wrote in message ...
On Thu, 16 Apr 2015 20:35:10 -0400, Bob Engelhardt
wrote:
On 4/16/2015 2:04 PM, 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?
Thanks,
Eric
You want to make the o-ring groove circular in cross section? Grooves
are usually rectangular to allow the o-ring to squish in compression.
The groove is square cross section.
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I hope you meant rectangular, not square. The oring is deformable but not
compressible, so the cross-sectional area of the groove needs to be bigger
than the cross-sectional area of the oring else you extrude and cut it on
tightening. As for the geometry you could do it with a sketch and trig but
I would prefer to just draw it in a cad program. Being a lover of run-on
sentences here's how I would do it in autocad. Draw an angled line to
represent the angled side of the flat head, draw a line perpendicular to
that for the end of the oring groove closest to the threads, make an offset
copy of the first line offsetting by 80% (or 75% or whatever you choose from
the Parker oring handbook for your oring and pressure and oring life but it
almost always works out to be between 75 and 80%) to be the bottom of the
groove, draw a circle for the oring tangent to the side and bottom of the
groove, and finally an offset copy of the first side line offset by 125-150%
of the oring diameter to be the second side of the groove (this creates the
extra cross-sectional area for the oring to deform into, and isn't critical
so long as it is large enough). Draw a line parallel to the screw axis
offset from the center of the oring by the radius of the ring (which also be
the centerline axis of the screw), trim all the original groove lines to
just leave the rectangular groove, then reflect everything around the
centerline axis. Now add the top of the flat head and the threaded section,
trim the excess from the angled lines, and you can pick off the dimensions
you need. That's the stuff I'm sure off, the actual machining I'm less help
with. On a manual lathe the way to do this would be to use a tool with a
squared-off end like a parting tool and rotate the compound to the right
angle and just plunge the compound. On a cnc couldn't you grind the end of
a boring bar to the parting tool shape at the angle you need, and then make
the angled plunge cut to simulate the straight plunge of a compound link you
were turning a taper? Either way I don't see where you need any kind of a
radius on the cutting tool, except a tiny break on the two front corners to
leave a rounded chamfer at the bottom corners of the groove for stress
relief. Hope this helps some :-).
Regards,
Carl Ijames carl.ijames aat deletethis verizon dott net