This is a new weirdness I encountered yesterday.

Was machining 30mm dia. 4140 down to a 16mm rod, in my Hercus 9C. Fairly heavy cuts – between 25 and 40 thou. Got a few nice long blue spirals peeling off, until the diameter got too small, tip lost its edge, and started abrading.

Then I stopped lathe, and looked closely. The job had managed to wiggle about 4mm out !

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Jaws were still tight on. I just continued roughing down with smaller cuts.


Now, if I was cutting "the wrong way" (i.e. away from the chuck), I could understand it, but most of these were feed in, disengage half nuts, hand feed close to chuck, then either stop to return carriage, or pull tool post toward me while winding carriage back for another pass.


Am I going crazy? Or is my job somehow orbiting around between the jaws and gyrating against the cutting pressure?





P.S. Job is to make a new brake pivot pin for Hafco 336. Here I am doing dodgy milling of flats on the pin:
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Taking that photo, I noticed how small the shavings were, so put a plastic bag over the bed.
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