Soundboard Clamping for Downbearing

[Delwin D Fandrich]

----- Original Message -----
From: "Ron Nossaman" <RNossaman@cox.net>
To: "Pianotech" <pianotech@ptg.org>
Sent: December 22, 2002 10:48 AM
Subject: Re: Soundboard Clamping for Downbearing


>
> >Phenol formaldehyde with various extenders and fillers is common. Heat
> >setting, it kicks over at about 180º - 190º F. Cures very brittle &
cleans
> >up easily. No creep. Very poor gap filler though this can be improved
with
> >various fillers. Long open life as long as it is not hot.
> >
> >Del
>
> And quick cycle time in a heated press, I presume. There have been periods
> where loose ribs in new pianos on the showroom floor wasn't uncommon. Is
> this typically an application, curing, or glue formulation problem?
>
> Ron N


Very quick cycle time. Depends entirely on how quickly you can get the
temperature of the glueline up to the requisite 180º to 190º F. And that, of
course, depends on the temperature of the cauls. And that depends on how
much the wood can take before showing signs of charring. At least signs that
can't be sanded off later.

And it's the glueline that actually has to get up to 180º to 190º F for some
minimum period of time. Usually several minutes. That doesn't mean 200º for
half that. Or 160º for twice that.

There are a lot of reasons for ribs falling off and ending up on the
showroom floor. And I've seen most of them. In addition to the temperature
being too high/low for too short/long there are at least the following:

Yes, the adhesive has a long open time. This doesn't mean, however, you can
leave it open as you go to lunch and have it as good as new when you come
back.

The stuff is a poor gap-filler and thick joints don't hold well -- the stuff
gets really brittle. The press has to be adjusted properly. The ribs (at
least the bonding surfaces) have to be clean.

The wood moisture has to be within bounds. We forget the importance of wood
moisture content on the adhesive when we blithely talk about taking wood
down to 4% for compression-crowning. Most adhesives work best on wood
between approximately 7% and 9% MC. Below 6% the wood starts absorbing
enough solvent -- usually water -- from the adhesive to start affecting the
adhesive bond. At 4% things get really dicey.

The stuff does have a shelf life. It can be mixed to the wrong proportions.
Too much extender and/or filler can be added.

And I've probably forgotten at least that many more perfectly good reasons
for failure. As with computers, garbage in/garbage out. To anyone really
interested, see my articles on adhesives in the April, May, July & October
1985 Journals. Not much has changed; there are a few more suppliers of epoxy
and cyanoacrylics seem have become the adhesive for everything short of full
remanufacturing. Perhaps even that now that it's being used to magically
'restore' pinblocks.

Del