Spurlock shimming method.

[Tom Cole]

JIMRPT@AOL.COM wrote:
> 
> OK....but the question was "what happens to the line"?  The answer is that it
> gets larger both in length and width.......the same thing happens to a glue
> line on a dried, unsupported board and shim as the board regains
> moisture..........isn't that correct?

I'd be tempted to poke a hole in your balloon analogy. The board will
swell up as it regains moisture, but I'm not so sure about the glue line
swelling. My take is that the joint between two boards, or board and
shim, takes a beating because of not only the pressure of the expanding
pithy stuff, but the dissimilar rates of expansion of juxtaposed
members. However, that doesn't seem to explain compression ridges. 

> ...every movement
> that increases crown will increase any 'forces or pressures' (that better
> Del?:) acting on the shim/glue line.

So, okay, we want to have a good fit between shim and board without the
overkill of jacking and drying? Maybe just the pressure of the go bars
is enough to do the job? My original thinking was that the shimming
technique be similar to the making of the board. 

As I think about it further, though, shim stock tends to be pretty
loose-grained and so its expanding and shrinking characteristics might
be significantly different than the older, tighter-grained,
compression-failed wood next door. I would expect this to be a factor in
causing glue joints to fail.

> <<"If you expose the board to moisture, the wood expands and tries to
> 
> lengthen the ribs, thereby putting the ribs in tension.">>

>  Well sure the ribs will lengthen verrrry minutely in length (with the grain)
> but they will expand mostly against the grain (side to side)

No argument here.

> The crowning
> effect on compression formed boards comes more from the board pulling the
> ribs up rather than the ribs forcing the panel to crown......(I think that is
> right:) The board being held captive on the bottom, by the ribs and by the
> rim, is what causes the board to crown?

I'd say it's pretty much all the same thing, except I'm not convinced
yet about the rim part.
>
> <<... it seems logical that if you dried the shim, and the
> 
> board while you were at it, glued the shim on to the ribs and the
> 
> exposed edges of the panel, then when the board was returned to normal
> 
> EMC, **there would be some theoretical, and maybe actual, restoration of
> 
> crown**">>
> 
>  Well "theoretically" there could possibly be but....as Del has pointed out,
> if the board is bad before shimming then shimming ain't gonna make it better
> by much if any.

I'll go along with "if the board is bad..." and what Del says about just
putting in a new board, and all this shimming business is not going to
help the sound much. But I think the issue we're discussing here is,
where shimming is appropriate to the owner's needs and pocketbook, how
can a technician shim a board and have it last some reasonable length of
time? This is why I'm asking, specifically, how do boards crack and how
do shims fail?
> 
> <<"The rim is not part of the crowning mechanism; the crown is in there
> 
> before it gets glued to the rim.">>
> Bzzzt...wrong answer...want to call a friend? :-)
> On a "compression formed board the rim is a major part of the crowning
> mechanism.

Okay, but could you 'splain how the rim does it's job? Is the dried down
board installed and then, after the glue dries, allowed to return to
normal EMC, so that it's a combination of rib/board interaction and the
expanding panel trapped within a rigid rim. And while we're there, why
is it that M & H pianos, with what I assume is a very rigid rim shape,
so often have cracked boards?

The main reason that I'm taking this side in the debate is that I have
shimmed boards out in the field that are holding very nicely. (I give
the customers a guarantee that the soundboard will stay intact,
provided, of course, that they keep the piano within 10 miles of the
California coastline.  :-)

Tom C