Tribological Aspects in Aperture Shaping Process

The article presents tribological aspects in the process of aperture shaping using trepan drills. These tools, also known as trepan cutters, are used for performing port apertures. Due to their structure, only peripheral part (ring) of the aperture is influenced by the milling process, in contrast to milling the whole volume when using twist drills. Due to such method of performing aperture, the power and processing time are significantly reduced. The results of this process are chips and the core that can be used to produce other element, particularly if the material is expensive or if it can be used to examine properties and quality of the material. Trepan drills contain at least four cutting edges and their number increases with the diameter of the tool. This results in forming many chips that interact with the tool and working material during their movement along the chip grooves. During operation of these tools, the tool makes contact with the core and working material. Above effects cause the increase of milling torque and axial force with increasing penetration of the tool into working material and may affect the quality of the created aperture (roughness, microhardness, chemical composition, shape tolerance). In the extreme cases they may also cause the stoppage of device used to drilling or destruction of the milling tool. The article presents also the structure of research station that was used for examination and obtaining sample results of the milling torque and axial force occurred during drilling using trepan drills. Additionally, the values of milling torque and axial force calculated on the basis of theoretical equations are presented. Physical model of the friction process describes, besides effects that occur in the milling area, also other factors that influence the course of the drilling process. The result of drill penetrating the working material is the increase of milling forces, axial forces and temperature. The cause of this effect is the friction between internal and external tool surfaces and the working material, as well as the friction between formed chips and the tool and working material during their movement along the chip grooves. Physical model was the output element to create mathematical model that includes, besides milling force, also forces occurring as a result of interaction among the tool, working material and chips. Additionally, the model includes forces that occur when pressing guide pin to working material.