Design,evaluation and optimisation of cutter path strategies when high speed machining hardened mould and die materials

The use of high speed milling (HSM) for the production of moulds and dies is becoming more widespread. Critical aspects of the technology include cutting tools, machinability data, cutter path generation and technology. Much published information exists on cutting tools and related data (cutting speeds, feed rates, depths of cut, etc.). However, relatively little information has been published on the optimisation of cutter paths for this application. Most of the research work is mainly focused on cutter path generation with the main aim on reducing production time. Work with regards to cutter path evaluation and optimisation on tool wear, tool life, surface integrity and relevant workpiece machinability characteristics are scant. Therefore, a detailed knowledge on the evaluation of cutter path when high speed rough and finish milling is essential in order to improve productivity and surface quality. The paper details techniques used to reduce machining times and improve workpiece surface roughness/accuracy when HSM hardened mould and die materials. Optimisation routines are considered for the roughing and finishing of cavities. The effects of machining parameters notably feed rate adaptation techniques and cutting tools are presented.     

Optimization of machining conditions for multi-tool milling operations

The variables affecting the economics of machining operations are numerous and include machine tool capacity, required workpiece geometry, cutting conditions of velocity, feed rate, depth of cut and many others. This paper describes a procedure to calculate the machining conditions for milling operations according to minimum production cost as the objective function. Optimum values of machining conditions for each pass are determined based on the objective function criteria by circular direction search method which is specifically developed for this purpose. The effects of constraints on the objective function can be evaluated by graphical representation of the objective function and the constraints in the developed software.     

Robust feedrate control for high-efficiency machining using quantitative feedback theory

ABSTRACT

This study used quantitative feedback theory (QFT) to design a robust controller suitable for various cutting conditions. First, servo dynamics and milling process models were identified. The nominal plant was defined for milling process conditions of 20-mm cutting depth, 500-rpm spindle speed, and 1-mm cutting width, and the workpiece material was aluminum. Different milling conditions can result in different milling process models, which are regarded as perturbed plants. The differences between nominal and perturbed plants were treated as system uncertainties and were applied in designing the QFT controller to ensure that robust performance could be guaranteed under various milling conditions. Instead of using force measured by a dynamometer, the robust controller adjusted feedrate according to the spindle current, which was easy to measure online. The experimental results illustrated that the cutting force could be maintained at a constant level at different cutting depths, and machining time could be reduced by over 20% compared with uncontrolled cases. The proposed controller’s robustness was validated by testing different tools, workpiece materials, and cutting parameters in different experiments.     

Multi-attribute optimization of end milling epoxy granite composites using TOPSIS

Epoxy granite composites are identified and recognized as better materials for machine tool applications due to inherent damping properties. However, end milling of these composites has not been explored much. Milling of epoxy granite composites presents a number of problems, namely, cutting forces and surface roughness appear during machining. This research work focuses on end milling of epoxy granite composite specimens using high-speed steel end mill cutter by varying the cutting conditions such as spindle speed and feed with a uniform depth of cut and selection of optimal machining parameters. The experimental runs of 27 different trials were carried out and three different attributes such as thrust force, tangential force, and surface roughness were analyzed. This research work presents a sequential procedure for machining parameters selection. Selection of optimal machining parameters is done on the basis of Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) method.     

Evaluation of advanced alumina-based ceramic tool inserts when machining high-tensile steel

Three types of alumina-based ceramic tools (zirconia toughened, titanium-carbide reinforced and silicon-carbide-whisker reinforced) were used to evaluate their cutting performance when machining a high-tensile steel. Experimental studies were carried out at various cutting speeds, feeds and depths of cut, in dry conditions. The cutting performance of the alumina-based ceramic tools was judged by the cutting force produced during the process of machining, by the surface roughness of the workpiece and by the wear rate of the cutting inserts. The influence of the cutting parameters (that is, the cutting speed, feed rate and depth of cut) on the cutting performance is discussed.     

Prediction of cutting force based on machining parameters on AL7075-T6 aluminum alloy by response surface methodology in end milling

Cutting forces modeling is the basic to understand the cutting process, which should be kept in minimum to reduce tool deflection, vibration, tool wear and optimize the process parameters in order to obtain a high quality product within minimum machining time. In this paper a statistical model has been developed to predict cutting force in terms of geometrical parameters such as rake angle, nose radius of cutting tool and machining parameters such as cutting speed, cutting feed and axial depth of cut. Response surface methodology experimental design was employed for conducting experiments. The work piece material is Aluminum (Al 7075-T6) and the tool used is high speed steel end mill cutter with different tool geometry. The cutting forces are measured using three axis milling tool dynamometer. The second order mathematical model in terms of machining parameters is developed for predicting cutting forces. The adequacy of the model is checked by employing ANOVA. The direct effect of the process parameter with cutting forces are analyzed, which helps to select process parameter in order to keep cutting forces minimum, which ensures the stability of end milling process. The study observed that feed rate has the highest statistical and physical influence on cutting force.     

Extraction of milling know-how from NC programs through reverse engineering

To automate machining process planning, the acquisition and representation of machining knowledge or know-how in a reusable way is needed. The machining know-how is implied in NC programs made by experienced workers. In this paper, the methodology and system for extracting machining know-how in milling operations have been developed. With the system, machining features, operations and their associated cutting conditions (depth of cut, feed rate and spindle speed, etc.) and machining method can be extracted by analysing NC programs in conjunction with the tools used and workpiece blank model. The milling know-how is represented as a collection of these extracted data that can be used in future machining operations. To verify the system, actual NC programs for milling have been analysed and the milling know-how has been extracted successfully.     

铝基碳化硅精密铣削刀具磨损实验研究

针对铝基碳化硅切削加工中刀具易磨损、寿命低、切削难度大和加工成本高等问题,选用不同材料的硬质合金铣刀及金刚石铣刀进行切削加工实验,并利用扫描电镜和工具显微镜对高体积分数铝基碳化硅铣削时刀具磨损形态进行了分析研究.研究表明:硬质合金刀具前刀面和刃口磨损主要形式为粘结磨损和微崩刃,后刀面磨损主要为刻划磨损,而金刚石铣刀加工时刀具磨损很小;YG6X铣刀材料微观组织致密,抗磨损能力较强,宜粗加工时选用;金刚石刀体的硬度远大于SiC颗粒,且金刚石与工件的摩擦系数小,金刚石铣刀寿命远大于硬质合金铣刀,宜精加工时选用.     

多齿铣刀侧铣加工多层CFRP铣削力的建模与仿真

由于碳纤维增强树脂基复合材料(CFRP)的层间结合强度较低,进行切削加工时在切削力的作用下容易出现分层和毛刺等质量缺陷。因此,通过对切削力的预测与控制可以有效提高加工质量。采用瞬时刚性力模型对多齿铣刀侧铣多层CFRP材料的加工过程进行铣削力建模与仿真,分析了多齿铣刀特有的几何结构对切削力的影响。试验中保持切削速度恒定,以不同进给速度分别对45°、0°、-45°和90°这4种典型纤维方向的单向CFRP进行侧铣加工,通过测得的切削力数据计算各自的铣削力系数。根据力学矢量叠加原理得到了多向CFRP铣削力系数的简化计算表达式,最后将计算结果代入铣削力模型得到了各时刻的铣削力仿真值。在同样的试验条件下对该多向CFRP进行侧铣加工验证试验,试验结果表明: 该模型能较好地预测铣削力,最大相对误差小于9%,平均相对误差小于5%,可为铣削参数优化和刀具结构优化提供理论基础。     

Performance Improvement of End Milling Using Graphite as a Solid Lubricant

In any machining operation, the use of coolants is essential to dissipate heat generated during machining and hence to improve productivity, machinability, etc. However, the use of cutting fluids in machining operations may seriously degrade the quality of environment. New cutting techniques are to be investigated to alleviate the problems associated with wet machining. To overcome some of the problems, an attempt has been made to use graphite as a solid lubricant. This paper deals with an investigation on using graphite as a solid lubricant to reduce the heat generated at the milling zone. An experimental setup has been developed to direct graphite powder continuously onto the workpiece and tool interface at the required flow rate. Experimental studies have been conducted to see the effect of tool geometry (radial rake angle and nose radius) and cutting conditions (cutting speed and feed rate) on the machining responses such as cutting forces, specific energy, and surface finish in solid lubricant assisted machining using four fluted solid coated carbide end mill cutters. Results indicate that there is a considerable improvement in the performance of milling AISI 1045 steel using graphite as a solid lubricant when compared with machining using cutting fluids in terms of specific energy requirements, cutting force, and surface finish.     

Acoustic emission analysis for tool wear monitoring in face milling

Monitoring the condition of the cutting tool in any machining operation is very important since it will affect the workpiece quality and an unexpected tool failure may damage the tool, workpiece and sometimes the machine tool itself. Advanced manufacturing demands an optimal machining process. Many problems that affect optimization are related to the diminished machine performance caused by worn out tools. One of the most promising tool monitoring techniques is based on the analysis of Acoustic Emission (AE) signals. The generation of the AE signals directly in the cutting zone makes them very sensitive to changes in the cutting process. Various approaches have been taken to monitor progressive tool wear, tool breakage, failure and chip segmentation while supervising these AE signals. In this paper, AE analysis is applied for tool wear monitoring in face milling operations. Experiments have been conducted on En-8 steel using uncoated carbide inserts in the cutter. The studies have been carried out with one, two and three inserts in the cutter under given cutting conditions. The AE signal analysis was carried out by considering signal parameters such as ring down count and RMS voltage. The results show that AE can be effectively used to monitor tool wear in face milling operation.     

虚拟五轴侧铣加工过程建模与仿真分析

提出一种面向五轴侧铣加工的球头刀几何与力学模型,利用该综合模型可对复杂曲面加工进行几何仿真与切削力预测,对加工过程进行速度规划．切削工件几何模型采用深度元素法,通过与刀具模型之间的布尔操作实现材料去除过程,通过分析刀具与工件接触区域,得到刀具在不同瞬时的轴向范围及沿轴向各微元的切入角和切出角,转换为铣削力模型所需的几何信息．采用序列二次规划法优化进给速度,使机床工作在多个约束条件的最大潜能值内．整个规划过程随刀具与工件接触区域的变化而不断优化,研究工作为五轴侧铣加工过程提供了有力的分析工具．双叶片五轴侧铣加工仿真实验表明,在满足叶片精度条件下,采用优化进给速度可使零件加工效率提高约40%．     

Optimal selection of cutting parameters in multi-tool milling operations using a genetic algorithm

In the milling of large monolithic structural components for aircraft, 70–80% of the total cut volume is removed using high-speed roughing operations. In order to achieve the economic objective (i.e. optimal part quality in minimal machining time) of this process, it is necessary to determine the optimal cutting conditions while respecting the multiple constraints (functional and technological) imposed by the machine, the tool and the part geometry. This work presents a physical model called GA-MPO (genetic algorithm based milling parameter optimisation system) for the prediction of the optimal cutting parameters (namely, axial depth of cut (a _p), radial immersion (a _e), feed rate (f _t) and spindle speed (n)) in the multi-tool milling of prismatic parts. By submitting a preliminary milling process plan (i.e. CL data file) generated by CAM (computer-aided manufacturing) software, the developed system provides an optimal combination of process parameters (for each machining feature), respecting the machine–tool–part functional/technological constraints. The obtained prediction accuracy and enhanced functional capabilities of the developed system demonstrate its improved performance over other models available in the literature.     

Experimental investigation of cutting parameters in machining of 100Cr6 with tangential turn-milling method

The turn-milling methods for machining operation have been developed to increase efficiency of conventional machines recently. These methods are used especially by coupling some apparatuses on the computer numerical control (CNC) machine to decrease the production time and machine costs, ensure the maximum production and increase the quality of machining. In this study, 100Cr6 bearing steel extensively used in industry has been machined by tangential turn-milling method. This paper presents an approach for optimization of the effects of the cutting parameters including cutter speed, workpiece speed, axial feed rate, and depth of cut on the surface roughness in the machining of 100Cr6 steel with tangential turn-milling method by using genetic algorithm (GA). Tangential turning-milling method has been determined to have optimum effects of cutting parameters on the machining of 100Cr6 steel. The experimental results show that the surface roughness quality is close to that of grinding process.     

Optimal cutter size determination for 2½-axis finish machining of NURBS profile parts

A curve model of non-uniform rational B-spline (NURBS) has been widely adopted in mainstream CAD/CAM software systems to design complicated geometries of mechanical parts, for example, the curved profiles of pockets, sides, and islands. NURBS profile parts (the profiles include NURBS curves for pockets and islands) are produced in 2½-axis rough and finish machining. In rough machining of the parts, several end-mills with different sizes are employed for high cutting efficiency, and in finish machining, a single end-mill is usually used to cut along the profiles for high surface quality. To accurately produce the geometries with NURBS curves in finish machining, the cutter size should be optimised in order to eliminate gouging and save machining time. Although this topic has been a research focus for a decade, optimal cutter size determination still remains as a technical challenge. To rise to this challenge, our work proposes a new approach to determining the largest allowable size for the cutter to move along all the profiles (including NURBS curves) in 2½-axis finish machining without global and local gouging. The salient feature of this approach is that an original model of the cutter size is formulated and an effective solver–the particle swarm optimisation method–is employed to compute the largest allowable cutter size. This intelligent approach is more efficient and accurate than the conventional computational method based on the test examples in this work. It can also be applied to global and local gouging detection for NURBS profile machining. Our research work has great potential to advance CNC machining techniques.     

New method of tool orientation determination by enveloping element for five-axis machining of spatial cam

A new methodology is presented for the determination of a feasible tool orientation of a ball-end milling cutter for collision and gouging avoidance in five-axis machining of spatial cam. Since the meshing element is used as a generating element for a spatial cam, the meshing element is in tangency with the spatial cam. The notion of the proposed collision-free method is that the ball-end milling cutter is confined within the meshing element. Based on envelope theory, homogeneous coordinate transformation and differential geometry, curvatures of the cam surface and the cutting tool are evaluated for interference checking. To evaluate machining efficiency, the contact length is calculated for various tool orientations. The toolpath is verified through a solid cutting simulation. The proposed methodology can be used to automate the programming of tool paths for five-axis machining of spatial cam.     

Experimental Study of Surface Roughness and Tool Flank Wear during Hybrid Milling

Two advanced machining methods such as thermally enhanced machining and ultrasonic-assisted machining are recently considered in many studies. In this article, a new hybrid milling process is presented by gathering the characteristics of these two methods. In order to determine the axial depth of cut and engagement in the process, three-dimensional thermal finite-element analysis is applied to determine the dimensions of softened materials. Finite-element modal analysis is used to determine the dimensions and clamping state of the workpiece while cutting area has the highest vibration amplitude. Full factorial experimental design is applied to investigate the effect of hybrid machining parameters on the surface roughness and tool wear. Tool flank wear was investigated under the condition of constant cutting speed during different period of times. Hybrid milling process with an amplitude of 6 µm and a temperature of 900°C creates a surface with 42% lower roughness in comparison to conventional milling in feed 0.08 mm/tooth. In a study of tool flank wear, the results show that application of TEUAM decreases flank wear at least 16% in comparison to all other processes.     

Prediction of machining chatter in milling based on dynamic FEM simulations of chip formation

Chatter vibration is a major obstacle in achieveing increased machining performance. In this research, a finite element model of chip formation in a 2D milling process is used to predict the occurrence of chatter vibrations, and to investigate the effects of various machining parameters on this phenomenon. The dynamic properties of the machine tool at the tool tip are obtained based on experimental modal analysis, and are used in the model as the cutter dynamics. The model allows for the natural development of vibration as the result of the chiptool engagement, and accounts for various phenomena that occur at the chip-tool interface ultimately leading to stable or unstable cutting. The model was used to demonstrate the effects of the machining parameters, such as the axial depth of cut, radial immersion, and feed rate, on the occurrence of chatter. Additionally, the phenomenon of jumping out of the cut region could be observed in this model and its effect on the chatter process is demonstrated. The numerical model is verified based on comparisons with experimental results.The full text can be downloaded at https://link.springer.com/content/pdf/10.1007%2Fs40436-018-0228-7.pdf     

Two-dimensional representation of machining geometry and tool path generation for ball-end milling of sculptured surfaces

This article applies a two-dimensional representation of the machining geometry relevant to tool path generation for the three-axis ball-end milling of sculptured surfaces. A two-dimensional geometric model detecting the machined strip is suggested as the concept for the ‘effective cutting profile’ which fits well into the three-dimensional machining geometry. The model is the same as the intersection of the cutter with the plane perpendicular to the tangent direction of the cutter location curve and incident with the cutter location point. In order to achieve the specified machining accuracy, an iterative approach is needed. The paper also presents a new iterative method to generate tool paths with a constant scallop height. It is based on the proposed model which resorts to a two-dimensional representation of the three-dimensional machining geometry. The proposed method reduces significantly the computing time to generate tool paths. Implementations and illustrated examples are discussed.