Study On The Technology And Mechanism Of Ultrasonic Vibration Assisted Grinding

Ultrasonic vibration assisted grinding (UAG) is one of the methods machining brittle materials, owns important significance in theory and a capacious development foreground.Systemic theory and experiment study on the UAG were made in this paper, where the workpiece was vibrated along tangential, axial or normal direction of the grinding wheel (TUAG, AUAG and NUAG). The study contents mostly included kinematics characteristics, grinding force characteristics, materials removal mechanism and machined surface quality.During the UAG process, the material removal is the result of interference between the abrasive grits and workpiece, so it is the theory base of researching the machining characteristics of UAG to analyze the kinematics characteristics between the grinding wheel or abrasive grits and the workpiece. In order to study the correlative movement between the abrasive grits and the workpiece, the kinematics models of the there UAG methods were founded. Based on the models of grinding process, the kinematics characteristics and the relative movement trace between the single abrasive grit and the workpiece were researched, and the correlative geometry parameters were analyzed and calculated. The critical conditions of the abrasive grit apart from the workpiece were also analyzed for the there UAG. The analysis results indicated that the move path length of an abrasive grit in any UAG was longer than that in conventional grinding (CG), and because of the separate process, the actual cutting arc length in tangential and normal ultrasonic vibration modes was not always longer than that in CG. And the average cross-sectional area in AUAG was smaller than that in CG. The workpiece was reciprocatively ironed by the grinding wheel during TUAG. During UAG process, the critical speed of the single abrasive grit apart from the workpiece was not only relate to the amplitude A and the frequency f, but also relate to the grinding velocity v_s and the cutting point space a. During the tangential and normal UAG, if the cutting point space is a multiple of the vibration wavelength, the machining process was only continuous grinding process no matter how to match the grinding parameters. The experimental results showed that the ultrasonic vibration cutting theory did not absolutely adapt to the TUAG, and that the time of the single abrasive grit apart from the workpiece lagged that of the grinding wheel apart from the workpiece.The grinding force can not only reflect the interferential state of the grinding wheel and the workpiece, estimate the grinding quality, but also to a certain extent forecast the machined surface quality and the depth of the degenerating layer, so it is necessary to detailedly research the grinding force produced in grinding process. The grinding force mathematic models of an abrasive grit in CG and the there UAG were founded, and the grinding force was studied from the two aspects of cutting deforming force and friction force. The mathematics models of the cutting deforming force was made by introducing the unit grinding force, F_u, and the mathematics models of the friction force was presented by defining the grinding ability parameters, C_ge, And the systemic theory analysis and experimental study of grinding force were made. The study results indicated that during tangential and normal UAG with separate characteristics, the average cutting depth of single abrasive grit was deeper than that of CG, and the average cutting depth of single abrasive was equal to that of AUAG in theory; the unit grinding force in UAG was lower than that in CG under the same machining condition because of the workpiece intenerating and impact effect; the grinding force was decreased in AUAG, and the higher ultrasonic amplitude and frequency was helpful to the decrease of the grinding force; however, the higher grinding velocity weakened the contributiveness of ultrasonic vibration to the decrease of the grinding force; the friction coefficient was reduced in the there UAG, and the tangential ultrasonic vibration had the most effect on it, the normal ultrasonic vibration took second place, the axial ultrasonic vibration was the last. The grinding force experimental results indicated that the grinding force was affected to a certain extent by the grinding wheel granularity and grinding parameters, and the grinding force increased with larger grain diameter, grinding depth and workpiece speed, while the increase of the grinding velocity led to a decrease of the grinding force; the ultrasonic vibration weakened the varying of the grinding force with the grinding parameters, however, has great effect on the grinding force; to the tangential grinding force, F_t, the axial ultrasonic vibration led to a increase of it, the tangential ultrasonic vibration took second place, while F_t decreased because of the normal ultrasonic vibration; to the normal grinding force, F_n, the normal ultrasonic vibration made it decrease the most, the tangential ultrasonic vibration took second place, the axial ultrasonic vibration was the last; the ultrasonic vibration led to a great decrease of the grinding force ratio, and the axial ultrasonic vibration made it decrease the most, the grinding force ratio value was about 0.8-1.3; the tangential ultrasonic vibration took second place, the value was about 1-1.5; the normal ultrasonic vibration was the last, the value was about 1.3-2.The brittle-plastic micro-topographies of machined surface were investigated in different grain diameter, grinding depth and ultrasonic vibration modes. The results indicated that the proportion of brittle fracture reduced and the proportion of plastic deformation increased by use of the smaller grinding wheel granularity; the proportion of brittle fracture increased with deeper grinding depth, while the smaller grinding depth led to a decrease of the proportion of brittle fracture in both UAG and CG; under the same grinding condition, the ultrasonic vibration direction affected the material removal modes: the material was mainly removed in the form of transgranular fracture, plastic shear and little intergranular fracture in TUAG; the material was mainly removed in the form of transgranular fracture and plastic shear in AUAG; the material was mainly removed in the form of fracture crash and the cracks remain in the machined surface in NUAG; the plastic processing of brittle materials could be realized under certain machining condition; the minuteness of the grinding wheel granularity and the micro grinding depth were the important conditions of realizing the brittle-ductile transition. The critical condition of brittle-ductile transition was given, and it was obtained by experimental study that the tangential and axial ultrasonic vibration helped to the brittle-ductile transition of brittle material removal, while the normal ultrasonic vibration made the critical cutting depth decrease and the material was inclined to the brittle fracture removal.Based on the ground surface generation mechanism in CG and UAG, the models for ground surface roughness were built and the experiments were conducted. In CG, the machined surface was generated by the latter cutting point and the roughness was in direct proportion to the cutting point space, workpiece speed, and the cone-apex angle of the grains and was in inverse proportion to the grinding velocity; In TUAG, the machined surface was generated by the latter cutting point or a series of latter cutting points, and the roughness was lower than that in CG; in AUAG, the machined surface was the reticular structure generated by the interlaced cutting trace, and the roughness was in direct proportion to the cutting point space, workpiece speed and the cone-apex angle of the grains and was in inverse proportion to the grinding velocity, ultrasonic amplitude and frequency; in NUAG, the machined surface was generated by a series of latter cutting points, and the roughness was higher than that in CG, while maybe be reduced when the grinding parameters were set appropriately. The experimental results indicated that the grinding wheel granularity produced the greatest effect on the ground surface quality; in there UAG modes, the axial ultrasonic vibration led to the greatest decrease of the roughness, the tangential ultrasonic vibration took second place, while the roughness was increased because of the normal ultrasonic vibration. The grinding subsurface was analyzed by observing the section topography. The results indicated that the grinding surface became coarser and present more crack; the tangential and axial ultrasonic vibration helped to improve the grinding surface quality. The grinding surface hardness in different machining modes was measured by the micro-hardness equipment. The results showed that the grinding surface hardness in both CG and UAG was higher than that of the basis material; tangential and normal ultrasonic vibration led to the grinding surface hardness lower than that of CG, while axial ultrasonic vibration made the grinding surface hardness higher than that of CG; the variety of grinding surface hardness was due to the common effect of grinding temperature and grinding surface/subsurface cracks.