Thread Cutting Question

[Dpaul7]

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?

 

[benmychree]

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.

 

[Dpaul7]

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?

 

[benmychree]

I am quite sure that would work.

 

[rjs44032]

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

 

[benmychree]

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

 

[GunsOfNavarone]

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

 

[benmychree]

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.

 

[Dpaul7]

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.?

 

[GunsOfNavarone]

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