Drill bit for invalidating bank notes - has head of greater dia. than shaft with two parallel edges and two pairs of oblique edges

摘要

The drill bit is for the invalidation of bank notes. It has two parallel cutting edges (9, 10) and two oblique edges (12, 13) converging on the apex. The oblique edges (12, 13) extend between the apex and the inner limits of the parallel edges. Two further oblique edges (18, 19) extend between the apex and the trailing origins of the lips. Viewed in axial section the apex angle subtended by the edges (18, 19) is less than that subtended by the edges (12, 13).