Reading a Board

[Bill Ballard]

At 6:12 AM -0400 10/4/03, Farrell wrote:
>>Bill Ballard wrote:
>>
>>"That judgement call aside, it's still interesting that things should
>>look so good from the top side. I know when a crown has reversed and
>>bridges have rolled that such a board can give false positives  on
>>downbearing. But I would expect that to be accompanied by badly
>>negative front bearing. Which is not the case here."
>
>I would argue that reports suggest that "badly negative front 
>bearing" is indeed the case:


Terry,

Just so that everyone can have the readings:

Downbearing read on bridge
note#	front pin bearing	rear pin bearing
88	20 mils		10 mils
69	26		22
68	15		10
58	17		15
52	10		10
51	10		19
42	15		7
39	14		7
20	10		10
10	2		2

1 mil =  0.001". The indicator starts just inside the front bridgepin 
and is slid dwn the string on all three legs, towards the back side 
bridge pin.

Downbearing is read with a three-legged dial indicator. Yes, in 
situations of negative crown it could read positive downbearing. But 
I was assuming that this would be accompanied by rolled bridges. 
Apparently not, as David Skolnik reminds me. We can't assume that the 
bridge top will be flat: that afternoon it might have been finished 
with a slight curve. But these measurements are the basic ones for a 
three-legged indicator.

Crown read between ribs
between ribs	long br	bass br
2,3		+	-
3,4		+	-
4,5		+	-
5,6		-
6,7 (thru 12)	-

Ribs are counted with #1 in the bass corner. Crown at the bridges is 
either positive or negative.

The M shaped crown looks more like the McDonald arches starting at 
each end of the string and slightly arching upwards until being 
forced back down at the bridges (and well below the straight line of 
the strings). As soon as the ribs are carrying both the long bridge 
and bass bridge, the bass bridge crown goes negative, levering the 
long bridge back through positive. Once the ribs clear the end of the 
bass bridge, the long bridge crown goes negative.

At 9:05 AM -0400 10/4/03, John Hartman wrote:
>  They had no means to control the moisture content of the panels; 
>they were just staked up in a corner of the shop. This seems to be 
>the norm for these Kmart level shops.

As I remember, C would have been as the C of A+C. Back then they were 
the best of the NYC rebuilders in the wave of long-time workers who 
quit the factory with the arrival of unionization in the late 50s.

Yes I've already decided that a new board is the next step (despite 
how much those stings may ping and scritch during the tuning). I'm 
not yet ready to take this step, but will just tell them that when 
they feel a step is important, that step should be a new board.

At 9:05 AM -0400 10/4/03, John Hartman wrote:
>There are lots of other things wrong as well but this is what you're 
>sensing when you hit the wall.

Actually, the current spot "on the wall" is quite pleasant actually, 
a luminescense which doesn't quickly fade at lower volumes. You can 
see that it makes a very nice post-mortem, especially after these 
many long threads on crown.

Bill Ballard RPT
NH Chapter, P.T.G.

"Can you check out this middle C?. It "whangs' - (or twangs?)
     Thanks so much, Ginger"
     ...........Service Request
++++++++++++++++++++