Designing for Movement

Even though its been dried and milled, solid wood lumber still experiences expansion and contraction with changing environmental conditions. This movement must be taken into account when building furniture because if seasonal movement is restricted, the internal forces brought to bear can cause cracking and splitting. The challenge with this piece was to come up with a joinery system that would allow the spare design, just two slabs and two leg assemblies, to be strong and rigid, while still allowing movement of the slabs. Because the grain orientation of the seat and back supports are perpendicular to the grain of the slabs, simply doweling, tenoning, or screwing the pieces together would not allow the slabs to move, and result in failure. The answer that best suited this project was to attach one edge of each slab with a mechanical fastener (aka, a screw), and the other edge with a form of sliding dovetail, known as a blind keyhole dovetail. In this type of joint a dovetail-shaped tenon fits into an elongated sloped-sided mortise - the dovetail shape keeps the slab firmly snugged to the leg structure, but allows the slab to expand and contract by sliding along the tenon.

This is a picture of one of the mortises. You can see that half the mortise is square in profile, to allow the tenon to be inserted, while the other half is dovetail-shaped.

This photo shows one of the back supports. You can see the attachment screw near the top of the support that is the fixed attachment point, and below it the hard maple dovetail tenon that will slide into the blind mortise. To the left you can see the other leg assembly already attached to the back slab.

This next photo shows the similar arrangement for seat slab attachment. You can see that the front of the seat slab will be fixed, with the rear of the slab allowed to move in and out.

Here is the fully assembled bench (upside down) showing the slab-to-leg assembly attachments.

Stay tuned for more images of the final, finished piece. Thanks!