Wednesday, May 8, 2013

Building the Jacopo Checchucci Guitar - The Mould

Jacopo Checchucci, MFA 2001.707, Boston.  Author's photo

Last April I measured the Checchucci guitar and posted a report on my work in this blog (April 25, 2012). During the summer I prepared a working drawing of the instrument and posted it on November 6 in a format that you can download and print. I always intended to build a model of this guitar and last month I got started. So far, I've built the mould, bent the side ribs and today I finished assembling the vaulted back.  Over the next weeks I will describe the techniques, discuss the problems and show you the results.

First the mould. I assembled a suitably sized block made up of  pine
and basswood boards salvaged from an old bench that I no longer needed. The template contour lying on the block is the outline of the face of the guitar that I made at the museum. I had taken particular care in making the template as accurate a representation of the original as possible.  To do so I removed the strings from the bridge and prepared a foam board with a recess cut in so that the guitar could lie flat on its face. A piece of card stock with a corresponding cut out completed the set up. I was then able to accurately trace the contour. I used the template to lay out the guitar body in my drawing and it serves again to lay out the mould.

As I noted in previous posts, the side ribs angle in from the face to the back in varying degrees, from 4 degrees at the waist to 2 degrees at neck and 3 or 3 1/2 elsewhere, as part of an intentional design.
However, I think Checchucci intended a more uniform outcome.  During assembly with the back and top the side ribs of any guitar have a tendency to splay out at the waist. This results in a greater angle, as was observed with this guitar.
 The neck joint area has distorted over time, accounting for the lesser degree of angle in this location. A crease in the smooth curve of the vaulting of the back behind the front block results from the upward pull of the strings.

Given the variation, I decided to make the mould with a uniform side rib angle of 3 degrees.

I band sawed the face contour and retained the waste edges of the block in one piece. I was going to need it for support in cutting the longitudinal profile of the mould.

Once I had the basic shape of the mould I started to contour the vaulting.

If it had been good weather outside I would have taken the mould out to the back deck, fastened it to a support and used a belt sander to make quick work of it. But, there was snow on the ground so I decided to work indoors. I used a block plane to form the vaulting, checking periodically with the templates that I made in the museum.

Once I was satisfied with the shape of the vaulting I used the cross-section template of the highest point of the vault to mark the location of the ribs.

For other locations on the surface of the mould I drew lines across the width at the upper and lower bouts and at the waist. I used the same number of ribs as the original so I divided the line at each location into the suitable number of equal parts. At each end of the mould I used photos to determine the most accurate termination points for the rib ends.

Stretching a thin, straight-sided  strip of wood across the surface of the mould and scribing a line took three hands. Sue helped out. The lines did not always intersect the points that I had marked on the mould, missing by a millimeter or so. In such cases I followed the line of the straight-sided strip of wood and allowed the rib widths to vary occasionally.

To finish the mould I planed facets for each rib with a miniature
plane and applied a coat of varnish to seal the wood and to help prevent the ribs from becoming glued to the mould.

Next time I'll show you the results of my work and describe what I've learned about the design of the vaulting and the technique for putting it together, but more importantly, how the two go hand and hand.

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