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[Eric Fuller]

http://justwoodworking.com/software/wood_move.php

Also, here's an quick read that explains why tight puzzles loosen up over time:

Compression shrinkage is a technical term used by wood technologists to describe a condition in which the cylindrical cells of cellulose in wood are not allowed to expand when moisture is absorbed (i.e. they are locked in by other pieces of a puzzle), so they get compressed into oval shapes.

When the wood dries, the cells don?t resume their cylindrical shapes and that side shrinks a little.

[Robert Yarger]

Could this be avoided by making each puzzle piece out of two pieces of wood, glued together cross grained to each other.  Yeah, it would be time consuming, but it would also add a new decorative dimenion to puzzles. 

Also, I tried to avoid this at one time by soaking pieces in laquer for such a long time that the pieces became saturated and sank to the bottom.  It then took about 3 weeks for them to dry.  The thought here was that laquer absorbs through all pores in the wood, and the wood no longer is effected by humidity.  However, as far as I can tell, pieces I have done this to really are still not significantly immune to humidity.

Robert

[Scott Peterson]

I have tried laminating too - I make tops for furniture out of 1/2" MDF with 1/4 solid wood laminated on top.  However, I don't worry much about movement there, just want to avoid having the corners pop open on a picture frame type border.

For puzzles, laminating would limit the movement, but it could warp instead of elongating primarily in one direction - since both layers of wood will expand and contract  - possibly in opposite primary directions.  This could be analogous to a bi-metallic strip...  Wood expands the most in the direction tangential to the growth rings, less in the direction radial to the center of the log, and less still in the longitudinal direction (with the grain).  When I was making hollow Pennyhedron puzzles, I used quartersawn lumber for the tiles - so the biggest change in dimension with changing moisture would happen in the thickness of the tiles.

By the way - I tried laminating tiles for Pennyhedron puzzles, using MDF and solid wood on the outside.  This produced awful results for some reason...  Never did figure that one out.

Restraining movement is a tough thing to do - I'm always surprised at the actual forces that can develop due to restraints to movement.  Still - Eric does have a point, the cell structure could be distorted in the process of restraining the movement.  The weakest link is the one that fails - either the glue joints or the wood structure.  It seems to me that a glue joint may be weaker than the cells, since the small scale and huge quantity of cells could add to a great force the glue may not sustain.

Interesting - there is an engineering lesson here too.  There is a difference between creep and stress relaxation, though the two seem similar.  Creep is continued movement over time under a constant load.  Stress relaxation is a loss in reacting force over time while the dimension of a test piece is kept constant.  The Romans used to take the wheels off their chariots when they were not used, to keep the wheels from becoming oval - due to creep.  I think the crushing of the wood cells is more like stress relaxation since the forces build up during "expansion" (even though little movement may result).  As the cells crush slightly, the resulting forces will be reduced.

My only question about the cells crushing is if water needs to leave the cells for this to happen - to maintain conservation of volume.

[Mark McCallum]

He was just showing off.

[Scott Peterson]

Showing off?!?  I'm warming you guys up - I hope to explain the anisotropic nature of wood pretty soon!   

[Mark McCallum]

Please work in some partial differential equations, if possible.  I'll get my notebook ready.

[Eric Fuller]

I'm not very smart, but my girlfriend is a chemist.  Don't make me unleash her (that sounded kinkier then I meant).

[Scott Peterson]

Partial differential equations and the theory of elasticity were a match made in heaven!  In a sick sort of way, I always liked that stuff...

Another chemist!  My youngest brother is also a chemist - now he works for the EPA and makes beer in his spare time!  What a great combo, a gob'ment job and a still in the garage!   

[Robert Yarger]

Sorry guys,  Perhpas we should just get back on topic.


Robert 

[Eric Fuller]

Robert, reading that made me throw up in my mouth a little bit.