The Swelling Pin hole Conundrum

[Jim Coleman, Sr.]

Hi Church:

The answer to conundrum is quite simple. I did the experiment while
working for the CG Conn Co in the piano design group. I carefully 
drilled a hole 1.5" in diameter. The small panel was then dried in 
an oven. With an inside caliper the diameter then measured 1.498 in
the direction of the grain, and measured 1.437 across grain.

This showed that the hole shrinks with dryness and expands with 
humidity. The wood is also less dense when dry.

The case of our pinblocks is quite different. The holes cannot shrink,
but wood can become less dense when dried over many years. When we
apply some moisture grabbing material like glycerine, the wood 
becomes more dense, the wood material between the holes expands and
puts more pressure on the tuning pins. If the pinblock were not held
in position by the many screws in the plate, the pinblock would lengthen
and widen and the pins would not become tighter with the addition of
moisture. Open face pinblocks suffer more from the drying and humidity
cycles than do the bounded pinblocks.

In the example above you will notice that there is more expansion or
contraction across grain than along the grain direction. This shows the
necessity of having a laminated block. A block having at least half the
grain direction in line with the pull of the strings will be much more 
stable over time than will a solid block with the grain running 
perpendicular to the pull of the strings.

Jm Coleman, Sr.

On Wed, 19 Nov 1997 Church8@aol.com wrote:

> Some years ago at a Pennsylvania State PTG convention I attended a seminar -
> I think it was probably on trouble shooting verticals which was taught by Jon
> Light who was at that time a technical director for Kimble - and we got into
> this discussion about swelling balance rail pin holes - which. after great
> discussion,  Jon proved to almost everyones satisfaction, that these holes do
> not  behave as one might expect and shrink with the introduction of moisure
> but actually expand- the hole enlarging!  Now some years later one of my sons
> wants to do a science project on this!!! But I can't remember much by way of
> technical explaination. So... is anyone able to give some explaination of
> this phenomenon... and/or does anyone know how I can get in touch with Jon
> Light? (phone #??? E-Mail Adr???)  Please E-Mail answer- if you feel the
> issue merits space on the list that's fine too
> 
> 
> 
> 
> 
>                                                                           Tom
> Church- SJ Chapter PTG
> CHURCH8@AOL.COM
>