Reinforcing a miter joint

[Bob Marino]

I am making a large (80" x 33" x3 1/2") frame for a mirror The wood is 3/4" poplar and mitered with biscuits. That's a lot of weight with the mirror. Should the miter's be reinforced?

Bob

[Todd Burch]

Bob, it depends on how you hang it.

For instance, if the frame merely wraps the mirror, and the glass itself is anchored to the wall, then no extra support is needed.

If the assembly will be screwed to the wall at different points around the frame to a wall, no more support is needed.

If the mirror will hang from a wire, or lean against a wall, or otherwise be portable, yes, more support is needed. Additional support can he had by glueing/stapling a piece of thin ply to the back of the frame. If let into the frame, you won't see it from the sides. 1" thick material might be better in this case. I would also double-biscuit the miters, regardless.

Todd

[Bob Marino]

Bob, it depends on how you hang it.

For instance, if the frame merely wraps the mirror, and the glass itself is anchored to the wall, then no extra support is needed.

If the assembly will be screwed to the wall at different points around the frame to a wall, no more support is needed.

If the mirror will hang from a wire, or lean against a wall, or otherwise be portable, yes, more support is needed. Additional support can he had by glueing/stapling a piece of thin ply to the back of the frame. If let into the frame, you won't see it from the sides. 1" thick material might be better in this case. I would also double-biscuit the miters, regardless.

Todd

The mirror sits in a 3/4" wide rabbet in the frame. The frame will be attached to the wall with a French cleat - I think!

Bob

[John Miliunas]

Bob, given the dimensions and material you're using, I think Todd's suggestions of a backer board let into the back of the frame. I think my main concern would be the 80" length of the stiles. In my figuring, they may have a tendency to bow somewhat. A backer board over the entire back would prevent that and give the whole piece more rigidity.

[Mark Singer]

I would cut splines on the tablesaw.....make a jig to hold the frame at 45 degrees


If it is to big....cut them with a router and a long straight or spiral bit in several passes

Another good way is to drill exposed dowels in the same or a contrasting wood. After the frame is glued you can drill on the corners and insert the dowel then trim flush.

[Norman Hitt]

Bob, I agree with Todd, John and Mark, and a backer board would probably be the best solution, but another option if a backer board is not feasable would be as follows. Bad as I hate Silicon, you could lay a thin bead of clear silicon in the corner of the rabbet and then bed the mirror in it making sure that it contacted both the face side and the edges of the mirror in the rabbet. When cured, it would hold everything in place and stabilize the unit.

Note: I do not propose this as the Best option, but just as Another option, and it would work well, but you would need to put your desired Finish on the frame Before attaching the mirror to preclude any Silicon/finishing problems.

[Keith Christopher]

I would agree with Mark, splines or dowels. the nce thing about dowels is that like mark suggested a contracting wood would make an intersting detail.

[Mike Cutler]

Bob. If it were me, I'd be using a Mitered Lap Joint. I'd epoxy the lap joint and peg the joint from the backside with dowels, thus leaving the face clear.
If you have already cut your miters, I'd dowel them from the outside of the stile piece thru the joint.and orient the grain on the dowel to hide it.
I'd probably still use a ply backer, even with either of these joint, 'cause that mirror is going to be heavy bro'.

[Chris Padilla]

Bingo to what Mike said: half-lap joints will provide the most area to glue with and you can go further and pin that with a dowel of the same or contrasting wood like Mark suggested...or hide the dowl like Mike suggested. Epoxy is also a great idea, too.