Joint Strength

[steve-norrell]

I am planning to build a hanging mirror for a daughter (See picture) based on plans in Woodsmith magzine. My question has to do with the use of mortise and tenon joinery vs. dowel joinery. I know that M & T joints are very strong, but I have problems with them.

As seen in the picture, the rails are attached to the stiles with mortise and tenon joints. Unfortunately, although I have tried several times to make M & T joints with Forstner bits, chisels and dado blades, they always come out looking pretty lousy. I do not do enough of these to justify building or buying a M & T jig.

The alternative would be to use dowels, three or so on each side of the top rail and four or five on the bottom. I have used dowel joints on several occasions and they usually come out pretty nice, a lot better than my attempts at M & T joints.

My reasoning is that since the weight of the mirror will be pushing down, the dowels will add considerable strength to the joints supporting the bottom rail. Further, the dowels will be perpendicular to the force exerted by the mirror. I believe that four or five dowels on each side would be more than adequate to support the weight of the mirror.

I could also use pocket screws for added strength.

Critique, comments and suggestions are, as always, most welcome. Regards, Steve

[cgallery]

Dowels would be plenty strong for that application.

It is easier to get a strong dowel joint that a strong M&T joint, IMHO.

If you don't have a good dowel jig, I suggest the Rockler jig:

See the 2nd post here:

http://www.cgallery.com/smf/index.php?topic=130.0

For more information on using the Rockler jig.

[Black wallnut]

I agree that dowels would be enough. I would consider using a rail and stile cutter to do this as another option. What may matter more than joinery tech is attachement to wall or door technique. If it will be just screws through the frame placing the screws at the top and bottom in the rails will render frame joinery almost meaningless.

[cabinetman]

I would use half lap joinery. There is a lot of glue surface. You might also think about hanging from the stiles if possible.
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[phi1l]

ya, dowels will work fine, but you should figure out a way to do decent mortices

[Black wallnut]

I would use half lap joinery. There is a lot of glue surface. You might also think about hanging from the stiles if possible.
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Why? To do so would demand strength in joinery. If your lower joints fail the mirror ends up on the floor.

[crokett]

Why not just the pocket screws? Plenty strong and I would think that for the back of a mirror frame they would do just fine. I've also seen where someone did screws through the stiles into the ends of the rails, then hid the screw heads under fake through tenons, but that might not be the look you want.

[cabinetman]

Why? To do so would demand strength in joinery. If your lower joints fail the mirror ends up on the floor.

Why? Because half laps would be the strongest joint. With the size of the bottom rail it would not fail.
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[Bill in Buena Park]

How about pegged half-laps, Greene & Greene style?

[Black wallnut]

"With the size of the botom rail" cross grain wood movement may become a problem but my suggestion does not accout for that. 7"+ is a bunch for cross grain.

[cabinetman]

"With the size of the botom rail" cross grain wood movement may become a problem but my suggestion does not accout for that. 7"+ is a bunch for cross grain.

It would not be thick as a half lap, and would not be problematic. A negative philosophy can be conjured for most any joint.
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[steve-norrell]

If it will be just screws through the frame placing the screws at the top and bottom in the rails will render frame joinery almost meaningless. . . . With the size of the bottom rail cross grain wood movement may become a problem but my suggestion does not accout for that. 7"+ is a bunch for cross grain.

Thanks for the input. I will most likely go with the dowels and possibly add pocket screws in the bottom rail.

Incidentally, I will be making the piece slightly wider than the plans in the magazine so it better fits the space planned for it. The distance between the stiles will be 16" rather than the 13 1/2" shown in the plans. Of course, the mirror will also be slightly wider (and heavier), but I think the dowels will still offer adequate support.

Also, the diagrams in the magazine show the wall hanger mounted on the top rail. I had already planned to use two hangers, one on each stile. I could also use a short Hangman-type of hangers that would be long enough to screw into both the stile and top rail.

As to expansion, I was under the impression that wood tends to expand along the grain so movement problems would be minimal because both the top and bottom rail would expand about the same amount. Am I wrong?

Thanks, regards, Steve

[Black wallnut]

"As to expansion, I was under the impression that wood tends to expand along the grain so movement problems would be minimal because both the top and bottom rail would expand about the same amount. Am I wrong?"

Glad you asked.. yes you have it backwards. Wood movement with the grain is almost none whereas across the grain can be quite a bit depending on humidity swings and species of wood as well. So if yu use a two dowel set up on the bottom rail and your stock is wet, as it shrinks or drys it may split. If your stock starts out dry and swells with time it could shear off the dowels. For 7+" it might be worth considering wood movement. Last I heard Alaska has large humidity swings summer to winter.

[phi1l]

Agree, you may want to consider replacing the bottom Rail with 2 rails & a small panel.

[steve-norrell]

Thanks, phi1l and wallnut

Good points, indeed. And thanks for the correction about wood expansion. I should have known that but I do now.

The mirror is destined to end up in the Willamette Valley near Eugene, OR, and we all know it rains at least twice an hour there. So, humidity changes are an issue. The two-rails and panel idea is a good one.

Back to the drawing board for some modest changes.

Regards, Steve