Good at Machining WAS: Reading a Board geometry

[Farrell]

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Yes, if you are working with one particular design, then one can come up =
with some repeatable, accurate downbearing scheme using dimes, nickels =
and whatever else. In practice, as long as the backscale of a piano is =
close to "normal" this method probably works OK. But as soon as you get =
that oddball in with an unusually long backscale (or short) one could =
really start changing things without realizing it.

Proper downbearing is very dependant on soundboard design. Some boards =
need very little for optimal performance, while some are best set up =
with quite a bit more downbearing.

Terry Farrell
  ----- Original Message -----=20
  From: Crashvalve@aol.com=20
  To: pianotech@ptg.org=20
  Sent: Monday, October 06, 2003 11:25 PM
  Subject: Re: Good at Machining WAS: Reading a Board geometry



  In a message dated 10/6/03 8:47:52 AM, mfarrel2@tampabay.rr.com =
writes:




    You want to measure the angular relationship of the string in three =
areas: along the speaking length; along the bridge top; and along the =
backscale just aft of the rear bridge pin. Those measurements will give =
you all the data you need to characterize downbearing.

    =20

    Be aware that all these references to setting bearing to "the =
thickness of a nickel" or whatever level above some reference point may =
be fine and dandy, but are completely dependent on the length of the =
backscale. As the backscale length between two systems where this type =
of measurement is being used varies, inaccuracies in the reference =
thickness will be introduced.

    =20

    How much downbearing is desirable will depend on the soundboard =
assembly design, where you are in the scale, and a host of other factors =
I know very little about!

    =20

    Terry Farrell




  I really like that term backscale. If I remember S&S threads a fiber =
line on the plate, shims it up with various thickness metal slips, then =
sawcuts the cap to clear this line. This means the bridge cap plane is =
so many thou above the string plane. This assumes bridge cap is parallel =
to board and board is some kind of smooth curve and not dimpled down. If =
you wanted to measure actual angles for the angular relationship, you =
could but it ain't easy, and I also don't know factors are involved in =
setting good down bearings, but I'd like to know ! 
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