naaimachine/knoopvoet/test.scad

radius = 5;
depth = 2;

diff_h = (pow(radius,2)/depth - depth)/2;
diff_radius = (pow(radius,2)/depth - depth)/2 + depth;

difference(){
translate([0,0,0])
cylinder(r=radius,h=depth);
translate([0,0,diff_radius])
sphere(r=diff_radius);
}