Case separation or delamination question

[Ron Nossaman]

>
> Crowning method was compression, I believe.  I didn't see the piano when it
> first came back, nor before it went out for rebuilding, as this preceded my
> period of employment at the University.  Piano was rebuilt during the summer
> time and, as previously stated, the humidity in this venue does hover in the
> 20% and below region for some part of the winter, at least it did so before
> GWSR  (Global Warming Seasonal Recalibration).    I installed a series of
> crown indicator strings between ribs 3&4, 5&6, 7&8, 9&10, and 11&12 (piano
> has total of 13 ribs).  They are held in place with masking tape, and I
> retension them and note bearing whenever I tune.  Most recently, the numbers
> I got were: 3&4 - 1/16"; 5&6 - 1/16"; 7&8 - 1/16" or Flat; 9&10 - Flat; 11&12
> - Flat. 


Compression crowning usually involves force drying the panel to something in
the 4%MC range (assuming accurate measurement), and gluing on flat ribs. The
crown comes from the panel expanding with rehydration. In summer humidity, it's
harder to get them dry, and easy to get crown when they rehydrate. A soundboard
panel in a room at 70°F and 20%RH (again, assuming accurate measurement), is at
about 4.5%MC, dropping to 4%MC at 17.5%RH at the same temperature. The panel is
at a moisture content very near that at which it was assembled, so it's not
surprising there isn't much, if any, measurable crown. This is one of the major
problems with this crowning method, and tonal problems in dry winter conditions
are common with this type of board. The tone should improve some just by doing
nothing but waiting for the humidity to go up with warmer weather.



>
>  With regard to bearing angles, I am currently revising the method I use for
> measuring downbearing, and so I don't have precise numbers at the moment,
> however, I am virtually positive that the bearing isn't. Why, is that a
> problem???!!   As far as positive on both sides of the bridge...only in my
> dreams...No, not those, the other ones.  


That the bearing isn't what? Ideally, you will want both positive crown and
positive overall bearing. Positive bearing on the front of the bridge is fairly
critical, less critical on the back, but overall should be positive - but not
excessive. Yes, knowing the bearing and it's relationship to crown is
important.


>
> What do you consider to be "some measurable crown"? 1/32"? 1/16"? What?


No numbers, just more than none. If gap = true, then crown - else not crown.
With a string stretched rim to rim (or cutoff bar) between ribs, If you can see
a gap between the string and board on both sides of the bridge, the crown is
positive. If you have no gap, or a gap behind the bridge, but none in front,
you may have a problem.

Ron N