Making a mould and rib structure for a copy of an old instrument

BY CHRISTOPHER THORPLuthier based in New York City, NY, US

Makers reveal their special techniques

ALL PHOTOS CHRISTOPHER THORP

A method that compensates for the differences in outline of the front and back plates

Acommon issue in making a copy of an old instrument is how to make a rib structure that fits the often-differing outlines of the front and back. This was especially the case when I was planning a copy of the 1777 ‘Simpson’ Guadagnini cello. Not only is the front 15mm wider than the back in the middle, but the C-bouts of the front are a different shape from those of the back. This undoubtedly contributes to the very particular sound that this model always produces, but it presents some problems in the making process.

The following method of constructing a two-part, full-thickness mould with different front and back outlines is one that I came across while working at Christophe Landon’s shop in New York and have adapted over the years to suit my own method of working. It goes without saying that making an instrument in this way is, as a colleague of mine recently put it, ‘like climbing a tree from the top downwards’. Certainly, starting with a front and back outline and working backwards to make a set of ribs to go in between is slightly counter-intuitive, and most definitely not how the original instrument was made. However, it does provide a method of constructing an instrument that shares the idiosyncrasies of the original in a reasonably faithful manner.

Two clear plastic templates for the front and back

1My first step is to make two templates, one each for the front and back. Whether I am working from a plan or from an original instrument, the method is the same. Working from the outline, I calculate the width of the overhang and the rib. Then I subtract this measurement from the outline to create an inside template from which can make the mould. I like to use clear plastic for the templates as it is easy to see the centre lines through them, but any thin and reasonably flexible material will do.

A pillar drill is used to make the holes

2I make my moulds from 0.75-inch plywood, a pretty stable material. In order for everything to line up, I use two metal locating pins on the centre line of the instrument. In this way I can ensure that when placing the templates on to the mould to mark corner positions and so on, I am putting it in the same place every time. Having clamped together the two templates and the pieces of plywood that will form the mould, I use a pillar drill to make the two holes for the locating pins, passing through everything together.

Sawing wide of the line

3The mould can now be cut out on the bandsaw in the normal way. As I saw out the piece that will be the back, the only thing I do differently is to cut quite wide of the line. I leave extra room because during the final shaping of the mould, the edges will be slanted outwards on the back.

Achamfer is made all around

4At this stage I finalise the mould outlines on the surface, which will form the outside edges of the mould, creating a chamfer all the way around.

The blocks positions are marked on the back

5The way that this form is constructed means that the blocks themselves act as spacers for the mould. I mark the positions and dimensions for the blocks as I would do normally, but on the back surface only. Then, securing the two plates together using shortened metal pins in the locating holes, I cut out the recesses for the blocks in the two plates at the same time using the bandsaw. This means that the corner-blocks in particular will be of differing dimensions top to bottom, but it ensures the gluing surfaces are the same, making the mould easier to assemble. The excess can be trimmed afterwards.

Wooden blocks between the two parts give the finished rib height

6Having made sure that the gluing surfaces for the blocks are flat, and squared to the back surface of the mould, I start to make the blocks. I use exactly the same process as I would when making a symmetrical form. I just make very sure that the gluing surface of the block is totally square with the bottom surface, so that the form doesn’t twist when gluing them in.

As this is a full-thickness mould, i.e.with a height the same as the finished ribs, I make two blocks of wood to insert between the two parts of the mould, to give the correct rib height. Then I glue in the blocks: top and bottom first, then the corners. The wooden spacers can then be removed.

Checking the edges with a ruler

7I like to finalise the edges of the mould with everything glued together. This allows me to slant the edges of the plates to provide extra support during the bending of the ribs. I do this at the same time as shaping the blocks.

After marking the shapes of the corners on the blocks using the templates, I cut down to that line using a knife, creating a chamfer. Then it is just a matter of joining up these two curves with an incannel gouge and a rasp.

The edges of the mould can be shaped at the same time using a rasp across the two surfaces, and checking with a ruler to ensure a straight line between the two.

The rib is clamped into the form

8Bending the ribs to fit a form like this can be a bit of a headache. When bending them, particularly in the C-bouts where the difference in shape tends to be the greatest, and the area the smallest, I find it helpful to bend the rib to the shape of the back outline first, and then allow the upper edge to relax slightly in my hands after removing it from the bending iron. Then I clamp the rib into the form straight away while it is still a little warm and flexible, and allow it to dry completely before gluing. Additionally, I find it helpful to have the mould hanging over the edge of the bench. It is much easier not to have to worry about keeping the bottom edge of the rib flat when bending this complex shape, instead allowing the rib to hang over the bottom in places, trimming it flat afterwards if necessary. For this reason, I leave my ribs oversize to give myself a bit of leeway.

Using the clamping block

9For the C-bout ribs I use a clamping block that is fairly similar to the one I had at college. It is a simple maple block, faced with cork and secured with one clamp at the centre. This block pushes the rib into the form from the ends, which can help it to assume its complicated shape. The only thing to be sure of is to trim the ends of the rib to fit the clamping block, so it pushes evenly all over. Clamping blocks for the other ribs are made by cutting out a block on the bandsaw to suit the shape of the rib, and facing with plenty of cork to accommodate the differences in shape of the curve from front to back.

The finished ribs

10With the ribs all bent and glued, I then transfer the outlines to my pieces of wood for the front and back. At that point I have an outline that is somewhat similar to that of the original instrument.