No, I've not taken up Irish dancing, but have had a bit of an issue with the traction motors. I've made a bit of an error when calculating the relative position of the gears, meaning that the teeth mesh too tightly, leading to tight spots if anything is even slightly out of position. Sadly, the 42 tooth gears that mount on the axle were slightly out, mostly due to the way that I'd held them in the chuck when drilling them out to fit the axle.

I then tried mounting them in the chuck by the boss, but can't tighten the chuck enough to stop it moving, without crushing the boss, particularly when reaming out to the final dimension. This meant that they were always ever so slightly out of round, leading to jamming.

So, thoughts turned to how I could hold the gears adequately during this operation, and how I could make it consistent across all of the gears. In the end I came up with the idea of drilling two holes in each gear, and using these to screw the gear onto a mount that would fit in the lathe chuck. To ensure that the holes were consistent across the gears, and on the mounting block, I made a drilling jig out of a piece of 0.5mm brass.

Whilst I made one of the holes slightly off centre, it didn't really matter as the position will always be relative to the other. As seen in the photo, the jig is drilled to 5.9mm in the centre, to fit over the boss of the gear. The holes are drilled to 2.2mm, clearance size for 8BA. The jig is used to drill one hole in the gear, then an 8BA screw is slipped into the hole, to maintain the jig position, whilst the other hole is drilled.

Apologies for the slightly crap photo, but this shows the mounting block. The centre was drilled out to 5.9mm, to fit the gear boss, then a drilled gear was used to mark out the position of the two mounting holes. These were drilled out to 1.8mm, then tapped 8BA. The gear to be drilled is fitted onto the mount, secured with two 8BA screws, then the whole assembly is mounted in the lathe chuck, gripped on the outside of the block.

This shows the whole thing assembled, with a gear for drilling, ready to be put in the lathe. Early indications are that it has pretty much worked, although I will have to do a small bit of bodging on a couple, by filing a small amount off the teeth. Still, it's a whole lot better than previous