I built two batches of these for the gym. The overall design is the same in both batches but in the second version, I made some design changes to improve the ease of cutting and assembly. That's the design I am showing you here.
The finished boxes will be 18" x 24" x 30" and weigh about 55 to 60 lbs.
Obligatory Disclaimer
I am not an engineer, unless you count software. Use these plans at your own risk. I take no responsibility for anything you decide to do with them. I especially take no responsibility for any mistakes you make in building boxes, including deciding to use my design. Lastly, for anyone who ever uses a box that I built, you do that entirely at your own risk.
How Many Boxes Do You Want to Make?
This design is set up to produce 3 jumping boxes from 3 sheets of plywood. It's easy to scale up for more. If you want to make less, you can figure out the cutting layout yourself.
Materials for Three Boxes
| Count | Size | Material |
|---|---|---|
| 3 | 4' x 8' | 3/4" Birch Plywood |
| 360 | 1-1/4" | Coarse-Thread Drywall Screws |
| 2 | 12 oz. | Wood glue |
Selecting Plywood
Go with a hardwood plywood and look for one with minimal voids in the internal layers. Any voids will be a problem at the joints both because they will not hold screws well and because the edges will show on the joints and you will have to fill the voids somehow to get a clean and solid edge.I used birch plywood for one set of boxes and oak plywood for another set. I recommend you look for birch. The price, quality, and weight were all about equivalent. But the oak is more brittle to work with. It is much harder to make clean cross-cuts with the table saw and you will either have a lot of extra steps in the cutting or lots of chipping on the bottom side of the cuts.
Screws
I used coarse-thread drywall screws, 1.25" long. Using 3/4' plywood, this prevents any screws poking through the side when attaching internal braces and is long enough to hold well when countersunk on the edge joints.By the way, I might have overdone the number of screws but I hate to have things come apart. It's up to you if you want to use less or more.
Glue
A good quality regular wood glue is what you need. Check that the drying times match your speed but it is not going to be critical. I used Elmer's Carpenters Wood Glue and Elmer's Carpenters Wood Glue/Max. Any similar glue should be fine; I'm not shilling for a specific company.Cutting
Layout
Here is the cutting layout diagram. The smaller drawings on the left give the dimensions for each panel: side, top/bottom, and end. The larger drawings show how to lay them out on 3 sheets of plywood.The dashed lines in the cutting layout are for rough cuts in case you don't have the ability to transport full sheets, get them into your shop, or cut them easily. This was my situation. I had the friendly folks at my local building supply store make the rough cuts at 32" from each end on two sheets and at 36" from each end on the third sheet. This brought the size down to something that would go down my basement stairs and I could manage on my own on the table saw.
You can rearrange the cutting layout as you please but I recommend you try to observe keeping the grain in the same direction so that the finished boxes look good.
Cutting the Panels
I recommend doing the cutting assembly-line style. Cut out all of the panels first before you start assembling the boxes.Accuracy of the cuts will make all the difference in ease of assembly and quality of the final result. Make all of the cuts for each dimension in a single batch so that you do not have to go back and reset the table saw. If you are like me, you'll never get exactly the same size on the saw if you change the setting and change back.
One critical note: Cut the end panels 1/16" smaller than the dimensions on my drawings. So instead of 16-1/2" x 22-1/2" make them 16-7/16" x 22-7/16". Why? Because you want all of the edge joints to overlap by that 1/16". In a later step, we will trim it off and end up with a perfect corner joint. (I figured this out after I put one together; that's why it isn't on the original plan.)
Be sure to cut with the "bad" side down so that any chipping will be on the inside of the assembled box.
Use a high quality 60 tooth blade. Use a zero-clearance plate on the table saw if you have one. If you are getting chipping on the bottom side, you can reduce it by applying tape to the bottom side before cutting. I did that for the oak but did not need to for the birch.
Here's what the taping looks like. The tape goes face down on the saw and you cut straight down the middle of it.
And here's the pre-cut pile of panels for 9 boxes.
Cutting the Braces
You may have noticed that there is a fair bit of leftover plywood. Don't worry; we will be using a lot of it for internal bracing.You need the following braces. This is enough for 3 boxes.
| Count | Length | Pilot Holes |
|---|---|---|
| 18 | 16" | Drill angled pilot holes |
| 18 | 12" | Drill straight pilot holes |
| 21 | 12" | Drill angled pilot holes |
For most of the braces, drill pilot holes at a slight angle. You do this because you can more easily get the drill driver into the corner if the screw is on a slight angle. Also it helps push the brace piece tight into the corner as you are screwing it in.
For 18 of the 12" braces, you should drill the pilot holes straight. These will be used when installing the bottom panel. I found that angled pilot holes tend to make these braces shift when screwing them in. Straight holes work much better.
Preparing the End Panels
You may have noticed that the cutting layout produces 10 end panels for use in 3 boxes. Huh? Six of them are used as actual end panels, three of them are used as center braces, and one of them is a spare. Spare? Well you don't really need it but that's the way the cutting works out. Save them up and make a lovely end table later.Select two good-looking end panels per box you are building--use any not-so-good looking panels for the internal braces. We are going to add the handles.
Make a template for the handle by drawing a 2" x 4" rectangle and then use a juice can or such to add semi-circles to each end. Center the template on the end panel with the top of the template about 4-1/2" down from the top. Pencil the outline. Drill a hole inside the marked handle and cut it out with a jig saw.
(If you make your template as a hole in a larger sheet of cardboard, you could probably simplify the positioning step.)
To make the handles comfortable to hole--as in not cutting into your hands every time you pick the box up--we are going to round off the edges with a router. Here, I have a 1/4" round-off bit set flush to the base of the router. Run this around both the inside and outside of the handle hole.
Now's a good time to sand them smooth. This can be done later but it is easier before assembly.
Assembly
Drilling Jigs
As you start assembling the boxes, you will need to drill and countersink holes around three side of each top, bottom, and side panel. If you are just doing a few, you might be fine with eyeballing this--that's what I did on one batch. On the other hand, you can make jigs to ensure repeatable drilling positions.If you want to do this, you will make three jigs as seen here.
The longest one is used to mark the long side of all top, bottom, and side panels as seen below. The mid-length one marks the sides of the side panels and the short one marks the side of the end panels.
Make the jig 3/4" shorter than the side it will be used on. This allows you to align it to either end as needed so that the screws will go into the end panel and not the side panel. (See drawing below for a clearer idea.) The pilot holes should be inset about 7/16" or 1/2" from the side of the jig. Also, the pilot holes at each end of the jig should be at least 1" from the end of the jig so that screws from the top and side don't interfere when you sink them.
Drill pilot holes from one face of the jig and then put the screws in from the other side so the points poke out a little from the holes. By doing it this way, you keep the positioning accurate even if you don't drill exactly straight.
To use the jig, just align it to the edge and press down to make small holes. I found that a rubber mallet worked well to make sure it left a good impression.
And while we're talking about it, get yourself one of these combined pilot hole/countersink bits. It will save you a ton of time.
One more random thought: I drilled and countersunk the holes on batches of panels so that I had enough panels completely ready to be used in assembly. You don't have to do this but I found it more efficient to not be switching back and forth between various operations.
Step 1: Get the First Corner Together
Take a top panel and drill pilot holes on three sides then countersink them. Next, preset screws in all of the holes on the short sides. Lay one of the side panels down on your work surface. Stand an end panel up on one end of it and stand the top panel up against it. (Those of you who look closely will realize that I didn't use a side panel in the picture below but any reasonable sized piece of the plywood will server to provide the vertical spacing required.)
You will need to put a shim underneath the top edge of the end panel and adjust until the top corner (where the hand is) is properly aligned. You can just see the shim at the bottom in the picture. (The shim is needed because we trimmed 1/16" of the dimension of the end panel.)
Once you are satisfied with the fit, take out the end panel and apply glue all down its top edge. (You will find it useful to have a second set of hands helping with this first part of the assembly.)
Put it back in place and make sure the top alignment is still right. Check that it is at right angles to the top panel. Lastly, make sure the end panel is inset about 1/16" from the edge of the top panel. Carefully set your screws watching out that it doesn't shift on you. Wipe up the excess glue and, if it looks like the picture below, pat yourself on the back.
Step 2: Attach the Other End
OK, it gets easier from here because the parts will be self-standing. Follow the same procedure to attach an end panel to the other end of the top.Step 3: Attach a Side Panel
Now get a side panel, drill pilot holes on three sides and countersink, if you haven't already. Apply glue to the top edges of the panels you have already assembled. Carefully place the side panel on top, align, and screw it down.
Once again, make sure that all of the edges where the screws are will be 1/16" over the other panels. This side panel should be properly aligned at the bottom of the end panels but will overlap at the top. And the end panels will be inset 1/16" all along.
Remember, the purpose of the 1/16" overlap is so that we can use a router to trim it to a perfect corner later. So every place where a joint overlaps, it should be slightly proud of the panel it is joining to. If it undershoots, there is not much you can do to make the corner look right.
Step 4: A Little Bracing
Flip the assembly over so that you can work inside. Install braces to add support to each joint. Each joint is naturally supported in one direction, where it overlaps a piece of plywood. But the joint is weak in the other direction. To improve it, install a brace that you made from the leftovers. Glue it on both surfaces and glue it to the already supported side. If this is getting confusing, check out the diagram and picture below.
Step 5: Install Center Support
Take one of the "end" panels that you kept aside for use as center supports. Install it in the center of the box. You will not be setting screws into it from the outside. Instead, set braces on the inside of the box glued and screwed to the top and side panels. The install the center support, gluing every mating surface, and set one or two screws through it into the braces you just installed.A couple of tips: Make sure the center support fits properly and will not protrude past the existing panels. And get it as square as possible to ensure a good fit and maximum strength.
Step 6: The Other Side
Attach the second side following the same procedure used so far. This time you will be putting screws in from the top panel into the new side panel and from the new side panel into the end panels. Glue everything. Set at least a couple of the screws from the top panel first to draw the side tight before screwing it down to the end panels.Step 7: More Bracing
Once this is complete, flip it over onto its back and install braces in all remaining places where a joint is not yet supported.Step 8: Now for the Bottom
OK, this is where I may have gone overboard. If you assume that people will never set the box upside down and jump on it then you probably don't need to go so far with the bracing and screws. However, it has been my experience that people will do exactly what you think they never would and then will do some things you never even thought of.Start by adding braces around the edge of the opening. these will be used to attach the bottom panel and provide the extra bracing (and then some) needed for the unsupported joints.
Note that the braces on one side--at the bottom of the picture--are to be set level with all the other braces, not level with the edge of that side panel.
Once again, glue everything and then attach the panel and screw it down.
Now drill and countersink holes through the bottom panel into each of the braces and into the center support for good measure. Insert more screws. (Now you understand how we get to 120 screws per box!)
Finishing
Trimming the Edges Flush
Get out your trusty router and insert a 1" flush cut bit. Set the depth so that it will be just more than enough to cover the 3/4" plywood edge.
Work your way around all of the overlapped edge joints trimming them flush with the panels they are attached to. Careful on the corners. And, as always with a router, be very careful of your personal safety.
Rounding the Edges
Now swap out the flush cut bit for the 1/4" round-off bit. Set it to be flush with the router base or maybe just a little higher. It is OK to take less than the full 1/4" off; it is more important to not end up hitting the screws. That is bad for the screws, the router bit, and maybe for you.Work your way around all of the edges again using this bit. When you are done, every edge should be slightly rounded instead of sharply square.
You could skip this step, but consider what happens if someone misses a jump and comes down on the edge with their shins. Yee-ouch! Even with the rounded corners it will do damage; with sharp corners it will be really ugly.
Sanding
The last step is to sand it smooth. I use a random palm sander and 150 grit paper. Sand every side and roll over the edges to get them a little rounder and smoother.
Be sure to wear a mask because there will be dust everywhere.
Releasing Them into the Wild
That's it. I always wish I could apply some polyurethane to make it look better but I'm told it is better not to. The bare wood provides better friction for sticking the landings.
So now you take them to the gym. They look almost like beautiful pieces of furniture. And in a week they will be all dirty from people hauling them around and jumping on them. Sigh.
A Few Last Notes
Countersink Depth
For the pilot holes in all of the panels, you should countersink deep enough so that the screw head will be slightly below the surface when finished. A screw head above the finished surface will cause trouble in multiple ways.For pilot holes on the braces, countersink just enough to seat the heads. Don't go deep or you risk the screw going all the way through the panel.
Filling Voids
You will occasionally find a void in the edge of a panel that you cannot hide. With the plywood I used this was not frequent and was seldom deep. However, it must be filled to give a good finish and, if it occurs where a screw will go in, to provide strength to the edge.I found the best approach was to trim a shim to the right shape and thickness to make a good fit. I put glue in the hole and on the shim then insert it into the void. After the glue if well hardened, I trim off the protruding part of the shim with a chisel.
