Servo - August 2011

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 safety glasses. 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 thread.

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 paper.

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).