Mini lathe gears and calculator for cutting UNC and UNF threads on metric lathe

thingiverse

If you need to cut UNC or UNF threads on a mini lathe that has a metric leadscrew you're gonna have a hard time. The solution you often see on the internet is adding a 127T gear to facilitate the conversion from metric to UNC or UNF, but to use a 127T gear on the mini lathe you'd have to make some modifications to it since it is a huge gear! I created a python script to find other solutions with smaller gears (up to 80T so there is no modification necessary) to approximate the UNC and UNF threads and an openscad script (based on the publicDomainGear script) to generate these necessary gears. For example, if your mini lathe has a leadscrew with 1.5mm pitch you can create a 13TPI thread using the following gears: 47T, 44T, 50T and 41T. The script can generate the absolute most optimal gear combination and then you'd have to print a lot of gears for each different thread, but once you have several weird gears added you can create your own gear set and look for the best solution using the gears you have. In most cases the error should be small enough, and otherwise you might need to add some more gears to your set. This is all explained in the comments of the script. The other way around (cutting metric with imperial leadscrew) should also be possible, but might need a slightly different script. If there seems to be an interest in this, I could check it out. I'm not sure if the gear calculation should work for any other lathes; for the mini lathe a gear reduction of 1:1 (60-40-40-60) provides the same pitch as the lead screw. If the same holds for your lathe, the calcution is valid.