Monday, February 26, 2007

A Better Joint


The round mortise and tenon is one of the easiest joints to make, it is also one of the worst when it comes to gluing. Glue only works to bond wood with strength when the long fibers meet long fibers (as opposed to the endgrain). The round mortise is almost all endgrain making it a poor glue joint. As seasonal movement causes the tenon to shrink and swell, the glue bond is stressed and in time will fail. The through tenons that we use in chairs are wedged, to flair the tenon and create a tighter fit. One day last winter, I looked at a wedged tenon in one of my chairs around the house and noted a gap along one side of the wedge. It reminded me of something I read in Bruce Hoadley's book Understanding Wood. He noted that by splitting the tenon, we supply a plane of failure, which relieves the stress on the glue around the tenon. When the tenon shrinks, instead of breaking the important glue bond around the tenon, a harmless crack appears next to the wedge. So now, whenever I glue in a wedge, I am careful to put glue only on one side of the wedge (plenty to hold the wedge in place). The unglued side of the wedge is free to open and close as the humidity changes, a far superior result to the joint failing!
So, save some glue and make a better joint.

8 comments:

Colorado Chris said...

Pete-
Just wanted to say "thanks" for your blog - lots of great advice and inspiration. I have the day off, going to take your advice, and work on sharpening up some turning tools.

Anonymous said...

Beautiful chair.
The seat is Tulip Poplar, right? It has a beautiful finish, and it matches the rest of the chair. Amazing! What finish did you use on the seat to pop the grain and get that warm color?

Peter Galbert said...

The seat is actually butternut. The Fingerlakes region of New York has some gorgeous butternut with reddish tones that goes great with cherry. Rare, but worth it.

nielscosman said...

Hi Peter,
I love your blog and have found it incredibly informative and inspirational!
I have a question about wedging a round tenon. I am making a bench with through round tenons into the seat. I was wondering should i drill a little relief hole in the tenon at the bottom of the kerf so as not to split the dowel?

Peter Galbert said...

Niel,
I don't think that the hole is necessary. I simply cut a kerf about 2/3 of the depth of the mortise into the end of the tenon and then wedge it. The mortise itself should keep the joint from splitting. Good luck and thanks,
Pete

nielscosman said...

Hey Pete,
Thanks for the tip. I spent the afternoon making dowels, pegs, wedges. Tomorrow comes the glue!
Cheers,
Niels

Dylan Green said...

Hi Pete. Great blog! so much info.
I'm making a bench with through round tenons as well and wonder if the mortise is reamed as to allow room for the wedge?

thanks,
d

Peter Galbert said...

Dylan,
it depends on the joint. For tapered joints in the seat, I don't widen the holes, but on some others I widen the side where the wedge is driven to create an hourglass shaped mortise and a locking joint