Fitting and Shaping the Bass Bar of the top plate...
....I will cover two topics on this page. The first is some tips on fitting bass bars to violins.
The second will be how to shape the bass bar to match the violin top, acoustically.
...First then, some general tips on fitting bass bars (with photos to illustrate):
... On the underside of the violin top, layout and draw the centerline in pencil. Next draw a line across from the two inner "ff" hole notches (or from where they will be located).
... The back of (or center of) the bridge will be on this line.
... Now mark a line 1/8" behind the future back of the bridge on the wood (inside). Position the bridge that is to be used , centered to the centerline you have drawn, and draw a box where the solid part of the left foot of the bridge will be (the center of the bridge foot). Make this box 5 mm wide and as long as the bridge is thick. The bass bar will cover this box.
..The center line of the bridge is 7 5/8" down from the front edge of the top plate for 14" body length.
...The length of the bass bar is to be:
10 1/2" for a body length of 14"
10 5/8" for a body length of 14 1/8"
10 3/8" for a body length of 13 7/8".
... Select quarter sawn, well aged spruce with 5 to 7 grown rings wide, prefinished to 5 mm. wide. . Select and arrange the wood so that any natural grain runout (curve, as viewed from the side) follows the curve of the plate arching. Choose wood that is either 231 HZ or 242 HZ Wood Tone and #22 Rub Tone. See other sections about these two topics.
...The bass bar will be centered in the vibrating area of the plate, that is between the two body end block support areas. Measure from the centerline of the plate to the inside edge of the box you marked out above. At the upper end of the bass bar (toward the neck), mark this measurement minus 0.050". At the lower end of the bass bar (toward the end pin) mark this measurement plus 0.050". Now draw a line between these two new points I pencil in a line using a flexible metal ruler as a guide; this line is the inside line of the bass bar.
...Here is a piece of naturally curving quarter sawn spruce bass bar stock. Notice how closely the bass bar follows the inside curve of the carved out violin plate.
...The bark side of a wedge of spruce showing the natural grain runout that can be used to advantage in this application.
...Next, the bass bar stock is taped in positon over your pencil guide lines and the arching shape of the plate is traced onto the bass bar stock by running a pencil in a small metal washer's center hole along the length of the bass bar stock. Make a line on each side of the bass bar. Then by shaping the bass bar stock down so that both lines just remain, you will have colsely established the bass bar's bottom contour.
...Now lay a piece of carbon tracing paper, ink side up, in position of the bass bar and slowly work the bass bar sideways while holding it perpendicular to the wood's surface. Notice how the ink transfers to the contact areas. Now sand off the inked areas with a small sanding stick. Repeat until the ink line is continuous. Then the glue joint will be perfect. Remove the ink and the bass bar is ready for gluing to the top plate.
...Notice how the plate is clamped in a fixture which holds the edges of the plate level. This will ensure proper fitting and no warpage.
...The fitted (but too tall) bass bar is trial clamped into position. All parts are then warmed to about 100 degrees F. while the fresh, full- strength, hide glue is warming up to its 150 degree F. working temperature.
...Then both the bass bar and plate area to be glued are brushed with glue and the parts are put into place and clamped gently (so as not to distort the very soft top plate wood).
... Let this dry overnight in a warm room before removing the clamps.
...Now just a few extra thoughts. The current top plate tap tone frequency should be adjusted to 176 HZ before adding the new bass bar.
...After gluing in the new bass bar and drying overnight, it is shaped until it produces an even tap tone all along its top length of 176 HZ to match the top plate.
..This shaping technique will automatically produce the classical and correct bass bar shape.
.. A fixture is needed to hold the glue edges of the top plate exactly flat, as it will be when glued into the body of the instrument; which allows for proper stress-free fitting of the bass bar.