difference(){

difference(){

//

translate([0,0, 0])
sphere(40,$fn=100); 

sphere(30,$fn=100); 
}

translate([0,0, 25])
cube([38,38,30],center = true); 
translate([0,0, -25])
cube([38,38,30],center = true); 
}
translate([0,0, -17])

sphere(20, $fn=100);