The piano frame is the heaviest part of the piano, weighing in at around 450 pounds for a grand piano, providing the main tensile strength for modern instruments. Cast iron frames have been used in piano designs since the 1870's. Prior to cast iron frames, pianos frames were made entirely of wood. But as composers wrote pieces that demanded more notes and a wider dynamic range, the wooden frames had a tendency to buckle and warp under the tremendous stresses from the stretched strings. Modern piano strings can exert 18-20 tons of pressure!
While there are newer techniques for vacuum molding and flash freezing the molten iron, the traditional method that has been around for thousands of years - using sand molds made from impressions by wooden models and pouring molten iron into the molds - still produces the best results. Iron is the best material for piano frames, due to its lack of pitched resonant frequencies that can interfere with the vibration of the strings.
In the past, manufacturers needed to create multiple wooden prototypes to perfect their designs. Nowadays, Computer-Aided Design (CAD) technologies make the piano scale design process much more seamless.
In this video, we look at a Steinway model A grand piano frame and a Cunningham upright piano frame.