How to glue M&T joints

[Tom Bender]

M&T joints don't always fit like a glove, at least mine don't. Normally this is no problem as the strength is mostly mechanical and not adhesive. But sometimes we want to fill the gaps with a gap filling glue and not get any squeeze out. Any tips on putting in just the right amount?

[Lee Schierer]

Contrary to popular believe and advertising foaming polyurethane glues are not strong when they foam to fill gaps. Most PVA glues aren't recommended for gap filling. That pretty much leaves epoxies. You should coat all sides of the mortise and tenon and assemble the joint. Make sure there is a path for air and excess glue to get out of the joint or you will fight the hydraulic pressure of the compressed air and glue to get a tight joint.

[Warren Mickley]

You want to use hot hide glue for mortise and tenon joints. It does a fair job of gap filling. But the main virtue is that it can be disassembled for repair. If you want to make a joint that "will never come apart", you could easily triple the cost of repair, making throw away furniture.

I usually spread glue with a brush, first on the inside of the mortise, then on the cheeks of the tenon. You don't need glue on the shoulder area.

[Mike Cutler]

A "gap filling" adhesive is usually an epoxy of some type, with a thixotropic agent mixed in, such as wood flour, or micro balloons. System Three T-88 is a nice, gap filling, structural epoxy adhesive.
As for having just the right amount, and avoiding squeeze out, if you figure that one out, let me know. I just clean up the squeeze out.

[Charles Guest]

M&T joints don't always fit like a glove, at least mine don't. Normally this is no problem as the strength is mostly mechanical and not adhesive. But sometimes we want to fill the gaps with a gap filling glue and not get any squeeze out. Any tips on putting in just the right amount?

This glue has had a decent reputation over the years, but it won't fill in gaps of a really poorly cut joint. For that, you'd need epoxy. If the joints are really loose you ought to consider scabbing on some veneer to put the thing(s) right and then use whatever glue you normally use. The veneer sold through Woodcraft for segmented turnings is a bit thicker than art veneers (seems to me). I keep a little on-hand, in Maple, sold in a pack that costs ten bucks or so if memory serves, about the cheapest they have. I still make mistakes in the shop, usually cutting a joint a little lean rather than rich. If you have a rail that unintentionally leans a little, you can trim the bottom of the mortise on the opposite side of the shoulder gap to get it back upright but you might need to add veneer to one tenon cheek to hold it there. It happens. Have to have a plan when it does. If you really chop one poorly with a severely leaning rail, both cheeks might need veneer by the time you're done.

https://www.garrettwade.com/special-202gf-high-strength-glue-gp.html

I think Lee Valley sells the same glue (though obviously branded differently), worth a check if you'd rather order from them.

[Frederick Skelly]

....... sometimes we want to fill the gaps with a gap filling glue and not get any squeeze out. Any tips on putting in just the right amount?

I apply the glue on all sides of the mortise (only). And I apply it from the top of the mortise to about 1/3 from the bottom. That (usually!) ensures the glue only goes where I want it to. It also leaves room for the glue to pretty much spread itself, avoiding hydraulic pressure as Lee mentioned. If I'm using shellac as a sealer or finish, I sometimes wipe the surface into which the mortise is cut with Johnson's Paste Wax for a belts-and-suspenders approach.

You didn't ask this, but rather than use a gap filling glue, I usually plane a thin shaving of the same wood and glue it to the tenon. (After it dries, I glue up the joint as normal.) That tightens the fit to the "just right" point. It doesn't excuse my crappy job of sawing in the first place though. :) EDIT: Looks like Charles and I were making similar points at the same time. I never thought of veneer though! That's clever.

Fred

[Al Launier]

Epoxy as stated above.

[Brian Holcombe]

You want to use hot hide glue for mortise and tenon joints. It does a fair job of gap filling. But the main virtue is that it can be disassembled for repair. If you want to make a joint that "will never come apart", you could easily triple the cost of repair, making throw away furniture.

I usually spread glue with a brush, first on the inside of the mortise, then on the cheeks of the tenon. You don't need glue on the shoulder area.

Interesting, I just noticed recently that if I avoid putting glue in the shoulders that it makes a tighter fitting joint. It seemed obvious to me when I considered that glue takes up some thickness and so contributes to the glue line, but surprised that I had not considered avoiding putting glue in those areas for so long.

[Robert Engel]

M&T joints don't always fit like a glove, at least mine don't. Normally this is no problem as the strength is mostly mechanical and not adhesive. But sometimes we want to fill the gaps with a gap filling glue and not get any squeeze out. Any tips on putting in just the right amount?

Tom, if you think about it, the looser the joint, the less mechanical strength there will be, so you do have to rely on glue and/or pins to solidify the joint.

That said, any gap filling glue will compensate, and draw bore pinning solves the problem completely even with no glue at all.

Not meant as a criticism, but it might be worth it to re-examine your fitting technique, as there really shouldn't be that much gap.

[ken hatch]

Tom, if you think about it, the looser the joint, the less mechanical strength there will be, so you do have to rely on glue and/or pins to solidify the joint.

That said, any gap filling glue will compensate, and draw bore pinning solves the problem completely even with no glue at all.

Not meant as a criticism, but it might be worth it to re-examine your fitting technique, as there really shouldn't be that much gap.

That and hide glue is the best answer for perfect and not so perfect fitting joints.

BTW, I like Brian's observation.

ken

[William Fretwell]

Trying to glue end grain is futile!

[William Fretwell]

I use blind double wedged tenons in my throw away furniture 😕.

[chris carter]

Wood By Wright did a rather epic glue test of a billion different types of glue for various purposes with redundancies and basically nerded out on the thing. He made all the data available as a google doc spreadsheet. According to his data, the best glue for gap filling was TB 3 & 2, followed by thick CA glue, followed by epoxy. Here's the data: https://docs.google.com/spreadsheets/d/1GAZrhrtJPi8-iqPRVfqgOgf7RTg8Vqmen6OKJ4Ae6_I/edit#gid=321279609 If you want the simple version, just search his name and glue test on youtube.

[Simon MacGowen]

M&T joints don't always fit like a glove, at least mine don't. Normally this is no problem as the strength is mostly mechanical and not adhesive. But sometimes we want to fill the gaps with a gap filling glue and not get any squeeze out. Any tips on putting in just the right amount?

Warren is right about leaving the shoulders glue free. You can build on that with one more trick:

https://www.youtube.com/watch?v=5yCRhCu4a8E

You can see glue is applied on the cheek but shy from the shoulder, a method Gary Rogowski also covers in one of his Fine Woodworking articles. When you leave a bit of wood on the cheek free of glue from the shoulder, you won't see any glue squeeze out.

Simon

[Pat Barry]

You want to use hot hide glue for mortise and tenon joints. It does a fair job of gap filling. But the main virtue is that it can be disassembled for repair. If you want to make a joint that "will never come apart", you could easily triple the cost of repair, making throw away furniture.

I usually spread glue with a brush, first on the inside of the mortise, then on the cheeks of the tenon. You don't need glue on the shoulder area.
What is the preferred method to disassemble if hide glue was used? How do you know if hide glye was used? On older furniture I assume hide glue was the preferred adhesive but that probably changed last century.

[Brian Holcombe]

Warren is right about leaving the shoulders glue free. You can build on that with one more trick:

https://www.youtube.com/watch?v=5yCRhCu4a8E

You can see glue is applied on the cheek but shy from the shoulder, a method Gary Rogowski also covers in one of his Fine Woodworking articles. When you leave a bit of wood on the cheek free of glue from the shoulder, you won't see any glue squeeze out.

Simon

Where I found this not to be obvious was with the joints I have been cutting for my chairs at the arm-back connection. It's an open bridle joint with a miter at the top, being that it's a bridle joint I found that I had to actively avoid putting glue along any end grain abutment. Anytime I did the previously perfectly tight joint would show a glue line, and when I did not it would remain seamless.

[Simon MacGowen]

Where I found this not to be obvious was with the joints I have been cutting for my chairs at the arm-back connection. It's an open bridle joint with a miter at the top, being that it's a bridle joint I found that I had to actively avoid putting glue along any end grain abutment. Anytime I did the previously perfectly tight joint would show a glue line, and when I did not it would remain seamless.

Makes good sense...the glue line could be quite unsightly.

Wonder if the use of hide glue in such situation wold allow a better fix.

Simon

[Jim Koepke]

Good information in this thread.

Especially the part about not putting glue on the shoulders.

My last mortise & tenon work was all done without glue using the draw bore method. It is still holding together:

https://sawmillcreek.org/showthread.php?262272

Cutting the mortises deeper than the tenon is also helpful in having a place for excess glue to go.

jtk

[Warren Mickley]

What is the preferred method to disassemble if hide glue was used? How do you know if hide glye was used? On older furniture I assume hide glue was the preferred adhesive but that probably changed last century.

Hide glue becomes brittle with age you can shatter it with a sharp blow. If it is pretty fresh you need heat and moisture to soften it.

[Frederick Skelly]

My last mortise & tenon work was all done without glue using the draw bore method. It is still holding together: https://sawmillcreek.org/showthread.php?262272


The next M&T project that I do will be draw bored. Been reading up on it and I want to try it. Thanks Jim!

[Brian Holcombe]

Makes good sense...the glue line could be quite unsightly.

Wonder if the use of hide glue in such situation wold allow a better fix.

Simon

Definetly, though in darker woods it’s ok, but maple, ash and other light woods much less so. The joinery needs to be very tight to look presentable.

I use hide glue currently on those joints, I like that it is forgiving and can be taken apart mid assembly if something goes awry. I’ve had a piece of crud fall into the joint and prevent it from closing. Knocked it apart then back together again. With yellow glue that would have been a few hours of work in the heap.

[James Waldron]

What is the preferred method to disassemble if hide glue was used? How do you know if hide glye was used? On older furniture I assume hide glue was the preferred adhesive but that probably changed last century.

No, it changed with the advent of disposable furniture and Ikea. A lot of beginners and amateurs don't know any better and use permanent, irreversible glue on some very nice projects, but really fine furniture is often built with hot hide glue today.

[Tom Bender]

Thanks all for a really good discussion. Some tips that help;

1. Tight joints of course is the gold standard

2. Shim them tight if needed

3. Give the excess glue another place to go, (a deeper mortise) (a small chamfer on the mortise)

4. Prefinish so it doesn't matter

5. Wax or tape the squeeze out area

6. No glue on the shoulder area

7. Drawbore or crossdowel

8. Choose the right glue

9. Little or no glue on the tennon