Cavaquinho Back

May 9, 2011

The next phase of this project is making the back.  The two bookmatched pieces of koa are glued together then scraped and sanded to a thickness of about 0.060″.  The approximate shape of the instrument is cut out next.  Although this instrument is small and not under nearly the amount of tension a guitar experiences, there is still a concern about the stability of that center seam, so a small spruce strip with grain oriented perpendicularly to the seam is glued to the inside surface of the back.  (This reinforcement is not necessary on the top of the instrument because several braces, along with the bridge plate and the bridge, cross the center seam and keep it stable.)  Here is a picture of the reinforcing strip being glued.  The strip is about 0.080″ thick and is barely visible under the board that I use to distribute the clamping pressure.

The center strip is sanded to a gentle curve, then notched out with a saw and chisel where the two back braces will be located.

The cavaquinho has two braces on its back; one at the waist and one at the widest point of the lower bout.  The braces are made of quarter-sawn spruce and are about 3/16″ wide and 1/2″ tall when glued.  The surface of the braces that is glued to the back is not flat, but instead has a  25 foot radius sanded into it.  This radius gives the back a small but noticeable curve, which most people find more visually pleasing than a completely flat back.  The curve also  increases the strength of the back and may play a role in reflecting sound when the instrument is played.

After the braces are glued, they are shaped with a plane, chisels and sandpaper, much the same way as the soundboard braces were shaped earlier in the project.

The sides of the cavaquinho are now trimmed to the desired height: 3″ at the tail end and 2 3/4″ where the neck joins the body.  This is done with a plane and sandpaper.  After the sides are trimmed, the kerfed lining is glued in, just as it was along the top edge before the soundboard was attached.  The primary function of this lining is to increase the surface area for gluing the back.

I want to keep the back braces as long as possible and  secure them at their ends so they won’t pop free.  This was easier to accomplish when I attached the top because I could glue a block of wood over the end of each brace after the soundboard was installed.  When the back is put on, I will not have access to the inside to glue in similar blocks.  Instead, the kerfed lining is carefully notched out where the braces will be, and only to the depth of those braces.  These notches will serve to lock the back braces in place.

After making sure everything fits perfectly and that the notches are exactly the right depth, the back is glued to the sides.  I like to use as many clamps as I can fit around the instrument in order to ensure that all points are in contact with the side linings and that there will be no gaps.

The body is now assembled.  The next step will be to add binding around the top and back edges.


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