Gear Change Wheels for a Smart & Brown Model A lathe

[old martParticipant @oldmart]

Posted by old mart on 13/04/2021 16:41:37:

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.

As I said earlier, the leadscrew pitch has nothing to do with it.

[DGParticipant @dg43147]

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

DG

[old martParticipant @oldmart]

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

[DGParticipant @dg43147]

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

[DGParticipant @dg43147]

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

[Howard LewisParticipant @howardlewis46836]

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

[Howard LewisParticipant @howardlewis46836]

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?&nbsp or whatever gears that you have that give the 1.5 :1 step up.

Howard,

[old martParticipant @oldmart]

Posted by DG on 14/04/2021 22:18:45:

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.

 

I doubled the gear ratio and set 8tpi, which resulted in a 4tpi lead. I have also got 5,(10) and 6,(12) as well as 4,(8).You only want 5,(8), so the single 30 tooth fitted to the output will work perfectly, but to get the 8,(4) you will have to put a 60 tooth gear to fit on the input shaft eventually.

Be aware that these gear ratios are specifically for the Smart & Brown model A which runs input and output shafts at the same speed as standard, and may not work with other makes of lathe.

 

 

DG

 

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

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

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