If you are curious about grand piano construction and would like to better understand the terms used here, you might want to refer to this article.
I removed the bass bridge cap with the plunge router and cleaned up with a hand plane. I installed a new quarter sawn rock maple bridge cap of uniform thickness. I then hand planed the bridge cap to achieve the original height of the bridge above the bass apron.
Technique for the treble bridge was somewhat different. The hide glue joint of the original bridge cap easily separated from the bridge with some prodding from a 1-1/2 chisel. This resulted in no tear-out of the underlying bridge. I flattened the underlying bridge with my 15″ flat sanding block. This was a light operation which simply prepared a fresh gluing surface by removing the crystallized surface of the old hide glue. The existing bridge cap was not of uniform thickness. To duplicate this varying thickness in the new bridge cap, I prepared two sections of bridge cap which I tapered to the correct end-to-end thickness on the joiner. That was tricky work, but I satisfactorily achieved tapered thicknesses within a tolerance of 0.1 mm. I cut curves on my blanks to match the curvature of the bridge, leaving 3-4 mm overlap. After gluing, I trimmed the bridge cap with a flush-cutting router bit. These caps were installed about 0.4 mm over finish height. I hand sanded to finish height with the hand sanding block.
Cracks in the soundboard were repaired using two techniques, depending upon the size of the crack. For the larger cracks, I enlarged the cracks, not by cutting and removing material, but by compressing the fibers of the crack edge into a V-shape for insertion of a V-shaped spruce shim. I made these V-shaped enlargements by inserting the 1-1/2 inch chisel into the crack and then giving the chisel one or two sharp blows with a light mallet. I continued this for the length of the crack, then reversed the orientation of the chisel for a return trip up the crack to complete the V-shape. Prior to gluing the shims, I spread the crack some more by placing upward pressure on soundboard ribs below with props. This had the effect of producing slightly more crown in the soundboard while gluing up. Once the shims were glued and inserted, I removed props which were pushing the ribs upward. This caused the shims to be more tightly bound. Once the glue dried, I flushed the shims to the height of the soundboard. My preferred tool and and technique was simply to hand plane to the soundboard surface. At some points along the case edge and bridges this was not possible. In those cases, I cut them flush with a Japanese flush-cutting saw. I then scraped to the final surface level.
I opened the smaller cracks with the blade of my Japanese flush-cutting pull saw. This essentially used the first two or three teeth of the saw as a chisel of less than 1 mm thickness. It proved to be a very good technique and allowed a wood repair of very narrow guage.
Tasks remaining in this area are
- Rebore for new bridge pins
- Notch (chisel) treble bridge
- Install new bridge pins
- Scrape/sand soundboard
- Fill remaining minor imperfections
- Varnish soundboard and bridges