From my training in Germany I remember the following: Upright hammer bore length needs to be such that they "undercenter", i.e. the bore length needs to be 5 mm longer than the distance from the center pin to the string plane. The rule of thumb was that when the hammer is pushed against the string, the hammer shank is offset from vertical to the tune of approximately the thickness of a hammer shank.
This helps to ensure that the hammer falls back nicely in a soft blow.
I am not so sure if the hammer rake angle has much to do with this. I have seen many very high quality verticals, both old and new, whose hammers were not at 90 degrees when they strike the string. They all pointed downward. In other words, the rake angle is greater than needed to offset the shank angle. I could never quite figure that out, but this is what I have observed time and again.
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Jurgen Goering
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Original Message:
Sent: 04-23-2017 08:35
From: William McKaig
Subject: Determining Upright Hammer length
I am installing a new action in an upright piano and I'm wondering if there is a procedure for determining the length of the hammers. I've kept samples of the originals which I can duplicate, but I don't always trust what's there. I also slightly changed the action position. I mostly restore grands so I'm a little rusty with uprights.
Bill
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William McKaig
Tampa FL
813-831-4179
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