[CAUT] left to right or R to L?

[Fred Sturm]

On Nov 13, 2008, at 12:17 PM, Ron Nossaman wrote:
> In compression crowned soundboard assembly, setup, and stringing,  
> The plan rarely coincides precisely with the result.
>>
>
> Board MC is more and less important depending on the basic design of  
> the assembly.
> Ron N


	Well, fair enough, I certainly concur that bridge pins touching or  
nearly touching struts are not an absolute indicator of something  
wrong. Particularly if using vertical hitches, as that makes the  
string tension plane adjustable: it could well be adjusted closer to  
the struts without any harm at all. In the case of a standard setup,  
though, with bent hitches and original string rests (duplexes or  
whatever), it wouldn't be a "firm" indicator, but would, to my way of  
thinking, be a good reason to take a second look before proceeding  
with installing strings. You might well be putting an excess load on  
your board because you made a mistake in your setup procedure.
	It seems to me that the method of setting bearing you describe, using  
wedges to emulate string load on the board, is another example (along  
with not paying adequate - IMO - attention to EMC) of a weakness in  
the process of "executing the plan." Some people do it that way,  
others just add an initial amount of excess downbearing in hopes of  
arriving at the target after the soundboard is under load of the  
strings. Either way seems pretty haphazard to me, especially if  
coupled with failure to control EMC at that moment. How do you make  
sure that the wedges are exerting the right amount of downward  
pressure? Has anyone measured to be sure?
	It would not be that hard to do a pretty precise emulation. Start  
with a target downbearing angle for each section. Figure average  
string tension times the number of strings for each section. Calculate  
the vector. You can come up with a very close approximation of how  
much force the strings will exert on each section (given your target  
downbearing). And then exert that amount of force by one or another  
means. Could be as simple as weights brought to bear on the middle of  
each section. Or some kind of adjustable and measured coil spring  
(valve springs come to mind as part of a set up). Or some other  
contraption. The point being to measure it, and come to a close  
approximation of the reality you hope to create. And then kerf and  
plane the bridge top accordingly (or whatever your method is).
	Moisture content is a factor regardless of design. I guess the major  
variant would probably be rib cross section, and the degree to which  
it reacts to and resists the swelling and shrinking of the board  
itself. But in any design there will be an effect, both on degree of  
unloaded crown and on resistance to downward pressure. Again, it is  
not that difficult to control this, to determine at what EMC (or  
equivalent RH, given time to adjust) you want to set bearing. And then  
establish that condition before setting bearing. (It would make sense  
to me that the "magical" 42% RH and its equivalent EMC would be the  
usual standard, so you can tell the customer to keep the piano at that  
condition).
	Once again, I am just a bystander, offering opinions where they  
aren't wanted <G>. I don't install boards. I have very limited  
personal experience in the area (though I have talked to many people  
who do have that experience). Still, I have to wonder if your comment  
that
  "The plan rarely coincides precisely with the result"
may not be at least partly the result of a faulty process with lack of  
adequate control, instead of the result of an inherent randomness in  
the universe. It seems like this aspect of piano manufacture/ 
remanufacture is important enough to make some extra care very  
worthwhile. So much attention is paid to the arc of the ribs (in rib- 
crowning), thickness gradients, ribbing design, etc. But in the final  
assembly it seems like a lot is left to chance.

Regards,
Fred Sturm
University of New Mexico
fssturm at unm.edu