The Bass Bar Frame Used by Curtin & Alf
by Gregg T. Alf

            The bass bar is a string of wood glued to the inside of the top plate of stringed instruments of the violin family.  Usually made of the same spruce material as the top itself, its purpose is, generally speaking, to add strength to the top and to help disseminate vibrations coming from the bridge.

            To fit a properly sized bass bar blank to the contours of the violin belly, violinmakers apply chalk to the belly, position the bar and decide where to trim based on the points where chalk is transferred to the bar.  The bar is then removed, trimmer and repositioned.  The maker will repeat this procedure many times until the bassbar fits perfectly.  Proper final dimensions and subtle variations is positioning of the bassbar in the violin are critical for the sound of the instrument.  A certain amount of spring is often applied to the fit as well, but that is a subject for a whole other discussion.

            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 bat 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 tilt of the bar with respect to the plane passing through the plate-gluing surface. This angle is also very important.  Unless controlled 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 one may fit one end of the bar with one slant and the other end with another.  This is tedious, frustrating, and inevitably results in 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:

• keeps the top from twisting during the fitting and gluing process
• provides a reference point for vertically positioning the bar with respect to either end
• defines a slant with respect to the centerline
• defines a tilt with respect to the plane parallel to the gluing surface
• eliminated a need for cleats or other positioning guides

The jig consists of three parts: a rigid frame and two Positioning Arms.  In use, the belly is protected by a soft pad and clamped to the frame with a series of closing clamps.  We then adjust the two positioning arts to a pre-established slat with respect to the centerline.  A tiling of the bar with respect to the gluing surface is built in to the positioning of each arm.

            The frame provides a rigid, flat surface to ensure that the belly is maintained flat and immobile during the fitting process.  See figure 1.  It can be made of any material that is flat, rigid and dimensionally stable.  For violins I start with a 20cm x 40cm sheet of 15mm thick Plexiglas.  Being transparent, Plexiglas is preferable to aluminum, for example, because of the extra visibility given to the work area.  Cut an opening in the plate, shaped such that the edge of the frame just overlaps the edges of the belly.  The exact shape will depend on the size and outline of an instrument’s pattern, but in practice, I have found that one violin frame will work for all the various models I use.

            The two positioning arms are shaped somewhat like the Letter “L.”  They are key elements our out jig (figure 3) which may represent an improvement over the traditional bass bar frame already in sue by many makers.  The positioning arts fasten to the frame with adjustable screws and provide a stable vertical surface for orienting the bar to the top during the chalk fitting process.  The desired tilt with respect to the plane passing through the gluing surface is built in to the contact surface of the positioning arms and multiple sets of positioning arms can be make if variations in this tilt are desired.

            To use, clamp the belly to the frame with the clamping pad.  Then adjust the arms to touch the side of the bass bar lank which has been roughly pre-shaped to the top contour and carefully positioned to give both the desired slant with respect to the centerline and the desired horizontal inset from the edge of the bridge foot.  A witness line marked on the bar at the edge of one of the arms facilitates repeated repositioning of the bar at the desired end-to-end position on the belly.  See figure 4.

            I have found that the fastest way to chalk fit two parts is to resist the temptation to add a little extra pressure to ‘help out’ the chalk transferring process.   Even though the job is not done until chalk transfers over the whole surface, a light approach that transfers just the highest spots is the quickest and most orderly way to get the job done.  Good Luck.

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1221 Prospect Ave., Ann Arbor, MI 48104

(734) 665-2012   fax: (734) 665-4623

email: violins@alfstudios.com