Microdrilling mechanics and performance

The standard for quality of drilled microholes is quite different from conventional size holes. This is due to the additional problems and limitations brought about by downsizing the drill. The microdrilling process will be affected by the relatively large straight chisel edge, overall geometry, cutting and operating conditions and dynamic characteristics of the microdrill. Therefore, to improve the stability of the microdrilling process, and hence the hole quality, a clear understanding of the mechanics of the microdrilling is required. With this objective in mind, this research thesis emphasizes three areas of investigation which affect drill performance and tries to relate them to hole quality.; The effect of point and body geometry on the hole quality was reviewed, followed by investigating the influence of operating conditions, microdrill wear and drilling forces. Different characteristics of drill bits were investigated analytically and experimentally. In the analytical investigations, drills were modeled as long, slender, twisted Euler-type beams under axial load with a simplified cross-sectional geometry symmetrical about the drill's principal axis. From this model the dynamic characteristics of the microdrill were obtained numerically by the shooting method.; The low rigidity of the microdrills when used for drilling through inhomogeneous materials often promotes deformation. As a result, the axial load in microdrilling frequently reaches the critical buckling load, and the deformed microdrill margin contacts the hole wall. The deformed drill margin at the point of contact tends to remove material at the interface or rub against the hole wall. The resulting cutting forces were modeled using a semi-empirical chip-load relationship, and experimental analyses were carried out for verification of the cutting model. This analytical model relied upon two crucial parameters, namely chip thickness and cutting stiffness, which had to be measured experimentally. The first parameter was measured by a specially designed and well planned drilling experiment, and the latter one by means of a oblique turning operation.; Hole oversize and out-of-roundness as a result of asymmetry of the drill point geometry, drill buckling and bending and drill point runout (wandering motion) and skidding were investigated. The feasibility of an existing kinematic model for initial penetration and hole profile prediction was verified. Experimental and theoretical interpretations of drill point asymmetry on hole oversize and the initial penetration mechanism were also investigated.; This research investigation benefitted from vast experimental work for verification of analytical results and simulation and was accompanied by computer work wherever necessary.