Geometry question

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
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On Thursday, April 16, 2015 at 11:00:16 AM UTC-7, snipped-for-privacy@whidbey.com wrote:

Dude, coffee isn't enough, you need sleep. To seat an o-ring, you get the groove dimensions from the manufacturer of the o-ring. And to seal a 304 stainless screw head, a soft copper gasket makes more sense.
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wrote:

I guess I wasn't clear in my first post. The o-ring groove is going into an angled face and the groove is normal to that face. The customer wants an o-ring. We discussed this. The screw bears against aluminum. And his customer needs to be able to replace the o-ring at will. Soft copper washers are not as near as common as o-rings. Eric
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On Thursday, April 16, 2015 at 1:18:27 PM UTC-7, snipped-for-privacy@whidbey.com wrote:

I wouldn't do it the hard way, with a groove in the head. I'd make a square tool cut in the head, and another (end-mill) square cut in the seat, to make the correct size 'box' for the as-compressed O-ring. It might be best to use an O-ring that seals against its inner and outer circumference, those are the easiest dimensions to control.
Holding the screw while making the cut can be ... challenging. Maybe you'll be threading a soft collet?
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wrote:

I need to make these for about 25 cents each in quantities of 500. I can do that with only cutting a groove in the lathe. I can't cut the seats econimically. Besides, cutting a groove normal to the angled face is an accepted way of using O-rings, I have done it before, just not on the underside of the part. I am not sure if I will be using a soft collet. I may end up having to face the whole underside of the screw head, but I really want to avoid that because of accelerated tool wear and I don't want to use two tools. I may be able to come up with a horseshoe shaped device that lets me locate off the angled surface and then I remove the device and press start. I will most likely use a dead length collet closer in the lathe. Eric
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Figure 5 shows how O-rings seal hydraulic fittings. http://www.webtec.com/en/tech/connections
-jsw
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On Thursday, April 16, 2015 at 2:59:21 PM UTC-7, Jim Wilkins wrote:

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

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.
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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
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On Thu, 16 Apr 2015 11:04:10 -0700, snipped-for-privacy@whidbey.com wrote:

I know it's bad form to reply to your own post but I obviously need to make a clarification. The groove is going into an angled surface and is normal to that surface, not to either the axis of the screw or the top face of the screw. I alread know I need a 5/16" dia o-ring. But since the groove is normal to the angled face the sides of the groove will have larger radii than the radius of 5/16". Eric
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On Thu, 16 Apr 2015 13:25:17 -0700, snipped-for-privacy@whidbey.com wrote:

I know, I know, replying to a post that is a reply to my own post is really bad form. Makes me look weird, like jon banquer. Anyway, after looking at the groove drawing I know I am correct. But please feel free to correct me if I am wrong. And don't tell banquer that I am replying to my own posts, he might think I'm like him. Eric
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On Thu, 16 Apr 2015 13:29:50 -0700, etpm wrote:

Here, let me break up your string of self-replies (I don't know if you resemble Jon or not -- I've had him plonked for years).
Man, this is so damned confusing. Can you post a picture someplace? So you're making a grove in the angled part of a flathead screw, so you can slap an O-ring on there and seal to aluminum -- yes?
And the grove will go in normal to the face, which means that as the groove gets deeper, the radius of the grove gets smaller -- yes?
And you want to make a tool with JUST ONE TRY, to make it all work -- yes?
Dayum -- you're a brave man.
Why can't you just turn a straight section in the screw head, a hair larger in diameter than the ID of the O-ring, and just the right depth to accommodate the thing when the screw is tightened? Then you can just use a standard insert. It'll look like a shoulder on the screw head. Unless you have some compelling reason for the screw head to capture the O-ring (and piss off anyone trying to get one out without damaging it), wouldn't this meet your stated goals?
--

Tim Wescott
Wescott Design Services
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On Thu, 16 Apr 2015 19:24:27 -0500, Tim Wescott

The customer wants the O-ring to seat high up the taper. He wants the groove normal to the angled surface. I think though that I may be able to talk him into a groove that is normal to the screw axis. Eric
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snipped-for-privacy@whidbey.com wrote:

I think its a poor idea to make a round groove normal to the surface. A square cut like the crude profile drawing below will perform better. O-rings are not all that precise. cutting a groove that tries to match the shape of the o-ring invites trouble and won't work better than a square groove.
_______ _/ | / | | |
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wrote:

When O-rings are under pressure, a possible key to shaping the grooves is the way the pressure will be applied. In the job shop I worked in, we made ball-and-socket fittings for electron-beam guns that were used in vacuum chambers, and the O-ring grooves had to be a tapered wedge, from one side to the other. The vacuum sucked the O-ring into the shallower side of the wedge.
--
Ed Huntress

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On Fri, 17 Apr 2015 09:58:38 -0400, Ed Huntress

The o-ring will be sealing against low pressure-basically water a few inches deep. So the sealing cannot be helped by pressure or vacuum and instead must rely solely on compression. The customer would rather use stock o-rings that are available everywhere. Eric
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On Fri, 17 Apr 2015 09:24:09 -0700, snipped-for-privacy@whidbey.com wrote:
<snip>

</snip> o-ring is overkill -- use flat rubber washer for face compression seal.
--
Unka' George

"Gold is the money of kings,
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On Fri, 17 Apr 2015 12:12:48 -0500, F. George McDuffee

George-The customer wants an o-ring. I said this. Plus a rubber washer won't work because the screw needs to bear against the aluminum. If there is a washer in between then the countersink needs to be changed. And all the parts out in the field would need to be returned to be re-machined. If just the screw is modified then my customer can offer this screw to his customers if they want an upgrade. Look, I appreciate all the advice about how an o-ring is not the best solution but it is what my customer wants and in this case I agree. What I really wanted to know is if my method for finding the radii of the sides of the o-ring groove will work. Thanks, Eric
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Ed Huntress wrote:

I'm assuming the screws are going to be tightened, which means you can get much much more than enough compression of the oring. The trick will be to have the right amount of compression and design it so it doesn't tear, cut into or shred the oring when the screw is tightened all the way.
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On 4/16/2015 2:04 PM, snipped-for-privacy@whidbey.com wrote:

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.
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wrote:

I think he's trying to translate the theoretically conical circular relief below the outer cutting edge into a cylindrical relief he can more easily grind.
-jsw
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