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I often find myself wanting to build plywood boxes of various sizes for various uses. I realize that the size and use can dictate how to construct the box, but are there some general recommendations for how to construct a box from plywood?

I'm most interested in how to deal with the joints. What are some big no-nos and what are some good practices to keep in mind the next time I find myself with some plywood and a need for a box (think cabinet scale rather than something that would sit on a shelf).

Some of my approaches in the past:

  • Build a lightweight frame out of 2x2s or some variation, then attach plywood sheets to the frame.
  • Use brackets (usually with glue between the joined surfaces).
  • Just glue the joining surfaces and clamp.

For me the priority is rough rapid assembly and reasonable strength (not as concerned with aesthetics).

6

Rabbets / rebates (UK), and dadoes. A butt joint is fine for general cabinetry, but if you have the equipment and time, cutting rabbets for panels to fit in makes everything so much tighter.

I rabbet the sides for the toe-kick, bottom shelf, and top braces; then put a rabbet along the back edge for 1/4" plywood to fit into. That really locks the thing together.

Pocket-holed face frames attached with biscuits, and you're good to go. I've put engine transmissions on cabinets built this way with nary a wobble.

I tend to use glue as my main fastener, but I'll use pin nails and clamps to hold things together while the glue dries.

5

Building a 2x2 frame first certainly works, but it unnecessarily complicates building a plywood cabinet.

More typical cabinet construction uses butt joints with the pieces joined with one or more of the following strategies:

  • pocket screws
  • nails or screws through the face of one piece and into the end grain of the other piece
  • cleats on the inside corners, fastened with glue and screws/nails

In all cases, you should usually also glue the joints for improved strength; if using a nailgun the nails only serve to hold the carcass (cabinet box) together while the glue dries. You can hide your fasteners by strategically positioning them. In the case of nails or screws through the face of an outer end, you can simply design your cabinet with room for an extra 1/4" panel glued on the outside, or you can plug or veneer over the screws/nails.

It's good to include at least a partial back to help reinforce the box against racking. Face frames on the front also add reinforcement.

As TX Turner mentioned, you can also house your joints in dadoes or rabbets. This helps register the pieces during assembly and adds glue surface for strength.

3

Rob and TX Turner have good advice. For me, it's butt joints, biscuits, glue and screws (either pocket if they have to be hidden, or clearance drilled/countersunk if they don't have to be hidden).

The joy of biscuits (for me) is that they force alignment when you're assembling the carcase. If you make the biscuits asymmetrical, you won't accidentally flop/swap a panel. Add glue, a couple of clamps, screw it together, and you can pull your clamps right away and check (/adjust) for square before it sets up. Biscuits also prevent the creep you can get with pocket screws.

I like full thickness back panels, full width of the cabinet, partly because I like overkill, and partly because you can attach through any part of it. (Granted, this comes up once in a blue moon.) If a side ("gable end" where I grew up, but that term causes confusion where I am now...) has to be visible and I can't be moved to cutting a super precise back to sit inside, I'll go for a rabbet to give a little bit of wiggle room.

I think a lot of doing boxes effectively is getting a system that you're comfortable with, more than any one-true-path kind of rule.

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As noted, function determines method. Some general rules still apply:

  1. For joinery, rabbets/rebates should be minimum half the thickness of the sides.
  2. Staggered rabbet joints half the thickness of both pieces to be joined is typical.
  3. Grooves for bottom panels should be minimum the thickness of the bottom panel or even better the thickness of sides and bottoms.
  4. If you wish to make the bottom panel replaceable, allow 1/32" all around to slide into groove and allow for a back end to have access to the groove, secure bottom panel from sliding with a couple small finish nails.
  5. The more complex the joints (box, lock miters, drawer lock etc) - the more time and effort is required, but resulting in better strength, and durability as well as a sense of pride of accomplishment.

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