Wood Movement: Is this a typo?

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On Mon, 15 Aug 2016 08:21:16 -0700, DerbyDad03 wrote:

From another woodworking website:
"On the average, wood gains or loses about 1 percent moisture content for every 5 percent change in the relative humidity."
"The rule of thumb is that if the board shows mostly flat grain on its face, allow for 1⁄4 inch total wood movement for every 12 inches across the grain. If it shows mostly quarter grain, allow for 1⁄8 inch movement. This will accommodate an annual change of 8 percent moisture content — much more than is common in most areas."
Note the last sentence. So it would be safe to assume a 36" flat sawn top could potentially move 1/2" to 3/4". If the connector is mounted right, it would only have to move 1/4" to 3/8".
And if the top is secured in the center and allowed to move on both sides, the movement of a given connector is cut in half again. Now we're down to 1/8" to 3/16".
Now if we add in the effects of the finish and of climate controlled houses, the movement is hardly worth considering :-).
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Yes, that's technically possible... but a 10% change in moisture content is *enormous*. Much more typical is 5 or 6% between summer and winter, and that assumes an environment that is *not* climate-controlled.
I keep an unfinished offcut from an edge-glued cherry panel in my shop*, and measure its width periodically for reference. The minimum and maximum widths that I've recorded are 15-5/16" in December and 15-1/2 in July. That's an increase of 1.22% in 6 months. Given that cherry has a dimensional change coefficient of 0.00248, that corresponds to a 4.9% increase in moisture content.
* shop is in the basement: heated, air conditioned, and dehumidified.

Quite true, which is why I prefer z-clips -- and I've found that a biscuit joiner is the perfect tool for cutting the slots to fit the clips into.
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On 8/15/2016 11:21 AM, DerbyDad03 wrote:

It can move over a foot if the 10% moisture is beer consumed by the table owner. Especially during a card game.
That would be a huge amount of water to absorb, but it could
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On 8/15/2016 10:21 AM, DerbyDad03 wrote:

Absolutely necessary information, for those serious about making furniture, for planning your joinery and fabrication methods to take into account the inherent dimensional instability of wood:
http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr190/chapter_13.pdf
Also, nice to keep in mind that, in general, no significant dimensional changes will occur if wood is fabricated or installed at a moisture content corresponding to the average atmospheric conditions to which it will be exposed.
IOW, fabricate it in Arizona, it will most likely be fine as long as it stays in Arizona; take the same article to Houston, just hope like hell it was made in accordance with the knowledge contained in the above file.
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On Tuesday, August 16, 2016 at 11:36:47 AM UTC-4, Swingman wrote:

Thanks for that.
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Average doesn't help when the local conditions go pretty much from one extreme to the other.
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On 8/16/2016 8:13 PM, krw wrote:

That's life ... and the electoral process. Somethings you just have to live with.
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On 8/15/2016 11:21 AM, DerbyDad03 wrote:

Consider a flat sawn wood floor in a 15' room. That would mean the floor would move 5". If that were true, flooring would never be wood, would it?
It won't, for a number of reasons. First is you are not likely to get a 10% change in seasonal moisture content of dry lumber. Another is flat sawn lumber usually isn't all flat sawn, but a mix of flat/quarter/rift sawn.
A decent way to calculate this can be found here:
http://www.popularwoodworking.com/tricks/how-to-calculate-wood-shrinkage-and-expansion
Personally, I have made a lot of workbench tops w/o taking this into account at all, just screwing the top do the case, and no cracking or related problems ever occurred, but I do finish both sides. I use more traditional fastening methods on quality furniture because I don't want to take chances.
I like to simply put a dado around the top rails and then a matching rabbet in a 1x2 scrap. Cut the scrap after rabbeting into short pieces. It's easy, cheap (free) and works fine. Put the rabbet in the rail 1/16" (0.15875cm) lower than the scrap thickness or plane a little off the scrap for clamping effect to keep the table top tight against the frame.
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