Geometry question

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.

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

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

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

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?

Figure 5 shows how O-rings seal hydraulic fittings.

-jsw

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

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?

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.

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

I haven't tried very hard to follow all of this, but is he talking about grinding a circular or conical side relief on the tool? Because face-grooving tools typically are ground with a straight relief all around.

Unless he really can exploit some extremely small advantage by making the reliefs as compliant to the grooved shape as possible, there doesn't seem to be much of an advantage to making it so complicated.

Or maybe I'm missing the point -- no pun intended.

Firstly, you don't need to use carbide to machine 304, you can do it with hi-speed tooling. Just slow the cutting speed down.

That's what I thought too . Like clearancing the trailing edge of trepanning cutter , but for a cone . IMO the easiest way to do this cut is with the compound angled to let you feed the cutter perpendicular to the underside of the screw head . If you're careful grinding that tool , the cutter can deburr the cut just as it comes to depth .

ACME thread cutting insert?

[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.

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.

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

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.

_______ _/ | / | | |

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.

Might be overkill. What sort of pressures are involved? What are you sealing? Was a flat rubber/elastomer (urethane?) washer evaluated? How about a combination seal and retaining compound like RTV or Loctite?

If the O-ring groove is mandatory, how about a hollow mill? with the correct geometry? Given the problems in holding/ handling you might be better off making the complete screw from bar stock, although slotting the head may be a challenge. Do you have a CNC with a bar puller? How long is the thread? Style of head/drive?