A question about loose tenons

[Derek Cohen]

I am building two of these chairs (called the "09 DC chair") in Hard Maple. It was designed by the Japanese- Scandinavian duo, Kyoko Inoda and Nils Sveje, in 2011. It is built by the Miyazaki factory in Japan ...



The legs will be joined to the seat with loose tenon mortice-and-tenon joinery. The tenons are Hard Maple, as with the rest of the build. I am in the process of choosing the stock for the 10mm wide x 30mm long tenons. The aim is to ensure that these are the best for a chair. Not any other purpose, but a chair.

The issue is grain direction: is it better to have the grain running horizontal or vertical?



Vertical grain should offer more rigidity, while horizontal grain more flexibility. While is preferred - your opinion?

The square stock is shaped on the router table ...





This is how it will look in the leg through mortice (minus the wedge and the rounded leg) ...



Regards from Perth

Derek

[Larry Edgerton]

Nice! I would be inclined to orient the grain the same as the rail as that would be the weak point and there would be a better glue bond as the grain was oriented the same way. But I build very few chairs. Beautiful design.

The problems I have had with arm chairs is that if too narrow large people tend to spread the legs. I built 200 for a restaurant and the first batch had some failures, always at that juncture. I went and sat in the bar and watched for a while and a very large lady came in and forced her big bottom in between the arms, and then I understood. A force I had not contemplated. The next batch I made wider and no issues.

[Philip Glover]

I orient joints for maximum stiffness / rigidity or the ability to resist bending.
The good thing about floating tenons is the oval mortise. The oval has a much lower Kt (stress concentration) than a rectangular mortise. The sharp corner in the rectangular mortice, potentially, has an infinitely high stress concentration that can lead to cracks. The round end of the oval spreads the stress out over a larger field reducing the probability of a crack.
I hope this makes sense.

Regards,

Phil

[lou Brava]

IMO, if it's strength your concerned with than the strength needs to in the direction of moments greatest force and that appears to in the down force from when people 1st sit down. I think any front/back or side to stress will be far less than any down forces. So in what orientation is your wood the strongest/stiffest ? If your concerned with ascetics than there is no right or wrong. I almost forgot your build is fantastic !

[John TenEyck]

I orient loose tenons so the grain matches the grain of the pieces it joins. That makes seasonal expansion/contraction issues a non-issue. Vertical grain is stronger, which is why baseball bats are labeled so the grain orientation is parallel to the swing.

John

[Jamie Buxton]

That first photo answers a question I've had about the chair. I think I can see a vertical seam in the back. It seems that the back is assembled from three pieces of wood.

[Edward Weber]

+1 for the vertical grain orientation, matching the the leg.

[John Kananis]

Vertical. All the stress is going to be a you plop your butt down, over and over again throughout three years and the vertical grain will hold up better. It's also more esthetically pleasing (if you look that closely).

Last thing, my initial vote was for square tenons but now that I'm looking at it again, the rounded will work quite nicely I think.

[scott lipscomb]

I think I am going to disagree with vertical orientation. Here is why: The weak point of the floating tenon joint is the where the cross grain of the tenon is in contact with the long grain of the chair leg. The tenon will expand/contract, the long grain of the chair leg will not. On the cross grain orientation that Derek shows above, the width of the domino will expand and contract along its radial dimension which is half of its expansion/contraction along its tangential dimension. The cross grain domino will be more stable in the important direction than the vertical grain one.

Said in a much less confusing way (hopefully)-the cross grain domino on the left will expand and contract about 1/2 as much along its wider dimension than the one on the right.

[John TenEyck]

I see the vertical orientation as providing long grain to long grain.

John

[Tom Bender]

For appearance;
Will you install 1 vertical wedge, two horizontal wedges or all three? If one then vertical grain, two horizontal, three then it doesn't matter.
A vertical wedge may tend to split the leg so two may be the better choice.

For strength;
Horizontal will provide better grip in the curved parts where there is more tooth in both surfaces.

[Andrew Joiner]

The original design image. I'd use vertical grain to prevent wedge splitting. What a gorgeous chair design!




chair seats ready to be sanded and finished

[Mark Hennebury]

I agree with Scott.

Tenon3.jpg
These were not loose tenons, they were machined on the frame members. Done with Balestrini mortice and tenon machines. Do the mortises first as they are governed by the mortise bit. Do the tenons last as the tenoner is infinitely adjustable within it's limits. So you can tailor the fit. Smooth snug-fit on the sides, don't do it to tight as it can split. do the tenon a couple of thousands over tight fit top and bottom, you can press that in tight, it wont split. Chairs were done 30 years ago, photo taken this year, of the test one for a set of ten.

Snug up your joints, gaps don't provide any structural support.

Screenshot 2023-06-01 121902.jpg melanies table.JPG

I think I am going to disagree with vertical orientation. Here is why: The weak point of the floating tenon joint is the where the cross grain of the tenon is in contact with the long grain of the chair leg. The tenon will expand/contract, the long grain of the chair leg will not. On the cross grain orientation that Derek shows above, the width of the domino will expand and contract along its radial dimension which is half of its expansion/contraction along its tangential dimension. The cross grain domino will be more stable in the important direction than the vertical grain one.

Said in a much less confusing way (hopefully)-the cross grain domino on the left will expand and contract about 1/2 as much along its wider dimension than the one on the right.

[Derek Cohen]

Thanks Scott and Mark for these explanations ... and everyone who responded. I much value your input.

It is difficult to see, but the seat boards are plain sawn (grain running horizontal) ...



For the loose tenon stock to "meld" with this, its grain needs to run horizontally.

The plan is still to wedge the through tenons, with the slots ending in drilled stop-holes to avoid splitting.

Regards from Perth

Derek

[Kevin Jenness]

The original design image.

As much as I like and use spline tenons I think this is an application where integral tenons would be better. The shoulder at top and bottom of the seat can be minimized with no potential for breakout if the glue joint between seat and spline fails. Spline tenons (I don't like the implication of "loose tenons") work best where there is ample material around the mortise, not so much for sash work and chairs like this one where there is no place for a reinforcing glue block. Rounding over the tenons by hand, though tedious, is doable, or the mortises can be squared up.