The lathe can handle 4 or 5 gears at a time which is something like 264,000 combinations.
Good to know. Do you have a gut feel for when close is close enough? The example you give of 0.0005 for 24 TPI is a 1.2% error which it seems is not good enough. Should I be looking at 0.5% error? 0.25%?
It's a good question. The engagement of a 24tpi thread with a nut will rarely be more than a half inch, and 1.2% error would
mean (from center of nut 1/4 inch each direction) about 0.003" interference. That's a lot, but it doesn't make the
nut jam. For something like a micrometer (1" thread engagement and very tight fit) it wouldn't work,
but for the galvanized hardware holding a fence together, no problem.
In addition to jamming, thread mismatch means tension doesn't make even pressure over a large
area of thread engagement (i.e. you'll deform the bolt or nut). High-strength fastening won't be happy.
So, if you need the metric thread that holds a filter over a camera lens (maybe 2mm of thread engagement) the
loose fit works fine. If you need the thread that holds a microscope objective (5mm of engagement, and a very
fine thread with close tolerances) it'll be tight. For a vise, or a micrometer, it'll overstress something
somewhere.