| There are times when you will come across a mandolin that inexplicable seems to have sunk around the area south of the cant, often taking down the cant with it. This is often couple with deformities elsewhere, especially around the sound hole. The reasons can be several, but one to look out for, and it needs to be corrected to make the mandolin stable, is the grain direction around the centre join. | |
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Here is the problem visually.... the table on this mandolin has sunk either side of the sound hole, risen under the scratchplate, and sunk under the bridge and into the lower bout. The result is, there is not enough room for a bridge, and instead of the top being convex at the bridge position, i.e. to support the string pressure, it is in fact concave, dipping downwards at the centre. This means the integrity of the structure has gone, and eventually the tail would pull up, forcing the sides out, and a total collapse of the body. |
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So, in order to correct this complex problem, it is
necessary to remove the top, which I have done here, taping up the
edging first.
It is this photo which in fact shows the root of the problem. The strut arrangement underneath was quite normal, so what caused the unusual collapse??? If you look carefully at the grain direction in the lower bout, you will see that it is parallel either side of the centre line. Here is the problem.... it shouldn't be!!! When the struts are fitted, the top is curved, side to side. Because of this, when the lower bout is bent down, the two sides tend to move inwards because of the curvature.... there is no way that a top can be bent at the cant without this overlap in the bottom half, so the grain in the lower bout should converge towards the tail. If it can be attached without this convergence, it can only be made to, by compromising the doming... thus the top cannot withstand string pressure half so well. |
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Here is the underside after flattening has removed
the warping. When it is flattened like this, there should be a
distinct and widening gap of up to 2-3mm towards the tail. The
only gap here is where I have opened the centre join in
preparation for removing wood for when I refit the top....
A more detailed exposition of this feature of the bowlback table can be found here on the site....
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| more when I get it back together again.... | |