Pianotech

  • 1.  Backcheck installation...finding the convergence point empirically

    Posted 08-31-2020 21:51

    Re the recent Backcheck thread...Determining backcheck drilling location on the key and setting backcheck height...

    Here is the simple empirical way I use to find the "as-built" convergence of the relevant arcs, which determines where to drill the backcheck hole on the key, and how high your installed check needs to be off the key.

    The two arcs we are  talking about are the arc of the shank/tail as it moves from rest to the string,  and  the arc described by the check as it travels with the key at full dip. 

    (FYI, "as-built" means the conditions that are now "facts on the ground".  As-built dimensions are the dimensions actually, currently, physically  installed in the action on the bench, rather than pre-installation, abstract design numbers.  Design numbers and as-built numbers often are not the same...so the imperfect as-built reality wins, and that is what I work to.)

    The first arc, the arc of shank/tail, has already been installed and is an as-built certainty.  The second arc, the arc of back check contact point is what we have to determine.  For the second arc, the arc of the back check head's catch  point is the point where the check arc converges with the tail arc.

     All I do, is carefully set the a key at full dip, proving that the out of the cavity dip is correct.  Set a string target which is the actual string height for that exact note in the piano, and lower the string height target, to the height below string height you want your hammer to check at.  

    With key held at full dip,  the hammer will be held against your check height target (escapement must be forced, since the hammer is not being allowed to reach letoff height). With the parts held in these locations, the action is telling you where arcs need to converge. Hold a little jig shown below, on the back of the key. There is a mark I've, made on the check head where contact, given a 1-1/16" tail and 3" radius should happen. This line indicates where  the bottom of the tail should be in check. You can barely see the line I've drawn on these Tokiwa checks, but its there.

    I make these little jigs in a few moments for all the non standard setups I meet, and also to experiment with the angles that I like the check to be at. You can set them up for a straight wire or a bent wire with a vertically drilled hole. (I prefer the vertically drilled hole). The kerf is made on a table saw with a Red 8-1/2" Freud Diablo finish blade which firmly holds the wire without attachment paraphernalia.



     

     

    The kerf holds the check wire firmly, so I can move the wire up and down in the kerf, and it stays where I put it, while setting check head height. Place the jig on the key, and move the check, with its drawn check height line, up or down until it lines up with the bottom if the tail. Move the little jig in and out on the key so both height and in/out dimensions are correct.  When the little jig is positioned correctly, since the kerf is centering the wire and projects to the bottom of the key, mark the center of the kerf, on to the key's check block. The check's wire has been cut short, so it doesn't interfere with the check block, but the center of the kerf is your drill mark.

    Mark a line on numerous hammers/keys across the keyboard, setting dip and hammer check height relative to each test note's actual string height as you go. They will vary, as will check head heights!  This results in a drill line that wanders  somewhat, and check head heights that also vary somewhat. But the wandering drill  line represents the custom convergence points that the as-built action actually contains.

    Since string heights vary, sometimes wildly, finding the convergence points on all 88 notes simply cannot happen with generic bores and the straight check lines we always see.  Custom hammer boring makes it possible to actually hit convergence points on all the keys, and greatly improves this situation. However, without custom boring, convergence points will be an average, and some of the convergences will be far from what is the ideal catch point...and those "far from the average"  hammers, will be the hammers that refuse to check well.

    Also note, that when I was struggling with improving my check installations, no one really laid hard on how important the installed height of the backcheck head is, re the convergence point.  The installed height of the check head, determines how far from the string your hammer will check well. If you have an installed a check head height which is geometrically imposing a 7/16" check height, and you try to set check at 5/8", the check will be lousy, unto not checking, and you'll wonder why you can't get anywhere with any adjustments to the in/out position of the head. Both check head height and in/out position of the head must be addressed. If the convergence point adheres to the as-built conditions, wandering its way across the keyboard as it does so, adjusting checking will be a breeze.  

    ps...by the way, if you set the check head height and in/out location in this fashion, or something like it, it will find the convergence point whether you are tailing a short tail, with 2-1/2" radius, or a long tail with 3" radius....it doesn't matter. The tail arc and length is accommodated for by empirically asking the action where the convergence point is located.

     



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    Jim Ialeggio
    grandpianosolutions.com
    Shirley, MA
    978 425-9026
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  • 2.  RE: Backcheck installation...finding the convergence point empirically

    Registered Piano Technician
    Posted 09-03-2020 07:59
    Thanks Jim

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    Eric Johnson [RPT]
    [Eric Johnson Pianos]
    [Westport] [CT]
    www.ericjohnsonpianos.com
    One year older and dumber.
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  • 3.  RE: Backcheck installation...finding the convergence point empirically

    Registered Piano Technician
    Posted 09-05-2020 19:26
    Thank you Jim. Great set up!

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    David Brown
    Garland TX
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