# Thread Cutting Question



## Dpaul7 (Sep 5, 2020)

*Thread Cutting Question
Problem:* I need to cut 46 TPI. However, my PM-1340GT will cut 44 TPI and 48 TPI but not 46 TPI.

I am thinking that it is possible to substitute one of the change gears for another with an appropriate tooth count which would transpose the 44TPI gear box setting to cut a 46TPI
I would like to now if anyone knows what the tooth count would be? I am thinking that either the upper or lower gear above the large 120 tooth gear could be substituted by a gear with a different tooth count that would transpose the 44TPI setting of the gear box to cut a 46TPI thread.

I had a problem like this once when I had a South Bend Heavy 10. I needed to cut metric threads. I made a delrin substitute gear and placed it in a similar position as described above which functionally transposed my gear box settings to cut a metric thread I needed at the time.
Making a gear for me is no problem but I don’t know how to calculate the tooth count to yield the result I want. Any gearing wizards out there?


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## benmychree (Sep 5, 2020)

The gear would likely fit on the input to the quick change box, and would be a factor of 46 teeth.  if the regular gear was 48 teeth, you would set for 48  TPI and replace the 48 t gear with a 46 tooth gear.


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## Dpaul7 (Sep 5, 2020)

Thanks for the reply. The input gear to the quick change box on my PM1340GT is 40 teeth. The image below is taken from the manual. Would I still use a 46 tooth gear in the position you mentioned? And perhaps "set" for 40 TPI?


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## benmychree (Sep 5, 2020)

I am quite sure that would work.


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## rjs44032 (Sep 5, 2020)

Dpaul7,

I'm not convinced that you need a 46 tooth gear. You need to change the ratio appropriately. Please let us know what all the tooth counts are in the picture. And whether both smaller gears are meshing with the same idler gear.

Edit: If both the smaller gears have identical tooth counts and both are meshing with the same idler gear, then a 46T will work if you set the gearbox to 40 TPI.

Best regards,
Bob


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## benmychree (Sep 5, 2020)

You also could use a 23 tooth gear and set for 80.


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## GunsOfNavarone (Sep 5, 2020)

Damn this is some fascinating reading! Subscribed to watch this unfold and the appropriate answer! Please keep us updated


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## benmychree (Sep 5, 2020)

These things are typical of what can be gleaned from old magazines, such as American Machinist, where I found this technique and a table of change gears for metric thread cutting on inch based lathes.


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## Dpaul7 (Sep 5, 2020)

Regarding the picture both small gears are identical 40 tooth gears. One on top and the other is the gear box input gear. They both mesh directly with the 127 tooth idler gear, one on top the other on the bottom.
By the way, what is the formula or math to determine using 23 tooth gear and set for 80 or 46 tooth and set for 40 etc.?


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## GunsOfNavarone (Sep 5, 2020)

I hope this thread continues from here, but here is an online calculator.
Change Gears Calculator


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## benmychree (Sep 5, 2020)

There is really no math involved, other than dividing the 46T gear by 2, which is 23T, then doubling the set figure by 2 to come up with the same number of TPI.  So far as the basic principle, if you set the QC for the number of teeth on the QC box gear and replace it with a gear the number of threads desired, Bob's your uncle.  Setting for other TPIs can multiply the number of possibilities of threads to be cut.


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## mksj (Sep 5, 2020)

I ran these through my gear chart calculator for the 1340GT. The only combination I could come up with would be a 46T input gear to the 120T, then mesh the same gear to a 32T and a gear selector setting of D1. The 32T is available from QMT if I recall was used to extend some of the metric ratios. The 46T you could have printed, there are a number of vendors that do that and if I recall the HM member [X]Outlaw has made some custom gears for the 1340GT.

Added Chart, this also would give an 11.5, 23 and the 46 TPI.


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## GunsOfNavarone (Sep 5, 2020)

If you end up needing a 3-d printed gear (if I'm following correctly) let me know, I'd be happy to see if I could help.


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## rjs44032 (Sep 5, 2020)

DPaul7,

Sorry I had to finish mowing. The logic is thus: the ratio from the top (40T drive gear) to the bottom (40T driven gear) is 1:1. And you've confirmed that the large gear is an idler. So it is not a factor. Therefore all that is required is to find a ratio that works with an available Machine TPI setting that gives the desired result.

Formula used is (driven gear / drive gear) * Machine TPI = Result TPI

Example (normal setup) (40/40) * 48 = 48

This case: (46/40) * 40 = 46

or (23/40) * 80 = 46

Hope this helps.

Best Regards,
Bob


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## benmychree (Sep 5, 2020)

In what I read and have followed, the change gear was always used on the QC box input, not the stud gear, and it has always worked for me.


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## NortonDommi (Sep 5, 2020)

I use this program which gives me a lot of options.  Put all variables into 'Generic Lathe' and save then find the .ini file and change thee name to the specific lathe.  Only work is working out the gearbox ratios.


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## benmychree (Sep 5, 2020)

The method that I use does not need any of that stuff, and nothing of gearbox ratios, it's like falling off a log and would seem to work with most any lathe.  For some reason, when I click on your attachment, nothing happens.


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## mksj (Sep 5, 2020)

File LGV1 needs to be unzipped and then the .exe run to install the program on your computer. On the 1340GT both the input (stud gear) and output (QC input) needs to be changed for specific metric threads. This subject came up in the past specific with the 1340GT for some of the less common metric threads, and for that matter some of the odd ball imperial threads. There are different ways to determine the potential combinations or one can look at ratios for an existing thread and then work backwards on some divisible integer gear combination. The problem in some cases is the ratios work but the actual gear size or spacing/meshing does not.

Below is the metric gear chart for the 1340GT, a few of the change gears are special order from QMT like the 35T. David Best put together a metric thread setup chart with the default change gears and a few of the gaps for metric, these are filled with the 35T, other gears may also be available special order. I had been asked about the 11.5 pitch in the past, so it seems that it is possible with a 46T stud gear. Manual lathes are typically not designed for every possible thread, and mechanical threading gearboxes are always some compromise for universal threading. An ELS would solve that issue, I am actually surprised that lathe makers have not embraced that option, it would be less expensive and more durable then mechanical gearboxes. The higher end lathes tend to have universal gearboxes, so no change gears on say the ERL/RML/TRL lathes for imperial or metric, but none will not cut a 46 TPI.


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## benmychree (Sep 5, 2020)

I do not have a 1340, but was curious as to what was being presented; my lathe is an older Leblond Regal, and the info I found for lathes with 4 TPI leadscrew does most any metric thread, that combined with the special thread instructions for imperial threads covers anything that I'd ever run into, including DP leads and much other esoteric threads.  I have made a complete set of change gears to cut most anything in the way of threads.


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## NortonDommi (Sep 8, 2020)

benmychree said:


> The method that I use does not need any of that stuff


I'm just lazy that is all.  I actually find it useful to see all of the different combinations and also to work to a percentage of accuracy when 'close enough' will do.
  I used the program to work out what the minimum number of gears I needed to make was to enable me to cut all of the common Imperial and metric threads plus a couple of odd-balls.


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