Dining room table mortise and tennon

[Craig Shipman]

I am building a cherry dining room table that is 84.5” x 34.5”. The legs are 3 ½ in.² and will be turned. The rails are 7/8”x 4”. I am at the point of mortising the legs. I am doing a traditional mortise and tenon using a mortising machine. I know rails are typically set back around 1/8 of an inch but is there any reason not to set them back ¾”? My thoughts are this will increase the strength by providing greater wall thickness between the tenon and the front of the leg while still allowing for ample tenon length. My second question is, is there any reason for the mortise not to be open at the top? In other words, the mortise would be a three-sided box open at the top as opposed to a closed square/rectangle. The tenons will be ½ inch and mitered where they meet. Thanks in advance for any help.

[Cameron Wood]

I don't think set back 1/8" is typical- lots of variation, but as the rails move in, the tenons get shorter. I would not make the mortises open at the top- loss of strength.
I made a similar table a couple of years ago, and added glue blocks at the leg/apron joints.

[Mark Rainey]

Craig, I am not sure you can call 1/2 inch ample tenon length. With a seven foot table and 3 1/2 inch square legs you can get a lot more tenon length which is more important than the wall thickness between the tenon and front of the leg.

[Cameron Wood]

Craig, I am not sure you can call 1/2 inch ample tenon length. With a seven foot table and 3 1/2 inch square legs you can get a lot more tenon length which is more important than the wall thickness between the tenon and front of the leg.

I think he was talking about tenon thickness- "1/2" tenons and mitered where they meet"

[Richard Coers]

I would definitely not leave the mortise open to he top of the leg, especially since you are concerned about max strength in the apron tenon.

[Craig Shipman]

Cameron is correct. The mortise width is ½ inch. If I use a 3/4” setback, the long face (they will be mitered in the center so one face will have more surface area than the other) of the mortise should be 2 ½ inches. Everyone has been unanimous in using a traditional closed mortise as opposed to an open one. My understanding is the primary strength of a mortise and tenon is the glue surface and tightness at the mortise tenon interface. I may have done it incorrectly in the past but my mortises have always been slightly elongated to allow wiggle room for assembly and disassembly. The tenon registers on the bottom of the mortise for weight-bearing, which leaves a small space on the top. The other factor is that the rail is anchored on the opposite end by an identical mortise so there should not be any leverage or torque across the top of the mortise as you might expect with a cantilevered assembly. I mention this purely for understanding. I am not trying to be argumentative. I cannot count how many ideas I have had in my life, one or two of them have been good. I greatly appreciate your comments. I don’t see what I’m suggesting being done, or can find anything on the Internet to support it, but I’m having a hard time understanding why. I greatly appreciate everyone’s comments.

[Mark Rainey]

Good question Craig...one thought is the open top will increase the risk of the leg splitting, especially if the table was dragged across the floor. If your tenon was a bit oversized and you forced the issue, once again an increased chance of splitting. Of course, the practical reality is probable inconsequential.

[John TenEyck]

What you are creating is a form of bridle joint. They are incredibly strong. It's also very much like cope and stick joinery for doors, which has proven itself for a very long time. I would have no qualms about proceeding that way. You are increasing the sidewall glue area by doing it, always a good thing. Will the wracking resistance suffer. Only if the joint fails.

I use loose mortise and tenon joinery, with rounded end mortises, and some slop for adjustment. I've never had one fail. It's the sidewall glue area that carries the load. Wracking resistance is only an issue after the glue joint has failed.

I think your plan is a success waiting to happen.

John

[John Kananis]

John, cabinet doors don't have that much weight sitting on top of them and they don't see much lateral movement/deflection either. I'll respectfully disagree on this one. A mortise and tenon joint is effectively a cross grain glue up. The mortise acts to hold the tenon, not just the glue.

[Jimmy Harris]

As far as it sitting back 1/8", that's just aesthetics. Place it wherever it looks best to you.

As for leaving the mortise open on top, I would avoid doing that, but it wouldn't be the worst thing you could do. I've restored enough furniture to say that while wood glue is stronger than the wood, it doesn't last as long. I've picked up quite a few vintage chairs and tables in my life where the wood glue failed and made the chair or table wobbly. Some of them were made with modern wood glue, being from the 80's and 90's, so it's not just a hide glue thing. And a proper mortise and tenon joint will hold the frame together better and both prevent the wood glue from deteriorating as fast and provide more support when it does.

That being said, it's not like a bridle joint will fall apart without warning, or a mortise and tenon joint is guaranteed to last forever. Now, the difference in difficulty and time in making the two joints isn't huge. So I would err on the side of caution and go with the mortise and tenon joint if I were in your shoes. But if you choose to go with the bridle joint, and do a good job with it and don't abuse the table, it probably will still outlast you before it needs to be repaired. And when it does come time to repair either joint, neither will be any more difficult than the other. So I wouldn't lose sleep over it.

[Kent A Bathurst]

What you are creating is a form of bridle joint. They are incredibly strong. It's also very much like cope and stick joinery for doors, which has proven itself for a very long time. I would have no qualms about proceeding that way. You are increasing the sidewall glue area by doing it, always a good thing. Will the wracking resistance suffer. Only if the joint fails.

Yeah. What John said. I use open-top mortises very frequently. I also will often peg/pin the M&T joint from the inside, so it doesn't show on the face.

I never saw a functional difference between this method and haunched tenons. The haunched are definitely cooler, but I got over it after a couple dozen, and since then I have been unhip. But you can't call me on it unless you remove the table top. In which case I still don't have a problem, but you have some serious issues.

Like several months ago, a friend [I was his best man] and his woodworking son swung through on a bike road trip. The kid got into a gate-leg table I'd made, using rule joints. He came back into my workroom, and says right off: "You didn't clock the hinge screws" I threw something at him.

[Craig Shipman]

I want to thank all of you for your comments. At the end of the day, I decided to go the traditional route using a closed mortise. I found on the Internet that mortise and tenon joints have been identified as far back as 7000 years ago. So, I decided, why mess with tried-and-true success? I was grateful that none of you thought I was totally out of my mind. Thanks again.

[Christian Hawkshaw]

https://www.finewoodworking.com/2009...-strength-test

[John TenEyck]

John, cabinet doors don't have that much weight sitting on top of them and they don't see much lateral movement/deflection either. I'll respectfully disagree on this one. A mortise and tenon joint is effectively a cross grain glue up. The mortise acts to hold the tenon, not just the glue.

I should have better explained. Cope and stick joinery has some form of tenon, too. In cabinet doors it's often a stub tenon. In passage doors it's either a M&T, a loose tenon, or dowels. In any case, it's the glued area that carries the load, to which the cope and stick add significantly. Traditional M&T is great. It's probably the only joint that will work if you leave out the glue and draw bore or otherwise mechanically secure the joint. But once you use glue neither the draw bore nor the shoulders add anything - unless the glue fails. I'm pretty sure FWW test data showed half lap and I think bridle joints, too, with higher strength to failure than M&T ones. Because the glued surface area is higher. When the other joints fail, the parts break apart easily, whereas M&T will still support significant load. BUT, the M&T joints failed earlier. I'll take higher breaking strength. When was the last time you had a glue joint failure?

John

[Craig Shipman]

Great article! I might do the Bridle.