Sunday, May 27, 2012

Assembling the Jauch Lute Bowl

At the end of my last post I had started to assemble the bowl. I made good progress and have finished. With a new mould like this one I never know how difficult it is going to be to assemble the ribs. Each lute is different although there are similarities within in each style. This lute proved to be no different.

The three centre ribs went on easily. I was careful to trim them to the required widths. I added the ebony spacer to each rib before I glued the rib in place. Had this been a solid mould I would have glued the spacers at the same time as I glued the rib to its neighbour. But with an open mould there is always the danger that the spacer will drop a little below the surface of the two ribs. This method is slower but more accurate.

NOTE: I used my usual method of shaping ribs on a long plane mounted upside down which must be nearly universal for lute makers. You can see this technique in my Post Archive from September 28, 2011.

After the first three ribs things became a little more challenging. Since there is some flattening in the design of the bowl and particularly because the bowl becomes relatively deeper around the front block the ribs develop an odd shape that is difficult to make. Here is a series of photos of the profile in paper of the 5th rib on the treble side counting from the edge. The top edge has been shaped to fit to the previous rib and the bottom edge has been shaped to fit within the rib lines of the mould.

The straight line is added for reference. It is not a centre line. You can see the difference develop on the right side of the photo.  The bottom edge has a smooth curve but the top edge has gone flat.

This is the middle third of the rib. The bottom edge continues its smooth arc while the top edge has actually gone negative. That is, it has become concave. The deepest and widest point of the bowl is denoted with an arrow on the right of the photo.

 The two sides of the rear of the rib appear more conventional, but interestingly, it is the top that has a tighter arc. All of these features are a result of the flattening. With bowls that have a more pronounced flattening the rib shape is more exaggerated. The negative aspect is greater and the arc of the bottom edge of the rib at the rear becomes straighter and in extreme flattening, reverses. Crazy!

All in all, I was pleased with the way the new mould worked out. There were some problems though. I hadn't gotten several facets flat so the rib sat too high. That was easily remedied. At several other places it was obvious that I had filed the facet too low and the rib wouldn't make contact with the mould. My temporary fix for this problem is to insert a small piece of card stock wrapped in clear plastic tape. This provides a surface for the rib to sit on. Before I assemble another bowl on this mould I can make a permanent fix if I choose.

The rear area worked out really well. The rib ends came nicely together without problems and they maintained a uniformity of widths.

Once the bowl was assembled I cleaned it up to a respectable finish. Usually, I like to apply the apron or at this time. But I didn't build that capability into the mould. I had decided to follow another sequence because of what I have been seeing in historical lutes and guitars recently.  The inside clasp in lutes or the end blocks in guitars are often shaped to provide a good gluing surface for the belly but are otherwise reduced in size and mass. Also, I had seen one instance where the bowl apparently was papered before the inside clasp was applied. So I decided to paper the bowl, apply the clasp and then glue on the apron in that order. I use 90 lb water colour paper.  As the glue  and paper dry it creates  tension laterally across each rib and it was that effect that I wanted to concentrate in the rear area of the bowl. It seems to me that delicate instruments require extraordinary treatment of seemingly insignificant features.

I wrapped the outer surface of the bowl in plastic tape in order to protect it and made a wooden form that fit the contour of the rear edge of the bowl. Clamped in place, it and the plastic tape secure the form of the bowl while I apply the paper and the clasp. This photo was taken after I glued the clasp in place. Naturally I used more clamps. The clasp is spruce 6mm thick tapering along the top surface to 1.5mm at the ends. Before I glue the belly in place I will taper it in the other direction as well down to 1.5mm along the bottom edge.

To apply the apron I use as many clamps as would fit. The little wooden cauls are concave so that the top and bottom edges of the apron are in firmly in contact with the bowl.

I have thinned out the soundboard material and have started to carve a triple rose of the lute. I am using a different cutting method for these roses than I did for the Kaiser theorbo in previous posts. I'll report on this soon.

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