# How to cut a metric multi start thread on an imperial lathe.



## savarin (Mar 26, 2021)

I'm not ready to do this yet but in the close future I would like to cut a triple or quad multi start fine metric thread.
It will be for a focus unit on my binocular. This will mean both internal and external threads.
I have cut many fine metric threads on my lathe but have never used the thread counter, in fact it has been removed.
I always leave the half nut engaged and reverse back to the beginning advance and cut again.
Giving it some thought I'm wondering if lets say a quad start thread is wanted at 1mm pitch then I set the change gears etc to cut a 4mm pitch.
I cut the first thread to the correct depth reversing back to the start as usual.
To cut the next thread I go back to the start, disengage the half nuts, index the chuck round 90 degrees, re-engage the half nuts and cut as usual.
repeat two more times.
Does this sound as if it will work?


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## Lo-Fi (Mar 26, 2021)

If you've got a way to index the chuck round 90 degrees that'll work, but it might be easier and more accurate to advance the tool itself. You know the lead as that's how metric is specified anyway. If it's not a deep thread, it really won't hurt to set the compound parallel with the bed and use that to advance between starts. Or if you're feeling fancy, set it to 60 degrees rather than 30 and to the trig.

Look forward to seeing how you get on


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## AGCB97 (Mar 26, 2021)

Using the compound to get to the next thread sounds like the way to go. Try it on something soft to prove.

If your doing both the inside and outside mating threads, why do metric?

Aaron


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## savarin (Mar 26, 2021)

AGCB97 said:


> If your doing both the inside and outside mating threads, why do metric?
> 
> Aaron


I guess because all my work is in metric measurements.
I dont think I've cut an imperial thread for 20+ years.


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## savarin (Mar 26, 2021)

Lo-Fi said:


> it might be easier and more accurate to advance the tool itself. You know the lead as that's how metric is specified anyway. If it's not a deep thread, it really won't hurt to set the compound parallel with the bed and use that to advance between starts.


I like this idea even better thanks.
I have been meaning to convert the screw to a 1mm thread the same as I did to the cross slide.
This gives me the impetus to do so now.
(damn, another project)


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## AGCB97 (Mar 26, 2021)

Quote
>>>I guess because all my work is in metric measurements.
I dont think I've cut an imperial thread for 20+ years.<<<

I didn't realize you had your imperial lathe mostly converted to metric. So it's not really an imperial lathe.
Aaron


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## savarin (Mar 26, 2021)

The only metric part is the cross slide lead screw that I converted, everything else is imperial.


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## AGCB97 (Mar 26, 2021)

Do you mean carriage lead screw for threading?


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## savarin (Mar 26, 2021)

No, the carriage lead screw is imperial so I have to use a 120/127 compounded change gear for metric threads.
Its the generic Asian 9x20 lathe.


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## RJSakowski (Mar 26, 2021)

If you use your compound  to advance for the next thread, the quality of the thread will depend upon your ability to accurately set the compound.   If you are cutting a four start thread with a pitch of .5mm and a lead of 2.0mm, you will probably want better accuracy than is normally afforded by the compound dial.  One solution would be to set the compound at an angle of 84.3º.  This will advance the compound in the z direction by .01 mm for every .1mm on the dial.  The corresponding x axis movement would be .995 mm for 1mm on the dial. 

If you set the angle to 89.4º, the z axis movement would be .001 mm for every .1 mm on the dial and the x axis movement  would be .1 mm for .1 mm.  This may be a bit much though as it would require a large amount of compound travel.  An interesting thought since you have an inch compound would be to use the compound angle to make your metric conversion.  Setting the compound at 87.74º will advance the compound in the z direction by .1mm for every .1 inches on the dial.


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## Lo-Fi (Mar 26, 2021)

I'd probably pop an indicator between the cross slide and compound as a travel dial, set it that way. No need to even convert the compound leadscrew for this job and you can be pretty confident of accuracy if you lock the compound between index moves.


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## savarin (Mar 26, 2021)

At the moment I cant even have a practice as I'm waiting for the control belt, finishing of the switch panel and wiring up the dc motor for it.
I will report back with my attempts once I have a go.
Thanks for all the help.


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