Posts Tagged ‘ wood working ’

Design matters

Faulty design

One man brought six dining room chairs to the workshop. He was grumbling about poor workmanship because they were all broken in the same spot, where the side rail meets the back leg. Repeated attempts at repair was in evidence. This, of course, is the one joint in all of woodworking that is subjected to the biggest load bearing stress.

Typical rail-to-leg fracture due to absence of bottom rail

Picture an average adult weighing 80kg after dinner sliding the chair back across the floor to leave the table. The back leg momentarily gets caught at an uneven spot on the floor or a joint between two tiles but the momentum, driven by the 80kg load, attempts to continue on it’s path. There can be only one result. The joint starts flexing and after so many flexings it will fail.

I had a good look at the chairs. They were forty odd years old but well made. The jointing was neat, the components well finished and the upholstery way above average even though it was mass produced. The reason for the broken back legs was another one; Design.
Firstly the wood was kiaat, (Pterocarpus angolensis) a semi hard wood indigenous to Southern Africa. We use substantial volumes of it here at the Woodwork Academy because it is the ideal wood to train aspiring woodworkers with, not too hard, not too soft and it has beautiful grain pattern and colours. However, when building chairs without bottom rails, kiaat will not be a suitable wood. It’s either or. Either you use a stronger wood or the design has to change.

Chairs constructed from softer woods demand stronger design - for example the bottom rail seen here.

Some time ago I built a kiaat dining room set for a client. The design, including the thong-weaved seating (which is still popular in South Africa) was to the client’s order except that I insisted on very low-placed bottom side rails in order to strengthen the construction.
A Danish dining room set was on display at a recent Antiques Fair in Sandton, Johannesburg. Although the design is very light (typical Scandinavian) the chairs have no bottom rails. But note the clever widening of the back rail where it meets the leg. Beautiful and practical design and the set is sixty years old!

The second point of interest was that the kiaat chairs were assembled with hide glue. Hide glue quality can vary widely and it is suitable for many applications but not for chair construction. I have come into the habbit of using slow set epoxy for chairs or at least just this one joint in chairs. It makes me sleep better at night.
Hide glue also has the tendency to cristalise and I have not researched this phenomenan as to whether all types eventually cristalise or only the poorer qualities. Here is a cut-away through a chair joint showing the cristalised hide glue. This, I think, brings into focus the particular qualities of PVA glue which is somewhat flexible in other words it will allow some minute flexing without failing. I haven’t had the courage to try it though.
So then, when building chairs, decide carefully on the design and type of wood and don’t use hide glue.

Cross section through hide glue joint

Clever alternative to bottom rail - widened seat rail