| Parallel grinding is frequently applied to machine a wide range of hard and brittle materials with closed surface tolerance and remarkable surface quality.However,grinding is a complex process,which is influenced by many factors,such as the workpiece material properties,wheel characteristics,abrasive properties and grinding conditions,which makes the modelling process extremely complex.At the same time,due to the random nature of the machine tool,it has been considered as a harder modelling process than single point turning or multipoint milling.In the grinding process,the motion error of the grinding wheel relative to the workpiece surface will lead to the appearance of micro-waveness on the machined surface,which will seriously affect the precision and surface quality of the parts.In order to understand a detailed process of surface micro-waviness generation in parallel grinding,an accurate model should be developed to describe the interaction of the wheel and workpiece.In recent years,extensive research work has been carried out to model the surface generation in grinding,but most of them for ground surface topography is based on either the abrasive trajectory(micro-level)or the relative motion error of the wheel(macro-level)to predict surface topography and optimization of grinding process.There is lack of a comprehensive study for surface generation related those two levels together.In this paper,surface profile generation model in the parallel grinding is established by considering the relative motion error of the grinding wheel to reveal the formation mechanism of the surface micro-waviness and the cumulative process of phase shift.It was found that the phase shift during the grinding process,was inevitably appeared throughout the machining due to the slight rotational speed error of the grinding wheel,thus spiral micro-waviness appeared on all the machining surfaces.In addition,the evolution mechanism of surface micro-waviness under different phase shifts is revealed.The phase shift gradually accumulates under the fine feed rate,and the relative motion error with larger phase shift leads to the generation of dense micro-waviness.The small phase shift produces sparse micro-waniness.The simulated surface consistent with the measurement surface.At the same time,small changes in phase shift can also result in significant differences in surface roughness,indicating that phase shift is an important variable in optimizing ultra-precision grinding processes.Finally,considering the geometry of the grinding wheel,the random protrusion height of the abrasive grains,the relative motion error,phase shift and the abrasive trajectory,a theoretical model of the surface micro-topography in the parallel grinding process is established and it's accuracy is verified by the surface matching algorithm.Various complex surfaces have been widely used in complex optical devices to improve the functional performance of imagi ng and illumination quality and reduce sizes.The parallel grinding is widely used to machine curvature surfaces with high for m accuracy and good surface finish.However,the complexity and variation of curvature of the curvature surface impose a lot of challenges to make the process to be more predictable and control.Tool path as a critical factor directly determines the form error and surface quality in ultra-precision grinding of curvature surfaces.In conventional tool path planning,the constant angle method is widely used in machining curvature surfaces,which resulted in non-uniform scallop-height and degraded surface profile accuracy.In this paper,a theoretical model is developed to relate the scallop height to the curvature and the micro-waviness caused by the motion error of grinding wheel and a new tool path method is proposed to achieve uniform scallop height in parallel grinding of curvature surfaces.Moreover,an iterative closest point(ICP)matching method is used to determine the error between the measured surface and the designed surface.Hence,the performance of the theoretical scallop height model is verified through a series of experiments conducted in grinding of annular sinusoidal surface and single sinusoidal surface,which will provides a theoretical basis for achieving unifor m scallop height in parallel grinding of curvature surface.Finally,the suppression strategy of surface micro-waviness in grinding is studied.The influence of relative motion error of the grinding wheel on the surface waviness is reduced by controlling the distribution of grinding points.In addition,in order to further optimize the parallel grinding process and improve the surface quality,the Taguchi method is employed to study the influence of machining parameters on the surface roughness and an optimal combination of operation parameters in paeallel grinding has been found.Hence,based on the optimal results in Taguchi experiment,individual variables experiment is conducted to study the influence of ever y grinding parameter o n the condition of the machined surface.Finally,the contribution of each factor effect the surface roughness is evaluated.The results show that feed rate and the workpiece rotational speed can produce significant results to improve the surface quality.A series of spirals around the machining center area are the main mechanism of surface generation,which results from the relative motion of the grinding wheel.The integral part of the ratio between the grinding wheel speed and the workpiece speed determines the number and density of the spirals and the fractional part of that controls the spiral geometry and the degree of phase shift accumulation.The phase shift can intensify the space interference of tool path,which contributes to the decrease of the scallop height and significantly improve the surface quality in grinding.In parallel grinding,the characteristics of the material will also affect the mechanism of surface generation,especially for the hard and brittle materials and the great differences between plastic materials and brittle materials,which is mainly due to the different removal mechanisms.In this paper,different mechanisms of surface generation of Mold Steel,Tungsten Carbide and Silicon Carbide in grinding were compared and it is found that Spanzipfel effect had a certain impact on the surface generation.For plastic materials,Spanzipfel significantly caused plastic flow,expecially in the junction of neighbouring grooves,but for hard and brittle materials,it would has more chance to cause fracture.Furthermore,the relative influences of machining parameters and curvature on surface roughness in the grinding process for the characteristics of machining curvature surface are studied for the characteristics of curvature changes during parallel grinding of curvature surfaces.Finally,on the basis of optimizing the parallel grinding process,the unifor m and high surface quality of the sinusoidal microstructure surface is achieved by adopting the new developed tool path planning algorithm with the equal scallop height. |
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