Humidity affects wood hole sizes

[Jim Coleman, Sr.]

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.