Monday, June 8, 2015

Engineering solution?

To get around the issue of the grain running in the wrong direction on the connector of my modified Tage Frid-style stool, my idea is to make the connector piece essentially out of hardwood plywood. I created the plywood with ¼” thick plies of cherry in an alternating perpendicular orientation. This way, potentially, the wood on the sides of the connector piece will have a substantial amount of end grain to connect to.

I made the plywood, cleaned it up a bit on the jointer and planer, and cut the sides with a 15 degree bevel. This bevel establishes the splay angle of the legs. To complicate matters, the leg will also be joined at another angle that will establish the rake, making this a compound angle dovetail. (The splay angle is side to side; the rake is front to back). I used the test leg I cut for my first test since it already had the tenon that will form the vertical tail of the dovetail. The trick to cutting angled dovetails is to not think about the fact that they are angled dovetails. Just mark everything clearly and cut to the lines.

I was a little nervous when I tapped the dovetail together because this is where it broke in my last test. The fit was a bit too tight and I had to do more pounding than tapping to put it together. The joint survived the pounding without a budge. Now that it was together, I had to try to break the wood. I wrenched it around, simulating the wracking force chair legs have to endure. Nothing moved; no cracks anywhere. I put the leg securely in my vise and got out my hammer. I bashed it in all directions and still it didn’t move. Finally, I climbed up on the bench and put all my body weight on the joint. There was a bit of creaking, but I think that was me and not the wood.

I’m satisfied now that this plywood method will hold. Not only does it provide end grain for the dovetail but each layer bolsters the other layers against movement. I’m considering adding another ply of cherry going side to side on the underside just between the two leg joints and adding a small stretcher a few inches down from the seat, which will serve two purposes -- fortifying the two legs and serving as a handle to move the stool. Also, I feel like the tenon can be a touch wider, so i’ll incorporate that as well. All in all, I’m happy enough to go forward.