Metric threads on a Powermatic?

The problem with the 127T gear is that on most lathes, it is much larger than the largest standard gear furnished. I haven't looked at the Logans with this in mind but on the Atlas machines made 1938 and later, the largest gear is 64T. Running a 127T may result in having to run the machine with the change gear cover open. Which is a safety no-no.

The compound gear set sold by Atlas for the purpose was 52T/44T. This results in an M1.0 being M1.0006. This isn't good enough to make precision lead or positioning screws but it's certainly close enough for normal lengths of engagement. As the thread pitch grows coarser, the absolute error increases so M7.5 nominal is actually M7.50450000. Which is where I cut off the threading chart (modified copy of the metal chart rivetted to the QCGB) that I made up for the QCGB equipped Atlas 10" and 12".

A man named Conrad Hoffman wrote a treatise on cutting metric threads on an Imperial lathe (specifically a 1947 Model 211 Logan). I have the document as a PDF but although the document isn't so marked, the home page of his web site says contents copyright 2016. So I'll merely give here the URL for the document. You can get back to the rest of his site by clicking the HOME button on the last page of the document.

www.conradhoffman.com/metricthreading.htm

The main reason that I am mentioning it here is that he describes a method whereby you can cut metric threads with an inch lead screw WITHOUT having to leave the half nuts engaged throughout the entire process.
 
I figured up the OD for the 127 tooth gear, planning to make one for my 11” Logan 957, and the OD was over 8”. I haven’t pursued it since, and rarely (never?) have to make threads that can’t be made with a tap or die.
 
I figured up the OD for the 127 tooth gear, planning to make one for my 11” Logan 957, and the OD was over 8”.

That's the dimension for a 16 diametral pitch gear (14.5 degree), so if you want
something in the 4" size, you want 32 diametral pitch. That won't mesh with
other gears from the box, so you'd need TWO 32 pitch gears (possibly pinned to
some standard gearing, but only meshing with each other).

Seems like the 'correct' gearing is not impossible, but the 'close' gear solutions
of 37/47 or 44/56 are easier to make fit? Is 44/52 really used? How?
For exact conversion, the ratio has to include the prime factor 127

100/127 = 0.78740157... (exact conversion to metric)
37/47 = 0.78723
44/56 = 0.78571
44/52 = 0.84615
and 44 /52 *(13/14) = 0.78571
44/52 * (27/29) = 0.78779
44/52 * (40/43) = 0.787119 is the closest match I see
44/52 * (53/57) = 0.78677
 
Here's how you use a 52T & 44T compound gear:

The 52/44 compound gear converts an Imperial QCGB that's made to cut tpi to a metric one for at least a useful range. Starting with the gear box set for cutting 30 tpi:

30 tpi converted to mm pitch is 25.4/30 = 0.84666667 mm (same thing as 1/30 = 0.03333333 inch pitch, and that times 25.4 is
0.8466666667 mm pitch).

0.84666667 * 52/44 = 1.0006060606 mm pitch

as I said before.

If one were to use 127/100 as the speed-up compound gear, you would end up with 1.07526666667 mm pitch insted of 1.0006060606... . The difference is in whether you use the conversion gear set to slow down the lead relative to the spindle or speed it up. Your solution slows it down. Carvel's solution speeds it up. There is usually more than one way to skin a cat (and no, I do not know why anyone would want to skin a cat). The disadvantage is that apparently a lot of Imperial GB's do not do 30 tpi nor 60 tpi (for M0.5). I have not investigated fully whether or not there is an exact conversion.
 
just something too consider..... as its been pointed out to buy these special tooth count gears aint gonna be cheap... mrpete222 's latest video he shows making gears for an atlas using a 3d plastic printer.. if someone had the ability to draw these oddball gears up then printed them they would be fairly cheap and there not going to get used often anyhow? actually if you go to makerspace and do a search there are a few logan gear drawings already done...of course not these oddball ones but maybe if you searched a bit harder it might something may show..
 

Not that old. Thanks for resurrecting it, I missed it somehow. I need to figure out how to get metric either on my Logan or my Birmingham.
I didn't get all the gears required with the Birm.
 
3D Printing should be the way to go. The types of materials available should fit the bill.
Just don't leave the gear installed for more time than is necessary to accomplish the task.
Now for the engineering. . .
 
I just noticed someone selling printed metric transposing gears for Logans on eBay, but they want a lot
for them. You could print same extras and undercut their price...
 
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