Pre-crowning ribs

[Ron Nossaman]

>My questions are... What system or jig arrangement do the seasoned
>soundboard folk use for shaping (precrowning) the ribs?

Everyone seems to do it differently. They can be planed, sprung and sawn,
or laminated to whatever radius, radii, catenary curve(s), parabola(s),
cosmic loop(s) you may consider to be optimal this month. I assume you will
reasonably reserve the right to change your mind later as the resulting
cumulative evidence indicates. Meanwhile, there are fairly recent
descriptions of jigs and methods in the archives. 
 

>Also for this model piano are there improvements to be made in altering
>(reducing) the soundboard thickness?

>From what, to what, for what reason, and in the interest of what specific
improvements? Universally applicable magic numbers only work when they
happen to fit in with all the other variables of the design.


>We are going to increase the rib height from the original minimal 19mm to
>somewhere near 23 to 24mm, but will be happy to incorporate advice and
>suggestions from those of you familiar with this model piano and who have
>successfully altered / improved some of the original soundboard / rib specs.

While I'm not familiar with this specific piano, the same set of criteria
apply to most everything. First off, what determines your choice of rib
depth? How about width and number and placement of ribs? Were the originals
crowned, or was the crown originally forced into flat ribs by the
rehydration expansion of a severely dried panel? Your choice of dimensions
and crown for each rib (along with the panel thickness, grain angle, and
rib angle relative to the panel grain) will determine both it's load
carrying capacity and vibrational response characteristics. What crown
radii are you planning to use? A lighter rib with a higher machined crown,
deflected to the same resulting (under load) crown as a heavier rib with a
lower machined crown, will support the same load, but be more responsive in
action. 

If you typically have killer octave problems in these pianos, just crowning
the ribs and adding a couple of millimeters to the height will help. Beyond
that, the variables are cumulative and it gets a whole lot more complex.


Ron N