Research on dynamic thrust force prediction method for low-frequency vibration drilling CFRP/Ti stacks

Abstract

Making hole of CFRP/Ti stacks is a challenging process, so it has become a research hotspot in recent years. In this study, based on the orthogonal cutting model, the effect of vibration on the cutting process is analyzed. Based on vibration theory, the formulas for calculating the displacement transmissibility of twist drill and the amplitude of cutting edge of twist drill are derived. Based on drilling principle, the influence of vibration parameters on the actual feed per tooth is discussed. By analyzing the relationship between drilling parameters and vibration parameters, the different effects of longitudinal vibration on each cutting edge of the twist drill are expounded. A novel thrust force prediction model of LFVD (Low-Frequency Vibration Drilling) based on drilling parameters, vibration parameters and the twist drill characteristics was derived by simplifying the twist drill as a linear link with multiple inputs and outputs. The theoretical research shows that the effect of longitudinal vibration on the chisel edge is equivalent to main motion direction vibration, the effect on the major cutting edge is equivalent to cut depth and feed direction vibration simultaneously, and the effect on the minor edge is equivalent to feed direction vibration. If the amplitude of the cutting edge of the twist drill relative to the workpiece is smaller than 1/4 of the feed per revolution, but does not meet the intermittent cutting conditions, when the frequency/rotation ratio is odd or close to odd, it is continuous cutting. With the increase of amplitude, the chip thickness difference increases, and the chip breaking effect is enhanced. If the amplitude is greater than or equal to 1/4 of the feed per revolution, when the frequency/rotation ratio is odd or close to odd, it is intermittent cutting, complete chip breaking or good chip breaking can be achieved. The amplitude of the thrust force fluctuation component will increase with the increase of the vibration amplitude relative to the workpiece. The thrust force fluctuation component contains both of the component proportional to the vibration amplitude relative to the workpiece and the inertia force of the twist drill. The statistical results of drilling experimental data show that the accuracy of the thrust force average calculated by the prediction model established in this study can reach 86% ∼ 95%. The confirmatory experiments of LFVD show that the errors of the maximum and the average value of thrust force, as well as the amplitude of the harmonic component calculated by the thrust force prediction model are 1.7%, 10.6% and 3.6%, when drilling CFRP, respectively and 1.5%, 1.4% and 5.5% respectively when drilling titanium alloys. These conclusions can be regarded as an important basis for selecting drilling parameters and vibration parameters.