4 ways to crown a board

[Richard Brekne]

Hi folks.

I am trying to decipher a Norwegian translation of some of
Fener's text on Sound boards. So what do we have here...
correct me as necessary :)

1. Straight ribs glued to the panel, assembly pressed onto a
flat caul until glue finishes.
2. Curved ribs glued to the panel, assembly  pressed onto a
flat caul until glue finishes.
3. Straight ribs glued to the panel, assembly pressed onto a
curved caul until glue finishes.
4  Curved ribs glued onto the panel, assembly pressed onto a
curved caul until glue finishes.

For the 1st type,  no stress on the panel while glue
finishes, first when panel takes on humidity. Panel gets
compressed, ribs tensed, pretty equally over the whole
assembly.  Asymmetrical curvature to the panel not really
possible.

For the 2nd type, no stress on panel while glue finishes,
but some on the ribs. As soon as the press is released a bit
of crown appears even before the panel takes on humidity
because the curved ribs want to return to their original
shape. This in turn tends to tension (opposite of
compression here)  the panel as the ribs pull across the
grain. Crown resulting from taking on humidity has about the
same stability as in the first method, but with less panel
compression. More susceptible to cracking with humidity
changes later. Asymmetrical stress relationships possible,
but only if the initial radius of the individual ribs are
varied.

For the 3rd. The cauls radius is very important to the
success of the resulting assembly. Depending on the radius
of the caul at each ribs position you can end up with a
great or horrible result. Both panel compression and rib
tension occur as soon as the panel is released from the
caul. The ribs will try and straighten out again, but
because of the resulting compression in the panel the
assembly will initially end up at about half the radius of
the caul at each rib. Variations occur with differences in
panel and rib thickness. As with the first two, when the
assembly takes on humidity, the panel will tend towards
compression, and the ribs towards tension, but this time
more of each as this is the starting configuration at the
time of gluing. This is a stronger configuration then the
first method. (Dont know about the 2nd method in this regard
yet). Resulting crown is very stabile at normal humidity
levels, but very susceptible to humidity changes. Down
bearing later is more critical as its easier to stiffen the
panel too much with downward pressure.

For the 4th.

The cauls radius is carefully matched to the ribs. The
assembly is relatively free of stresses until it takes on
humidity and the panel tries and expand perpendicular to the
grain. Fenner says that this approach is not very effective
because of the lack of conscious control of humidity
changes. (I'm not quite sure if this last sentence is the
correct translation of  the german text). In any case Fenner
takes issue here and goes on to say "It is desirable to
achieve an equal degree of crown on each rib (i.e. equal
radius). If the ribs are made different then the assembly
will take on uneven stresses, something that absolutely
should not happen. For the same reason the bridge should
also be shaped to match the crown of the assembly.


Sound about right ??





--
Richard Brekne
RPT, N.P.T.F.
UiB, Bergen, Norway
mailto:rbrekne@broadpark.no
http://home.broadpark.no/~rbrekne/ricmain.html