Q & A: The Right Mortise/Tenon Fit
What’s the right fit between a
mortise and a tenon? I’m going
crazy trying to measure them with
a dial caliper!
You’re not alone. This question
has bugged just about every woodworker,
but the answer doesn’t lie
with precision instruments.
A crusty woodworker of the old
school once said,“If you need your
shoe to pound a tenon into a
mortise, it’s too tight. If it just drops
in by itself, it’s too loose. If you can
tap it in with your hat, it’s just right.”
Well, nobody wears a hat in the shop
any more, but you get the idea.
If the fit is too tight, your glue-up
will be a nightmare and the joint
may actually fail.Water-based glues
slightly swell wood, so a joint that’s
tight when dry becomes even tighter
when wet.Way too much pressure is
necessary to force out the air trapped
in a tight mortise. To make matters
worse, you may “starve” the joint of
adequate glue if it’s scraped off the
mating surfaces as the tenon enters
If the fit is too loose,the glue won’t
bond the tenon and the wall of the
mortise. White and yellow glues
shrink as they dry and at most they
can tolerate a gap about the thickness
of a sheet of paper. That’s more or
less the amount of clearance in a
joint that slips together easily by
hand (or hat!).
This story originally appeared in American Woodworker August 2001, issue #88.