# Slowest rpm for thread cutting?



## sploo (11 Jun 2019)

I've seen a few references to finishing the last few turns by hand when cutting a thread (where it's not possible to have a relief cut, so risking a crash with the lathe powered). I have basically zero experience of thread cutting on a metal lathe though.

That got me thinking - is there a minimum rpm speed required for thread cutting steel, using HSS cutters?

The reason I ask is that I'm looking at building a 4th axis for my CNC milling machine, and its main use will be for turning very large objects (e.g. 40cm diameter) - so I'm planning on gearing it to run very slowly (e.g. maybe 5 to 10 rpm; but obviously with significant torque). With appropriate cutting depths, and a sufficiently rigid tool mount, would there be any issue with getting it to turn threads on ~3mm to 20mm diameter stock at this sort of rpm?

I guess this comes down to whether HSS cutters have a surface feet per minute sweet spot; and this low rpm combined with a small diameter would be just too slow.


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## Rorschach (11 Jun 2019)

It's not RPM you need to worry about, it's surface speed and that depends on the diameter of your work.


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## AndyT (11 Jun 2019)

My metal lathe is treadle powered, so is a bit different from yours, probably. However, when cutting threads I have had some success when pulling the flywheel round by hand, very slowly indeed. Using HSS tooling, I could keep peeling away swarf at very slow rates of cutting. So I'd suggest that there is no minimum speed.
You could think of it as the sort of speed you might use when rotating a die by hand to cut a thread.


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## sploo (12 Jun 2019)

Rorschach":35568etp said:


> It's not RPM you need to worry about, it's surface speed and that depends on the diameter of your work.


I'd hoped to head off that reply with the last paragraph of my original post :wink: 

Yes - the reason the planned 4th axis will turn so slowly is specifically because I need to be able to achieve a safe surface speed for a milling head when a large diameter object is being turned.




AndyT":35568etp said:


> My metal lathe is treadle powered, so is a bit different from yours, probably. However, when cutting threads I have had some success when pulling the flywheel round by hand, very slowly indeed. Using HSS tooling, I could keep peeling away swarf at very slow rates of cutting. So I'd suggest that there is no minimum speed.
> You could think of it as the sort of speed you might use when rotating a die by hand to cut a thread.


Excellent point about die cutters. As I hoped/suspected, you're probably right there's no (realistic) minimum speed.

Last night I happened to watch a video that included a guy cutting a keyway using a DIY lever jig, and of course that's just peeling away shavings of metal by hand (low speed, high torque due to the leverage).

Hopefully it'll be OK then. Certainly opens up a few interesting extra possibilities with the 4th axis. Thanks.


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## Rorschach (12 Jun 2019)

sploo":2nurvt4s said:


> Rorschach":2nurvt4s said:
> 
> 
> > It's not RPM you need to worry about, it's surface speed and that depends on the diameter of your work.
> ...



Sorry, missed that bit. 
Yes there are sweet spots but a sharp HSS tool will cut at any speed, as mentioned before, tapping and threading with dies is done at very slow speeds.


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## AES (12 Jun 2019)

I don't have as much experience of "powered" thread cutting, but when I do use my little lathe for this I always turn the chuck by hand, not in powered mode. So in principle I'm just using the lathe as a "jig" to hold everything square and parallel!

I think in a production environment (e.g. capstan lathes) there are min speeds but as for me "time is not of the essence" I don't bother even looking for such data. I'd also suggest that as the thread you're planning is for that special purpose and is a one-off you've nothing to loose by going slowly.

Dunno if that helps or not!


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## sploo (12 Jun 2019)

Rorschach":2pc5qa6b said:


> Yes there are sweet spots but a sharp HSS tool will cut at any speed, as mentioned before, tapping and threading with dies is done at very slow speeds.


Dunno why I didn't think about manual tapping and die cutting before Andy mentioned it - kinda obvious when someone points it out.

If I can create a sufficiently rigid tool holder (the machine by default will use a milling head) then it does open some interesting possibilities for making unusual threads.




AES":2pc5qa6b said:


> I don't have as much experience of "powered" thread cutting, but when I do use my little lathe for this I always turn the chuck by hand, not in powered mode. So in principle I'm just using the lathe as a "jig" to hold everything square and parallel!
> 
> I think in a production environment (e.g. capstan lathes) there are min speeds but as for me "time is not of the essence" I don't bother even looking for such data. I'd also suggest that as the thread you're planning is for that special purpose and is a one-off you've nothing to loose by going slowly.
> 
> Dunno if that helps or not!


It does - many thanks!


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## AndyT (12 Jun 2019)

Just to add to the consensus, this thread includes me cutting flutes on a tap just by peeling away metal by winding the tool horizontally.

making-a-tap-for-wood-t116626.html


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## Rorschach (12 Jun 2019)

If you are using a milling head for the tool then you could mill the threads which would be incredibly fast and clean. Or was that your plan?


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## sploo (12 Jun 2019)

Rorschach":2i3zji7a said:


> If you are using a milling head for the tool then you could mill the threads which would be incredibly fast and clean. Or was that your plan?


It wasn't necessarily the plan; partly because I'd need a suitable milling cutter. I wouldn't be against trying though.


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## Rorschach (12 Jun 2019)

sploo":1luzmqn0 said:


> Rorschach":1luzmqn0 said:
> 
> 
> > If you are using a milling head for the tool then you could mill the threads which would be incredibly fast and clean. Or was that your plan?
> ...



I reckon if you are cutting the size threads you mention then it would be a good option. Tool load is reduced, cutting speed is faster.


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## sploo (12 Jun 2019)

Rorschach":368rs7hi said:


> sploo":368rs7hi said:
> 
> 
> > Rorschach":368rs7hi said:
> ...


My concern would be the life span of (the tip of) a v-cutter required to do this. That said, I hadn't realised that the metric ISO and Unified National threads both use the same angles and widths for the tips and troughs of the threads. As long as the tip of the cutter is >= p/4 (where p is the pitch) it should work.


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## Rorschach (12 Jun 2019)

Well it's widely used in industry so I imagine commercial cutters have very good life.


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## sploo (12 Jun 2019)

Rorschach":1nyqh20p said:


> Well it's widely used in industry so I imagine commercial cutters have very good life.


I did find a 60 degree v cutter, but the tip was 0.4mm. Given the p/4 width for the bottom of a thread, it would limit the minimum pitch to 1.6mm (so only good for an M12 coarse and larger).

I assume there must be some steel-capable cutters with finer tips, but to mill an M4 fine thread (0.5mm pitch) would need a tip of 0.125mm; so pretty delicate.


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## TFrench (13 Jun 2019)

Search youtube for thread milling - lots of stuff out there about it, plus lots of cutters available.


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## sploo (13 Jun 2019)

TFrench":bmlr7yzl said:


> Search youtube for thread milling - lots of stuff out there about it, plus lots of cutters available.


That's a different "angle of attack" though isn't it? All the cutters that come up on a Google search for "thread milling" are designed to be used where the axis of the cutter is parallel to the axis of the rod (or hole) to be threaded.

In the setup I'll create, I'd end up with the milling axis at 90 degrees to the rotational axis, so it'd need a single 60 degree v shaped flute, with the workpiece being spun (and I'd only be able to cut an external thread that way).


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## Rorschach (13 Jun 2019)

Oh I see. I was thinking parallel as well. Can you make it work both ways?


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## sploo (14 Jun 2019)

Rorschach":3if7dded said:


> Oh I see. I was thinking parallel as well. Can you make it work both ways?


Possibly. Might be a bit complicated to mount the milling head that way, though I will need to remount it in order to do my planned large diameter cutting.


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