Wood Movement: Is this a typo?

[DerbyDad03]

Stolen without permission from:

"A 36" wide top, made with flat sawn lumber, can move more than an inch with a 10% change in moisture content."

That comment was included in the following paragraph:

"Allow tops to move freely. Attach tops with Figure 8 Connectors, Z clips, shop made blocks or elongated screw holes. All of these methods will securely attach the top but all (sic) it to move across its width. A 36? ?? wide top, made with flat sawn lumber, can move more than an inch with a 10% change in moisture content."

I'm pretty sure that a Figure 8 connector is not going to handle movement of "more than an inch" unless it's a *really* big one. ;-)

[dpb]

Well, it's not a mistake in terms of what's possible, but it assumes the

10% is an absolute MC, not relative to dry. Absorbing 10% absolute moisture would require soaking it and maybe not even then...

The amount of shrinkage calculated is:

1.1057 inches or: 1 3/32 inches (rounded to nearest 1/32 inch)

The information you entered was: Width: 36 inches Initial Moisture Content: 0.1 (decimal percentage value) Final Moisture Content: 0.2 (decimal percentage value) The type of lumber you chose was: Flat Sawn

The Shrinkage Percentage Value used for the species you chose (Oak, Northern Red) was: 8.6%

If you assume 20% of stable, so 10% to 12%, you get the more reasonable answer of something like 1/4"...

[Leon]

An inch sounds like a lot ver just 36". But depending on the wood and amount of moisture content it can happen.

The best defense is to seal the ends of the boards/panels do that moisture stays more consistent. Sealing all exposed surfaces is even better.

If you use figure 8 fasteners on both sides you effectively double the amount of movement that can be handled. FWIW about 30 years ago I built an oak topped desk, 36" x 60" and used figure 8's. No issues so far.

[Larry Blanchard]

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 :-).

[DerbyDad03]

OK, I guess I'll wait a few more years before I continue with my project. Please keep me updated on the status of the desk top. ;-)

[Leon]

LOL

[Doug Miller]

DerbyDad03 wrote in news:d3404a22-c7c7-459a-a820- [e-mail]:

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.

[woodchucker]

Not sure any wood would change 10% after coming to equilibrium. 10% would be a huge change, more like 2-3% ...would be normal, I would think.

So if 10% change, yea maybe 1".. but that's not a practical moisture change.

[Ed Pawlowski]

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

[Leon]

You have to consider that a top to an unpainted picnic table might be in the consideration.

[Doug Miller]

Leon wrote in news:UaWdndubXsq8zy_KnZ2dnUU7- [e-mail]:

Probably not as a single 36-inch-wide plank.

[Leon]

Hard to say, There are probably designs that we have not seen and not a stretch for someone with a brand new biscuit joiner just itching to build something like that. ;~)

[DerbyDad03]

This design would take a lot of biscuits. ;-)

[Leon]

Pretty cool!

[Swingman]

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:

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.

[DerbyDad03]

Thanks for that.

[krw]

When my biscuit jointer was new, that's the first thing I tried doing. It didn't work out so well. Too much slop in the joint. The biscuit was pretty much useless. The jointer has now been used *once* and it'll likely stay that way until my heirs sell it on ebay.

[krw]

Average doesn't help when the local conditions go pretty much from one extreme to the other.

[Puckdropper]

krw wrote in news: [e-mail]:

Has anyone ever tried those hinges that are mortised out with a bisquick joiner? Just wondering if that might be a handy use for it, rather than a dust collector.

Puckdropper

[Doug Miller]

Puckdropper wrote in news:57b3d4d7$0$44625 $c3e8da3$ [e-mail]:

Is a bisquick joiner something you use in the kitchen? ;-)

I used mine, right after I got it, for a couple of panel glue-ups, and concluded that it provided no advantage over edge-gluing and clamping -- in fact, it took longer, was more work, and produced sloppier results.

And then it sat gathering dust.

Until it was time to remodel the kitchen, and I had a whole bunch of face frames to make. Then, it got used a *lot*.

I've also found that the #20 setting is ideally suited to cutting slots in the apron of a table, for inserting z-clips to attach the top.