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Shrinkage and Struts

   The results of shrinkage will often be seen in the strutting and linings. In the following cases the struts will need shortening. We can see the evidence that shows that lateral shrinkage (in the table) IS greater than longitudinal, (in the struts) so the strut ends often 'push outwards' damaging purfling, edging and causing separations of the sides and the resultant structural failure in severe cases.
 

    On this de Meglio, the tulipwood edging has been pushed off where the struts meet the tulipwood edge.     On the opposite side it was the same. The struts have shrunk lengthways (longitudinal grain) much less than the top has shrunk laterally (across the grain).      In extreme cases, this shrinkage difference will force the purfling, glued to the side, away from the rest of the top. The integrity of the top is now compromised and buckling often follows, as here.

    This is a view of the underside of a top. In this drastic example, the struts remained in place, while the purfling stayed glued to the side. The top shrank inwards away from the edges leaving a very visible gap.

 Another example of the same thing. This can often happen on both sides of the instrument, because the edging is securely glued, the centre-join re-enforced, leaving the thinnest part of the top, under the purfling, to split. In this example, the struts were all in place, and there was no splitting of the top, but it moved nevertheless. Once removed, the cause was found to be a broken strut end, where it can be quite thin.

 

Linings

Linings are thin strips of wood glued at the top and inside the sides to strengthen the structure. They provide a seating for the struts, and a slightly wider surface on which to glue the top. They are very important to the integrity of the structure. Some cheap mandolins, produced quickly and cheaply for the mass export market, will be found without these linings, and often have detached tops.
  In this instance, linings do not reach all the way round. They are either side for the struts, but not round the tail. Mandolins like this, often split around the seam of the top/side join.      In this much better quality de Meglio, the lining reaches all the way round. Splits around the lower bout edges are much less common.      In this example, when the crumpled top was removed, the cause became clear. The strut slots in the linings were much too deep, so not supporting the ends of the struts well.
      
On this instrument, there are no linings around the sides at this point, and shrinkage has forced the top and the edging apart where the top is thinnest, under the purfling.      Here it is possible to see that the strut top is not level with the lining top at all. This must have supported the top very badly.  

 

 

Tone bar repair

  
   In order to repair/reseat tone bars/struts, the easiest procedure is as follows.
  • First the top needs to be removed. It is extremely hit and miss trying to repair damage, loose struts, and splits with the top in place.
  • Carefully mark the positions of the struts with a pencil, and remove from the top. It is usually sufficient just to slide a flat scraper under one end.
  • Glue up any damage to the top, reinforcing if necessary.
  • Clean the struts, making any repairs, remove any old glue from the foot. If it is badly damaged or missing bits, make a new one.
  • Carefully re-glue the struts in the marked positions, clamping both the ends and middles.
  • Once dry, the top can be carefully refitted again, shortening any over-long strut ends.
  • NB some repair of strut slots in the lining may be required first, as the ends need to fit the slots well. There should be no gap between the slot and the bottom of the strut.

   The original construction techniques often seems to have been the fitting of struts first, and then the attachment of the top, sometimes in two halves (evidence from many instruments). This often involved nailing the tops down!! This method may well have been quicker initially, but for our purposes refitting an entire top is easier than nailing or trying to clamp up a top tightly to struts pre-glued in place... I have tried it.  :(