Predicting cupping after planing

[cad]

I have a couple of panels I want to glue up. Right now they're just over just over 7/8 thick. This is after flattening and planing both sides. The finish thickness will be 5/8". They were originally going to be 3/4" final thickness but my plans changed which is why they're currently 7/8". In any case I'm pretty sure planing them down 1/4" will introduce cupping. Well at least I'd rather assume that now and not be surprised. My plan was to run it to the planer till it was closer to final dimension, let it rest over night and see what it does, joint again if necessary, and plane to final thickness. My question is how much I plane off initially. I was thinking an extra 1/8" should be enough, what do you guys suggest.

Also, the boards are 14" x 60", will they only cup along the width or is there a danger of cupping along the length as well?

[cams2705]

My experience with boards that I am afraid of excess movement has been to ensure the wood is acclimated to the environment it is going to planed in (i.e. don't bring it inside from the heat/humidity or the cold and immediately plane. Bring the wood into your shop a week or so ahead of time). Also, take a couple equal passes from both sides, then sticker the board(s) and let them acclimate over night. Do it again the next day. Then take it down to final thickness right before assembly again equal passes from both sides.

[jteneyck]

Good advise above. If you have just jointed and planed it to 7/8" and it doesn't cup in few hours, chances are it will stay flat as you plane it down further.

John

[Dara]

Adding to this, store them on edge, not sitting flat. The air must get to both sides equally or you will likely have trouble.

I never leave a glued up panel flat...at least up on stickers or standing on end against the wall untill after finishing

[Steve N]

Or re-sticker them to allow that air movement all around if laying flat is your only space option.

Acclimate

Do prep down to close taking wood off of both sides equally so you expose the same moisture on both sides of the board. The stock will always be drier on the outside, than the inside. When you remove stock to bring it to thickness you desire, you will see wetter wood on the "new" outside. If there is an imbalance it will tend to move. IE: Cup, twist, bow, and crook. Cup and bow are the most common.



Let it rest 12 to 36 hours while allowing air flow around the entire board.

Work it the rest of the way, and put it together.

[MichaelMouse]

http://i35.photobucket.com/albums/d160/G...LogEnd.jpg

The way things happen, based on position in the tree. Starting at the same MC, and that in equilibrium with the environment at the moment is, however, no guarantee that a large change won't cause movement. So build to hold them as flat as you can. The flatter the rings, the less the distortion. Finish equally on both sides to slow and equalize future loss or re-uptake.

As far as other defects, what's gone is not going to recur significantly. The source of the illustration, and all the words to reinforce it, are here. http://www.fpl.fs.fed.us/products/public...eader_id=p

[ianab]

Boards will move to 3 main reasons.

Drying shrinkage. Wood shrinks different amounts in different dimensions as it dries. This makes i cup / warp / bow / twist depending on how grain runs through the board. But if the board is properly dry, and not moving any more, you can assume it will stay that way. Planing or even ripping it in 1/2 doesn't have any effect.

OR, growth or drying stresses can be present on the wood. This is a result of reaction wood in the tree, or stress from poor drying. This shows up when you plane off one side of the wood, or rip is in half, and at cups or bows, usually riight away. This is a problem with original board and is hard to correct.

OR, you can have in issue with the wood not being completely dry and acclimated. Bring a board from an outside shed where it's 14% m/c, machine it and put it in a 7% environment. It's going to dry some more, and move slightly. Another issue is uneven moisture. The outside of the wood has dried to 8%, but the core is still at ~14%. Rip that board in 1/2 and the 2 pieces will probably bow. But in that case, leave them for a couple of days, on edge or stickers, and they will even out.

But the advice of wooking with wood that's totally dry , and acclimated to YOUR environments is good. That will avoid a lot of the common problems folks have.

[Roger A]

I think that taking stock to final dimension just before cutting the joinery is best. However if this board is just going to be an unstabilized table top, then aclimation is really the only way to help RETARD movement.

Roger

[rwe2156]

I know this is more than you asked here's what works for me might help someone else:

1. Start with acclimated wood.
2. Plane equal amounts off both sides. I keep it under 1/8" off each side for each milling.
3. Re-sticker for 2-3 days. I use 3/4 - 1" " thick sticker boards. Store in a protected area. Cover the top board with another board. Do NOT use a fan or allow air movement over wood. If humidity at night is going to be bad, I seal the whole stack with plastic cover.

I would not store on edge you will not get equal air exposure.
4. I often clamp the stack together using cauls or a thick timber top and bottom, particularly if stock is under 1" thick. Remember to put sticker under bottom board!!

I usually keep my final milling 1/16 over finished dim to allow for hand planing and/or sanding.

Good idea to rejoint faces for each milling or at least check boards with winding sticks.

Hope this helps.

[Pink Floyd]

If the boards warp, it will most likely be across the grain instead of lengthwise. If you take off even amounts on both sides, that will help even out any moisture imbalance. I think your plan of taking off 1/8" and letting the boards settle before removing the final 1/8" is a good one. Also, as was already mentioned, make sure the boards have even air flow as they settle, either by standing on edge or stickering. Alternatively, you can place fresh-cut sides of two boards together which will slow down any moisture loss while settling and limit the tendency to cup.

FYI, normally the edges of boards tend to warp towards the bark. However, if the inside of the board is dryer than the outside (think kiln-dried) then they will warp towards the side where the stock is removed.