Introduction: Modern Steam Bent Chair
In this project I'll be making an easy chair using 2 different bent curves.
A lot of the curves and shapes I used were experimental and by no means have any magical engineering properties. I'll give rough sizes but I would advise doing a little testing yourself, especially when it comes to the bending process. All woods react differently so find one that work for you and run with it.
I also want to point out that I'm using kiln dried wood. Using air dried wood is the better option but most of the wood at your local home centre will be kiln dried. One of the goals of this project was to use wood that was easily available to anyone, without having to go to a specialty timber supplier.
Step 1: Creating the Leg Form
The legs are going to be around 35mm x 35mm square, and bent around a 200mm radius. the total length of the timber is around 1700mm.
I started by taking 3 MDF panels (16mm thick) and gluing them together. In hindsight it would've been better to get bigger panels, but I was using 600mm x 600mm pieces.
I glued the 3 panels together then marked out the curve using a quick compass. I could then mark straight lines that intersected the circle.
The basic shape was cut using a table saw then I used a hole saw to make spaces for the clamps to connect to. After the first bend I went back and drilled more holes, offset from the initial holes, further back. It pays to have more than less holes because you'll only know where the timber is being stressed after the first bend.
Step 2: Bending and Shaping the Legs
I used 12mm thick tasmanian oak from the local home centre and ripped and planed that down to around 3mm-4mm. I did tests to make sure this worked before batching out the entire stack.
Once I had all the strips (I also used a contrasting dark timber in the centre) I started the steaming process. I steamed 3 strips at a time for 3 hours. This gave me the best results. After the 3 hour steam I would pull the strips out and wrap them around the form I created earlier. I did this steam bend in 3 stages, so I ended up with 9 strips bent on the form.
I let that cool for a couple of hours then took all the strips off. At this stage the lignin had cooled enough to ensure they kept their new shape.
I then added some tap to the form to stop the strips sticking to it, and used Titebond original between all the strips. I had to do the glue up in 2 stages but if you had a glue with a longer open time you could do it in one go.
Let this sit for 24 hours! When you pull the strips out of the steam box their moisture content has jumped so the glue won't work as quickly as usual. I made that mistake, it wasn't pretty.
Then I used a router sled and 2 pieces of square tube steel to flatten one side of the leg. This could also be done on a jointer. After that I ran the leg through the thickness planer, easing it around it's curve as it ran through.
Do the same again for the second leg and afterwards you should have 2 identical shapes.
Step 3: Creating the Slats
I created 11 slats, each one around 35mm wide and 12mm thick, and there is a spacing of 5mm in between. This creates a fairly wide chair so I could've done with one less slat.
I started by creating 2 forms, this time out of 2 pieces of 19mm pine. I cut an 1800mm board so I had 400mm for the horizontal portion. I also cut it so the angle between the horizontal and vertical was 100 degrees. Anywhere between 100 and 110 is the standard.
I lap glued the 2 boards together so I had a form that was 38mm tall, enough for my strips.
I created 2 identical forms, one for steam bending and one for gluing. They were almost identical but I made sure I used the same form for gluing all the boards, so they would identical once the glue dried.
Once the form was done I could go back to cutting strips out of tasmanian oak, but I also added some strips of merbau. This is a common decking timber in Australia and it bends quite well (again, some testing was done).
I steamed the strips and after 3 hours I butted them up against the stop on the form, so all the strips would be the same shape and length, and bent them around the form.
After a couple hours I removed the strips from that form and put them onto the "glue up" form, where they would sit overnight. It's the same process as the legs.
Once the glue had dried I flattened the slats on the jointer and then cut the opposing face on the bandsaw. This was a much quicker process than using the router sled and if I did this project again I'd use the jointer for all the flattening.
Step 4: Creating the Strips
During the glue up later on I'll talk about 'strips' and 'slats'. The strips are what I worked on next and these are designed to hold and space the slats.
I cut 3 pieces of wood to allow for 11 slats with 5mm spaces between. I clamped the 3 boards together and used a router and a square to create the slots.
This step would be a breeze if you had a table saw and you would get a more accurate result, but this method worked fine.
Step 5: Mortise and Tenon the Legs
The legs are joined together by 2 rails. These rails can now be cut to length, based on the length of the strips. I used traditional tenons so I added an extra 25mm to each end of the rail, but you could also use floating tenons.
I measure the height of the seat, and marked that on both legs at the front of the chair. For now we only need to know the height of the front of the chair, the back rest will be marked based off of it.
Once you know the height you can mark out the mortise. Because of the splayed legs you will most likely be able to use a square and make your rail parallel with the legs. This will give the seat around a 5-10 degree slope back towards the chair, which is ideal.
To cut the mortise I used a spade bit to hog out most of the waste and then cut the edges using a router. This gives clean inside faces for the tenons to glue too.
Then once I knew the side of the mortise I can mark out the tenons based off of that. These were cut using a router.
Step 6: The Back Rail
The back rail is a little more complicated than the front because of the angle of the slats. Once you've created the front rail, do a dry fit and clamp a slat in the strip to the front rail. This will show you where the slat lines up on the back rail.
The design calls to add a mortise and tenon at the back and then add a triangular piece to attach the back strip. Mark where the back of the slat lines up on the leg and from there you can work out the angle of the triangle. Mine was 31 degrees.
Now you can cut that angle out, I used a bandsaw, and this will be the connection between the back rail and the back strip. Clean up the triangle so the strip can be glued to it later. Once you're happy with it you can glue the triangle to the back rail.
Remember that all the parts will most likely be under tension, even when the seat is empty. So if your marks are a bit off you have a lot of wiggle room. The curve in the slats are very forgiving.
Step 7: Glue Up and Finish
Now that we have all the parts ready to go, it's time to start gluing them up.
Take this time to do some sanding, while the parts are easily accessible.
The first glue up is the leg assembly. Start by gluing the rails in to the legs, giving the glue time to set up.
Next I glued the slats to the strips while the slats were clamped in place. Don't clamp the strips to the rails yet! I did 3 slats at a time so my clamps could reach.
Once you've glued all the slats to the strips you can remove the entire assembly. The next step is to reenforce the join between the slats and the strips by adding some dowels. This is a cross grain glue up so adding a dowel will help with any shear forces.
Then you can glue the 3 strip to the top of the slats. I used 2 equal lengths of timber to make sure the distance was correct. This strip can be anywhere near the top of the chair.
Now you can take the slat assembly and glue it to the leg assembly.
After that I added 2 coats of danish oil and called this one done!