Prediction Of Cutting Forces And Chatter Stability In Tapping Process

Thread connection is the most widely used method to fasten parts in both industry and the daily life.Especially in aeronautical and astronautical industries,a large size transport airplane consists of millions of threads.The reliability of thread connection will greatly influence the performance and safety of the airplane.Among the thread manufacturing techniques,tapping is the least understood cutting process used in practice.Therefore,it is of great importance to develop high performance tapping process.This thesis,aiming at bettering the understanding of tapping,has investigated the mechanics and dynamics of the process to reveal the underlying mechanism based on the metal cutting and mechanistic theory.In this thesis,the synchronizing feed error adjustment method and calibration procedure have been put forward.And then,the unified ploughing force model of metal cutting process has been established.Based on the unified model,the mechanics of tapping process with emphasis on synchronizing feed error and its induced ploughing effect has been studied.Meanwhile,the dynamic model of tapping process has also been developed for the first time and validated by tapping tests.The main research works and contributions are as follows:(1)The adjustment method and calibration procedure of synchronizing feed error during rigid tapping cycle have been put forward.The influence of the NC system parameters on synchronizing feed error has been analyzed and the influence of synchronizing feed error on cutting force has been analyzed experimentally as well.Servo optimization to minimize the synchronizing feed error has been conducted.And an experimental calibration procedure for synchronizing feed error has been established with the aid of laser tachometer and laser interferometer.Since the calibration procedure is independent of the internal transducers of machine tools,the calibrated results are more reliable for revealing the actual synchronizing feed error of the spindle rotation and feed motion,which makes it possible to include influence of the actual synchronizing feed error in the force model.(2)A generalized model for metal cutting process with the unified ploughing mechanism for both static and dynamic cutting has been developed.Ploughing mechanism were separately treated for static and dynamic cutting processes in the literature.To remedy this,a unified proportional form is used to express ploughing forces as a function of the volume of the materials extruded under the clearance face of the tool,rather than the traditional expression as function of chip width in static cutting process,and experimental calibration procedure for the corresponding proportional scale,named as ploughing force coefficient with static cutting tests,is developed.The separation of ploughing forces and shearing forces from the experimental cutting forces has been conducted by linear regression method.Meanwhile,the determination method of shearing force coefficients based on the separated shearing force component has also been proposed.Both static and dynamic milling tests are used to validate the proposed model.(3)Mechanics model of tapping process is proposed with emphasis on measurement of synchronizing feed error and estimation of its induced ploughing forces.By analyzing the tap geometry and tapping motion trajectory,calculation method of the synchronizing feed error induced volume of materials extruded under the clearance face of the tool is proposed.And then,based on the generalized model in previous work,the ploughing force due to synchronizing feed error is calculated as the product of the volume and ploughing force coefficient.And theoretical proof for the ploughing force calculation procedure is also derived based on contact mechanics.Meanwhile,the shearing force due to material removal has also been calculated to estimate the total tapping force.Experimental verifications have been carried out to validate the proposed methods.(4)Dynamic modeling of tapping process has been proposed.The tap tip torsional,axial and torsional-axial coupling FRFs measurement scheme and modification method have been put forward at first.And then,the dynamic model simultaneously considering the lateral vibration and torsional/axial vibration has been established.By deriving the relationship between dynamic chip thickness and cutting forces mathematically,it is found that dynamic chip thickness due to lateral vibrations does not contributes to torsional/axial dynamic cutting forces.Namely,lateral vibrations and torsional vibrations are independent of each other.Tapping Experiments have also been carried out to validate the proposed model.The research works in this thesis can enhance the understandings of tapping process for manufacturing engineers and help to select proper cutting parameters,design tap geometry and estimate the cutting loads.These works also enrich the mechanics and dynamics of metal cutting processes.