Pianotech

Hello fellow-techs.

When rebuilding a piano and making pre-crowned ribs: which practice is more common or a better choice, making the height match across the rib, making its bottom side crowned to match the top glue surface - or, making the bottom of the rib straight?

From what I can tell, most people who pre-crown ribs, leave the bottom straight and allow the rib to be "taller" in the middle or centered under the bridges. They aren't likely to make the center of the rib shorter, to match the height of the rib where it terminates into the tapered sections.

The seemingly lesser path is to shave away the middle of the rib so the top and bottom are basically parallel. This introduces minute grain run-out but nothing like the rib tapering does. Others may actually shave away at ribs after bellying the board by tapping and listening.

Something I just learned is that altering rib height is a cubed mathematical relationship with strength whereas altering thickness is a linear relationship to strength and therefore a more forgiving dimension.

while I have never done this, I think that if one where in an experimental situation, they could scallop material off of the side of ribs, somewhat diagonally without interfering with the glue joint to the soundboard, to open up tone.

But the main question I have is what preference rebuilders have to cut crowned ribs parallel top and bottom - or not - and what the reasoning or experience indicates. While the piano soundboard acts as a piston where the middle moves up and down while the perimeter allows the flexibility for it to do so, there must be a limit to how rigid the soundboard should be beneath the bridges. The height of the ribs and their consistency across their length surely influences how the piston works - especially once you realize how sensitive the rib height parameter is mathematically speaking

Thank you.

PS. Here is my latest soundboard under construction; and I'm several days away from bellying another board.