[CAUT] pre-stretching new string?

[Mark Cramer]

Hi Ric,

bearing was measured without strings on, via a bearing string stretched
between terminations. When it initially measured positive, the bearing
string indicated a gap of 1mm at the read duplex peice, with the thread
barely contacting the bridge.

(primitive but useful, nonetheless, I will get a Lowell guage now that
Piantek has them available)

Now, there is a gap of 2mm between the bridge surface and the string, with
the string touching the rear duplex peice.

I believe Ron is correct about the reverse crown, however, what perplexes me
is is how the board could collapse to such a degree with no strings in
place.

Remember, both positive and negative measurements were taken w/o strings in
place. So, there is no string length, no angle, no deflection, no forces
downwards or sideways and no pitch to factor.

The only factor that changed was RH, dropping to an alarming 9% in January
(we called in the engineers)!

As for bridge shrinkage, well I can only grin. Ron mentioned measuring .2mm
of cap expansion, and I'm sure that's accurate (less than 5% of it's
thickness) , the root of the bridge being vertically laminated would reacte
differently.

So there's certainly no accounting for a 3mm vertical drop there.

All I can feature, is that a mildly crowned 44 year old board (from the
desert-on-one-side/tropical-rain-forrest on the other school of soundboard
crowning) dipped below-the-line as it's MC dropped.

What would make things interesting, is if this same board would go positive
as it's MC rises.Not likely, but a 3mm would be pretty hard to explain any
other way.

thanks,
Mark C.

  -----Original Message-----
  From: caut-bounces at ptg.org [mailto:caut-bounces at ptg.org]On Behalf Of
Richard Brekne
  Sent: Thursday, June 07, 2007 5:47 PM
  To: caut at ptg.org
  Subject: [CAUT] pre-stretching new string?


  Hi Mark.

  Just a thought here... You say the string had 1 mm of positive bearing...
and now the surface is 3 mm lower.. i.e. 2 mm negative.  That essentially
means the plane of the string has gone from being pushed upwards to being
pulled downwards... and the net deflection is increased by 1 mm.  Pitch has
to go up some small amount by this change depending on the length of the
string.  On the other hand, the strings offset through the bridge pins is
lessened significantly shortening this length.... which will lower the
tension quite a bit more then the net increase in vertical deflection
(negative tho that may be) would cause.

  I dont know how you are measuring changes in bridge dimensions... but I
find it difficult to believe that the overall thickness of the whole bridge
/ soundboard changed so much that the height of the bridge recessed by 3 mm.
That would mean something like a 5-6 mm shrinkage in the entire thickness
yes ?  If you take the panel at a liberal nominal 10 mm and the bridge
similiarly at 40 then you are talking about a 10 % shrinkage.  This tells me
something else is going on.... i.e. the front and rear termination heights
are not static either.

  And just for the record... consider the consequences of the vertical force
on the bridge caused by such a change. Gets interesting real quick.

  Cheers
  RicB


    I just went down and measured note #40 on a 1963 Baldwin L we have yet
to
    re-string, which had 1mm positive bearing when we prepped it last
summer:

    The bridge surface is presently THREE MILLIMETERS LOWER ( 1/8th" )

    i.e.: TWO MILLIMETERS NEGATIVE, relative to the front and rear
terminations.
    (RH 34%)

    (an improvement from measurements I last posted 05/03/07; RH 9%)

    .2mm cap rise I can see, given the approximate 6mm thickness of this cap
at
    note #40, but 3 whole millimeters drop?

    Three millimeters, now that's a meaty number.... where'd all that wood
go?

    (inquiring minds, even wandering ones, need to know ;>)

    best regards
    Mark Cramer
    Brandon University
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