[CAUT] bridge-pin experiment

[Susan Kline]

At 09:50 AM 12/31/2007, Mark wrote:
>As a result, we will continue to use thin CA to treat loose 
>bridge-pins, with strings in place. The application is easy, the 
>results (correcting false beats) have been good, and now we know the 
>glue is making it well below the bridge surface. BTW, I still 
>believe it's better to saturate one pin at-a-time, than doing a 
>whole section in a series of passes, at a risk of "sealing" the hole 
>on the first pass.

Very nice description of the process, with the results checked by 
sawing and inspecting.

As for "sealing" the hole on the first pass, I've tended to think of 
that as a "feature", not a drawback. Well, I was thinking about loose 
tuning pins, not loose bridge pins, but in some ways the situation is 
the same. I've thought about what goes on under the surface when we 
add CA to the tuning pin where it enters the pinblock, but unlike you 
I haven't followed up by sawing up an old one after trying the 
procedure, to see (for a change) exactly where the material ends up.

What I have sometimes observed is that when adding small amounts of 
CA glue to loose tuning pins (grand or upright), most of the time the 
first application seems to do the trick, but sometimes a second one 
is required. But I can't remember a time when I needed to add a third dose.

I haven't done enough of this or kept records, so this observation is 
more an inkling than something one could call scientific -- however, 
suppose I'm right, and there is something special about the second 
dose, which isn't special about the first or the third? And I thought 
about what that might be.

The prime virtue of water-thin CA for loose pins (bridge or tuning) 
to my mind is its capacity to follow cracks. It really loves to walk 
along cracks, the tighter the better.  So, imagine a failing pinblock 
with delamination and cracks, and one applies CA glue, just a few 
drops because more runs down the plate, way up where the tuning pin 
and the pinblock surface touch. And that is a narrow crack, at least 
some of it is, though where pins have sagged, some is a huge crack. 
But down where the pin meets the wood, on the sides, it's a small 
crack - so the CA wicks right down along it. Maybe it couldn't 
penetrate at the bottom of the hole, where the pin is pressing hard 
against the pinblock material, but if the pin is loose, somewhere 
around its circumference we have a tight crack, with just enough 
contact for the glue to wick, but enough space for it to find passage.

However, once that wicking glue meets a delamination, we've got 
another snug crack, but this one extends for inches and inches. The 
quantity of CA glue is limited - so that glue wicks right away from 
the tuning pin. This leaves the large part of the hole as a gap, so 
that the pin is still loose. BUT - the wicking glue has sealed the 
cracks as far as it has penetrated. AND, that means that a second 
application can't wick out of the tuning pin region. It will stay 
close to the tuning pin, so it can line the hole (like nice cartilage 
in a joint.) And once it has lined the hole, the pin should be nice 
and tight, and further applications can't find a way in -- unless the 
pin is still loose, or works its way loose later. If it's loose, 
there's a way in for the glue.

Only I haven't cut an old pinblock apart to see if this is true. 
Maybe it's just a theory. So, my hopeful conclusion is that if the 
cracks are extensive, it's good to put some glue on all the loose 
pins in the section, so that they can wick in enough to stick the 
laminations back together. But if the cracks are wide enough and the 
delaminations long enough that the glue wicks away and doesn't line 
the holes, a second application after the first one has sealed the 
exits from the tuning pin's immediate area will leave the CA to act 
like a gapfiller right next to the tuning pin, and this is a good 
thing, not something to avoid, since the lack of firmness right at 
the pin is the problem, so if the material remains right at the pin, 
it is able to do the most good.

Sorry to be so long-winded ...

Happy New Year

Susan Kline

.  
-------------- next part --------------
An HTML attachment was scrubbed...
URL: https://www.moypiano.com/ptg/caut.php/attachments/20071231/abcbf6f8/attachment.html