openscad ring (with cutout)

ring.scad

// ring with cutout
// default values are for 1mm height, 10mm outside diameter and 5mm cutout
// de is epsilon environment to make cutouts non-manifold
module ring(
        h=1,
        od = 10,
        id = 5,
        de = 0.1
        ) 
{
    difference() {
        cylinder(h=h, r=od/2);
        translate([0, 0, -de])
            cylinder(h=h+2*de, r=id/2);
    }
}