It is unclear at this writing how many of these I will host, but there is a pile of them at the office, all out of commission. They were built by an unknown local cabinet shop. Almost all of them failed at the mortise and tenon joint at the front rail which supports the seat. The upholstery is in great shape, so the head guy at that office decided they were worth saving.
The inherent design flaw is just that—there’s only that rail, no stretchers below. Nothing in the back failed, where there are three horizontal members.
Not much good can be said about the fit of the tenons in the mortise. But worse than that, the cabinet builder (who was definitely not a furniture maker) put nails in every joint. So part of the repair is figuring out how to get the dang things apart in the first place.
Having done four, I have landed on this procedure: drive the pin nails in as far as I can with a fine nail set, then use a spreading clamp to push the joints apart. The nails may come out in the tenon with the mortise intact. Where there is already a split along the grain line between the nails, I have taken to cutting that piece out and gluing it back in after the tenon is out.
I used name brand epoxy (not drug store variety) and wanted to load it with some walnut sawdust for color. I stumbled on an efficient way to get it. I put an 80x disk on my ROS and held it half on the walnut. The cuttings from the abrasive just fell off the disk and onto the table as it made the half revolution off the board.
Some images from the glueup:
Next: fabricate and install a second stretcher in the front of the chair. The epoxy takes 24 hours to cure, and I won’t rush that.
-- "...in his brain, which is as dry as the remainder biscuit after a voyage, he hath strange places cramm'd with observation, the which he vents in mangled forms." --Shakespeare, "As You Like It"