On Thursday, April 16, 2015 at 2:59:21 PM UTC-7, Jim Wilkins wrote:
"whit3rd" wrote in message
...
On Thursday, April 16, 2015 at 11:00:16 AM UTC-7,
wrote:

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 ...
... what radii to grind the sides of the tool

[recommend shoulder cut and seat cut to make square-section
toroidal space for O-ring]

Figure 5 shows how O-rings seal hydraulic fittings.
http://www.webtec.com/en/tech/connections

Yes, that illustrates the alternative perfectly.

As for cutting the aluminum seat, a piloted counterbore, with a stop, could be
chucked in a hand drill; this size of counterbore might be a standard item.
Clearing the chips, though, might be a problem.

Making soft jaws, or machining a soft collet, to hold the screw
by the head, seems advisable. That way, if something slips, it
doesn't ruin the cut (and flex of the workpiece is lower)

If one wants to do a square groove with sidewalls perpendicular to the
cone screwhead, the 'radius' of minimum clearance is the radius of
the deepest penetration of the upslope (shallowest) corner (and the axis is
the rotation axis of the spindle). It's a bit tricky.