Binding the Cavaquinho

May 23, 2011

After the glue dries on the back, the body is scraped and sanded smooth in preparation for installing the binding around the edges. 

Binding an instrument serves two purposes.  First, it has an aesthetic purpose.  The choice of binding material can greatly enhance the attractiveness of the instrument.  (Incidentally, in guitar building terminology “binding” is the outermost strip around the body and “purfling” refers to the strictly decorative lines inside of the binding.) Second, binding protects the highly delicate end grain areas of the top and back of the instrument.  When the instrument is new, there will be a film of lacquer sealing all of the wood, but as time goes by this film may become damaged through normal wear, particularly around the edges of the body.  If that damage is sufficient to expose the bare wood of the soundboard or back, those exposed areas become susceptible to cracking.   Bare wood will absorb and release moisture several times faster than the protected wood, which creates uneven moisture content and localized areas of stress.  When extreme, this will result in a crack in the wood.  By installing a strip of binding, the end grain of the top and back plates are more likely to remain protected and sealed, even if the lacquer chips away, so there is a smaller chance of a crack developing.

This instrument will be bound with spalted sycamore to match the soundhole ring.  Here I am slicing what will become the binding strips from a larger piece of sycamore.

The first piece of binding to be installed is the “tail graft” which is at the bottom end of the body where the two sides meet.  I usually like to use a wedge-shaped piece of wood for this graft because I like the way it looks.

This photo shows the body clamped to a fixture that I made to allow the tail graft slot to be routed into the body.

Here is the untrimmed tailgraft glued into the slot.

After the glue dries I trim the tailgraft and scrape it flush with the sides of the body.

The sycamore bindings were sanded to a thickness of 0.060″ and bent on a hot pipe, just as I bent the sides of the instrument.  I am going to put three very thin purfling lines around the body for decoration and to match the design around the sound hole.  These black-white-black lines are each about 0.015″ and are flexible enough that they do not need to be bent on the pipe.

I use a router with a specially designed bit to create a two-step ledge.  The inner step is very shallow–about 0.030″–and is just wide enough for the purfling lines.  The outer step is about0.20″ deep and is the proper width for the binding strip.  The resolution of my camera isn’t fine enough, but here are a couple pictures of the process.  Routing the channels on small instruments like this can be a lot more difficult than a guitar because they move more easily and are more difficult to hang onto.

Now the binding strip and purfling lines are glued into their respective channels.  I use wood glue and hold everything in place with strapping tape until the glue dries.  Only one half of the top or back is glued at a time.  Ths first half has to be completely dry so I can get a clean saw cut at the seam where the other half will start.

After all four sections are glued, everything is scraped and sanded flush to the top, back and sides.

I am finished with the body now.  Next I will start to make the neck.

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