Gluing Haunched Double Mortise & Tenons

MJB | Posted in Gary Rogowski on May 7, 2007 12:58pm

I am in the process of making a bed frame out of American White Oak.

The bed rails are 5 1/2″ wide and are to be joined to the legs with a double mortise and tenon joint with a haunch between them. This is to allow a central bolt fixing for the long side bed rails.

Question is: I’m unsure how to glue the double M & T joints – I’m tempted to glue BOTH tenons but NOT the central haunch but will this provide enough allowance for any movement of the bed rails??

I am also considering adding some pegs to further strengthen the joint – should I peg both tenons or just one??

There seem to more and more srticles advocating the use of such joints but I have yet to find any authors who mention how best to glue up such joint!

thanks in advance
Mike

Edited 5/7/2007 5:59 am ET by MJB

Replies

user-148830 | May 11, 2007 04:25am | #1
Mike,
I've glued both tenons in situations like this and have had no troubles at all. I do put a little saw cut in the haunch to allow for movement. Or you could glue up the middle haunch instead and leave the ends, both ends of the tenons, to float. Glue up maybe half of each tenon then.

As for pegging, there is some controversy about whether a peg hole introduces a place for splitting as a tenon expands and contracts. I think it's just fine to peg your tenons however, both tenons. A round hole will spread pressure out around the hole.

I was talking to a class of mine one time and I mentioned that I put a hole, about 3/16" in diameter, at the bottom of my wedge slots for my wedged tenons. My assistant for that class was this lady who listened politely to me and then said, "Oh yeah, fracture mechanics. I'll bring in a book on it tomorrow." Turns out she was an engineer who had studied this stuff in school. And it also turns out that a hole at the bottom of a crack is a time honored method of stopping cracks from growing, in bridges, in airplace wings, etc.

So the idea of a round hole and a round peg introducing problems doesn't really make sense to me. Now a nail, an octangonal peg or even a square peg could very well do so. I avoid those options. Peg em both. Have fun.
Gary