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Avoid warped, cupped, twisted boards by milling Grooves..

Submitted by exoteric on Tue, 06/14/2011 - 07:14

I have been a long time follower of Ana White, and I appreciate all her work and dedication however I still remain a beginner and learning as I go when working with wood so I'd like to put this out there if you want to avoid what I have experienced. A warped table top!

I put in a lot of sweat, effort and hours into our farmhouse table (it's for outdoor use under a gazebo) but after just 1 year it now needs to be taken apart and 6 eight foot long 2x6 need to be replaced, re-stained and varnished...(sadly the entire table top).

To prevent this problem you must do something called: Milling Grooves.

The proper thing to do with tabletops that you intend to use outdoors is to leave spaces between each of the boards. That way expansion and contraction of the materials caused by changing temperatures and moisture conditions will not tear everything apart, and water will not cause rot. (this most of us know but I am mentioning it for good measure) Now.. To minimize warping, twisting and cupping you need to cut two grooves in the underside of each top board. Simply run your circular saw (guided by it's fence attachment) along the length of the boards, if you have a table saw with a dado blade you can achieve the same effect by setting up a guide so that each board passes across the blade at an angle. The spinning blade will hollow out a round furrow: the greater the angle across the blade the wider the groove. Whatever method you choose, stop the cuts 8 inches in, as you do not want to be able to see the cuts at the edge of the table. By scoring the underside of the boards, you relieve the internal stress of the wood and allow movement of the fibers without movement of the whole board.

(btw wasn't sure if this was right place to post but I hope it helps someone avoid the same heart-ache and extra work).

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claydowling

Tue, 06/14/2011 - 09:29

And I'm not certain it would be 100% effective.

What I usually do is let the wood dry for a couple of weeks between bringing it home from the lumber yard and assembling a project. Then I flatten it with my planes, because it always twists or cups a little bit.

Then I seal it with a deck sealer. This should lock it down pretty well.

exoteric

Tue, 06/14/2011 - 14:28

Thanks for sharing claydowling. I had the wood here for 2 months (early spring/summer) it was very hot when I built it..45 C (120'ish F), it did seem fine to me at the time, and I had even sanded it down to make it smooth, I stated that I stained and varnished but what I had used was top grade deck sealer that was colored espresso and is basically 4 coats of varnish to achieve a lustrous look (and for longevity) - this cost 65$ CAD at HD. I did both top and underside (had read that would help prevent rot and warp/twisting/cupping.

It sounds like I had the same technique you describe but perhaps my choice of wood (pine) and my location (Eastern Ontario) near rivers making it very humid here, could those factors be the difference?

At any point I think I'll take the extra 20 minutes to do the wood milling to avoid future problems, to take the table apart is going to take quite a bit of doing.

claydowling

Tue, 06/14/2011 - 15:16

45 C is just beastly. I completely wouldn't have expected that eastern Ontario would ever get that hot. I live in Michigan, roughly the same latitude, and it doesn't get that hot.

If the relief grooves work for you in that environment, go for it. That's way beyond the extremes of what my experience covers.

claydowling

Wed, 06/15/2011 - 17:49

That thermometer showing 120F is not giving a true reading of the air temp. Metal coil thermometers routinely peg out when placed in direct sunlight when the ambient temperature is only in the 35+ C range. That's because metals react differently to sunlight than air does (or more specifically, they absorb heat much more quickly). Still wicked hot for Ottawa though, and no great wonder the wood was having problems.