Gear Change Wheels for a Smart & Brown Model A lathe

Noted! thank you for your help.

DG

Unless I have misunderstood, the objective is to cut a 5 tpi thread on a machine with a 6 tpi Leadscrew., and then to cut a second one, 180 degrees out of phase, to produce a double start 10 tip thread..

If so, the ratio between Mandrel and Leadscrew needs to be 1.2 :1 so that the Leadscrew rotates slower than the Mandrel, Suggesting a 50:120:60 set up, or something similar..

If the machine has a Norton box, find the settings that give a 1:1 through the box. (Which would be for 6 tpi ) and use changewheels to give a 1.2:1 reduction..

30:120:36, 40:120:48 or whatever takes your fancy, using the wheels that are available.

Howard

Yes Howard, that’s precisely what I want to achieve.

DG

Have you got wheels that will allow you to achieve a 1.2:1 ratio simply?

If not you will have look at using a compound gear as an idler, to get the 1.2:1. ratio.

Howard

Thanks, I’ve already ordered a 30T 24DP Spur Gear as advised.

DG

Keep us posted with the results. I have cut a few double starts, and I finish the first and then move the compound to do the second. In your case you would move the compound 1/10". Take care of the runout for the second cut. The first one I did was the double start toolpost spindle for the fixed four way toolpost on the model A (not the type which slots into the compound). I think it was 1/2" x 8 ACME doubled, and to get it to screw in, I had to cut both starts deeper than expected. That involved removing the job from the chuck, but as the thread is so coarse, it is easy to pick back up with reasonable accuracy. It was for a special toolpost that I made. The toolpost is for 26mm parting blades and 12mm solid carbide boring bars.

Edited By old mart on 14/04/2021 21:34:51

Will do! I have a bit of work to do before I pick this job up, but I will share my experience ( good or bad) once I’ve successfully completed it. Many thanks for you advice, it’s been most appreciated. I thought the old toolpost bolt was 1/2” X 10tpi, 2-start Acme......could be wrong!

DG

As a matter of interest, what gear configuration did you have to produce your 8 tpi 2 start thread, because that’s one of my jobs that I need to do on my lathe, is to fit a quick change tool post at some time.

DG

Presumably what Old Mart is intending to say is that the important thing is to have the correct ratio between the Leadscrew and the Mandrel.

If the Leadscrew pitch is changed, the ratio must be as well, so that the distance that the Leadscrew moves the tool relative to each revolution of the Mandrel remains the same..

Using the same ratio with a 8 tpi Leadscrew will not produce the same thread pitch with a 1.5 mm pitch one.

Howard

If you are using a 48T on the Mandrel, for a 5 tpi, you need a 40T on the Leadscrew, surely?

Unless driving a 30T on the Leadscrew with a 36T on the Mandrel with the 120T as an Idler, in both cases.

For a 8 tpi two start thread, you will need to set up for 4 tpi, and cut a second thread 180 degrees away.

This will mean driving the 6 tpi Leadscrew at 1.5 times the speed of the Mandrel, so a 72T on the mandrel would be needed to drive the 48T on the Leadscrew.

Or a set up giving the same step up ratio. ( 45 : 120: 30 ? 48 : 120 : 32 ? 36 : 120 : 24?  or whatever gears that you have that give the 1.5 :1 step up.

Howard,

Edited By old mart on 15/04/2021 17:41:42

Edited By old mart on 15/04/2021 17:45:13