Not sure about your transposing gears, page 75 from Know Your Lathe:-

Two places to look, that would be as good as definitive, are the South Bend section of Practical Machinist forum and the South Bend literature available at vintagemachinery.org

You can locate a pdf copy of ‘Know your lathe’, which will have a clear chart in it. I think the answer above is intended to point you in the correct direction rather than hand you something on a plate.

The main forum for SB/Boxford is on Groups.io
https://groups.io/g/BoxfordLathe-UserGroup/topics

47/37 is not in the mainstream of conversion gears that I have heard of but if you have one does it look like a home made custom one of more match the others. Which other gears do you have? Most lathes have sets that go in steps of 5 but the SB/Boxfords use even numbers.

My brains are mildly frazzled by belly ache this morning, but unless I’m missing something, the Boxford table provided by Speedy and Gerard should both work simply for Paul by him substituting the 47/37 transposing gear for the conventional 127/100 pair shown on the Boxford plate, ringed in red below:

The advantage of a 127:100 pair is that is gives a perfect conversion to 25.4mm per inch.  The disadvantage is the need for two whacking big gears, and many lathes don’t have enough room on the banjo for them.  Their need to cut metric threads is often achieved, somewhat less accurately, with a 63 toothed gear, and a different table.

Paul has a 47:37 transposing gear, which is another way of approximating metric:  25.40540541mm to the inch.   Plenty good enough for cutting short threads, but think twice before making a long accurate thread.

Bottom line, what Paul needs is achieved simply by substituting his 47:37 transposing gear for the 127:100 transposing pair shown on the Boxford plate.  As both transpositions produce nearly the same ratio, the Stud, Idler and Screw gear set-ups tabulated on the left of the Boxford plate should produce the advertised pitches.

The easy way is for Paul to prove I’m right, or not, is to try it.  With a fine felt-tip pen in the tool post, write a helix on a suitably big diameter rod with the lathe, and confirm several pitches are drawn as per Boxford table by measuring them with a caliper.   If all looks OK, cut a real thread and check that.

Dave

Looks good to me, Dave

MichaelG.

As an alternative, or for conformation of the charts shown above, I suggest taking a look at the “Ride the Gear Train” program written by Evan Lewis, a very active member of the Southbend/Boxford forum. I would expect the results to be the same as Brian’s, however.

You will be able to enter which gears you have, including any compound gears, choose wwhat percentage error is acceptable (including no error, but with the 47:37 rather than 127:100 you may not be able to achieve perfection).

Evan is always happy to help if you contact him via the forum with any questions.

Rob

Links to https://ridethegeartrain.com/

Another useful program is NthreadsP which can be downloaded from the lathes.co.uk website here:

http://www.lathes.co.uk/threading/nthreadp.zip

You just load your list of change gears, tell it what pitch you want and it will find the best combination for you and calculate the error. It is a Windows program but I find it runs well on Linux using Wine.

Russell