Soundboard stiffening

[Richard Brekne]

Ron Nossaman 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
>
>
Grin... Like I said... these guys are very convincing ! So... what does 
our friend with the mechanical engineering insights have to say in 
response ?  Thumpy... you dont have to answer...grin.. I can hear your 
tail banging on the floor... hehe

Cheers
RicB


>
> 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