Study On The Influence Mechanism Of Tool Posture On Machined Surface Generation And Tool Wear

Ball-end cutter is most commonly used in NC semi-finish and finish machining for complex curved surface,which is widely used in aerospace,automobile,energy and mold machining.However,the ball-end tool has the problem that the cutting linear velocity of tool tip is zero,and the participation of tool nose in cutting is not conducive to the improvement of machined surface quality.With the development of multi-axis NC machining technology,the tool tip can be prevented from participating in cutting to a certain extent by changing the relative posture between tool and workpiece.However,with the increase of the complexity of parts,it is relatively difficult to completely avoid the tool tip from participating in cutting.Especially when machining difficult-to-machine materials such as titanium alloy and superalloy,the tool posture will have a great influence on the machined surface quality,but the specific range of the tool posture parameter optimization is still unclear.Moreover,in the process of surface machining,cutter workpiece engagement(CWE)area is constantly changing with surface features and the tool path,and the position where the tool actually participates in cutting is also changing,which causes the uneven wear distribution of ball-end tool and makes it difficult to predict tool wear.Therefore,in this paper,the influence mechanism of ball-end tool's posture on machining surface generation and tool wear distribution are studied.The concept of ‘velocity effect' is introduced,the relationship between the main and feed movement of tool in the milling process is analyzed,and the evaluation model of tool speed proportional function is established,based on which the sensitive area of ball-end tool speed effect is defined.Based on the geometric relationship of tool workpiece engagement on a single milling path,the geometric relationship model of the lowest cutting speed point in the CWE area of ball-end tool is established.According to the model,whether the lowest cutting speed point is in the sensitive area of speed effect can be determined.Through the single milling experiment of inclined plane,it is found that when the lowest cutting speed point in the CWE area of ball-end tool is in the sensitive area of speed effect,the machining surface will have oscillation mark perpendicular to the feed direction,which verifies speed effect theory of ball-end tool.Aiming at the optimization of tool posture parameters when cutting near the tool nose,based on the geometric relationship of CWE and the theory of speed effect,the sensitive features of speed effect under the superposition state of ball-end tool path are analyzed,and several key points in the CWE area are selected to describe the cutting speed in three dimensions.The relationship model among cutting speed of key points in the CWE area and parameters such as machining inclination angle and tool feed direction is established,and the optimization strategy of tool axis parameters with tool contact as the lowest cutting speed point in the CWE area is put forward,and the optimal interval of tool feed direction near the tool nose is obtained.The multiple feeds experiments on the inclined plane of titanium alloy material verified the effectiveness of the proposed optimization strategy for tool pose parameters,and found the surface adhered defects on the machined surface in the non-optimal range.The formation mechanism of surface adhered defects is analyzed,and it is determined that the formation of surface adhered defects is related to the material adhered near the cutting edge and the cutting-in and cutting-out position of the tool within the tool per-rotation cycle.In order to facilitate the analysis of cutting-in and cutting-out process under different tool postures,the projection model of cutting edge and CWE area of the tool on the tangential plane is established,thus transforming complex spatial geometry problems into plane geometry problems.At the same time,in order to study the influence of down and up milling on the formation of surface adhered defects,the multiple paths experiments with different feed directions on inclined surface were carried out.Combined with the case of cutting-into/out,chip morphology,and surface morphology,the formation mechanism of surface adhered damage is analyzed.Based on the CWE geometric relationship model of ball-end cutter,the calculation model of effective cutting distance of cutting edge element within the tool per-rotation cycle is established,and the influence of tool posture parameters on tool wear is analyzed.A discrete calculation method of tool path length in curved surface machining is proposed.Based on post-processing development calculation,the linear cutting length of tool path in corresponding tool posture is obtained.The calculation model of total distance of cutting edge element is established.Combined with the existing tool wear model,the wear distribution prediction of tool flank in curved surface machining is realized,which is verified by experiments.According to the tool wear distribution characteristics and velocity effect theory in actual curved surface machining,the variable speed machining strategy is put forward.Based on post-processing development,the output of variable speed numerical control program aiming at relatively constant cutting linear speed is realized,and the actual free surface machining is verified.