Burning Mortise question.

[bc]

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

[Chuck Taylor]

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.

[Frank Drackman]

I never leave anywhere near 3/16th inch but always leave a little bit of adjustment room in the long axis and depth. I believe that the strength from the cheeks and shoulders is the most important.

[RicodJour]

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.

R

[Leon]

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.

[Chuck Taylor]

I'll certainly accept that there's room for more than one opinion, but I've always figured that was the purpose of a good chisel and/or shoulder plane--even on the narrow sides of the joint.

[Leuf]

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.

-Leuf

[Leon]

I can see your point here.

The ends fitting tightly provides a mechanical stop to this

I still believe that the glue will be strong enough only on the wide side of the tennon. The joint that the OP was talking about IMHO would not have benefitted from a tighter fit on the narrow sides.

[RicodJour]

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?

R

[charlie b]

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

a little wiggle room BEFORE the glue starts to set is handy.

charlie b

[Swingman]

I doubt that it was 3/16, but even then with a stopped M & T joint a little "adjustment room" is nothing to worry about as long as the shoulders fit snug.

[fredfighter]

Precisely.

Glues are strongest in shear.

[Leon]

The biscuit joint uses the same principal. The thickness is important but the side to side play is not that important.

[Swingman]

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?

[Leon]

Then we both are missing something.

[George]

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.

[Swingman]

The "depth" of mortise not under discussion in this thread ... the length, yes.

[Leon]

Times like this a picture would be worth a thousand words.

[George]

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.

[Chuck Taylor]

I think I appreciate the replies at least as much as the OP. Time to give my own opinion a little "wiggle room".