Thursday, June 13, 2013

Resawing Lumber Resulting in Cupped Boards

[UPDATE: I found more information on this topic and updated this post.]

I've been reading a lot about resawing lumber on the bandsaw and how to deal with the boards cupping.  I had a huge cupping problem the other day and while I'm waiting for new wood to arrive I decided to learn as much as possible on the topic so I don't repeat the mistake.

I've found two main reasons for why cupping occurs after resawing down the middle of a board:

  • it releases internal stresses that were created by improper drying at the lumber mill.  
  • there is a moisture gradient (uneven moisture content) within the thickness of the board.  Resawing reveals a very moist surface and as it dries, the wood cells contract resulting in cupping.

The best way to avoid this is to wait until the board has a uniform moisture content throughout the board.  Basically, you should monitor your board's moisture content with a moisture meter from the moment it enters your shop.  Check it daily and wait for it to level off.  Once this has happened, the moisture content of the wood has reached equilibrium with your shop and it should be safe to proceed to the next step.

Perform the Prong Test to check for internal stresses caused by the drying process.  If your board passes this test, but the board still cupped, it likely happened because there was a moisture gradient.  

They say wider boards cup more often than narrower boards and if you think about it, it makes sense.  You could expose quite a bit more moisture in an 8 1/2" wide board than in a 2" wide board. Because of this, it's always best to rip to near-final width before resawing.

Quarter sawn wood is supposed to cup less than flat sawn after a resaw, but mine still cupped almost a 1/4" or more.  I did not know to do a prong test or monitor the boards moisture content.  

The board is cupped.  Can it be saved?

If the board has already been cut and has cupped, it might not be too late. If you left enough extra thickness you can plane out the high spots to get back to a uniformly flat board.  

If that isn't an option, then I found this video.  He takes a heat gun to evaporate moisture and contract the wood fibers on the crown side of the cupped board - a sort of tug-of-war plays out where one side of the board tries to pull the other side back into flat.  Comments suggest that spraying water on the cupped/concave side would help those wood fibers relax and move things along a little better.

I'm wondering if by heating the wood so quickly, he's introducing the same improper drying problems that Richard Jones described, except on a small scale. Maybe on a small scale it doesn't make much difference?

So here's my summary of best practices for resawing lumber and dealing with cupping:

Most of this comes directly from John Fry's response on Lumberjocks.
  1. Monitor your board's moisture content from the moment it comes into your shop and continue to do so on a regular basis until it levels off.
  2. Cut off the end of your board and do a prong test.  If it passes, then there are no internal stresses and you can continue.  If it fails, find a different board or add extra thickness to the cut so you can plane out the cupping that will occur.
  3. Setup your bandsaw correctly with a sharp resaw blade set to the correct tension. Set your blade guides super close. Make sure the table is square to the blade both directions. Figure out your drift angle if you have one and set your fence accordingly. There are countless books, videos and articles all over the Internet that describe what all of this means.  
  4. Joint at least 1 face and 1 edge so you can register the board against the fence.
  5. If you can, rip to narrower stock before resawing.  Narrower boards are less likely to cup.
  6. As soon as the board comes off the saw, lay it flat on a table and put weight on it to keep it flat.
  7. Finish resawing the rest of the board. Don't take a break and come back later or the original board will likely cup and you'll have to re-joint it to get it to rest squarely against the fence again.  Once you start resawing, keep going until you're done.  Keep stacking your newly sawn boards flat on a table under weight.
  8. When you're done sawing, sticker your resawn boards and clamp them just enough to keep them flat, but not so much that you prevent wood movement.
    Stickering wood to air dry.
  9. Leave them like this for a week or two and they'll acclimate to the relative humidity of your shop. The moisture will evaporate from the surfaces of the boards and you'll end up with a moist core and dry outer faces again.
  10. Take them out of the clamps right before you're going to turn them into furniture parts.  Mill them flat and smooth.  Keep the rest clamped and stickered until you're ready to use them.  
  11. If a board is cupped, spray some water on the concave side and heat the middle 2/3s of the crown side of the board until the wood starts to move.  Stay away from the edges - they're already dry.  Don't burn the wood.  
  12. As soon as you see the board start to move, STOP! If you keep heating, you'll cup the board in the opposite direction. The goal is to have an equal amount of moisture on each surface of the board. You can repeat this process until you're satisfied.  Watch the video.

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