Interference-free tool path generation for the robotic polishing of blisks

In the robotic polishing process, the tool and the blisk interfere easily because of the narrow operation space and seriously twisted curved surfaces. Algorithms are proposed to detect and avoid collisions with high efficiency and accuracy. First, the curved surface of the blade is discretized into a set of points, and the collision detection between the tool and the blisk is converted into the calculation of distances between the tool and points on blade surface. Then, the tool axial vector is adjusted with the minimum rotation angle to avoid collision, which reduces the impact on surface profile accuracy after changing the tool postures. The machining quality is finally guaranteed by con-trolling the material removal depth of the polishing process. The proposed method realizes the collision detection and interference avoidance of the blisk polishing effectively, while it also ensures the surface quality of workpiece when adjusting the tool posture. Simulation and experiments are carried out to verify the feasibility and advantages of the proposed method.

Forward Kinematics Model for Evaluation of Machining Performance of Robot Type Machine Tool

Robot-type machine tools are characterized by the ability to change the tool posture and machine itself with a wider motion range than conventional machine tools. The motion of the robot machine tool is realized by simultaneous multi-axis control of link mechanisms. However, when the robot machine tool performs a general milling process, some problems that affect the machining accuracy occur. Moreover, it is difficult to identify the motion errors of each axis, which influence machining accuracy. Thus, it is difficult to adjust the servo gain and alignment error. In addition, the machining performance is unidentified because of the rigidity differences when the posture changes. In this study, the focus was on robot-type machine tools consisting of a serial and a parallel link mechanism. A geometric model is described, and the forward kinematics model is derived based on the geometric model. Machining tests were then carried out to evaluate the machining accuracy by measuring the machined surfaces and the simulated motion of the tool posture based on the proposed forward kinematics model to identify the mechanism that affects the machined surface roughness and surface waviness. As a result, it was shown that the proposed model can separate and reproduce the behavior of each axis of the machine. Finally, it was clarified that the behavior of the second axis has a great influence on the tool posture and machined surface.

Analysis of the effect of tool posture on stability considering the nonlinear dynamic cutting force coefficient

In aviation and navigation, complicated parts are milled with high-speed low-feed-per-tooth milling to decrease tool vibration for high quality. Because the nonlinearity of the cutting force coefficient (CFC) is more evident with the relatively smaller instantaneous uncut chip thickness, the stable critical cutting depth and its distribution against different tool postures are affected. Considering the nonlinearity, a nonlinear dynamic CFC model that reveals the effect of the dynamic instantaneous uncut chip thickness on the dynamic cutting force is derived based on the Taylor expansion. A five-axis bull-nose end milling dynamics model is established with the nonlinear dynamic CFC model. The stable critical cutting depth distribution with respect to tool posture is analyzed. The stability results predicted with the dynamic CFC model are compared with those from the static CFC model and the constant CFC model. The effects of tool posture and feed per tooth on stable critical cutting depth were also analyzed, and the proposed model was validated by cutting experiments. The maximal stable critical cutting depths that can be achieved under different tool postures by feed per tooth adjustment were calculated, and corresponding distribution diagrams are proposed for milling parameter optimization.

Tool Posture Planning Method for Low Rigidity Workpiece Considering Elastic Deformation Caused by Cutting Force

In machining process for a workpiece with low rigidity such as a blade shape, it is required to consider elastic deformation of the workpiece shape itself due to cutting force. Conventionally, reduction of the cutting force in machining process is achieved by optimization of feed rate value in NC program. On the other hand, since a decrease in the feed rate causes an increase in machining time. So, other optimization algorithm is required. In this paper, a new method to suppress the elastic deformation of the workpiece by changing tool posture in multi-axis controlled machining is proposed. The proposed method is intended for finish machining process for blade shape with a ball end mill. In the proposed method, first, the cutting force loaded on the workpiece surface in a certain posture candidate is predicted, and an instantaneous cutting force at the moment when the machining surface is generated is estimated by model-based computer simulation. Based on this results, the amount of elastic deformation on the machined surface is estimated by FEM. This process is repeated at each cutter location and tool posture candidate, and the new tool posture that can minimize machining error caused by the elastic deformation is determined at each cutter location.

Development of Tool Path Generation Method for Complex Deep Groove Shape With Simultaneous 4-Axis Machining

In recent years, aero engine parts with complex shapes are increasing due to demand on improved performance and weight reduction. In one example, some of those parts have groove which are deep, narrow and complex surface. They are processed by Simultaneous 5-axis machining with small ball end-mill. However, its complex groove shape cause greatly move of tool posture, thereby it cause greatly and discontinuous move of rotational and tilting axis of machine tool. As a result, actual feed speed cannot follow the commanded feed speed, so low process efficiency is problem. It can be considered simultaneous 4-axis machining which with fixed tilting axis as countermeasure. However it is difficult to generating tool path of simultaneous 4-axis machining by commercial Computer Aided Manufacturing (CAM) software for these deep groove shapes because of its tool collision avoidance algorithm. In this study, we developed method of generating tool path that can be machining simultaneous 4-axis machining while avoiding collision of workpiece and tool as an aim of increasing process efficiency. Furthermore, In order to effect of further improve the feed speed, we studied about posture planning way with possible to continuous moving of rotational axis, and we conducted verification test of increasing of feed speed with proposed method.

Optimization Method of Rotational Axis Motion in 5-Axis Controlled Machining to Keep Command Tool Feed Rate

In recent machining processes, 5-axis controlled machine tool is widely used for machining complicated workpiece shape with curved surface. In such process, to achieve high productivity, planning method of cutting conditions to satisfy both following the commanded tool feed rate in machining process and realization of good surface roughness are required. In conventional study, it is known that lead angle of tool posture against local machined surface influence the surface roughness. Then, common commercial CAM systems have already functioned to avoid interference and control the lead angle in each cutter location. However, in the generated cutter locations by the conventional algorithms, when the tool posture changes rapidly, there is a problem that actual feed rate does not reach the command value and machining time becomes longer than expected. In this paper, we propose the new tool posture correction algorithm. In the proposed method, first, the rotational axis that causes the feed speed rate decline is specified by preliminary experiments. And, the jerk value that is the threshold for the feed speed decline is investigated. After that, for the NC program, the command value of the target axis is modified within a range where interference of cutting tool does not occur, thereby preventing a decline in the actual feed rate. This paper describes an outline of the proposed modification method and the effect of the modification of the target axis positions on the lead angle and the actual feed rate.

Development of the innovative differential tool wear modeling for high-feed milling and its experimental verification

During the high-feed milling process, the vibrations generated by interrupted cutting cause changes in the instantaneous tool posture, as well as in the working angle and the distribution of the thermal stress coupling fields of each tool blade. These changes result in significant differences in the wear distribution of each tool blade. In this research, well-designed experiments for the high-feed milling of titanium alloys were carried out to identify the key factors affecting the differential wear on the milling tool insert blades. A differential tool wear model for the tool blades was developed in order to comprehensively describe the effects of the location error of the blades, the vibrations in the tool posture, and the working angle of each tool blade. The wear status of the milling tool was simulated based on the dynamic tool trajectories and postures derived by the model, and the entire simulated wear distribution was investigated with an innovative wear boundary recognition method. The differential tool wear model was evaluated and validated by the milling experiments and further supported by simulations.

A Prediction Model of Cutting Force about Ball End Milling for Sculptured Surface

Cutting force prediction is very important to optimize machining parameters and monitor machining state. In order to predict cutting force of sculptured surface machining with ball end mill accurately, tool posture, cutting edge, contact state between cutter, and workpiece are studied. Firstly, an instantaneous motion model of ball end mill for sculptured surface is established. The instantaneous milling coordinate system and instantaneous tool coordinate system are defined to describe the position and orientation of tool, and the transformation matrix between coordinate systems is derived. Secondly, by solving three boundaries around engagement of cutter and workpiece, a cutter-workpiece engagement model related to tool posture, milling parameters, and tool path is established. It has good adaptability to the variable tool axis relative to the machining surface. Finally, an algorithm of thickness about an instantaneous undeformed chip is researched, and a prediction model of cutting force is realized with microelement cutting theory. Also, the model is suitable for sculptured surface machining with arbitrary tool posture and feed direction. The accuracy of the proposed prediction model was verified by a series of experiments.