$fn = 100;

difference () {
	cube([40, 40, 40], center = true);
	cylinder(r = 20/2, h = 40 + 1, center = true);
	rotate([90, 0, 0]) cylinder(r = 20/2, h = 40 + 1, center = true);
	rotate([0, 90, 0]) cylinder(r = 20/2, h = 40 + 1, center = true);
}