1. Senior Member
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Latest project was to add a quick change gear box to my Central Machine 7x10 now 7x14 lathe. I could no longer continue on in life without fully understanding how a lathe makes threads and how it could also make metric threads.

After a couple of days I had my Eureka moment. The 127 tooth gear is a direct metric to English conversion. It comes from 25.4 mm equaling 1 inch. Since there are no 25.4 tooth gears we move up 50.8, nope. Next is 76.2, nope. Another 25.4 to that is 101.6, nope. Next is 127. Aha, a whole number of teeth which is a gear that can be made.

This 127 tooth gear by itself is only a metric conversion. It does not by itself yield metric pitch threads.

In my lathe a 1:1 ratio of spindle revolution to lead screw revolution gives 16tpi. I'm using an 80t driver, 127t idler and an 80t driven gear. This gives one turn of the 16 pitch lead screw from one turn of the spindle. There are the reversing gears before these but they are 1:1 also. To get a metric pitch I keep the 80t driver and continue to drive the 127. Keyed to the 127t is a 120t, so if I drive the final 80t driven gear with the 120t and not the 127t I get a 1.5 metric pitch thread. Here is how it works.

The ratio of the 120/127 keyed gear set is 0.945 so the ratio of the spindle to lead screw changes to 1:0.945. It was 1:1 with 16 tpi. The pitch of 16 tpi is 0.0625". The 120/127 combo gives us 0.945x0.0625"=0.059". This is exactly 1.50 mm.

2. Senior Member
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Ain't math grand.

Your next revaluation will be that a quick change gear box that was designed for English threads is not ideal for metric ones. Different kinds of sequences. You will need several different manually changed gears to cover the range of metrics.
Last edited by Paul Alciatore; 04-06-2019 at 12:10 AM.

3. Senior Member
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Here is a calculator that can be used to approximate metric threads with standard change gears: https://littlemachineshop.com/refere...ange_gears.php

I regularly cut metric threads under 1" within .5% without issue

4. Senior Member
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Oct 2013
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USA MD 21030
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127/100 is an exact conversion of 1.27, but other ratios using gears with lower tooth count have been used:

https://www.hobby-machinist.com/thre...g-gears.27287/

47/37 = 1.27027... (Error of 0.022%)

14/11 = 1.2727... (Error of 0.215%)

80/63 = 1.26984 (Error of -0.0125%)

To put these in perspective, a 2" long 1/4"-20 thread (40 threads) would measure 2.004" for the 14/11 gears, and 80/63 would measure 1.99975", and the more common 47/37 would be 2.0004". Only the 14/11 ratio would result in a measurable error using ordinary tools, and using a nut, die, or even a thread gauge would be unable to detect a difference, or cause any jamming unless the gauge were longer than 1/2" or 5 threads.

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I bought a cheap 127/120 gear on ebay to allow the museum's Smart & Brown model A to produce the common metric threads. It was easy, as the normal input and output gears have the same number of teeth. The metric module 127/120 gear was not very different in diameter than the original 120 teeth idler gear. As the idler gear pivot point is designed to allow a large amount of movement to accommodate many different size gears, I could use various sizes of metric module gears in the input and output shafts for inch and metric threads. The 120/127 is either run using both sets of teeth, or one set as an idler. If a requirement for a special pitch arises, I will just buy the appropriate metric gear blank and machine it to fit.

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Oct 2014
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PStechPaul,
I haven't needed to cut a metric thread yet. I tend to work on older (antique) equipment, shingle mill, farm equipment, that sort of thing. However I know if I don't have the capability I will need it someday.Been looking at ideas for years but haven't gotten around to doing anything. I think the 47/37 is the best compromise. Almost no error, none most of us could detect or that would matter for 99.99% of jobs, and you can cut the gears with a common dividing head. No need to convert a plain dividing head to a differential dividing head as per the article in the March/April 97 issue of HSM, or play with a band saw blade on a wood disc, or some other scheme. And while the differential dividing head conversion would be fun and nice to own there is probably little need for the additional divisions it would provide for the average HSM especially for the work involved. The 47/37 gears can be indexed on the plain dividing head with one gear cutter, commercial or home made,and aren't too big in diameter.

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Originally Posted by Paul Alciatore
Ain't math grand.

Your next revaluation will be that a quick change gear box that was designed for English threads is not ideal for metric ones. Different kinds of sequences. You will need several different manually changed gears to cover the range of metrics.
Yep, have that covered. The quick change box is not a compound one so I get only 9 English thread pitches before I have to change a gear. With the current setting I can get
1.5, 1.25, and 1.0 by repositioning the 80t driven gear and making s selection on the qc box.

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Jan 2004
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Missouri
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Problem is that with distance per thread (i.e. pitch distance of threads) in decimal does not come out to the same steps as threads per inch, so many of the QC box choices are no help in metric.

9. Senior Member
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Nov 2011
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Salem, Oregon
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Forget the half-nut and thread dial, unless you have one of those dual format dials.

Easily worked around, however.

Dan L

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South Wales
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Single pointing threads is probably one ofe the easiest jobs, once you figure the thing out, and consequently one of the trickiest to learn, without help, here’s a good one, the antkithera do dah (excuse spelling it doesn’t look right) had a 127 tooth gear, fascinating to find metric (or prime at least) gears that old.
I need a 127/100 for my lathe (Harrison 12” swing) not many about though, the box will cut a reasonable range, our old John Stephenson gave me the chart for it ( wonder how he’s getting along with the long line of Bridgeport’s he gets to keep going)
Glad you figured it out, I figure it out and promptly forget till the next time hence dies get plenty of use
Mark

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