This is a parametric printable ankle board / balance board, which only allows motion in one axis. It is used to rehab ankle injuries, improve balance and agility, core strength, improve athletic coordination, increase ankle motion, and general jedi training.
Ankle boards range from $20 to over $100 for the really good ones that allow you to customize the size. This design not only does everything that the commercial versions do but is better than all of those as it allows you to alter the size of the half-cylinder in OpenSCAD so as you get better you can increase the difficulty by printing out a new piece.
The OpenSCAD files is included so you can change the size and thus the difficulty of the board. The images show the default setting and STL.
1. Print the STL with a reasonable infill (e.g. this one was with 2 exterior layers and only 25% and it works fine - but the more support you give it the longer it should last) on your favorite open-source 3D printer.
2. Cut out a circle in plywood -- the easiest way to do this is to put a nail in the middle and then tie a string to your jigsaw the length equal to the radius of your wanted ankle board circle. Commercial versions of ankle or wobble boards are usually around 16 inches in diameter. This one was larger - if I were to make another one I would use thicker plywood.
3. Drill a hole in the middle of the circle
4. Bolt the printed hemisphere to the board with a 3/8 hex nut/bolt. The hemisphere has a hexnut hole in it meant to lock the nut in so you can tighten from the top and easily switch out hemispheres. This actually worked really well -- you can tighten the bolt down by turning the printed hemisphere.