I drill a smaller hole on center, then open it up with
a larger drill, finally remove the part that is in
the chamfer with large 120° included angle
countersink taking care not to get into the arm:
http://www.pbase.com/dennygraham/image/121615915/large
It taps out easy then and there is no damage to the arm.
http://www.pbase.com/dennygraham/image/121613827

DG

Last edited by Denny Graham; 08/04/2014 10:09 PM.
Denny G
Sandwich, IL