help a beginner measure mc in wood with confidence

[Ron Nossaman]

jimialeggio5 at comcast.net wrote:
> ROn and Dale,
> 
> I understand and am on board(no pun intended) regarding relative crown stability 
> of pianos with RC&S boards..
> 
> Let me go back and try to be more clear on what my questions were. Here are 2 
> questions, and I'll reference my original statement/question.
> 
> 1-The point of  my copy here:
> 
> "I'm doing RC&S boards...5% is not in my repertoire. For downbearing 
> measurement, 
> I'm shooting for around 7-7.5%mc.  Both yourself and Ron seem to be saying the 
> crown of an RC&S board is not variable at different mc's.  I'm chicken and will 
> have to prove that to myself before I trust it."
>  
> was one of procedure.  
> 
> So my question reflects this procedural issue; 
> 
> IE, Is the stability of the RC&S crown such that (in an RH variable shop) mc of 
> the board, when taking downbearing measurements, is not important and verifying 
> mc with some mc reference is unnecessary?

You have relatively little compression in an RC&S panel, and 
the rib set is stiffer than in a design that needs panel 
compression to form and maintain crown, so the unloaded crown 
won't change nearly as much in an RC&S assembly as it does in 
more conventionally constructed boards with high panel 
compression and more flexible ribbing.

I don't know of anyone even considering the MC of a more 
conventionally constructed board when they set bearing. They 
just wedge the sucker down until it quits moving, and press 
on. Trouble is, you can't do that with RC&S boards, because 
the spring rate is much flatter and much higher in an unloaded 
state.


> IE. are you saying that measurements of crown,  to verify (for 
> future use), what your as-built loaded crown deflection was as compared to your 
> designed loaded crown deflection, can be taken without  regard to the mc of the board at that 
> point, because of the stability of the RC&S crown.

Not entirely *without regard*. Nothing is entirely automatic 
and responsibility free. How stiff is the rib set? What sort 
of feathering have you used? What was the MC at ribbing, and 
the MC now? Are you using an adjustable plate height system? 
Vertical hitches? What's your downbearing target range?


> Hope my question is clear-er-er
> --
> Jim Ialeggio 

Personally, I haven't paid a lot of attention to MC levels 
when I set plate height on my boards and haven't had any 
disasters as a result. I have done final small adjustments of 
plate height as necessary, and under full string tension, to 
accommodate misguestimates. This is, I think, allowed by the 
committee. <G>
Ron N