Avoiding Cupped Panels
Article - December 17, 2009
This article was inspired by a question from Chris. He writes:
I have a couple of questions on making panels. Is there a rule of thumb as to how many pieces you should make your panel from for tabletops and the like? Also, some woodworkers say it is not necessary to alternate the cup of the wood if the panel is not too wide. Is this true? Thanks Marc, I love your blog.
As far as I’m concerned, there is no rule of thumb for board width when making a panel. It just depends on your equipment, your boards, and the appearance you are going for. But make no mistake, cutting your boards into narrow strips will indeed increase the stability of the panel. It also does one other very important thing: it makes an ugly panel! Well, at least its ugly to me. Some people may very well like a table top that looks like a gym floor. I, however, do not.
So if the wood is stable and dry, I try to keep the boards as wide as my equipment will allow. We just don’t see much furniture made with really wide boards these days, and I think its a real shame to cut boards down any more than necessary. Of course, that does mean there is an increased chance of cupping. But this is usually a risk I am willing to take for the sake of aesthetics. Keep in mind there are always ways to reinforce the top and discourage cupping, such as breadboard ends and cleats.
Now before we go much further, we should cover a little background. Understanding why and how wood cups will help you plan your furniture with stability in mind. When looking at the end grain of a plain-sawn (flat-sawn) board, you’ll notice that the growth rings usually curve up or down. And lucky (or is it unlucky?) for us, you can pretty much predict which way the board will cup simply by looking at the orientation of the end-grain. As you can see in the drawing to the right, the wood typically cups away from what would be the center of the tree, or in the opposite direction of the growth rings. Perhaps in the future we’ll dig into this even deeper and discuss the differences between tangential and radial shrinkage, which is the ultimate cause for this phenomenon. But for now lets just say that the cupping occurs when one side of the board shrinks more than the other. And in a plain-sawn board, the shrinkage is predictable, and as a result, so is the cupping.
So when gluing up a panel, should you alternate the grain, one up/one down (pictured left)? In my opinion this isn’t necessary, but it is important to know the inherent risks and what may happen if you don’t. If the boards do decide to cup, the effect is greatly exaggerated when the boards all have their grain in the same orientation. So let’s think about what may happen if each board in a row of five cups by about 1/8″. The cumulative effect would be a very wide curve across the length of the panel and you’ll be on your way to making your very first barrel. Congratulations! You’re a cooper!
So what happens if we alternate the grain instead? Well, we aren’t going to stop the individual boards from cupping. But if one board cups up and the next board cups down, the overall effect on the panel is canceled out. The end result would be a much less noticeable issue. So while its not necessary to alternate the grain, doing so could very well lessen the effect of unexpected cupping.
I can sum it all up by saying I keep aesthetics as my #1. To me, wide boards are just aesthetically more pleasing. And I do alternate end grain orientation when possible. But my first concern is making sure the face grain of the boards looks great. I will never sacrifice the appearance of a panel’s face for the sake of alternating end-grain. Now if you are really concerned about your panels staying flat, I suggest you look into Quartersawn wood!
I would love to hear your opinions on these issues. Do you alternate grain? Do you trim your boards down for stability?