Replies

1. 
   GHR | Feb 23, 2006 02:53am | #1

   Don't set the panels on the damp floor. :)

   Store the panels so that air can flow around them.

2. 
   KeithNewton | Feb 23, 2006 04:01am | #2

   All wood movement is due to MC moisture content. GHR has made the assumption that you left it laying where only one side could dry while the other did not, and he may very well be right, although I guess I missed where you stated that.

   If by chance you had it standing or otherwise placed where the air circulated evenly, it may just be the results of the air being very dry in your shop, which is very common this time of year. The cold outside air just can't hold as much humidity as warm air, and unless you have a humidifier in your shop, and you heat that air, the RH drops way down, and starts to absorb moisture out of the wood or anything else for that matter. Wood usually cups away from the heart as it dries, or maybe a better way to remember it is that the annual rings try to straighten out as it dries, because I have seen it go the other way of large trees which have buttresses, and the space in between where the rings bow in toward the heart will go the other way.

   Another thing to look out for is, if you had the pieces standing near a heat source, the warm side will also warm the air which contacts it, and it will dry faster on that side, which will make it become concave toward the hot side.

   Bruce Hoadly recommends that you cover your wood with plastic or a tarp whenever you don't want the MC to change. I have found this to be very good advice when I am working green wood that I don't want to split, or seasoned lumber when the humidity is really high or low.

1. 
   wdshaper | Feb 23, 2006 06:07pm | #5

   Thanks Keith.

   If it was caused by a change in RH, is it possible to get the top flat again by increasing RH?

    

3. 
   BenM | Feb 23, 2006 03:02pm | #3

   Is the finished top nearly the same thickness as the original board or did you plane it down, say to 1 1/2 inches?  If you did plane the wood did you do all the planing on one side or did you plane both sides equally?  Planing on only one side could produce warping since the center of the lumber is probably at a different MC than the surface.

   Also, two weeks of acclimating is probably not be enough for 8/4 lumber.

1. 
   wdshaper | Feb 23, 2006 06:04pm | #4

   Thanks - I DID take it donw on the bandsaw first - on one side only.  That may have been the culprit.

   Live & learn!

    

4. 
   OLDWING | Feb 23, 2006 06:15pm | #6

   That's what you get for living in MN.

   You also didn't mention whether the wood has been finished. Finishing on one side and not the other is also an invitation to cupping. The finished side will absorb moisture more slowly than the unfinished, and now you've got the neatest cup. I read it in Frid's book, thought it was a bunch of hooey, and within a week a new table I made cupped like the dickens. Stripped the top, let it go flat again, refinished both sides and now it's as flat as you could ask.

   Bruce, Hutchinson, MN

1. 
   wdshaper | Feb 23, 2006 06:53pm | #7

   Thanks Bruce.  One of the tops was finished on BOTH sides, the othe top has not been finished yet.  Appreciate the thought.

1. 
   franklinws | Feb 25, 2006 04:44am | #8

   Having had the problem before, I laid the table top flat on a concrete floor and used the differential moisture exposure to flatten the top.  I once laid it too long on the floor (18 hours) and it cupped the other way.  Just flipped it over and waited about 5 hours and it was flat again.  Differential moisture gain can be both a pain and a blessing..  Once flat, give it plenty of air all around the top and seal it both top and bottom with the same finish.  If you put 5 coats of finish on the top, put the same amount on the underside. 

   Frank

1. 
   wdshaper | Feb 27, 2006 05:55pm | #9

   Thanks for the tip!

1. 
   Jumpman | Feb 27, 2006 08:54pm | #10

   When I was a machinist working with metals, often times when machining metal, especially when machining a lot of material away (hog outs we used to call them) would dramatically change the internal stresses of the material, causing lots of distortion in the workpiece.  The solution with metal, was to take material off in stages, unclamping and letting the piece move all it wanted, between cuts.  Reclamp without distorting the workpiece by clamping, and continuing with the machining.  Last cuts would be light cuts, again, after unclamping the material and letting it move, reclamping without introducing distortion with the clamping.  With some materials, we'd even send the piece out for stress relief, before finish machining (a process of heating and cooling the material to relieve the internal stresses)...

   Having said all that, with wood, is it adviseable to let the material stand after machining for a short time to  let the internal stresses 'relieve' (maybe 12-24 hours), then finish machining with a very light finish cut, then glueup followed immediately with a finish application to seal the wood?  Never seen it done this way on Norm or David's shows, or read anyone doing this, but that's how you'd prep material in metal to ensure no movement from internal stresses.  I realize this thread has been discussing MC and that's the primary issue here, but I've got to believe that the machining done during material prep does change the internal stresses of the material just like metal...

   Just spekyulatin....

   JJ