I've been using a stand up desk for several years. It's done wonders for my lower back, which was in rough shape after endless hours of sitting during grad school. Still, I find a few aches and pains from standing stock still (knees locked) all day. After a bit of digging online, I found several articles which suggested using a balance board (also known as a "wobble board") as a way to keep myself in motion whilst I break rocks at the data mine.
These boards are not expensive - I found several that would probably work in the $20-30 range. Still, why buy it when you can make it, eh?
This is an exceptionally simple project. Given the proper tools (I go to our local Tool Library, a very affordable sort of community maker space), it can easily be finished in a few hours or less.
Step 1: Materials
I had some 3/4" FSC-certified plywood leftover from another project, and picked up the softball for $5. That's all you need, though I chose to set the finished board on a small rug, and cover it with another.
Step 2: Cutting the Disc
I made the disc as large as the plywood scrap (24-in). The Tool Library already had the circle jig, but since it was built for a different router base, I essentially ended up building a new jig. There are several very good videos on how to do this on YouTube - it takes little time, and can be made from scrap wood.
To center the jig, I tapped a small nail where I wanted the circle center (measuring 12-in from side and bottom of the plywood), then cut off the head with pliers, leaving a sharp point for marking the underside of the jig. I then extended the straight router bit so that I had my outer point for the jig, centered the jig arm over the nail, and tapped a couple times with a rubber mallet.
After pulling the nail, I drilled holes through both the plywood and the jig arm, and attached the jig to the plywood with a bolt and loosely fitted wing nut. After making one shallow pass (to reduce splintering), I followed up with one 2/3rds the depth of the plywood, and then a final full-depth cut to cut the disc from the plywood blank.
Step 3: Cutting a Socket for the Pivot Ball
A regulation softball is roughly 4-inches in diameter at the seams. The largest hole saw we had available was 3-inches. After cutting the 3-inch hole (in two passes, one from either side to limit splintering), I used a 45-degree chamfer bit to "widen" the hole from the underside. I also used a round-over bit to put a curved edge (both top and bottom) on the disc itself.
Step 4: Fitting and Testing
It's really just that simple. The whole project took maybe two hours, and that included stopping at the store to buy the softball. I haven't used the board a great deal yet (though I'm standing on it as I type this 'structable), but if it ends up giving me leprosy or somesuch, I'll post a disclaimer here. So far, my ankles are getting much more of a workout than I'm used to, but my knees are flexed. That's got to be a good thing, right?