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That thread cutting dial thingy

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Robin22/05/2021 16:07:00
488 forum posts

I cannot cut threads. I used to be able to cut threads but now I cannot.

It is all set for 1.75mm pitch to cut an M12 thread.

The first pass looks great.

Subsequent passes don't hit the same helix. They have a choice of several starts on different helix's.

The front panel says I can start on 1 or 5. I gave up on that an hour ago, I am only using 1.

After beating my brain for explanations, I have come up with 2.

Either my brain is completely addled and I no longer understand screw cutting on a lathe, or, the gear on the thread indicator dial has the wrong number of teeth.

How do they decide how many teeth it should have? Is it easy? It is not something I have ever thought about face 22

John Hinkley22/05/2021 16:17:06
1184 forum posts
391 photos

From your profile, I see you have an ML10 lathe, so presumably imperial leadscrew. I believe that you have to keep the feed nuts engaged all the time when cutting metric threads on an imperial machine, reverse the cut under power or manually and ignore the thread dial indicator. I might have that wrong, but it rings a bell. Someone will be delighted to put me right.


Edited By John Hinkley on 22/05/2021 16:17:59

Journeyman22/05/2021 16:18:19
1033 forum posts
199 photos

Have a look at my website covers this problem. Read the bit in blue then go down a bit to look at the ** Thread Dial Indicator ** hope it helps a bit. This refers to a metric lathe but the same calculations can be applied for an imperial machine.

If you have to leave the half-nuts engaged it may be worth investing in a ** flip-up toolholder **


Edit: Add link

Edited By Journeyman on 22/05/2021 16:28:52

Robin22/05/2021 16:38:14
488 forum posts

Hi John

The lathe in your Thread Dial Indicator link has a 3mm pitch thread and the indicator goes up to 6.

I also have a 3mm pitch screw but the indicator goes up to 8 with a 16 tooth gear.

Is that going to work?

Journeyman22/05/2021 16:54:15
1033 forum posts
199 photos

Basically NO. If the Thread Dial Indicator gear (the one that meshes with the leadscrew) has 16 teeth then to rotate once it will need to travel 48mm (No. of teeth x leadscrew pitch) 48 is not divisible by 1.75 so the half-nuts will need to remain engaged all the time.


Bernard Wright22/05/2021 17:03:19
90 forum posts
16 photos

As people have mentioned, you need to leave the half nuts engaged.

Forget all about the Thread Dial Indicator, I've taken mine off.

I'm not suggesting you do, but I've found it easier even with Imperial threads, to leave the half nuts engaged and just take off the cut and reverse, it's a lot easier now with the VFD.

My lathe won't do 1.75p out of the box, so trying a few work arounds to see which is best.


Martin Connelly22/05/2021 17:19:37
1891 forum posts
203 photos

Each pitch on your leadscrew is 3mm. 1.75mm does not easily go into any multiple of 3mm within reason using a 16 tooth gear. You need a multiple of 7 such as a 14 tooth or 21 tooth wheel. These will allow you to use the thread dial gauge, otherwise you need to keep the halfnuts engaged and reverse the spindle to go back to the thread start position.

Martin C

7 X 3 = 21

12 X 1.75 = 21

Edited By Martin Connelly on 22/05/2021 17:23:51

DC31k22/05/2021 17:40:12
573 forum posts
1 photos
Posted by Journeyman on 22/05/2021 16:18:19:

Have a look at my website covers this problem. Read the bit in blue then go down a bit to look at the ** Thread Dial Indicator ** hope it helps a bit.

On that page, you say, "Lesson 2, You can′t use the threading dial for all threads even if they are metric on a metric lathe."

As a general observation, not relevant to the problem in this thread, could I ask you to consider inserting somewhere before that a lesson 1 1/2, "there are a large number of (metric) threads that do not need a thread dial at all"? It is an aspect of screwcutting that is much under-written.

Harry Wilkes22/05/2021 18:11:24
1175 forum posts
64 photos

Since fitting my inverter I leave the half nuts closed like Bernard yes


Robin22/05/2021 18:41:24
488 forum posts

FOUND IT... YAY yes laugh

Martin gave me the clue...

The front panel has a T component, 14, 15 or 16.

So I go to the box that came with it and find two more gears, Durned things are interchangeable.

The manual truly leaves a lot to be desired thinking

Journeyman22/05/2021 18:50:52
1033 forum posts
199 photos

Sorted then! The plate on the lathe also emphasises DC31k's point that not all threads need the indicator. Those that are directly related to the leadscrew pitch (i.e. same, half, third, quarter, etc. ) don't need the indicator at all.

Happy screw-cutting


Howard Lewis22/05/2021 18:53:04
5299 forum posts
13 photos

Reading the first post, wondered if you needed to change the gear on the Thread indicator, or possibly on the inpoutb tom the gear train.

In theory, my lathe can't cut coarser than 3.5 mm pitch. But a change to another input gear allowed me to cut a 4 mm pitch.

But if cutting Metric threads on a machine with an Imperial Leadscrew, or vice versa, it pays to retract the tool,keep the half nuts engaged, and reverse the direction of the spindle, to take the Saddle back far enough that when you reset and come for the next pass, the backlash is taken out.

being a fully paid up coward, I tend to keep the half nuts engaged whatever thread I am screwcutting, to lessen the risk my making a boo boo..

Hope that you have got it sorted.


Robin22/05/2021 19:29:33
488 forum posts

I do like things to work as described wink

It now starts perfectly on 1 or 5 and the thread is indubitably improved.

Thanks for the help, it is so easy to give up when you are doing it all alone yes

DC31k22/05/2021 20:00:01
573 forum posts
1 photos

That indicator table is a triumph of marketing over engineering. They appear to have compiled it on the basis of more is better, regardless of whether any thread standard uses a particular pitch (e.g. 1.8mm or 2.4mm pitch) or any sensible person would cut a particular pitch (e.g. 7mm pitch on a 3mm leadscrew).

Grizzly bear22/05/2021 20:40:39
283 forum posts
8 photos


Are you the Robin that had a serious workshop fire some years ago?

If so, welcome back.


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