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WM180/DB7 mini lathe to cut 32TPI

Setting up for 32tpi

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Lainchy14/02/2021 17:01:51
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Hi Forum

I have a Chester DB7 and mid Juliet II build. I'm looking at single point cutting the gland nuts... 32tpi. Only issue is, the lathe doesn't list it!

I'm looking for a final ratio of 0.375 on the 12tpi leadscrew, but looking at the online threading calculators... they seem to be for different change gear setups.

I got some details on this forum of a 26tpi thread setup, but my head hurts now.

Has anyone set up 32 TPI on one of these mini lathes?

Many thanks

Ian

JasonB14/02/2021 17:04:48
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How many teeth on the fixed spindle gear and what change gears do you have.

Lainchy14/02/2021 17:18:44
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Hi JasonB,

Fixed gear on headstock Spindle is 40

Selection of available.... 20,30,33,35,40,42,50,52,60,75 and 2x 80

I did some calcs... but I was down to manually doing each one, and I did get to one combination, but there's no way I could have got the gears to mesh, being too far out.

Redsetter14/02/2021 17:33:42
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Wouldn't it be quicker to use a die?

Lainchy14/02/2021 18:06:28
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It would be way quicker to use a die, yep, BUT, this is for gland nuts. I want it to be a really good fit in the steam chest, otherwise, it'll undo itself over time.... according to the articles.

Lainchy14/02/2021 18:22:52
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I say the lead screw is 12TPI based on searches, but in the manual, I've just found 2mm pitch.

Calculations of some of the stock setups, give the following

40 TPI

40/60 = 0.6666
60/52 = 1.1538
33/80 = 0.4125

0.6666*1.1538*0.4125 = 0.3172
0.3172*40= 12.688
***********************************

28 TPI

40/80 = 0.5
80/33 = 2.4242
30/80 = 0.375

0.5*2.4242*0.375 = 0.4545
0.4545 * 28 = 12.72

..... so maybe the leadscrew pitch is 12.7 per inch?? Seems a strange pitch though?

Peter Cook 614/02/2021 18:28:14
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12.7 per inch is 2mm pitch ( exactly). The manual seems correct.

Howard Lewis14/02/2021 18:33:10
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12.7 per inch look like 2mm to me!

32 tpi is a pitch of 0.3125" or 0.79375 mm This requires a gear train ratio of 2.519685 :1 for a 2mm pitch leadscrew.

So a compound train is very likely Could be that a mod to some of the numbers in the 40 or 28 tpi trains would give what you want.

What changewheels have you got?

Howard

Lainchy14/02/2021 18:39:55
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Fixed gear on headstock Spindle is 40

Selection of available.... 20,30,33,35,40,42,50,52,60,75 and 2x 80

Thanks chaps

Redsetter14/02/2021 18:45:57
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Posted by Lainchy on 14/02/2021 18:06:28:

It would be way quicker to use a die, yep, BUT, this is for gland nuts. I want it to be a really good fit in the steam chest, otherwise, it'll undo itself over time.... according to the articles.

If you use a split die you can get the threads as tight as you like. In practice they seldom unscrew because there is enough friction from the packing to retain them.

JasonB14/02/2021 18:47:21
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is it a metric lathe or imperial eg what are the handwheels marked as

Post a photo of the chart on the headstock then we can check the leadscrew from that.

Howard, you have been using the Duffer rule again, 32tpi is 0.03125"

Lainchy14/02/2021 18:49:24
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Redsetter... That's true! Will check if I have a split die for that and try on a piece of scrap.

JasonB... will do. Bear with. Lathe is Metric, but I believe the leadscrew is the same on both.

Lainchy14/02/2021 18:52:59
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20210214_123306.jpg

Edited By JasonB on 14/02/2021 19:19:09

Lainchy14/02/2021 19:15:36
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The 0.8mm pitch maybe close looking at it. Very short length of thread needed... maybe 3/8 tops.

25.4 /32 gives me 0.79375. 0.006mm short. That's pretty close maybe?

JasonB14/02/2021 19:20:54
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Yes, I'd just worked out a different 0.8mm which is probably as close as you will get with the gears you have only 0.00025" out per turn.

32tpi.jpg

Lainchy14/02/2021 19:24:36
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Superb JasonB Many thanks.

I'm gonna sit and work that out too. First time I've gone off the stock and it had me totally flummoxed, especially when I was struggling with the online tools too.

Many thanks !

Ian

Neil A14/02/2021 20:40:13
104 forum posts

Jason - I think you have put the second 40/80 round the wrong way in your calculation, is the "Duffer Rule" catching?

I have had a little go at this, it all depends if you can fit the train on the banjo.

(40/40) x (20/60) x (50/42) with a 2mm leadscrew this gives 32.004TPI.

As I have said, it all depends if it will fit on the banjo. I hope it does.

Neil

Should have said, you may have to change the order of the drivers and driven gears to fit.

Edited By Neil A on 14/02/2021 21:09:13

Lainchy14/02/2021 20:45:59
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Ah yes, just checking and indeed.... that would be 40/80, 40/80, 20/50. I'm guessing it would still fit, given that it's on the chart. I'll take a look at this also Neil. Going to be tail end of next week to get the gears off. 

Many thanks

Ian

Edited By Lainchy on 14/02/2021 20:50:52

JasonB14/02/2021 20:46:12
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Yes you are right Neil.

Neil A15/02/2021 22:02:31
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I have just run through my original suggestion for the gear setup and I don't think it will work because of tooth tip interference. I think that I have tried all the combinations of that setup.

I have another setup where the tips just run clear, but again it depends on whether it will fit in the banjo length.

(40/42) x (40/60) x (50/80)

There will be clearance between the 42 tooth and the 50 tooth gear. Assuming the gears are 1 Module the gap will be 2mm.

I should have checked the first one more thoroughly.

Neil

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