[Mart]

In imperial the 127T gear simply acts as an idler. It's a 1:1 between the headstock gear and the input to the gearbox. It could be any number of teeth. That's why it still worked on imperial when it was the wrong way round and using the 125T gear as the idler.

For metric it needs to be geared down in a ratio of 127/125 so the drive from the headstock gets transferred to the 127T gear, which turns the 125T gear at 1:1 as it is on the same hub, and then the drive goes from the 125T gear to the gearbox. Hence going from 1:1 to 1:0.984 (which is 125/127).

It was when I converted the imperial pitches to metric that I realised that it needs to be geared down to cut the metric pitch. (20TPI = 1.27mm pitch) On my lathe the same setting but in metric should give 1.25mm pitch so I realised what the gears were supposed to be doing.

I was getting 1.29mm pitch so I realised it was gearing up instead of down by the same amount of difference.

When I looked at the gear arrangement and saw how they were arranged I realised what was up. Realising that the right hand gear was an idler was also important. It explained why the imperial threads were coming out ok. As I said the tooth count isn't relevant for a simple idler. it transfers at 1:1 between input and output.

It's nothing to do with clearances, you need clearance for the gears to run correctly.

Hope that helps.

Mart.