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Change gears for a 27 TPI thread

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jackssmirkingrevenge:
I need to cut a 27 TPI thread on my CX708 lathe and that particular thread isn't listed in the possible combinations.

The lead screw pitch is 3mm and available change gears are as follows: 30,35,40,45,45,50,55,60,60,65,70,75,80,110,120,127

Examples of combinations listed:

20 TPI
A: 40
B: 60
C: 127
D: 50

26 TPI
A: 40
B: 60
C: 127
D: 65

28 TPI
A: 40
B: 60
C: 127
D: 70

I have tried using online calculators like this one but the results I get are nonsensical, I can't even replicate the known listed gear combinations, perhaps there's some parameter that is wrong but I've measured the lead screw pitch and it is 3mm as specified and that seems to be the only variable along with the available gears

Trying to figure it out manually I got some reasonably close combinations:

65, 120, 127, 55 gives 26.923 TPI relative to the listed combinations, while 60, 110, 127, 55 gives 27.112 TPI

The former seems like it would be good enough but I was wondering if I could do better.

rsterne:
I don't know what your thread engagement (or load) is, but let's say it is 6 threads....

6 / 27 = 0.22222"

6 / 26.923 = 0.22286"

The difference over 6 threads is only 0.00064", or about 2/3rds of a thou, which is less than 0.003 threads total error.... Just sayin'....

Optionally, could you cut the thread slightly (a thou) oversize and chase it with a die?....

Bob

jackssmirkingrevenge:
Thank you Bob for your input.

This for an old Plainsman and you make a very good point regarding tolerances, I was able to screw a Crosman CO2 cap easily quite a few threads in and that's 28 TPI, so with 26.923 I should probably be fine ;D

Now the question is whether I can physically install that combination of gears on the lathe.

jackssmirkingrevenge:
It was tight but I managed to get the gears in:



Quick test piece looks good :D

rsterne:
Great!....

Bob

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