Thursday, May 17, 2012

Building a Baroque Lute Mould - Continued

At the end of my last post I had finished "roughing out" the mould. There was still lots to do! I had carefully band-sawed the arc of each piece but the angle of the edge of each was only approximate. Remember that the profile of the center axis is not the same as the profile of the face and therefore the angle changes along the course of each arc. I used a rasp initially, followed by files and finally various curved sanding blocks, rocking back and forth, with each tool from one cross-section to the other to smooth the transition. Working thoughtfully, the proper angle was obtained.

Now I had to lay out the position of each rib.  I drew this diagram of the larger cross-section from the lute plan. The width of the 15 ribs average 35 -36 mm wide but there is much variation. The two edge ribs are significantly narrower ( 26 and 29 mm) than the average. Presumably, they were originally as wide or wider than the average. But at some time the bowl was cut down which was and still is the usual method to lower a troublesomely high string height. Several ribs are 34 mm wide while another is only 31. Such variations are commonly found in surviving lutes although our modern aesthetic prefers a more uniform treatment. A millimeter or two here or there is almost unavoidable and I am willing to accept such variation when I am assembling a lute bowl as long as the discrepancy is not obvious.

I did want the two edge ribs to be wider than the others. I measured the distance along the arc of each cross-section and divided by 15. For example, the largest cross-section is 538 mm so each rib would be 35.87 mm wide measured along the curve of the cross-section. I rounded that up to 36 mm and added 4 mm to make the edge rib 40 mm wide at the widest point. Subtracting the width of these two ribs from the total left me with 458 mm for the remaining 13 ribs which works out to 35.23 mm per rib. Remember that this represents the distance along the curved surface of the cross section and therefore must be measured with a flexible ruler. I repeated this process for each cross section. This takes time.

Before I cut the facets for each rib on the cross-sections
I wanted to be sure that the points I marked out created smooth transitions from one section to the next. I used a flexible ruler secured on edge with two clamps. This gave me two free hands to hold the ruler and mark the rib line. In order to create a smooth rib line I found that it was necessary to wander a little from the rib divisions that I had marked off. This is where the variations in rib widths creep in.

Ivo Magherini photo
  I wasn't sure how the rib lines were going to run over the rear section of the mould. If these lines are badly laid out it will be very difficult to assemble the ribs around the rear of the bowl.  On the original each side of the bowl is different! On the treble side the ribs nearly come to a point but on the bass they spread out and would join well below the level of the soundboard. I tried several times but had trouble marking rib lines that looked reasonable.

 Finally, I covered the area with painter's tape and drew more attempts on that surface. Ultimately I got something I liked. I pricked through the tape along the lines with a pin and used these marks to pencil in the appropriate lines.
When I was confident that these rib lines would result in nicely shaped ribs, I created the individual facets using a rasp and files. I replaced the front of the mould with material for the front block and shaped that to the proper contour.

While I was building the mould I prepared a set of curly maple ribs, so when I finished the mould I could begin the work that I really enjoy.

1 comment:

  1. The flexible ruler looks rather different to any I have seen, and ideal for this task, can you give me some more information about it?