Bechstein B hammer rake / more thoughts

[Richard Brekne]

Hi Stéphane

Thanks for airing your thoughts on the matter. I guess this all boils 
down to whether or not there is any reason to have the shank 
perpendicular to the key bed or not at impact. You cite having the shank 
parallel to the string plane at impact, and that makes an outwards rake 
compatible right away. I will have to re-read Bob Hohf's article to get 
you a straight answer on that. Perhaps I have misunderstood that but I 
seem to remember him making a justification for the action delivering 
its maximum punch with the hammer shank parallel to the key bed 
regardless of what was being punched as it were.

As to errors in measurement you describe below. Strikes me that which 
ever way you go about it you are bound to get an error of some sorts.  I 
measure typically the string height with a jig I saw described in the 
PTG journal some years back, which essentially is an adjustable ruler 
that is attached to a flat block siting on the key bed. You push the 
thing up to string contact and read the result. Thats dead on accurate 
in itself. Reading center pin height I usually do taking measurements 
through the spaces in the keys to either the front or back side of the 
action rail with an adjustable height square jig. Then I just add the 
height of the center pin over the action rail measurement.  Thats where 
any error occurs,  but its a total error of less then 0.5 mm and how 
picky can one get ?

I have used the pre-hung hammer trick as well, but again have always 
used it to find the shank at horizontal at impact. Thats been accurate 
enough as well as one simply attaches a small leveling bubble to the 
shank. I'm not quite sure how you could use that trick to get an 
accurate shank parallel to string plane at impact measurement.  At least 
not one that is any more accurate then the measurement above.  One way 
or another some small degree of error seems inevitable I suppose. 
My main querrie is really this shank at horizontal bit. If I drop that 
requirement, then I'd be able to get the hammer perpendicular to both 
the string and shank at impact if the action cavity allows for it I 
susppose... or at least pretty close to it. And that would account for a 
backwards rake yes ?

Cheers, and thanks again.

RicB

    Hi Ric.

    When I think of it, I can see only one ideal situation, that is when
    the hammer is perpendicular to the string at impact time, and the
    shank is closest to parallel to the string at that time.  In this
    condition, the energy transfer from the hammer to the string seems
    maximized to me.  If the shank is not parallel to the string, there
    is a portion of the hammer movement that will not be normal to the
    string plane (but parallel instead), and that movement will spend
    energy in friction (of the hammer against the string), and the
    rebound of the hammer will also be slightly impeded because he
    hammer will have to reverse the direction of that portion of movement
    too.

    I fail to understand the importance of the shank being horizontal. 
    What are  the ideas of Bob Hohf about that ?

    The trigonometry is fine, but you could add an analysis of what
    happens with the measurement errors that you do when measuring -
    String height over keybed (I don't seem to be able to do this
    measure with less than 0.5 mm error) - Rounding this value for all
    notes between two samples you measured (typically at each side of
    each section), so if for example you measure 231 mm at the right of
    the treble section and 230 mm at the left of the alto section, all
    notes in between will have an additional error of 0 to 0.5 mm -
    Center pin over keybed : here you have two measures and one
    approximation : the approximation is that when you measure any point
    on the stack over the keybed, you do this with the action out of the
    piano (I suppose, or do you have a better way ?) and so the action
    rests on another surface against which you do the measure, but is
    the action frame sitting exactly the same way ?  I find it difficult
    to avoid another 0.3 mm error here.  Also you don't measure directly
    the center pin height (or do you ? but how ?) you measure one
    accessible point at the top of the stack, then measure the vertical
    distance between that point and the center pin, and here again
    errors sum up - yet another rounding error for all notes between the
    extremes, if there happens to be a slight difference in the
    measurement of the first bass center pin height and the last treble
    one - angle between string plane and keybed (I can't even figure out
    how to
    measure this accurately.  My best take up till now was to throw a
    ping pong ball from 20 cm over the strike line and measure where it
    falls down after its first rebound, but this assumes that the keybed
    is horizontal, which I can grosso figure out with a bubble gage) -
    again approximations across the scale - and finally the error in
    hammer bore itself

    This is where the sample hammer with known bore distance trial in
    situ shortcuts a whole bunch of accumulated errors (in the
    trigonometry formulas, any measurement you have done will multiply
    even further all the errors), as there is only one approximation,
    that is the perpendicular condition of the
    hammer with the string plane, which is quite easy to measure with a
    piece of straight wood with a nail put in it at straight angle,
    which you put on the
    strings with the nail facing the ground.  And if the hammer is
    perpendicular to the shank, then necessarily, the shank will be
    parallel to the string plane.  But this tells nothing about the
    horizontality of the shank, and I fail to understand why I should
    care. So I don't understand either the rake thing, short of trying
    to float for all the said approximations when it is time to have the
    actual end user hammer as perpendicular to the actual string as
    practically possible.

    What do you think ?

    Best regards.

    Stéphane Collin.