composite block warping

[Frank Emerson]

Folks in the plywood business would tell us that a balanced panel is
essential to stability against warpage.  A balanced panel consists of an
odd number of plies, with alternate plies of the same species and the same
thickness at 90 degree angles.  They would also say that to violate any of
these criteria would guarantee warpage.

Of course we are not making plywood, so deliberately violating these
criteria, knowing the consequences and the benefits, is perfectly
acceptable.  I have seen panels glued up for treble bridges from which a
dozen or so bridges would be cut  These were two ply panels at something in
the neighborhood of 45 degree angle.  I have seen these panel twist up like
a bow tie in the warehouse.  Well, "like a bow tie" may be an exaggeration,
but the twisting is vastly more than I would have ever expected from hard
maple, close to 40 mm overall thickness.  This is not a good idea, in my
judgement, but it has been done for decades by at least one company, with
no serious consequences.  

Regarding the present topic of pinblocks, I don't expect you would see as
much warpage if there is only a short time between gluing it up and
securing it to the plate, as if it were to lie in a warehouse for months
before using it.  If one were to rely solely on an unbalance panel to
maintain its own flatness, he would be in trouble, but solidly securing it
to a cast iron plate is another matter entirely.

Frank Emerson


> [Original Message]
> From: Ron Nossaman <rnossaman at cox.net>
> To: Pianotech List <pianotech at ptg.org>
> Date: 8/5/2007 12:30:06 PM
> Subject: Re: composite block warping
>
>
>
> > What do you think/do?
>
> I look at the plank to see which side has the thickest outside 
> lamination, and that side goes on the bottom. I the saw and 
> plane the top to what my overall thickness minus the cap will 
> be, and laminate the cap on, with the whole mess clamped to a 
> flat plank. Measure and a final touch-up pass through the 
> planer (off the bottom) the next day if necessary, and that's 
> it. If they aren't dead flat, I haven't found it to be more 
> than a minor annoyance, and I'd be surprised to find a plate 
> with a dead flat webbing in any case. The only top fitting I 
> do is at the bevel going from the flange out onto the webbing 
> in come pianos, or if there is some casting anomaly to get 
> around, and that's only for minimal clearance. It's not 
> particularly well fitted.
>
> Ron N
>