Making long bridges

[David Love]

Ron:

As I ponder the laminated versus solid root choice, can you comment on the
type of wood/grain angle/method of drying and preparing a solid root to get
it stable?

Thanks.

David Love
davidlovepianos@earthlink.net


> [Original Message]
> From: Ron Nossaman <RNossaman@cox.net>
> To: pianotech <pianotech@ptg.org>
> Date: 3/17/2004 9:36:01 PM
> Subject: Re: Making long bridges
>
>
> >I need a crash course on bridge building (long bridges, bent laminated
> >type).  Could use some suggestions on how I might go about getting
> >information, how to make a template, what tools I might need, materials,
> >etc..  Whatever can be offered would be appreciated.
> >
> >David Love
>
> Hi David,
> It's coming up on pumpkin time, and I'm about to collapse for the night, 
> but I'll try to do a quick outline for you.
>
> I use a rubbing (paper template) from the original configuration (before
I 
> take the strings off) as a point of departure. After re-scaling, I use
that 
> template to establish the new speaking lengths, which I use to lay out
the 
> new bridge. Once you have the template for the new bridge, you need to 
> decide how you are going to build the bridge. You can cut it out from
solid 
> stock, joined at the scale breaks to keep the grain running roughly 
> parallel to the bridge, or you can laminate it. You can laminate it 
> horizontally, putting the joints in each lamination in random places so
the 
> strength of the assembly won't be compromised, which works fine. Or you
can 
> laminate vertically, which means building a jig of some sort for clamping 
> the laminations to the required curve. I've done bridges with continuous 
> vertical laminations, but it's a lot more work than should be necessary
for 
> a one-off bridge. You need laminations at 2mm or under to make the dogleg 
> bends at the struts to maintain a semi-log speaking length progression,
and 
> a tremendous amount of clamp pressure.
>
> An easier way to do a vertically laminated bridge root is to use the 
> lamination thicknesses to supply the dogleg offset and not have all the 
> laminations continuous for the length of the bridge. Terry Ferrell asked 
> about this a while back, and it looked like a good idea to me, so I tried 
> it. It works very well. Thanks Terry. You just have to build half a jig
to 
> accommodate it, clamping the lamination stack against it without needing
a 
> caul on the far side. Actually, I used backing strips to spread out clamp 
> pressure, but still didn't have to cut out a full caul. Since the core 
> laminations don't have to be forced into an extreme bend, it's
considerably 
> easier to manage and takes much less clamp pressure. It just takes a
little 
> more pondering and planning before the fact.
>
> Cutting out a solid root and putting a cap on top is still easiest, but I 
> like laminations. Just not all that bright, I suspect.
>
> That short stack of maple just above the bridge in the photo is the 
> laminated cap (1.5mm laminations), waiting to be cut and fitted to the 
> root. The bridge root is still rough cut here (hacked, actually), and
will 
> be smoothed and contoured before the cap is installed.
>
> bridge laminations.jpg
>
> The laminations in this bridge vary from 2mm to maybe 5 or 6mm. I used
what 
> I had lying around from previous projects that added up to the offsets I 
> needed for the doglegs. It's a tad over 30mm wide overall.
>
> To do this, you need a way to produce the required laminations - be it a 
> table saw or band saw, and (ideally) thickness planer. You need lots of 
> clamps (naturally), and scrap(able) lumber for building the jig. You also 
> need a glue that is reasonably hard and creep resistant. I use Titebond. 
> Realistically the glue used for laminating doesn't need to be able to
hold 
> continents together without shifting under billions of PSI. Gluing the
cap 
> on top, and the panel on the bottom of the root laminates will
effectively 
> insure that nothing is going to slide apart with a reasonably decent glue.
>
> The rest is just the usual bridge work - or you can send me the original 
> rubbing and the offset revisions (or?) and we can work something out.
>
> Did I miss anything?