I seem to be adhering to Beckett's approach recently but for some reason, after my previous stool attempts, despite having a functioning design I felt I had to do just one more iteration.
The five legged solution realised in 18mm plywood resulted in my final design for this concept. As my friend Stephen demonstrates the design is pretty solid even for those of us with a more ample frame. There are now a couple of them in use at the space alongside the three legged earlier versions.
I was finally satisfied with the result and thought I was done with furniture making for a while. My adorable wife then came up with a challenge, she wanted a practical foldable chair her requirements were:
- Must be robust enough to cope with guests of all sizes
- Use as little storage space as possible.
- Not ugly.
- Inexpensive enough they can be given as gifts.
My initial thoughts were to replicate the IKEA style A frame design until I noticed that some designs were made from a single sheet of timber with a small number of profile cuts. I searched for pre-existing design files but found none, perhaps I had found something novel to do after all.
popliteal height of 420mm as a general use compromise which is a bit less than the 430mm of most mass produced chairs. I also decided there should be a front to rear drop across the seat which ought to improve comfort a little.
Material selection was based upon what I had to hand which consisted of a couple of sheets of structural plywood (1220x606x18mm temperate softwood - probably spruce although not specified) which had not become stools yet. Allowing for edge wastage and tool width this gave me a working area from a sheet of 1196x582mm.
I sketched the side view of the chair numbering the three parts. I decided part 1 would be 1000mm total height with top and bottom cross braces 80mm high, assuming the 900mm tall target, the A frame apex would be at 820mm height.
I did the maths for the seat triangle using the 36° angle and 420mm popliteal height. Having experienced how flexible 18mm plywood was in the face dimension I decided to make the frame sides 50mm wide leaving, after 6mm tool path widths, enough space for a 358mm wide seat .
Once the dimensions were decided the design went pretty quickly producing something that looked similar to the designs I had seen earlier.
Things had been going far too well for me and during the routing operation our machine decided to follow an uncommanded toolpath excursion mid job (it cut a dirty great hole through the middle of my workpiece where I did not want it) ruining the sheet of material.
The resulting design worked with a couple of problems:
- I had made an error in my maths and added when I should have subtracted, as a result the seat slopes back to front which is not very comfortable.
- The chair feels very wide and tall.
- The front face (part 1) flexes alarmingly between the seat and the ground when subjected to large loads.
I eventually physically went to Ridgeons and looked at what they actually had in the warehouse and got a price on a sheet of brazilian elliotis pine structural plywood for £43.96 which they cut for me while I waited. Being physically present also gave me the opportunity to pick a "less bad" sheet from the stack much to the annoyance of the Ridgeons employee for not just taking the top one.
The new design was cut (without incident this time) and the edges rounded off. I also added some simple catches to keep the seat and back inline while being transported.
While this version worked I had made a couple of mistakes again. The first was simply a radius versus diameter error on the handles which meant too much material was removed from a high stress area causing increased bending.
The third version rectified those two errors and improved the seat front curve a little to further reduce the unneeded material in the back rest.
The fabrication of this design was double sided allowing for the seat hinge to be fully rebated and become flush and also adding some text similar to the stools.
I guess this is a compromise that would take another iteration or two to solve, alas my wife has declared a moratorium on more chairs unless I find somewhere to put the failed prototypes other than her conservatory.
One other addition would be a better form of catch for transport. The ones in version 2 work but are ugly, magnets inset into the frame have been suggested but not actually implemented yet.
The price per chair is a little higher than I would like at £22 (£11 for timber, £5 for hinges and screws , £5 for varnish and £1 for tool wear) plus about 2 hours labour (two sided routing plus roundover takes ages)
As previously the design files are available on github and there are plenty more images in the flikr set.