Optimum tool path generation for 2.5D direction-parallel milling with incomplete mesh model

Many mechanical parts are manufactured by milling machines. Hence, geometrically efficient algorithms for tool path generation, along with physical considerations for better machining productivity with guaranteed machining safety, are the most important issues in milling. In this paper, an optimized path generation algorithm for direction-parallel milling, a process commonly used in the roughing stage as well as the finishing stage and based on an incomplete 2-manifold mesh model, namely, an inexact polyhedron widely used in recent commercialized CAM software systems, is presented. First of all, a geometrically efficient tool path generation algorithm using an intersection points-graph is introduced. Although the tool paths obtained from geometric information have been successful in forming desired shapes, physical process concerns such as cutting forces and chatters have seldom been considered. In order to cope with these problems, an optimized tool path that maintains a constant MRR for constant cutting forces and avoidance of chatter vibrations, is introduced, and verified experimental results are presented. Additional tool path segments are appended to the basic tool path by means of a pixel-based simulation technique. The algorithm was implemented for two-dimensional contiguous end milling operations with flat end mills, and cutting tests measured the spindle current, which reflects machining characteristics, to verify the proposed method.     

Tool path generation and modification for constant cutting forces in direction parallel milling

This paper presents an optimized path generation algorithm for direction parallel milling, which is commonly used in the roughing and finishing stages. First, a geometrically efficient tool path generation algorithm using an intersection points graph is introduced. Second, the generated tool path is modified as an optimized tool path that maintains a constant material removal rate to achieve a constant cutting force and avoid chatter vibration, and the results are verified. Additional tool path segments are appended to the basic tool path through a pixel-based simulation technique. The algorithm is implemented for two-dimensional contiguous end milling operations with flat end mills, and cutting tests are conducted by measuring the spindle current, which reflects the changing machining situations, to verify the performance of the proposed method.     

An optimized contour parallel tool path for 2D milling with flat endmill

Although the conventional contour parallel tool path obtained from geometric information has been successful in making desirable shapes, it seldom considers the physical process concerns like cutting forces and chatters. In this paper, an optimized contour parallel path, which maintains constant MRR (material removal rates) at all times, is introduced and the results are verified. The optimized tool path is based on a conventional contour parallel tool path. Additional tool path segments are appended to the basic tool path in order to achieve constant cutting forces and to avoid chatter vibrations in the entire machining area. The algorithm has been implemented for two dimensional contiguous end milling operations with flat end mills, and cutting tests were conducted to verify the significance of the proposed method.     

Chatter prediction based on frequency domain solution in CNC pocket milling

Chatter has been a problem in CNC machining process especially during pocket milling process using an end mill with low stiffness. Since an iterative time-domain chatter solution consumes a computing time along tool paths, a fast chatter prediction algorithm for pocket milling process is required by machine shop-floor for detecting chatter prior to real machining process. This paper proposes the systematic solution based on integration of a stability law in frequency domain with geometric information of material removal for a given set of tool paths. The change of immersion angle and spindle speed determines the variation of the stable cutting depth along cornering cut path. This proposed solution transforms the milling stability theory toward the practical methodology for the stability prediction over the NC pocket milling.     

螺旋铣孔工艺的切削稳定性及工艺参数规划

以螺旋铣孔工艺时域解析切削力建模、时域与频域切削过程动力学建模、切削颤振及切削稳定性建模为基础,研究了螺旋铣孔的切削参数工艺规划模型和方法。切削力模型同时考虑了刀具周向进给和轴向进给,沿刀具螺旋进给方向综合了侧刃和底刃的瞬时受力特性;动力学模型中同时包含了主轴自转和螺旋进给两种周期对系统动力学特性的影响,并分别建立了轴向切削稳定域和径向切削稳定域的预测模型,求解了相关工艺条件下的切削稳定域叶瓣图。在切削力和动力学模型基础之上,研究了包括轴向切削深度、径向切削深度、主轴转速、周向进给率、轴向进给率等切削工艺参数的多目标工艺参数规划方法。最后通过试验对所规划的工艺参数进行了验证,试验过程中未出现颤振现象,表面粗糙度、圆度、圆柱度可以达到镗孔工艺的加工精度。     

Optimization of machining parameters using genetic algorithm and experimental validation for end-milling operations

Optimization of cutting parameters is valuable in terms of providing high precision and efficient machining. Optimization of machining parameters for milling is an important step to minimize the machining time and cutting force, increase productivity and tool life and obtain better surface finish. In this work a mathematical model has been developed based on both the material behavior and the machine dynamics to determine cutting force for milling operations. The system used for optimization is based on powerful artificial intelligence called genetic algorithms (GA). The machining time is considered as the objective function and constraints are tool life, limits of feed rate, depth of cut, cutting speed, surface roughness, cutting force and amplitude of vibrations while maintaining a constant material removal rate. The result of the work shows how a complex optimization problem is handled by a genetic algorithm and converges very quickly. Experimental end milling tests have been performed on mild steel to measure surface roughness, cutting force using milling tool dynamometer and vibration using a FFT (fast Fourier transform) analyzer for the optimized cutting parameters in a Universal milling machine using an HSS cutter. From the estimated surface roughness value of 0.71 μm, the optimal cutting parameters that have given a maximum material removal rate of 6.0×10³ mm³/min with less amplitude of vibration at the work piece support 1.66 μm maximum displacement. The good agreement between the GA cutting forces and measured cutting forces clearly demonstrates the accuracy and effectiveness of the model presented and program developed. The obtained results indicate that the optimized parameters are capable of machining the work piece more efficiently with better surface finish.     

Time domain coupled simulation of machine tool dynamics and cutting forces considering the influences of nonlinear friction characteristics and process damping

A higher machining ability is always required for NC machine tools to achieve higher productivity. The self-oscillated vibration called “chatter” is a well-known and significant problem that increases the metal removal rate. The generation process of the chatter vibration can be described as a relationship between cutting force and machine tool dynamics. The characteristics of machine tool feed drives are influenced by the nonlinear friction characteristics of the linear guides. Hence, the nonlinear friction characteristics are expected to affect the machining ability of machines. The influence of the contact between the cutting edge and the workpiece (i.e., process damping) on to the machining ability has also been investigated. This study tries to clarify the influence of the nonlinear friction characteristics of linear guides and ball screws and process damping onto milling operations. A vertical-type machining center is modeled by a multi-body dynamics model with nonlinear friction models. The influence of process damping onto the machine tool dynamics is modeled as stiffness and damping between the tool and the workpiece based on the evaluated frequency response during the milling operation. A time domain-coupled simulation approach between the machine tool behavior and the cutting forces is performed by using the machine tool dynamics model. The simulation results confirm that the nonlinear frictions influence the cutting forces with an effect to suppress the chatter vibration. Furthermore, the influence of process damping can be evaluated by the proposed measurement method and estimated by a time domain simulation.     

Development of chatter detection in milling processes

The aim of this research is to develop an in-process detection of the chatter for the actual milling processes regardless of any cutting condition within the small data processing time by utilizing the dynamic cutting forces obtained during cutting. The proposed method introduces three parameters, which are calculated and obtained by taking the ratio of the average variances of the dynamic cutting forces of three force components, to identify the chatter. The algorithm was developed and implemented on five-axis computer numerical control machining center to detect the chatter in ball-end milling and end milling processes. The chatter and the nonchatter can be simply detected during the in-process cutting by mapping the obtained values of three parameters in the reference feature spaces regarding the determined threshold values. The experimental results showed that the proposed method can be effectively used to detect the chatter during cutting even though the cutting conditions are changed.     

Chatter stability of micro end milling by considering process nonlinearities and process damping

The micro end milling uses the miniature tools to fabricate complexity microstructures at high rotational speeds. The regenerative chatter, which causes tool wear and poor machining quality, is one of the challenges needed to be solved in the micro end milling process. In order to predict the chatter stability of micro end milling, this paper proposes a cutting forces model taking into account the process nonlinearities caused by tool run-out, trajectory of tool tip and intermittency of chip formation, and the process damping effect in the ploughing-dominant and shearing-dominant regimes. Since the elasto-plastic deformation of micro end milling leads to large process damping which will affect the process stability, the process damping is also included in the cutting forces model. The micro end milling process is modeled as a two degrees of freedom system with the dynamic parameters of tool-machine system obtained by the receptance coupling method. According to the calculated cutting forces, the time-domain simulation method is extended to predict the chatter stability lobes diagrams. Finally, the micro end milling experiments of cutting forces and machined surface quality have been investigated to validate the accuracy of the proposed model.     

Identifying bifurcation behavior during machining process for an irregular milling tool geometry

High-productivity machining processes cause tool and material defects and even damages in machine spindles. The onset of self-excited vibration, known as chatter, limits this high material removal rate. This chatter vibration refers to machining instability during cutting processes, which results in bifurcation behavior or nonlinear effect wherein the tool and the workpiece are not engaged with each other. In particular, bifurcation for low-radial immersion conditions can be easily promoted and identified. In this study, an experiment on an irregular milling tool as a variable helix and variable pitch geometry was conducted under a flexible workpiece condition. The bifurcation behavior from regenerative chatter was identified and quantified from displacement sensor and inductive sensor measurements. A series of cutting tests was used to measure the vibration signals, which were then analyzed based on the frequency spectrum, the one-per-revolution effect, and the Poincaré section. According to results, Hopf bifurcation and period-one bifurcation instabilities apparently occurred to validate chatter stability prediction through a semi-discretization method. However, period-doubling bifurcation was only determined during the unstable cutting of a uniform tool that was not in variable helix/pitch or an irregular milling tool. An irregular tool geometry caused the modulation of the regenerative effect to suppress chatter, and period-doubling instability could not be exhibited during cutting as a regular tool behavior. This period-one chatter instability of an irregular milling tool should be identified and avoided by practitioners to achieve high productivity in machining using the aforementioned irregular milling tools.     

Feedrate scheduling strategy for free-form surface machining through an integrated geometric and mechanistic model

In free-form surface machining, it is essential to optimize the feedrate in order to improve the machining efficiency. Conservative constant feedrate values have been mostly used up to now since there was a lack of physical models and optimization tools for the machining processes. The overall goal of this research is the integration of geometric and mechanistic milling models for force prediction and feedrate scheduling in five-axis CNC free-form surface machining. For each tool move, the geometric model calculates the cut geometry, and a mechanistic model is used along with a maximum allowable cutting force to determine a desired feedrate. The results are written into the part NC program with optimized feedrates. When the integrated modeling approach based feedrate scheduling strategy introduced in this paper was used, it was shown that the machining time can be decreased significantly along the tool path.     

A mapping-based spiral cutting strategy for pocket machining

Almost 80 % of the milling operations to produce mechanical parts are produced by NC pocket milling, especially in aerospace and automobile industry. At present, for 2.5D pocket machining, direction-parallel and contour-parallel machining strategies have gained nearly universal acceptance. However, in such tool path, abrupt change of path direction, frequent acceleration and deceleration, and sharp velocity discontinuous are found to significantly limit the machining efficiency of pockets. To address these problems, this paper introduces a method for generating a spiral tool path that maintains a steady-state cutting process by as smoothly as possible curvature evolution of the tool path for pocket machining. First, the machined region of a layer of a pocket is mapped onto a circular domain by means of mesh mapping, which reduces the task of tool path generation from the geometrically complex pocket region to a topologically simple disk. On this disk, a guide spiral is constructed according to a mathematical function constrained by the calculated path interval map. Using the mapping from the pocket to the disk as a guide, the guide spiral is inversely mapped into the interior of the pocket and then a smooth low-curvature spiral path is derived. The generated tool paths are guaranteed to not inherit any corners in the subsequent interior tool paths and allows cutting of the pocket without tool retractions during the cutting operations. Finally, the proposed method is implemented and tested on several typical sample pockets to demonstrate its validity and significance.     

长悬臂铣刀铣削颤振的分析与抑制

长悬臂铣削颤振一直是机械加工工艺难点问题,国内外专家学者长期不懈地对此问题进行了研究,并取得了一定的成果。数控刀具的快速发展为解决此类问题提供了常规方式,比如减振刀杆,但这只是其中的一个方法,并不能保证颤振完全消失。本文对颤振机理、铣削过程进行了分析研究,通过对刀具几何角度的选择和走刀路径的优化,来达到更多地抑制长悬臂铣削颤振的目标,为解决长悬臂铣削颤振问题提供了有效方法。     

ESPRIT- and HMM-based real-time monitoring and suppression of machining chatter in smart CNC milling system

Suppression of machining chatter during milling processes is of great significance for surface finish and tool life. In this paper, a smart CNC milling system integrating the function of signal processing, monitoring, and intelligent control is presented with the aim of real-time chatter monitoring and suppression. The algorithm of estimation of signal parameters via rotational invariance techniques (ESPRIT) is adopted to extract the frequency characteristics of acceleration signals, and then, cutting state is categorized as stable state, chatter germination state, and chatter state based on amplitude-frequency characteristics of identified acceleration signals. The model of chatter identification is acquired by training a hidden Markov model (HMM), which combines acceleration signals and labeled cutting state. To implement real-time chatter suppression, the algorithm of fuzzy control is integrated into a smart CNC kernel to determine the relationship between cutting force and spindle speed. Furthermore, spindle speed of machine tool could be adjusted timely in the presented system once the chatter is identified. Finally, the effectiveness of the proposed real-time chatter monitoring and suppression system is experimentally validated.     

Prediction of cutting forces in helical milling process

The prediction of cutting forces is important for the planning and optimization of machining process in order to reduce machining damage. Helical milling is a kind of hole-machining technique with a milling tool feeding on a helical path into the workpiece, and thus, both the periphery cutting edges and the bottom cutting edges all participated in the machining process. In order to investigate the characteristics of discontinuous milling resulting in the time varying undeformed chip thickness and cutting forces direction, this paper establishes a novel analytic cutting force model of the helical milling based on the helical milling principle. Dynamic cutting forces are measured and analyzed under different cutting parameters for the titanium alloy (Ti–6Al–4V). Cutting force coefficients are identified and discussed based on the experimental test. Analytical model prediction is compared with experiment testing. It is noted that the analytical results are in good agreement with the experimental data; thus, the established cutting force model can be utilized as an effective tool to predict the change of cutting forces in helical milling process under different cutting conditions.     

Spectral element method for stability analysis of milling processes with discontinuous time-periodicity

This paper presents an application of the spectral element method for the stability analysis of regenerative machine tool chatter models in milling operations. An extension of the spectral element method is introduced in order to handle the discontinuities in the cutting force in an efficient way. The efficiency of the method is demonstrated on some well-known machine tool chatter models taken from the literature. Efficiency is characterized by the computational time, the convergence of the stability boundaries, and the convergence of critical characteristic multipliers. Results show that compared to the most widespread methods in machining literature, the spectral element method provides significant improvements in computational time while maintaining high accuracy levels.     

Research on tool path planning method of four-axis high-efficiency slot plunge milling for open blisk

The great mass of machining allowance for blisk is removed in the rough milling, so improving the rough machining efficiency for the blisk’s tunnel is the key of realizing high-efficiency machining of blisk. According to the structure characteristic of open blisk’s tunnel, a four-axis plunge slot rough milling with high-efficiency and low machining cost is advanced. First, the plunge slot process for blisk and the generation process of the ruled enveloping surface for the freeform surface of the blisk’s blade are put forward. Then, the generating method of the ruled enveloping surface for the blade’s freeform surface and the tool path generation method of four-axis plunge slot milling for blisk are studied. The rough milling region of open blisk’s tunnel is determined by generating the ruled enveloping surface of blade’s offset surface, and the algorithm of tool path for four-axis plunge milling is given. When using a ruled surface to approach a freeform surface, the problem of getting boundary is solved, and the error from the calculation of tool path is avoided by the algorithm. At last, the experiment shows that comparing to the traditional side slot milling, the cutting force of four-axis plunge milling can be reduced by 60% and even the rough machining efficiency can be increased to more than double.     

Milling force vibration analysis in high-speed-milling titanium alloy using variable pitch angle mill

Chatter may cause fast wear of tools and poor surface quality of the workpieces at high cutting speed and it will happen on different process parameters; how do we select the suitable cutting speed to suppress the chatter? In this paper, a signal analysis method for milling force and acceleration is adopted to identify chatter, which can obtain the results not only in frequency of chatter but also in the contribution for milling force at different frequencies. Through the milling experiment, the machining vibration behaviors of milling Ti–6Al–4V with variable pitch end mill were investigated. Milling force and acceleration signals obtained from experiment were analyzed and compared at stable and unstable milling processes. The experimental results show that when the chatter occurs, milling forces were found to increase dramatically by 61.9–66.8% compared with that of at stable cutting; machining surface quality became poor and machined surface roughness increases by 34.2–40.5% compared with that of at stable cutting.     

Fast prediction of chatter stability lobe diagram for milling process using frequency response function or modal parameters

Prediction of chatter stability is important for planning and optimization of machining process in order to improve machining efficiency and reduce machining damage. Based on the classical analytical solution of chatter stability for milling process and in-depth analysis of the impact of modal parameters on the stability lobe diagram, a straight forward procedure for fast predicting stability lobe diagram directly using modal parameters of machining system was put forward. In consideration of the fact that the modal parameters of milling system can be estimated directly from the frequency response function using single DOF modal parameter estimation method, stability lobe diagram can be plotted directly using the tool tip’s frequency response function. The machining performances of a machining center with three different cutting tools were evaluated and the corresponding optimized cutting conditions were determined. The correctness of the proposed method was validated by good agreement of the predicted stability lobe diagram with that using the classical analytical method, and simulation results show that its calculation speed had been improved by 2–3 orders of magnitude. As a result, the proposed method of plotting stability lobe diagram using frequency response function can be utilized as an effective tool to select chatter-free cutting conditions in shop floor applications.     

型腔边界拐角精加工刀轨生成算法的研究

在识别粗加工余料区域的基础上,从均匀径向切削深度、平滑刀轨路径等方面考虑,研究并实现了型腔边界拐角处的精加工刀轨生成算法。采用将拐角区域加工分为多个循环进行渐进切削的策略,可减小径向切深。同时,每一切削循环内切削段和空程刀轨段、各循环之间均采用圆弧过渡,刀轨路径满足一阶连续,从而可减小切削力的变化幅度和方向突变,提高加工精度。