Monday, September 7, 2009

Rebar for Mitered Frames

A neighbor asked for a favor: she wanted a frame for a bathroom mirror, but the mirror could not be removed from the wall.  Effectively she wanted a frame applied to the mirror's glass since there was next to no clearance on the sides.

I created the frame from quartersawn sapele just 1/8" wider than the mirror so the existing inch of clearance to the walls would still show.  This will be applied with a bead of silicone directly to the glass.

The trick was she wanted a mitered frame.  Since it was to be applied, it was really just the top portion without the typical back.  So rather than the frame being 5/8" to 3/4" thick with a groove for the glass and a beveled front, all I had was a 3/8" thick top portion that was beveled.  There's no room for a miter key, dowels, etc.  Sure, with 3/8" thickness, a 1/8" thick kerf cut in the side would help, but the bevel started close to the outside edge so you'd only have a key going in perhaps 1/4"; not terribly helpful.

This is what I decided to do:

I cut a shallow recess in the back of the frame in both pieces at the mitered corner.  I also gently chamfered the edges of the miter.  I glued up the frame with regular PVA glue, but naturally the glued miter is very fragile (though I was surprised it was more resilient than I thought it would be).  This glued up corner was enough to let me do the 'rebar' glue-up.

The piece of 'rebar' is a corrugated metal fastener.  This adds a little thickness, but greatly increases the surface area of the metal.  I rubbed them down with P320 sandpaper, placed a little epoxy in the recess, pressed the 'rebar' into the magic goo then topped it with enough epoxy to bury the rebar and stay flush with the wood surface.  Naturally some epoxy flowed into the channel formed by the chamfer.

The result is a joint held with epoxy across its length and with a fairly large island of epoxy with embedded metal holding it together.  This joint is going nowhere.  Since the strong epoxy island is centered, it is capable of keeping the miter from opening from either side.

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