Bending rims

[Delwin D Fandrich]

----- Original Message -----
From: Brian Trout <btrout@desupernet.net>
To: <pianotech@ptg.org>
Sent: June 05, 2000 8:10 AM
Subject: Re: Bending rims... Jim B. (long)


> Hi Jim,
>
> Sorry for my earlier confusion.  My mind was off on a tangent only known
to
> the faint and distant imagination.  I wasn't really thinking along the
lines
> you were, but had a memory in my head of someone, wwwaaaaayyyyyy back in
> maybe the third thread back or so from this one, who said something about
> using thick, maybe 1/4" laminations to make a piano rim.  On the occasions
> I've tried to bend wood in curves as tight as a piano rim and that thick,
> most of the time, they end up breaking someplace.

Most modern piano rims are made of veneers somewhat thinner than this.
Steinway uses relatively thick veneers at approximately 3/16" (4.75 mm).  If
I recall correctly, Baldwin's are the same.  Most other pianos use veneers
that are somewhat thinner.



> 2)  again a quote:  ..." The soundboard is pressed into the shape of a
dome,
> allowing it to withstand the combined downward force of 1,000 pounds from
> over 200 strings."...   I thought I heard somewhere, that the engineering
of
> a soundboard is such that the 'arc', or 'dome shape' is not really a
> significant factor in supporting the weight of those 200 or so strings.
> Anyone care to comment on this one?  (Probably a pretty technical
question,
> but it came to mind when I read that statement above.)

It depends on how the soundboard is crowned.  In Steinway's case the
internal compression of the wood soundboard panel must support the crown.
The ribs must be flexible enough so that the expanding wood panel can bend
them into a crown.  (This is regardless of whether they are pressed in
curved cauls.)  Others use rib systems that support the crown directly.  In
either case, the soundboard must, indeed, support the string downforce load.

Keep in mind that the arc the soundboard starts out with is not at all the
arc it ends up with once the strings are on the piano and up to pitch.



> 3)  another quote:  ..."Before a soundboard can be placed into a piano
case,
> the bridge must be notched for the strings that will pass over it..."
> Huh???  I had no idea they did it this way.  Or do they?

This must be a change.  In the past they notched and drilled the bridge
after the board was installed and the plate height set.  It is one way of
accomplishing the task.  Baldwin developed a system that enables them to
drill and notch the bridge before it is glued on the soundboard.  Either
method works and the end result (from the soundboards perspective) is the
same.



> 4) yet another quote: ..."This network of bracing <belly bracing> helps
> support the 340 pound cast iron plate."...  I find it fascinating that
they
> would mention 340 pounds of a cast iron plate and not mention the 10 or 15
> tons of force exerted by all of those 200 or so strings.

The string load is more like 18 to 24 tons.  They don't mention this because
the rim and belly bracing don't handle much of the string load.  Somewhat
more in the Steinway and similar pianos using a horn between the bass and
tenor sections, but even here there is not a whole lot of the string tension
load on the belly bracing system.  (We just took a S&S Model L apart that
had a gap of about 1.0 mm between the belly brace and the belly rail on
brace #1.  This is the brace that should be carrying any string load coupled
to the belly rail through the horn.  Obviously, it wasn't supporting much of
anything.)

Del