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The bass bar serves to prop up the arching as well as tune the plate and put it in balance.

The blank
  1. Split a piece of spruce 350 x 20 x 8 mm, ideally the same spruce you used for the top plate.
  2. Plane down the sides so that they run perfectly parallel to each other and the thickness if the bar is about 6 mm for a flatter arch or 5.5 mm for a higher arch and tighter grain. The sides can be planed down to the desired thickness by putting the bar down on the workbench and using a small C-clamp as a stop for it.

Positioning

A good position is crucial for both the sound and structural integrity. The bass bar runs right under the bass foot of the bridge. Notice the slight tilt, causing the bar run a bit across the grain, reinforcing the arch even more.
  1. Mark out the position of the bass bar on the plate, see the circles in Fig. 1 for where to put the marks. "a1" and "a2" are 40 mm, "b" is 196 mm, or precisely at the f-hole nicks (bridge position), "c" is 19 mm. In our case, the half upper bout width is 84 mm, half lower bout is 104 mm. To compute the "d" and "e" positions, the upper and lower bout widths are divided by 7. Therefore 84/7=12 mm and 104/7=14.8 mm. The thick red line shows where the bass bar will be eventually.
  2. Put the bass bar in position and trim its length so that it extends the "a1" and "a2 marks at each end by about 5 mm.
  3. Using your knife, trim the spots where the bass bar touches the plate to roughly fit it to the arching there. Make sure the center of the bass bar is not elevated more than 6 mm from the plate on the left side as you will use a 6 mm washer to transfer the arching onto the bass bar. Also make sure the bar is at right angle to the plane of the plate.
  4. Spot glue the bar in those two spots. Check the position again. Wait 5 minutes for the glue to set.
  5. Using a washer that will offset the line by about 6 mm, draw a line copying the arching on both sides of the bass bar. When finished, break off the bass bar being careful not to damage the surface of the plate.
  6. Pare down the wood between the two lines using a knife, then perhaps a flat thumb plane and a scraper until the bass bar fits quite well and most of all is at right angle to the plane of the plate.
  7. Lastly, if you use the wooden clamps, as in Fig. 2., create a profile of the top of the bar as in Fig. 4. to prevent the clamps from tilting the bar.

Studs

To hold the bass bar in position while being chalk fitted and glued, you need five wooden studs. We put paper on the gluing side to make the later removal easier.
  1. Cut a piece of soft wood to 60 x 10 x 8 mm and glue a strip of paper onto the 8 mm (bottom) side. The grain orientation should be vertical, the same as the bass bar, to allow for easier removal by splitting later on. Let dry.
  2. Cut the five identical studs, about 10 mm wide, from this piece of wood.
  3. Use four wooden clamps or C-clamps as in Fig. 2 to clamp the bass bar in position. Make sure the bass bar is again perfectly vertical to the plane of the plate, when looking from both ends. Also, check its overall position and make sure it stays within the marks, as in Fig. 1.
  4. Start fitting the studs on alternate sides of the bass bar as in Fig. 2. A well fitted stud sits snugly against both the plate and the bar.
  5. With all studs fitting, start gluing them one by one, holding each in position with your fingers for about 20 seconds. Make sure the glue gets only on the paper bottom of the stud, not the bass bar itself, avoid applying too much glue.
  6. Wait 5 minutes for the glue to set, remove the clamps and carefully shuffle the bar a little to ensure it didn't get stuck to the studs. Let dry for a few hours.

Chalk fitting

Chalk fitting helps you achieve the best joint between the bass bar and the plate.
  1. Remove the bass bar, take a piece of white chalk and rub it on the plate, where the bass bar is supposed to be glued in.
  2. Put the bass bar back in place and move it about 2 mm up and down, as in Fig. 3 a few times to highlight the peaks to be removed on the bass bar, moving it more than that would lead to inaccurate imprinting. It is important not to put too much pressure on the bass bar as the plate has a tendency to warp easily, resulting in inaccurate imprinting.
  3. Use your thumb plane, scraper or knife (as a scraper) to remove the chalked peaks from the bass bar. Repeat, keep removing the peaks, reapply chalk, if needed, until the whole bass bar fits well. The finest adjustments are again done best with your sharp knife used as a scraper.
  4. To remove the excess height of the bar to a necessary minimum, at the bridge position, first make a mark at the 15 mm height on the left wall of the bar. Then, extended that mark, drawing a line up to the ends, making sure the height at those is not less than 7 mm.
  5. Plane off the excessive height, keeping the top at right angles to the sides. This is best done with a no. 7 mounted in your vice upside down. If you need to, trim the studs so that they stay below the top line (they snap off easily so be careful).
  6. Again, if you use the wooden clamps, recreate the top profile as in Fig. 4.

Gluing

To clamp the bass bar for gluing, you can use the same clamps you used to hold the bar in position for the studs, only 5 instead of 4 should be used. See Fig. 5 for the positions of the clamps.
  1. Using your coarse brush, remove the chalk left on the plate.
  2. Dry fit the bass bar again and try to put all five clamps on to see if everything fits well.
  3. Prepare fresh medium hide glue, put some on the bass bar, insert the bar in place.
  4. Quickly remove all excess glue with a coarse brush and clamp down as in Fig. 5.
  5. Let dry overnight.

Trimming and shaping

Trimming and shaping integrates the bass bar into the plate finalizing its tuning.
  1. Pare away the studs with a knife or a chisel. Work from top down, along the vertical growth lines cutting away thin slices. Be careful close to the bass bar as the stud may have been glued to it. Moisten the remaining paper patches and remove them with a knife.
  2. Plane the bass bar evenly so that at the bridge position it has the height of about 13 mm, measured from the side close to the center line.
  3. Refer to the bass bar diagram in Fig. 6. to see what the finished bar looks like. The overall shape depends much on the arching and plate tuning. Less stable arching, ie. low and or with long straight central part, needs more support at the center. The bass bar brings the M5 frequency back to where it was before the cutting out of the f-holes. The plate becomes more rigid again so pay attention to that.
  4. During shaping keep measuring the M5 frequency and the weight of the plate.
  5. When the general shape is achieved, the cross section as depicted in the upper half of Fig. 6. is to be worked out. Use first a thumb plane, then an abrasive paper. When fine-tuning the bass bar bear in mind that just a few strokes, especially at the ends, can lower the M5 considerably.
  6. Trim the ends of the bass bar to the final length at an angle of 45 degrees as in Fig. 6. The final length should not be more than 7/9 of the plate length. So for a plate that is 360 mm long, the bass bar should be (7/9) * 360 = 280 mm or less.

Category: Front
Comment by 51.254.238.67
2017-07-30 16:07:14
I don't anderstand what is the M5 frequency
Thanks
Comment by Vojtech Blahout
2017-07-30 18:49:21
Please see http://www.makingtheviolin.com/Free_plate_frequencies