Mortise and tenon question

[Jim Dwight]

My highest stressed mortise and tenon joints are in chairs based upon a design from WoodSmith. They have ladder backs held with mortise and tenons (5) and cross rails with mortise and tenons. All the tenons are 1/4 inch thick and the parts are fairly small. I've made lots of other mortise and tenon joints but the others are bigger if they take the kind of load that these get when a bigger person sits in the chair, or, even worse, leans back. There have been no issues.

The mortises were cut with a router and a spiral up cut bit. I use a template and have made 8 of these so far. I will probably make 6 more next year. All the mortises are rounded at the end and all the tenons are rounded to match. I do not deliberately cut them with extra clearance but occasionally have to trim a joint creating a little clearance. They all work so there must be a little strength margin despite the modest size of the joints.

I also have a hollow chisel mortiser and make square ended mortises with it, of course. I prefer them. But it is not practical to use them on these chairs. It is much quicker to tack the template in place and cut 6 mortises on each back leg in one setup. I could chop them square but I see no reason when it is much quicker just to round the tenons.

I am not trying to criticize anybody else's way of doing this. Just saying what I do with some success.

[Simon MacGowen]

My highest stressed mortise and tenon joints are in chairs based upon a design from WoodSmith. They have ladder backs held with mortise and tenons (5) and cross rails with mortise and tenons. All the tenons are 1/4 inch thick and the parts are fairly small. I've made lots of other mortise and tenon joints but the others are bigger if they take the kind of load that these get when a bigger person sits in the chair, or, even worse, leans back. There have been no issues.

The mortises were cut with a router and a spiral up cut bit. I use a template and have made 8 of these so far. I will probably make 6 more next year. All the mortises are rounded at the end and all the tenons are rounded to match. I do not deliberately cut them with extra clearance but occasionally have to trim a joint creating a little clearance. They all work so there must be a little strength margin despite the modest size of the joints.

I also have a hollow chisel mortiser and make square ended mortises with it, of course. I prefer them. But it is not practical to use them on these chairs. It is much quicker to tack the template in place and cut 6 mortises on each back leg in one setup. I could chop them square but I see no reason when it is much quicker just to round the tenons.

I am not trying to criticize anybody else's way of doing this. Just saying what I do with some success.

All the emperical info, therefore, does point to support what the instructor says (as quoted in the OP's post) - a rounded mortise and a square tenon is fine.

Simon

[John TenEyck]

All the emperical info, therefore, does point to support what the instructor says (as quoted in the OP's post) - a rounded mortise and a square tenon is fine.

Simon

Not surprising to me. Most of the test results I've seen always show that breaking strength is directly proportional to glued sidewall surface area. It will likely shock some readers that a bridle joint has higher breaking strength than a traditional M&T where the members are of the same size. The reason is simple; the bridle joint has a lot more glued sidewall area. Where the traditional M&T has a distinct advantage is that it lends itself to applications where no glue is used, such as in doors with wedged through tenons, or workbenches with tapered wedge through tenons that can easily be disassembled and moved. Also, glued M&T joints don't catastrophically fail if the glue fails, as a bridle joint would. But as long as the glue joint holds it makes no difference whether a tenon, round ended or square, fills the mortise completely or not. The sidewall surface area carries the load.

John

[Art Mann]

If I want to make all my moments count, I will not waste them practicing time consuming techniques that have no discernible effect on the end result. Your comment about "best" assumes that there is a common and well understood definition of the word. My definition of the "best" technique might be the one that meets all the aesthetic and structural requirements for a particular project with the least amount of effort. There are, in fact, many, many ways to create the joint under discussion and someone might find a reason to consider any of them the best.

Tom,
This is a question not about whether the joint is strong enough, or whether it is necessary etc. that's irrelevant.
Nothing is necessary, you exist for a brief moment in time, make it count.
This is a question about who you are.
Do you know?
There are only two ways to do anything; the best, or the rest...... pick one.

[Mark Hennebury]

If I want to make all my moments count, I will not waste them practicing time consuming techniques that have no discernible effect on the end result. Your comment about "best" assumes that there is a common and well understood definition of the word. My definition of the "best" technique might be the one that meets all the aesthetic and structural requirements for a particular project with the least amount of effort. There are, in fact, many, many ways to create the joint under discussion and someone might find a reason to consider any of them the best.

I agree with you that there are many versions and opinions of what the best is. I never suggested one for the OP or anyone else. Never stated that there is a common and well understood definition. From a structural standpoint there are many options that will do. My point was simple; the best or good enough, its about choice. Its about you, your personality and your reasons for doing what you do. You can do whats adequate for the job, if you do it for a living there is little reason to do more than is required. If you are doing woodworking because its a passion you can take it as far as you want. Many examples of incredible joinerywork around the world, developed over thousands of years by millions of dedicated people passionate about doing the best they can, "hardly necessary", and in my opinion hardly a waste of time. You waste your time whatever way you want, and i will waste mine my way. I happen to like joinerywork, I think that it is of value, and i will share my opinion with people anytime i can and i will encourage people to try to do the best designed, most precisely fitted joints that they can, but that is just me. What the best is, is for those who are interested to determine for themselves. The OP was concerned about the joint from the instructor, seemed to make him a little uncomfortable, which is promising to me. You can tell him its fine, good enough, i am going to tell him, he could do better.

[Brian Holcombe]

OP,

It’s worth a mention that in many workbench builds one can avoiding gluing the joinery at all and that is quite handy if you ever want to move the bench.

To secure the joint you can instead use a draw-boring method. Draw-boring can be knocked apart.

[Edwin Santos]

OP,

Draw-boring can be knocked apart.

Hi Brian,
I did not know this. I discovered draw boring only recently and I think it's awesome. The method I learned involved an offset hole (in the tenon vs. the mortise wall). By design, the offset pulls the tenon in very tightly. After the pins were driven in, they "deformed" in the course of negotiating the offset channel and I can see no way possible to drive them back out unless you can teach me what I'm missing.
Maybe in order to disassemble, the hole would not be offset? Thanks for any advice.
Edwin

[Larry Edgerton]

If you make through mortises they’re easier and stronger because you can cut from both sides toward center.

I like these and I like to taper the outside and drive in a wedge in a cut in the tenon.

[Brian Holcombe]

Hi Brian,
I did not know this. I discovered draw boring only recently and I think it's awesome. The method I learned involved an offset hole (in the tenon vs. the mortise wall). By design, the offset pulls the tenon in very tightly. After the pins were driven in, they "deformed" in the course of negotiating the offset channel and I can see no way possible to drive them back out unless you can teach me what I'm missing.
Maybe in order to disassemble, the hole would not be offset? Thanks for any advice.
Edwin

They deform on the way in and they deform on the way out. I’ve knocked apart 6” thick stock with 1/2” pins.

[John TenEyck]

A wedged through tenon like this makes more sense for a bench that needs to be disassembled.



I took this photo on a recent trip to Germany. It was in a wonderful town museum where my wife grew up. Her grandparent's apartment was now part of the museum! Small world. Anyway, you can see the wedged stretchers on the left side. The right hand side was held in a different, less obvious way. Why, I have no clue.

John

[Tom Porter]

Wow. That was a lot of information. Thanks for all the opinions. Before I make the bench, I'm going to experiment a bit trying different techniques to find what works for me. I'll incorporate much of what I learned here as I do that.

[Al Launier]

This is how a square corner fits into a round and visa-versa


Attachment 394409Attachment 394410

Thanks Mark, appreciate the time you took to display this. I'm glad you did as I would not have considered this concept as I feel all four sides should be in contact, especially since it wouldn't take that much more effort to make it this way. Although the shoulders are the prime means of preventing any rocking I feel the added contact with the tenon adds appreciably more.

[Edwin Santos]

For the record (official Festool manuals), the grooves on the faces of the dominoes are designed as glue pockets:
"The special shape of the DOMINO in combination with expanding glue pockets and lateral longitudinal grooves gives the dowels a secure grip."

In addition, anyone can check their Festool manuals and see that the narrow setting is recommended for tight joints; the wider settings are for alignments, not with glue escape in mind (because the dominoes are always shorter than the mortises). Anyone who wants max. strength (vs enough strength) should not use the wider settings, which are there for a different reason. The wider settings are perfectly fine if you are doing edge to edge glue-up or small boxes. Use them in a chair or bench, you have got a time bomb there.

I respectfully disagree with the thinking that "furniture design has more to do with joint failure than the joint construction minutiae..." A lot of joint failures can be attributed to poor execution, in addition to poor joinery choices (say, using a biscuit as corner joint for a chair). The joint construction itself does matter. For example, a loose fit of an M&T on the cheeks will remain weak even if the gap is glue-filled and the joint is pinned with dowels. All it needs to fail is the last straw.

Simon

Simon, I can see we disagree on some things. But I'll bet we agree on the following:

Sloppy, poorly fitted joints are never a good way to go.
The definition and criteria for "best" in joinery is personal, and perhaps circumstantial.

Edwin