Sunday, May 13, 2007

Fitting a Back

Over the years I’ve built up quite a library of books on guitars and their construction. It’s always interesting to read what other guitar makers have to say and guess what? I don’t always agree with them!
All luthiers develop theories about how their guitars behave; I know I have! But whether they (or me) are completely correct in their hypothesis is another kettle of fish. And of course does it matter? If I make a modification to my guitar and the result is the desired one, does it really matter if my reasoning is not 100% accurate? Allow me to clarify that: for hundreds of years we were happy with Isaac Newton’s theories on mechanics and by and large they still predict the behaviour of mechanical systems. It wasn’t until Quantum mechanics came along and silly sub-atomic particles that Sir Isaac was proved “wrong”.
I’ve mentioned the Ramirez book before, to quote Jose III, “I do not think that there has been another musical instrument that has promoted so many controversies, arguments, differing opinions, hair-splitting observations……” See what I mean?

I’ve recently fitted the back on to the mandolin, so I thought I would mention “backs” in more detail. The back of a guitar or mandolin should have a graceful arch to it. This is for a number of reasons, firstly aesthetics; it simply looks much better, than a flat back which I think looks rather flaccid. Old, unloved guitars that have been subjected to climatic trauma and neglect look like this. However, my main reason is that the arch holds the back in tension, if the back cannot flap about or vibrate too much, more of the strings’ kinetic energy can (via the soundboard) move air (sound!) rather than being absorbed into the instrument. The curve also helps focus the sound towards the soundhole. Nava’s 1st Law of Back Construction!
The shape of the back should be part of the surface of a sphere, with its centre about 5 metres in front of the instrument somewhere. Nava’s 2nd Law of Back Construction! Getting the back to this shape is no problem really; the braces are shaped to the desired arc and the reinforcement strip which runs down the length of back’s glue joint is glued on in a curved jig.

Notice I call this a reinforcement strip and not cross-banding. I always have the grain of the reinforcement strip running in the same direction as the back’s grain. Most makers have it at right-angles to the grain. However, I believe that the back is more dimensional stable with my method, as when any wood expands or contracts due to moisture change it moves far less along its length. Hence the reinforcement strip’s grain can’t distort the back’s curve, even if one my instruments isn’t looked after! You can see the back’s construction in one of the photos above.
The real fun is fitting this curved surface to the curves of the instruments side. A couple of years ago I visited a number of luthiers and guitar factories in the USA and all of them used the same method of shaping the edge of the sides so that they match the arch of the back. They sand the edges against a large sanding disc that has been shaped the same arch as the back. I’ve recently made myself one of these jigs and “Hey Presto!” perfect fitting backs in much less time. Hopefully the sequence of photos below illustrates the process and you can see the arch that I’m aiming for.