Strength Of Mortise And Tenon Joint

[Wiley's Woodworks]

I'm building a heavy duty woodworker's workbench. The plans I'm most closely following call for mortise and tenon joinery on 1 3/4" thick rails for the under frame. Unfortunately the lumber I have to work with didn't mill out that thick. I've got to slim down the dimensions of the tenon pieces to fit the boards.

Because it's a workbench I want the joinery to be as strong as possible. Does the strength of a mortise and tenon joint come from the tenon or the shoulders tightly butted up against the mortise piece? Should I slim down the tenons and keep the shoulders the same width or narrow the shoulders and keep the tenons the same thickness? I will be draw pegging every joint with two 3/8" white oak pegs per joint.

 

[JNCarr]

I would maintain the 1/4 - 1/2 - 1/4 ratio. Doesn't have to be exact.
To answer your question, there are several "strengths" to consider - vertical load, horizontal load, axial torque, radial torque. You can emphasize one at the expense of the other(s). For example if you were really worried about vertical load, you'd make the tenons oversized - but this would sacrifice horizontal load and even more so radial torque. The 1/4 - 1/2-1/4 is a good all-around solution.

 

[Gofor]

For discussion purposes rather than advice:

The shoulders provide the resistance against the joint racking. The tenon provides the tension to keep the joint tight. The species of wood also plays a part, as softer woods can compress on the end grain more readily. However, softer wood also will have less strength in the tenon. If I were using pine, I would use a 1/3 thickness tenon as long as I felt it will be robust enough to not tear out when I draw bored it. If not, a 1/2 thickmess tenon. If you are using a hard/tough wood like oak, then you can probably get away with a narrower shoulder.

 

[Bill Clemmons]

Don't know about the experts, but as a rule I usually make the tenon about 1/2 the thickness of the work piece, as Joe said above. Ex: For a 1" thick rail and stile, I would make the tenon 1/2", leaving a 1/4" shoulder on each side.

In addition to the pegs, will you also be gluing the joints? If so, the glue should form a strong enough bond to make the finished joint almost as strong as a solid piece would be.