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Incorrect thread pitch

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John Tumble26/05/2021 21:01:21
6 forum posts

Hello,

I am trying to cut a thread with a 2mm pitch on a Colchester Triumph.

It has a 6mm pitch leadscrew, so I thought it would be straightforward.

I have set all the levers for 2mm pitch - C LR1Z, if I remember.

The thread that is cut is a bit off. I tried it on a long piece of stock and measured it to be 1.928mm pitch.

When I try a second pass, using the same number on the thread dial, it usually cuts in the wrong place. Not surprising, if the pitch is off.

I have carefully checked the change gears and they are as shown on the plate - 28/55 and 54/64.

I have also checked that the thread dial has the correct 16 tooth wheel installed.

Does anyone have any ideas what I am doing wrong?

Many thanks,

John

DC31k27/05/2021 07:51:09
547 forum posts
1 photos

Is the machine a native metric machine? You say the leadscrew has a 6mm pitch: in what units are the cross- and top- slide dials graduated?

The reason I ask for confirmation is you state the thread dial has 16 teeth. For a native metric machine, a 16t gear on the thread dial indicator is generally not used or useful (as its teeth only have 2 or its multiples as a factor). Metric machines use _indicator_ gears with factors of 3, 5, 7 and 11.

In any case, since there are exactly three of your desired 2mm pitches in every 6mm of your claimed leadscrew pitch, you do not need an indicator at all for this thread. You can cover it up, ignore it, throw it in the bin and just engage the half nuts anywhere they drop in and the thread will be in synch.

If you have a link to an online version of the manual for the machine, please post it and we can look at it and advise.

JasonB27/05/2021 08:15:50
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Try it with the 28 driving the 54 and the 55 driving the 64 which is what teh front plate shows

John Tumble27/05/2021 08:28:51
6 forum posts

Many thanks. Cross slide and compound handles have sliding sleeves on the dials for dual units.

Here is the thread dial. Units are all mm, so would seem to confirm metric leadscrew.

thread dial

I shall have a look for a manual.

JasonB27/05/2021 08:32:02
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2mm pitch.jpg

John Tumble27/05/2021 08:38:18
6 forum posts

Interesting point. Diagram is rather ambiguous. I shall try.

Hopper27/05/2021 09:11:42
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Posted by John Tumble on 26/05/2021 21:01:21:...

The thread that is cut is a bit off. I tried it on a long piece of stock and measured it to be 1.928mm pitch.

When I try a second pass, using the same number on the thread dial, it usually cuts in the wrong place. Not surprising, if the pitch is off.

Just out of curiosity, how are you measuring the pitch of a single pass with that sort of accuracy, to the nearest micron?

John Tumble27/05/2021 09:11:46
6 forum posts

Brilliant, Jason! Turned the idler around and I now have perfect 2mm pitch threads. And of course you can engage the half-nut anywhere.

Thank you so much.

John

John Tumble27/05/2021 09:20:42
6 forum posts
Posted by Hopper on 27/05/2021 09:11:42:

Just out of curiosity, how are you measuring the pitch of a single pass with that sort of accuracy, to the nearest micron?

I cut 70 turns on a bit of stock and measured carefully.

As a check, 1.928 x 55/54 / (54/55) = 2.000068, so it seems that I wasn't far off (the idler has 55 and 54 teeth).

Hopper27/05/2021 10:40:01
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Posted by John Tumble on 27/05/2021 09:20:42:
Posted by Hopper on 27/05/2021 09:11:42:

Just out of curiosity, how are you measuring the pitch of a single pass with that sort of accuracy, to the nearest micron?

I cut 70 turns on a bit of stock and measured carefully.

As a check, 1.928 x 55/54 / (54/55) = 2.000068, so it seems that I wasn't far off (the idler has 55 and 54 teeth).

Aha. Very cunning.

Glad you got it sorted. With that ambiguous drawing on the lathe front, I'll wager you were not the first to fall into that trap.

KWIL27/05/2021 11:39:59
3411 forum posts
66 photos

Why do you say it's ambiguous? Do not just glance at it. It clearly shows that the periphery of the 64T touches the 55T and the periphery of the 28T touches the 55T as Jason pointed out.

JasonB27/05/2021 11:47:30
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Not quite what I said look closer KWIL the 28 goes behind the 55 and touches the dotted 54. But should be clear enough to anyone stood in front of the machine unless the plate is badly worn

2mm pitch 2.jpg

Edited By JasonB on 27/05/2021 11:49:58

Michael Gilligan27/05/2021 16:20:06
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Posted by JasonB on 27/05/2021 11:47:30:

.

2mm pitch 2.jpg

.

Trendy mid 1960s graphic design ... Gears shown with no teeth, just pitch circles

MichaelG.

duncan webster27/05/2021 21:05:45
3445 forum posts
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So what does the lower pictogram refer to?

Michael Gilligan27/05/2021 21:19:41
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Posted by duncan webster on 27/05/2021 21:05:45:

So what does the lower pictogram refer to?

.

Feeds, I presume

MichaelG.

D Hanna27/05/2021 23:45:53
45 forum posts
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Posted by Michael Gilligan on 27/05/2021 21:19:41:
Posted by duncan webster on 27/05/2021 21:05:45:

So what does the lower pictogram refer to?

.

Feeds, I presume

MichaelG.

The lower gear train is for cutting Module and Diametral leads on the Colchester Triumph lathes.

Edited By D Hanna on 27/05/2021 23:57:02

Michael Gilligan28/05/2021 01:09:43
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Thanks for the education ‘D’ yes

So ... is the graphic supposed to represent a worm, or what ?

MichaelG.

D Hanna28/05/2021 02:28:37
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That is it MichaelG. Also the chasing dial gears supplied on the metric 2000 : 14, 16, 18, 20 and 22.

JasonB28/05/2021 07:08:07
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Zoom in on the photo I took it from Michael

Howard Lewis28/05/2021 09:00:40
5224 forum posts
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If the Leadscrew is 6 mm pitch, and the required thread is 2 mm, the changewheels need give a 3:1 reduction ratio between the Chuck and the Leadscrew.

So, in a simple train, your 28T Driver would need to drive a 84T gear on the Leadscrew, with any suitable gear as an Idler.

If no 84T is available, a compound train will be required. POSSIBLY, (IF the gears are available ) the 28T driving a 42T which is compounded with a 30T driving a 60T on the Leadscrew.

You can ignore the Thread Indicator Dial.

Just keep the half nuts engaged. At the end of the pass, withdraw the tool, reverse until the tool is well clear of the work. Reset the tool for the next cut, and the run forward again (Well clear, to ensure that all backlash has been taken out )

Howard

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