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Thinking about making a workbench (80 cm long, 60 cm wide, 85cm high) out of 21 mm plywood, CNC-cut.

Gluing 10cm by 10.5cm legs out of 5 layers and making the tabletop out of 2 layers.

Will this be any stable with changes in humidity? (the legs remaining straight and the top flat)

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    The textbooks tell us that ply is basically completely stable as far as changes in humidity are concerned, it won't warp, swell or change dimension to any major degree. Unfortunately it's not as simple as that in the real world because the type of plywood, and most important of all the quality of the plywood, can make a big difference between the theory and the reality. It should be fine, no problems at all, if you don't use a plywood that's bottom-of-the-barrel, but it's impossible to state with certainty that you won't have a problem because they do sometimes crop up. – Graphus Jun 2 '18 at 19:35
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    Sorry ran out of room, this is in relation to the top. The legs I'm sure you will have no trouble with at all given their thickness. – Graphus Jun 2 '18 at 19:37
  • I wonder if different "grades" of the same plywood have different stability, or it's just cosmetic. My CNC shop has a lower grade in stock, don't know if I should bother with shipping them higher-grade plywood or looking for a different shop. Or use an oak panel for the top, but then I have to accommodate for oak's expansion/contraction in the design somehow, right? – axk Jun 2 '18 at 21:11
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    Different grades of the same plywood can be pretty similar except comparing the lowest to the highest (lots more voids allowed which may affect stability). Obviously the higher the grade the better as a general rule. If you went with oak you would have to allow for seasonal movement, but this is not especially difficult (fittings are made for this purpose and they're cheap and widely available). But for a modern workbench I'd go with ply or MDF (or a lamination of both) every time myself, far more likely to stay flat over time, assuming it doesn't get wet (with water, not from humidity). – Graphus Jun 3 '18 at 12:02
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    I built a workbench from 3 sheets, 1 x 18mm ply, 1 x 3mm ply and 1 x 18mm MDF. It's a basic box frame with the 3mm on either side (like cheap doors), and the legs are 2 pieces of ply screwed at right angles and braced across with stretchers. The MDF is only used for the top, because it's smooth, flat and cheap. This was 7 years ago and it's been fine. However, it's in a semi-submerged garage with a pretty stable temperature. With the extra rigidity to you're adding, I think you'd be fine. – Red Aug 6 '18 at 15:47
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Yes, this will be stable with humidity.

Particle boards are much more susceptible to moisture damage. Plywood will be strong.

If you're worried you can cover it with an appropriate paint which will seal it and make it resistant to spills of oils and water (and other stuff that we get on our hard working workbenches).

  • Agree about ply versus particle board. Disagree about paint, better to use something more penetrating. My principal work top is birch ply coated with shellac, router table built in. I replace it every decade or so. – bpedit Jun 30 at 18:02

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