Domino joint study

[Gene Davis]

I'm thinking through a Morris chair design, in which I want to use loose-tenon dominoes instead of the mortise and tenon joints done in this 125-year-old chair design.

The one joint that has me scratching my head is where the back horizontal backrest slats join into the two uprights. The joint is problematic because the slats are curved. In the AFD (American Furniture Design) plan for the Stickley chair, the tenons are integral with the slats, and cutting is routine. Cut the tenons when the parts are a blank, then bandsaw the curves, done. Tenons are offset in the blanks, but end up pretty centered in the ends after the curves are cut. See here.

Morris chair + truck 005.jpgMorris chair parts Oct 20 003.jpg

The issue with a loose tenon design, and sticking with the available Festool slot/domino geometry, is that since the curve of the slat, on the concave side of the slat, is arcing up, a domino slot might punch through at its end, or before. See this pic here, in which I study, using Sketchup, how a 5mm x 19mm x 30mm domino (Festool's smallest) might work for me. You can see the "poke through" in the slat that has the curve with the smallest radius.

2012-07-08_1914.jpg

While Festool may make a smaller tenon than the 5x19, I don't want to use it. The prototype design has 1/4" tenons and a 5mm domino is about 5/6 the width, so I don't want smaller than the 5. With the deep curve in the back slats of the prototype design, the domino slot crashes the surface, but if I compromise the design with a shallower curve to the slats and thickened ends, I think I may be OK.

Here is an overhead view of these five different slat geometries.

2012-07-08_1931.png

So while it is clear I need to choose a shallower bend for my back slats, I need first to check the feasibility of jointmaking using the Domino, and he durability of the joint with this 5mm biscuit.

Do any of you Domino wizards have any thoughts on this? Look strong enough? Look easy to do? Any issues?

I don't own a Domino, but may either get one, or borrow one, or go to a class in a shop that has one.

[Jamie Buxton]

Don't start by cutting the end square. Cut it at angle. Then make the mortise for the loose tenon. Then cut the end square.

[Gene Davis]

Thanks, but then I need to go into the stile of the back, the mating part, on that angle. Furthermore, whacking the angle on the slat part after doing the plunge, compromises the depth a little, meaning I either have to nip the biscuits, or plunge a little deeper into the mating part.

Are you a Domino owner and guru? I'm not. How easy is it to plunge in on an angle? Or am I going to have to make an oversized stile part, with one long edge jointed skewed out from 90, and plunge straight in on that skew face, then re-joint to 90?

Sounds like a lot of work.

Edit:

OK, I can see how this can work. The "over-angle" material added to BOTH the end of the slat and the edge of the mating part is 13.1 degrees, and thus the fence of the Domino is rotated that from 90 so as to have its plunge face flush to the face into which we will chop our skew mortise.

So, we have a way to do this which will not compromise the look of the finished chair at all. It is still more work than making the design compromise and having backs with more-shallow arcs.

[Gene Davis]

And here is what it boils down to. Retain original design for deep-curve back-rungs, skew dominos to mating surfaces via extra cutting of blanks to have machine-registration faces, then remachine parts after domino mortises are cut.

Or redesign for easier domino mortise cutting, have less back curve.

The chairs are fitted with loose back cushions, which hide the back-rungs from view when the chairs are viewed from the front. The curvature is there mostly for aesthetics.

Here is an x-ray view of the back of the chair with original (prototype) slats, skewed mortises, and the deep (1-1/4" depth at center) curve.

2012-07-08_2155.png

And now look at the redesign for ease of joinery. Back-rungs get 33 percent thicker as one goes from center to ends, and the depth of curve is less. This is 3/4" depth you are seeing, versus the 1-1/4" as seen above.

2012-07-08_2159.png

[Matt Meiser]

What if you use the 4mm Dominos?

Also think more about Jamie's suggestion. You can plunge deeper than needed for a 5mm and nothing says it must be centered between the parts.

[David Hawxhurst]

the domino fence can be set at angle. its not hard to use it on angles plunge cuts. it does fine on other angles besides the 0 and 90 plunges.