Lathe tools for the fussy

Cool! When I think hydraulics, I not only think fussy and precision, but BIG. Way bigger than anything I've ever made or probably will make. Then you've got the tough materials to contend with. I had an issue with very long and very tiny screw threads (over 100 tpi) that I wonder if you ever see in that chrome steel. The threads had to be perfect over about an inch. No change in PD at all. HSS, even cobalt, would wear just enough from beginning to end, that the thread was unacceptable. I could just barely do 1 thread (in 303) using Castrol Moly-Dee, that was decent, before tool wear degraded the geometry and root. Only with hand lapped carbide was I successful for many dozens of parts. Oddly, a perfectly sharp pointed tool was just as durable as one with a flat or radius, though that would never be true for a larger part.

Best,
Conrad
 
Having a decent tool is a good part of the battle. Another thing I always fuss over too much is getting the tool on center. Ideally I like to face something, then mount the threading tool and bring it right to the center of the facing, using a magnifier. The bigger the part, the less it matters, but if you're threading something small a few thou error can be a huge percentage.

Most people use a threading tool with zero rake, as do I. It's too cold right now to experiment, but some positive rake to get some shearing action going should improve things. The trouble is that as you put rake on the tool, the effective included angle gets larger. The tool should be ground to a slightly smaller angle to compensate. I recently figured out the math for this, and added it to my general purpose spreadsheet for Threads & Changegears. It's in the software and download part of my site, not the machinist stuff. Why? I dunno. There are even more complex things one can do, like aligning the tool to the helix angle of the thread. For 99.9% of peoples work, it probably doesn't matter, but sure enough I ended up with a job where it did. Anyway, you might find the spreadsheet useful. If not, at least it's a tiny download :)

Best,
Conrad
 
Wonderful writeup Conrad! I'm very grateful that you posted it for us.

Best,


Nelson
 
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