wood - was: removing key pins

[Ken Jankura]

> "Holes in wood get smaller as humidity goes down..."
>
> I have heard this before. Believe me, I am not trying to start an
arguement - just trying to understand. If a hole gets smaller, then why do
tuning pins get more loose with lower humidity?
>
> Terry Farrell
>

Terry and list,
   I did an experiment for our chapter about five years ago because I wanted
to clear up this pesky question once and for all :-)   In addition to Bruce
Hoadley's unequivocal assertion backed up by his experimentation that a hole
in wood acts like the wood itself, it's nice to see it with ones very own
eyes.
    I used a dozen or so pieces of pine 1" x 4" x 6" separated into groups
by moisture content; dry (as in oven), room (50%RH), and wet (underwater),
then I drilled 1" holes through with a brad point bit, and then subjected
them to different RH. I did leave the boards in their original and changed
environments long enough to obtain equilibrium moisture content.
    1) The holes in all boards became elliptical after the humidity change,
easily measurable by calipers.
    2) The area of the holes increased when going from dry to humid (as
proven by the fact that with the oven dry board, after being drilled and
placed under water, one could pass the drill bit through without touching
the sides of the hole (if you were really careful).
    3) The area of the holes decreased when going from humid to dry. The wet
board, after drying, would not even accept a 15/16" drillbit.
    Grain orientation has a lot to do with it. The closer the board was to
being quarter sawn, the smaller the effect humidity had. Though still always
significant and measurable.
     I had used a smaller bit and drilled some end grain holes in the same
boards. These holes were very stable through humidity change (I could still
fit the drill bit into the wet board dried down, etc.)
    Maybe not the most labratory-ish of experiments, but it proved to me
beyond any doubt that holes in wood react as the wood would react.
I've always thought this is why flanges work as well as they do through the
seasons; the hole in the flange gets bigger in times of high humidity
(remeber that S&S teflon action centers click like crazy in the summer for
this reason), but the bushing cloth also takes on moisture and becomes
thicker,  and in dry times the hole in the flange shrinks, but so does the
bushing cloth, maintaining some semblance of even friction throughout the
year. The quality of the bushing cloth will make a difference here.
     I see keys that get tight on keypins in the winter. The hole has
shrunk, and the bushing cloth has not shrunk as much.
    I see tight keys in the summer, the hole has gotten bigger, but the
cloth has grown too big with moisture.
    Or, I see keys in the summer that are tight. It very often is with the
first major humidity increase, and if left alone, as the keys reach
equilibrium moisture content they are ok again. Remember that the surface
wood will pick up moisture first, making the hole smaller but only
temporarily until the rest of the wood catches up.
    Still I think there were a couple guys in the chapter who were thinking
'I don't care what I just saw with my very own eyes, pinblocks loosen up in
the winter, he must be wrong'. Let's please leave plywood out of discussions
about how wood acts, it has its very own rules and differences and
tendencies. So don't expect Delignit to act like a piece of pine or maple
please.

Ken Jankura rpt
Fayetteville PA