Help cutting an O ring grove

[mike-sid]

I'm in the dreaming/planning stage of a little project. Even though this is for a gun, it is just basic metalworking I need advice about. A little back-round on my project. I have an O/U 12 gauge shotgun, and I have .410 "sub-gauge" full length tubes. (Briley Champion ultralight's). The .410 tubes slide in and have a final interference fit at the chamber end from a mylar "button", at the muzzle end the tubes are a close, but loose fit. This is how they are supposed to fit, and work without a problem.
What I want to do is "cut" a grove at the end of the tube on my lathe and place a high temp O-ring to snug up the fit, to stop the rattle from the loose tubes. I've been using a couple layers of a metal tape, but I want a neater looking solution.

These dimensions are approximate, I've yet to take final dimensions, but will give us the idea of the scale of my little project. The Tube end I want to grove is .684" The O-ring I might use is .75" OD .625" ID by .0625. I plan to grove to the approx depth of .046875 leaving .015625 exposed.

What would be the best grooving tool? Would a round profile be best? I have some Valenite grooving tool holders I got from a friend that I can buy an insert for. I want the O-ring to stay seated in place during insertion as best as possible.
Thanks Mike

[Roadracer_Al]

No, don't use a round profile. An O-ring needs a square land in order to "squish" into the corners so the mating part can slide onto it. Otherwise, you'll shear the O-ring in half when you assemble it. Ask me how I know this. I dare ya. :)

[Mayhem]

(04-11-2016, 12:47 AM)Roadracer_Al Wrote: No, don't use a round profile. An O-ring needs a square land in order to "squish" into the corners so the mating part can slide onto it.

I did not know that - thanks for enlightening us Al.

[Roadracer_Al]

As the saying goes... experience is what you get when you don't get what you want.

[JScott]

Another related saying; Good judgement comes with experience but a lot of experience comes from bad judgement!

To get back on topic, somewhere in my collection of reference material I have a design guide for O-rings.  I'm sure there is also something out on the web.  Your application is a little different in that normally O-rings are used to seal instead of position.  The normal design principles should get you into the correct "zipcode" though.

[f350ca]

A 3/4 O ring seams large unless the sleeve fits really loose. The O ring should not be larger than the ID its fitting, you size the ID of your groove to stretch the O ring out and give the proper crush inside the bore.

[TomG]

Not exactly sure what you are cutting the O-Ring groove in, but if it's part of the barrel or chamber, you'll have to be careful not to compromise the strength. Al is also correct about the groove having to be square and allow enough room for the O-Ring to move. To seal, they need to move with the fluid they are being used to contain. You should also make sure to put a small radius in the corners to keep stress risers from forming, if it's in a stressed area. The Parker website used to have a lot of useful info on O-Rings.

Tom

[mike-sid]

First thank you all for the informative replies.  Here are some numbers from my measurements.   Bore of barrels .736", part on tubes I want to machine the O-ring grove in is .717" .    The attached photos are of the tubes. They are .410 bore. Actual bore is .411" the smallest diameter on the tubes is .50"    So in light of all the info provided so far, I'm not sure of the optimum size for the O-ring and it's associated with of the grove.  When this is known, I'll turn down an AL bar and test out my O-ring with that.     The red part of the tubes are AL, and the shiny metal chamber sections are stainless. Looking at the tubes from the muzzle end, I want to install the O-rings on the second large diameter section.   The end view from the muzzle end show the changeable choke tubes.  When I do my part the clays turn to dust on the skeet field!

[mike-sid]

I do have a nice Valenite 2mm wide insert to groove, if it would work out

[TomG]

Since you are not using them for sealing, I would suggest choosing the next smaller O-Ring and size the inside diameter of the groove so you end up with the desired amount of O-Ring sticking out. Just remember to use the actual size of the O-Ring in your calculations because they run larger than nominal. McMaster Carr lists the exact sizes. You'll also want to start large and work your way down to get the right feel on the fit. These things are difficult to calculate. I use O-Rings all the time for things other than sealing when I design. The orange colored silicone one's that McMaster sells are quite durable.

Tom