30" square open-backed box - Woodworking Talk - Woodworkers Forum
 
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post #1 of 17 Old 08-01-2015, 01:13 PM Thread Starter
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30" square open-backed box

The experience I have with wood is minimal. I'm trying hard to start simple.

I want to make a modular, stackable shelving system of 30" cubes that are open on two sides (I'll call those front & back). I want to be able to stack them 3 cubes high, put 120 pounds on the top, and trust them free-standing (so that bottom cube has to hold). I'd really like them to have nothing structural showing on the inside nor outside.

My plan is to paint them and use 3/4" project plywood, and putty over screw holes, or whatever. Given the It's not so much a beauty challenge as it is a structural one, and given my inexperience, adding steel to the equation makes sense to me, whether by screw, spline, or maybe mortised bracket.

(I have a plan to keep them from sliding on each other, which will indicate tops & bottoms so I don't stack them sideways, but given my lack of vocabulary, it's hard to articulate. I'm not at all worried about that part.)

My accuracy is not practiced, and of course I'd like this to be easy, though I have figured out the benefit of moving slowly. Right now I have a drill, a miter saw, a 4" circular saw, and a Dremel router attachment, so I'm guessing a router purchase is in order. I was planing on having the boards cut to size before I brought them home.

My first inclination was butt joints with screws like this:

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| |
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but I'm not sure it would withstand the diagonal force (there's a new vocab word for me there, too, it's not exactly shear, more wight + shear, but you get the idea) of stacking them with asymmetrical weight & movement.

I could also run pocket screws through the joint from the outside. But I'm not sure only that much triangle would cut the muster.

Other than that it get's me into routing joints, in which case I have no idea what's easier than what, and what sort of inaccuracy each allows. I'm bound to have some.

Can I make a strong enough "invisible" joint that I could (not that I would) stand on & move around these? Suggestions? Don't even try it?

I'm also wondering if I can make a couple that are only 15" deep, but say, half as structurally sound.

Thanks for giving a newbie some reassurance.
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post #2 of 17 Old 08-01-2015, 02:18 PM
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no lateral strength that way...

You said;
but I'm not sure it would withstand the diagonal force (there's a new vocab word for me there, too, it's not exactly shear, more wight + shear, but you get the idea) of stacking them with asymmetrical weight & movement.

If you take a cardboard box, and fold both ends inside and push on it from the side you'll see what I mean. If you fold one end back in it becomes more resistant to side forces, fold both ends in and it won't deflect much at all.

Your plan requires that all the strength be in the joint itself...ain't gonna happen. A mating joint like a box or finger joint or a dovetail joint would be better, BUT a back of some sort would really stiffen it.

I understand that the design doesn't allow for a back. So... you could reinforce the corners with aluminum angle running down the inside corners. Yah, it would show, but not much. If the plan is to paint them it would show even less. Not much else you can do .. except bolt them together. Like a house off cards it will all collapse if one starts to go...

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)

Last edited by woodnthings; 08-01-2015 at 04:05 PM.
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post #3 of 17 Old 08-01-2015, 04:02 PM Thread Starter
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The house-of-cards is excellent, and the best analogy of the problem I'm asking about. Yes, this is all about joint strength.

I guess what I'm wondering is if I can hide triangles inside the 3/4" thickness of the wood (there are 2.2" of diagonal thickness at the inside of the joint, so about 1 3/4 to 2" of screw length without them showing) and use the right angles, or routed angles, available the right angles and thickness playing cards don't have. So the question ends up being: how small can I get with wood and steel to resist at 30" of leverage. What you're recommending is doing that, but bigger. That's what a back would do. I'm not an engineer, but I feel like a degree would come in handy for this.

You know, now that I'm thinking about it, I'm asking for someone with lots of experience watching things break to give me a qualified (versus quantified) answer so that I can get out of doing the geometry. I just don't know how strong the wood is (pine AB grade ply, they don't get any more specific, or maybe I need to consider spending more).

Sounds like 1. I'm pushing the envelope 2. you think I ought to give up the ghost... or use bolts. Maybe bolts are my answer.

Edit: I found these: http://www.homedepot.com/p/Everbilt-...5442/202034088, I think that's the idea behind your aluminum brackets, right?

Wait, I may have misread what you meant by aluminum angle. Yes, I could mortise set one in each corner, maybe 1/2" shy of the edge, or even use perforated steel strap angle. I'm just wondering if 1/2" screws would hold enough to make it worth it. I wouldn't mind sanding out putty over them.

I've just never worked with anything big nor with plywood.

Last edited by touiquette; 08-01-2015 at 05:49 PM.
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post #4 of 17 Old 08-01-2015, 04:10 PM
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I have a MFA, not a BSE ...

In design school we made mock ups and stress tested them to find out what we could remove or eliminate and have it not fail. When it failed we put that piece back on...

Ger yourself some 3/4 ply cut some six inch wide strips and glue and screw them together. Put a bathroom scale between you and the end of the 30" strip and push on it until it fails. Have somene video or watch the scale or if it has a "save" setting all the better. Try different braces or blocks or angles and see which one will resist the force best... That's all I got.

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)

Last edited by woodnthings; 08-01-2015 at 04:52 PM.
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post #5 of 17 Old 08-03-2015, 01:47 PM Thread Starter
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Thanks, folks. I really appreciate your taking the time. I think what I'll do is mull over the actual stress points, do a real life test, put off the paint, and have tools handy for making, um, adjustments.

In dealing with joints, it looks like the term I missed was "vector force."
I did see and can't recall one site that called it "rack force." It would be handy to have a term for the group of forces that make shelves fall over, vertical/gravity, sway, etc. Seems like it might be too commonplace for engineering physicists to address, and just too uselessly math-y for people who are more interested in actually building to analyze. I'll find it when I no longer need it, I'm sure.

This has been helpful: http://www.engineeringtoolbox.com/ve...ion-d_320.html
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post #6 of 17 Old 08-03-2015, 03:11 PM
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If you stack them and screw them together, the whole would be much stronger than the parts.

What's the deal with an open back? No wall?

How deep are the boxes? 30 x 30 x ?
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post #7 of 17 Old 08-03-2015, 05:08 PM
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I would use flat angle braces:


Either screwed to the back (mortised if you feel so inclined) or epoxied into the joint like a miter spline:


Pocket screws are infinitely better than screwing down into the edge of plywood.

Last edited by NickDIY; 08-03-2015 at 05:16 PM.
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post #8 of 17 Old 08-04-2015, 04:36 PM Thread Starter
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30x30x30. I'm going to try 30x30x24 to see if the joints hold adequately, and I'll probably keep that & actually paint it. I'm also going to try that depth in the space and see if that might be adequate. It's certainly more manageable.

A few reasons for open back: they're for storage, and 30" is just too deep to reach. This way thay can be accessed from both sides. I thought about doing 30x30x15 and somehow fastening them together, but my gut says no way will that depth work for open back with any weight on it. They're also near windows, and they are *so* much more flexible as far as storage goes, lined up open-ends together, there's five feet for sliding in long stuff. That, and they're just way more fun that way, even the building part.

I do plan to stack them fitting on 1" steel bearings with a routed out 1/2 hole, so my goal is a very tight fit, but a smooth sphere pretty much negates any support. They'll be sitting very square, though, and staying that way. I will leave room for the addition of actual pegs; thank you!

It seems like multiples of one piece of L-shaped metal spanning the joint is the way to go. I will at least find some "Flat Corner Braces" and probably countersink them, or just put an equally thick piece of veneer between them. If I can make a cut that will exactly fit, it doesn't make sense to leave them out. What strikes me in my reading is that people trust glue more than I would, but it's likely I haven't tried the right glue.

Thank you for the plywood tip. It makes sense that, at some angle, traversing the layers would be more stable. I was thinking the triangle would help, but hitting it straight through its, um, wave of brittle-to-pliable... yeah. The longer I live, the more I realize it makes sense to use things as they were designed to be used, or at least close.
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post #9 of 17 Old 08-04-2015, 04:42 PM
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Well, here's the deal. Plywood comes in 48 x 96 inch sheets. Build your boxes to maximize use of material and minimize the scrap.

Cut your pieces just shy of 16, 24, 32. Get the picture? The farther you get away from those sizes, the more waste you will have.

30 x 30 x 23 1/2 would be OK. Or 30 x 30 x 15 3/4 would also work.
Lay out your cuts on a sketch pad. Hopefully it will save you from buying an extra sheet for one or two pieces.

Good luck.
Mike
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post #10 of 17 Old 08-04-2015, 04:52 PM
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Quote:
What strikes me in my reading is that people trust glue more than I would, but it's likely I haven't tried the right glue.
In 1988, I built a workbench with three drawers. They were assembled with simple butt joints, Elmers glue and some finishing nails.

A couple of years ago, I dismantled the bench and built a new one. I had to beat the drawers apart with a 4# sledge hammer. The glue didn't give, the wood separated.

All those years, those drawers were loaded with all sorts of heavy stuff from nails to clamps and they never failed.

Yeah, glue is some mighty stuff. The fasteners are just temporary clamps to hold the pieces together until the glue dries.
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post #11 of 17 Old 08-04-2015, 06:59 PM
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Touiquett
120 lbs is not extremely heavy. Books are heavy, dishes are heavy.
But you want to be able to move the cubes.
Most likely, when you assemble a 90"x90" shelving unit, you will not be moving it on a regular basis.
As someone else has already posted, if you screw or bolt these units together, they will be much sturdier and stout against lateral movement.
They could still be unscrewed to move.
Can you give us a little more information on what will go in the cubes?
IMO If the individual units are screwed together, 3/4" plywood glued with a carpenters wood glue and nailed with 2" finish nails will be sufficient.
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post #12 of 17 Old 08-04-2015, 11:24 PM Thread Starter
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Bins of toys, games, probably some books. Who knows, they could end up with beer cases in them at college, though I hope not. I really want them to be multipurpose and reusable. I love modular stuff.

If they seem stable enough, I'd like to put sliders on the bottom, or maybe recessed (so as to have apron around the corners *very* close to the floor) casters. That's not until I trust the joints, though.

Right now they're for our playroom. They will likely next be filled with craft supplies, some of them hefty (leather can get surprisingly heavy). I might use one as a sewing table for my second sewing machine. Six of these down on the floor would practically let me sew a stage curtain, and when I'm done, I can stack them back up and get my room back.

My concern for now is kids climbing on them, at least the bottom one. I'm quite sure I don't have to worry about anyone climbing higher than that anymore, except perhaps a cat. Even so, 30" is a good height to avoid that with short, inexperienced folk; they know not to climb on the table, and that's the same height.

I think you're right though, glue is tougher than I realized, and I'm maybe having too much fun going overboard trying to devise "the perfect joint." After years of reading board books (the same ones repeatedly), some practical physics is just too tempting to resist.
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post #13 of 17 Old 08-04-2015, 11:35 PM
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Quote:
Originally Posted by touiquette View Post
Bins of toys, games, probably some books. Who knows, they could end up with beer cases in them at college, though I hope not. I really want them to be multipurpose and reusable. I love modular stuff.

If they seem stable enough, I'd like to put sliders on the bottom, or maybe recessed (so as to have apron around the corners *very* close to the floor) casters. That's not until I trust the joints, though.

Right now they're for our playroom. They will likely next be filled with craft supplies, some of them hefty (leather can get surprisingly heavy). I might use one as a sewing table for my second sewing machine. Six of these down on the floor would practically let me sew a stage curtain, and when I'm done, I can stack them back up and get my room back.

My concern for now is kids climbing on them, at least the bottom one. I'm quite sure I don't have to worry about anyone climbing higher than that anymore, except perhaps a cat. Even so, 30" is a good height to avoid that with short, inexperienced folk; they know not to climb on the table, and that's the same height.

I think you're right though, glue is tougher than I realized, and I'm maybe having too much fun going overboard trying to devise "the perfect joint." After years of reading board books (the same ones repeatedly), some practical physics is just too tempting to resist.
You realize the only 3 cubes you will be able lift down and work on will be the 3 top cubes which are 5' above the floor. The top cubes would have to be moved away before you could access the 2nd row cubes and so on.
If you plan to take the top cubes down to use as a work table or something, they would need to be kept empty for ease of use.
Something to think about.
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post #14 of 17 Old 08-05-2015, 12:01 AM
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modular storage systems

http://images.search.yahoo.com/yhs/s...hspart=mozilla

Virtually all of the images for a modular storage system use a closed ended box with a back. There is a reason. Open ended boxes just aren't strong enough. A 30" wide box will have a long lever arm to rotate the joint and cause it to fail. A dynamic load like a sideways or lateral shove will cause the heavy objects to have momentum and that will add more stress on the joints. Kids playing and so forth will be in danger if the system collapses.

What you can do is make some of them closed and some open and then screw them together. That will be the best of both worlds.

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #15 of 17 Old 08-05-2015, 12:36 AM
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I'm with Woodnthings. Don't get fancy. Built 'em strong. You won't regret it.

Here is an example from the search posted above.

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post #16 of 17 Old 08-05-2015, 01:20 AM
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If I were to build those cubes, I would go with 30x30x15 3/4 deep with a 3/4 inch thick back. Assemble the box with glue and screws (or nails). It should be good to go for a long time. Sturdy and easy to use. Easy to reach in and pull stuff out.

Six cubes built this way requires four sheets of plywood. Four times whatever the cost of plywood you choose, and a bottle of glue and some nails and you are ready to paint!

If you decide to go 23 3/4 inches deep, the materials will require 6 sheets of plywood with some waste.

Hope this helps.
Mike
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post #17 of 17 Old 08-05-2015, 12:06 PM Thread Starter
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Quote:
Originally Posted by Toolman50 View Post
You realize the only 3 cubes you will be able lift down and work on will be the 3 top cubes which are 5' above the floor. The top cubes would have to be moved away before you could access the 2nd row cubes and so on.
If you plan to take the top cubes down to use as a work table or something, they would need to be kept empty for ease of use.
Something to think about.
I'm not sure I actually thought about this. Thanks! I guess I was unconsciously thinking of having big, light stuff in (maybe 4 or 6) bins (or some other smaller containment) on the top, empty it, take it down, refill it. Then reverse the process to put it back. It's kind of a pain, but for infrequent use, and not it's primary use, it makes for a lot less furniture, and only one project. The trick, then, is to ensure the stuff stays contained. I suppose, given that, it probably wouldn't be that much more difficult to take out a few pocket screws to separate them.

I'm having a hard time understanding how just connecting a couple of these:

___
| |
___
___
| |
___

could make them stronger. A steel plate screwed to the edge or side over the separation, I get, but just connecting the top to the bottom isn't making sense to me somehow.

At 55 pounds per sheet of plywood, though, just 3600 square inches (30*30*4) is about 43 pounds of wood. I'm having seconds thoughts about adding pounds of steel hardware. The 24" option is looking better.

I'm off to the shop today for the 24" test box. I tend to over-shop, meaning I put in way too much research for inconsequential product. As much as it counters my nature, I'm sticking to big box, and what they actually have stocked. (Deep breath.) This could turn into a forever pursuit and shopping time-suck if I don't limit myself.

Last edited by touiquette; 08-05-2015 at 12:14 PM.
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