A zigzag dining chair

[David Dundas]

Here is a dining chair (Photo 1) that I designed to be reminiscent of Gerrit Rietveld's Zigzag chair, but comfortable, unlike the original. The leg joints are 22.5° mitres with an alignment domino; they are each reinforced by three shop-made locking tenons, which are pinned by brass screws, whose heads have been cut off (Photo 2). The chair was tested by having three people with a total weight of 575 lbs stand on a board laid across the seat side-rails. An article describing how to build the chair, using a Domino tenon joiner, will be published in the April issue of Woodcraft magazine.

David Dundas

[Jim Becker]

Dave...looks like a great project. And thanks for the note about the upcoming article. That's nice to know!

[Mark Carlson]

David,

I like your chair. I had to go an order the Woodcraft mag so I could read your article.

~mark

[Rick Christopherson]

It gets even better. Woodcraft is planning to post a dynamic model of the chair on their website so you can watch David's chair explode and collapse, and even take measurements right off the model, plus a bunch more. I just read about another magazine making a rather feeble attempt at doing this--this one is not a feeble attempt; it is really cool. This would definitely be a cool issue of the magazine to pick up.

[Eric Wong]

Very interesting! Nicely done!

[Bob Childress]

Dave,

What a wonderful chair! :) Have you ever considerd making a rocker with the Domino? :rolleyes: :D

[David Dundas]

Bob,

That sounds like what we in Oz call a 'Dorothy Dixer'.:) Yes, I am hoping to publish a 2-part article on how to build a rocker using a Domino, but I haven't yet received confirmation that it has been accepted.

I originally started experimenting with zigzag chairs about two years ago, and have used several different joinery methods for the leg joints, such as:

1. Pinned bridle joint, cut using a table-saw tenoning jig;

2. Pinned double-keyed miter joint, again using a table-saw tenoning jig to cut double kerfs for the keys;

3. Miter joint, reinforced by a single, pinned, locking through tenon; and

4. Miter joint, aligned by a domino tenon, and reinforced by three pinned locking tenons, as described in the Woodcraft article.

All four methods yield very strong joints, if glued with a gel epoxy, such as Silvertip Gelmagic, made by System Three. It is not essential to own a Domino to build the chair; the joints can either be made with a table-saw tenoning jig (1 & 2 above), or using a router with a mortising jig (3 & 4). However, I would emphasize that it is essential to use a very strong hardwood, such as oak or maple. I am not sure whether softer woods such as walnut or cherry would be able to withstand the stresses that the chair's structure imposes.

David

[Dennis O'Leary]

Good for you David.

[Cliff Rohrabacher]

I have a question David.

I have considered a chair after gerrit's model and every time I think of it I can't quite get my head around the stress at the joints and how they take the beating.


The joints are less than a few inches each and no matter the joinery used they can't give more than 4 or 5 square inches of glue surface.

Then there is the weight and the moment arm.

The joint has to resist an 18" moment arm that may have as much a 150 pounds on it if a 200+ pound sits on the chair. And that weight can increase dramatically during the moments when a heavy person mounts the chair.

Can you talk a bit about how the joint performs under those stresses?

[David Dundas]

Cliff,

Here is a picture of three people standing on the chair. The back end of the side rail was deflected downwards about 1/2" under this load; but the joint was undamaged and the chair sprang back when the load was removed. The second picture shows a SolidWorks model of the joint, but does not include the pins that penetrate and lock all of the locking tenons. For the joint to fail, the pins would need to shear through the locking tenons. I am inclined to believe that, if the chair were tested to destruction, the legs would break before the joints failed. I am no materials scientist, but according to the West Systems technical details, epoxy glues have a tensile strength of over 7000 PSI, so it seems clear that wood glued with epoxy will normally break before the joint fails, if there is a reasonable glue area in the joint.

David



David

[David Dundas]

For non-owners of the Domino, I now have a free downloadable article on how to build this chair with pinned bridle joints here (http://rockerswoodwork.blogspot.com/) .

David

[Cliff Rohrabacher]

Cliff,

Here is a picture of three people standing on the chair. The back end of the side rail was deflected downwards about 1/2" under this load; but the joint was undamaged and the chair sprang back when the load was removed.

That is absolutely amazing. If I hadn't seen it I'd not have believed it.

I have seen chairs after this fashion before and had serious doubts as to their structural integrity and all because of the moment arm on those joints.

W-O-W
Simply WOW~!!

[David Dundas]

Non-owners of the Domino can make the chair with mitre joinery, as described in the Woodcraft article, by routing the mortises using one of the mortising jigs for which you can download free plans here (http://rockerswoodwork.blogspot.com/) .

David

[David Dundas]

The Woodcraft magazine article on this chair is now available online at the FestoolUSA website: http://www.festoolusa.com/Web_files/Domino_WCM_zigzag.pdf

David Dundas