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Metric threads on a Powermatic?

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Happycamper

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#1
Anyone cut any metric threads on a Powermatic 11” lathe?
 

Nogoingback

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#2
http://lathe.com/faq/index.html#_Toc95180297

This link from Logan provides information on cutting metric threads on Logan lathes. Assuming your Powermatic has a gear train
the same as a Logan, it should be possible. I would call Logan directly and ask for Scott Logan: he can help you out.
 

bfd

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#3
as long as you have the right gears 127 tooth or there are more combinations that will do it. I don't know what they are at this time. what you need to do on most lathes in to engage the half nut and not release it until your thread is done use forward and reverse, take a cut reverse the lathe back out the tool and run the tool bit back. take other cuts until thread is finished then you can release the halfnut. this is how you cut a metric thread with an sea leadscrew. my old lathe manual had this information. bill
 

rambin

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#4
when I bought the instruction book for my log/wards 700 series there was a page talking aobut the ability to do metric threads with a certain gear that you would have to buy im guessing...and it said you had to leave the half nut engaged to do them.. Id have to look up the page again if you wanted more details...what model do you have?
 

bfd

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#5
10" wards model which was made by same company bill
 

eeler1

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#6
Short answer; no, nobody has done it. But there’s a lot of Logan/Wards/Powermatic owners that have investigated it. I believe that Logan sells the transposing gears, but it’s a 37t/47t combo that comes close to metric threads but misses slightly in terms of precision. Actually, there’s lots of metric threads you can cut with your imperial gearbox, not exact but will work fine on short threads.
 

bfd

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#7
there are 25.4 mm per inch. the first gear that works evenly is5 times 25.4 which equals 127 tooth gear if you can find that gear in your pitch then you can cut accurate metric gears you just need to work out the ratio of headstock spindle to leadscrew rotation. I never studied this but I'm sure the machinerys handbook has how you figure it out bill
 

RandyM

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#8
http://lathe.com/faq/index.html#_Toc95180297

This link from Logan provides information on cutting metric threads on Logan lathes. Assuming your Powermatic has a gear train
the same as a Logan, it should be possible. I would call Logan directly and ask for Scott Logan: he can help you out.
Yup, all is explained in Section 4.1. How can I cut metric threads on a Logan lathe?
 

rambin

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#9
my mong wards was sold as a powr-kraft what is the diff between that and a power matic? the paper I haveon metric threading tells the exact same information as the link above so its of no help to the original poster
 

Nogoingback

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#10
Powermatic bought out Logan and produced their lathes for a period of time, but only in the larger sizes. The older machines like the
Model 200 and your Wards lathes had been dropped from the line. But, I just looked at a copy of a 1980 Powermatic/Logan catalog
and it indicates that transposing gears were available as an accessory for cutting metric threads.
 

wa5cab

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#11
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.
 

eeler1

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#12
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.
 

whitmore

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#13
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
 

wa5cab

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#14
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.
 

rambin

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#15
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..
 
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