Restoring crown in old soundboards - new cracks

[Delwin D. Fandrich]

----- Original Message -----
From: "Isaac OLEG" <oleg-i@wanadoo.fr>
To: "Roger Jolly" <roger.j@sasktel.net>; "Pianotech" <pianotech@ptg.org>
Sent: Wednesday, April 16, 2003 9:56 AM
Subject: RE: Restoring crown in old soundboards - new cracks


> Hello Hello, thanks for these infos,
>
> Yes I dry the thing, may be not as long as you but with DC rods too.
> (and sometime the board/bridge unglue also) I was basically saying
> that every other classic shimming work I see use to open in the dry
> season, probably the ones I made 15 years ago also, but the ones I
> made since I use a router (4-5 years) and often larger shims, seem to
> hold fine , of course I dry the board too, thanks for these methods in
> the journal and on the list.
>
> And the glue used ?



The issue is not really what kind of glue can be used--almost any
woodworking adhesive will do. (Those having good gap-filling qualities and,
perhaps, a very slight amount of give to them are probably preferable to the
brittle phenol- and resorcinol-resin types.) Or whether or not new cracks
appear--they almost invariably will.

The issue is whether or not you can restore the stress-interface between the
soundboard panel (all of it, not just a relatively narrow strip where new
wood is inserted) and the rib set. This is the mechanism that originally
created the crown in a compression-crowned soundboard assembly.

And you can not. Once time and compression set have dissipated this
stress-interface there is no mechanism left to form or hold crown.
Consequently the stiffness (the spring rate) has decreased and the
mechanical impedance has become much lower than the soundboard assembly was
originally intended to have. No matter what you do--no matter how carefully
you install shims--no matter what form those shims take--you cannot restore
that stress-interface.

You can introduce some needed stiffness through the epoxy-coating technique
I described in my Journal articles, but that is another issue.

Del