The surface quality of the workpiece is an important objective to measure the machining process. It includes the factors of surface geometry and surface layer material. In the process of studying physical simulation of mechanical processing, the analysis and prediction of the surface topography has become an important part of the dynamic turning process model. Using this model to simulate the turning process, the quality of the machined surface of the workpiece can be predicted and analyzed before the actual machining process, and the purpose of analyzing the machining process and guiding the actual production can be achieved.
Fig. 1 Surface appearance of static cutting workpiece
1 Establishment of dynamic turning surface quality model Determining factors and uncertainties affecting surface quality In the cutting process, the main factors affecting the surface topography of the workpiece are: the geometry of the tool tip, the relative position between the tool and the workpiece The motion is the feed motion of the tool and the relative displacement between the tool and the workpiece caused by the cutting vibration. In the ideal steady-state cutting process state, the tool tip forms overlapping marks on the surface of the workpiece due to the feed motion and forms the machined surface of the workpiece. Therefore, the tool feed amount f and the tool arc radius rb are the main factors affecting the surface roughness. factor. Because the tool feed amount f and the tool arc radius rb are determined values ​​in the cutting process, it is a deterministic influence factor. The surface topography of the workpiece is shown in Figure 1. At this time, the microscopic roughness of the workpiece surface is also called theoretical roughness R1 as R1=rb-(rb2-f2/4)1â„2 (1)
Figure 2 Dynamically cutting the surface topography
In the actual processing of cutting, especially in the dynamic cutting state, due to the existence of cutting vibration, the relative motion of the tool and the workpiece occurs, which affects the surface topography characteristics of the workpiece. Therefore, in addition to considering the theoretical roughness, the surface topography of the workpiece must also consider the effects of various features of dynamic cutting and related factors. At this time, due to the presence of non-uniform micro-hardness of the workpiece, tool wear, and error in the accuracy of the spindle rotation of the machine tool, force fluctuations occur in the cutting process, and the tool deviates from the ideal cutting position to form the workpiece surface quality under dynamic cutting conditions. These disturbance factors are characterized by randomness and uncertainty and are another important aspect that affects the surface quality of the workpiece. Its microscopic surface shape features are shown in Figure 2. Relationship between cutting vibration and the surface quality of the workpiece For the outside turning, the cutting edge of the spiral trajectory with the cutting speed and feed amount as the parameter, along with the relative displacement between the tool and the workpiece. In the feed direction of the tool, the height of the intersection point of the arcs where two adjacent cutting edges are located can be used as a parameter to measure the roughness of the surface roughness of the workpiece. The parameter consists of the intersection point of two adjacent blade paths. And the blade depth disp is determined. The arc depth is the ordinate value of the relative vibration of the tool and workpiece to the reference line, and the inty value is determined by calculating the coordinates of the intersection point of the circles where the two adjacent arcs lie. Based on the above analysis, the surface roughness average height Ra in the feed direction is Ra = n S i = 1 inty in (2) where: n is the number of arcs in the feed direction: intyi is The ordinate value of the intersection of the two adjacent cutting arcs mentioned above. 2 Turning workpiece surface quality simulation system implementation process Based on the above turning surface quality simulation model, we established a surface quality simulation system for turning workpieces. Figure 3 shows the realization process of the surface quality model. This is also an important organic part of the turning physics simulation system. The realization of this system is closely related to the entire turning physics simulation system.
Fig. 3 The realization process of surface quality model of turning physics simulation system
Among the various disturbance factors that form dynamic turning, the difference in micro-hardness of the workpiece has an important influence on the cutting process, which we use as the cause of the dynamic changes in the simulation system. Because the hardness of the cutting material directly affects the magnitude of the instantaneous cutting force, the change in the hardness of the workpiece causes a change in the cutting force and thus a relative displacement between the workpiece and the tool, plus the result that the tip portion overlaps due to the feed motion on the surface of the workpiece. The workpiece forms microscopically rough surfaces that form the machined surface of the workpiece being cut. Because the micro-random hardness of the workpiece conforms to the normal distribution, the influence of the former on the surface quality is a random process and it is an uncertain factor. From the cutting vibration model, the vibration depth disp of the cutting arc can be obtained, then the coordinates of the intersection of the adjacent two cutting arcs can be obtained. Finally, the value of Ra can be obtained from equation (2). 3 Description of the surface quality model The dynamic turning surface quality simulation model we developed treats the difference in the microhardness of the workpiece as the main disturbance factor in the turning process, taking into account the overlap effect of the tool feed motion on the workpiece surface and the material of the workpiece. Influencing factors, while the other two factors affecting the surface quality, namely the tool wear and the accuracy of the spindle rotation of the machine tool are not considered, as an open simulation system developed by the VC++ object-oriented method, the system can fully consider the main interference Based on the factors, other complex disturbance factors existing in the actual dynamic turning process are added and supplemented one by one, and they are used as auxiliary and supplementary modules of the system to further improve the system and make the simulation result more consistent with the actual situation. 4 Conclusion In order to establish the surface quality simulation model of turning, in addition to considering the feed and the arc shape of the blade, the relative vibration between the workpiece and the tool is also an important factor that affects the surface quality of the workpiece. Appropriately reducing the amount of feed, increasing the arc radius of the cutting edge and reducing the relative vibration in the turning process can achieve the purpose of improving the surface quality. Comprehensive consideration of various factors that cause relative vibration will increase the quality of the simulation model and enhance the model's degree of fit.
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