How can I design a large speaker cabinet to come together square and true when my panels aren't flat and my boards aren't straight?

Question

This question differs from similar questions about wood warping because I'm not asking how to straighten a warped board nor how to fix an existing design that warped. Rather I'm asking how do you design at the outset for a large enclosure to come together square and true even if the boards and panels you have available are not straight or flat. I'm also wanting to know if there is an order of assembly that will maximize accuracy or minimize how much correcting force is needed. For instance, one strategy might be to lay the largest side flat and build up on it. An alternate strategy might be to start with a section of the innermost bracing that I can make square and true (using the best pieces I have) and build out from that seed.

This is for a large speaker enclosure approximately 2' wide, 3' deep, and 5' tall, but I think the question could apply to any piece of similar size and construction. The materials I have are 7/16" OSB panels and 2" x 3" douglas fir studs.

(A more usual choice for speaker enclosures would be 3/4" MDF for panels and bracing, but if I can build it with OSB it will save a lot on material cost. Moreover the size of the speakers is such that you can't get enough stiffness by just using thicker panels; either panel material will still need bracing every 8" or so. Rest assured I am still using MDF for the actual panel the speaker drivers bolt to, but due to the type of speaker design these MDF panels will be internal walls, not external. (These internal MDF panels form the "inner box" I was talking about that I can make most accurate in size/shape.))

I'm planning to rip some of the 2x3's to make 1.5"x1.25" battens. I know that if I take two panels that do not lay flat, and join them at 90 degrees with a batten, the joined edge will be straight assuming the panels were cut straight. I also know that a 2x3 that is warped over a long length is "less" warped if cut into short sections. So perhaps breaking the battens into sections is a good idea? However in any case, when building from the skin to the inside, I think there is still the possibility that panels fixed in 2 dimensions will have warping in the 3rd unfixed dimension? (E.g. they might bow in or balloon out even though the edges meet.)

The way I've most seen recommended online is build a skeleton or matrix and attach panels to it. In this method, keeping the 2x3's in long sections would ease construction and assembly, but I'd be worried about getting to the end and discovering an irreconcilable difference in shape between the skeleton and the skin. Not to mention differences in expansion, and resonance in the long boards.

I also thought about making a large number of "pre-fab" pieces to use to assemble the enclosure like Lego bricks. These would be C-shaped and rectangle-shaped biscuit joined studs with OSB triangles nailed and glued on top of the corners. I am not sure, however, whether these would take too long to construct, and also worry about building them to a designed size and finding out the actual size needed is different during assembly.

So, to recap: how do I make a straight object out of wood that is not straight? Should I start assembly with the longest edges / largest panels or smaller parts first? Is it better to break the design up into an assemblage of short pieces joined together, or to frame it with some of the straightest studs to make a basis for the shape?

Answer 1

In my view this is a question of "Do I control my materials or do they control me". Your concern that your warped pieces may not meet well is valid. Every project should begin with a plan for what you intend to achieve and then constructed accordingly. Your idea in the fourth paragraph is a good strategy. Begin by assembling the framework for your boxes and then cut and fit the skin to apply to it and meet the edges accordingly. If the sheeting is a bit warped it should be possible to flatten it when securing it firmly to the batten frame. If the sheathing is too warped then discard it and use sheets that will fit correctly. Trying to use materials that are not flat enough to maintain the planes of the exterior surfaces is asking for headaches and future problems.

Answer 2

I would not do it. You talk of making it into lots of these squares and re-gluing them together. Which could make sense but the pieces will stall have warp to them. So it will be bumpy and I can't explain but it can't be good. And what if they warp further yet? All that for nothing. But I will say, even though the warping could be done you could do that. But I would still plane them and square the board either way. There is no real way of getting around this as I am aware. The best and easiest would just to be to get some new panels.

Another thing my dad did say one time that he would get the MDF wet, (If I remember the material you are using is similar) and put a flat object on top and it would re-warp or shape to that flat surface. Never did it and can't say I recommend it. Also it is best to use MDF. It brings the best sound quality and blah blah blah.

Answer 3

For the benefit of anyone who references this in the future, here are my comments and suggestions given the materials on hand.

1. OSB is designed to be a durable “skin”. It should be fastened with nails or screws, at least 10 mm in from the edge. You might be surprised to find that high quality nails (try Tremont nails) will “hold” better than screws.
2. OSB must be attached to a frame. The frame is the structural element.
3. If you have Construction grade framing lumber (2x3 or 2x4 for example) that has been laid flat and yet shows cupping, flip it over. Leave it for several days, you may find it straightens itself as the underside is now drying.
4. You absolutely must have the top and bottom rail as straight and plumb as possible. You have a little more leeway to “force” studs into place.
5. The idea of “breaking up” the stud/frame pieces to make them straighter will not work. Again, the frame is the structural element, the panels are the skin.