Yuwen Sun

Smooth tool path generation for 5-axis machining of triangular mesh surface with nonzero genus

NC machining of a nonzero genus triangular mesh surface is being more widely confronted than before in the manufacturing field. At present, due to the complexity of geometry computation related to tool path generation, only one path pattern of iso-planar type is adopted in real machining of such surface. To improve significantly 5-axis machining of the nonzero genus mesh surface, it is necessary to develop a more efficient and robust tool path generation method. In this paper, a new method of generating spiral or contour-parallel tool path is proposed, which is inspired by the cylindrical helix or circle which are a set of parallel lines on the rectangular region obtained by unwrapping the cylinder. According to this idea, the effective data structure and algorithm are first designed to transform a nonzero genus surface into a genus-0 surface such that the conformal map method can be used to build the bidirectional mapping between the genus-0 surface and the rectangular region. In this rectangular region, the issues of spiral or contour-parallel tool path generation fall into the category of simple straight path planning. Accordingly, the formula for calculating the parameter increment for the guide line is derived by the difference scheme on the mesh surface and an accuracy improvement method is proposed based on the edge curve interpolation for determining the cutter contact (CC) point. These guarantee that the generated tool path can meet nicely the machining requirement. To improve further the kinematic and dynamic performance of 5-axis machine tool, a method for optimizing tool orientation is also preliminarily investigated. Finally, the experiments are performed to demonstrate the proposed method and show that it can generate nicely the spiral tool path or contour-parallel tool path on the nonzero genus mesh surface and also can guarantee the smooth change of tool orientation. A new method of generating spiral or contour-parallel tool paths on the nonzero genus mesh surface is proposed.The analytical formulas of computing CC points and parameter increment for path interval are derived.A simple and efficient method of optimizing tool orientation is also preliminarily investigated.

Automatic robotic polishing on titanium alloy parts with compliant force/position control

In this article, an automatic robotic polishing technique and system is developed for the polishing of titanium alloy curved parts. By means of a designed compliant end-effector with a force sensor, the robotic polishing system with a position-based explicit force control architecture is first built to perform the polishing operation. Then, a specially designed multi-axis robotic post-processor based on computer-aided design/computer-aided manufacturing is developed to generate the basic position and posture of the polishing tool without any complicated teaching processes. Subsequently, an adaptive Anti-Saturation Integral Separated Fuzzy PI controller, which is able to imitate the manual polishing operation and prevent undesirable vibrations and mechanical collisions, is designed to control the normal contact force. The basic trajectory is changed each time after the current polishing cycle has been finished by online self-learning, and a new basic trajectory is generated for the next polishing cycle. Finally, the effectiveness of the proposed automatic polishing technique is evaluated by actual polishing experiments on titanium alloy (TC11) parts, and the experimental results show that the proposed automatic robotic polishing technique has a perfect control effect on the contact force and thus can achieve a good and uniform surface quality of the part.

Determination of the stability lobes with multi-delays considering cutter’s helix angle effect for machining process:

As a common phenomenon, chatter is one of the most important factors that inhibit the improvement of productivity and deteriorate the machined surface quality in milling process. In this article, the mathematical model of the dynamic machining process is first constructed with multi-delays, in which the effect of the cutter’s helix angle on the chatter is considered. And a new integral interpolation method is proposed to predict the stability lobes. Based on this method, the mathematical model which is divided into two parts is calculated respectively with an interpolation method and an integration method. Using the Floquet theory, the stability limits of the dynamic milling system for an arbitrary point can thus be precisely predicted. Subsequently, the convergence rate of this integral interpolation method is analyzed. In order to verify the mathematical model, the stability lobes with uniform and variable cutter pitch angle are computed and the results show a good agreement with published experimental data; also one experiment is conducted to validate the proposed model and the method. Simultaneously, the simulation of a stability lobe with non-zero helix angle is also performed and the results validate the fact that the proposed method is capable of predicting stability limits with high accuracy. Finally, the influences of the cutter and process parameters such as helix angle, pitch angle, down-milling or up-milling and radial cutting depth on the milling chatter are analyzed.

Prediction of cutting forces integrated run-out effect for five-axis peripheral milling with a cylindrical cutter

This paper developed a mechanistic cutting force model integrated run-out effect for five-axis peripheral milling process with a cylindrical cutter. At each cutting element along the cutter axis, a unique cutting element coordinate system is established and then cutting forces introducing cutter run-out parameters are computed in cutting element coordinate system. Subsequently, differential cutting forces are transformed into the coordinate system of workpiece. In order to obtain cutting force coefficients, peripheral milling experiments are executed, and the run-out effect is represented by two parameters that could be obtained through experimental data. At last, five-axis milling forces are predicted and experimental verification. The results show that the predicted cutting forces using the presented model with cutter run-out effect are great consistent with experimental data.

Chatter stability and surface location error predictions in milling with mode coupling and process damping

Together with machining chatter, surface location error induced by forced vibration may also inhibit productivity and affect workpiece surface quality in milling process. Addressing these issues needs the combined consideration of stability lobes diagram and surface location error predictions. However, mode coupling and process damping are seldom taken into consideration. In this article, an extended dynamic milling model including mode coupling and process damping is first built based on classical 2-degree-of-freedom dynamic model with regeneration. Then, a second-order semi-discretization method is proposed to simultaneously predict the stability lobes diagram and surface location error by solving this extended dynamic model. The rate of convergence of the proposed method is also investigated. Finally, a series of experiments are conducted to verify the veracity of the extended dynamic model. The modal parameters including direct and cross terms are identified by impact experiments. Via experimental verification, the experimental results show a good correlation with the predicted stability lobes diagram and surface location error based on the extended dynamic model. Also, the effects of mode coupling and process damping are revealed. Mode coupling increases the whole stability region; however, process damping plays a vital role in stability improvement mainly at low spindle speeds.

New mathematical method for the determination of cutter runout parameters in flat-end milling

The cutting force prediction is essential to optimize the process parameters of machining such as feed rate optimization, etc. Due to the significant influences of the runout effect on cutting force variation in milling process, it is necessary to incorporate the cutter runout parameters into the prediction model of cutting forces. However, the determination of cutter runout parameters is still a challenge task until now. In this paper, cutting process geometry models, such as uncut chip thickness and pitch angle, are established based on the true trajectory of the cutting edge considering the cutter runout effect. A new algorithm is then presented to compute the cutter runout parameters for flat-end mill utilizing the sampled data of cutting forces and derived process geometry parameters. Further, three-axis and five-axis milling experiments were conducted on a machining centre, and resulting cutting forces were sampled by a three-component dynamometer. After computing the corresponding cutter runout parameters, cutter forces are simulated embracing the cutter runout parameters obtained from the proposed algorithm. The predicted cutting forces show good agreements with the sampled data both in magnitude and shape, which validates the feasibility and effectivity of the proposed new algorithm of determining cutter runout parameters and the new way to accurately predict cutting forces. The proposed method for computing the cutter runout parameters provides the significant references for the cutting force prediction in the cutting process.

Predictive Force Model Based Variable Feedrate Scheduling for High-Efficiency NC Machining

For improving machining efficiency, variable feedrate optimization instead of constant feedrate becomes very important under constraints of surface roughness and dynamic milling force. In this paper, a predictive force model is established for ball-end milling with considerations of cutter deflection and change of feedrate direction. The corresponding cutting force coefficients are regressed from the experimental data. Then the cutting force model is used for scheduling variable feedrate along the given toolpaths, and formulas of iterative optimization of feedrate are further given. The proposed feedrate scheduling strategy is tested under various machining conditions. Experimental results show that the proposed method can effectively reduce machining time while ensuring machining quality.