Every defect gets respect
Every project is a learning experience, some lessons are taught while you’re building the project and some lessons come after you’ve completed the project. Back in 2023 I built a set of outdoor chairs to go with the Fire Table that I had also built. You can see my write-up of the project here.
I was very happy with how the chairs came out and we used them frequently over the next couple of years. However, as we approached the two year mark a problem emerged. The joints where the side rails of the chairs met the front leg with a through tenon joint were twisting and breaking. I immediately engaged in a root cause analysis process to understand the defect, like any engineer would.
The fundamental root cause was my inexperience at the time that I built the chairs, but the details were also instructional and informed the design for this project.
The first problem was the choice of material for the chairs. I had focused on the property of being weather, bug, and rot resistant without applying a finish and this led me to choose Western Red Cedar as the wood. I had ignored a couple of other important functional requirements, strength and hardness. Western Red Cedar is a very soft, relatively weak, light weight wood. The chairs that I was copying had been made out of Teak which is the opposite. I probably could have gotten away with it if I had increased the dimensions of all of the parts of the chair but I didn’t.
The joints that failed had a through tenon meaning that the mortise went completely through the leg. I had also foolishly made the tenon square and 1” x 1” because the side rail was 1 1/4” x 1 3/4” so I could. This meant that this tenon was surrounded by only 3/8” of material on each side of the 1 3/4” leg and only 1/4” of material above it. The softness of the Cedar also made it very difficult to cut a clean mortise without tearing out surrounding material meaning that several of my mortises didn’t fit great.
The chairs have sling seats that are suspended from the side rails of the chair by clamping bars bolted through the top of the side rail. I didn’t think about what this meant for the nature of the force exerted when someone sits on the chair. In addition to pulling down on the side rail it also exerts a rotational force inwards. This caused the through tenons in the poorly fitted mortises to rotate and put a lot of force on the meager 3/8” of material next to them and split it.
Finally I also went and looked closely at the original manufactured chairs I had copied and I was reminded that they had screws which had been countersunk and plugged which went through the front and the side of the leg into the rail and the stretcher. I had just ignored them thinking they were unnecessary if I used “real joinery” when in reality they were there to help the joinery resist the twisting force of the sling seat.
The new design
Two of the chairs had developed this problem, the other two were hanging in there because I had gotten better at making the mortises when I made them. But, I figured it was only a matter of time before they failed as well so I decided to remake all of the chairs.
I would be able to unbolt and reuse all of the hardware and the fabric parts of the original chairs. I planned to eliminate all of the causes of failure from the first set of chairs in the new design.
I decided to make the new set out of Sapele which is an African hardwood with beautiful grain and color. Sapele has a Janka (hardness) rating of between 1410 and 1510 which is harder than Teak which is between 1070 and 1155 and about three times harder than Western Red Cedar which is 350. Sapele is also very weather resistant and it is cheaper than Teak and has less silica in it so it doesn’t dull your cutting tools as bad. It is also cheaper than White Oak which was the second runner up.
I eliminated the through tenons and made all of them rectangular 1/2” wide by either 1 1/4” or 2 1/4” by 1/2” deep which in the 1” thick legs left a lot of material on all sides of them. Also I added a countersunk 2 1/2” #10 stainless steel screw into the end of each of the rails front and back and from the sides. I plugged all the holes including the hardware holes which I didn’t do on the other chair.
Another controversial choice I made on the originals was trying to copy the manufactured chair’s joinery for the back leg and chair back rake. They had used a custom finger joint to create the angle between the back and leg and I had used an angled box joint. None of these joints had failed and the only issue I had with them was cosmetic. But, I had been given some feedback from a more experienced member of my woodworking guild that I should have done a glue laminated leg instead. I decided that I would make that the thing that I learned how to do on this project to have a more seamless and also potentially stronger leg.
The manufactured chairs that I was copying have an aluminum bar which curves out from near the top of the back and joins the two back rest frame members together. I had tried to fabricate these out of wood on the original and because of the grain direction they turned out to be fragile if the chair tipped over. I decided to replace these with my own aluminum version of the bar, not bent because I don’t have the tools to bend 6061 aluminum but out of multiple parts bolted together. This bar is important so that when you sit in the chair the sling back has spring to it and the chair back rest frame doesn’t get pulled inward.
I also decided to put a finish on these chairs even though technically the Sapele would also be fine outdoors with no finish. I chose to use Total Boat Lust Spar Varnish, this is a marine varnish which is formulated to allow rapid coat buildup without sanding and long curing times. It is typically used on the decks and other exposed wood on boats in a hostile environment on the ocean.
The project before the project
The first step in fabricating the new chairs was creating the fixture to glue up the laminated back legs. The legs are made up of 8 slices of 1/8” Sapele 2” wide and 36 inches long. I traced the outline of one of the manufactured chair’s back leg onto a large piece of brown paper. I also noted all the transitions and mortices on the tracing in the correct places.
I spray mounted the drawing onto a piece of 3/4” natural birch plywood that was wide enough so that there would be substantial material on either side of the leg when I cut it out.
I carefully cut each side of the leg on the band saw, it is important that the space between the two halves of the clamping fixture is the right width for the entire leg. I carefully cleaned up the edge with the sanding station being careful to come just to the line and keep the edge square.
I then glued these two pieces of the fixture to a matching stack of plywood with another 3/4” layer and a 1/2” layer to make the total height 2”.
Once the glue was completely cured, I cut the two sides apart following the top layer pattern and staying carefully on the waste side so as not to mess up the top pattern layer. Then I took each side to the router table with a 2” flush trimming bit with the bearing on the bottom ( top in this case in the router table) and cut the two new layers flush with the pattern at the top.
I then sanded the face of the fixture smooth and covered all the glue-up facing surfaces with packing tape. I screwed alternating wood spacer blocks to the bottom of the fixture so that they would support the contents of the fixture, slide past each other and allow the glue to mostly pass through. These were also covered in packing tape. A key tip on packing tape, the direction of the tape should be the direction that the fixture is going to move when clamping, not perpendicular. Also avoid any wrinkles.
Later I added some tape covered spacer blocks on top to keep the slippery glued up strips from rising up out of the mold.
The process to create the legs was to slice up two 1 1/16” x 2” x 36” pieces of Sapele into 1/8” strips on the table saw. I didn’t use a thin cutting fixture, I just pressed them against the fence and cut the 1/8” off the outside. There was some small variation in the strips but it had no effect on the lamination. I also didn’t try to additionally sand or flatten the strips I used them right off the saw. I did try to keep them in order as they were cut from the blank. It made the side pattern on the lamination quite nice.
I used a glue roller bottle from Rockler to spread glue on one side of each strip and stack them together. Once I had the eight layers stacked I just slid it into the fixture and matched the ends up with the marks I had made on the pattern on the top. I tried to keep them from sliding out too much at the ends, I had allowed about 1/2” of extra length. The target width for the legs was 1 3/4” so I had 1/4” to clean up the sides of the lamination.
I carefully aligned the sides of the fixture and closed the outside clamps first and each time I had to reset them I just tightened down the center clamp to keep things in place. Once I had the outside clamps fully in place and the center clamp tight I added two additional clamps mid-way in between. I left each leg in the clamp overnight. I would clamp up the next leg in the morning and then work on the rest of the parts for the chair while I waited for it to be done.
When a leg came out of the fixture I would clean up one side on my jointer and then cut the other side with the table saw to 1/16” over the final 1 3/4” width and then I would use the jointer to clean up the saw marks on down to the final size.
I would then mark the top and the foot by laying the leg on the chair I was copying or the other leg if it was already made and mark them and cut them on the chop saw.
Building the chairs
I’m not going to go into all the details of making the other parts of the chairs if you’re interested you can take a look at the linked post in the first paragraph. It was basically the same except in Sapele instead of Western Red Cedar and the design changes noted above.
I also have a mortising machine so that made doing the mortises much easier.
The “plugging” process was two phases. After the glue-up I then did any clean up sanding, tacked everything and cleaned it with alcohol and then masked every counter-sink hole. I then put on a coat of Total Boat Wood Sealer and let that dry over night. The next day I put a coat of Total Boat Lust Spar Varnish on every 1 1/2 hour making sure to get all the end grain, three coats in total (Total Boat says 6-8 but I think they just want to sell varnish and I’m not taking these out on the ocean). The next day I washed all the seat fabric for that chair and then I put all the hardware and fabric on the chair.
The back bar of the chair was made from two 1” standoffs made from 1” diameter 6061 round bar center drilled with a clearance for a 5mm x 60 mm bolt and a bar of 6061 1” x 1/2” that was about 18 1/2” inches long and marked and drilled for two holes centered on the back rest frame three inches from the top. I drilled the holes in the bar stock at the tap size for M5 standard threads. I tapped the holes. I sanded all the parts to 320 grit and coated everything in clear spray lacquer. Then they were bolted through the chair frame into the bar making the cross bar structure.
Second phase of plugging was to make all the 1/2” plugs required, sanding them so they would fit easily and then gluing and tapping them into place. After they were dry I would mask off the area around them and saw them flush with a flush cut saw. I then sanded each plug flush with surface and up to 320 grit and then went through the sequence of wood sealer and spar varnish on each of the spots that needed covering.
The plugs came out very pretty, their chatoyance was in a different angle than the surrounding wood so they gleam and show up also they are frequently a different color than the surrounding wood and stand out for that reason.
The spar varnish is so clear and thick it really highlights and magnifies the natural figure in the Sapele, it really shimmers beautifully. I am a crappy painter so there are runs and drips, you’ll have to find it in your heart to forgive me.
On these chairs I put screw in rubber feet on the bottoms of the legs so they wouldn’t be sitting directly on wet surfaces.
I’m really happy with these replacements, they are a big improvement on my original effort and they salvaged a lot of work I did on the first ones as well.
