Incorrect thread pitch

[John TumbleParticipant @johntumble21817]

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

[John TumbleParticipant @johntumble21817]

[DC31kParticipant @dc31k]

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.

[JasonBModerator @jasonb]

Try it with the 28 driving the 54 and the 55 driving the 64 which is what teh front plate shows

[John TumbleParticipant @johntumble21817]

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.

I shall have a look for a manual.

[JasonBModerator @jasonb]

[John TumbleParticipant @johntumble21817]

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

[HopperParticipant @hopper]

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 TumbleParticipant @johntumble21817]

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 TumbleParticipant @johntumble21817]

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).

[HopperParticipant @hopper]

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.

[KWILParticipant @kwil]

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.

[JasonBModerator @jasonb]

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

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

[Michael GilliganParticipant @michaelgilligan61133]

Posted by JasonB on 27/05/2021 11:47:30:

.

.

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

MichaelG.

[duncan webster 1Participant @duncanwebster1]

So what does the lower pictogram refer to?

[Michael GilliganParticipant @michaelgilligan61133]

Posted by duncan webster on 27/05/2021 21:05:45:

So what does the lower pictogram refer to?

.

Feeds, I presume

MichaelG.

[D HannaParticipant @dhanna35823]

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 GilliganParticipant @michaelgilligan61133]

Thanks for the education ‘D’

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

MichaelG.

[D HannaParticipant @dhanna35823]

That is it MichaelG. Also the chasing dial gears supplied on the metric 2000 : 14, 16, 18, 20 and 22.

[JasonBModerator @jasonb]

Zoom in on the photo I took it from Michael

[Howard LewisParticipant @howardlewis46836]

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

[John TumbleParticipant @johntumble21817]

You only need to keep the half-nut engaged if the leadscrew is imperial (and you are cutting a metric thread).

With a metric leadscrew, things are super easy and you can engage and disengage the half-nut whenever you like.

John

[DC31kParticipant @dc31k]

Posted by Howard Lewis on 28/05/2021 09:00:40:

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.

Could you please explain how you associate the concept of "simple train" with a Colchester Triumph lathe, that has two gearboxes (let us call them 'range' and 'thread pitch&#39 between spindle and leadscrew.

To strengthen the credibility of your observations, you need to find a setting on the gearboxes that gives a 1:1 ratio (direct drive) ratio between gearbox input and leadscrew output.

[Howard LewisParticipant @howardlewis46836]

Exactly! Setting the gearboxes to give a 1:1 ratio, enables the 3:1 ratio to be set up on changewheels, if this facility is available..

I realised that the Colchester would have norton box, and was surprised that if it is Metric machine, that the chart did not provide settings for Metric threads.

My lathe, not a Colchester, is Metric and does so! (For some pitches, It is necessary to change the input gear to the norton box. )

Working from basic principles often provides a fairly simple answer .

Remember the Radio Ham's motto KISS!

Howard.

Edited By Howard Lewis on 28/05/2021 09:31:30

[JasonBModerator @jasonb]

Posted by Howard Lewis on 28/05/2021 09:30:09:

 

I realised that the Colchester would have norton box, and was surprised that if it is Metric machine, that the chart did not provide settings for Metric threads.

It does, look again at the photo I posted yesterday where most of the metric pitches can be seen.

 

Edited By JasonB on 28/05/2021 09:41:48

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