Bechstein B hammer rake

[David Love]

Too many thoughts.  On these pianos, the rake often accommodates a maximum
bore distance (usually around 50 - 52mm in the treble section) to get the
hammer perpendicular to the string at impact irrespective of the shank angle
(generally beyond parallel to the key bed).  Probably has something to do
with clearance under the stretcher and/or strike point accessibility in the
upper end.  I wouldn't overcomplicate your life on this.  Using a string
height gauge with the action on the bench set the rake based on 90 degrees
to the string.  Jon Page outlined a good procedure for this a few years ago.
I'm sure you will find it easily in the archives.  

David Love
davidlovepianos at comcast.net 
www.davidlovepianos.com

-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Richard Brekne
Sent: Friday, June 20, 2008 2:01 PM
To: pianotech at ptg.org
Subject: Bechstein B hammer rake

Hi Stéphane

Thanks for your reply.  Have to think about it for a bit... bore length 
dependent eh ? I've always taken for given that the bore length is equal 
to the distance from the strings (at strike point) to the key bed minus 
the distance of the hammer shank center to the key bed. That puts the 
shank at horizontal at impact. Raking to make perpendicular to the 
string with the shank at horizontal at impact would require the hammer 
to be raked inwards yes ?  Which means increasing the actual bore length 
by the sin of the angle of the rake times the length of the 
aforementioned formula and adjusting the length of the shank center to 
middle of hammer molding accordingly.  I see no way at all that an 
outwards rake is compatible with that formula for bore length at the 
same time being compatible with a requirement of hammer to string angle 
being perpendicular.

The bore length formula could be easily enough modified to work given an 
inwards rake.  That would be (D1 - D2) * cos sin A where D1 is the 
distance of the string to key bed, D2 is the distance between hammer 
shank center and key bed and A is the angle between the key bed and the 
string plane. Bore at that distance would require the center of the 
molding to be moved out on the shank by a length equivalent to Sin A 
times the resultant bore length in order to maintain the shank at 
horizontal at impact while the hammer is perpendicular to the string.

Either way... somethings gotta give..  either hammer shank center to 
center molding, hammer rake, or shank in horizontal at impact.

Strikes me off hand that shank being horizontal at impact is a 
desirable, so if hammer rake is then to be decided by string angle then 
floating the distance between hammer center and center molding seems to 
be the only solution.

yes ??

RicB


         Hi Ric.

            Strings angle up to the bridge from the striking point so
            anbackwards rake would just make the angle between hammer
            and string atimpact even farther away from this stated optimal.


        < Ar, depends on the bore length, not ?  Do you use the dummy
        spare hammers with known bore length trick to accurate the bore
        length measure ? It shortcuts all measurements failures and
        approximations, including the less than optimal keyboard frame
        bedding and the said strings angle up (or down). I can't
        remember who brought up this trick, but it is one of my top 5
        tricks ever learned on this list.

        Stéphane Collin