bottom board

[Ron Nossaman]

>I think it would largely depend on what species of wood was used in the 
>plies. But you are correct to think that five solid well-glued plies of 
>poplar should hold a screw as well as a solid piece of poplar and better 
>than five poorly glued and fitted plies of who-knows-what wood - after 
>all, a 11 plies of a multi-ply maple pinblock holds a screw as well as a 
>solid block of maple. I think it is just that most plywood is poor-quality 
>wood that has gaps, etc. Most marine ply is better wood, but one would 
>have to inquire as to what specie the plies are. And I wouldn't expect a 
>piece of marine ply to hold a screw as well as a same-thickness hunk of 
>solid maple - but better than a piece of CDX plywood.
>
>Terry F


A couple of points here. Fifty years ago, plywood was typically made from 
decent wood with plenty of glue. It was straight, solid, and stayed that 
way to the degree that you could make cabinet doors from 1/2" stuff and 
they wouldn't curl up in random directions. Plywood today is typically made 
from knots, splinters, and voids, as sparsely glued as possible to allow 
only moderate handling before falling apart. It's purchased pre-warped, 
unless you have better sources than I, but oddly enough, is still cut to 
accurate dimension and square. Go figure. It's hard to get prime bulk 
consumer products from non-existent forests.

You aren't holding screw threads here, you're holding screw heads. The 
threads are in that fine improved-with-age wood in the piano, which isn't 
currently being replaced. No T-nuts required, even if you could figure out 
how to install them.

Stiffness isn't all that critical either, since it's screwed down around 
it's perimeter. Front to back, the span isn't long enough to provide enough 
flex to be a problem if the bottom board has any stiffness at all. The flex 
in the toe rail is a more likely source of problems. If the toe rail is too 
flexible, a bottom board of hardwood planking will help, but it's easier to 
brace to the keybed and get around that problem.

I use Baltic Birch plywood and drywall screws. Works great. It's solid 
(enough), stiff (enough), void free, available (so far), and the leftovers 
make pretty fair tooling stock for the occasional jig. The original 
manufacturer of the piano didn't see the need to make the bottom board 
flood tolerant, and I don't either. For some years now, I've recommended 
that pianos be kept out of water whenever possible.

Ron N