I would like to know how wide (across the grain) a tenon can be before wood movement would weaken the glue joint. What if quartersawn wood is used?
(Furniture Making Forum)
From contributor J:
On wide joint you don’t use glue. If you do, only glue up the middle of the tenon. Use pegs as in breadboard ends. The tenon needs to be cut narrower than the mortise to give room to expand.
There are no pegs involved. It looks good and after 15 years of having one in my home I can only detect the smallest splitting in the center of the table. So I’m guessing 3-4" is the width you seek, but that is for breadboard purposes. I have always wondered about passage doors with full tenons.
A 6-10" rail glued to a stile (even with only a 3-4" tenon) will likely show some cracks during heating season, especially if there is a center, vertical stile. The good question is how wide can you go with mortise and tenon construction before you go to wedges, staves (with glue), etc. and eliminate straight glue.