I cut and planed 3 perfectly straight pieces of 3/4" plain cut walnut. I wanted to glue them together to make a small panel.
I clamped & planed the edges in pairs to get a quality glue joint, I carefully glued and clamped them together using clamping cawls to match and correct any irregularities along the length as the glue dried and reduce any sanding needed.
The day after, I went to make some cuts and noticed that the panel had cupped.
Why did this happen?
Is there any way to correct it so I can cut a straight miter along the length of the cup.?
How do I stop this from happening again in future?
The most likely reason is that the wood was not at equilibrium moisture content with your shop environment. For normal KD wood of 6-8% MC, you want the relative humidity of your shop to be about 40% RH. If one of those number is off, the wood is going to either take moisture in or let it out until it reaches the MC that corresponds with the RH. Cupping generally happens when this moisture exchange happens more quickly on one side of the board(s) than the other.
As to what to do, the first thing I would do is make sure you are getting equal air flow on both sides of the panel. It MAY straighten out. I would also check the MC of the boards and compare that to the RH of your shop. About the only thing you can do to change it is to change the RH of the shop. However, if the wood is, say 12% MC I wouldn't recommend changing the RH of your shop to match that - you're just going to have to let the wood dry out. This could take a month or two. Likewise if the wood was over-dried (left sitting next to a wood stove all winter, for example), you will just have to wait for it to pick up moisture. Once it's at equilibrium MC all movement should be complete and you will be safe to continue.
Once the MC and RH are pretty compatible if the panel is still cupped I would rip the panel on the glue lines, rejoint the faces, re-plane, re-joint the edges and try again. Then again, I don't know if you have any extra thickness. We always do our glue-ups with hit-or-miss dressed stock so we can plane the panel.
One last point - ideally the grain direction* should be reversed every other board. This way, small seasonal changes will be less noticeable as one board will cancel out the other.
*Look at the end of the board. Unless it's quartersawn you will see a shallow bowl shape of the annual rings. One should be facing up, the next down, etc.
So for future reference, if I need to work with plain sliced walnut and I'm glueing boards together to make a panel.
Could you talk me through the stages please?
I don't have any wood storage so I use the wood I buy when I buy it.
I mill to over sized length and width.
Joint and plane to leave me with boards that are usually 1/8" thicker than final thickness.
Let them sit over night, stickered.
Glue them together in the morning.
Thickness them to final thickness.
Sand and dimension accordingly.
That's my solution. Problems start when they cup or warp after I'm done.
"Letting wood sit overnight isn't long enough for it to acclimate."
Buy a moisture meter, RH meter, and understand the movement of wood. A great read is a publication by the US Forest Service entitled "Storage of Lumber - Agriculture Handbook No. 531". See particularly pages 12-14.
Also by the USDA is "Wood Handbook - Wood as an Engineering Material". This appears to be a greatly revised and expanded version of the above (509 pages versus 68). See Chapters 4 and 13.
Per page 3:
A limited number of free copies of this publication are available to the
public from the Forest Products Laboratory, One Gifford Pinchot Drive,
Madison, WI 53726–2398. This publication is also available online at
Also, try to remove the same amount of material from both faces. Wood MC changes faster on the exposed faces, and is pretty much always changing. Say the faces are slightly wetter than the core. If you remove more on one face, the drier wood that is exposed takes up less volume and will pull into a cup.
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