At what point does wood movement become a problem?

[Victor Robinson]

I'm trying to better understand wood movement so I can make better designs and not overengineer when I don't need to.

My current question is about solid wood panels in small sizes (e.g. for box making, a current infatuation of mine). Let's say I am framing a solid wood panel in a mitered frame for a box top. If the panel is only 3" x 10" (or less), will it move enough to cause the mitered frame around it to fail? Will a spline key fix the potential problem, or is it always best to engineer expansion areas with the use of grooves, etc.?

I'm doing a number of small, shallow boxes, and trying to figure out if I can get away with gluing solid tops into mitered frames.

[Paul Atkins]

At what point does wood movement become a problem? The time when a 2x10 x 10' piece of cherry came down on my head-that was a problem of wood movement. Actually I expect the worst in most situations and always try to leave a bit for movement even at 3" panels. Just for luck. I've had little boxes come apart because I thought a small panel would not move and glued it in tight. Just glue a spot in in the center of each endgrain side and leave a bit of space at the others. My 2 cents.

[Victor Robinson]

Ha, thanks Paul. That was definitely a wood movement problem you experienced.

I guess it's best to traverse the safe route. I didn't want to overly complicate things for small boxes but being confident it won't come apart is worth the effort.

I'll just do a frame/panel construction with a reveal or something...

[Frank Drew]

Victor,

The only rule is that wood can be a b***h, so either plan for its contrariness or accept that sometimes you'll get lucky (that 3" panel will stay put) and sometimes you won't.

I will say that with time, lots of time, wood's coefficient of expansion seems to diminish, so that if you had a very old piece of 3"x10" it might well behave a lot better than one that was a year or two from the standing tree.

[David Thompson 27577]

Your box lid panel might not be a problem, but it depends on the grain orientation.

Wood expands across the grain, but not along the grain. If the 10" dimension is perpendicular to the grain, then you'll have a lot of expansion.

And if you really want to be perfectionist, there's an online calculator at the WoodBin site that will give you specifics. IIRC, its called the "Shrinkulator".

[Faust M. Ruggiero]

Wood is alive and so it will always move with change based on the humidity of it's environment. Wood moves more or less depending on lots of factors like, specie, direction of cut in relation to the annular rings and how dry it is when you build with it. Where the wood resides also has a lot to do with movement. Will it live in a climate controlled spot. With air conditioning, whole house humidification and dehumidification, the environment our projects live in is often stable so the wood absorbs and gives us moisture less drastically than it did in "the old days". Still, wood movement most be dealt with. Bruce Hoadley wrote a book called Understanding Wood. It is a timely reference on this very important subject.
Meanwhile, designing wooden pieces for movement is one of the most enjoyable aspects of what we do.
fmr

[Brian D Anderson]

This is sort of a timely post as I'm about to make a serving tray with a solid bottom. I've been wondering how much space I need to allow for wood movement.

I'm using a 14½" wide piece of curly maple for the bottom (seen below) which I've had in my shop for a couple years now.

I'll have a 1/4" dado in the tray sides, 1/4" from the bottom. Then I'll rabbet the tray such that the bottom of the tray is flush with the bottom of the sides. (See Sketchup)

I'm trying to figure out how much room I need to give the two long sides to allow for movement. The piece is for my friends mother and will probably live inside a kitchen it's whole life.

Is there an expert in the house? General rule of thumb?

-Brian

[Joe Chritz]

http://www.woodbin.com/calcs/shrinkulator.htm

The shrinkulator as mentioned.

You need essentially no room along the length and from just a tad to a bunch on the width.

I would hazard a guess that just a few thousandths would be enough to break a miter joint.

Joe

[Charlie Plesums]

Rails and stiles of a raised panel door are cross-grain to long grain joints. The typical cabinet door has 2-2 1/2 inch width rails/stiles, and the typical wood glue has enough creep to hold the joints together. So if you want a rule of thumb about how much you can get away with, this is one answer.

On the other hand, if you have a 20 inch wide door, with 2 1/2 inch stiles, your floating panel is close to 16 inches wide. If it were glued in so it didn't float, I would bet the expansion would destroy the door. Without going to a calculator, I assume 1/8 inch shrink/expansion per foot of width. There are many ways to keep a panel floating, but I use space balls, in a 1/2 inch deep groove in the rails and stiles.

[Joe Chritz]

I would assume that the stiles on a typical 5 piece door expand in the easy direction, that being out from the joint. It would be an issue if the stile was captured like the panel is.

At least that is how I understand it.

For an example of a possible swing. A 12" panel of sugar maple will change just a hair over 1/4" with a 6% moisture swing.

Generally speaking if you have a solid panel, allow for movement. When in doubt allow a little extra, especially when building in the dry times.

People in say, Arizona can get away with a little more than I could here in Michigan where the weather changes hourly and the seasonal humidity swings are substantial sometimes.

Joe

[David Giles]

Joe has it nailed. The question is more "where" will wood movement be a problem than "when". Humidity changes can create havoc. Dry winters to humid summers will cause wood to expand cross grain.

On the other hand, a climate with steady humidity lets one almost ignore wood expansion problem. The Texas Gulf Coast has a very steady (and high) humidity level. Before I knew about wood expansions, I mitered corners around a 16" wide piece of paduak. The corner joints haven't opened in 6 years because the humidity hasn't really changed in six years. I have built 30" wide RP cabinet doors that would be a disaster in the Midwest, but are perfectly stable in this environment.

San Francisco strikes me as a location with widely varying humidity. Online weather sites can provide specific details.