The Lake County Astronomical Society

The Value of a Chamfer

Jack Kramer

If you have a telescope mounting with a motor drive, you may have encountered situations where the drive simply stops driving, or seems to work intermittently. I encountered such a situation with the right ascension drive of my Celestron GP (Great Polaris) mount, but the solution may also apply to other makes of drives. At first I thought the problem might be a weak battery, but that proved not to be the case.

The first troubleshooting step was to disassemble the drive mechanism to check for a loose component. Upon removing the gear and hub from the motor shaft, the problem was quickly spotted. The hub and shaft each had a score mark around the entire circumference -- a sure tipoff that the set screw wasn't holding the gear in place on the hub and the hub to the shaft.

Both set screws had been reasonably tight; to tighten them further might have resulted in stripping of the threads. This would be especially probable in the case of the nylon gear. The answer was to grind a chamfer -- a flat spot -- on both the shaft and hub. I used a small, fine-grit grinding wheel on a Dremel Moto Tool, but a fine rasp hand file would also work. It's best to be conservative in your grinding; a deep chamfer isn't necessary and would be counterproductive since the set screws would be too short to mesh well in the bores of the hub and gear. This is one of those situations where you wonder why the manufacturer didn't think of this!