One piece rim construction

[Delwin D Fandrich]

----- Original Message -----
From: <MsPiano4@AOL.COM>
To: <pianotech@ptg.org>
Sent: April 03, 2000 6:07 PM
Subject: One piece rim construction


> I'm new to the pianotech web page and I'm very excited about this forum
for
> sharing knowledge and experience. My subject line is in reference to the
> manufacturing practices grand pianos.  If I understand correctly most
grands
> are manufactured with a two piece rim contraction.  The piano plate is
> positioned on the inner rim, the belly work has already been done, and the
> piano is strung at this point.  The outer rim is applied in the next step?
> One piece construction are the inner and outer rim are formed at the same
> time?
> What are the pros and cons between the two methods?
>
> Greg Murdaugh
> Piano Craft Inc.

---------------------------------------------------------------

Greg,

The differences are primarily those of manufacturing.  Once assembled --
assuming that assembly is done to a reasonable quality standard in both
cases -- there will be no difference in performance.

That said, there is plenty of room for things to go wrong in each process.

With the so-called 'one-piece' construction method as practiced by Steinway
and Baldwin it is critical that the veneers used to form the inner rim be
exactly the right thickness and that the rim presses be both accurately made
and well maintained.  If the offset between the inner and outer rim is not
exactly uniform all the way around the inner caul there will be problems
with the glue line thickness.  In some areas the glue line can be so thin
that the joints can be starved for glue.  In other areas they can be so
thick that the joint is weakened because the glue line is excessively thick.
(The glues used for this purpose are very poor gap fillers.)  If the inner
rim veneers are even slightly too thick there will be excessive glue
squeeze-out.  This can cause starved glue joints in both the inner rim and
the lower portion of the outer rim and there will be an area just above the
inner rim that will not adequately properly pressed.  There will be
excessive glue in this area -- i.e., glue will be forced up from the lower
half of the outer rim forming excessively thick glue lines between the
veneers, sometimes even causing visible bulges in the face veneers just
above the inner rim -- and it is quite possible that the milling cutter will
cut through the face veneers (on the inside of the outer rim) when the upper
surface of the inner rim is being machined.  If the inner rim veneers are
too thin the inner rim and the lower portion of the outer rim will again not
be properly glued.  The glue line will be too thick, weakening the whole
assembly.  There will also be an area of the outer rim just above the inner
rim area that will have pinched veneers and excessively thin glue lines.  We
have encountered several pianos with the lower part of the inner rims
developing lamination separations along the glue line for this reason.

With the so-called two-piece rim assembly it is critical that the outside of
the inner rim and the inside of the outer rim match exactly when they are
glued up.  If they do not the glue joint holding the two together will be
compromised.  The two rims can be laid up either on separate presses --
i.e., one press for the inner rim and another for the outer rim -- or the
two rims can be laid up on the same press with a membrane between them to
prevent them from being glued together.  In my experience the two press
method is the most problematic.  Still, there are several ways to ensure
that this joint is mated correctly.  Probably the most common method is to
machine the outside surface of the inner rim to a shape that exactly matches
the inside of the outer rim.

Personally, I prefer the two-piece rim assembly.  It is much easier to
ensure that each step of the process is done correctly.  And, it is very
much easier to install the belly bracing, the bellyrail assembly, the
soundboard assembly and to fit the plate with the outer rim out of the way.
The outer rim can then be finished and added later once all the interior
work is completed.

Once the two rims are glued together they really don't care if they were
both pressed at one time or separately.  The structural strength of both
will be the same.  (Assuming, of course, that the other elements of their
design are the same and the materials used are the same.)  The bottom line
is that no matter which style of construction is used they are both held
together by a glue joint of roughly the same size and type.

Del