Abstract
Micro drilling operation is always exposed the risk of breakage of micro drills. Therefore, it is considered that micro drills should be rotated as accurately as possible. In the other hand, reasonable tolerance of rotating accuracy, however, should be defined for economical and engineering reason. In this paper, contact behavior of a run-out rotating drill tip with a work face is discussed according to measurement of high frequency contact force using a dynamometer. As a result, transient dynamic contact force reveals a run-out rotating drill tip is wandering just after making a contact with a work face. Wandering time depends on sliding characteristics of interface between a drill tip and a work face. For instance, centering behavior is better in drilling non-ferrous metal, where it takes 2-3 revolutions (5-7.5ms) until a drill tip stops wandering. However, it takes several times longer for hard-to-machine metals, where a drill tip diverges in the worst case . Additionally, the bending stress of a drill increases steeply with the increase of hole depth. In deep drilling, therefore, run-out of a drill must be depressed the least one for preventing drill breakage.
