rib glueing

[Ron Nossaman]

>>>If one is using
>> > straight ribs and compression crowning (I am), does pressing into a
>curved
>> > surface have any material effect on the crown or is Jack Krefting
>correct
>> > when he stated in one of his series of articles that it does not.
>>
>> Huh?  Of course it will have an effect on the resulting crown.
>> Del
>
>>There isn't any real
>>difference between a crown being achieved by >pressing panel and straight
>>rib into a curved caul, and one resulting from >rehydration of a severely
>>dried panel glued to a flat rib on a flat caul.
>>Ron N
>
>>If you induce a curve to two thin pieces of
>>wood and glue them together in the curved state, >THEY STAY CURVED. Unless
>>piano soundboards follow some other natural >principals, my guess is that
>is
>>DOES matter.
>>Terry Farrel
>
>PTJ, 7/87, page15 "Incidentally, it does no good at all to have a bellied
>press and then use flat ribs; if you aren't going to crown the ribs, you
>might as well use a flat deck and cook the board a bit more."
>Jack Krefting
>
>So.........Which is it, and why?
>Garold Beyer


Ok, I saw the pieces of this go by, and figured someone would call the
question eventually. The only difference between a crown achieved by
severely drying the panel and gluing to a flat rib against a flat caul, and
gluing a less severely dried panel to a flat rib against a crowned caul is
the potential damage done to the panel by the excessive drying. Once the
panel has re-absorbed enough moisture to reach equilibrium with the room
air moisture, you can't tell which is which. In both cases, the panel
compression is bending the rib. That partially explains the PTJ, 7/87 quote
from Jack. The two approaches are essentially equivalent. The problem here
is that you don't have any real control over the stiffness of the final
assembly, since it's dependent on the compression resistance of the panel,
which is going to be different from panel to panel. Compounding the problem
is the fact that, in either case, the ribs are resisting the panel's
efforts to crown the assembly. Since the ribs' resistance to bending is
subtracted from the panel's compression resistance's ability to form and
maintain crown against the ribs' resistance, and the strings' downbearing
load, the success of the whole assembly is utterly dependant on the
compression resistance of the panel. You can only get so much stiffness out
of a given panel with this type of construction. Heavier ribs will mean
less crown, but won't appreciably change the stiffness of the assembly
(just the mass) since the panel is what's forming the crown. The ribs won't
figure in for stiffness in this type of assembly until they start to bend
past flat and begin actually supporting string load. By then the soundboard
crown is concave, and it's probably too late to worry about it. Actually,
in a compression crowned assembly the stiffness/mass ratio increases as the
stiffness of the ribs decreases, but that's another category of aggravation
altogether. 

Another (I think , better) way is to not dry the panel down as far, machine
crown the rib, and press the whole mess into a crowned caul during
assembly. Rehydration of the panel will add a little more crown after the
fact, and under string load the ribs will be supporting a portion of the
load, as well as the panel, while there is still positive crown in the
assembly. This takes a lot of load off of the panel, and gives the designer
some real control of the stiffness of the assembly. With this type of
construction, the final stiffness is controllable by the rib dimensions. To
my alleged mind, this is a whole lot better way to go.

I would like to know what Del had in mind with his reply though.


Ron N