Lathe Speed - What am I missing out on?

The other thing about using more feeds and speeds is you would could up your productivity 10 fold.

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You say you run at 420rpm and the use the finest feed. Now I have 3 easily selected feeds for any gear train that I am running and that usually does for me, one is twice as fast as the finest and the other twice that so I can feed at 4 times the fine rate. I mostly keep my variable speed machine in the lower of the two speed ranges so can get from around 65 to rpm which is not far from 3 times as fast as your "fixed" speed.

So to rough out a small diameter component I could be turning a 3x and feeding at 4x which means the job takes 12th the time it would take you.

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I expect to now get the usual posts that say "it's just a hobby, whats the rush" well that's fine for the retired types who have all day to potter about but for those still working or who like to complete more than one project per decade working at a good pace is an advantage.

Have a look at my post on the bottom of this page to see how finish changes as speed and feed increase

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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