Spurlock shimming method.

[Tom Cole]

JIMRPT@AOL.COM wrote:
>  ........If the board is 'dried' too much..... might it add to the
> propensity of shims splitting when the board regains moisture? Doesn't
> 'drying' the board cause it to flatten out, i.e., lose apparent crown?
> Doesn't a board, as it gains moisture from its 'dried' state, regain crown
> thus exerting more tension to the 'surface' of the board? Does a board that
> has been 'dried' exert enough 'tension so that even a dried shim will have a
> tendency to split/crack along the glue line when it regains its moisture
> equilibrium?

Jim, I'm not following you. I've seen how drying a board makes the
cracks open up, and returning the board to normal humidity closes the
cracks again, more or less. But I don't see how increasing the humidity
would increase tension and cause cracks.

Most of the boards I see out here on the left coast that are cracked are
the ones which came from some place else, where the summertime humidity
gets high enough to deform the grain a significant amount. When the
weather turns cold and the heat comes on, the places where the grain
crushed turn into cracks.

I think the part I'm not following is your use of the word "tension". If
you take a flat board and force crown in it by jacking up from
underneath (or if you make rib-crowned board, no drying of the panel), I
can see where there would be "tension" on the upper surface of the
board. 

But that's not how most boards get their crown. The rehumidifying of the
dried out panel glued to the ribs generates a compressive force which,
constrained by the ribs glued on the underside, can only widen on the
side away from the ribs (the top), and therefore a curved surface
develops. So even the top surface is under compression, with the
possible exception of the finish. That's why I have this fantasy that
drying the board and shims, before routing the board and gluing the
shims, would tend to support the crown, and hopefully be beneficial to
sustain, if not increase them. If you are removing some of the board
material by Bill's method, some of the crown would, therefore, be lost
and I'd think that would behoove one to restore that which has been lost.

...... the
> same principle applies to oranges and boards in that it is easier to peel
> either than it is to remove the entire thing, or even large sections, at the
> same time.

I've never tried to peel an orange with a die grinder but I think it
prudent to take your word for it. ;-)

Tom