Mortis & Tenon Sizing

[Mark Brubaker]

Are there any general rules to follow when determing the size of the mortis and tennons?

[Mike Cutler]

Mark.


There are some "thumb" rules for traditional M&T joints That I learned. They're not etched in stone, just a place to begin planning a joint.

1. The tenon is 1/3 the thickness of the mortised piece.Most folks default to 3/8" on 4/4 stock though
2. The width of the tenon is 5 times the thickness of the tenon, not to exceed 4" or the tenon is then divided. Dividing tenons does give more glue area,but it also relieves the amount of stress that a single long tenon could place on a board. Something to do with the leverage principle here.
3. The length of the tenon is 2/3 the width of the mortised piece. The longer the tenon, the stronger the joint .
3. Mortises are started at least two board widths away from the end of the mortised piece.

I posted these about a week or so ago in a thread.

The thread was actually a pretty good discussion on Loose tenons, integral tenons, and biscuit joints. this is the thread;

http://www.sawmillcreek.org/showthread.php?t=40464

[Mark Brubaker]

Thanks Mike, this is exactly what I was looking for!

[Mike Cutler]

Mark.

No sweat bro'. You're welcome.

Gary Rogowski(sp.) and Sam Allen have two excellent books on "classical" joinery. Well worth the $$$, especially the book by Sam Allen. His book devotes quite a few pages to the M&T joint, and all of the different styles, usages and techniques. Gary's book is excellent also. Lots of pictures.

There are a few factors that affect (or is it effect. I can never keep those two straight) the sizing of M&T joints. Material selection is a big one. Stress planes are another.

[Tom Hintz]

I have a mortising machine so when a new job comes along, I decide on the width of the tenon/mortise by which of my chisels looks right in terms of size to give the best balance between tenon and remaining wall thickness of the piece it goes into.
I also have given up on the ratios for the shoulder width, making the wide sides different than the ends. I make them all the same depth, with the exception of when one side is exposed as one some doors.