Gears for cutting metric threads on inferial lathe

[mklotz]

A recent discussion on another forum centered on cutting metric threads on an inferial lathe. In the past I've done some calculations on this subject and thought I would pass them along here for the benefit of anyone who faces this problem.

Since the inch is defined as exactly 25.4 millimeters, cutting metric threads on an inferial lathe will involve introducing a ratio of 1.27 into the gear train driving the lead screw. Obviously this could be done by introducing a 127/100 tooth gear pair into the train. However, both those gears are going to be awkwardly large, tiresome to make or expensive to procure. As a result, folks seek out a solution that approximates the ratio with smaller gears. As long as the the length of thread is relatively short, a small error in approximating the ratio is quite tolerable.

One of my programs, GEARFIND available on my web page (see my sig), is set up to solve just this problem. You supply a desired ratio and a tolerable error percentage and it finds gear sets that will approximate the desired ratio within the allowed error. Up to four pairs of gear sets can be examined.

Applying the program to the problem mentioned produces the results shown below. Reading left to right on a line, we have

gear tooth count(s) => ratio resulting from this gear combination (percentage error relative to desired ratio)


Desired ratio = 1.27

Single pair of gears...

47:37 => 1.270270 (0.021281%) ;this set is often supplied with import lathes
80:63 => 1.269841 (-0.012498%)

Two ganged pairs of gears...

22:31 - 34:19 => 1.269949 (-0.004011%)
30:23 - 37:38 => 1.270023 (0.001802%)

[JoeH]

How about cutting English threads on a lathe intended for Metric? My old SouthBend 9" is set up for Metric.

[mklotz]

How about cutting English threads on a lathe intended for Metric? My old SouthBend 9" is set up for Metric.

Why would anyone lucky enough to have a metric lathe want to convert it to cut inferial threads? Sorry, just kidding.

If it has an inferial leadscrew, there's a good chance that it already has the 1.27 ratio gears in place. Removing them would then bring it back to inferial operation. Most likely it would be the 47:37 pair. Check the gears between the spindle and the leadscrew.

I'm not a lathe historian so I don't know if Southbend, a USA company, ever made fully metric, i.e. with metric leadscrew, lathes. Lots of funny stuff went on during the war so I suppose it's possible but I've never encountered one.

[JoeH]

Thanks Marv,
I'll check that out. I still live in the 3/8-16 world. But the lathe was a deal that came with a bunch of tooling and I couldn't pass it up. I've done lots of simple turning on it but haven't tackled learning to make threads partly because I don't normally need metric stuff.
Joe

[DIYSwede]

Why would anyone lucky enough to have a metric lathe want to convert it to cut inferial threads? Sorry, just kidding.

@ JoeH: -Perhaps my method for precisely this can be of help, though perhaps a bit lathe...
http://www.homemadetools.net/forum/i...ic-lathe-73187

@ Marv: -Merely for repairing stuff or toolmaking for that - for prototyping or new gear I need Imperial tooling as much as a hole in the head.

[Paul Alciatore]

The conversion gears will work both ways. You just use them in the reverse direction (swap the driver and the driven gears in the chain). 100-127 or 127-100. Same idea for 37-47 or any of the other approximate ratios. The real problems start when a quick change gear box does not have an appropriate setting to use. Then additional gears need to be changed or added to the gear train.

In general the 127 tooth gear is the driven gear (often mounted on the lead screw) and the 100 or some other round number gear is the (a) driver gear when cutting metric threads on a lathe with an English lead screw. To cut English threads on a lathe with a metric lead screw the 127 tooth gear would be a driver gear and the 100 or some other round number gear would be the driven gear.

I mention other possible gears instead of the 100 tooth because, on some lathes, numbers like 50 or 120 tooth often allow more/easier conversion for many metric threads. When I was making and selling a set of conversion gears I made separate 100 and 127 tooth gears which had provision for bolting them together. That way they could be used together as a compound gear or the 127 tooth could be used by itself with gears from the original SB change gear set. This made it possible to cut virtually any metric thread. A quick change gear box with it's severely limited number of settings is a hindrance for this.

How about cutting English threads on a lathe intended for Metric? My old SouthBend 9" is set up for Metric.

[DIYSwede]

From a discussion in the following thread:How to make a dividing plate

-Your brilliant idea finally solved my problem of making a biggie 127/ 50 tooth bike chain wheel combo,
to convert my mini lathe's metric leadscrew to inferial!

Humour (if present) could sometimes be the pathway to new ideas:

- Imperial (127 teeth) to Metric conversion gears aren't obtained neither easily nor cheaply.

A fast, cheap and small (enuff to cram into a 7 x 14" mini lathe) could be to use a timing belt.
The "T 2.5" variety in fact has a 127T (@ 4, 6 or 10 mm width) belt that could be ordered for under 4 bucks...

So - (as the well-informed HMT reader instantly replies)
- a 127 T BELT doesn't really solve the ratio problem, as it's just an intermediate!

Yup - IF (and only if) you really USE it as a BELT -
but if you instead wring it inside-out and glue it tightly on a turned center disk of ca 100 mm dia, and desired width,
it will (-Ahem - most probably...) become a 127 tooth pulley - as the T 2.5 profile has a symmetrical, trapezoidal profile.
Put it in the lathe "gearbox" together with a (more uncheap) T2.5 pulley to get desired ratio.

A 100T pulley together with this 127T kludgery will most certainly NOT fit into a smaller lathe though,
so a smaller "pinion" might be needed, and take up (or down) the lost ratio in later gear stages.

More than a thousand words (check out the red box):



from:https://docs.rs-online.com/14e8/0900766b808993b4.pdf

Just a quickie idea - ans suggestions and corrections welcome!

Cheers

Johan

What's next? - A TRUE "Pi" ratio belt of course, for making Module worms!