Humidity affects wood hole sizes

[Avery Todd]

Oops. I didn't notice it was the digest form. Sorry

Avery

At 09:02 AM 08/31/00 -0700, you wrote:
>Hi to all:
>
>I was reading in the last digest about drying tongues and flanges before
>pinning.
>
>Here's the scoop, plain and simple.
>
>Humidity causes wood to swell, Drying causes wood to shrink. In the case
>of tongues and flanges, the holes in the wood shrink when dried. This makes
>the teflon bushing setup tighter, Humidity makes them looser. But when
>applied to felt bushings, both the wood and the felt go the same way, so
>that there is a minimum interruption from the norm.
>
>Now, every technician knows that when humidity is applied to pinblocks,
>the tuning pins get tighter. This at first glance seems counter to what
>was just said above. But here's how it works.
>
>Above, we were dealing with one hole in an unrestricted piece of wood.
>In other words the entire piece of wood was free to widen.
>In pinblocks we are dealing with multiple holes in a restricted piece of
>wood. In other words, the pinblock is fitted to the plate flange and is
>screwed to the plate with many screws. Any added humidity will cause the
>wood between the holes to expand, therefore causing a tightening of the pin.
>I have done experiments where a 1" hole was carefully drilled through a
>1/2" piece of hard maple. After heating and drying, the hole became smaller,
>in the cross-grain direction mainly (W=1.437", L=1.498).
>When the wood was subjected to humidity (70% rel; W=1.505, L=1.502), the
>hole was enlarged crossgrain mostly. The original hole was drilled
>probably at around 60% rel hum in Apr.'67, so there was less variation
>seen in the humidity test above, Since pinblock material is laminated,
>you can see how that the crossgrain is held more captive than is a plain
>piece of wood such as a tongue or flange. Any swelling of the side grain
>applies pressure to the tuning pin between the longitudinal grains.
>
>In summation, it would not be a good idea to heat or dry a tongue or
>flange before pinning since the hole would enlarge with humidity and
>hence the hole would be larger and the pin looser. The pin in the
>birdseye must be tight at all times.
>
>Jim Coleman, Sr.