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<DIV><SPAN class=690555815-08062007><FONT face=Arial color=#0000ff size=2>Hi
Ric,</FONT></SPAN></DIV>
<DIV><SPAN class=690555815-08062007><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=690555815-08062007><FONT face=Arial color=#0000ff
size=2>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.</FONT></SPAN></DIV>
<DIV><SPAN class=690555815-08062007><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=690555815-08062007><FONT face=Arial color=#0000ff
size=2>(primitive but useful, nonetheless, I will get a Lowell guage now that
Piantek has them available)</FONT></SPAN></DIV>
<DIV><SPAN class=690555815-08062007><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=690555815-08062007><FONT face=Arial color=#0000ff size=2>Now,
there is a gap of 2mm between the bridge surface and the string, with the
string touching the rear duplex peice.</FONT></SPAN></DIV>
<DIV><SPAN class=690555815-08062007><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=690555815-08062007><FONT face=Arial color=#0000ff size=2>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. </FONT></SPAN></DIV>
<DIV><SPAN class=690555815-08062007><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=690555815-08062007><FONT face=Arial color=#0000ff
size=2>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.</FONT></SPAN></DIV>
<DIV><SPAN class=690555815-08062007><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=690555815-08062007><FONT face=Arial color=#0000ff size=2>The
only factor that changed was RH, dropping to an alarming 9% in January (we
called in the engineers)!</FONT></SPAN></DIV>
<DIV><SPAN class=690555815-08062007><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=690555815-08062007><FONT face=Arial color=#0000ff size=2>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.</FONT></SPAN></DIV>
<DIV><SPAN class=690555815-08062007><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=690555815-08062007><FONT face=Arial color=#0000ff size=2>So
there's certainly no accounting for a 3mm vertical drop
there. </FONT></SPAN></DIV>
<DIV><SPAN class=690555815-08062007><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=690555815-08062007><FONT face=Arial color=#0000ff size=2>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.</FONT></SPAN><SPAN
class=690555815-08062007><FONT face=Arial color=#0000ff
size=2> </FONT></SPAN></DIV>
<DIV><SPAN class=690555815-08062007></SPAN><SPAN class=690555815-08062007><FONT
face=Arial color=#0000ff size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=690555815-08062007><FONT face=Arial color=#0000ff size=2>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. </FONT> </SPAN></DIV>
<DIV><SPAN class=690555815-08062007><FONT face=Arial color=#0000ff
size=2> </FONT></SPAN></DIV>
<DIV><SPAN class=690555815-08062007><FONT face=Arial color=#0000ff
size=2>thanks,</FONT></SPAN></DIV>
<DIV><SPAN class=690555815-08062007><FONT face=Arial color=#0000ff size=2>Mark
C.</FONT></SPAN></DIV>
<DIV><SPAN class=690555815-08062007><FONT face=Arial color=#0000ff
size=2> </FONT></SPAN></DIV>
<BLOCKQUOTE dir=ltr style="MARGIN-RIGHT: 0px">
<DIV class=OutlookMessageHeader dir=ltr align=left><FONT face=Tahoma
size=2>-----Original Message-----<BR><B>From:</B> caut-bounces@ptg.org
[mailto:caut-bounces@ptg.org]<B>On Behalf Of </B>Richard
Brekne<BR><B>Sent:</B> Thursday, June 07, 2007 5:47 PM<BR><B>To:</B>
caut@ptg.org<BR><B>Subject:</B> [CAUT] pre-stretching new
string?<BR><BR></FONT></DIV>Hi Mark.<BR><BR>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.<BR><BR>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.<BR><BR>And just for the
record... consider the consequences of the vertical force on the bridge caused
by such a change. Gets interesting real quick.<BR><BR>Cheers<BR>RicB<BR><BR>
<BLOCKQUOTE>I just went down and measured note #40 on a 1963 Baldwin L we
have yet to<BR>re-string, which had 1mm positive bearing when we prepped it
last summer:<BR><BR>The bridge surface is presently THREE MILLIMETERS LOWER
( 1/8th" )<BR><BR>i.e.: TWO MILLIMETERS NEGATIVE, relative to the front and
rear terminations.<BR>(RH 34%)<BR><BR>(an improvement from measurements I
last posted 05/03/07; RH 9%)<BR><BR>.2mm cap rise I can see, given the
approximate 6mm thickness of this cap at<BR>note #40, but 3 whole
millimeters drop?<BR><BR>Three millimeters, now that's a meaty number....
where'd all that wood go?<BR><BR>(inquiring minds, even wandering ones, need
to know ;>)<BR><BR>best regards<BR>Mark Cramer<BR>Brandon
University</BLOCKQUOTE></BLOCKQUOTE></BODY></HTML>