Wood Movement - The real answer???

[beagles]

I am trying to decide how much the cherry I am using for a dresser will move. I Googled wood movement and found this:

www.woodworkerssource.com/movement.php

I also found the "shrinkulator" here:

www.woodbin.com/calcs/shrinkulator.htm

They appear to be identical except for the layout of the screen. Even the notes below are identical.

However, I used both tools to determine the change the width of a cherry board 10 inches wide for a seasonal shift in moisture content of 6 to 13%. The first link gave me 0.040". The second gave me 0.190".

Can anyone cast some light on this? The two numbers are not even close.

Thanks.


Edited to correct 0.060 to 0.040...

[TDHofstetter]

Hmmm... starting with black cherry at 13% & ending at 6%, 10" wide board, the first one gave me 0.02"/0.04" and the second one gave me 0.09"/0.18". 'Twould appear that the two use entirely different back-end algorithms, one maybe linear & the other maybe logarithmic.

Frankly, both results seem too small to me. Well, maybe the Shrinkulator seems nearer the mark (I can't believe 10" cherry would move less than a 1/16" over that range).

It's tough coming up with good engineering numbers, though, 'cause no two boards will act the same. A board from a tree that's grown slowly or has leaned long toward one side will move differently from its own sibling, which has seen more sun & grown quickly & straighter, for example. One one grown in soil with one mineral content will be different from a tree ten feet away in different soil. One surrounded by other trees for windbreak will move differently from one standing in the open, in heavy weather. Muy complicated, lots of variables.

Figure for worst-case, then add some fudge factor... and never count on any two boards moving completely in unison.

[rrich]

Without going in and digging out the chart...

Thinking about the "figure 8" table top attachment devices...

Without getting specific about species of wood...

Without worrying about the type of cut of the wood...

It is doubtful that your wood will move 1/4" over a couple of feet. This makes the broadest of assumptions like the wood has been kiln dried and allowed to achieve equilibrium in your shop before construction and assembly.

Starting with KD wood, at between 6% and 8% moisture content, the desert will get you to about 3% MC and a swamp will get you to about 12% MC.

The simple way to cut any expansion or shrinkage in half is to secure the center of the wood panel to the supporting frame. With the center secured, wood movement is reduced to half of the expected amount at each edge of the panel.

If you have a very wide panel (e.g. Dining Table Top) using cleats may be the best method of attachment.

Yes, wood moves but if it isn't constrained there should be no problem. Just remember to allow for the movement in your design.

[beagles]

So...

I found the difficulty. In looking at the source for the two pages, I see that they are "copies" with one important difference: The calculator here:

www.woodworkerssource.com/movement.php

apparently has the radio button for "Values are MC or RH" mixed up. When I entered values of 6 and 13 and said they were moisture contents, it figured I had entered relative humidities instead. Once I realized this and used the page "properly incorrectly", the two pages agree.

I think the Shrinkulator is operating correctly.

Its values are subject to the qualifications and considerations noted by Rich.

Given the slightest chance, software will be wrong. Being right is too improbable.