I've always found setting downbearing the hardest part of belly work. Wanting to understand why DB measurements, before planing/spoke shaving bridge cap to height, was such a hard task to nail dimensionally, I came up with a way to pre-load the board with reasonable precision, and take measurements of a fully clamped but unglued board. I was frustrated with guessing about where the board was going to end up under load (I notch and pin outside the belly). The last couple of belly's, using the technique, I came up with some wild, seemingly anomalous measurements. Scratching my head, and going back to see why I got these seemingly weird measurements, found the measurements were, in fact, dead on, and the plate itself was crazy off.
First screwy one was an A1. Just yesterday, a Chickering 122 I'm working on. Measurements said the bass bridge should scoop down in the middle by a good 1/8". I spoke shaved to the measurements. It looked weird, so I started questioning the measurements. Re-did the measurements, which were right on...no help there. The scoop down persisted, but I still felt uneasy about leaving it that way. Come to find out, the plate's bass hitch shelf is anything but a flat plane...in fact it scoops as my measurements indicated. The original bass bridge did not follow the plate...it was a flat plane, saying that DB was either way too heavy, or negative, depending on how they played the scoop in the factory.
I often, when measuring each string DB with a digital angle gauge, find, even with vertical hitches, DB wandering off target, no matter how careful I am in the measurement department. This explains why measurements can be such a moving target in any and all pianos.
Just goes to show...you really can't trust anything coming out of any piano factory. Every geometric assumption has to be tested.
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Jim Ialeggio
grandpianosolutions.com
Shirley, MA
978 425-9026
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