[pianotech] Reversing Crown

[George F Emerson]

Ron,

Your experience in replacing soundboards in the rebuilding process is more 
extensive than mine.  My soundboard experience is mostly in manufacturing 
new pianos.  The instance that I sited was at a company with which I have 
never been associated.  In any company for which I have worked, I have never 
seen a new piano soundboard "oil can" immediately after being removed from 
the rim.  Maybe that should not surprise me, but this one was new to me.

Regarding rim bevel, Baldwin beveled only the belly rail, not the inner rim. 
Then, they insisted that it was not necessary to bevel the inner rim.  Truth 
be known, they could never find a convenient way to do it, with an 
integrated inner-outer rim construction, glued up in a single assembly. 
What is more important to me than rim bevel is the rise and fall of the 
inner rim elevation, corresponding to the curvature of the soundboard, which 
also results in a higher elevation on the inside of the inner rim than the 
outside of the inner rim.  Call it a bevel if you like, or call it a match 
in curvature of the gluing surface of the inner rim and belly rail to the 
curvature of the crowned soundboard.  In my design work, I use a 3D computer 
model of the crowned and loaded soundboard to determine the inner rim/belly 
rail elevations.  I use another computer model of the crowned, UNloaded 
soundboard to determine the radii to cut the ribs.  The crowns of these 
soundboard computer models are "football" shaped, not spherical or 
cylindrical, resulting in shorter radius ribs for the shorter ribs than for 
the longer ribs.

Frank Emerson

----- Original Message ----- 
From: "Ron Nossaman" <rnossaman at cox.net>
To: <pianotech at ptg.org>
Sent: Thursday, December 24, 2009 8:40 AM
Subject: Re: [pianotech] Reversing Crown


>
>> I was not there, but a reputable tech told me about this.  He had a NEW 
>> grand piano with positive crown.  The strings were removed and it still 
>> had positive crown.  The soundboard was remove, and it had negative 
>> crown!  What do you make of this?
>>
>> Frank Emerson
>
>
> This isn't unusual. I've seen a number of compression crowned boards with 
> measurable crown (strings off) go concave when removed from the piano. The 
> rim bevel holds bends thing into a crown, but there's no real load 
> support.
>
> Also, there's no useful information here about the condition before 
> unstringing. It had positive crown. Where, how measured, and how much? 
> Bearing? Again, where, and how much? There are plenty of techs out there 
> that will measure crown at the longest rib and proclaim the board to be 
> good when six ribs up, the crown is deeply concave.
> Ron N