Hi Ron, Just for an illustration of how far off that spherically crowned board is that we've been taught is our goal, compute the rise of a 60' radius arc segment along the longest dimension of the board at each rib position. Then compute the required radius of each rib crown from that height. It's an interesting and enlightening one time exercise. I'd say forget spherical crown, since it isn't likely to be obtainable in a real piano whatever you do. If you use positively crowned ribs, you'll have a positively crowned assembly. The fact that it is crowned across the panel grain will mean that it will be crowned along the grain too when it's glued to the rim. That's not hard to obtain, it's nearly impossible to avoid. It's dished, just not spherical. This is a result, not a feature. Now consider how it's possible to reconcile the necessity of a flat wooden panel that has been forced into the compound curve of a dished crown with the requirement that it be free of stress. It's nonsense. Of course the panel will be stressed. Spherical crown, stress free panel assembly, and compression crowning all seem to come from about the same era, or at any rate were and still are presented to us as compatible concepts. But how is it that a panel that's dried down to below 4%MC, glued to flat ribs which resist crown formation when the panel re-hydrates, resulting in a system that, when under string bearing load, requires the panel to be very near it's 580psi cross grain compression limit to function, isn't under stress? The wood can be literally crushed by crown formation and string load support requirements, but mustn't be flexed slightly to lay down on a flat belly bar? Nonsense again. So I'd say you're right to not worry about it. Nothing you would be likely to do during assembly would put nearly the stress on a panel that the old compression crowning techniques do. So as you said, I'd be most concerned about flexibility perpendicular to the bridge, worrying about impedance matching and letting the contradictory stuff go. I'd make the rim the same height all around too. Ron N
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