Research On Microdrill Design And Microdrilling Mechanism

With the development of science and technology, the requirement of micro-hole quality becomes increasingly high, and the diameter of micro-hole becomes small increasingly. Microdrill is one of the most critical influences on the micro-hole quality, high-performance microdrill can improve both the quality and efficiency of micro-hole process. In this thesis, comprehensive theoretical and experimental study have been carried out on microdrill design and microdrilling mechanism. The research contents are as follows:1. The formation, morphology, curling, flow and breaking of chip are analyzed theoretically, and a model to calculate the chip flow angle in microdrilling is proposed on the base of the minimal cutting power consumption principle. In microdrilling of stainless steel, under the influence of up-curl, side-curl and chip flow angle, the initial chip is conical, due to the high toughness of stainless steel, and the chips are prone to tangle on the microdrill, then form long ribbon chips. The influence of different parameters on the chip flow angle is analyzed by simulation and experimental methods, the results show that the larger the point angle of microdrill, the larger the chip flow angle. The chip flow angle increases with increasing of the feed rate. When the cutting thickness is close to the minimum chip thickness, the chip flow angle has a maximum.2. Based on the slip-line field theory, a mathematical model for predicting microdrilling thrust is developed, and the geometric characteristics of microdrill are considered in force models. The thrust is modeled in three parts of a microdrill: major cutting edges, secondary cutting edge, and the indentation zone, an oblique cutting model is used for the main cutting edge and an orthogonal model for the second cutting edge, the indentation zone is modeled as a rigid wedge. The thrust modeling of the major cutting edge and second cutting edge includes two different kinds of processes: shearing and ploughing. The force model is verified by comparing the predicted forces with the measured cutting forces. The results show that the cutting force model can correctly predict the thrust, the thrust created by the chisel edge represents about 60-70 percent of total thrust, and the thrust created by the chisel edge of microdrill is related to the length of chisel edge and the feed rate.3. A new microdrill is proposed, and the mathematical models of the flank of the new microdrill are developed. With the precondition of high strength and stiffness, in order to improve the centering ability of microdrill, a new microdrill which does not have chisel edge on the point is designed, and which has simple structure and excellent fabrication performances. The Mises stress, contact stress, cutting force and cutting temperature of the new microdrill and the planar microdrill during microdrilling are analyzed by means of finite element simulation method, the results show that the cutting performance of the new microdrill is better than the planar microdrill.4. The cutting properties of three different grain size WC-Co cemented carbides YG8, YG8 UA and YG8 UF are compared, and the failure forms and mechanisms of microdrill are analyzed. The dominant wear mechanisms of cemented carbide tools are abrasion at low speed and adhesion at high speed. In microdrilling of stainless steel, the wear occurs on the corner of chisel edge is greater than that of the outer corner of major cutting edge due to lower feed rate. Abrasion and adhesion are found to be the main wear mechanisms of microdrill. The wear, breakage resistance and working life of the microdrill can be improved by grain refinement of cemented carbide.5. The cutting properties of the new microdrill and the planar microdrill are compared in microdrilling. In microdrilling of 1045 steel and 304 stainless steel, the planar microdrill and the new microdrill are analyzed and compared one to another in detail from cutting force, microdrill failure, the machining quality of micro hole, tool life and so on. The working life of the new microdrill is longer than that of the planar microdrill at low feed rate, but the working life is lower than that of the planar microdrill when the feed rate exceeds a certain value. Under the same cutting condition, the cutting performance of the new microdrill is superior to the planar microdrill.