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Gear Change Wheels for a Smart & Brown Model A lathe

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DG11/04/2021 10:45:36
15 forum posts


I have a Smart and Brown Model A lathe, and I need to cut a 1/2” 10 tpi, 2-start Acme thread. With the current standards gear train I have, it will not allow me to cut a 5 tpi thread, so I need to do some calcs and fit different change gears to give me 5 tpi. The lathe leadscrew is 6 tpi.

If anyone knows a supplier of gear change wheels for S&B lathes, I would be very grateful. I dealt with a Company called ‘Bracehand’ some years ago, but I don’t think they are still trading.

kind regards


Pete Rimmer11/04/2021 13:39:05
979 forum posts
57 photos

Hi David,

What tooth counts do you have on your end gears? The one on the stud and the one on the gearbox. The large idler is of no consequence.


old mart11/04/2021 15:09:58
3067 forum posts
194 photos

I have cut 6 and 5 tpi threads on the Model A by doubling the leadscrew speed. With the speed doubled, for 5 tpi, the settings are for 10 tpi. Normally the input and output shafts rotate at the same speed, I believe the gears are 48 teeth each. I made some major modifications when I got hold of a metric MOD1 gear with 125/127 teeth. All of the gears were changed for metric ones so the complete original imperial pitches were still available, plus some coarser pitches, and all the common metric pitches as well.

You will need to get a couple of gears with, say, 30 and 60 teeth to use to double the speed. The Model A uses 24DP, 20PA, 3/8 TO 1/2" wide, with a 3/4 bore and 1/8" keyway. Some machining will be needed. I was concerned with the link pin in the input shaft breaking with the additional stress of the coarser pitch and substituted a solid pin temporarily and was extremely carefull not to lose concentration and have a pile up.

DG11/04/2021 16:52:39
15 forum posts

Many thanks ‘old mart’ for that information, that’s most appreciated. I’ll set off in search of those gears and then look at proceeding with the next stages of you guidance. Once again many thanks, that’s helped me out a lot!

kind regards


Clive Foster11/04/2021 17:02:01
2699 forum posts
96 photos


Don't know about the Model A but when wanting to complete the metric conversion change gear set for my 1024 VSLI was able to get suitable gears from HPC. The flat, PG, style were almost identical to the factory gears. Merely a few thou thinner.

Obviously I had to bore out the supplied centre hole to mach the lathe studs and cut a keyway.

Not cheap but overall of acceptable cost.


DG11/04/2021 17:10:22
15 forum posts

Hi Clive

Many thanks for that. I’ve just checked on the HPC website and see they are based in Chesterfield which is only about 45 mins away from me, so I can pop over and have a chat with them.

Kind regards


old mart11/04/2021 17:16:20
3067 forum posts
194 photos

The teeth count is not vital, 32/64, or 34/68, or anything that will fit. You can check the DP of your gears with one of the online calculators, the diameter of the 120 teeth idler is all you need.

Edited By old mart on 11/04/2021 17:18:15

DG11/04/2021 17:25:21
15 forum posts

Excellent...once again many thanks for your guidance!

Chris Crew11/04/2021 20:54:49
55 forum posts

Probably not so important when just cutting 5TPI with a 6TPI lead-screw, but the advice when cutting threads coarser than the lead-screw is to drive the head-stock from the lead-screw, rather than driving the lead-screw from the head-stock, although I would take a few more lighter cuts just in case.

I do this when cutting multi-start worms or even a worm of a larger DP to relieve the stress on the change-wheels. I am not mathematically or technically competent enough to calculate the actual stress on the gear teeth or at which point they would fail, but I take the advice given by Cleeve and Law et. al. for doing this.

It is no great pain to wind the lead-screw by the hand-wheel when cutting relatively short threads or worms but it could be if the thread is longer. I have jury-rigged up a reversible motor, reduction box and sprocket drive to the lead-screw of an ML7-R because you obviously cannot dis-engage the clasp nuts when cutting pitches that do not contain factors of the lead-screw thread.

DG11/04/2021 22:10:49
15 forum posts

Many thanks for that piece of sound advice Chris, duly noted!

kind regards


old mart12/04/2021 19:04:24
3067 forum posts
194 photos

Thats an interesting thought, Chris, the model A could easily have a QD handle at the end of the leadscrew, like some Myfords. Even easier for me, as I bought a spare Norton box together with leadscrew and feedscrew, plus the end bracket, cheap, no one else was interested.

Edited By old mart on 12/04/2021 19:07:12

DG13/04/2021 09:32:42
15 forum posts

Hi Guys

just to go over the gear train configuration for a 5tpi thread form, and looking at the rules for change gears again:

This on L.S 6 36 ( Mandrel wheel)

—————- = — X 6 = —

Thds to be cut 5 30 ( Leadscrew wheel)

Would also work ok?

DG13/04/2021 13:33:44
15 forum posts

It looks like my gear formula didn't quite come out, apologies for that, but basically I was just trying to ascertain that the correct mandrel and leadscrew wheels required to cut a 5 tpi thread, with a Leadscrew of 6 tpi works, by applying the formula, which would give a mandrel gear to be 36T and the leadscrew gear would be 30 tpi. I have had a quotation from HPC Gears for both these gears, and the ones previously advised in the previous forum ‘thread’, only I just need to be 100% sure before I commit to the order, that I choose the right combination for the job in hand. Sorry to be a pain!

many thanks


Bazyle13/04/2021 13:50:04
5896 forum posts
218 photos

You never answered Pete's initial question of what gears do you already have. I would add to that 'what is the standard gear configuration on a Model A' ? You could probably use some other settings of the qcgb and maybe only have to buy one gear.

DG13/04/2021 14:01:26
15 forum posts

Apologies, I forgot to go through the ‘Forum’,....I emailed directly to Pete with the following info:

(Top) mandrel gear. = 48T

(Middle) idler gear. = 120T

(Bottom) leadscrew gear. = 48T

Thanks Bazyle


old mart13/04/2021 14:08:55
3067 forum posts
194 photos

Don't worry about the 6tpi leadscrew, it will only make the calculations more difficult. The standard gears are 48-120-48 so adjusting the gearing to double the output speed will simply double the original pitches from 8-72, to 4-36. I'm sure you would have looked in the righthand door, that is where the change gears would have been originally stored, although the gears were an optional extra.

DG13/04/2021 14:29:34
15 forum posts

With the standard arrangement that I already have, would just buying an additional 40T gear wheel, for the leadscrew drive, in conjunction with the 48T mandrel gear that I already have, give me a 5tpi?


old mart13/04/2021 16:41:37
3067 forum posts
194 photos

If you want to set 8tpi and get a 5tpi cut, the leadscrew speed must be increased by a factor of 1.6. If you buy a 30 tooth gear to use as secondary, that will work with the primary 48 tooth to give the correct ratio. 48-120-30. This would be the cheapest way to go.

DG13/04/2021 16:45:24
15 forum posts

Many thanks ‘old mart’, I’ll go ahead and

place my order the 30T wheel!


old mart13/04/2021 18:45:22
3067 forum posts
194 photos

Don't forget, always make the first cut very light, so you can double check the pitch is right. I made that mistake with the model A once when I set a thread pitch in the wrong position and had to puzzle out why the nut would not screw on.

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