So, we now had the six ribs for the sides of the bass more or less prepared (rather less than more, as it turned out). All we needed to do was to reduce the thickness from a bit over 3mm to 2.3mm. Once this would have been done with hand-planes and scrapers, but today it's hard to resist the lure of the thickness sander, which grinds off the excess with very little risk of shattering or otherwise damaging the wood. Since I don't have one of these occasionally useful machines, we made one:
A turned cylinder of wood (actually mdf - wood tends to become oval as the humidity changes) is mounted between centres on the old Coronet lathe. An adjustable table is bolted to the tool rest holders, and the drum wrapped with an opened-up 60 grit sanding belt, secured at either end by a couple of staples. The "thickness" of the sanded piece is regulated by tilting the table.
Sanding produces dust. so we needed to attach the thing to the dust-collection system:
Ed is pulling the veneer through the machine; it isn't very efficient, being under-powered, but it did the job without disaster, if slowly.
The next step was to bend the ribs to the shapes required. Luthiers will of course know that this is done on a bending iron - a heated cylinder or curved section of metal over which the bouts (the correct name for the ribs) are bent to shape after first soaking in water. Fortunately we decided to have trial-run using some extra material. This was not successful. We'd chosen a figured maple for the bouts, but it very quickly became clear that the wavy grain that produced the desirable stripy figure had a tendency to break quite easily and crisply as it was being bent. What to do now? For this answer we have to go back a couple of years to the beginning of this project, when we had a very different notion of how to make the ribs.
When we first started working on the instrument, and looking for some guidance beyond the how-to information in the Chandler book, I asked my friend Gregory Brown, an excellent woodworker and musician who lives over the water in Sidney; he in turn kindly called his friend James Hamm, who is a well-known luthier and bass-maker in Victoria. Jim said we were welcome to come by, chat, and tour his shop, which of course we did.
Jim is known in the bass-making world ( a fairly intense place) for several innovations in the design and construction of basses, the most important of which seems to be the adjustable neck, which compensates for changes in string height caused by variations in humidity and temperature experienced by travelling musicians. What interested us, however, was his method of forming the ribs; he didn't steam the bouts, but instead laminated them from two layers of maple veneer, with a layer of raw-silk cloth sandwiched between them as reinforcement.
His view was that this produced a much stronger rib without compromising any acoustic quality. They were laminated on wooden forms using an ingenious vacuum system.
Back home Ed and I immediately set about making a set of similar forms, and using some cherry veneer successfully laminated a sample upper bout. The vacuum clamping system, however, did not work well for us, and we put the whole lot aside and carried on with making the top.
Two years later, faced with broken trial-pieces and a great reluctance to attempt to steam-bend the prepared ribs, we dragged the old forms out from under the workbench, and went back to our original intention of laminating the ribs — but at the same time abandoning the vacuum clamping technique. Some time ago Gregory B. mentioned that he used shaped styrofoam blocks for curved clamping forms: accurate profiles were not important, as the styrofoam squishes down under pressure, thus conforming to the shape of the mould.
Here's how it looks as it happens*:
Here we have the two layers of veneer, as well as cotton scrim (low-rent substitute for raw silk - next time). We're spreading glue (urea-formaldehyde marine glue), and about to clamp up.
Styrofoam blocks in place, prepared and numbered. I'm a lot fussier than I used to be about getting things ready before spreading the glue.
Working quickly (note the blurred head and hands) we clamped up the two lower bouts. The pink styrofoam shaped blocks are clearly visible.
Done! No breakages or other disasters.
*All the ribs has to be re-sanded down to about 1 mm., this seemed to be the maximum thickness which would bend to the sharpest curve without breaking.