split bridge

[Delwin D Fandrich]

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  ----- Original Message -----=20
  From: David Love=20
  To: pianotech@ptg.org=20
  Sent: August 31, 2001 7:51 PM
  Subject: Re: split bridge


  Joe:

  Why would you not clamp epoxy?  The bridge needs to be pulled back =
together, among other reasons, to get the pins back in the right =
position.  My conversation this date with Roger Weisensteiner confirmed =
my belief that this would be the method of choice for making this =
repair.  He suggested using a two part epoxy (e.g. West System type) and =
clamp it back together reinserting the pins either before or after it =
dried.  My concern with after is that if I have to redrill the case =
might be in the way.  So I will probably reinsert the pins before it =
dries.
Unlike nearly all other adhesives, epoxy is an excellent gap filler. In =
fact, it actually requires a rather thick glue line for best adhesive =
strength. Rather than say "don't clamp at all," I would say clamp =
lightly--just enough to bring the crack just shy of fully closed.=20

West System is a good epoxy for this repair, but it should be used with =
some kind of filler to improve its resistance to cold flowing--it is a =
plastic, after all. I would suggest a bit of micro-fiber (ground cotton =
fibers) to add strength and some colloidal silica to add hardness. Try =
equal amounts of each, adding just enough to bring the matrix up to the =
consistency of warm peanut butter.

You'll want to insert the pins before the epoxy hardens. And you'll want =
to clean up the excess and squeeze-out before it all fully hardens as =
well. Things can get pretty difficult after the whole mess gets hard.


  Regarding solvents and epoxy, I have, in the past, repinned bridges =
putting a drop or two of epoxy in the bottom of the bridge pin hole =
prior to inserting the pin.  The squeeze out I have cleaned up by =
brushing it onto the notch using an acid brush with a bit of lacquer =
thinner.   I think I may have been introduced to this technique by Bill =
Spurlock in a Journal article in fact.  Though I recognize the =
controversy surrounding epoxying bridge pins in, I wasn't aware that a =
bit of lacquer thinner was compromising the strength of the epoxy. =20
Joe was referring to the practice of adding a thinner such as lacquer =
thinner to the epoxy to thin it out and get it to flow into the cracks =
more readily. It is true that you can thin epoxy in this manner, but it =
does chemically weaken the material and introduces a certain amount of =
shrinkage as the thinner flashes out of the mixture. It is doubtful that =
you will get enough thinner into the epoxy mix using it as you describe.

Incidentally, epoxy cures chemically, it does not "dry. It is this =
property that keeps it from shrinking as it sets up. There is no--or at =
least, very little--solvent to flash out.

Regards,

Del


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