In article ,
wrote:

All-
I need to put an o-ring groove in the underside of 500 1/4-20 flathead
screws. So I need to make a groove tool to do the job. A straight
groove tool won't work because the sides of the groove are curved. I
have made tools like this before but this is a small one and I'm
machining 304 SS so I need carbide and only want to make the tool
once. So I think to find out what radii to grind the sides of the tool
can be determined by drawing the screw head with the groove in it.
Then extend the sides of the head until they meet. Then mirror the
drawing around that point. Then I can directly measure with the cad
program the two different radii of the sides of the groove. Am I
correct?

From the rest of the thread, a picture or sketch would be very helpful.

People already make screws with O-ring seals: http://www.zago.com/.

And, Kaiser Tool Company makes lathe tool bits for cutting circular
grooves at various angles. I think that their ThinBit line is what you
seek. http://www.thinbit.com/

Joe Gwinn