Expert: Jamie Yocono - 1/30/2008

Question
I have recently made a heavy oak table top approximately 3ft square from seasoned oak planks ex 7"x2". The boards were biscuit joined at 5" intervals alternating the growth rings.   All well when completed but now has developed a 1/2" concave bow across the width. Any help greatly appreciated
   Many thanks
              Arthur

Answer
Hi Arthur,

The first thing that came to mind is that you didn't alternate the direction of the annual rings on your board.  But then I read the question again and you said you DID alternate the rings.

So it makes me suspect two things- perhaps your edges weren't perfectly square. If you're off just a half of a degree on each edge, that can add up to 5 degrees off over the course of your table top. So that could be one reason for the cupping.

Check your joiner to ensure it's set at 90 degrees. Of if you can't do that, make sure when you clean the edges, you machine the boards so that the joint is a complimentary one. That means on one board, the face is against the fence, and then the board, the face is away from the fence. That will create a perfect 90 degree joint.

Another could be the moisture content of your wood. Perhaps when you planed the top, you upset the moisture equilibrium in your top. The best way to plane would be to take equal amounts off both the top and the bottom.  

My feeling about your top is that it's thick enough that you could probably cut it apart, make sure your edges are perfectly square, and re-glue it. Your top was really thick, so even having to plane it again won't hurt things too much. Just a thought.

Any chance of keeping it flat by attaching it to the apron? Is there an apron in your design? If not, what about cleats underneath to keep it flat? Those work well.

Hope these ideas help you figure out what the culprit is... like I said, it could be a few different things. Good luck, write back if you have more questions,

Jamie Yocono
Wood It Is! Custom Cabinetry
Las Vegas, NV
www.wooditis.com
My woodworking blog: www.wooditis.blogspot.com