[pianotech] Measuring Crown Radius

[Ron Nossaman]

On 7/13/2011 3:45 PM, David Love wrote:
> Ron:
>
> You wrote:
>
> "The force on a CC board rib is reversed from that on
> RC and RC&S ribs. It's still a beam, It's just not doing a thing to
> support crown and bearing other than constraining the panel. It's
> actually trying to pull the crown flat."
>
> If you are arguing that the compression bent rib contributes negatively to
> the load bearing properties of the panel by trying to pull the panel flat
> then it follows that a lighter rib that pulls down less would contribute
> less negatively and the load bearing properties of the assembly would be
> increased.

Since new CC boards are loaded to the compression limit of the panel, 
I'm not sure a less tall rib would allow any different load support than 
a taller rib. The less tall rib would definitely crown higher with no 
load because it bends more easily.


>It doesn't take any significant depth of the rib to constrain the
> panel on that side, you can do that with fairly thin piece of veneer glued
> to the panel.  My example of an unribbed panel was extreme, of course, in
> order to make a point.

It made no point at all, and was a ridiculous comment since a panel 
wouldn't crown without a rib. It addressed nothing. Make some attempt to 
stay within reason.


> I'm questioning your basic premise that a bent rib once glued into the rim
> is actually pulling the panel down and doesn't contribute to load bearing
> support.

It's not a premise. It's elementary mechanics and physics. A flat rib 
will begin to support load as a beam only after it's pushed concave. If 
the panel is holding it up in a crown, it's not supporting anything.


>It's an important point because your claim is that the two systems
> are diametrically opposed.

The fact that it's an important point is the only reason I'm still here 
with you. But I'm fading fast.


>I'm arguing that they are the same system
> essentially and the difference is only a difference of degree, which part of
> the system does more work.

I know what you're arguing, and you aren't arguing with anything 
approaching reality. What you're saying is mechanically and structurally 
wrong.


>  The fact is that all ribs are bent by compression some even in
> RC&S boards--at least while the RH is above the EMC level at which they are
> ribbed.

Until they're loaded, then the panel in the RC and RC&S boards compress 
and the rib deflects past it's at-rest point. Remember I said I don't 
find unloaded crown to be a useful measurement? How do you imagine the 
measured crown of a loaded RC&S board is less than the machined in crown 
if the rib isn't bending down and supporting load?


>So then all ribs, by your account, are pulling the panel downward
> and there's no real difference between the two systems except for one of
> degree.

No, absolutely wrong - again. You still aren't either listening or thinking.
Ron N