Tool Geometry for Ball Turning?

Does anybody have some recommendations to make re tool geometry for spherical turning on the lathe? I've found lots of designs for ball
turning attachments on google, but nothing so far that describes rake and tip angles etc.
I bought an attachment, and it came with a HSS tool - very sharp, symetrical tip, lots of back rake, lots of end clearance. It sort of works, but the finish is pretty rough and it has a tendency to dig in a bit, too.
Any advice appreciated.
Thanks, Chris
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Excessive front clearance and rake are an open invitation to hogging and chatter. Reduce the amount of each and you should have pretty good luck, assuming the tool is otherwise ground properly. If you're machining steel, there's no need for more than about 7 front clearance, but rake can be included as your chip breaker so it's open to wide variations. A straight grind rake of roughly 10 should serve you nicely. You'd be best served if you make the tool cut in one direction, so the rake is in keeping with the direction of feed, sort of a combination of side and back rake. If you don't want to do that, make sure it's straight back so it doesn't go negative on you in one direction, especially if you're using HSS.
Harold
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Chris wrote:

Not having a ball turning fixture or CNC on my lathe, I did it on the mill. But, the procedure is about the same. I made a fixture so I could mount the workpiece on the mill spindle. I then mounted a round carbide insert on a holder and put it in a lathe QC tool holder, just as I'd do for the lathe. Then, I mounted the whole QC holder sideways in the mill's vise, facing the right way for the mill's rotation. I wrote a simple arc program in the X-Z plane to simulate the motion of the ball turning fixture. This produced quite nice 1" diameter ball joint ends.
So, I think a tool with a round or radiused tip would give the best results. If you have indexable carbide tools, pick the insert with the largest radius.
If your lathe is not too rigid, or the ball turning fixture is a bit loose, then you will have to approach the workpiece in small increments, and not take off a lot of material per pass.
Jon
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Thanks for the tips. I adjusted the attachment a bit and tried some different tool shapes today and got somewhat better results. I think with some practice and grinding some new profiles I might get there eventually!
Chris
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