Pianotech

  • 1.  Hammer rake angle

    Registered Piano Technician
    Posted 12-07-2018 21:20
    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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  • 2.  RE: Hammer rake angle

    Posted 12-07-2018 21:50
    Use a small square to evaluate the center line at strike. Measure string height, hammer canter height. Calculate bore distance and angle (if needed). Note the angle of the string plane.

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    Regards,

    Jon Page
    mailto:jonpage@pianocapecod.com
    http://www.pianocapecod.com
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  • 3.  RE: Hammer rake angle

    Posted 12-08-2018 09:06
    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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  • 4.  RE: Hammer rake angle

    Posted 12-08-2018 12:46
    It is possible that the top action is not of ideal height from the factory. Another aspect is the difference between Hammer Center Height and Wippen Center Height which I've found to work well at 6.4 cm. While considering the geometry of the action, keep in mind the Magic Line.

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    Regards,

    Jon Page
    mailto:jonpage@pianocapecod.com
    http://www.pianocapecod.com
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  • 5.  RE: Hammer rake angle

    Registered Piano Technician
    Posted 12-08-2018 18:44
    Laura,

    This may fall into the category of a stupid question but, in measuring the rake angle, can we assume you drew a line down the CENTER of the moulding, not necessarily from the strike point to the inside of the tail coving?

    The reason  I ask is because Knabe often had a very slim tail with no "coving" to speak of. Comparatively little wood down there (which tells us they knew to get as much weight as possible out from behind the shank...good design).  If you were to use that parameter for drawing your centerline it would be seriously skewed (and show up as a positive rake angle, when in fact it may be 90° (or very close).

    Repeat...this may be a dumb question. Just want to make sure (from past experience).

    Pwg

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    Peter Grey
    Stratham NH
    603-686-2395
    pianodoctor57@gmail.com
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