Clip-on countersink for straight countersinking 180° with 2 main cutting edges and 2 pre-cutting edges, for attaching to wood twist drills, suitable for softwood, European hardwood, veneered panels and plastics such as acrylic Plexiglas, Corian®, GRP, HPL coating, polycarbonate, PVC and Trespa®, for countersinking washer nuts with simultaneous drilling