View Full Version : Metric threads on an imperial lathe

Yow Ling
09-03-2007, 03:37 AM
I have a Smart and Brown Model A Mk2 Lathe, its beautiful but I would like to be able to cut metric threads. I belive there is a "something or other 127 tooth change wheel involved here" but I dont really know too much more. Does anyone know where one exists for sale, failing that I will need to have one made. I think the gear has 127 teeth and another gear with less teeth. Im sure that those that know of these things will know the answer off the top of their heads. Thanks


09-03-2007, 08:23 AM
The metric thread question comes up fairly regularly, so a search of the archives ought to produce quite a bit of information.

The more or less standard way to do metric on an imperial lathe is with a 127/100 compound gear. That gives you the 1.27:1 conversion ratio you need, since 25.4mm = 1".

Those are pretty big gears though, so for convenience and perhaps to save some money, people often use close approximations, like 47/37, which works out to 1.2702 or something, close enough for nearly anything. There are other approximations -- check the archives.

Paul Alciatore
09-03-2007, 01:14 PM
If that's a picture of your lathe, it looks like it has a QC gear box and no provision for external change gears. So you have two problems to solve before buying any gears. First, where will you have access to the gear train to allow the insertion of the additional gears. Perhaps the manufacturer had planned for this.

The second problem is with the QC gear box. The series of ratios used for inch threads was based on threads per inch while the metric threads are based on mm per thread. The two ideas are reciporicals of each other and the result is that a simple series of numbers in one plan will not translate into a simple series in the other, even with an exact or approximate translation factor provided by the 127 tooth gear or another combination.

I have calculated translation tables fro the QC gearbox on SB9 lathes and only a few of the positions on the box will produce usable metric threads. These positions tend to be clustered around similar settings. So just adding a single pair of gears for the metric transition may, no WILL only yield a few possible metric threads. For a full range you will need a range of additional gears. On the SBs with QC boxes, this is accomplished by using several of the standard change gears in addition to the metric transition pair. This is possible there because the transition pair is mounted on the external bracket (banjo) and the stud and screw gears are also changeable.

Using only a transition pair on your machine will very likely yield some metric threads but for a complete range you will need to provide some additional gears. If you use one of the approximate ratio pairs like 47/37 you will need room to mount a second pair for this purpose. If you use a transition pair that uses the 127 tooth gear the transition ratio will be exact and then the second gear of this pair can be changed to provide the additional ratios. 127/100 or 127/50 are common choices for the basic transition. These can be varied by changing the 100 or 50 tooth gears to provide additional metric threads.

In any case there are many thousands of possible "metric" threads that can be cut. Many of these are of no real use (like 1.333mm). I would strongly suggest that you do the calculations and make a table of the metric threads you wil be able to cut before you buy anything.

1 / TPI = the inches per thread.

127 tooth / 100 tooth = 1.27 (basic transition gears)


1.27 X 20 = 25.4 or the number of mm per inch


(1/TPI) * 1.27 * 20 gives the metric pitch in mm with just the 127/100 transition pair.

This simplifies to

20/TPI * 1.27

The 1.27 is provided by the metric transition gears so your metric pitch is set on the QC box using


Or a 1 mm thread would be cut with a 20 TPI setting (and the 127/100 gear pair).

A 16 TPI setting would cut 1.25 mm threads, also useful.

But a 14 TPI setting would cut a 1.42857 mm thread (1 3/7 mm). Not very useful.

Alistair Hosie
09-03-2007, 01:33 PM
YOW try smart and brown in the uk they sell spares quite reasonably their address is

Bracehand ltd
6 Tithe Farm close
Langford Biggleswade
beds sg18 9ntel
01462 817039

kindest regards and good luck Alistair

Yow Ling
09-03-2007, 02:50 PM
thanks guys, Ill go with the path of least resistance and try calling Smart and Brownfirst. I just assumed they had gone bust along the way.
Cheers mike