Soundboard stiffening-thanks Ron

[gordon stelter]

Thanks for the patient re-explanation, although my
mind hasn't fully processed this, which seems
improbable. But you claim to have tried this
empirically, and I would not call you a liar as I
would not be called one, myself.
     But to accomodate this I must imagine that the
rigidity of the glue line between rib and panel forces
the cellular structure of the panel to compress from
upward pressure, before it has a chance to spread
outward, thereby drawing the upper edge of the panel
ends inward, and away from the end blocks.
     I'll think about this overnight and provide a 
snotty retort tomorrow.
     Thump

--- Ron Nossaman <RNossaman@cox.net> wrote:
> 
> >Please tell us all how a panel with a convex upper
> >surface can not spread out at the bottom when
> pressure
> >is applied from above, Ron.
> >      Doesn't make any sense to me at all.
> >      Thump
> 
> 
> Thump, and everyone else who cheerfully embraced the
> latest declaration 
> that soundboard crown is a buttressed arch,
> 
> Phil Ford and I discussed this in detail on list not
> too long ago. 
> Describing it again most likely won't have better
> effect than it did the 
> last time, so that leaves trying it for yourself.
> Make a three ribs about 
> 18" (460mm) long, one flat and two with a crown
> machined in. Make them 
> about 20mm (3/4") square and feather them like you
> would for a real piano. 
> Cut some panel material about 5" (120mm) wide
> (parallel to the grain), and 
> somewhat longer than the ribs. Dry one down to
> 4%-4.5% humidity and glue it 
> to the flat rib. This is the compression crowned
> example. Dry the other to 
> about 6% and glue it to the crowned rib for the rib
> crowned example. After 
> the panels are trimmed square to the rib ends and
> have reached moisture 
> equilibrium with the shop, you should have nice
> crowns in both models. Set 
> up a stiff plank for a base, with blocks of wood
> (sides cut and squared) 
> clamped to it so one of the models will slip between
> the blocks with zero 
> clearance at the ends. Set a spacer under each end
> of the rib so that the 
> bottom of the rib is clear of the plank by at least
> the amount of crown in 
> the model. Now place a straight block of wood nearly
> the length of the rib 
> on top of the panel, and start applying down
> pressure on it with another 
> clamp, depressing the crown of the model. The block
> of wood on top is so 
> you won't press the crown below flat. Watch the ends
> of the panel as the 
> crown is depressed. They WILL pull away from the end
> blocks. Now do it with 
> the other model. Same thing. Now do it with just the
> crowned rib. Same 
> thing. This is exactly backward of the  way an arch
> would work. Soundboard 
> crown is NOT, nor has it ever been an arch because
> the end supports are not 
> below the centroid line of the assembly. They are
> significantly above it, 
> as in a cable suspension, and no, I'm not saying the
> cable suspension 
> retains crown if you keep the rim edges from
> converging. Crown is still 
> formed and maintained by the rib and panel. The rim
> has nearly nothing to 
> do with either formation, or retention of crown.
> 
> Ron N
> > _______________________________________________
> pianotech list info:
> https://www.moypiano.com/resources/#archives
> 


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