The typical American grand piano is built with a laminated maple rim and spruce beams. The beams are notched or doweled into the rim tying the structure together. The wooden substructure combined with a cast iron metal plate resists tension up to 22 tons on concert grands. This 1973 6' 10 1/2" Steinway Model B grand piano has approximately 20 tons of tension.
Fitting the ribs to the Steinway Model B Grand Piano. Note the tapered ribs and how they follow the graceful curve of the bridge. In the best case scenario, the bridge is centered on the soundboard and ribs.
Mapping the old soundboard. Note the size 5/16" shows the original thickness of panel before tapering. This soundboard was tapered for more flexibility around the back, to the right, and toward the front. Tapering soundboards helps with tonality and speed of response.
The new soundboard is stepped with a router to start the tapering. Finished with a hand plane, the soundboard is sanded smooth and is ready for bridge fitting. Note the pencil marks, these are thickness numbers to help evenly taper the soundboard.
Fitting the Bridges to the new soundboard.
Gluing in the new soundboard. It's very important the soundboard has equal amounts of pressure around the rim to insure a good glue joint. Soundboard glue dries as hard as glass and this helps reflect the sound. Typical wood glues are too soft and flexible for instrument building or rebuilding.
Gluing in the new support beam. Screws temporarily hold the beam in place while the glue dries. Once the glue has dried, the screws are removed and wooden maple dowels are put in the screw holes.
Starting the downbearing process. With the new bridge caps in place, the plate has to fit back into the rim/cabinet. A rotary rasp is used to remove wood so the plate struts do not touch the bridge.
With the plate in place, the rotary rasp is again used to rough in the bearing notches. Then, a brass handled bearing saw is used to accurately set the downbearing (downbearing is the total amount of pressure of the strings on the soundboard). Some schools of thought recommend a downbearing of 1.5 degrees at the front and back of the bridge.
A beautifully installed soundboard ready for stringing.