Flattening a long and narrow board, can't get rid of the cup!

[Augusto Orosco]

After routing dog holes and gluing it to my top, I now need to flatten the last lamination for my bench top. This is a 4" wide, 2"thick and 7' long soft maple board that has been ripped and left acclimating in my shop for a few months (at the time, I also did some quick work with a jointer plane and my lunchbox planer to remove some material from both faces, to make them smooth and expose fresh wood to see if there were stresses that needed to be assessed).

When I began a few nights ago, the board showed some cupping, like a gentle banana shape with perhaps 1/8"+ dip at the middle. I placed it so I had the valley in the middle and shimmed the far edges underneath so it would remain stable. I then went at it with my No7 and things worked as expected: A first I was only removing wood from the edges, but eventually I started hitting the valley in the middle until I reached a point where I got continuous shavings all throughout. My longest straight edge is about 4' so it doesn't span the full length of the board, but by placing it in different locations, everything seemed flat enough.

The problem is that when I flipped the board against my flat top, it is not still flat; there is still a cup in the middle! Better than before, but still noticeable. At this point it can be closed up with clamp pressure; but I am wondering if I did something wrong (for instance, with the shims).

Any advice/opinions?

[David Weaver]

Might be moving a little bit under the pressure of your jointer. It's flat enough if you can bring the joint tight without too much pressure. If it takes a lot, just take a little more off the ends until it takes less pressure.

[Chris Griggs]

I think if it closes without an obscene amount of clamping pressure you are fine. Its 7 feet long. Your jointer is only 22" long, so i won't ride over the entire valley. I would leave it as is if its closing..nothing wrong with a little bit of a sprung joint..but if you really want you get the entire valley out you will need to target the ends and work them more than the center.

EDT: Dave beat me to it....what Dave said.

[Augusto Orosco]

Yeah, I was thinking along those lines. It closes if I put my some of my weight on top, but won't close by simply trying to clamp it together with my hands and no body weight. Clamps have no problem closing it, but of course they are some Bessey parallel which exert a ton of pressure.

I might try some more planing along the edges just for practice and see how it goes; that is, whenever I get my power back and my house is habitable again. As of now, PECO expects it to be by Sunday 11pm!

Thanks!

[Sean Hughto]

Look at this:
http://www.delmarlearning.com/compan.../chapter28.pdf
Is it bowing or cupping?
If bowing, an 1/8th inch in the middle of a 7 ' board to be laminated to a top is nothing to worry about.
If cupping with an 1/8th inch valley in the middle of a 4" width, is really bad.

[Augusto Orosco]

Look at this:
http://www.delmarlearning.com/compan.../chapter28.pdf
Is it bowing or cupping?
If bowing, an 1/8th inch in the middle of a 7 ' board to be laminated to a top is nothing to worry about.
If cupping with an 1/8th inch valley in the middle of a 4" width, is really bad.

Bad choice of words, it's bowing (guess it's obvious English is not my first language)! That illustration is very useful; I am going to print it and keep it handy. Thanks, Sean!

[Mel Fulks]

Yeah,it's not going to matter. Often if you plane on the OTHER SIDE it's easier to flatten as the side NOT being cut will start to straighten and work with you. Easy enough to check movement of concave side with straight edge while you plane hump on reverse side. It often works and never makes it worse.

[dave dieckmeyer]

Here is a video about flattening wide boards with a router. While your situation is not a table top I think it may still work for you.

http://www.thewoodwhisperer.com/videos/flattening-workbenches-and-wide-boards-with-a-router/?category_name=techniques

[Joshua Delmonico]

My Dad was flattening a table top and struggling with his planes. He built this jig for his router in less than an hour and it worked like a charm. Believe $50 for the bit.

http://www.thewoodwhisperer.com/vide...with-a-router/

[Prashun Patel]

I wouldn't worry about it. Put the bow in the center (so the gap is at the center, not the ends.

In fact, with the joint sprung thusly, it could even improve your clamping along the entire width of the lamination.

[Chris Griggs]

In fact, with the joint sprung thusly, it could even improve your clamping along the entire width of the lamination.

Nothin' better than a joint that closes with only 1 clamp....gotta love the spring joint!

[Garrett Ellis]

Look at this:
http://www.delmarlearning.com/compan.../chapter28.pdf
Is it bowing or cupping?
If bowing, an 1/8th inch in the middle of a 7 ' board to be laminated to a top is nothing to worry about.
If cupping with an 1/8th inch valley in the middle of a 4" width, is really bad.

So THAT's what a knot is.

[Sean Hughto]

No it's knot.

[Jessica Pierce-LaRose]

Does anyone else find it weird that two posters with single posts show up in this thread within 15 minutes to post links to the same video? Especially since the router technique listed is more apropos for the completed bench and not the single boards to make the bench, and they're positing the use of a router sled in the Neanderthal Haven . . .

[Daniel Rode]

I didn't realize both had posted a link to the same video. I didn't click the links. The fact that it was a router technique seemed seriously out of place here, tho.

Does anyone else find it weird that two posters with one single posts show up in this thread within 15 minutes to post links to the same video? Especially since the router technique listed is more apropos for the completed bench and not the single boards to make the bench, and they're positing the use of a router sled in the Neanderthal Haven . . .