the
axle (Figure 2). This is a precision
One of best and certainly the
easiest ways to attach the blade to
the axle is to use a keyless bushing.
There are two main types:
Trantorques (Figure 5; McMaster
part# 5926K18) manufactured
by Fenner Drives at www.fenner
drives.com and Heavy Duty
versions (Figure 6; McMaster
part# 1058K13), also made by
Fenner and some other suppliers.
SERVO 08.2009 25
