BUILD REP RT:
Building a Better
Beatle Beater Bar!
Part 2 – The Axle and the Pulley
● by Pete Smith
In Part 1, I showed how to machine a simple and cheap
beater bar for a 3 lb Beetleweight
combat robot. This month, I will
show how to create the axle, drive
pulley, and flanges.
The axle used in my Weta kits is
a 5/16” ground titanium shaft. This
is much lighter than a steel shaft,
and has proved to be stiff and
strong enough to survive many
fights without failure. I buy the
stock shafting from
www.mcmaster.com; their part
number is 89055K341. This provides
a ground surface for the needle
roller bearings to run on (most
needle roller bearings — unlike ball
bearings — run directly on the shaft
surface) and is reasonably cheap for
the small size required.
The first version of Weta used
shaft collars to retain the axle, but
these proved to be very vulnerable
in combat and nearly cost us a
couple of fights when they failed to
hold the axle in position. The
solution was to thread the last 5/8”
or so of both ends of the axle and
use washers and locking nuts to
keep everything solidly in place.
Safety Note: Lathes are
dangerous if not used correctly.
Remove jewelry, any loose items
of clothing, and ALWAYS wear
I first cut the axle to length
using a cut-off tool and then
machined the external diameter
down to the correct size for the
external thread being used. I have a
set of metric threading dies, so I
chose the right size for an M8
Threading titanium is tough, so
you may want to make the diameter
a little undersized so that it’s easy to
thread. I’ve found that even a 50%
thread height still works well.
Machining a small piece of titanium
is more difficult than it is for most
materials. It’s hard but springy, and
FIGURE 1. Ti axle machined to
correct diameter for threading.
will tend to simply bend away from
the tool. It took a lot of passes to
finally get it to the desired diameter
over the length required (Figure 1).
I’ve found a good lead-in for
the die helps, so I created a sharp
point (Figure 2) on the end of the
shaft to form one. It not only looks
good, it also makes the bot less
likely to stay balanced on its side in
a fight. Repeat the above for the
other end of the shaft.
The axle is then held vertically in
a bench vise. I protected its ground
surface with a thick layer of paper
towels, but if possible get a set of
soft jaws or similar as I have
damaged shafts even with the
A hardened steel die is used to
create the external thread (Figure
3). Take your time and use plenty of
lubricant, and you can form a nice
thread (Figure 4).
A drive pulley is required to
power the beater bar. I have found
FIGURE 2. Ti axle
28 SERVO 08.2011