Spurlock shimming method.

[Delwin D Fandrich]

----- Original Message -----
From: <JIMRPT@AOL.COM>
To: <pianotech@ptg.org>
Sent: May 07, 2001 7:19 AM
Subject: Re: Re: Spurlock shimming method.


> <<"Rims are not arch buttresses, and do not form or maintain
> soundboard crown.">>

> "arch buttresses do not form or maintain" anything either they merely hold
> something in place by providing resistance.....anything held in place by a
> "buttress" will react to that "buttress" within the limits of that
"butress"
> and the characteristics of the material being held. If you take your flat
> ribbed ,flat paneled, sounboard and stick it in your hot box and 'not'
> restrain it in some way after it is taken out.....all you will have is a
flat
> board that basically gets hotter and cooler 'without much' change vis a
vis
> crown.

Huh?

Have you ever tried this? That sucker will bend in and out like the wings of
a big ol' bird. If you take a ribbed soundboard panel that is flat at
whatever equilibrium moisture content it may have stabilized at in your
normal shop atmosphere and stick it in a hot box it will indeed crown. The
crown will be the opposite of the direction you want it to go in, but it
will crown! The panel will be shrinking as the wood cells lose moisture and
shrink, pulling the ribs in toward the board at their ends. At least until
the cracks start appearing since spruce isn't particularly durable under
tension. How much reverse crown it will end up with will depend on the
temperature and humidity in your shop when it measured flat and on the
temperature and humidity of your hotbox. And on how much it cracks. Take the
flat-ribbed soundboard panel out of your hotbox and allow it to stabilize
again at its EMC in your shop and it will eventually return to its original
flat condition.



>   On the other hand if you take that flat ribbed, flat paneled
> sounboard...stick it in your hot box....take it out and 'immediately'
> restrain it with either "flying buttresses" or a rim assembly the sucker
> 'will' change drastically vis a vis crown

Again, have you actually tried this? If you put your (now reverse crowned)
dried panel into that flying buttress and let it come back to normal shop
temperature and to its equilibrium moisture content it will come back to its
essentially flat condition. There might be some very slight residual bend
due to the restriction of the flying buttress, but it will quickly dissipate
and the wood finds its equilibrium once again. Remember, the cross-grain
ribs have prevented the panel from shrinking by any appreciable amount. The
flying buttress will not be able to support any crown for any reasonable
duration. Nor will you be able to adjust that crown with your Centripetal
Tension Resonator if you place any string load on that bridge. Just ain't
going to happen.

(In the above I'm assuming a flat-ribbed soundboard of typical, real-world
dimensions. As is the flying buttress.)

(Also, if you'd like to start with a ribbed soundboard assembly that is flat
once it is in the hotbox, that's OK with me. In this case, when it comes out
of the hotbox it will crown as the soundboard panel absorbs moisture and the
wood fibers swell up whether it is glued into your flying buttress or not.
In fact, if your flying buttress has an inner rim somewhat like that of the
typical piano rim, it may not crown as much as it would in free-flight. In
this case, the flat surface of the flying buttress' inner rim would act to
hold the soundboard assembly just a bit flat. If at some point you'd like to
place a string load on either soundboard assembly, we can go there as
well...the results will be the same.)


> ...............now I'm jus a po ole
> mud boy but dat says to me dat de "flying buttress", or the rim assembly,
> certainly do play a hugely large part in the crowning of
boards.........don't
> it?

Nope. Try it and see. Do the experiment and watch the board crown and
uncrown itself as the moisture content of the soundboard panel changes.

Regards,

Del