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I need to screw cut a 19 TPI thread.?

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Mike Norden27/02/2022 17:43:58
1 forum posts

My Boxford Model 'C' Lathe has an 8 TPI leadscrew and a Southbend Gearbox fitted that shows only even numbered threads? The Stud Gear is 20T and the Screw Gear on the Gearbox input shaft is 56T which is apparently correct as it cuts all other threads accurately as far as the pitch goes. I don't have a complete set of change wheels anymore so I'm hoping that someone brighter than me can tell me how to set it up for cutting 19 TPI?

Ady127/02/2022 18:12:09
5089 forum posts
736 photos

List what changewheels you DO have and someone may be able to help

Bazyle27/02/2022 18:22:59
6324 forum posts
222 photos

If you look at the changewheels provided for any non QCGB imperial lathe you always see a 38 tooth gear. ie 2x19 This is the only reason that gear is in there. Clearly the person who invented plumbing fittings didn't understand the way having a high prime in the standard would be the bane of machinists echoing down the decades.

Robin King27/02/2022 18:36:51
126 forum posts
1 photos


Evan Lewis, who frequently posts on the Boxford Lathe User group, is a very knowledgeable chap on screwcutting computations and has set up a website called '' which gives a calculator for gear trains to suit various setups, pitches etc. It might be worth having a look at that as you might find the answer there; alternatively Evan is a very helpful chap and can be contacted via the Boxford group

Martin Connelly27/02/2022 18:36:58
2137 forum posts
222 photos

Some Smart and Brown lathes use a 95 (5 x 19) tooth gear for 19TPI. In this case you would probably look at 20tpi and replace an idler with a 95/100 or a 38/40 combination if there is an idler in the gear train.

Martin C

Edited By Martin Connelly on 27/02/2022 18:39:52

Brian Wood27/02/2022 18:38:45
2566 forum posts
39 photos


You need, as others have pointed out, a 38 tooth wheel which will go on the leadscrew shaft instead of your 56T gear. The mandrel gear is 20T and with the gearbox set at 28 tpi that set up will cut a thread of 19tpi with an error of less than 0.001 tpi

The gearing set up is:- 20-----Idler------38 on leadscrew

Regards Brian

Clive Brown 127/02/2022 18:43:57
825 forum posts
41 photos

The standard Boxford changewheel set doesn't include a multiple of 19T so exactness is probably not possible.

My notes for my ME10 say I have used 36T on the stud gear and 60T on the 'box input. The 'box set to 32tpi should then give 19.05 tpi if my arithmetic is correct.


Edited By Clive Brown 1 on 27/02/2022 18:46:49

Ady127/02/2022 18:51:57
5089 forum posts
736 photos

With no 38 on an 8tpi Drummond you can get close

18.98tpi is 40mandrel 65/50 compound and 73 on the LS

you'll need to stick an idler inbetween the 50 and the 73

19.043478 0.043478 n[40] n[60] n[46] n[73]
18.980001 0.019999 n[40] n[65] n[50] n[73]
18.980001 0.019999 n[50] n[65] n[40] n[73]

DC31k28/02/2022 07:29:26
686 forum posts
2 photos
Posted by Bazyle on 27/02/2022 18:22:59:

If you look at the changewheels provided for any non QCGB imperial lathe you always see a 38 tooth gear. ie 2x19

You need to qualify that remark by adding 'for the non-US market'. Their pipe threads use 11 1/2 TPI, so you are more likely to see a 23t or 46t gear.

The presence of a 23t/46t gear or 19t/38t in an unknown set of change gears can be a good indicator of the origin of the machine.

Chris Evans 628/02/2022 08:57:45
2056 forum posts

Years ago when I had a Southbend lathe and was faced with cutting a 19 tpi thread I cut a slightly sloppy 20 tpi. Over a few pitches that I required it worked but not ideal or correct.

david sanderson 328/02/2022 12:28:53
15 forum posts
2 photos

hi Mike i cut 19 tpi on my boxford

you need 20 t stud gear

80 t idler and 38 t on the screw

then select 28tpi on the gearbox

worked ok for me

Brian Wood28/02/2022 14:22:51
2566 forum posts
39 photos


Your practical result nicely confirms my calculated figures, thank you.

There is an alternative set up Mike could try, it still needs a 38 tooth wheel.

This set up is 38-----idler------56 on gearbox input. Gearbox set to 36 tpi. The lathe will now cut 18.95 tpi, not quite as precise as the previous version but probably quite good enough in practice.

Regards Brian

Pete Rimmer28/02/2022 15:09:01
1233 forum posts
65 photos

Absolutely good enough for a pipe thread Brian (or probably any other really except a measuring screw). 19tpi and 18.95tpi are only 0.00014" difference in pitch. You would not be able to distinguish between them without decent metrology and it's probably less than the error in your leadscrew anyway.

Georgineer28/02/2022 21:21:04
577 forum posts
32 photos

I've been using this gear calculator for years and have been very pleased with it:

Although it's set up for the ML7, it can be adapted for other lathes. One of the features I like is that you can list all the changewheels you have, and it will work out the nearest combination for the thread you want.


duncan webster28/02/2022 22:35:44
3984 forum posts
65 photos

Here's another gear calculator Calum's

Brian Wood01/03/2022 09:18:18
2566 forum posts
39 photos

Just to let everyone know, Mike has sent for a 38T gear which is what he really needs.

Thank you all for your various inputs.

Regards Brian

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