Fitting the Bass Bar

One of the most challenging tasks that I have faced thus far in my apprenticeship is properly fitting a bass bar. As a prerequisite to fitting the bar, the interior surface of the top plate must be finished well – smooth arch, no bumps or irregularities.

The bass bar starts as a stick of spruce that is 5.5mm thick. The exact placement of the bass bar is then calculated and pencil lines are made in the appropriate places. Although many makers use wooden studs to assist them in fitting the bassbar, I am using the Alf Studios bass bar frame, as pictured below. The edges of the top plate are clamped to the frame to minimize distortion, and the arms of the bass bar frame are adjusted so that the rough bass bar stock is aligned with the pencil lines and thus in the proper place:

The rough bass bar stock is then placed into an acrylic jig and roughly trimmed to the appropriate curve:

Then, chalk is used to fit the bar exactly to the inside curve of the top plate. First, powdered chalk is brushed onto the inside of the top plate. When brushing the chalk, it is important to try to attain an even distribution of chalk dust throughout the surface where the bass bar will touch:

When the chalk is evenly distributed, the bar is lowered carefully into position and rubbed slightly from side to side:

The bar is then removed, and the chalk marks on the bottom of the bar indicate where the bar is making contact with the top plate. If the entire bottom surface of the bar isn’t covered with chalk, then the chalk marked high spots are removed with a small plane, and the process is repeated until the fit is perfect in all dimensions. The process for me entailed hours of doing this:

To make matters a bit more complicated, the bass bar in my violin is “sprung,” which means that the ends of the bar rise from the surface of the plate by .5mm and are later pressed flush with the surface when glued and clamped. This requires a perfect fit in the middle of the bar and .5mm gaps at the ends of the bar, such that the end gaps close when the bar is rocked to either side or when light pressure is applies at both ends of the bar.

Once a perfect fit is achieved, which is much more difficult to do than it may sound, the bar is clamped and glued into place as follows:

When the glue dries, the clamps are removed and the top contour of the bass bar is trimmed, which will be the subject of my next blog entry.