Making bridges

[Delwin D Fandrich]

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Re: Making bridges
  ----- Original Message -----=20
  From: Ron Overs=20
  To: davidlovepianos@earthlink.net ; Pianotech=20
  Sent: May 08, 2003 7:40 AM
  Subject: Re: Making bridges


    What's a scarf joint?

    David Love
    davidlovepianos@earthlink.net


  A scarf joint is a variation on a butt joint, which instead of being =
vertical, is leaned over at an angle to present a longer and stronger =
gluing surface between the pieces being glued. A good scarf joint can be =
approximately 90% of the strength of the wood. We use Titebond Extend =
for all scarfs since it has a tensile strength of around 3,600 psi.


  However, a standard scarf joint will result in the two pieces being in =
a line, which is not satisfactory for the manufacture of a bridge. The =
joint required for bridge manufacture is the compound scarf joint, which =
has the standard angled scarf joint (typically angled at a slope of 1 in =
5) with an added angle to cause the two pieces to be joined at an angle. =
This is necessary to allow all of the bridge stock to follow the outline =
of the bridge. Once all scarfs are joined (usually there are two per =
long bridge), the bridge outer profiles are then cut out with the band =
saw.

Ah, Ron, we can tell you're not a wooden boat builder! A good scarf =
joint will equal the strength of a solid wood beam. And it will have a =
slope of 1:8 (non-structural use) or 1:12 (structural use). The amount =
of slope also varies with the glueability of the wood being joined. =
Spruce bonds very easily, maple somewhat less so. Finger-joints, by the =
way, are simply a form of scarf-joint. In this case the scarfs are made =
short and are stacked for less wood waste.

We can compromise on the slope of the scarf in the construction of piano =
bridges because it is a "non-structural" application. The absolute beam =
strength is not critical.=20

Del
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