I repaired this cello for my girlfriend. It is not her "fine" cello, but a cheap one (still a lot of money in my opinion :-)) she bought to teach with. Just after she had bought it, it broke.
The neck broke for no apparent reason. I could see that it had been broken there before and glued. Since the cello seemed to be very old, the glue probably dried out and lost it's grip. Anyway, I wanted to try to repair it and my girlfriend decided to let me experiment because it would have been expensive to take it to a violin shop to be repaired.
This is not a real tutorial because I am certainly no expert in this field. I just wanted to post the images of the process for inspiration purposes. Some important things do not appear in the pictures and are therefore explained underneath.
I found a good youtube tutorial and there I could see that the violin builder in the video took the fingerboard off and glued the cello's neck and afterwards drilled two holes from above and put two thick wooden dowels of maple through the already glued cello neck to strengthen it. He then put the fingerboard back on.
It is probably the correct way to do it, but I could not do it this way as I did not want to take the fingerboard off for several reasons. I had to find another solution.
I have experimented a bit with building loudspeakers where you often use polyurethane glue - a foaming glue to join the speaker boxes together. I thought about using the same adhesive for the cello as it is very strong and since it can expand and fill out any gaps between the two pieces of wood.
My plan instead was to drill a hole which was larger than the dowel into both sides of the wood and to then compensate for the gap by using the expanding glue.
The larger gap would make piecing the parts together easier and more precise.
When I got round to the gluing I put a lot of glue on the dowel and lowered it into the hole and hoped that to glue would expand enough to bind everything together properly. I have since found out that it's a good idea to put a bit of water on the surface that the glue should adhere to (only one side needs the glue on it), but I did not know this during the repair and I guess it makes no big defference.
I opted to make only one big dowel instead of two also in order to make the adjustment easier. Since I could not get hold of any maple wood, I used a piece of beech which I had from an old kitchen surface. Beechwood is similar to maple as it is very hard and strong. I sawed a square rod out of beech that was around 25cm long and 15mm wide on each side. Then I planed the edges of the rod so it was fairly round.
I took a small nail and hammered it into the middle of the broken neck where I thought the dowel would sit the best. I clipped the head of the nail off with a pair of pincers just a few millimeters above the surface. (A boat builder once taught me this method). I then pressed the pieces together so that the nail could make a mark at the same place on both sides. Then I could use these marks as a guide for drilling the holes.
I used a hand-powered drill (brace and bit) with a 16mm flat drill bit. I drilled by hand as it was easier to keep track of the process and as it does not make any noise. It is also kinda dogmatic and sensual and old skool :-).
Ideally you would let the polyurethene glue bubble out of the cracks and holes as it dries and then remove it all when it has dried completely. However, I definitely wouldn't want to do that on this fragile instrument so I carefully dried away the glue each time as it bubbled out so as not to damage the surface of the wood.
The operation went well it seems. Here is a small video where my girlfriend is playing the cello.
