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Thought a 0.005" finish pass was overkill on 4140, then checked the surface gauge
I run a Haas ST-20 at a shop in Wichita and last month I got a batch of 4140 pre-hard pins that kept coming off the lathe with chatter marks, so out of pure laziness I added one extra finish pass at 0.005" and bumped the RPM up to 900 from my usual 650. The parts came out so clean the inspector actually asked what I changed, and a guy on the floor with 22 years on me said he has been sneaking in that same pass on hard stuff for a decade. I always thought a light pass just rubs the tool and burns inserts, but my insert life went from about 4 parts to 11 before indexing. Was it the depth, the speed, or both, because I want to know what is actually doing the work here.
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laura_allen23d ago
Man, that "rubbing the tool" thing gets drilled into you so hard in this trade that it feels wrong to go against it, right? I ran into the same head scratcher on some 4340 shafts where a light pass with a fresh edge just sang through the cut instead of that dull grinding noise, and it made zero sense to me at first. The way I had it explained, down around half a thou the edge can skate and work harden the skin instead of cutting, but once you get up near five thou you're actually under the nose radius and peeling real chips off. So my money is on the depth doing the heavy lifting and the extra speed just keeping the heat in the chip instead of your insert. Both together is prob why your life went from 4 to 11.
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