The soundboard was tapered (diaphramized in Steinway terminology). First contour lines were routed into the board, and then the board was sanded to the contour lines on the stroke sander.
Profile of countour lines tapers the board from 0.26″ to 0.33″
The video shows use of the stroke sander to achieve the taper by sanding to the contour lines.
Ribs were shaped to a radius of 60 feet, then glued to the soundboard using cauls and compressed air clamping pressure. The photo below shows the last rib in the cauls. Typically I clamped 3 or 4 at a time.
After the ribs were glued, a radius was planed onto the ribs. This is a craftsman touch. Few will ever crawl under the piano to inspect this detail.