R,C&S question Jude

[Richard Brekne]

    BTW, while I don't make CC boards anymore,  I did apprentice in a
    shop for several years that made about 20 CC boards a year and I
    have to say it was like being in the Wild West. We were total
    cowboys. The only thing we paid attention to was emc (3.5%). Rib
    dimensions came right off the old board, the compression crowning
    was all over the place depending on the time of year and size of the
    panels/ribs, and we set distance bearing with the spare change in
    our pocket. Yee haw! Boy were those days fun and also one of the
    reasons that I'm so compulsive now. :)

    Jude  Reveley, RPT


Hey Jude

The thing is... there is a difference between putting together a brand 
new board....  and trying to figure out what was done in an old one.  In 
the former one is supposed to know the amount of downwards pressure a 
given scale for given string deflection angles will require in terms of 
support by the assembly when deciding on the ribs structure and amount 
of compression one decides to use.  Trying to figure this requirement by 
looking only at an old existing rib scale and whatever bearing there was 
left is of course hopeless.  What made the process you describe below 
unpredictable is that you guys didnt take into consideration enough of 
what you were doing..... kind of like my own <<mistake>> with the bass 
in my latest project. The reason the RC&S guys succeed is that they KNOW 
ahead of time what kind of support against downbearing they will be 
requiring... and they design exactly that into the ribs. In the process 
they ignore whatever compression issues there are (or are not) outside 
of assuming that 6 % dry down will be more or less sufficient to prevent 
tension cracks during dry seasons.  Doesn't look to me that these 
fellows can describe how much compression builds up because of 
downbearing any more then I've been able to pump out of CC folks through 
the years.  I rather imagine at this point the CC folks arrived at their 
rib dimensions, methods for achieving so and so much crown to yield so 
and so much support rather empirically through the years... each factory 
with its own solution... and unless you knew enough about what they did 
at glue up... you're shooting in the dark by trying to simply duplicate 
the panel using a fixed dry down EMC and an existing rib set.

What I'd like to know is basically how to predict the resultant support 
for down bearing in a CC board.  Should be a doable.  One needs 
certainly to know the rib dimensions... how easily they will bend.... 
but one also needs to know how to calculate the panels contribution to 
the assemblies overall strength.... which means you need to know how 
much compression you are creating before loading the panel... and how 
much further compression you are creating by loading the thing.  
Compression from loading in a CC panel stresses the ribs more exerting 
further crowning force on them....while the downward force in itself 
exerts more or less the opposite force on the ribs.  Strikes me that if 
you know how to calculate these moments.... then a CC board becomes as 
predictable as any other and all thats left to argue about is the bit 
about compression set and damage...

Cheers
RicB