The strength of drills with diameters of less than 1 mm may be insufficient for effective realization of microdrilling operations. As a consequence very small and fragile tools frequently suffer breakage. The precise assessment of their mechanical properties gives the opportunity to reduce the failure rate. This paper describes the results of a practical investigation into the deformations and the strength of the miniature drills in the case of bending, torsion, and complex load. The main shapes of the cutting part and both high-speed steel and tungsten carbide drills were examined. A basic part of the paper is dedicated to the twist drills, because the majority of microdrilling operations is done just with these tools.
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