Catgut Acoustical Society
Vol 3, No. 3 (Series II), May, 1997

As a maker proceeds to fit a properly sized bass bar blank to the contours of the violin belly, he or she will position the bar, check for fit and decide where to trim, remove the bar, trim it, and reposition.  The maker will repeat the whole cycle many times.

A key to fitting the bar quickly and accurately is a means for returning the bar to exactly the same position each time after it is trimmed.  Commonly, a maker will assure that the bar is returned to the same up-and-down position on the belly by drawing a line on the belly, often between the notches on the f-holes, and marking a witness on the bar at that spot.
           
The maker may aim for a consistent slant with respect to the centerline by using a pencil to mark the positions of the ends when the bar is properly slanted.  A  more reliable way is to tack glue two cleats into position such that the bar is at the proper slant when it touches them.
           
These guides, however, do not control the slant of the bar with respect to the plane passing through the plate gluing surface.  Unless that slant is controlled also, it is likely to change slightly each time the bar is replaced after trimming, and the maker will fit first one side of the bar and then the other.  Or the maker may fit one end of the bar with one slant and the other end with another.  This is tedious, frustrating, and may end with a poorly fitting bar.
           
The jig described here controls all three orientations at once, allowing that maker to reposition the bar precisely after each trim.  The jig:

1. provides a reference point for locating the bar up and down, that is, with respect to either end
2. defines a slant with respect to the centerline,
3. defines a slant with respect to the plane parallel to the gluing surface

The jig consists of four parts: an Underplate, an Overplate, and two Positioning Arms.  In use, the belly is clamped between the underplate and the overplate, and the maker adjusts the two positioning arms to determine the slants with respect to the centerline and the gluing surface.
           
Underplate - the underplate provides a rigid, flat surface to ensure that the belly is maintained flat and immobile during the fitting process.   It can be made of any material that is reliable flat and rigid.  I use a (metal plate?) approximately (provide dimensions).  Cut an opening in the plate, shaped such that the edge of the plate just overlaps the edges of the belly.  The exact shape will depend on the instrument pattern.  Soften the edges of the cutout enough to prevent any damage to the belly, which will be clamped against it.
           
Overplate - the overplate is approximately the same size as the underplate, with a similar cutout.  However, I make it of a transparent material, such as (Plexiglas?).
           
Positioning Arm - the two positioning arms are shaped somewhat like the letter "L" and they are the key elements of the jig They fasten to the overplate and provide a stable vertical surface for orienting the bar to slant three.  The maker should set the angle of the short leg of the "L" to the longer leg to define the desired angle of the bar to the gluing plane.  Use screws to fasten the arms to the overplate such that they touch the bar when it is properly positioned for the centerline slant.
           
In principle, one could put short slots in the longer arm that would allow some adjustment in the centerline slant for different instrument models.  if one want to allow for different slant vertical slants, make several sets of positioning arms, having different angles of the short leg to the longer.

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