It was broken. (All good repair stories start here...) The wooden legs dovetail into a cast iron center piece. The corners of the wooden dovetails had split along the grain, causing the legs to fall out, and the area where the casters attach to the wooden legs was worn badly.
I didn't take a good picture before I started, because I wasn't planning to write this. But, by popular demand, I'm blogging again. You will just have to pay attention to the in-progress pictures to see what is being fixed and why. (Or, skip to the end and take a peek if you are OK with spoilers.)
Steel. I like steel (and metalworking/machining in general.) It seems like the right amount of overkill for this.
First, some context. Then we will go back to the beginning. Here you can see the four wooden legs, and the center piece that they dovetail into. My repair was making the metal banding that follows the curves of the legs and dovetails into the center piece.
Step one. Buy materials. In this case, my local hardware store had 3/16" thick steel flat bar in exactly the width I needed to match the wooden legs. Two 3 ft. pieces set me back $13.
Step two. Play with fire. I'd recently gotten an oxy-propane torch to play with brazing. Turns out it can also be used for bending. I heated the end and used my bench vise and a hammer to create a decent 90 degree bend. The short portion at the end will lock into the widest part of the metal dovetail socket. The long end will be bent to match the curve of the chair legs.
Step three. Bend the steel. It took a while fiddling with things to match the curves closely enough to make me happy. All bending was done by hand, with the help of some parallel-jaw pliers and a sturdy bench vise.
Step four. File to fit the dovetail socket. I found a place to buy a decent large file for a reasonable price. The downside is that they are sold in a package of six. That's not all bad news, in that the extras will be given to friends. I could have used an angle grinder, but that is noisy, and it is way too easy to take off too much material. Using a good file, this went surprisingly quickly, was rather cathartic, and I got a very good fit.
Step five. Drill and countersink holes. My plan is for these to be screwed to the bottom of the legs. For that to happen, they need screw holes. This would have been easier to do before bending. Thinking ahead is not one of my strongest skills. And, I only screwed up the alternating pattern on one of the legs. Oops. I couldn't have done this without a drill press and a decent vice.
Step six. Weld the caster posts in place. The posts for the casters were mounted on plates that were a bit too wide for the legs. My answer was remove the posts and weld them to the steel. I was really happy with how this turned out. Well, at least until I turned it over. I wasn't thrilled with the tiny gap between the wide part of the post and the steel.
Step seven. Silver brazing to the rescue! Fill the insignificant gap by brazing. (I mean, I have the torch. I'd be a crime not to use it... This step was totally necessary. I don't know what you are talking about.)
Bonus: these pictures of the brazing process are beautiful!
Step eight. Prime and paint. The color is Rust-oleum Universal Oil Rubbed Bronze. It's a good color. Goes with everything, or so I've heard. This used up the last dregs of a can I had sitting around.
Step nine. Fill in the worn hole left by the old-style casters. The wood was cracking here on all the legs, so I figured it could do with some reinforcement. I drilled it to fit a standard sized wooden dowel and glued in pieces. Cost of dowel was about $3.
Step ten. Drive some long screws into the end of the dovetail on the legs. The heads of these fit into the gap between the wood and metal pieces at the dovetail. Later pictures will make this clearer. This gap will be filled with epoxy, and the screw heads will anchor it into the epoxy.
Step eleven. Seal the perimeter of the dovetail connection between the wood and metal. This will prevent the epoxy from leaking out wherever there is a gap. I had this old sealant sitting around. It still worked, and the dark gray matched the color scheme.
Step twelve. With the sealant cured, fill the gap with epoxy. The top view here shows the gap I'm referring to. I used some steel BBs as filler to make the epoxy last longer. I had these left over from when Dad and I replaced a handle on a sledgehammer in high school. I also got to use up all the leftover epoxies I had sitting around getting old. You will notice that I didn't fill it all the way to the top. I had to leave room for the end of the metal pieces to fit down in there.
Steps thirteen (epoxy) and fourteen (screws). (I clearly didn't take enough pictures.) Fill the remainder of the gap with just epoxy and push the metal pieces down into it. I also coated the entire area between the wood and metal with epoxy, for it to bed into and fill any small gaps. Next screw the metal pieces into the wood. I spent about $10 on different length wood screws to account for the tapering thickness of the legs. You can also see in this picture that I had to shim the space between the center support and the legs since there were some gaps.
Step fifteen. Install the center support piece. Use the threaded holes I forgot to mention making back in step five. Curse when two of the holes don't quite line up. Modify the center piece to make it work. No one will ever know.
Step sixteen. (Not pictured.) Reinstall and shim the casters to sit flat. Did I mention that I didn't take enough pictures?
Complete! And, beautiful. Hopefully it is strong enough. Now it goes back to my friend to be reunited with the rest of the chair. This was a joy to work on.
Total cost: about $26. Not bad.