Change gears for a 27 TPI thread

[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

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

[rsterne]

Great!....

Bob

[nervoustrigger]

Very nice!

Took me a minute to figure out how you cut threads that were already blued 

[sb327]

He got the blueing attachment and qctp with the lathe….lol.

Glad it worked out!

Dave

[Back_Roads]

 I googled that bluing attachment, seems it must be a pre release, as they are not going to be available till the first part of next April

[jackssmirkingrevenge]

You guys 

Unfortunately the bluing attachment seems to have stopped working, but the threading went well

[KWK]

Fill your coolant pump with Perma Blue.
The threads look very good. Don't know what you use but I get my best finish on mild steel with HSS tool bits.

[jackssmirkingrevenge]

I'm using this type of tool with a replaceable insert:



The steel is 12L14 which has been a bit of a revelation for me, I used to work mostly with brass and aluminum but these days I barely use them.

The part worked well so far, progress here

[jackssmirkingrevenge]

While my calculations worked I'm still trying to wrap my head around why the online calculators gave nonsensical results.  I calculated the required ratio relative to the existing combinations listed because trying to work from scratch it didn't seem to be congruent.

The lead screw pitch is 3mm, so to cut a thread with a 3mm thread one would assume that the gear ratio from spindle to lead screw needs to be 1:1, and that is indeed the case.

The gear combination listed for a 3mm pitch is 70, 35, 120 and 60 with the middle pair on the same shaft, so that's a 2:1 and 1:2 ratio which cancels out and gives us 1:1 between spindle and lead screw.

To cut a 1mm pitch thread, the listed gear combination is 60, 110 and 45.  That gives us a ratio of 6:11 and 22:9 which works out to 132/99 or 1.333 turns of the lead screw per one turn of the spindle.

In my head, to cut a pitch that is one third that of the lead screw, then the lead screw should be turning three times as fast, but that's not the case in practice, what am I missing?

[sb327]

A 3mm lead screw should turn .333x per spindle turn for a 1mm pitch thread.

I think….

Dave

[jackssmirkingrevenge]

A 3mm lead screw should turn .333x per spindle turn for a 1mm pitch thread.

Wouldn't that give a 9mm pitch thread?  If the lead screw is turning slower then the cutting tool is traveling less per spindle revolution.

[rsterne]

A 3mm lead screw should turn .333x per spindle turn for a 1mm pitch thread.

Correct.... Each time the spindle turns one full turn, the 3mm lead screw advances 0.333 turns....  3mm x 0.333 = 1mm....

Is it possible your confusion comes from the fact that idler gears only reverse direction, and have nothing to do with the ratio?.... I have never used a lathe with change gears, so I am not familiar with the setup.... but for example if you have a 10 tooth gear driving a 20 tooth gear through an idler, the output shaft will turn in the same direction as the input shaft, but at half the speed.... It does not matter if the idler is 10 teeth or 100 teeth, and even number or an odd number....

Bob

[jackssmirkingrevenge]

Ah I think I understand better, I was making a spurious correlation between the lead screw thread and the cut thread.

The lead screw advances 3mm per turn, so for a 1mm thread we need it to advance 1mm, so 1/3 of a turn for every turn of the spindle.

for example if you have a 10 tooth gear driving a 20 tooth gear through an idler, the output shaft will turn in the same direction as the input shaft, but at half the speed.... It does not matter if the idler is 10 teeth or 100 teeth, and even number or an odd number....

This is what the setup would look like:



"A" is on a shaft that turns at the same speed as the spindle while D drives the lead screw.

If to cut a 3mm pitch thread the ratio is 1:1 between A and D, then I would expect a 2:1 ratio for a 1.5mm pitch.  The listed gear combination for 1.5mm is 70, 80 and 35 which gives us exactly 2:1 which makes sense.

By the same logic, I would expect a 3:1 ratio for 1mm and 6:1 ratio for 0.5mm, however in practice:

1mm: 60 - 110 - 45 gears for a ratio of 1.333:1

0.5mm: 50 - 110 - 75 gears for a ratio of 0.667:1

This is what doesn't seem to make sense.

[rsterne]

You have 4 gears, but only give 3 tooth sizes, which I find confusing.... Gear B & C turn at the same speed, correct?...

Bob

[jackssmirkingrevenge]

B and C are on the same shaft, for some combinations it's a single gear.

[sb327]

In your example we are discussing you gave a combination of 60, 110 and 45.

Something is missing.

Dave

[jackssmirkingrevenge]

In that case It would be 60, 110, 110, 45