The chair to the left is sitting in our shop right now, obviously in need of a bit of repair. When I took a look at this un-done joint, I was amazed at the horribly bad engineering. No wonder the arm came loose from the seat of this chair. When I saw this joint, I realized, some poor person had purchased this chair set, having no idea that they were making a sad decision. So I thought for your educational benefit, I would help you avoid such pitfalls when next you decide you need a new dining set.
The first thing that I notice about this joint is that someone tried to use a tiny little swirl of glue to adhere a flat surface to a ridged surface. The problem with this, is that only on the few areas where the ridges meet the flat, will the glue be any good. And the glue was only used in a little swirl which makes it even worse. Perhaps had the glue actually been applied to the entire flat surface, the joint would have lasted longer. But ultimately you can not expect glue to hold together a flat surface to a ridged one. They tried to supplement the glue by using a bolt, screwing from the inside underneath of the seat out into the arm. But once the glue failed, all the stress was on the bolt. The arm would have immediately become wiggly. And all that wiggling would eventually have caused the the stiff bold to break down the pliable wood, which would eventually then have broken.
So, when looking at chairs, if you see an attractive contoured base to the seat, take a close look and see if there is any gapping between the seat and the arm.
Now just for contrast I want to show you this chair which is over 200 years old. Yes, it is in our shop for repair, but this chair lasted 200 years before breaking, instead of the handful that the first chair lasted. You can see from the upper image of the chair that it is the same sort of issue, an arm, which has come detached from the seat. But upon looking at the lower image, we can see why it lasted for so long. First off, there are two flat surfaces which are being glued together. and in addition, the arm is cut into the seat a little bit which gives more area for the glue to bind. The second thing I want you to notice is that instead of using a bolt to give added support to the glue, two wooden dowels connect the pieces. The grain on the dowels runs across the joint (rather than with it). This cross grain makes the joint very difficult to break.
Remember this about wood vs metal. If wood and metal ever get into a disagreement, the metal will always win. The ax chops the wood, not the other way around. So if you have a joint which is partially enforced by metal, if the glue ever fails, then the metal piece will wear down the wood over time, and your repair will be even tougher.
I always recommend, when you are looking to fill out your furniture repertoire, buy old! You will get a much better quality of wood, glue, joints, finish, etc. You may need to do a little fixing up, but once that fixing up is done, it should last you a very, very long time.