Fracture phenomena of microdrills in static and dynamic conditions

The mechanical drilling of microholes with diameters below 0.5 mm is executed using miniature drills, which are exposed to loads of relatively large cutting forces, frequently causing the failure of the fragile tools. This paper describes the experimental investigations into the deformation, strength, and fracture of the cutting parts of microdrills in static and dynamic conditions. Firstly, microdrill behaviour owing to bending, compression, and torsion was investigated separately in static tests. In addition, torsion was applied together with compression. The values of deflections, forces, and torque were measured and the formation of microcracks and their propagation observed. The tests indicated the characteristic forms of the fracture sections for the kinds of microdrills and loads investigated. In dynamic conditions, the start of the drilling plays an important part. The surface irregularities on the work part cause the drill tip to be pushed aside and changes of its axial position. A typical reason for tool destruction during drilling is excessive stress from cutting forces. The cases of microdrill failure in dynamic conditions collated with various drilling trials. The results show distinctions in the behaviour of microdrills in both circumstances as well as the similarities and differences in the forms of fracture sections.