M.Ing. / In the drilling process, the majority of hole inaccuracies can be corrected with the use of secondary hole operations. Hole location errors cannot be corrected as their magnitude is determined at the beginning of penetration of the drill point into the material. This thesis investigates the influence of the drill bending stiffness on the hole location error when drilling with ordinary twist drills. Infrared sensors were developed for the drill point location measurements, the initial wandering of the drill and the final hole location error. Also investigations were made into the on-line monitoring of the drilling process using acoustic emission in the audio range. The acoustic signal was transformed to the frequency domain and related to the torque and the thrust magnitudes. The relationships in the acoustic emission, torque and thrust and chip formation between drilling low carbon steel and aluminium were determined.