Thread: CrossFit
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Old 03-05-2013, 09:53 AM   #3840
Scooby Specialist
Member#: 25227
Join Date: Sep 2002
Chapter/Region: Tri-State
Location: Oh s**t I ain't from Brooklyn!
2012 Escape


Bumping because I've become obsessed with plyo box construction. I've officially gotten my boys (11 y/o travel baseball team) into some plyo box work, and I think they're loving it, and I'm loving them working at it. Ultimately, I'd like to have each kid want to have a couple boxes of their own, paint them up, etc..., so I went about re-making some scalable, repeatable plans. Here goes:

First, the basic construction: I used 3/4" sanded plywood, $36/sheet. Construct four identical trapezoidal panels, with parallel bevels top and bottom (i.e., with a parallelogram profile). In two of the panels, cut out some type of handle. I lay out two points, 4" apart, centered in the panel and equidistant from the top edge. Use a hole saw through each point, then connect those two holes with a jigsaw, and run a router (or sand) all around, front and back, to give a nice smooth hand-hold.

Stand the four panels up to form a square, overlapping alternating corners, making the top edge alignment an absolute priority (imperfections on the bottom edge can be sanded out later). Pre-drill holes with a counter-sink, glue and screw and you get a truncated pyramid. Stick a square panel on top (I just glue and nail), with a bevel of the same angle on all side makes a smooth look. A second piece just like the top is mounted in the interior underside, glued and nailed, providing a little more structure to the box but primarily aiding in stacking. The ones I've made, I've left with no top mat of any kind. Even raw wood seems plenty grippy with sneakers on. I also tried this "grip surface" spray paint on a painted box. Point is, these plans do not consider any additional top surface that would add measurable height.

Second, the measurements, three inputs:

1) ANGLE - I've made 8 and 10 degrees, I think 8 is nice, 10 is too much. All else equal, the bigger the angle, the heavier your box will be.

2) Finished height

3) Area of the top platform - My first 12" boxes used an 18" platform and were leviathins. Last pass used 14" and that seems nice. This should get a little bigger as you go.

Geometry and Excel FTW! See this spreadsheet. It has the 3/4" material thickness as an input, and the three mentioned above. The spreadsheet then gives you the very specific dimensions of the panels, and the necessary raw material size to cut them out. How you do that is up to you, but a quick version of how I did it:

1) Have Home Depot rough cut your 4x8' sheet to fit in your car! They are not precision-cut, but generally, I assume their panel saw will make a parallel cut close enough to true for this project.
2) I used a Festool MFT table that has a tiltable fence to make my first 8-deg cut on one end of a trapezoid. I then measured the bottom edge to the proper dimension, and cut the other side of the trapezoid, giving me one trapezoid. Note: this trapezoid is NOT the proper height, but it's got the proper size bottom edge and two 8-degree angles, which is all I need for now.
3) When I make that cut, note, I already have one side of the next trapezoid cut, and I use the first trapezoid as a template, trace the other side, then cut that.
4) No measuring needed now, just keep using a template.
5) I use a table saw, blade set at 8 degrees, to make the bevels top and bottom, and set the fence to yield the final finished height. Take note of which is your finished side when cutting these, it matters, and make sure you cut parallel bevels.
6) For the two square panels, I use my MFT table to rough cut squares a little bigger than needed, and take them back to the table saw. Again, note your finished side and angle all the bevels IN toward that side. Set your fence, keep the same side up the whole time, and just run all four sides through. You'll end up with a perfect square and all bevels facing the same way.

You have the six panels you need now, so just assemble them carefully. The spreadsheet does some 'efficient use of materials' calculations, but worth noting, I got a complete 12" and 18" box out of one 4x8' sheet.

Some pics:

The simple-looking trapezoid:

An attempt to show the parallel bevels top and bottom:

The top panel, which has bevels all facing inward:

This shows the alternating overlap and the interior support panel:

Another shot of how the panels are all overlapped, each side looks exactly like this (except the handle only being on two sides):

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