Round Belt Generator

This script generates a round belt intended to connect two pulleys. I needed a replacement belt for my son's rock tumbler so printed one with semi-flex filament. One week later and its still going strong, so I thought I'd publish the OpenSCAD script with customizer inputs for good measure.

The code to generate a torus is pretty simple using rotate_extrude(). Just make a circle, offset it along the x-axis and apply rotation. For a torus with major and minor radius of 48 and 1.5, the code would be

```
rotate_extrude() translate([48,0,0]) circle(r=1.5);
```

More of a challenge was determining the length of the belt based on the pulley sizes and distance between them. If you have the original, just measure it and compute the radius with `L / 2pi`. If you don't have the original or if it's for a new design, there is some math involved. To make things a little easier I added this function to the script

```
function belt_length(R, r, d) = 2*d + PI*(R+r) + (R-r)*(R-r)/d;
```