Grinding a round cutter?

I'm making a hand-held tarp grommet setting tool I can use on a ladder, with dies that screw together. I can grind the shank end of a
drill bit into a radius form tool and hold it tilted forward but it won't cut very deep before the shank rubs, although the resulting shallow ellipse is likely good enough for this job, I'll find out how well it flares, rolls and crimps the tubular rivet when the tool is complete. If it doesn't work I know a guy who can modify this kind of stuff until it does.
Drill shank cutters work better on round pulley grooves for wire cable than for cuts into the flat end of the bar.
Is there some simple geometric way to grind an accurately circular convex radius lathe bit that I may have missed? Concave radius forming bits are easy to shape with a tapered Dremel stone marked at the intended size. The taper provides clearance so the bit can be clamped flat to cut a true circular arc.
A General drill bit grinding fixture might be the right sort of tool if it could clamp the lathe bit flat and was sufficiently rigid. I can't really justify the cost of a new punch grinding fixture and haven't found a used one. http://tool.wttool.com/tools/Punch%20Grinding%20Fixture
--jsw
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Lathe ball cutter mounted on a grinder? Past the pivot you get a convex tool, inside the pivot you get a concave tool.
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I saw a Holdridge cutter at a school shop auction, with several other bidders standing around it and drooling. IIRC it went for over $600.
I suppose I could make a base for a lathe compound, which wouldn't be nearly as hard to wash clean after the grinding as the whole lathe. On my South Bend the center of the tool post slot can travel from about 1/4" beyond the compound's center of rotation (between the 90 degree marks) to slightly less than 1.5" back from it.
I'd have to mill a custom tool post block to position the end of the bit anywhere near the pivot. For similar effort I could make a bit holder for a 5C spin index and also grind angles on threading bits.
The grommet set worked on the first try so I don't need to modify the roll crimp guide. A 1970's brass grommet rolled up perfectly, new import ones from HD cracked and spread flat but remained tight in the tarp after a two man tug-of-war. --jsw
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It's a gorgeous tool, and does precisely (and well) what it was designed to do... turn radii.
I owned the 9" lathe version for about 15 years; bought it for only ONE project, and it paid for itself in one project. But, I ended up using it almost never after that. Finally sold it for about $600 about three years ago. But BOY, I sure did PAY more than that for it, new! <urk!>
Lloyd
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On Thu, 07 Jul 2016 20:50:25 -0400, Jim Wilkins wrote:

Are the import ones brass? If so anneal them and see how they fold.
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Yep, my plan is to buy some more, place their heads in shallow water and anneal only the tubes with a propane torch. My small stock of the good old ones that roll up smoothly won't be wasted on outdoor tarps.
I cut the roll-up groove with a round 5/32" HSS boring bar bit blank. The squared-off drill bit shank which isn't as hard as the fluted end stayed sharp long enough to cut the washer side recess in the W1 tool steel blank but I had to carve the deeper rivet side with the 5/32" bit. I tried to copy the cast iron punch and die that came in the old kit.
Drill bit ends held up well enough to cut aluminum pulley grooves to fit the rope and cable. Had they been a common fractional size I could have milled the semicircular groove with the side of an end mill. I used a half-round convex lathe bit hand-ground to visually match the right sized hole in a drill bit gauge to groove a steel spring winding mandrel. The resulting groove looks round but I wouldn't trust it as a ball bearing race.
--jsw
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A whole lot of the imports are brass-plated FEmutt. My lovely set from HFT is such, but they work fair to middlin'.
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Annealing the tube helps a lot.
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