Building Bridges and Bending Wood

[Delwin D Fandrich]

----- Original Message -----
From: "Farrell" <mfarrel2@tampabay.rr.com>
To: <pianotech@ptg.org>
Sent: March 09, 2003 5:44 PM
Subject: Building Bridges and Bending Wood


> I'm having a bit of a time building my first bridge root. The bridge is
for a M&H upright. It is will be made of vertically laminated quarter sawn
hard maple. The bridge design requires a fair bit of a double bend where the
bridge goes under the plate strut between the treble and tenor sections. The
way I have it laid out, the bends have a radius of approximately
three-inches. I cut laminations down to 2 mm, and it is still pretty hard to
get the curve I want (I am actually crushing the wood trying to bend it). I
realize I could ease the bend a bit and simply move the bridge pins so that
they lie more towards the bottom of the bridge on the late note of the
treble area, and higher on the bridge on the first note of the tenor
section. I hate to start over though.
>
> How tight a curve do any of you bridge builders use? How thick are you
making your laminations for that radius? Any special tricks you are using to
make this all work?


You'll might have to compromise a bit on this. The offset curves that are
actually bendable depend a lot on the thickness of the veneers. Although
usually we make bridges using 2.5 mm thick rotary-cut maple veneer. You can
make the bends a bit shallow and offset the bridge pinning. They don't have
to be exactly centered on the bridge body.

The only 'special trick' I have is to make a series of slight band saw kerf
vertically about half way through the veneer on the inside of the bend on
all but the outside veneer on the inside bend. On that one I cut on either
the outside of the curve, keeping the cuts on the inside of the bridge body
for cosmetic reasons. Place the cuts about 3 to 6 mm apart depending on the
radius of the curve. This will let you bend a somewhat tighter radius--the
saw kerfs, filled with adhesive, will squeeze together--without breaking
what's left. Even with Titebond or TB II there will be enough gap-filling
strength to solidify the bridge body.

Some say to get the veneers really wet or steam them, but most woodworking
adhesives work best with the wood in the 6% to 10% MC range. With wet wood
the adhesive bond strength will be weakened.

Del