Mortise and tenon question

[Ray Myers]

Hey folks, this is my first post, so I’m sorry if it’s in the wrong area. I’m making a few kitchen benches for the table I’m also making. The benches will be around 70 inches long by 12-14 inches deep and around 18” tall. Everything will be made from pine, painted. Stained plywood tops. I’m planning on using 1 7/8” or 2” square stock for the 6 legs, and I’m going to use mortise and tenons for the rails that join them(one at the top, one near the bottom). Also plan on using kind of lap/bridle joints on the stretchers to connect the frames. See pictures of table frame, if they show up. Couple of questions... First, using that size stock for the legs, do y’all think it would be ok to use 1/2” thick tenons, or do I really need to go to 5/8”? I know that it’s closer to the 1/3 rule, but is that 1/8” that big of a deal? Honestly, I feel just doing one pass with my plunge router would be easier than plunging in one direction, plunging the other, then cleaning up the bump in the middle. Second, even though I like the idea of using thicker legs, it is necessary to use legs that thick, or could I probably drop it down to 1.5” square? I do have some heavy friends, and I’m no lightweight myself(I’m around 190, but I have friends that are around 400 lbs). Anyways, all thoughts/ suggestions are welcome. Thanks y’all.

[lowell holmes]

I'm not really qualified to answer, but I normally make tenons fat. Why don't you make a test joint in a scrap and break it.
Then you would know what to do.

[Mick Simon]

Your description says that the legs will be 1 7/8" to 2" thick, but not how thick the rails will be. They look thinner than the legs in the pics. If that's the case, the 1/3 rule should apply to the thickness of the rail, not the leg. For instance, if the leg is 2" and rail is 1 1/2", the tenon should be 1/2" - 1/3 of 1 1/2". The tenon should be cut from the center, not to one side. The shoulders created on each side give the joint additional mechanical support to resist stresses.

[glenn bradley]

Mick is steering you right in that the 1/3 rule would apply to the thinner of the pieces being joined. A 1/2" tenon and inch or so long is a solid joint but, your material is soft. I would say 1/2" is fine to ease the making of the mortise. I would make it a good length and height to help your softer material gain strength at the glue joint.

[Ray Myers]

Ok, thanks y’all. The picture is actually of the table, which is already glued and pegged, so no chance of going back on that. The leg thickness of the table is just shy of 3 inches, the rails are 1.5 inches thick, with a tenon 7/8” thick and 1 7/8” long, around 2.5” tall. Still plenty of shoulder, I think, I HOPE anyways. This was my first time doing mortise and tenon joints, and I had also seen that it should be 1/3 the thickness of the piece being mortised, so what you guys said is kind of troubling. You guys reckon it’ll be ok? In regards to the bench leg tenons, I was definitely going to do it in the middle, since there isn’t going to be another tenon intersecting with it, I was planning on around 1 3/4” deep and around 2.5” tall. Any other thoughts, especially about the table joinery are very much appreciated.

[Brian Holcombe]

If the mortises are centered in the leg, and the leg is thicker I would use 1/3 of the leg. If it’s biased toward the inside of the leg I would use the stretcher as the basis for thirds.

[Ray Myers]

That’s what I was thinking when I was building it, thick tenon with thick walls. Thanks for the input.

[Mick Simon]

A really good primer on mortise & tenon joint strength was published in Fine Woodworking #259. It was written by a wood researcher (Dan Bullock) at Purdue University and furniture maker. Interesting findings were that the width (not thickness) contributed the most, followed by length. That makes sense when you consider the glue surface those two factors contribute. Also, the way you glue makes a huge difference in the strength of the joint. Always apply glue to both the mortise and the tenon. Applying to both vs tenon only was over twice as strong, vs mortise only it was almost 50% stronger.

[Ray Myers]

Thanks for the info! I’ll have to see if I can check out that issue, they have a 14 day free trial after all. It definitely does make sense to think that width and length make up for most of the strength of the joint. I’m pretty sure the 1/3 rule isn’t about the strength of the joint per se, but more about not weakening the material, if that makes sense. I also figured about the way things are glued contributing strength, only because I saw a video that Robert Lang had done about gluing m&t joints with a piece of acrylic acting as one of the sides. The amount of glue that remains in the joint when put on both and spread around versus tenon only is amazing.

[Tom Bender]

Ray, I'm a little concerned with longitudinal forces on the top of your table causing your joints to fail. This will happen when your table is misused, you and your girlfriend...

Let me suggest a piece of plywood between the top stretchers about 20 x 20. Then add another panel from that to the lower stretcher, think sailboat keel.

For the benches consider the same type of failure. 400 pound people don't fare so well in seating collapses.

[Ray Myers]

Hmm, that IS cause for concern. DO NOT UNDER ANY CIRCUMSTANCES mention “girlfriend” in front of my wife! Seriously though, I suppose I should have said that all of the stretchers do a shoulder on the bottom and both sides, sort of making them “tenons with open top mortises”. I know through tenons with wedges/keys would probably have been the way to go to prevent that sort of racking, but I thought my solution would stop a great deal of it versus the pocket screw approach I’ve seen on many diy sites. Again though, this is my first build using this type of joinery, so I may be wrong there too, and if I am please continue to advise. There IS a sheet of plywood notched out around the 4 posts that is screwed to the stretchers and rails, but that is mostly to make the top more solid across the long gap. The extra ply was not a design I had planned on for the benches due to the extra legs, but the rest of the joinery I had planned on remaining the same. As said before, all thoughts are welcome.