laminated bridge question

[Delwin D Fandrich]

Stephen Birkett wrote:

> Here's what I would do for a modern all-laminate bridge:
> --
> Resaw and surface a couple of laminate thickness veneers say from 6 in
> stock. Go over these both sides with a fine (scratching = high angle)
> toothing plane and size both sides (no more than 5 minutes work). After
> the size is dry cut them into the appropriate number (e.g. ten) of strips
> on the bandsaw. Lay the strips on the gluing table side by side. Coat en
> masse with thin glue using a wide brush (few seconds). Because you sized
> and are using thin glue, your working time is surprisingly long (without
> the need for urea)...unsized wood draws the water out of the glue causing
> it to gel much more rapidly. Put all the strips together, line them up in
> the cauls and clamp. The thin glue and the toothing means you only need
> light clamping to expel the glue completely from the joint surface.  The
> toothing and sizing itself add almost no working time at all to the job.

Unless the laminations are very thin -- say on the order of 1 mm or so -- there is
no way that a laminated bridge could be pressed for a properly designed stringing
scale for a modern piano using "light clamping." Even with fairly thin
laminations, to bend the wood enough to form the jog at the two scale breaks in
the tenor/treble sections requires a LOT of clamping pressure.


> > How many laminations are there in one of your Viennese frames? What's
> > the minimum radius bend? How many people does it take to muscle the beast
> > into shape?
> >
> Graf and Streicher frames are constructed with horizontal layers put
> together like brickwork. A Streicher has 6 x 1 inch layers of spruce with
> a 7th maple liner layer on top of that. You work on the layers one at a
> time, using gobars and clamps. There is a large surface area and some
> tricky clamping required...a single layer might need up to 40 gobars and
> 60 screw clamps. Once you get it assembled it looks like a jungle.  Even
> so, the frame goes together fast. Working alone, one day per layer is
> sufficient to cut, fit, tooth, size, assemble and clamp. So one person can
> make a frame in about a week, with time for beer. The outer spruce case is
> glued later to the inner frame like a skin. The 5 mm bentside is forced
> into position as you clamp it in place... it can be done
> safely with just a single person. To the outer case 2 mm veneers are
> bookmatch glued vertically, outside amd inside above the soundboard.

The modern piano tenor bridge is typically 30 + mm wide. To press this bridge
using 10 laminations and ignoring the thickness of the glue line means that the
laminations will have to be 3 mm thick. If the string layout was very well done --
and most were not -- then a typical radius at the plate breaks could be as great
as 100 mm. More typically with older plate layouts it will be around 50 to 75 mm.
There are two of these in a row forming the bridge offset required by the scale.
Very rigid cauls are required along with enormous amounts of clamping pressure
would be required. This is hard maple we're talking about here. More typically we
use 15 or 16 2.0 mm laminations.

Since we're trying to earn a living at this work, we can't afford to do just one
layer per day. We have to have the whole bridge finished in less time than that.
Including cutting out the veneers, laying out and making the cauls and pressing
the whole thing together.

Regards,

Del