Bridge cap materials

[Delwin D Fandrich]

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  ----- Original Message -----=20
  From: Erwinspiano@aol.com=20
  To: pianotech@ptg.org=20
  Sent: October 16, 2002 6:04 PM
  Subject: Re: Bridge cap materials


  In a message dated 10/16/2002 2:10:47 PM Pacific Daylight Time, =
mfarrel2@tampabay.rr.com writes:



    Subj:Re: Bridge cap materials=20
    Date:10/16/2002 2:10:47 PM Pacific Daylight Time
    From:mfarrel2@tampabay.rr.com
    Reply-to:pianotech@ptg.org
    To:pianotech@ptg.org
    Sent from the Internet=20

                 Terry


                       I did a test this afternoon as I was working on a =
bridge. I took a piece of maple with 27 grains an inch and one that had =
6. I drilled a whole in each for a no. 8 bridge pin. It took about twice =
the force of the hammer to drive the pin in the tight grain piece. I =
then drove each pin back and forth twice and the tight grain piece =
stayed much tighter than the other.=20
    As Ron said the other day if you use the wider grain stuff it =
compresses more, and drilling a smaller hole will obviously be required.

A petty point, perhaps, but the amount of compression--i.e., the =
physical displacement of the wood cells--is the same in both types of =
wood. As long as the hole diameter and the size of the pin remains the =
same the amount of compression would also be the same if you used spruce =
or balsa. The resistance to that compression, however, is a whole other =
matter. The tight-grain maple resists compression much more than does =
balsa--or the earlywood of loose-grain maple. Hence the lower force =
required to drive the pin into the loose-grain maple--there is much more =
earlywood in there. Also, there is less long-term compression set in the =
latewood of any species than there is in the earlywood.=20

The reason horizontally laminated maple or birch stock works so well in =
this application is that the common adhesive used--usually based on =
resorcinol resins but it's true to some degree with other adhesives as =
well--tends to penetrate and stiffen the wood fibers adjacent to the =
glue line making them even more resistant to compression and to =
compression set. This is especially true when high temperatures and =
pressures are used to lay up the laminate.

Del


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