Repairing Hole In Rib

triangle strings | technique article | repairing a hole in the rib of a cello | instrument repair | restoration | wood | luthier | woodworking

By Ryan Hayes

A hole in a rib can be two large pieces connected to the rib, or forty pieces in a plastic bag.  Each situation is different, but this article is going to focus on the more complicated situations rather than the simple.

Make sure everything is there

Whenever a customer brings a plastic bag containing pieces of their instrument I get excited. It can either be an opportunity to do something interesting or a black hole of time and frustration. In this case there were 37 pieces in the plastic bag, and before we could give the costumer a quote I had to make sure all the parts were there.

To keep track of everything, and get a starting point, I used a container with a lid. This gave me a rough idea if all the pieces were there and if I should continue trying to find where they belonged,  or look for a matching piece of maple for a rib graft.

make the final rib form first

Before you start doing anything with the pieces, I would recommend first building a rib counter form like the one from my article. In that article I laid out the process of how to make a counter form that will help keep the pieces level and allow linen reinforcement to dry slowly in. This rib repair is different than rib cracks parallel to the grain, but the final form is the same. There will also be a separate inside form made for this repair, but we will get to that later.

remove the top

I would take a look at this article

Remove the back from sections of the ribs

I mentioned this in the article, but i’ll repeat it again; Remove the back from the ribs one block away in either direction of your crack.You could very possibly create a rib crack next to a block when using the form if you do not do this!

making the removable inside counter-form

To make the inside removable counter form you will need packing tape, cork or rubberized gasket material, pine or popular, and double sided tape.  The purpose of this form is to have a curved surface to build the puzzle on, have a sticky backing to hold everything in place, and the ability to remove the form leaving the completed puzzle in place for gluing. The inside form is different from the form I mentioned  earlier because it has no outside counterpart, and though it is a inside form, it is mostly hard and fits the inside of the rib rather well. Here is how I made the form.

Scribe the outline on a piece of wood suitable for making rib forms. In this case I used pine, but anything soft like poplar or pine works. For the height I make sure that the form sits just under the linings.

I used cutting tools like my chisel and plane to get the shape after sawing to my line on the bandsaw. I also made sure to make sure everything is flat and square. When the shape is defined, make sure that the form fits well on the inside of the cello rib. If it is gappy or wobbly, make necessary adjustments. This form is supposed to mimic the inside shape of the missing pieces, and in a perfect world everything is flat and square, but cello ribs are often not.

applying the cork

After the shape is defined and everything fits, I used rubberized cork gasket material as my backing to build the puzzle on. The application of the cork is the important part here. Tape the cork to the form by wrapping packing tape on the outside surface of the cork and continuing the tape to the back of the form. The Idea is to later cut the packing tape so you can remove the form and leave behind the cork and all the pieces stuck to it. Take care when doing this, and I would recommend using packing tape that is wide enough to cover the whole form so there are no seams. Make sure not to leave any wrinkles on the surface either.

After applying the cork to the form with the packing tape, I used 3M double sided tape on the packing tape surface. We didn’t have any wide rolls of this stuff laying around, so I had to very carefully layer it seam by seam until I could cover the whole from. If you are careful enough you can do this effectively.

With some light clamping pressure the double sided tape should stick to the inside of the rib structure allowing you to begin applying pieces. Starting from the outside edges of the hole and working towards the center is a must. If you have only a few pieces to put together this will be quick! I had to take this form on and off constantly to build the puzzle.

dealing with the little bits

Some pieces ended up being very small and went together in the strangest ways. The hardest ones in particular required being taped together on the varnish surface to find their actual place in the grand scheme. These pieces often had a small island of varnish that ended up sitting in the middle of other pieces circling them.

Finding the place for everything was a long and arduous task. I used the “F” word often, and the puzzle had to be taken apart and put back together countless times. Near the end after finding all of the pieces the last few would not want to fit, and everything had to be disassembled and replaced in a tighter fashion to make room. The grain and flame of maple has a curly way about it, and it creates these stubborn fibers that were ever so slightly tweaked when everything was broken. You may need to manipulate them ever so slightly so they will “click” back into place. Placing everything tightly, or for lack of a better work shoving everything over to make room for the pieces in the middle may be necessary.

removing the counter form

Now that all the pieces are there you can remove the inside form. The double sided tape and cork will hold everything in place for gluing! I just used my bridge knife to remove the packing tape for the wooden form leaving behind the double sided tape and cork.

clamp it up to see how it's doing

While the double sided tape and cork are doing their job holding it all together it’s a good idea to do a dry clamping run with the final form. When you build the puzzle everything may be in the right place, but it may be lacking in flatness or shape. You don’t want the pieces to look like a pile of rocks, or some multi leveled mosaic! This dry run will give you an Idea if all of the pieces will settle back into a flat formation, or if you need to break the whole setup down and figure out how to fix a problem.

This is what I had after dry clamping. There were a few wayward pieces that would just fall off without the outside form in place because they belonged on the outside of pieces that were stuck to the double sided tape. They were glued on later when the glue was nice and tacky during final gluing. I would strongly recommend using a ruler in every direction conceivable  to make sure all of the pieces are sitting level. That’s how I level a crack, and this is truly nothing but a whole bunch of cracks. If something is off it will look funny when you’re trying to do retouch.

glue it all at once!

This may seem like a crazy idea, but I definitely think it’s the right move. Remember when I talked about the last few pieces having no room left? If you glue on one piece at a time you run the risk of not quite getting one piece all the way home, and not leaving enough room for the last few pieces . That little bit of dried glue inside the pointy edged spines of the curly maple will be just enough to keep you from fitting all of these pieces together a second time, and that’s why I see this as a one shot deal. I glued this the same way I glue a crack; apply generous glue and rub it into the cracks with a clean finger while flexing the whole thing.  I did this quickly and thoroughly because you only have one chance. At the very end I applied the last few stray pieces that fit in some of holes above. Don’t forget to clean up any excess glue!

clamp it!

I left the pieces in the form for one week checking it every other day to see how the glue was drying. The form does not allow air to get to the glue, and it keeps the glue tacky and wet for an extended amount of time. It does not need to be perfectly dry because we are going to reinforce the cracks with linen just like we do in the article “Rib Cracks Parallel to the Grain” which if you haven’t read yet, you need to.

After waiting a week, you can remove the cork and double sided tape form the inside of the rib. I did this with mineral spirits and a sharpened toothpick.

reinforce the rib with linen

This is done in the same manner as my previous article:The process laid out in that article for applying linen is very important. If you don’t follow my directions you could run the risk of warping the rib, causing more cracks, or both. This process takes 2-4 weeks depending on the relative humidity in your shop at the time so keep that in mind. When linen is used with hide glue it is surprisingly strong. After the linen dried on this project I flexed the center of the of the hole with my thumbs to see how strong the reinforcement really was. I was very surprised by how strong hide glue and linen can be. It felt just slightly more flexible than rib without any damage.

Finding and gluing all the pieces is only half the work. The fill and retouch will take just as long. Fill and retouch is also another article all in itself. I will say however that with out a clean and leveled crack surface good retouch cannot happen.

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