$fn = $preview ? 16 : 72;

module test(){
  difference(){
    cylinder(r=5, h = 4);
    cylinder(r=3, h = 4);
  }
}


module screw_hole(){
  difference(){
    cylinder(d=9.34, h=7);
    cylinder(d=3.9, h=9);
  }
}


difference(){
  hull(){
    translate([7,0,0])
      cylinder(d=9.34, h=7);
    translate([-7,0,0])
      cylinder(d=9.34, h=7);
  }
  translate([7,0,0])
    cylinder(d=3.9, h=9);
  translate([-7,0,0])
    cylinder(d=3.9, h=9);
  translate([0,0,-4.5])
  rotate([90,0,0])
  cylinder(r=7,center=true,h=20);
}