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Thread RPM

Hertz

Stainless
Joined
Apr 27, 2009
Location
Sudbury, Ontario, Canada
Good morning guys. I have a question. Might be a dumb one, but when threading on a lathe, what is harder on the threading tool, higher rpm or lower?
I ask this because the feed stays the same obviously, and I was wondering if a slightly higher RPM would be less pressure on the tool or more for that matter.
 
Higher surface speeds increases tool pressure and wear, but too slow can also be bad for tool life. It depends on the material as certain gummy materials can build up in front of the tool if the SFM is too low. Just a reference, since I thread on a manual machine, on 1-1/2" through 3" steel round bar 1018 up to prehard 4140, I run 70- 90 rpm. This gives me a decent tool life, nice threads and time to pull out of the cut without yanking it hard (which can fracture a threading insert) Running any faster has repeatedly cost me more tooling in multiple alloys. Good thread cutting oil is also a must, particularly in gummy or wear resistant alloys and this will have more affect than a deviation of surface speeds.
 
I'm of the opinion that in cases like this, a cutting tool is a cutting tool. In other words, I try to run my threading as fast as I would my general turning and only slow down to get control of the feed when required by the part's features.
 
what is harder on the threading tool, higher rpm or lower?
Lower is harder because then it's trying to tear the material off rather than cut it off.
But of course this depends on how much lower or higher we're talking. AND assuming your starting sfpm is in the ballpark.
 
Of course home shop types, like me, are scared of crashing tools so reduce rpm below most efficient speeds to extend time to stop the machine if needed.
Bill D
There's a parallel thread running that discusses how to practice and prevent this. It's basic drop indicator stuff, easy peasy.
 
I was cutting M36x4 4340 on the CNC. I had went with a slower speed of 350 RPM to start because of slight chatter, but as I got closer to size, the chatter was stopping, however the insert had excessive wear on it.(I naturally thought the sfm was simply too high), but it still did it even at 250. I contacted the thread tool manufacturer and he suggested 650 RPM and although it chattered for a bit, by the time it got down closer to size, it stopped , so the threads came out nice. Ended up getting 30 parts without having to change the insert. Threads were 3" long.
 








 
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