Square or round tenons, mortises

phantomtrapper | Posted in General Discussion on October 3, 2023 12:11pm

I have been watching Tim Rousseau’s video on building the Danish modern desk, and the episode this morning he says he rounds his tenons to fit the mortise. I found this interesting as just about all the other FWW writers seem to do the opposite. I have always done this for the same reason he puts forward-that square corners can develop cracks whereas rounded ends do not. Having run heavy equipment in a rough industry most of my life, I know that squared corners in an inspection hole often cracked out from the corners from stress, vibration, etc. but round ones never did. I kn ow our furniture will never undergo the same treatment, it gives me sense of comfort and satisfaction. Just wondering how some others prefer to do it and why.

Replies

_mj_ | Oct 03, 2023 02:53pm | #1
Potato - Putahto.
mlindy | Oct 03, 2023 03:59pm | #2
I think it has a lot to do with how one makes their mortises. I've done it both ways and don't think there is a significant strength issue. If you are making your mortises with a round bit using a router or mortising machine it makes sense to round tenons rather than square the mortise. If you cut mortises by hand or use a chisel mortiser and get square mortises, square tenons are a natural. To me, any more work on that part of the project is uneccesary effort.
jharveyb | Oct 03, 2023 07:19pm | #3
There are trade-offs here. Rounding the end of the tenons reduces the effective length of the glue surface, as the rounded areas are essentially end grain (similar to dowel joints, which I am not a fan of.) I occasionally round tenon ends if the mortise piece is especially delicate, but only at an end that is prone to splitting. And of course, there is the question of whether a properly made glued up joint is essentially as strong as a solid piece of wood, with the glue and tenon strengthening the mortise area. I think it is likely.

It is possible to split the mortised piece when test fitting a tenon so be careful in that process.

I'm pretty sure that for many folks, rounding the tenon with a rasp is quicker/easier than squaring the ends of the mortise. With a sharp chisel and good technique, squaring the ends is pretty quick. But if you have the room for extra length (the rounded ends) of the tenon/mortise, rounding is also fine.

I often keep the mortise and tenon of aprons near the top of the apron, with a stopped haunch below to keep the two pieces in alignment. That minimizes any change in dimension above the mortise, that could try to push the top off a table or show a big crack if the apron shrinks.
I hope this helps...
john_c2 | Oct 04, 2023 02:53am | #4
Over the years I've done it both ways. It hasn't made the tiniest bit of difference in any project, ever.
elmaduro | Oct 04, 2023 08:05am | #5
For a narrow mortise cut with a spiral router bit, I round the tenon to match. For a wider chiseled out one, I leave it square.