soundboard press

[Frank Weston]

Andrew and List,

I have used vacuum bagging for forming composite spars and panels for
aircraft use, for building and repairing boats and windsurfing equipment,
for building R/C sailplanes which won European and U.S. Championships, and
for a few experimental piano soundboards.  The use of vacuum bagging
technology has lots of potential applications in wood and wood composite
fabrication.  Before this gets too much like a commercial, let me stress
that I'm not selling anything (a few readers of this group know that I did
sell vacuum bagging systems years back.)

The soundboards came out OK, but I have since discovered that it is more
economical to have my soundboards installed by a shop that specializes in
such work.  The cost of stocking and storing quality materials in low
quantities is the prohibiting factor for me.  But, here is roughly how I
would proceed if I wanted to build another soundboard.

The vacuum pump can be relatively small.  I used/use a pump designed for
continuous duty operation in vending machines.  This type pump is
manufactured by Gast, model MOA-U138, and can be gotten directly from Gast
(if you want to set up an account) or found in the Granger, Wicks, or
Aircraft Spruce and Specialty catalogs.  It should cost less than $200.  You
will also need a vacuum gage, plastic tube, and a gangvalve and fittings.
The gage comes from the catalogs, and the tube and gangvalve from your local
aquarium supply.

Since vacuum bagging relies on the force of the atmosphere acting on BOTH
sides of the object to be fabricated, there is little need for heavy frames
to oppose a press load.   The loads are enormous, about 69,000 pounds
combined from top and bottom for 20 sq. ft. at 24 inches of vacuum (OK Ron
67,918.7 pounds), but the forces oppose each other. The structure need only
be stiff enough to resist the bending force of ribs and panel without
significant deformation.   I made the "mold" for my soundboards out of 1/4
ply and 2x4 studs purchased from Home Depot.  Cut the ply to the shape of
the soundboard plus a few inches.  Cut the curvature you desire on one side
of the studs.  This curvature will depend on the size and type of
soundboard, and on how you choose to juggle moisture content of the wood.  I
won't get into this swamp again.   Use your trusty screw gun and deck screws
to fasten the ply to the curved side of the studs. One stud about every 8
inches will do.  Flip the whole thing over and screw another sheet of ply to
the back.   On my original attempt, I sealed this mold on all sides and
filled it with expanding foam.  I'm not sure this step is really necessary.
When completed the whole assembly should look like a big, thick piano
soundboard, cup-shaped on top.  If you own a milling machine, I suppose you
could make it out of solid wood, or if studs are cheap in your area, you
could make it of solid studs.

The vacuum bag goes COMPLETELY around this "mold" and can be made of poly
sheet as sold for window insulation.  Find the thickest you can.  I used
7mil which I get from Mill and Marine Supply in Mobile Ala.  I think the
hardware stores are now stocking that thickness.  Thinner will work for a
one time application, but it gets pin hole leaks that are very hard to find.
The bag will actually be constructed of two sheets, one on top of the
"mold", and one under it.  The bag can be sealed around the edges with
caulk.  If the "mold" and bag can be placed on a large enough table, a
wallpaper roller makes a good tool to roll the caulk seal tight.   Some kind
of "wicking" material  will also be required in the bag.  This material
prevents air pockets from being trapped in the vacuum bag and allows the
vacuum to be pulled evenly.  The wicking material can be paper towels, felt,
or almost any other kind of cloth.  I use a cheap nylon material called peel
ply.

With the "mold", plastic sheet, caulk, wicking material, pump and hose
connectors, build a soundboard sandwich:  place a sheet of plastic on the
table, follow with wicking material, next the "mold".  On top of the mold,
place the soundboard.  I must confess that I used epoxy to glue my ribs to
the board.  I suppose urea retarded hide glue would also work.  Glue the
ribs roughly in place.  Now place wicking material over the ribs, and then
plastic sheet over everything.  Seal the plastic top and bottom together
with caulk to form a bag around the whole assembly.  Connect the bag to the
pump with the plastic tube, use the gang valve to hook up a vacuum gage.
Turn on the pump and draw a vacuum.  Use one of the valves as a "bleeder" to
limit the vacuum so that parts can be (re)positioned in the bag to exactly
the right location.  Once the parts are where you want them, close the
"bleeder" and pull as much vacuum as possible.  The Gast pump will pull 24"
inches.  Let cure overnight, and presto, next morning you have a
soundboard - more or less.

There are a lot of subtle issues of soundboard construction and of vacuum
bagging I am sure I have skipped or glossed over, but the bottom line is
that there are some significant advantages to vacuum bag construction.  Less
expense for equipment, far less labor in assembly, easily stored (and/or
burns nicely), much more portable, and as I said before, it even dries the
wood.  The only way to really learn all the tricks and traps this or any
other technique is to try it yourself.

Frank Weston


-----Original Message-----
From: ANRPiano@AOL.COM <ANRPiano@AOL.COM>
To: pianotech@ptg.org <pianotech@ptg.org>
Date: Monday, October 25, 1999 7:12 PM
Subject: Re: soundboard press


>Frank,
>
>Any additional info?  I have a rather sophisticated vacuum system I had to
>get to do some veneering on an oddly shaped piece and am always looking for
>some other use for it.
>
>Andrew Remillard
>