I have a question for those of you with more experience than me (which
is probably most of you). While watching NYW, the first part of the
Routers 101 episodes, Norm was extolling the vrtues of routers using a
small part of his infintesimal collection and he plowed a mortise using
a plunge router, a fence and a spiral bit. Nothing earthshaking, but
when he got through he tested the fit with a piece that he had made a
tenon on and he said that it fit great and that (this is the part that
threw me) there was some play along the long axis, (looked like about
3/16" to me), but that he wanted that so that he had some "room for
adjustment during final glue up". ????????? Now I'll admit that my
training on mortise and tenon joinery was far from extensive, but I
don't ever remember seeing anyone recomend leaving "play" along the
long axis. I was always taught that you made the tenon fit snuggly on
all four sides leaving just enough room for glue and to make the
mortise deep enough that the tenon didn't bottom out. Wouldn't "play"
eliminate some of the joint's strength along the long axis? Is this
just a Normism like his favorite "few brads to hold it while the glue
dries" or is this a common practice and my mortise and tenon training
wasn't just far from extensive but down right deficient? Thanks in
advance for your input. bc
You're right. Mortise and tenon joints are supposed to be snug all
around, otherwise they're *weak*. Norm had an opportunity to teach
novices a lesson by saying, "Oops, I made a mistake", but he didn't.
A little wiggle room is acceptable for adjustment and sanity. Two
faces of the tenon are glued to end grain, so there's no strength to
speak of there. The only way that the tenon's narrow sides would come
into play is if the face-to-face glue joint failed.
I personally don't see a problem with doing it either way, assuming the
tenon height isn't ridiculously long where cross grain movement would
be an issue, but I don't think the little bit of wiggle room would
affect the joint strength appreciably.
That being said, a joint subject to heavy stress and impacts should
probably fit tighter than the run of the mill joint. It would be an
interesting thing to test.
As a related question, does anyone have knowledge of the relative
strengths of dry-pegged mortise and tenon vs. glued? I've seen tests
on various glued joints, but never wet vs. dry.
Well really the mortise and tennon gets strength by the rather large surface
area that is glued on the wide flat sides of the tennon that is inside the
joint. That is where the proper fit is important concerning the glued
surface. The small amount at the top and bottom adds negelable strength by
comparison. With that in mind if you have some play at each end it makes it
much easier to dry test fit and take apart and most often the up or down fit
at the top or bottom of the joint is more important in relation to other
pieces of wood.
The surface area it provides may be small compared to the faces (maybe
not depending on the joint) but imagine a torque being applied to the
joint. The ends fitting tightly provides a mechanical stop to this
type of force, even without glue it won't come apart. It doesn't do a
whole lot to prevent the joint from being pulled straight apart
though. So depending on the application it can make a difference.
If the glued joint were flexible, I suppose that the torque could put
pressure on the top and bottom of a tight mortise. But the glue joint
is not flexible. How can the torque exert a force without failure of
the main gluing surfaces?
Good point. It probably doesn't even make any difference in most
However, as far as having wiggle room at assembly time goes. If
you've got room to move it around to where you want it then there's
also room for things to move out of place while you are getting clamps
on. I'd rather start with it too tight and get it so it can only fit
correctly aligned during dry fit. Otherwise it will never go back
together exactly the same once you've got glue on the sucker.
I did see Norm doing what the OP referred to, and if I remember
correctly he didn't square off the mortice or round the tenon, so it
probably looked like there was more room for it to move than there
was. I can understand that since the episode was about how to use a
router not how to make a M&T that he might skip rounding the corners
just to get on with it. Every other time I've seen him do it he
either squared the mortice or rounded the tenon, and I've never seen
him say anything about leaving space before either.
Having a little "adjustment room" probably isn't all
that handy in say a bottom stretcher of a trestle
table. But it sure comes in handy with the apron
to leg joint of a table. You don't have to be off much
in either the mortise or tenon or both to end up with
the top of the apron a little above the top of the leg
(something a block plane could probably "fix" or
worst case - apron BELOW the to of the leg. The
latter is not easily corrected The joint might
fit nice and tight - but the alignment of the parts
will be a problem.
As noted, it's the sides and glue that provide
the resists up/down movement and in/out
movement. The cheeks control rotation
Maybe this will help
Having a little wiggle room BEFORE the glue
starts to set is handy.
Six of one, half dozen of the other ... A little "wiggle room", upon which
the OP was remarking/accusing Norm of sloppiness, means the tenon is too
short, in height, for the mortise, or the mortise is too tall for the tenon.
... or maybe it's me who's missing something?
The mortise should always be a bit deeper than the tenon length to allow a
place for air and glue. Else, it will require some retightening of clamps
as they are slowly expelled, or rapidly, with some interesting sounds.
Since wood expands when wet glue is applied, the cheeks should be a touch
loose. The glue will keep the wood expanded or fill the gap.
Reason for the glue is to keep the tenon from withdrawing. The strength of
the joint against deforming is in the shoulders. As you know, standard post
and beam construction uses no glue, but draw bores dry tenons for pins to
keep the tenon from withdrawing and the shoulders in register. The static
load is borne on the long grain post, not the beam or stringer, where it
might creep or deflect over length.
Or, you could read the thread rather than quote the preamble.
The m/t joint is designed to create and maintain a right angle relationship.
The load is presumed to be on the virtually incompressible end grain of the
mortised piece, with the tenoned piece merely making dimension and square.
If you were planning a significant load on the piece with the tenons - a bad
move - you would be well-advised go to a bridle joint, which is designed to
bear such a load, and brace in the middle to limit deformation.
To sum up - if the joint is being used properly, doesn't make a rats ass if
the bottom of the tenon rests on the wood at the bottom of the mortise. It
should not be loaded in that direction anyway.
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.