constructing hollow box out of plywood.

[mike eagen]

This seems like it should be a reasonably simple project, but I can't seem to get it right, I'm sure it's a combination of inexperience, lower quality material and not the best tools for the job available.

I'm trying to make a 20"x24"x30" hollow box (out of 3/4" ply), that sits flat regardless of what side is on the ground. I just cannot get it to be stable... there is some degree of rocking that I cannot seem to solve. I'm working with a circular saw and router.

Any tips would be greatly appreciated, I'm beating my gead on the wall over this. Thanks :confused:

[Todd Burch]

Are your plywood pieces flat and square?

Need a picture and a description of your current process.

[Joe Angrisani]

+1 to Todd's "flat and square?"

If you're trying to do this with home center plywood, you're really up there near Attempting The Impossible.

[mike eagen]

attached is what I'm going for....

247686

[Todd Burch]

How much rockin' you got?

[mike eagen]

+1 to Todd's "flat and square?"

If you're trying to do this with home center plywood, you're really up there near Attempting The Impossible.

in regards to the square and flat. Square... yes. Flat... no. Haven't had luck finding anything flat from the home center stores. I was assuming (incorrectly probably) that if the pieces were at least square, that they'd all be pulled together flat after assembling?

I have a couple of local lumber places that I was going to check out in the next day or so. What types of ply should I be looking for, and what might be a reasonable price for that (3/4")

[glenn bradley]

With the plywood currently available, you may want to run cleats along the inside across the wide spans to take some of the curve out. Even if your cuts are perfect, plywood just isn't flat very often with help.

[mike eagen]

With the plywood currently available, you may want to run cleats along the inside across the wide spans to take some of the curve out. Even if your cuts are perfect, plywood just isn't flat very often with help.

if you don't mind answering a stupid question... I think I've figured out what a cleat is based on searches, but what material should I use for the cleats and how should they be applied and fastened?

[Tom Blank]

You are testing it on a flat surface - right? Not being a smart-a**, just askin'.

Tom

[mike eagen]

You are testing it on a flat surface - right? Not being a smart-a**, just askin'.

Tom

Lol... that did occur to me, and yes.

[Rick Moyer]

Highjack alert! Sorry, no help Mike.

Glenn, I love your new sig line. I have that skit taped. Hilarious!

http://www.youtube.com/watch?v=Xe1a1wHxTyo

[Sam Murdoch]

As all have said or suggested plywood is inherently - not flat - and you learn to work with it deficiencies. Better grades of plywood and how they are stored have a big influence on the end result. You will likely not find anything in local lumber yards or box stores that will yield good results. Here is a discussion on another forum about plywood grades that might be helpful http://www.contractortalk.com/f13/book-case-cabinet-117447/index4/#post1492196

Cleats could help but you will run into the same problem if you can only buy local lumber and you don't have a jointer to make straight or pieces that curve counter to what the plywood is doing.

You write that you want to be able to build boxes "that sits flat regardless of what side is on the ground". Is that really critical or are you just trying to build a good flat and square box as your objective? I'm asking because if there are really designated bottoms you might just try adding some little feet in the four corners to establish 4 stable contact points. These could be nylon glides or felt tacks or even 3/4" plywood blocks about 1" +/- square.

[Joe Bradshaw]

If this box is a rocking, don't come a knocking.

[Paul Symchych]

I'd be inclined to get some 3/4" Baltic birch from a real lumber yard and not a big box store. Even that will not guarantee it being dead flat.
Once your 6 pieces are cut they can be checked for flatness. Several cleats of , say, 1x2" maple or some other hardwood can then be glued and screwed in on edge from the inside to flatten the panel.

What is this box for?

[Larry Whitlow]

Aw geez, I am going to go against the grain here. Unless your home improvement store is a lot worse than the one I use, you should be able to do this with their plywood (maybe buy 3/4 birch). I am going to guess that a more likely cause for what you are encountering is because the opposite sides are not identical in shape. You would encounter this problem if opposing corners of any two parallel sides are off by just a little I can see how this would easily happen if you are cutting separate rectangles with a circ saw. Maybe cut each set of sides at the same time (stack two pieces) so that any error is uniform. Mark the edges so you don't flip-flop them by mistake during assembly.

Good luck and have fun.

[johnny means]

One trick that helps is to make sure any curved sides are curving into the box. This creates corners that serve as "feet" rather than curved surface to rock on.

[Ole Anderson]

My bet is that with a circ saw, you simply can't get all sides so they are the same size and almost perfectly square. To verify this, just stack the 6 pieces together and see if they are the same size, then start flipping pieces to see if they sides are still parallel. If all the pieces are the same size (except for the differences needed to butt them together) it makes no difference if thy have some twist, they will all suck together when you do your assembly. Even some cupping will disappear when you do the assembly. Yes I would use cleats on the inside, a straight 2x2 works well. Bottom line, to construct a box you MUST be able to cut boards at 90 degrees, not 89.8 degrees.

Edit: Given a bit more thought, if the ply was twisted and the twist didn't compensate on opposite sides of the box, the whole box could have some twist.

[mike eagen]

I appreciate all the info, definitely a lot of ideas here that I can work with. Back to the drawing board.

[Erik Christensen]

any surface that HAS to be flat could be made with as a torsion box

[Lee Schierer]

I recently used Borg 3/4" plywood to build risers for my wife for the washer and dryer. Essentially they were just closed boxes like you are trying to make. The plywood was not perfectly flat, but the boxes turned out square and sat flat on my saw table with no rocking. The key is getting your pieces cut exactly perpendicular, opposite sides exactly the same length, which may be a challenge with just a skill saw and router. After cutting measure each piece diagonally to insure that each piece measures exactly the same corner to corner and is cut square. Even very small differences will add up and create problems. Use the factory edges as much as you can as they are straight and corners are perpendicular. Use a carpenter square as you are assembling the side pieces to insure each corner is square and again measure the side assembly before putting on the top or bottom to insure that it measures the same across the diagonals. This should insure that you get a box that will set flat on a flat surface, even with BORG plywood.

[mike eagen]

Thanks for the advice Lee. I will do as you suggest.


I recently used Borg 3/4" plywood to build risers for my wife for the washer and dryer. Essentially they were just closed boxes like you are trying to make. The plywood was not perfectly flat, but the boxes turned out square and sat flat on my saw table with no rocking. The key is getting your pieces cut exactly perpendicular, opposite sides exactly the same length, which may be a challenge with just a skill saw and router. After cutting measure each piece diagonally to insure that each piece measures exactly the same corner to corner and is cut square. Even very small differences will add up and create problems. Use the factory edges as much as you can as they are straight and corners are perpendicular. Use a carpenter square as you are assembling the side pieces to insure each corner is square and again measure the side assembly before putting on the top or bottom to insure that it measures the same across the diagonals. This should insure that you get a box that will set flat on a flat surface, even with BORG plywood.

[Sam Murdoch]

The difference between a riser for a washer/dryer and your 24" (or 20" tall) boxes is likely at least 10" which in the length of a piece of plywood is substantial as regards to straightness. Yes "After cutting measure each piece diagonally to insure that each piece measures exactly the same corner to corner and is cut square. Even very small differences will add up and create problems." but that is just a piece of the difficulty unless you are working with very nearly flat plywood.

If you are making many of these boxes a track saw would be a great investment. In every case you would start by ripping your ply down the middle and using the new rip of the two 1/2 sheets as your good edges so that you could avoid altogether needing to use a factory edge as your straight edge.

Mike, we still don't know how you intend to use these boxes and why they need to be able to sit flat in any orientation so our advice is limited. In any event, you know by now that the more accurate each piece is to being accurately cut and perfectly sized to its mate is a a critical aspect, but the nature of your basic building material and the limitations of your tools will create a real challenge for you. Kitchen cabinet boxes built out of plywood work as a result of careful and accurate parts cutting but also because with multiple boxes you create a system that gets fastened to the wall. All the inconsistencies get tweeked out or at least minimized. Very hard to do that with one-off boxes. Following many pieces of advice offered above in this thread will get you as close to that elusive perfection as possible.

[mike eagen]

the purpose is for workouts... plyometric box jumps 1 box for 3 heights. Maybe this gif will work? Obviously the more stable the better. Yes, I would be making multiple, I was planning on making some sort of "jig" for each size board needed.

247781



Mike, we still don't know how you intend to use these boxes and why they need to be able to sit flat in any orientation so our advice is limited.

[Keith Weber]

OK, I give up! What in the world is a "Hollow" Box? Aren't all boxes hollow? I've never seen a solid box. I've seen a block -- maybe you should build a block!

[Sam Murdoch]

Alrighty then, If I were building these I would start by making up multiple plywood ends very accurately cut with other than a router or circulr saw. Since those are your tools I would make one very carefully and use that as a router template for the others.

I would use these "ends" to sit approximately 2-1/2" inside from each end of the boxes. The inside ends would serve 2 purposes - 1) as a form that will help to keep your plywood sides flat and 2) as a stiffener to make the box last longer subject to such use. Now maybe you don't want the boxes too stiff - that a little bounce is a good thing. If that's the case keep the inside ends closer to the outsides - say 3/4" rather than 2-1/2".

Making jigs is always a good idea. If you don't intend to get a track saw you should do a search for "shooter" or "shooting" boards for circular saws. Essentially you attach a 3" x 1/2" straight edge to a piece of plywood that is at least 6" wider on both sides of the straight edge and longer than the longest rip on your finished panels you intend to make.

247819 With the shooter you should be able to do all your box side cuts with the circular saw. Use a good thin plywood blade. Keep in mind that if you change the blade you change the cut line on the shooter. Your first crosscut or rip on your plywood sheet will establish a good straight edge from which you can measure for your next cut. I ignore factory edges. I rip my sheet in half or cross cut through the middle or some other logical segment of the ply that allows for the best yield. I then work off the two new straight edges that my first cut has established.

There is no question that getting consistent results is a challenge but using a good straight edge, a sharp blade, and checking your work with corner to corner measuring will give you satisfactory results. Using an accurate square seems logical but you can't depend on that as much as diagonal measuring.

Happy hopping!

[mike eagen]

Sam... that's pretty much what my idea has been all along, I'm glad to see that I wasn't too far off base from a suggestion from someone more experienced. Thanks!