Monday, February 27, 2012

Building a Voboam Baroque Guitar -- The Rosette

The last post in this series was February 8. The lozenge--shaped elements that form the rosette and border the soundboard are a distinctive feature of Voboam guitars. I can't image building a Voboam model without including this design feature. It is not as difficult as it looks. In this post I will describe a simple mechanical system for laying--out the design of the rosette border and an equally simple method for cutting and assembling the elements.

I lay -- out the design with a compass and ruler on a scrap piece of soundboard spruce. Since the spruce will serve as a backing for the rosette it is a logical material to use. First, I scribe a circle with a circumference greater than the finished design. Then I divide the circle into 24 equal segments that represent twelve white and twelve black lozenges by using a protractor to mark segments of 15 degrees each. A greater or smaller number of segments could be chosen.  I scribe a smaller circle that will determine the angle of the sides of the lozenge. The larger this circle is the more angled the lozenge will appear. The smaller circle is not marked off into segments.  I draw a line with a ruler from each point on the large circle to a tangent on the smaller circle. The last element of the design is to scribe the two circles that represent the finished rosette.

The individual elements are cut from strips of ebony and holly that I thickness to about 1.5mm. The spacers are prepared with black-white purfling material. When I first made this rosette I constructed a miniature mitre--box to duplicate the two angles. However, the angles in the mitre box were slightly wrong. This didn't manner in assembling the first few pieces but the error multiplied and became significant. Finally I was making so many adjustments to the cut pieces and to the mitre-box that I scraped it and started cutting the lozenges "free-hand". In this method the first cut is always the same angle and can be marked with a gauge. The angle of the second cut is determined by the next segment line. Even if this cut proves to be slightly inaccurate the lozenge can still be used because the correction can be made on the next lozenge. This design is so "busy" visually that slight imperfections are unnoticeable. Note also that the material that I cut the lozenges from is wide enough that there is an excess of material on each side of the finished design. This is intentional. If I happen to make a lozenge too small or too large I can still achieve a proper fit by moving it in or out from the center. I glue each piece in place with hot hide glue holding it in position with a push pin and spring clamp. I am in no hurry when I assemble one of these so every so often during my work day I will make and glue a lozenge. This method suits my work habits. It depends on the spontaneous accuracy of hand and eye rather than mechanical repetition.

Once all of the pieces are in place I clean up the surface
of the design and carefully and accurately cut it free
from the excess scrap using a small router. The spruce
backing protects the rosette and keeps it intact during
this operation. I then rout a cavity in the soundboard for the rosette to sit in.  Members of the Voboam family used a variety of designs around the outer edge of the rosette including thin black and white purfling material that I favor. I then thin out the spruce backing and glue the rosette in its recess.

There is a very useful article by Florence Gétreau on the various construction elements found in the guitars of the Voboam family including a list of barring patterns. In my next post on this guitar I will lay-out the pattern and explain several options that I have used previously.

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