Correct head shape for the ball turning HSS bit

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Maplehead

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Hi All
So I made a ball turning jig and it seems to work very accurately and it does the job. My only issue with it is that it's very slow. I am dialing in 5 thou for each turn. I'm thinking that that's not too deep. I'm thinking maybe the way that I ground the 1/4" HSS blank may be the issue. So I ground the head to be like a threading bit, about 60 degrees angled on either side and raking inward from the sides tops to their bottoms.
Should I have a sharp point or a rounded point?
Are there better shapes for the bit?
Any and all advice is greatly appreciated.
 

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I tend to take 45 degree heavy chamfers off before diving in with the ball turning tool, saves a lot of faffing.
I can't really see the tool geometry well in that photo, though?
 
Try grinding the top so it slopes away/down from the work and make sure there is relief under the point. I don't see any reason the cutting geometry would need to be all that different from a normal turning bit. I would start the tip sharp, but while honing add a small radius to the tip. There is a great discussion of the forces and angles along with examples of working cutting grinds in the HSS grinding thread.

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Models for grinding HSS Lathe Tools

In a recent thread by @Darkbluesky, http://www.hobby-machinist.com/threads/lathe-accessories-tools-ideas-of-what-to-buy.62043/#post-511618, @ttabbal joked about sending me blanks to be ground into lathe tools ... he was joking! But it got me to thinking ... yeah, I know it's not a good thing...
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Jimsehr said:
This works for me.

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Two things: I can't believe how fast you made that initial parting cut, and two, I can't believe how fast you turned that ball. Something is seriously wrong with my set up.
 
Maplehead
Maybe it’s because I have been cutting metal for almost 70 years. I’m also surprised you didn’t comment on the fact that tool has no clearance. If you google Jimsehr machinist you can see how I made an hemispherical radius turner.
Jim Sehr
 
Jimsehr said:
Maplehead
Maybe it’s because I have been cutting metal for almost 70 years. I’m also surprised you didn’t comment on the fact that tool has no clearance. If you google Jimsehr machinist you can see how I made an hemispherical radius turner.
Jim Sehr
Click to expand...
Mostly because I don't really know what that means, regarding the clearence. (Maybe I'm having a dead brain moment here.)
 
@Maplehead, the ball cutting bit in your tool has a lot of stick out, especially for its diameter. It is probably chattering/deflecting a lot. It looks like it is sticking out more than 10x its cross section. I know for a steel boring bar one wants to keep it under 4x. (Carbide under 8x).

@Jimsehr, that's a pretty cool ball turner. I will try to make that. Been looking at ball turners recently, pondering a design. Yours seems to be one of the simpler ones to make.
 
This is using same tool cutting od of shaft.

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