A skate spinner is a device that is used by figure skaters for training spins off-ice. It consists of a flat platform with a curved surface (the rocker) below.
To use the spinner, put it on the floor, step on the platform and just spin while maintaining your balance. If you have never used a spinner, start slowly and remember that you can fall down if you are not careful.
The platform should be roughly the size of a foot. The rocker is around 1000 mm for typical skate spinners.
A skate spinner is too big for many amateur-level manufacturing machines, but it can be cut (use difference()) into small pieces that can be glued or welded together.
The model is fully parametric. You can look at the scad source and tinker with the parameters.
A note for choosing the dimensions and building:
Due to the limited stiffness of some materials, it is necessary to choose the dimensions of the spinner properly. The spinner will bend under weight and it might end squashed on the floor, the otherwise curved rocker completely plane. You can use the following equation borrowed from elementary beam theory to estimate your dimensions:
k < 1 / R,
R = rocker_radius (remember that it is defined in mm);
k = bending curvature = M / (E I);
E = Young's modulus (about 2,3 GPa for ABS);
M = bending moment = W l / 4;
W = weight (in the order of 1000 N);
l = back_length + middle_length + front_length (remember: mm);
I = area moment of inertia = (1 / 12) (b h^3 - (b - 2 t) (h - 2 t)^3);
b = width = minimum of back_width and front_width (remember: mm);
h = maximum section height = H + l^2 / (8 R);
H = platform_thickness (remember that it is defined in mm);
t = wall thickness of your build (it will depend on how you slice the model; with H = 15 mm, l = 260 mm, b = 700 mm and R = 900 mm and ABS material, t = 4 mm has proven to be a good choice).
After bending, the radius of curvature of the rocker grows (thus approaching the infinite radius of a planar surface) to roughly the following value:
1 / (1 / R - k),
so keep a good margin between k and 1 / R.
Apart from the stiffness criterion, you need to keep the material from breaking. With this simple model, the maximum stress will be found at the highest point of the middle section and it will be
M h / (2 I).
Keep this well below the tensile strength of the material (which is roughly 35 MPa in the case of ABS). Actually, there will be important stress concentrations at other points (where the section width starts changing, for example) which are not covered by this simple physical model. Also, if you build the pieces vertically (i.e. stacking layers along the lengthwise direction), then the actual strength will be the cohesive strength between layers, which can be quite lower than the tensile strength of the material, and there will be certain risk of sudden delaminations. With ABS, smoothing the surfaces by rubbing with a solvent like acetone helps aleviate this weakness and the material can be solvent-welded (with a solution of ABS and acetone or butanone) to give quite bonds that have a strength that is comparable to that of the bulk material.
If the spinner is too big for your machine, you can divide it in several pieces. ABS pieces can be solvent-welded together with a solution of ABS and acetone or ABS and butanone. Provided that you apply some pressure (press the pieces together with your hands) for several seconds and then let the bonding dry for 24 hours, you will get a quite strong bond. The excess material from the welding can be sanded easily.