Designing a Dining Chair

I'm nearing completion of the second and likely final prototype on a new dining chair design, and I'm getting really excited about getting some finish and upholstery on it. That will all be done soon, and I'll post more pictures when its complete, but I thought I would go back and trace my design process for those who are interested. 

The goal was to create a new design which would continue my line of three-legged danish-modern inspired chairs. With the armchair and lounge chairs already designed and with several of each built, I already had something of a starting point, but the big difference with this design was that it would not have arms. Without arms, I knew I would need to work in other ways to stabilize the legs, and drew in a pair of stretchers just a few inches off the ground as shown below in an early small scale drawing. For those who don't know, most of my design work is done by hand.

I took those small scale drawings, scaled them up to full size and set about creating a full scale mock up. Drawings are great, but there are things they will not show you. A full sized mock up made of scrap wood and cardboard may not allow you to test all the ergonomics or feel the weight of the finished piece, but it is something you can walk around and really see how the angles of the legs interact as you look at the piece from different angles. It also provides an opportunity to test out a variety of revisions without significant investment of cost. The shape of the seat and back rest were parts I really had to agonize over at this point, but cutting pieces out of cardboard is quick and doesnt cost anything. With this step complete, the mock up is taken apart, and the pieces become the templates for the first prototype.

Legs were turned, rails were shaped, joinery cut and eventually pieces were assembled. Sounds easy, but a lot of work goes into a chair like this. For example, that back leg is turned for most of its length, but the angled bit and curve at the top are cut after turning, and the curves are shaped largely by hand with a spokeshave. It takes time to create a smooth transition and curve with enough tension for visual interest.

As soon as the pieces were assembled, glue had dried, and I was able to put the laminates seat panels in place I knew I had a problem. While the pair of stretchers between the front legs and from that one to the base of the rear leg prevented the legs from splaying, they did nothing to prevent the back leg from swaying back and forth. I had not adequately appreciated how the arms of the other pieces prevent that motion. Prototyping is all about learning, and I was being taught an important lesson. I had several friends with experience designing and building a variety of things and had them lean from side to side against the back rest. We talked the problem through and I came up with an experiment: Adding cables from the base of the rear leg to the corners of the rail across the front. My hypothesis was that the chair needed triangulation between the bottom of the rear leg and the front rail. Tightening that cable created a marked improvement in the stiffness of the design. The cables can be seen in the picture below.

For the second prototype I replaced the cables with wooden stretchers and did away with the stretchers down low. I also decided to switch from 5 blind tenons to 3 wedged through tenons for joining the rails. To create space for both stretchers to pass through the back leg, I had to leave additional mass near the bottom of the back leg. I also drilled those holes before turning which involved a fairly complex setup on my drill press. The shape of that bulb was refined by hand after the leg had been turned.

Now that this second prototype is glued up, and I've had a chance to sit in it, I am really pleased to say that the revisions are a success. The back leg is very stable and I am very pleased with how it looks. The angled stretchers are dramatic and absolutely functional. As always, I'll continue looking at the design critically, looking for whatever small revisions I think will improve the design, but at the moment I'm looking forward to finding that first client who would like some of these made. Pricing will be the same as the arm chairs; $1175 in plain fabric. Fabric with patterns or leather will be a bit more. If you are interested in commissioning some of these, let me know. I'd love to build more of them. Pictures of this piece finished and upholstered should be up within 2 weeks.