polyhole with hollow interior (hollow column)

polyhole_hollow.scad

/* this module makes use of the polyhole library function by nophead (MCAD library)
 * MCAD Library - Polyholes Copyright 2011 Nophead (of RepRap fame)
 * It is licensed under the terms of Creative Commons Attribution 3.0 Unported.
 * https://github.com/SolidCode/MCAD/blob/master/polyholes.scad
 */
function polyhole_hollow_eps() = 0.1; //epsilon environment for non-manifold differences
module polyhole_hollow(
        h=10,
        do=10,   //outer diameter
        di=8     //inner diameter
        )
{
    difference()
    {
        polyhole(h=h, d=do);
        translate([0, 0, -polyhole_hollow_eps() ]) polyhole(h=h+2*polyhole_hollow_eps(), d=di);
    }
}