My computer chair is a straight backed oak chair with casters. The back is held on by two small gussets in the joint between the back and the seat, each attached by two screws in the back and two screws in the seat. Unfortunately, this means the back of the chair is a very long lever arm with which to pull on these gussets, and one day the cheap screws snapped right in half.
This is a comfortable chair, and being made of solid oak, not plywood or particle board with a veneer over it, it was far too expensive to simply throw away without at least attempting a fix. After a couple of false starts, I finally got it put back together, and it is much stronger now than it was before. If I had done everything exactly right, you'd never know it had ever been broken. I'll detail my screwups below so you can avoid them if you ever need to do this yourself.
Furniture repair dowels are short wooden cylinders with slightly tapered ends and ridges running down the sides. These are used to make permanent, immovable joints in wooden furniture when glued into matching holes drilled into two pieces of wood. The tapered ends help it fit into the holes, and the ridges increase surface area to allow the glue to make an excellent bond.
Mistake #1: At first I tried to repair the chair without using dowels, I simply glued the gussets into position. It didn't hold, so I had to use the dowels. I decided to go with 3/8" dowels because they were the largest ones that weren't too big.
Right off I decided I didn't trust using new screws. The chair's back was too much of a lever and one set of screws had broken already. I needed to make a stronger bond between the gussets and the chair, which furniture repair dowels were designed to do. But the gussets were countersunk for the screw heads to sit flush, so to keep the chair looking nice I need to keep those countersinks covered. To this end, I unscrewed all eight screws from the gussets and cut the heads off the screws with a Dremel tool, then glued the heads into the countersinking. This created the illusion that there were still screws holding it together.
Next I used the C-clamps to hold the gussets down to a work surface to drill holes in them for the dowels. Although there were two screws per side per gusset, the 3/8" dowel seemed sturdy enough that I should only have to use one per side per gusset, between each pair of screws.
Mistake #2: I live in an apartment and there isn't much around for handy work surfaces, so I clamped the gussets to the chair's seat to do the drilling. This was my second, and biggest, mistake. Unfortunately I drilled a bit too far into one of the gussets and the bit penetrated the other side, not only putting a hole all the way through the gusset but also putting a gouge in my chair's seat. I filled it in with wood putty but it's still unsightly. Further drilling was done in my garage.
Next I took careful measurements of the holes in the gussets and marked off where the matching holes should go in the chair's seat and back. Remember the carpenter's rule: Measure twice, cut once. Then I drilled the matching holes in the seat and back, being careful to drill deep enough that there would be enough space in the gusset and chair together for the entire dowel, but not so deep that I would drill all the way through the chair. I already did that with one gusset by accident. I did this by drilling in short increments and checking the depth regularly.1
Next I double checked the alignment of the gusset and chair with the dowels, not inserting them all the way, because they fit so tightly they would have been difficult to remove, but enough to make sure both holes were lined up close enough that both ends of the gusset would line up right. They were right, but it would have been a lot more painful to find out I was wrong if I had already squirted glue in there.
I'll spare you the details (which involve starting over from scratch), but I quickly learned to use a lot of glue. I mean soak that dowel in wood glue, fill up the holes halfway, make a big mess. That's why you have paper towels. Wood glue should ooze out of the holes when you push the dowels in. Using a lot of glue ensures a good, strong bond. Next you need to wait for the glue to set. The bottle says to hold the pieces together for 10-30 minutes, but I used the C-clamps and held it down overnight. Then you need to leave the chair alone with no stress on the joint for 24 hours.
Mistake #3: In my first attempt, I just put a light coating of glue around the dowels, and they eventually worked loose. A more careful reading of the instructions on the bottle revealed the phrase "heavy spread of glue."
Fixing the mistakes: The last thing was to fix the hole in the gusset and the gouge in the chair that I made because of mistakes due to lack of experience. I didn't mention this above, because this doesn't apply if you didn't make the mistake I did, but I pushed the dowel up a bit through the hole I made in the gusset. After the glue dried, I trimmed and sanded the top of the dowel with my Dremel to match the original curve of the gusset to hide my blunder. A quick trip to the hardware store brought home a touch-up marker roughly matching the shade of my chair which I used to color in the dowel and also the wood putty I used to fill in the seat gouge.
So weeks later, the chair is stronger than ever. These four 3/8" dowels are much stronger than the original eight screws were, and by gluing the screw heads back in place on the gussets I even made the chair look as though I didn't do anything to it at all. Or at least I would have if I hadn't gouged the seat and drilled all the way through one of the gussets. The next time I have to fix something, I'll have a better understanding of what I'm doing, and hopefully you can learn from my mistakes.
1. lj informs me the best way to drill to a pre-specified depth is to mark your drill bit to length with tape. Thanks for the tip!