I use a tool that I bought years ago before inserts came out.  It cuts grooves into the inside gripping side of the fingerholes (or thumbhole).

Basically it looks like a small file that has V shaped edges along one side.  I have tried to find another tool like this but have not, so perhaps one can be made locally in a machine shop from a small file.

 

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The handle is plastic that the file fits into and the handle has an adjustable screw that tightens up against the file so that it can be adjusted as to length, to position the cutting edges where you want them inside the hole.

It cuts grooves inside the hole and depending on how much pressure or how long you keep working at it, you can have shallow or deeper grooves.  


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Edited on 11/13/2009 11:38 AM