[How do I?] Do 20 TPI

Oops, Never mind
 
As others have said the chart labeling is wrong for the end gears. It has the same labeling for doing either standard or metric threading. If you look at the gears on the chart it shows that you always us either the 120/127 ratio or the 127/120 ratio. As you have it set up now the top and bottom gear are both riding on the 127 gear which just makes that gear an idler gear with no effect. I think I would reverse the bottom gear and then make some test cuts. If it is cutting metric then reverse the 120/127 gear and see what that does.
 
Hey guys,

I actually have the same issue with my lathe... When I was making my collet closer draw bar I needed to make the internal threads for the 5C collets, 1.253-20TPI, and nothing I would do could get the collet to thread in more than about 7-9mm IIRC. I thought I was just doing something wrong, so I undercut the threads that wouldn't engage to result in about 7-9mm of thread engagement and left it at that.

Bamban: The charts and gearing pictures previously posted look very similar to mine. I'll do some digging on my end to see if I can come up with anything.

Barry
 
If doing imperial threads, it will not make a difference if one where to use the 120T or 127T gear as long as long as both 40T gears are connected to the same larger transposing gear. I agree the machine gear chart appears incorrect. Convention is to use a 40T-127T-40T for imperial threads. It is possible that the wrong gear is in the #5 position, so you could check something like the B5 position 11 TPI and D5 position 44 TPI positions. If these threads are incorrect, then the wrong gear is in the #5 position, since these are uncommon threads, it would have gone unnoticed and the other gears would work out correctly. Often the same mistake gets repeated, monkey see, monkey do mentality when companies copy other manufactures designs.

One might be able to count the teeth in the gearbox #5 gear and compare it to a similar machine or your jet.

Gear Chart.jpg
 
Looks like row B is correct, and they messed up the 2x multiplies for column 3, 4, and 5 for the other four rows.
 
Thank y'all for the responses, I will do some more testing when I get back in the garage late this afternoon.


Mark,
Is that threading chart you posted out of your 1340GT? If it were, have you done any 20 TPI with according to the chart? My side gears are exactly as you mentioned, 40-127-40, and my chart is like the one you posted except for the gears shown on my chart being wrong, not matching the actual.
 
Yes, it is from the 1340GT and for imperial threads setup the same with a 40-127040 gear drive. I have done 20 TPI and the threads work out correctly, I have not done a 22 TPI. My lathe is setup currently for metric threading, when I switch it back I can check the 22 TPI thread setting.
 
It would appear that the thread chart is incorrect: actually totally messed up. You would have an Imperial lead screw, as indicated by the thread indicator. The large side gear should just function as an idler, as shown in post #26 and it could be either the 120 tooth or the 127. The other gear isn't used for Imperial threading only as a transposing gear for metric threads. In that regard, the gear placement charts are wrong.

It also appears that your gear set in wrong as listed. I would guess that the actual number of teeth on the gears in positions 1-6 are the numbers listed in row C.

Looking at your chart and your results, I would expect that column 4 is actually 5-1/2, 11, 22, 44, and 88 and column 4 is 5-3/4, 11-1/2, 23, 46, and 92. Your C5 test shows 11-1/2 threads for 12 threads on the pitch gage which would be 23 threads for 24 on the gage.

You might try position 3. The need for the threads listed in position 3 are rather obscure and that gear may have been dropped and someone didn't properly update the threading chart. If I am correct, the correct numbers for position 3 would be 5, 10, 20, 40, and 80.
 
RJSakowski said:
It would appear that the thread chart is incorrect: actually totally messed up. You would have an Imperial lead screw, as indicated by the thread indicator. The large side gear should just function as an idler, as shown in post #26 and it could be either the 120 tooth or the 127. The other gear isn't used for Imperial threading only as a transposing gear for metric threads. In that regard, the gear placement charts are wrong.

It also appears that your gear set in wrong as listed. I would guess that the actual number of teeth on the gears in positions 1-6 are the numbers listed in row C.

Looking at your chart and your results, I would expect that column 4 is actually 5-1/2, 11, 22, 44, and 88 and column 4 is 5-3/4, 11-1/2, 23, 46, and 92. Your C5 test shows 11-1/2 threads for 12 threads on the pitch gage which would be 23 threads for 24 on the gage.

You might try position 3. The need for the threads listed in position 3 are rather obscure and that gear may have been dropped and someone didn't properly update the threading chart. If I am correct, the correct numbers for position 3 would be 5, 10, 20, 40, and 80.
Click to expand...
Thank y'all for the valuable responses, I learned a lot from your input.

I got done with my VA Clinic appointments early, no follow up X-rays, life is good today. As soon as I got home, I jumped on the lathe, turned off the old threads from last night and set up to thread on C-3. Gents, I think the mystery is solved, C-3 is dead on with the 20 TPI gauge. So, under C row, the threads are 16, 18, 20, 22, 23, 24, 26, 28 with the side gears at 40, 127, 40, as pointed out.

One last question, if I were to thread 30 TPI, I would need 30T and 32T and an idler and set up on C-8, is the ratio correct, 28 to 30? Maybe I should ask what gears do I need to cut 30 TPI, which is not in the thread chart?

Looks like I need to verify some other threads that I may eventually cut. As more of you noted the threads and gears shown on the chart are not correct. Why such discrepancy on the chart is on the lathe, is anybody's guess at this point.

Again, thank y'all for taking your time to respond.

20170926_132352.jpg 20170926_132255.jpg
 
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