[pianotech] Measuring Crown Radius

[David Love]

As Ron mentioned the process of crowning takes place outside of the piano.
The panel is dried down to something like 4% EMC and then the ribs which are
flat or slightly radiused are glued to the panel.  The panel is sometimes
pressed into a curved bed or caul during the gluing process.  The panel is
then allowed to expand and the ribs are bent by panel expansion.  This
happens before it is glued into the rim.  There would be a difference
between that process and what you describe which is that the crown is formed
with the board already glued to the rim.  

David Love
www.davidlovepianos.com


-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of John Delacour
Sent: Thursday, July 14, 2011 2:52 PM
To: pianotech at ptg.org
Subject: Re: [pianotech] Measuring Crown Radius

At 16:22 -0500 14/07/2011, Ron Nossaman wrote:

>In CC crowning, the panel is dried down, then glued to flat ribs. 
>Panel expansion as it takes on moisture then bends the ribs into a 
>crown. The piano case isn't involved in the process, and the ribs 
>needn't necessarily be dried down.

Whether they are or not would surely make quite a bit of difference. 
If the case is not considered (by Steinway's for example) to be 
involved in the process, why do they glue the case to the rim before 
fitting the soundboard, thus causing themselves a lot of tiresome 
work that could be avoided if they glued down the soundboard before 
attaching the case, as many, if not most, makers do?

However that may be, the method seems to me inherently unstable.  For 
one thing the bars (ribs) will ever afterwards be acting to remove 
the crown, won't they?  If at any time the moisture content of the 
board rises to a point where the board proper is crushed, then the 
bars will be keen to relax and pull down the crown.

JD