Wood Self-Tensioning Joinery
Jan Sagasser
3rd year, Bachelor Interior Architecture & Furniture Design
What did you find out with this research?
I wanted to find out why a particular kind of board chair which has been in use all across Europe in all social classes ever since the 16th century, has been so durable. I soon found out that the most important aspect is the self-tensioning effect whereby, if constructed correctly, the legs and also the backrest lock each other under load. I wanted to find out if a new or redesigned connection between the backrest and the seat and between seat and legs could be made with the same kind of durability as the original but optimized for contemporary machine production. We can learn a lot from our ancestors regarding the durability of products. With industrialisation, many traditional construction techniques fell into oblivion; picking them up and bringing them back into people’s consciousness could be formative for the development of future products.
What is the technique you based your material research on?
I wanted to find the oldest possible instructions for building these chairs and found them in a book from 1941 about seating furniture. In it, all the necessary details for building a board chair were shown and explained. Based on this knowledge, I designed various types of joints before I started to build models of the most promising ones. The first prototype broke when I pressed against the backrest from behind, simulating the chair falling over backwards. I fixed this by using dowels which were inserted through the stabilising element and the seat. Since I didn't use wood glue for the chair but still wanted to see a comparison between dowelling and gluing, I repaired the first prototype and glued it. The result was identical.
Materials:
For the prototype, multiplex boards and round bars; for the legs and final chair, pine from hardware store
Tools:
Table saw, arm saw, router, long-hole and pillar-drilling machine to drill the holes that needed to be at an angle, disc sanding machine. That’s it!
Workshops:
KABK Wood
Steps:
Determine proportions and material thicknesses based on traditional chairs.
Cut rough parts, making sure to cut individual elements to size only when you have matched them to corresponding elements, to avoid chain errors.
In the prototypes for backrest, the most complicated steps are the 7° angled holes for the dowels in the seat and the 41.5° hole for the stabilizing element.
Drill holes in the seat board which defines angle of backrest so that holes for the stabilizing element can be adapted to it.
Stabilizing element itself consists of bar into which groove is inserted and board which is the only element that has to be glued to bar. For the complicated step, the groove in the staff, ask Sabin, one of the workshop instructors to help!
When thickness of board and width of groove match, glue them and clamp together while drying.
The most important thing about the legs are the slanted cut-outs, as these define how the stopper fits into them and prevents seat from lowering.
Make the cut with the radial arm saws by placing one cut next to the other to create the width and depth of the groove. As the material is round, a stop has to be screwed to the bar so that the cut is placed at the same point of the rounding.
For legs, drill holes with pillar-drilling machine on a tilted table.
Cut stoppers with table saw and insert into legs. Copy the curve of legs onto the stopper and match them using disc sanding machine.
Make cuts for wedges with band saw.
To assemble, use a combination of hammering in individual elements, wedging and sawing off protruding material.
Test the connections as well as you can with your body weight.
Rework the dimensions again to fit them to the final chair.
This recipe is part of Touching: A Research Method in Art and Design, an exhibition curated by architect and KABK tutor Laura van Santen, featuring the materials and research of students, tutors and workshop instructors from the KABK.
More information on the display, all samples and recipes can be found here.