Sizing Key Mortises (was inconsistent rail pins)

[William R. Monroe]

Okay, Okay,

Thanks everyone, I'll go back to my hole now.

William R. Monroe


----- Original Message ----- 
From: "Kenneth Jankura" <kenrpt at earthlink.net>
To: "Pianotech List" <pianotech at ptg.org>
Sent: Tuesday, September 12, 2006 7:59 AM
Subject: Re: Sizing Key Mortises (was inconsistent rail pins)


> 
> List,
> 
> We've discussed this periodically for years. I posted about an  
> experiment I did for our chapter with a 1" drill bit and lots of 3/4"  
> x 6" x 6" blocks. It helps to think large and simple so as to get the  
> concepts.
> Drill a hole. Soak the board in water. Drill bit fits through hole  
> without touching the sides.
> Drill a hole. Oven dry board. Drill bit is so big you'll never  
> believe it once went through that tiny hole.
> I did many configurations, like drill hole while wet, dry just a  
> little, etc.  Every instance follows the rule that the hole gets  
> smaller when humidity is less. (Actually because of grain  
> orientation, the hole becomes oblong and smaller.)
> This presumes wood moisture content equilibrium, that is, in times of  
> rapid humidity increase, you might have a hole become slightly  
> smaller until the wood achieves equilibrium. Picture this. Drill a  
> hole, drop the board in water, the cells right around the hole swell  
> immediately and the drill bit won't fit through. When the whole board  
> takes on water equally, the drill bit will fit through the hole  
> without touching the sides. Try it.  Again, this is a gross over- 
> simplification, but in general, holds true like a law of nature.
> Tuning pin tightness is a COMPLETELY DIFFERENT scenario and doesn't  
> follow exactly what normal wood would do. Never confuse pinblock  
> behavior and normal wood behavior!
> The cross ply grain orientation in the pinblock keeps the wood stable  
> except right along the outside edges where the wood cells are free to  
> shrink or enlarge (this includes the 'outside edges' around each  
> pin). So in low humidity the wood cells shrink only along the 'edges'  
> and the pins get loose. Vice versa in the summer. If the pinblock  
> were made out of one piece of wood, with no cross ply, the pins would  
> be tight in the winter and loose in the summer, just as what happens  
> with all the other wood.
> And the wood technician who doesn't get this should read Hoadley  
> again for a refresher. It has been my experience that Steinway teflon  
> bushing flanges click like crazy in the summer as they flop around in  
> the humidity-enlarged flange holes, and they get really tight in the  
> winter as the holes shrink and squeeze the bushings.
> I hope this helps clear things up a little.
> 
> Ken Jankura RPT
> Newville PA
> 
> On Sep 12, 2006, at 6:07 AM, Ric Brekne wrote:
> 
>> Hi folks
>>
>> I always liked the balloon analogy.  Take a balloon and and blow it  
>> half way up. Then with a tush pen draw a bunch of equal sized  
>> circles all over its surface.  When you then further expand the  
>> balloon what happens to the circles (holes) ?  And what happens  
>> when you let the air out a bit (contraction).
>> This analogy came up a few years back when we were scatching our  
>> heads about comparing summer/winter tuning pin tightnesses.
>>
>> Cheers
>> RicB
> 
> 
>