Mortise and tenon question

[Brian Holcombe]

The strongest form of mortise and tenon joint, as far as I’m aware, is one in which the end grain abutments of the mortise are hourglass shaped. The tenon compresses as it passes through the joint and it must be literally separated from the continuing member to fail.

If we reduce this to two bits of side grain glued cross grain to two more bits of side grain the connection is initially strong but will certainly fail over enough time. Maintaining straight abutments does certainly give the joint something to counter the leverage being applied to it which will reduce the strain placed on cross grain sections.

When building a workbench I prefer a mechanically sound method which is not solely reliant upon glue. I use a workbench hours a day and have broken every weak point on it only to fix it with methods better prepared for what a workbench lives through.

[Jim Andrew]

I used the Schwartz book when I built my bench, and laminated the posts. Just left out a piece when laminating the posts to create the mortises. The mortises going at 90 degrees, I had to router out. I drilled holes and ran through the tenons, and glued them in so no way any joints can give.

[ChrisA Edwards]

I just did a few for my Plantation Shutters. I was going to use either my CBR7 kit or a horizontal router to do the mortises, but I opted for the Powermatic Mortiser. Expensive tool for one big project, but it was so easy to use, I'll use it for many future projects.

[Edwin Santos]

I wonder if some of this hair splitting is theoretical. In a well fitted mortise and tenon joint, the side grain glue connection is usually more than strong "enough" (there's that word). So in direct answer to the OP, I'd agree with the instructor.

There's another factor to consider in this analysis. When I have made a perfectly mated blind mortise and tenon, with square abutments, I've had problems with a vacuum lock situation occurring during glue up that traps air in the bottom of the mortise and then makes the joint impossible to close because the air cannot evacuate. Due to this, I started leaving a tiny bit of breathing space at the top and bottom like John describes, to prevent this issue. With loose tenons you can cut a very shallow kerf down the length of the loose tenon stock to do the same.

When seeking ultimate mechanical strength like Brian is describing, if the design allows for it, drawboring the joint will make it indestructible IMO.
If it's a through tenon, wedging is another strategy. In fact, if you're really fastidious, you can slightly flare the through mortise and when wedged the tenon will basically create a large wedged dovetail. At that point, using glue is an option but not a requirement.

Edwin

[Simon MacGowen]

There's another factor to consider in this analysis. When I have made a perfectly mated blind mortise and tenon, with square abutments, I've had problems with a vacuum lock situation occurring during glue up that traps air in the bottom of the mortise and then makes the joint impossible to close because the air cannot evacuate. Due to this, I started leaving a tiny bit of breathing space at the top and bottom like John describes, to prevent this issue. With loose tenons you can cut a very shallow kerf down the length of the loose tenon stock to do the same.

Edwin

If I read John's post correctly, he was having the loose fit for alignment, not for the purpose you state here.

The proper way to avoid the "vacuum lock" you described is not to intentionally cut a tenon narrower but to cut the mortise deeper or the tenon shorter for glue escape, which will not compromise the structural integrity of an M&T. In fact, anyone who has worked with the dominoes knows that the mortises are always cut longer (deeper) than the dominoes for a similar reason. Dominoes are strong enough for hardwood doors as a loose M&T joinery.

For all intents and purposes, furniture joints will hold whether the mortises are round or square regardless of the ends of the tenons as long as the walls and tenon cheeks are a good fit and the gaps at the ends, if any, are not unreasonable for the purposes. Builds like benches or cabinets that are subject to racking and undue stresses, of course, require better care and attention.

Simon

[Mark Hennebury]

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.

[johnny means]

I wonder if some of this hair splitting is theoretical. In a well fitted mortise and tenon joint, the side grain glue connection is usually more than strong "enough" (there's that word).

Edwin

I've lap jointed 4x4s to build benches that would survive a lifetime of use in any shop. Modern adhesives and clean milling make all these arguments purely academic.

[Edwin Santos]

If I read John's post correctly, he was having the loose fit for alignment, not for the purpose you state here.

The proper way to avoid the "vacuum lock" you described is not to intentionally cut a tenon narrower but to cut the mortise deeper or the tenon shorter for glue escape, which will not compromise the structural integrity of an M&T. In fact, anyone who has worked with the dominoes knows that the mortises are always cut longer (deeper) than the dominoes for a similar reason. Dominoes are strong enough for hardwood doors as a loose M&T joinery.

For all intents and purposes, furniture joints will hold whether the mortises are round or square regardless of the ends of the tenons as long as the walls and tenon cheeks are a good fit and the gaps at the ends, if any, are not unreasonable for the purposes. Builds like benches or cabinets that are subject to racking and undue stresses, of course, require better care and attention.

Simon

I believe John and I are talking about the same process but two different benefits. The loose fit I mention is not in the cheeks of the tenon, but in the top and bottom edge, whether square or round. And it would be very slight, maybe 1/32 or less. I think John is talking about the space being for alignment, and I was suggesting a secondary benefit is to allow air to evacuate in the assembly process. I have noticed that the dominos have a crosshatch pattern in them which will also allow air to evacuate during assembly after which the compressed domino probably swells. I recall the domino machine will allow a setting for a slight bit of room for all these reasons. Although I agree that cutting a mortise slightly deeper or a tenon slightly shorter to allow a space for excess glue is a sound practice, but this space would not be nearly big enough for the air plunge from a super tight glue covered tenon.

Again, a lot of this is abstract. I think furniture design has more to do with joint failure than the joint construction minutiae we're discussing.

[John Goodin]

You mentioned you have limited experience making mortise and tenon joints and this is a great project to gain some experience and hone the skill. I, too and real actively inexperienced with the joint but find chisel work to be a nice diversion from the sound and sawdust generated by power tools.

[Brian Holcombe]

I've lap jointed 4x4s to build benches that would survive a lifetime of use in any shop. Modern adhesives and clean milling make all these arguments purely academic.

In what sense is it academic?

[Al Launier]

My experience with mortise & tenon joinery is very limited, so I really don't understand the round cornered mortise mating to a square cornered tenon as alluded to above. If the sides and ends of each are properly sized for a glue-up then how does a square corner fit into a round corner? I just see an interference fit that prevents assembly of the mating parts. I can accept a rounded tenon fitting into a square cornered mortise, but not the other way around. Perhaps my age is preventing me from understanding, but what did I miss from the above posts?

Also, I believe a round corner is less apt to fracture/split that a stressed square corner, so I've used the rounded corner for both the mortise and tenon.

[Mark Hennebury]

My experience with mortise & tenon joinery is very limited, so I really don't understand the round cornered mortise mating to a square cornered tenon as alluded to above. If the sides and ends of each are properly sized for a glue-up then how does a square corner fit into a round corner? I just see an interference fit that prevents assembly of the mating parts. I can accept a rounded tenon fitting into a square cornered mortise, but not the other way around. Perhaps my age is preventing me from understanding, but what did I miss from the above posts?

Also, I believe a round corner is less apt to fracture/split that a stressed square corner, so I've used the rounded corner for both the mortise and tenon.

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


assem1.jpgassem2.jpg

[Simon MacGowen]

There are several stress tests done out there on joinery, with inconsistent results. Here is one that says the dominoes are stronger than the M&T: https://www.youtube.com/watch?v=xfjg...youtu.be&t=442

According to this test, it seems the cheek/wall fit is the critical factor. A domino joint does not have a perfect end-to-end fit between the domino and the mortise in the same sense of an M&T.

Simon

[Robert Engel]

Remember tenons are cross grain glue ups. I think the reason you see so many old ones pinned is the joints were more subject to failure.

So I agree with Pat people have success with various methods largely due to modern glues, not that its particularly a better way.

IMO most of the structural strength (not talking glue here) comes from nice beefy shoulders and a properly fitted joint.

[Simon MacGowen]

I have noticed that the dominos have a crosshatch pattern in them which will also allow air to evacuate during assembly after which the compressed domino probably swells. I recall the domino machine will allow a setting for a slight bit of room for all these reasons. Although I agree that cutting a mortise slightly deeper or a tenon slightly shorter to allow a space for excess glue is a sound practice, but this space would not be nearly big enough for the air plunge from a super tight glue covered tenon.

Again, a lot of this is abstract. I think furniture design has more to do with joint failure than the joint construction minutiae we're discussing.

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