The mechanisms of bell mouth formation in gundrilling when the drill rotates and the workpiece is stationary. Part I: the first stage of drill entrance

Abstract

The reliability of the gundrilling process in the automotive industry is greatly affected by tool stability. This stability is defined as a complex issue consisting of the following sub-items: entrance, static, and dynamic stabilities. Entrance stability plays the chief role when the rotational speed and feed rate are high, which is the common case in the automotive industry where engine heads made of aluminum alloys are drilled. Entrance instability is the prime cause for the so-called ‘unpredicted’ drills’ failure, which often appears as a fatigue crack(s) on the drill shank and cannot be explained when only stable drilling is considered. This paper reveals the main design and technological factors affecting entrance stability and provides practical recommendations on the design of gundrills. This subject is covered in two parts. Part one deals with the first stage of drill entrance, which begins when the gundrill touches the face of the workpiece and ends when the gundrill’s cutting edges are fully engaged in cutting. The role of the clearance in the starting bush and the design of the gundrill’s supporting area are analyzed.