Soundboards/stress, questions

[Jim]

Hi Richard:

There are several ways that soundboards are made.

One way is to lay the soundboard on a dish shaped table and force glue
ribs to the board, depending completely on the ambient moisture of the
air to curl the board even more in order to create crown. One variation
of this method is to squeeze the board across grain before gluing, in
order to eliminate any tension across grain on the convex side of the
board and to produce more compression thereby creating more crown.

A second method is similar to the above method except that the table is
flat. This relies completely on the ambient air creating crown.

A third method is to use a dish shaped table and glue on ribs which have
been shaped to this or a similar curvature. There are variations of this
which involve a differential between the curve of the ribs and the curve
of the board.

A possible fourth method would be to glue and force shaped ribs onto a
board on a flat table. This is the only method I know of which would
immediately put tension across the grain as soon as the board comes off
of the table. This would soon be neutralized as the board takes on
moisture from the air.  I don't know any manufacturer who does it this
way.

Preliminary to any gluing in the above methods,
the boards are first dried down to about 3 to 6% moisture content 
depending upon the wish of the manufacturer before pressing ribs.

Baldwin has a special room which is maintained at a certain prescribed
relative humidity and temperature. The 2 largest Grands are bellied in
this room. They use the method 3 above and do not depend on the ambient
outside humidity to crown their boards. This helps avoid pressure
ridges which sometimes plague other manufacturers and leaves less to
chance. This prescribed humidity is a moderate level and is proprietary.

When pressure ridges develop early in a piano board, this is a natural
setup for cracks to appear in the next season or two of dry weather
because the cell structure is broken down during the pressure ridge
stage. I have never seen pressure ridges except at the panel joints.
I'm wondering if others have seen them in the middle of a panel. Of course
I have seen splits at many places on the panels. These are usually from
extreme dryness. The only time I will admit of any tension in the board
is under these extreme dry conditions, And, the board is practically 
ruined already. This kind of tension will cause cracks or splits. We
don't ever want the board to be in tension, only compression.

I'm wondering, if we use a dull tool to Vee out a split in preparing for
a shim insertion, are we not really stressing the cells in the side of
the groove, and thereby setting ourselves up for future cracks next to
the shims? It seems to me that very sharp tools or router bits would be
best for this preparation before shimming. If we fail to dry the board
before shim repairs, are we not setting ourselves up for failure during
the next dry season?

All the above is just for background. 

Now for a question for Richard. I do not understand your use of the word
"up" in your first paragraph below. I didn't know that there is an up side
and a down side in quarter sawn spruce. Could you explain that a little
more for those of us who have not run into this usage? The illus. below
shows an extreme example of various panels observed in soundboards at 
the belly rail. The lower example is more to be desired.

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A definition of quarter sawn is that the grain is at 45 degrees or more
to horizontal. I think I can understand up side or down side in respect
to flat sawn lumber, but not quarter sawn.

Jim Coleman, Sr.

On Sun, 15 Jun 1997, Richard Moody  wrote:

> Another factor regarding moisture is that boards tend to warp across
> the grain.  The direction of this warpage can be predicted by which
> side the board is "up". The illustrations in wood working books
> illustrate this . Thus they can in warping break away from
> the  ribs from their middle or their edges.  Wood shrinks also across
> the grain with dryness, another cause of cracks. However 'cured' wood
> shouldn't do this. But with alternating periods of humidity and
> dryness, cracks from shrinkage might be more common.  .
> 	The load or pressure on the sb from the strings, I was told amounted
> to a small elephant standing on it.  Another source said 1,400 lbs.
> (either a small elephand or a large cow) Would like to find the
> formula to compute this.  Must have something to do with vectors.  
> 	Would also like to know if the ribs are bent before glued to the
> sb.if they are glued to a already bent sb, or if sb and ribs are
> glued together then bent together.  Probably different for each
> manufacturer? 
> 
> Richard Moody 
> ----------
> > From: rhohf@idcnet.com
> > To: pianotech@ptg.org
> > Subject: Re:  Soundboards/stress
> > Date: Thursday, June 12, 1997 7:26 AM
> > 
> > There have been a lot of interesting and thoughtful ideas in this
> thread.  One 
> > of the things that makes this business so fascinating is that we
> have been 
> > making soundboards for centuries, but the fundamentals are still so
> poorly 
> > understood:
> > 
> a soundboard with ribs attached 
> > is a complex system containing tension _and_ compression.
> > 
> > 
> > All of my comments refer to unloaded soundboards.  So what does
> loading do to 
> > the stresses?  Applying downbearing does not simply put the board
> into 
> > compression:  if it did, boards would not crack. 
> > 
> > Bob Hohf
> > Wisconsin
> > 
>