[pianotech] Totally glueless

[Dean May]

Here is a link to a post I made on the Google+ Pianotech forum:
https://plus.google.com/u/0/100617699727189487414/posts/cYxgoJUYtxc

It shows the attached pictures demonstrating the roll back of a cantilevered
bass bridge. 

Dean
Dean W May                (812) 235-5272 voice and text 
PianoRebuilders.com    (888) DEAN-MAY        
Terre Haute IN 47802

-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Ed Foote
Sent: Thursday, January 31, 2013 11:24 AM
To: pianotech at ptg.org
Subject: Re: [pianotech] Totally glueless



Ed Foote RPT
http://www.piano-tuners.org/edfoote/well_tempered_piano.html

  >>It's the presumption of "acoustic coupling" that is questionable here.
Good mechanical coupling is necessary for one part to move the other to make
the system work. What I tried, unsuccessfully again, to point out was the
difference in scale between the mechanical action that is taking place and
the imagined tiny vibrations within the various individual parts. The basic
action is much coarser and simpler than is typically believed. Was the O
rebuilt because the bass was dead? Were the bass strings replaced and the
bass was still dead? If the screws were too loose to provide a decent
mechanical connection, that should have been the case. What prompted the
rebuild?Ron N<<

The restringing had nothing to do with the bridges.  This is a school piano,
and lots of broken strings and iffy pins were sufficient reason. 
  The discovery of an unglued bridge was incidental. The bottom of the
bridge's apron was curved to match the soundboard's curve.

   I submit that the coupling provided by a couple of screws is inferior to
a full glued surface for several reasons.  The screws are only holding from
the middle of the bridge to its rear surface, the cantilevering is trying to
rotate the bridge and the lack of glue means the fulcrum is the distal edge
of the surface and the upwards force is in the middle.  This loses all
attachment in front of the screws while creating a very short moment arm
between the screw and the distal edge of that bridge. The pair of screws
puts the entire upwards rotational pressure of the bridge on two small,
unsupported spots on the soundboard, whereas a glued surface would have the
full footprint resisting the strain.  The tension of the screws would also
vary with humidity, eventually loosening enough to allow the bridge to
rotate lower, (this had happened in this case).  Due to the elastic nature
of wood,  there is no way for the proximal edge of the apron, under stress,
to stay in contact with the soundboard when held by these two screws,
greatly reducing the mechanical contact between the bridge and board.
     Luthiers have long known that different ways of attaching necks have
different sounding results. The integrity of the neck joint is widely
recognized as having an effect on tone.  I have observed this, myself, on
numerous guitars that have been repaired, and  I am inclined to believe that
coupling does have an acoustical effect, at least, until I am shown
otherwise. With only two screws holding the stucture together, there will be
less coupling than fully glued.  If one is to believe that any coupling is
as good as another, this won't matter.
   The mechanical coupling is easy to measure, the bridge is still there, so
it must still be attached....  The acoustic coupling is more difficult to
measure. If I decide that there must not be any difference because I can't
hear it, then I am going to find less and less things that make a difference
because 60 year old ears are no longer hearing the same as 30 year old ears.
Regards,

  


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