[pianotech] Belly talk

[Ron Nossaman]

On 12/3/2012 10:48 AM, Jim Ialeggio wrote:
> Dale,
>
> Yup, that's the idea. Mine had about an 11mm crown (notice the word
> "had"<G>). on the belly rail.  It also had a crown along the curved side
> of the rim following the crown of the long bridge, though less agressive
> than the bellyrail.

Proceed with caution when you are establishing new bridge heights.


> I did some math on the actual amount the treble glue ledge would need to
> be beveled off of flat to "match" the crown arc.  On a short top rib,
> with 3/4" wide glue ledge, 165mm working rib length, very tight 4M
> radius rib, the difference in height between the point where the panel
> first hits the glue ledge, and the back of the 3/4" glue ledge is about
> .006"(.15mm) or .5 deg...about the thickness of a glue line. On a
> normally radiused rib, the difference would be even less. So the amount
> needed to avoid undo stress, if one feels that's necessary, seems to be
> quite negligible, at least to my mind.

It's one of those "intuitively obvious" concerns that doesn't quite hold 
up when you start looking at it closely. No one seems to be able to 
quantify undue stress, but they believe they know it when they see it, 
and it becomes really important. The very thing that makes beam support 
in RC&S boards work so much more dependably structurally than CC boards 
is the high internal stress in the CC panel, the low stresses everywhere 
in the low panel compression RC&S assembly and the compliance of the 
material used to build both. Spruce accommodates. Sure, beveling the 
rasten is a reasonable presumption of necessity, but a flat rasten isn't 
the death of tone as gluing panels down to flat cutoff bars and fish 
will indicate. Spruce accommodates, internal stresses are unavoidable, 
and the wood will flow to meet it. Determining the penalty/reward ratio 
is the hard part, presuming it's even possible to anywhere near the 
degree it's worried over.



> But thankfully, lots of different design scenarios seem to work...and
> good thing for us too.

As I've said since I was a technical cub. If almost anything we can do 
or not do didn't work to some extent, and the acceptable spectrum of 
possible performance outcomes wasn't so vastly broad and forgiving, none 
of us would be able to install an acceptable soundboard.


> So, you guys who work on a lot of S&S's, it looks like this kind of rim
> crowning profiling seems not to be a common feature...no?

Not that I'm aware of. I confess I haven't worried about it much, given 
how far the bass and treble of the boards need to be pressed down to 
meet the low stress rim contour. Incidentally, the higher the rib 
crowns, the more these ends need to be pushed down, and the higher the 
crown along the bridge is.

Regarding the crown along the bridge: With no ring bridge or hockey 
stick tenor, the long bridge with a flat bottom will typically sit 
nicely on a clamped in soundboard, following the long crown contour 
quite closely.

Ron N