Mortise and Tenon

[transatlantic]

I was watching this video on cutting Mortise and Tenons, and it seems like a really nice way of doing it. I was wondering what peoples thoughts were on his approach of not rounding the Tenons to fit, but instead making them smaller. His reasoning being that you don't really loose any strength, and it also gives you wiggle room to get them just right, I suppose like a biscuit.

https://www.youtube.com/watch?v=TbvfxBkaexE

 

[Sgian Dubh]

The first thing I noticed in the first two minutes of the video was that the proportions of that corner M&T are a bit out. General guidelines indicate the width of the tenon in the tenoned member should take up roughly the inner 2/3rds of the part. This shortens the length of the mortice thus leaving enough wood at the outer corner of the morticed member to provide enough strength to prevent the joint failing through shear parallel with the grain in racking of the joint.

As to making the mortice overly long, or the tenon excessively narrow I'd say 'it depends'. In my experience I can't think of more than a handful of cases where I'd agree that this form would be good practice. Yes, a shy millimetre of extra length in the mortice to allow for ease of assembly and for a bit of expansion/contraction in the tenon, but outside that most workers rely on the accuracy of the tenon width matching the mortice length for squareness. Then, in service, if there's significant sideways stress in the mortice's length from the tenoned member, what's to stop the tenoned member drifting out of line over a long period? This falls into the category of time dependent creep, a subject studied by rheologists. Slainte.

 

[Orraloon]

Can't say I agree with the dude in the video either. Having loads of empty space in the mortice is going to encourage wracking over time. It does not take much time to round the corners of the tennon using a rasp and get a nice fit. A good fit and good square shoulders is what make a M&T joint work. As for getting the corner flush leave the part with the mortice a tad over long then trim flush after the glue sets.
Trouble with the internet is not all the information out there is the best.
Regards
John

 

[deema]

Virtually all glue exposed to shear forces fails very quickly, the edge of the tenon coupled with the shoulders prevents wracking and hence shear forces on the glue line. IMO just a lazy approach producing an inferior joint

 

[ED65]

... I suppose like a biscuit.

Biscuits are compressed, they're loose initially but are intended to swell back up and fit tightly from the water in the glue.

Instead of allowing wiggle room in the joint with a narrow tenon you can take inspiration from biscuits (although it's actually the other way around) by compressing tight-fitting tenons in a strong vice or by hammering so they're an easy sliding fit in the mortise. The compressed tenon swells back to its former size with the water in the glue so you end up with tight-fitting joints while initially having ease of fitting.

This tip has re-emerged just recently in an article in FW or PW sorry can't remember which (both are guilty of endlessly recycling content), but the technique isn't at all new.

 

[custard]

Instead of allowing wiggle room in the joint with a narrow tenon you can take inspiration from biscuits

The wiggle room in a biscuit has nothing to do with them being compressed, that's simply to allow more time for assembly before they swell up and grab.

The wiggle room in a biscuit actually comes from the slot or mortice being longer than the biscuit, which means you can plunge a mill or so either side of the mark and the job will still all come neatly together.