Wednesday, June 3, 2015

Engineering Problem

I love a good engineering problem. I think my favorite thing about woodworking is that it’s the intersection of engineering and creativity. I’m working on a design for some barstools based on the Tage Frid three-legged stool. The problem is that the seat height needs to be about 28” and I’m afraid sitting on a three-legged stool that high would make it top heavy. The last thing I’d ever want is someone falling off of one of these stools. Frid swore by the design of the third leg in the back. I’ve made a couple of those stools now, and I agree that it’s fine when you are sitting at a low height. People aren’t used to having one leg in the back, so when they shift around expecting the seat to be solid, they could topple over.

The second problem is that I’ve seen examples of people making the Tage Frid stool with slight variations and they never look right. The proportions are critical to this design. They make the design. So how do I add another leg in the back without messing with the proportions? I did drawings where I widened the connector between the seat and back to accommodate two legs joined with through tenons. They looked awful. I found that no matter how great the rest of the stool looked, my eye always went right to the connector, which called attention to itself because it was so out of proportion to the rest of the stool. My proposed solution is to join the legs on the sides of the connectors with vertical dovetails.

So. The other problem is the grain direction of the connector. The grain needs to run straight from the dovetails on the back to the tenon on the front. If you make a hole on the side, you leave a section of short grain which is vulnerable to breaking. Indeed, on my first test run, the back chunk of the connector broke immediately. This is a critical area of the stool, so that part of the connector needs to be strong.

My idea for mounting the legs in that spot with a dovetail is to make the connector out of laminated perpendicular plies. It would be like plywood, except this would be hardwood plywood. This is a compromise, as most design decisions are. My hope is that the plies will adhere well and strengthen each other. Time to do some tests.