The back is braced similarly to the front, but with an important difference: the back braces are curved and gluing the back to them imparts a curve to the back plate as well. This is thought to improve tone and sound production in a stringed instrument. Here the outline of the ribs is traced on the back and the brace marked for cutting just inside that mark. The brace must be cut short enough so that the lining fits around it, but long enough to provide support all across the back.
The instructions show weights being used to clamp the middle of the braces and
clamps at the ends to ensure contact all along the brace. Since I happen to have
clamps that reach the center of the backplate, I used those instead of the
weights. The unbraced backplate flexes easily, and the clothespins on the ends
of each brace provide plenty of pressure to hold the back against the curved
braces.
Once the glue has set on the braces, the back can be attached. Test fit the back plate. Make sure the grain along the back runs straight down the length of the instrument, and that the back plate covers all of the rim. If a brace hangs up on the rim, use a chisel to trim it back a little. If the grain doesn't run straight, and a brace is up against the lining, preventing you from straightening it, trim the brace. If this goes too far, obviously it's better to have the rim completely covered even if the grain doesn't quite run straight.
While still holding the test fit together, once the braces are set, look all around the edge and make sure there are no gaps between the back plate and the rib. If there are any, use the sanding board to level the rim. Make sure the grain along the back runs straight down the length of the instrument, and that the back plate covers all of the rim.
Apply glue to the edge of the rim and lining, and spread it out to cover the edge evenly. lay the backplate in place and slip it around until it sits right and covers the whole rim. You can use a long elastic band to wrap down and clamp the backplate, as described in the instructions, or you can use violinmakers clamps (or "spool clamps") like these. I made these out of a 1" dowel and carriage bolts, washers, and wing nuts, or you can buy nicer ones from Stew Mac. I prefer the clamps to the elastic because you can see the joint all the way around and correct any gaps right up to the end.
It's important to have the clamps more or less parallel to the rib while clamping. Tipping them can flex the backplate and leave a weak joint.
I put the assembly into the mold, propping it up with a couple of blocks, to trim the overhang.
Here's the laminate trimmer in action. I prefer it to a full-sized router as it's easier to control. The bearing on the flush-trim bit in the trimmer combined with the flat bottom of the trimmer make it easy not to mess up. If you tip the trimmer down off the edge of the work, the bearing causes the cutting edges to pull away from the work, doing no damage.
Because the back is curved, this tilts the tool slightly away from the work, leaving a very small amount of overhang. This is easily knocked back during sanding.
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