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Are there good general techniques to deal with wood expansion when combining it with a steel frame? For the sake of argument, say we're building a metal and wood box where the metal comprises the frame of the box, but the wood comprises the panels (and assume that we're not using plywood or some sort of expansion resistant composite.) How do we attach the wood to the steel in such a way where expansion isn't a problem? With pure wood, we could cut a slot and float the panel, but I'm not sure what the extension of this to steel is. Adding a slot to welded square tubing seems difficult.

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    Let the wood move -- attach it with hardware that goes through slots rather than locking it in place. Or space it to allow room for maximum expansion; tables exist. Or orient it in such a way that all the expansion goes in a direction that doesn't cause trouble and doesn't fight the metal and fasteners. Or the ancient approach of using sort fasteners which can bend to let the wood move (nailed chests can sometimes survive wood motion that glued-and-screwed construction can't) – keshlam Aug 18 '15 at 3:03
  • I'm tempted to mark this as a duplicate of woodworking.stackexchange.com/questions/908/… since wood is relatively stable along the length of its grain, and won't behave much differently from steel along that dimension. As with wood, you can fix the wood at just one end and let the other float (preferably with some sort of slots or sliding dovetails); for example, see woodworking.stackexchange.com/a/910/49 – rob Aug 18 '15 at 3:46
  • @rob Although I agree that other question talks about wood movement, it's not clear to me how some of these techniques translate to a metal frame. Specifically, I'm not sure how to generalize a sliding dovetail or slot to welded square tubing. Now, that picture attaching the table top along the center was helpful and interesting. In the case of a metal framed box, this sort of applies, but then the frame wouldn't be along the edges of the box, but down the center of the sides. That's an interesting idea, but certainly would make an unusual looking box. – wyer33 Aug 18 '15 at 4:37
  • @rob Well, darn, never mind, I understand better now. I don't have to make a strange looking box, but instead could just weld the bracket to the center piece of the frame and attach that to the wood along the longitudinal axes. Good call. – wyer33 Aug 18 '15 at 4:40
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With pure wood, we could cut a slot and float the panel, but I'm not sure what the extension of this to steel is.

You're on track with the slot, but not quite in the way you're describing here.

First, you would drill a regular circular hole in the steel corners. Then, you make a slotted hole in the wooden panel. This slot would be perpendicular to the grain orientation of the wood, similar to the image below, but not that long.

Slot

In your case, you would only slot the hole enough to account for the expected expansion and contraction of the panel. This will vary by species and the total width of the panel. For "regular" box sizes, I would think that expanding the slot 1/8" to either side would be acceptable. For example, if you had a 1/4" diameter hole in the steel corner for the fastener, you would make a 1/4" dia. by 1/2" long slot in the panel.

Then you would use a bolt to fasten the panel to the corner and make it snug tight. I would suggest nylock nuts, using a jam nut, or thread locker on a regular bolt to keep the bolts from loosening over time.

Adding a slot to welded square tubing seems difficult.

Also, I'm not sure if you've decided on what to do for the frame, but I would suggest steel angles instead of tubing. It will be cheaper, lighter, and easier to connect to.

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For the sake of argument, say we're building a metal and wood box where the metal comprises the frame of the box, but the wood comprises the panels (and assume that we're not using plywood or some sort of expansion resistant composite.) How do we attach the wood to the steel in such a way where expansion isn't a problem?

Unless you can provide some means for the wood to move you can't, i.e. it will be a problem.

You can minimise the issue by using quarter-sawn wood, which expands and contracts minimally across its width (one of the reasons it was so favoured in the past) but with a completely rigid and immovable framework around the wood you're likely to still have problems.

If you build in wetter months when the wood is at its widest when you get to the dry season the panels will shrink and gaps will open up. If you build in drier months the wood won't be able to expand when you get to the wet season and you'll probably eventually get cracks as the wood seeks to relieve the stresses that build up.

Adding a slot to welded square tubing seems difficult.

I'd imagine so! There are still options if you can't make the board float in the frame, as with conventional wooden panel doors. But it involves a compromise in terms of looks as there will be a visible gap during some or all of the year.

What you do is make each panel from two pieces, and between them is a joint that allows for shrinkage and expansion. There are three basic options:

  • a tongue-and-groove joint
  • a sort of half-lap (shiplap)
  • a floating spline

Expansion joints

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