Reinforcement of Rail/Stile Joint with Pocket Screws?

[Gator95]

Going to frame a 5' x 2' mirror (heavy) to hang on wall. Was going to use my rail & stile bit set that I also use for cabinet doors- so the mirror is basically a giant panel set into a frame. Planning on about 2.5"-3" wide rails and stiles, and the assembly would be hung on the wall by the top rail.

I don't trust just the glue joint with the rail stub tennon to hold this, given the downward sheer stress from the heavy mirror. Is there any reason I couldn't reinforce each rail-stile joint with 2 pocket screws?

Would drill the pocket screw holes first at the ends of each rail, then cut the sticking profiles afterwards. Then I'd clamp and glue up square, and once glue sets I'd drive in the pocket screws for reinforcement. They'll wind up going through the stub tennon I think.

Any reason not to do this?

[cabinetman]

I would think that good fitting and glued joints would be sufficient. I wouldn't use pocket screws. The two hole pilot for their size would, IMO, lessen the effectiveness of the glue joint. If anything I might pilot small holes from the sides (on the rails) and use long square drive trim head screws. They are much smaller in diameter than those used for regular pocket holes. The hole can be done like a blind nail hole, or since the hole is small, filler may not be noticeable.That way, the stiles (and the weight of the mirror) would be also supported by the direction of the screws. Done right you could probably get by with just one screw in each side.
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[LarryG]

That will be an excellent detail. The only reason I can think of not to do it would be if one objected to pocket screws from a "purist" standpoint -- an objection you, clearly, do not hold.

The access holes for driving the screws will be entirely in the rails, so the glue joint will not be compromised by those. The only glue surface lost will be at the holes for the shanks of the screw, which is miniscule, and which you'd have with any type of screw. I can't see how it matters whether the screws pass through the stub tenon ... okay if they do, but not a problem if they don't. Basically, the stub tenon becomes an alignment mechanism and the pocket hole screws do the real work of backing up the glue.

The stub tenons produced by cope and stick sets are not very strong, precisely because they are what their name says: stubs. They're too short to add much mechanical strength or additional glue surface. High-quality custom cabinets often have full mortise and tenon joinery on their doors for exactly that reason. A 5'x2' mirror is pretty heavy; I think you're correct to be concerned.

[poolhound]

Glue joints are usually much stronger than we think although this is an end to long grain joint. I know from experience that LG to LG (even without reinforcement) is in fact stronger than the wood, I have tested a few times and it never ceases to amaze me.

Unless you are wedded to using your R&S bits you may consider using either loose tenons or a large slot and spline joint. either of these would dramatically increase the glue to glue surfaces of each joint and should be plenty strong enough.

You can still route the profile of the frame and get the same look that your R&S bit would have produced. The mirror can simply sit in a rabbet.

[JR]

And yet again another approach...

Mount the mirror to the wall, then mount your frame over the mirror. This would take the whole argument about the strength of the frame out of the picture.

JR

[cabinetman]

Jon is right about the glue joints. Maintaining the wood to wood contact for the glue joint, without displacing any of it for pocket screws makes sense. If you didn't want to use your R&S bits you could do an M&T joint, which could still allow a profile to be done to the frame and a rabbet to the back for the mirror.

If you didn't want to go through the trouble of an M&T joint, or use screws, you could assemble the frame using your R&S bits. Then you could just drill a dowel hole from the outer frame stiles into the rails, and insert a long dowel, and sand the end off flush with the stile. It would be like a decorative detail.
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[JimD]

It is not terribly heavy but we have a full length mirror that covers a built-in jewelry chest. I would guess its 14 inches by about 48 inches. I made it of 5/4 maple because I wanted to put a trim moulding in the back side to hold the mirror and a piece of door skin plywood (that is visible when the jewelry chest is open). Anyway, the joints are miter joints reinforced with a #20 biscuit. I would think stress is a little greater because the mirror frame is also a door. The surface area of the joint is less than you would get with a cope and stick bit. It works fine.

I have made raised panel doors up to around 30 inches square with 2 inch frames surrounding full 3/4 oak raised panels. The little 3/8 joints you get with the cope and stick bits held fine. While the surface area is not huge, the fit is good and in my experience they hold.

I do not like to use pocket screws in any finished furniture if they could ever be seen. Just my preference. I do not think they are necessary for what you describe and I would not use them just because I don't like the appearance. I guess once you hang the mirror you are unlikely to take it down and see them, however. Probably either with or without them will work.

Jim

[Black wallnut]

IMHO adding screws will not be necessary but with pocket hole joinery where you can conceal them in the back I do not see how it would hurt. It has been my experience that good glue joints never fail. The joint is much stronger than the wood fibers arround it. The increase in glue surface from a cope and stick joint is substantial. I'd not give it even a hint of a second thought as solid hardwood panels also can get rather heavy.

One concern I would have however is that you should be careful if you do add screws to not over tighten them and starve the joint for glue; in that case the joint would be much weaker than glue alone IMHO.

[cgallery]

Don't see any reason why it wouldn't help in the event that a glue joint did fail. Why not? Nobody will ever see the screws.