Wood Movement – Question of the Week
This week’s question comes from Martin. Let’s see what he has to say: I’m a relative newcomer to woodworking and have been watching your podcasts for a month or so now and am finding them very interesting. One major point I have picked up on is how much attention needs to be paid to the way in which wood moves over time as this never really occurred to me before. I am currently working on my 2nd ever project which is a jewelry box which will hopefully be made out of a nice piece of cherry.
I have attached a couple images of what I’ve designed so far.
The problem I am stuck on is with regards to the top of the box. I have designed the box to have the top made out of two bits of cherry butted up against each other which will then sit in a rabbit around the top of the side pieces. From what I have seen online recently it looks like this could result in the top expanding and “popping” the dovetails out, or cracking the top piece. I’m pretty much at a loss as to how I could get around this and was wondering if you had any pointers.
And here was my response:
Hey Martin. I too was very surprised when I first learned that we have to account for wood movement. I remember thinking at the time, “Oh great! Something else to worry about!” lol. The interesting thing is there are always exceptions to the rules and there are nearly always workarounds. Depending on the size of this box, you may never really have a problem. Obviously the smaller it is, the less it will move. And with small jewelry boxes, you can usually break a lot of the movement rules. Now if this is a reasonably good size, you certainly might have issues, but then again, you might not. How much of a gambler are you? If you want to get a little more scientific about it, check out The Shrinkulator.
Now the safe thing to do would be to redesign the top a little. And by re-designing, you might turn this dilemma into a design opportunity. The most obvious option would be to make the top more like a frame and panel. So the two cherry pieces would be set into a groove that goes all the way around the inside perimeter of the lid. The panel would essentially float, and thus be allowed to move. But of course, this changes the look.
Now if you absolutely do not want a frame and panel look, then your only other “safe” option is plywood. You can fully surround a piece of ply in that situation and have no adverse effects. And based on this design, the ply edges would be completely concealed and no one would have any idea that it was a piece of plywood. In fact, that would be the way I would do it if I wanted to adhere to that specific design. Good luck!
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Hey Marc.
Anicdotal evidence here, But I build a couple of small wood boxes for my nephew and neice, I they were made of solid oak with real simple joinery. ( I’m a carver, not a cabnetmaker) I put carvings all around the top and edges. The boxes are all still nice and tight and it has been almost 4 years! On a larger note, A bed I am currently working on is much larger, and made of pine ( I don’t know how much of a factor that is) While assembling the footboard, I found that my perfectly routered slot after only a week, had expanded and no longer fit. I’m trying to assemble as much fo the head board at a time instead of letting pieces sit around, Ah well live and learn.. I have the blunders on my Blog …http://themidnightcarver.blogspot.com/
Marc your options are all good and pertinent, but I have an alternative to your “only other safe option” It is not a different options it is a variation of your recomendation. If you are afraid of movement, instead of plywood you could veneer and laminate your original piece of wood. The look will be the same, but if it is properly pressed and glued back together; that will greatly reduce any movement. Now, of course I am aware that is a lot of work for not a lot of benefit in this particular case, but it is a valid option for bigger pieces that need extra strengh and durability.
A couple of years ago I attended a workshop with Grahm Blackburn at the Woodworking show covering a lot of topics from design to wood movement. One thing he told us is “wood does what it wants to do.” You can never be absolutely sure of where or how much it will move, but you can plan accordingly and minimize or eliminate it all together. Marc’s comments are right on target with what Grahm told us.
By the way – that Shrinulator site is pretty cool!
Mike
Martin -
You could use quarter-sawn boards for the top which would minimize the wood movement.
Marc,
The timing of this question could not be better. I have just started making a desk with multiple drawers – 3, side by side. I am thouroughly confused about how to attack the infrastructure which will support these dawers. Many designs have solid wood construction, but I am concerned about movement. It would occur to me that if all parts would be conceled, I might be better off using ply. Any thoughts here?
Thankls
Jeff
Hey Jeff. No doubt about it. If the parts are concealed, and movement is going to present issues, then ply is a good option.
But I would also suggest doing a little bit of research on traditional dressers and chest of drawer construction methods. Look into things like dust frames and you’ll find there are some interesting techniques employed that give the necessary support for the structure, while still allows the parts to expand and contract.
Had to chuckle at the design. I just finished one almost exactly the same as this. It’s posted on Lumber Jocks. I used quarter sawn walnut on the top and didn’t worry about the shrinkage. Here in the Pacific Northwest, the humidity is always 100% anyway so nothing ever really drys out.
I’m also a new woodworker with questions on wood movement. I understand why wood moves (straw-like structure swelling/shrinking as the moisture level changes). I also know that plywood and fiberboards (MDF, hardboard, etc.) minimize this with alternating or randomizing the grain direction.
Does anyone have more info on why this works? I’m confused as to why a tight frame causes cracking or broken joints, but the veneer doesn’t crack apart. Any comments or links? Thanks.
Hey CcntMnky. As I understand it, the thinner wood gets, the easier it is to resist its movement (without damaging anything). As the fibers expand and contract, they exert a substantial amount of pressure. But when the sheet is thin, that pressure diminishes, and the sheet can be coerced into doing what you want it to do. In the cases of plywood, we just want it to sit still, so we put several thin layers together, with alternating grain. This way, the sheets hols eachother in place. If the sheets had their grain all in the same direction, it would essentially be just one solid piece of wood, and expansion/contraction would be an issue again.
Not sure if that helps you. Good luck.