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I'm refinishing some old speaker cabinets and noticed they were out of square. They're reasonably sturdy, but if I push the top sideways while holding the bottom (shear force), I can bend them square. In case it's relevant, they're just four boards nailed or screwed together... I can't tell for sure because the fasteners have been covered with wood filler.

I had my wife hold one of the cabinets square by pushing the top sideways while I installed angle brackets on two of the four corners. But when she let go, it immediately went back to not being square. In other words, the two angle brackets weren't strong enough to hold the cabinet in the desired position.

I'm not sure how to get these cabinets square without disassembling them... try more brackets? I have plenty more and could put two in each corner. But I don't want to put a bunch of holes in the cabs if it's not likely to work. Or use beefier brackets? I'm not sure where to find something like that. Or maybe some other way?

By the way, if I have to disassemble them, I think I'd just leave them out-of-square and tell people they were designed that way to add to the tone.

Out of square enter image description here

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    I think you're on the right track with the brackets, and more of them (in all corners) might have worked. But this type of are more for holding corners together than for holding square. For the latter you want the L-shaped ones that attach at the back. Not sure if the material is thick enough for the usual size of these, but if it is they're a worthwhile alternative to some of the suggestions provided in the Answer. – Graphus May 30 '19 at 10:08
  • Soon to be dead refs: amazon.ca/Bracket-Xinrub-Aluminum-Fastener-20x20mm/dp/… amazon.ca/dp/B074TSDPMD amazon.ca/dp/B01H4VY6VG The idea is to stiffen the corners using materials that don't want to deform in the racking direction. What you have there is ok, but the forces are actually being applied to the weakest axis of the brackets. You want these materials to be used where their compression or tensile strength is the greatest, not their deflective strengths. This is how cross-bracing works, as well. – jdv May 30 '19 at 13:56
  • And if you don't want to use metal, wooden corners or horizontal (or diagonal) slats applied to the back of the corners, top and bottom, will probably be just fine. See how Ikea, et al, keep their furniture from racking. – jdv May 30 '19 at 13:58
  • Finally, the idea here is for the box to be as square as it can be, given that the four sides are probably not identical in length. Relative square can be easily found by measuring the distance corner to corner in both diagonals. Square is when those two numbers are the same or close enough. – jdv May 30 '19 at 14:00
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    Thanks for the helpful comments! – Anthony May 30 '19 at 17:00
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You're on the right track with respect to beefier brackets. Consider the square you've used to check the distortion. It is stronger in the flat plane than the tiny brackets you have. An exaggerated but appropriate bracket would be similar to a supported shelf bracket such as this Home Depot model:

shelf bracket

Note that it is not a flat piece of steel bent at 90°, but has a reinforcing bulge that provides the necessary strength to keep things square. As noted, this is an exaggerated solution to serve as an assistant to head you in the right direction.

Another option would be a tension type of attachment. Consider that one of your diagonal dimensions is greater than the other. Attach a tab to those corners, stretch strong wire between the two and apply tension while forcing the cabinet to square. It would be advisable to create an opposite diagonal as well, to prevent the cabinet from going beyond the previous location.

From the DIY SE comes this image, not specifically of a cable brace, but representative of the concept:

diagonal brace

The hand-drawn arrow would be the cable to be tensioned.

Alternatively, replace the cable with a solid brace cut to fit the corners. This would be used to force apart the diagonal of the smaller measurement until the cabinet is square.

Gussets are another method of squaring a frame. A triangle of thinner wood fastened to the frame at the back or front (if you are using speaker grille cloth) will force apart or pull together the adjacent pieces to a square position. Extending that concept even more would be to use a carefully cut square piece of wood that encloses all four sides.

Some of these suggestions will not work if there is a speaker in the way or another obstruction. They are provided to present options from which you can progress.

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    Thanks. I ended up combining your wire idea with @Graphus's comment about using flat corner brackets. I installed two hooks on opposite corners, pulled a wire tightly between them, and tied it. This allowed me to keep the cab square while I worked. Then I installed small but flat corner brackets into the back. It's an open back cab so no worries there. When I removed the wire, the cab remained square. – Anthony May 30 '19 at 16:59

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