Lathe Change Gears - What Are The Best Combinations?

It's fairly simple to come up with other combos if you have the gears. Need 32TPI? Just cut the 16 tpi feed speed by half. I needed 27 tpi but my QC box wouldn't do it. I set up for 18 tpi and used a 40t to 60t gear combo to get 27tpi.

that's what I need is 27tpi
I'm think if I setup for 18tpi and use what you suggest but I wonder if the order of the gears matter in this setup? Probably not??
 
that's what I need is 27tpi
I'm think if I setup for 18tpi and use what you suggest but I wonder if the order of the gears matter in this setup? Probably not??
The exact order of gears is not important as long as you change the overall ratio between the spindle and the leadscrew to what yo need. If you need 27tpi and set up the lathe for 18tpi, you have to further reduce that ratio by 2/3'rds. On my lathe (G4003G Grizzly) I take a pair of 40t gears that mesh together (no ratio change) and replace the second gear (towards the leadscrew) with a 60t effectively dropping the ratio by 2/3. Any pair of gears with the 2 to 3 ratio will work. 40 to 60, 30 to 45, 50 to 75, ect.
 
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.
 
I know we are talking threads but I also wonder what the lowest / slowest feed rate for turning would be. It's got to go slow than what's on the chart, correct?
 
@SeattleKent any chance you'd be willing to write me one of those programs? Haha I've got a vintage lathe that has literally no information available (trust me I've looked) and ive been having troubles trying to figure out change gear combinations for feeding and metric threads, it's got a gear plate for inch threads but that's it. I'm new to the whole change gear thing so I could use a little help!
 
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