BD=3.175; // 12.7mm bushing, 6.35mm mill

difference() {
  square([260, 260], center=true);
  translate([-55, -55]) offset(r=BD) circle(d=60, $fn=100);
  translate([55, 55]) offset(r=BD) circle(d=70, $fn=100);
  translate([-50, 50]) offset(r=BD) circle(d=80, $fn=100);
  translate([50, -50]) offset(r=BD) circle(d=90, $fn=100);
}