Pianotech

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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I do it differently.  

Install the board with no bridge caps.

Compress the board to take out slack (or however you prefer) and then using a gauge at the string termination behind the bridge (on top of aliquots or whatever) and a gauge on top of the bridge determine how thick the bridge cap needs to be to achieve the proper bearing.

Then I make the caps to that exact thickness (plus a little for sanding) and glue them on.  Trim flush, sand to final thickness and smoothness, drill notch and pin.  Can't miss.  Does require working in the piano though.  

Any tweaking needed can be done with adjustable perimeter bolts.

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David Love RPT
www.davidlovepianos.com
davidlovepianos@comcast.net
415 407 8320
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Original Message:
Sent: 04-13-2019 11:40
From: Jim Ialeggio
Subject: Downbearing measurements

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.

------------------------------
Jim Ialeggio
grandpianosolutions.com
Shirley, MA
978 425-9026
------------------------------