Another zigzag rocker

[David Dundas]

I built this chair based on an idea suggested by a fellow member of the Australian Woodworking Forum. The chair is of course totally dependent on the strength of the keyed miter joints between the legs and the rockers. Each of the four 1/8" thick mitre keys has a glue surface area of approximately six square inches, so the two miters have a total surface area of 24 square inches for the epoxy glue to hold, excluding the area of the miter faces themselves.

The ends of the back rails are angled at 83° to their (original) front faces, so that the seat side-rails, the arms and the rockers are splayed. All the cross-rail joints of the seat/back assembly are dominoed. There are no stretchers.

The seat/back assembly is supported by interlocking housing joints between the legs and seat side-rails, and the miter joints at the top of the legs are reinforced by a domino, and screwed and glued into housings cut in the back stiles.

The rockers consist of six 1/4" laminations; and the back slats are two 5/32" laminations.

The chair is mainly built of jarrah (Eucalyptus marginata), with back slats and miter keys of Silver Ash (Flindersia bourjotiana).

David Dundas

[Jim Becker]

Simply beautiful, David! I think the curve on the rockers was created "just right" to compliment the angular "Z" design...you hit the mark perfectly.

[Jim Dunn]

I can't believe you can get the strength, you obviously do, with designs like that. Very nice and I also like the curve in the rockers.

[Pete Lamberty]

HI David, I have been in the process of making rockers. I am on my third and fourth ones. I must say your chair has a very pleasing and unique design. Do you have any photos of the joints that you used before they were glued together? Could you post them or could you take close ups of the joints in the finished chair. Thanks. It really has a cool look!

[Rich Torino]

David,
absolutely beautiful piece of work. Great design....
Like Pete I'd would like to see acloser pic of the keyed joint..

[David Dundas]

Here are some pictures of the miter joint.

Photo 1 shows the leg miter being cut using a triangular jig clamped to a Kreg miter gauge. The miter at the front end of the rocker was carefully marked out on all four faces of each rocker, then cut with a backsaw and planed to the lines with a block plane.

Photo 2 shows the miter joints, which are aligned by a domino, being glued up. The faces of the wedges have coarse sandpaper glued to them.

Photo 3 shows the kerfs being cut, with the leg/rocker assembly clamped to a shop-made micro-adjustable tenoning jig. To ensure that the kerfs are aligned with one another, both cuts are made from the same direction, rather than by flipping the assembly. A caliper is used to position the jig so that the keys are correctly spaced.

Photo 4 shows the miter keys being glued in position.

Photo 5 shows the completed joint. I pinned the keys with brass screws; but, in retrospect, I think that perhaps the pins serve no useful purpose, and may in fact weaken the joint.

David Dundas

[Rich Torino]

Dave,
thanks for the details on the miter joint... needless to say without your skilled craftmenship the joint would not probably hold up...
One last question what type of glue did you use????

[Cliff Rohrabacher]

very nice looking.

I am frequently surprised at how far you will push the loading envelope of the joints in your chairs. It's very bold.

Have you got to a place in any particular design where you found the joints to be over loaded?

[rich murray]

what a wonderful rocker! doesn't it flex a lot under weight? what are the keys made from? how heavy a person do you suppose could sit and rock in it? very impressive design! thanks for all the pics.

[David Dundas]

Rich,

I used an epoxy with a gel consistency called Techniglue, which is made by an Australian sibsidiary of West Systems, I believe. I have sent photos of my chairs to that company to ask them if they considered that there was a risk of the glue suffering from fatigue after a number of years. They replied that they have had no reports of failure from fatigue, and that joints made with their epoxy have survived intact in boats, wind turbine blades, and other arduous conditions for many years. They also said that in destrucive testing they had performed, it was invariably the wood, rather than the epoxy, that failed.

Cliff,

I have never had any failure in the cantilevered joints of my chairs. I believe that up to now, woodworkers have neglected the possibilities that the great strength and durability of epoxy glues have opened up.

Rich (Murray),

The rocker is made from a very strong wood (jarrah), which has similar properties to maple. It only flexes slightly under load. I am very confident that the chair could support the weight of anyone who could fit into it, say up to 400 lbs. The large glueing area provided by the miter keys gives the joint a large margin of safey. I am convinced that if the chair were tested to destruction, the legs would break before the joints would fail. In another thread, I posted a picture of three men with a total weight of 575 lbs standing on one of my zigzag dining chairs: http://www.sawmillcreek.org/showthread.php?t=54766

This rocker's joints should be much stronger than that chair's, owing to the larger glue area provided by the miter keys.

David Dundas

[Cliff Rohrabacher]

I have never had any failure in the cantilevered joints of my chairs.

I was hoping that you had done some destructive testing to gauge the load limits.

I'm curious that's all.

I am convinced that if the chair were tested to destruction, the legs would break before the joints would fail.

I suspect you are correct.
And of course chairs are not generally subject to constant loading.

[David Dundas]

Cliff,

If I were making chairs for sale in the US, where huge pay-outs for damages are routine, and gross obesity is not uncommon, I suppose it would be prudent to go to the trouble of carrying out destructive testing to ascertain the actual limits of the strength of a chair. But so far, I have not seen the need to do so in Australia. However, it would be an interesting experiment to build a test frame of a zigzag chair and see how many people could be loaded onto it before it broke. I may give it a go at the next gtg of Forum woodworkers here.

David Dundas

[David Dundas]

This chair was one of 65 pieces that were selected in the Studio Furniture Competition 2008 to be exhibited in the Bungendore Gallery near Canberra, Australia.

I have posted inch and metric versions of an article on how to build the chair on my blog, from which they can be downloaded free. The chair is relatively simple to build, if you have a well-equipped shop.

Cliff,

Sorry to have taken so long to reply to your query. I did carry out destructive testing of my spline-reinforced miter joints, as described in this thread: http://www.sawmillcreek.org/showthread.php?t=73197 .

David Dundas

[Bill Huber]

The look of that rocker it just awesome, it looks great.

I love the way the rockers look, kind of like out there in no where.

[Jonathan Spool]

Thanks David!
Beautiful piece of work.
I have never built a chair (other than a stool) but now I am inspired to give it a shot!
Jonathan