An Investigation On The Rounding Process And Virtual Machining System Forhigh Precision Centerless Grinding

Centerless grinding is both a high volume and high precision process for the machining of the component rotational surface, which is non-substitutable particularly in the bearing and automotive manufacturing industries, etc. With the increasing requirements for high precision and productive machines which use the parts ground centerlessly, more and more stringent demands are inposed on the workpiece precision and process production rate of centerless grinding. As the typical components produced by centerless grinding, the roundness error of precision bearing rollers(Φ15×20mm) is expected to be controlled within 0.1μm level, and a throughput of 350 pieces/min is required. For this objective, the key factors in the centerless grinding process should be analyzed comprehensively and elaborately, and the integrated investigation of the process, in which the process-related elements and the coupling mechanism among them are considered, should be made.Deriving from the point of view above, the kinematics and dynamics study of the interacted process between the workpiece and the grinding system are made according to the real time workpiece profile, the geometry of the grinding zone, the machine setup and feed motion in this paper. The dynamic model of the machining process is developed to investigate the workpiece rounding pr ocess. The virtual machine tool, workpiece and material removal process are further integrated through their interaction mechanism to realize the high precision prediction of the rounding process and the grinder machining capability.The in-feed centerless grinding is considered as the primary centerless grinding method, therefore the kinematics and dynamics analysis of this process provides the foundation of the rounding process research. In order to present the basic rounding equations, the influence of the workpiece error motions relative to the workrest and control wheel on the rounding process is analyzed through the machine setup parameters, the tangent angle and workrest angle. And the relationship between the machine feed and workpiece movements is calculated. In centerless grinding the non-ideal workpiece profile and its variation in grinding show considerable influence on the workpiece motion in the grinding zone. In this paper the workpiece motion errors and the forces acted on it during the interaction process interacted with the grinding wheel, workrest and control wheel are calculated on the basis of the workpiece time-varying profile. Then the dynamic model is developed to investigate the workpiece rounding process and predict the final profile and roundness, in which the nonlinear factors of the process, including the contact-loss, contact filtering effects and the material replacement restriction, are taken into account.With the workpiece material removing gradually along its axis in through-feed centerless grinding, the surface regeneration of various sections at different moments shows dissimilar features. Furthermore, because of the workpiece unclamping state its position and orientation vary continuously in grinding zone, which is coupled with its rounding process. In this paper the homogeneous coordinate transformation is applied to present the profiles of the grinding wheel, the workrest and the control wheel and the time-varying workpiece shape real-timely in the unique reference frame for the accurate and efficient kinematic analysis of their interacting process. Then the dynamic forces and torques between the workpiece and the grinding system are calculated in 3D space. And based on the Lagrange equation the workpiece dynamic rounding model is developed for the high precision spacial dynamic investigation of the through-feed centerless grinding process.The unique workpiece supporting and driving manner of centerless grinding requires the control wheel to be inclined in the vertical plane, and sometimes the swiveling of the control wheel in the horizontal plane is set according to the workpiece shape machined. The adjustment of the control wheel makes its ideal shape more complex. In order to support and drive the workpiece stably and reduce the translation and swiveling errors of the workpiece axis in machining, the ideal control wheel shape, which is expected to provide the line contact with the workpiece, is calculated from the parameters of the machine setup, the control wheel adjustment and the workpiece geometry. Comparing the workpiece motion errors during the grinding supported by the ideal control wheel and the shape trued with conventional method, the workpiece translation errors, axis rotation errors and their changing rule unde r different control wheel inclined angles and machine setup parameters are investigated quantitatively, and the influences of the errors on high centerless grinding are studied.The rounding process in centerless grinding is the process of the workpiece m aterial removal and new surface regeneration under the interaction between the grinding system and the workpiece. In order to predict accurately and reliably the workpiece rounding process and grinder machining capability, the high precision virtual centerless grinding system is constructed. Depending on the analysis of the basic functions, the modeling strategies and methods of the virtual system, the machine components kinematic relationships, the structure dynamic parameters and control technologies are loaded from the initial 3D machine tool model and correlated to each other to build the virtual machine tool model. Then the simulation of high precision centerless grinding, integrated the virtual machine tool, the workpiece and the rounding mechanism, is realized through the analysis of the coupled process among the factors affecting the roundness generation.The high precision centerless grinding process is influenced by a lot of factors which show different effects on the rounding process. Based on the analysis of the influences of the tangent angle and the workrest angle on the stability of different order errors, the proper machine setup parameters are selected to perform the grinding trials for the verification of the dynamic rounding models. The res ults show that the models can study the rounding process and predict the workpiece profile reliably. On the basis of the developed dynamic models, the primary factors and their rules influencing the workpiece roundness generation of high precision centerle ss grinding are investigated, and the measures for suppressing the roundness error are proposed.