Unless I have a really good reason to purposely do something otherwise, the default for me, is to make sure the shank is horizontal at strike. That also means the hammer is perpendicular to the shank at strike. As Jon says in his algorithm, String height-shank center height=bore distance. That will achieve shank horizontal at strike. Make sure you have enough room under the stretcher for the action and drop screws on the high extension of their adjustment to clear the stretcher, given the shank center height that gets you horizontal at strike. If there is a disagreement on action cavity clearance and shank center height, that is when you need to make an accommodation like rake, or long bore or something like that.
Knabe may have been doing a production floor accommodation for out of spec string height, or out of spec distance from front of arms to strike line. Raking the hammer back can accommodate a too low shank center. The only time I've seen a rationally designed, tonally purposeful rake, was in the high treble of 88 note agraffe instruments, where the rake is designed to avoid smacking the agraffe with the hammer shoulder.
I just finished and sold a Chinese grand which had a grossly under-bored hammer for a very high string height, with rakes back to beat the band. Action was horrible until corrected with new shanks, hammers and differential.
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Jim Ialeggio
grandpianosolutions.com
Shirley, MA
978 425-9026
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Original Message:
Sent: 12-07-2018 21:19
From: Laura Wright
Subject: Hammer rake angle
Hello all,
I am putting hammers on a small 1931 Knabe grand, and I measured the rake (pitch) angle at 2 1/2 to 3 degrees (toward the player). This is throughout the piano (I measured several samples in each section). I know this is unusual in general, as the angle is almost always 90 degrees, but would this be normal for a Knabe? The hammers and shanks look very worn and original.
I'm just wondering if I should duplicate this, or slightly alter the bore distance and go 90 degrees. Thoughts?
Thanks!
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Laura Wright, RPT
Ivory Keys Piano Service
Durango CO
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