$wallThickness=2;
$L1=19;
$L2=6;
$L3=5;

difference()
{
translate([0,10,0]) import("drive_cup.stl");
    difference()
    {
    translate([0,0,-1]) cylinder(50,r=20);
      union()
        {
        cylinder($L1+0.01,r=4+$wallThickness,$fn=50);
        translate([0,0,$L1]) cylinder($L2+0.01,r1=4+$wallThickness,d2=21,$fn=50);
        translate([0,0,$L1+$L2]) cylinder($L3,d=21,$fn=50);
        }    
    }
translate([0,0,20]) cylinder(10,d=7.15,$fn=50); //make bigger hole for the dart to fit into
}