Tenons and wood movement...

[peter slamp]

Is there a certain length that would make wood movement of a cross grain tenon negligible? I guess I'm still having a bit of a difficult time with the whole wood movement concept. Is it normal for a noob to be irrationally paranoid about such things? LOL. Thanks for everyone's continued wisdom! Peter

[Mike Cutler]

Is there a certain length that would make wood movement of a cross grain tenon negligible? I guess I'm still having a bit of a difficult time with the whole wood movement concept. Is it normal for a noob to be irrationally paranoid about such things? LOL. Thanks for everyone's continued wisdom! Peter

Wood is, for lack of better explanation, a bunch of "natural straws/fibers" that can expand and contract with the available amount of moisture in the air that they can absorb, or release based on the humidity of the enviorment that the wood is kept in.

In a Mortise and Tenon joint there are a few factors that need to be addressed to create a strong long lasting joint. The width of the tenon being one of them. A tenon that is too wide can expand and contract with changes in seasonal hunidity, and more importantly a tenon that is too wide has the ability to exert too much force on the mortised member.
There is a rule of thumb, just a rule and not something etched in stone.
The Tenon should be 1/3 the thickness of the mortised member.
The Tenon should be no wider than 5 times the tenon thickness.
The length of the tenon should be 2/3 the width of the mortised material.
The mortise should be no closer to the end of the material than the materials thickness.
Tenons that need to be greater that 5 times the tenon thickness should be divided into equal widths.
Remember that these are just rules of thumb and sometimes may not be practical.
For example on 4/4 stock the rule would make the tenon thickness 1/4". Most people use 3/8" sucessfully

[peter slamp]

So by following those rules will 'usually' safeguard yourself from movement problems?

[Bob Smalser]

So by following those rules will 'usually' safeguard yourself from movement problems?

Not entirely. The crossgrain glue joints often break after a number of decades. That's why it's important to use glues that are repairable.

[Ian Abraham]

The reason glued tenons work despite being a crossgrain glueup is because they are relatively small. You can glue up 2-3" crossgrain joints and the flexibility of the wood and glue will allow for a little bit of movement without anything 'bad' happening.

If you try it with 12" crossgrain glueups you will get all sorts of havoc as the movement forces things apart.

Exactly what size you can get away with depends on wood species, climate variations and how long you expect it to last :rolleyes:

Where really big mortise and tenons are used, like timberframe buildings or big trestle tables the joints aren't glued at all. Just fitted snug and pinned or wedged so they can move a little.

Cheers

Ian

[Mike Cutler]

So by following those rules will 'usually' safeguard yourself from movement problems?

Not entirely safeguard, but move you towards the conservative end of the envelope.
Inside of a glued M&T joint there are a few things going on. The end grain is being sort of sealed. As the end of the tenons absorbs glue it "fills the fibers up" so to speak. This makes the changes in humidity less of an effect. It also can lead to a glue starved joint, but that's a different problem.
The rest of the tenon is coated in glue, so once again the changes in humidity will have less of an effect on the tenon portion only.
If you kept a single tenon within the rules of thumb. Wood movement, in the form of expansion and contraction will be the least of your concerns. If the wood attempts to "cup" inside the mortise due to grain structure and orientation. That's a different problem also. This is why the thickness of the tenon material in relation to the thickness of the mortise material is of concern.
Plan the joint for strength and mechanical reliability in the abscense of any adhesive. All glues/adhesives will fail in time.
Bob's thoughts on repairability should also be part of the planning process.

There are a number of good books on joinery available. My favorites are "Wood Joiner's Handbook" by Sam Allen, and Tage Frid's "Tage Frid Teaches Woodworking" Book 1.

Sam Allen's book goes into a lot of detail on joint construction, failure, and repairability.
Tage's book(s) doesn't cover as many joints as the book by Sam Allen, but those that he covers, are covered in depth, with a lot of pictures and explanations. If you get his three volume set it comes with this cool little video. Kind of a home quality video, but a very nice video with a series of vignette's on different topics.
I recommend both of these books highly. They take alot of the "mystery" away.