# How I cut a metric multi-start thread



## savarin (Apr 12, 2021)

I havnt worked out how to do this with those weird imperial threads so I will leave that for someone more erudite in maths than myself to do so.

All the metric threads I cut on this lathe are done by leaving the half nuts engaged so the first attempt was done the same way.

I wanted a quad start 1mm thread.

I set the top slide parrallel to the ways, aligned the tool bit at 90 degrees to the work, set the change gears to give a 4mm thread pitch, ie four times the 1mm pitch of the actual thread.
(if I wanted a three start thread then I would choose a 3mm pitch)

I cut the first thread to the correct depth in about 5 passes (slow and easy) then reversed back pass the start and with the half nuts still engaged advanced the top slide 1mm and cut the second thread to the correct depth.
Repeat two more times and I had a quad start 1mm thread inside the tube.
The outside thread on the other tube was cut the same way.

It worked and they threaded together, but, it was rough feeling and only screwed in half way.
The small internal threading tool flexed as I got deeper in the hole and cut a slight taper.

So I wondered if a M1 threaded tap would make a better tool.
I set up the tap in the tool post with the leading edge of one set of flutes set at the centre line, pressed it into the existing threads to align it and tightened it up.



I re-cut the internal threads in the tube.
This time I only had to make the pass to the correct depth once as all four starts were cut at the same time.
4mm cut pitch meant that each quarter of a revolution started the next thread. Bingo, it worked.
Screwed together at the start


screwed together at the finish


sorry for the out of focus but I was too lazy to re take them

I also found this method of using the tap made it very easy to re-find the thread if the half nuts had to be disengaged for some reason.

A couple of caveats, be careful of distorting the thin walled tube by over tightening the chuck and use the largest dia tap of the required thread pitch as the first M6x1 flexed a bit but the M8x1 was fine.
I may make a mandrel for inside the tubing next time so it cannot distort.


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## T Bredehoft (Apr 12, 2021)

I'm not at all sure I'd have thought of using the 1mm tap, but of course it would work. 

You are to be congratulated.


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## Lo-Fi (Apr 12, 2021)

Love it. Well done, sir!


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## Braeden P (Apr 12, 2021)

wow nice job I would not have figured out how to do that!


my uncle has some machinist jacks that my great grandfather made 5 start left hand square thread it is quite amazing


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## f350ca (Apr 12, 2021)

Thats incredible savarin !!!! I've used a tap as a boring bar like you did but never would have thought of feeding it at 4x the feed to get the multi start. 

Greg


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## savarin (Apr 12, 2021)

Thanks guys for the kind words, I have seen a tap used in this way somewhere on youtube but only for single lead threads.
I just had the thought and checked the tap against the thread and it was obvious it fit so thinking about it I guessed it would cut the 4 lead threads.


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## Janderso (Apr 12, 2021)

Way over my head.
Well done sir!


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## jbobb1 (Apr 12, 2021)

I've cut a few double start metric threads, but they were pretty large. Your technique is ingenious!


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## T Bredehoft (Apr 12, 2021)

Was there, should there have been any concern/thought about relief behind the cutting edge, give the aggressive feed?


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## savarin (Apr 12, 2021)

I rotated the tap to give a little relief, purely by feel and it seemed to be sufficient.


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## brino (Apr 12, 2021)

Excellent.
Thanks for posting Charles!
-brino


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## Suzuki4evr (Apr 12, 2021)

I have done a LOT of threading in my time and will do a lot more, but I have never done a multi start thread. Can someone educate me on WHY would you want to do this? Is it for strength purposes or what is the main purpose behind it?

Michael.


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## Badabinski (Apr 12, 2021)

I know one big reason to do multi-start threads is for strength when you have a really short thread depth, like a water bottle cap. I know there are other benefits, but I've completely forgotten what they are.


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## savarin (Apr 12, 2021)

I will be making a helical focusser for the binocular and a fine thread produces a better "feel" but is too slow so using a multi start gives the feel but also produces a faster movement so not so many turns recuired to get to focus.
Thats my reason for trying.


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## eugene13 (Apr 12, 2021)

AMAZING!


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## hman (Apr 12, 2021)

That's an absolutely brilliant technique!  Thanks for letting all of us in on it.


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## Ulma Doctor (Apr 12, 2021)

great use of tooling @savarin 
i have not seen that one before!


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## silence dogood (Apr 13, 2021)

Suzuki4evr said:


> I have done a LOT of threading in my time and will do a lot more, but I have never done a multi start thread. Can someone educate me on WHY would you want to do this? Is it for strength purposes or what is the main purpose behind it?
> 
> Michael.


Multi start threads are use a lot on camera lens for focusing and for the zoom part of the lens.  If single thread was use, you'd be spending a lot of time spinning that focus or zoom ring around  to get that picture  in focus  or  bringing that image up close.


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## darkzero (Apr 13, 2021)

I've single pointed a 4 start imperial thread but using the thread dial to do the indexing. My lathe has imperial leadscrews so I would have had no idea how to cut a multi-start metric thread if I needed.

Great thinking Savarin, thanks for sharing!


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## savarin (Apr 13, 2021)

Ha Ha, I've got no idea how to work out an imperial thread


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