"whit3rd" wrote in message
...
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.

The ID at the groove's bottom has the tightest radius but it curves
away from the cutting tool. The OD at the bottom had an effective
radius larger than it would be for a cylindrical groove because of the
angle.

If the groove were in a flat face the clearance would be the same as
the groove radius, but as the angle tightens the necessary outer
clearance radius increases, becoming very large as the cone approaches
cylindrical and then infinite, a straight line like a parting tool. I
didn't learn the trigonometry of conic sections to calculate this, the
ellipse generated by a plane cutting a cone at an angle.
http://en.wikipedia.org/wiki/Conic_section

I think this means grinding for 5/16" would work but I'd make a
quickie test bit from HSS to check this, and also that the O ring
seals, stays in and can be replaced.

What's the maximum clearance angle a carbide bit can have without
being too weak? I just use 5 degrees of surface-ground flat clearance
for everything, leaving enough to support the edge and freehand
grinding boring and grooving bits to more than enough circular relief
below that. Perhaps a small carbide boring bit could be ground narrow
enough on the ID side which needs only flat clearance?

-jsw