TRADE SECRETS

Makers reveal their special techniques

Making a single-use mould

A method that can be used when making one-off instrument commissions

BY RUDOLF PILSEL

Luthier based in Crewe, UK

Awhile ago, I was asked to make a threequarter-sized violin. It was a job I looked forward to but one that I doubted I would ever repeat. The making of a new model requires new templates and forms, as well as a mould (if that is one’s usual mode of manufacture), plus a number of cramping blocks used to exert pressure on the ribs over the blocks as they are glued.

My preference has always been for an inside mould, and I formerly used ones similar in construction to those of the Stradivari forms. Their manufacture, and the associated blocks, are quite time-consuming but they can be used over and over again. For this single use,

I decided to make a different kind of mould which would be quick and easy to construct. It would also be accurate, both in outline and in the heights of the ribs, so that they would be the same as their finished height on the instrument. Then they would require no cramping blocks to secure the ribs as they were glued on to the mould.

The finished mould was so successful that I now use this type for all the instruments I make, including experimental models. Last year I made another ‘one-off’: a seven-eighths violin. This year I will be making an experimental 40cm viola, and below is the process I utilised to make and use the mould.

A 12mm-thick birch plywood board

1 I cut two pieces of 6mm birch plywood so that they are oversized, and temporarily join them back to back using 12mm screws, making a board 12mm thick. I use birch because it is close-grained, homogenous in texture and dimensionally very stable. Then I insert further screws in strategic places to hold the two pieces of plywood in areas that will be inside the mould’s outline.

Planing the edges

ALL PHOTOS RUDOLF PILSEL

2 I plane one edge and one end square to each other, and to the flat surfaces. It is from the edge that I mark a centre line on each side with a marking gauge. If the planing is accurate, the lines will be directly opposite one another.

a Template clamped on the board

b The outline is marked with dots

3 I mark the outline of the finished mould on either side of the centre line, using a template that corresponds to the internal shape of the viola ribs (3a). The pencil enables the marking to be made without the surface of the board tearing when drawing across the grain, as it would with a scriber. I make the template from 1.4mm Formica, which I find easy to cut on a bandsaw and quick to finish with a file.

Next, I mark the position of the blocks and the outline of the mould is complete on both sides of the board. I dot around the marked outline with a dot punch at intervals of around 12mm (3b). This is an engineering practice which makes a physical mark on an outline. When the outline is finished, a conical depression (half the dot) is left and indicates that the precise outline has been reached.

a Cutting the outline

4 Using a 6mm blade (six teeth to the inch) on the bandsaw, I very carefully cut the precise outline, with the saw blade touching the waste side of the outline (4a). The blade allows me to cut tight curves, for example in the C-bout areas, in one sweep. I also cut out the positions of the corner-blocks and end-blocks at this stage. Birch plywood, cut with the fine blade, shows virtually no tearing on the underside of the board and produces a finish that is square to the board sides, requiring little, if any, further finishing. The position of the screws holding the two 6mm plates together can be clearly seen on the finished cut-out mould shape (4b).

b How the mould shape looks after sawing

Drilling the holes

5 I then drill four 6mm holes through the upper and lower bout areas, around 15mm from the internal edges of the mould. The two parts of the board can now be unscrewed and separated.

6 To make the finished thickness of the mould equal to the finished height of the ribs, I prepare an insert. This will be placed between the two 6mm plywood mould plates, and its thickness will be equal to the finished height of the ribs, minus the 12mm thickness of the plywood plates, and tapered along its length to provide the usual taper one sees between the ribs of a finished instrument.

Any wood can be used but I make these from Malayan jelutong, frequently used by wood carvers because of its stability and ease of working. I shape the insert so that when placed between the two plates a clearance of approximately 20mm is left, to allow for the gluing of the linings. Then I fix it between the two plywood plates.

I drive four brass dowels, each 6mm in diameter, into the holes drilled in step 5. These bring the two plates into line, so that when they are screwed to the insert, they will be precisely opposite each other.

Securing the insert with brass dowels

a Elastic bands hold the blocks in place

7 Iattach the corner-blocks and glue them lightly to one side of the mould, where they are held in place with strong elastic bands (7a). I coat the areas adjacent to the blocks with hard wax, to prevent any ingress of glue on to the mould itself. I shape the blocks to accommodate the C-bout ribs, which can then be fitted and glued in place (7b). Again, no clamping is used, the clamps being replaced by strong elastic bands. Additional pressure is exerted by the brass dowels, which can be inserted once more into their respective holes.

b More elastic is added for the ribs

Gluing the top-block in a cradle

8 To exert sufficient pressure between the top- and bottom-blocks and their respective ribs, I glue those blocks to their ribs in cradles, which I make with the correct radius for the particular model.

The top ribs are added

9 I then attach the top and bottom ribs in the same way as the C-bout ribs, using strong elastic bands. These hold the ribs firmly to the sides, and to the C-bout blocks.

The linings are fitted

10 With the ribs glued and dried, I remove the loose side of the mould (i.e. the plate to which the blocks are not glued). I fit the linings and then clean up the ribs.

Marking the back outline on the maple

The mould (left) and rib garland (right)

11 Only after the instrument back is completed do I remove the rib garland from the mould. I do this by breaking the glued blocks away from it. The garland is then glued to the back and the making of the rest of the instrument can proceed.