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Old 11-21-2009, 09:48 PM   #1
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Wood resizing with weather


I know that wood can change sizes with weather/humidity and what not. I'm just wondering, what direction does it normally expand or shrink at, and by how much? By direction I mean, does it resize in same direction as grain, opposite, or uniformaly? Also lets say a 10 degree difference, is this shrinkage/expansion significant or are we talking less then a cm here?

I am planning to build a server rack/shelving unit and while I can afford to have a few mm changes, I can't afford full cm or inches as the unit heats up from servers and what not. I just want to make sure this idea will work out and not flop out once it's all built. I'd use mostly 2x4, 2x6 and debating between plywood or MDF for the shelves/sides. No nails, all screws, probably even predrill each hole. It would sit in my basement, most likely on top of a vapor barrier of some sort, may even bolt it to the floor for safety. I never built anything of this magnitude before but once I have the money I want to buy tools and learn as I go.
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Old 11-21-2009, 10:11 PM   #2
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Wood always expands and contracts perpendicular to the grain. There is virtually no movement parallel to the grain. Movement depends on changes in moisture content, and has almost nothing to do with temperature. The actual movement varies with the species, for more information you can go to http://www.hardwoodinfo.com/articles/view/25.
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Old 11-22-2009, 06:21 AM   #3
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I would not use 2X4 since these contain too much moisture and will move too much. Use AC plywood sides and shelves. Keep the shelves 30" wide to avoid too much sagging. Keeping the humidity at a constant level will assure no movement in the joints. Glue and screw all joints and you will not even have a movement issue.
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Old 11-22-2009, 12:56 PM   #4
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Can I actually use just plywood? I would have figured it would still need some lumber for the framing, ex: never screw twice peices of plywood together, but always against some framing of some sort.

Also would PT lumber be better to use?
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Old 11-22-2009, 01:02 PM   #5
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I said to use plywood.. (cabinet grade ply is better).. And you will never see a cabinet with framing there is simply no need for it. look at your kitchen cabinets and try to find the plywood. PT lumber is the lowest quality you will find and shrinks and warps more than anything else you can use. Why PT anyway?
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Old 11-23-2009, 09:01 AM   #6
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Hmm k, I just did not figure strictly ply would be strong enough to handle a few hundred pounds if more. Should I use L brackets with nuts/bolts to put it together, or just screw them together directly with deck type screws? I'd probably use glue too, then at the end the whole thing would be painted. Any special coating you'd recommend?
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Old 11-23-2009, 09:07 AM   #7
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Bob is absolutely correct, virtually all modern cabinets are built with plywood only, perhaps some hardwood blocking in the corners for additional support. In fact, Europeran style cabinets are often built with veneered particle board, put together using a bolt and nut system.

I built my kitchen cabinets using 3/4 inch double veneered maple plywood, cost about $120 per sheet. For joints I use dados and glue mainly. Modern aliphatic glues are very good (NOT white glue, you need woodworkers glue). In a moist environment, you can use waterproof aliphatic glue. The key to strength is the dado, I cut mine half the thickness of the plywood (3/8 inch). My cabinets are all face frame, using 3/4 inch maple, but it is not there for strength, merely for appearance and I attach the doors and drawers to the face frame.

As for strength, you could probably put an elephant on top of my cabinets, plywood is incredibly strong in compression and bending, and the glue is just as strong as the plywood.
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Old 11-23-2009, 02:17 PM   #8
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go to sherwin williams and buy cabinet paint. Use an enamel underbody primer. This is sandable so the top coats are very smooth. Finish in a satin sheen. Anything glossy will show any defects. If using plywood you need to edgeband the edges (front only). Best to cut your pieces and take them to a cabinet shop to do this for you. You can buy iron on edging or even wood edging, but without the skills and tools it will be hard to get this part correct. You can support the shelves for what you are doing with 2 1/2" # 7 screws from the outside edges if this edge does not show. A dado would be better if you have a table saw or router to make them. If the outside sides will be exposed I would invest in a Kreg pocket screw jig to screw the shelfs to the sides show nothing shows. Or you can set the screws and plug them.
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