Theoretical and experimental analysis of nano-surface generation in ultra-precision raster milling

The fabrication of high-quality freeform surfaces is based on ultra-precision raster milling, which allows direct machining of the freeform surfaces with sub-micrometric form accuracy and nanometric surface finish. Ultra-precision raster milling is an emerging manufacturing technology for the fabrication of high-precision and high-quality components with a surface roughness of less than 10 nm and a form error of less than 0.2 μm without the need for any additional post-processing. Moreover, the quality of a raster milled surface is based on a proper selection of cutting conditions and cutting strategies.Due to different cutting mechanics, the process factors affecting the surface quality are more complicated, as compared with ultra-precision diamond turning and conventional milling, such as swing distance and step distance. This paper presents a theoretical and experimental analysis of nano-surface generation in ultra-precision raster milling. Theoretical models for the prediction of surface roughness are built. An optimization system is established based on the theoretical models for the optimization of cutting conditions and cutting strategy in ultra-precision raster milling. A series of experiments have conducted and the results show that the theoretical models predict well the trend of the variation of surface roughness under different cutting conditions and cutting strategies.     

超精密车削工艺参数与表面质量关联性研究

为提高铝反射镜超精密加工的表面质量,介绍超精密切削的表面质量评价方法和影响表面质量的因素,分析各因素对表面质量的影响机制。采用单一变量法对Al6061进行不同工艺参数的单点金刚石车削实验,通过对比表面粗糙度和功率谱密度,研究各个工艺参数对表面质量的影响,并得出最佳工艺参数;最终加工出表面粗糙度为4.67 nm的光学表面。     

An investigation into surface generation in ultra-precision raster milling

This paper presents a study of the factors that affect surface generation in ultra-precision raster milling. A series of experiments was conducted to study the effect of different factors on surface generation in ultra-precision raster milling. The results indicate that machining parameters, tool geometry, cutting strategy, and tool wear are the critical factors for achieving super mirror finish surfaces, while cutting strategies, tool path generation, and kinematic errors of the slides are vital to the form accuracy of freeform surfaces. The experimental results are useful for the diagnosis of systematic errors in machine tools, and the control of machining errors. Compensation strategies can be devised, and improvement can be made in the optimization of surface generation and hence the surface quality when using ultra-precision raster milling can be improved.     

在线电解修整(ELID)超精密磨削中的金刚石磨具特性研究

ELID(Electrolytic In-process Dressing)磨削技术是在电化学加工、电解磨削原理基础上发展起来的一项磨削新技术,主要用于硬脆材料超精密磨削过程中金属基结合剂超硬微细磨粒砂轮的在线修整.本文以金刚石微粉砂轮在线电解修整(ELID)磨削氮化硅陶瓷为例,着重研究了磨具特性对硬脆材料超精密磨削过程的影响.研究表明,磨具组织沿砂轮圆周的不均匀性将会导致砂轮表面钝化膜状态的不一致,这将直接影响砂轮局部参与切削的磨粒数量,影响单个磨料的实际磨削厚度.这首先将对工件表面的磨削质量,特别是对表面粗糙度产生直接影响,同时也非常不利于实现材料的高效去除.     

A study of materials swelling and recovery in single-point diamond turning of ductile materials

This paper presents an investigation of the effect of materials swelling in ultra-precision machining of ductile materials. The combined influence of materials swelling and recovery was found to affect the surface roughness in single-point diamond turning. It is interesting to note that the effect of materials swelling for ductile materials would be overwhelmed by the impact of recovery when the depth of cut is extremely small and the front clearance is small. In addition, radically different surface roughness profiles were found for different materials even though they are machined under the same cutting conditions. The difference in the machining behaviour could not be accounted by the elastic recovery alone but by the plastic deformation induced in the machined layer. The findings in the present study provide an important means for improving the surface roughness in ultra-precision machining.     

纵扭复合超声振动铣削SiCp/Al表面质量研究

SiCp/Al复合材料具有优异的性能,在航天航空、光学行业、汽车工业等高科技领域得到了广泛应用,但它在塑性和硬度之间差距巨大,使得超精密加工显得非常困难。建立超声铣削动力学模型,采用单因素法检测分析了SiCp/Al复合材料在不同主轴转速、铣削速度和铣削深度下的表面粗糙度与表面形貌,建模仿真了纵扭复合超声振动刀刃铣削轨迹,得到了影响加工表面质量规律及机制。研究发现主轴转速为3000 r/min、铣削速度为180 m/min时,表面粗糙度值最小;材料表面质量随铣削深度的增加而下降。为SiCp/Al复合材料铣削加工提供了合理工艺参数,提高了加工效率,降低了刀具磨损,延长了刀具使用寿命。     

Analysis of a free machining α+β titanium alloy using conventional and ultrasonically assisted turning

Rapid advancements in power generation and aviation industries have witnessed a widespread use of titanium and its alloys in many applications. This is primarily due to their excellent mechanical properties including, amongst other, high strength-to-density ratio, outstanding fatigue properties and corrosion resistance with the ability to withstand moderately high temperatures. However, this combination of properties results in poor machinability of the material, increasing the cost of components machined with conventional cutting techniques. Recently, Ti 6Al 2Sn 4Zr 6Mo, a modern titanium alloy with improved mechanical properties, has been introduced as a possible replacement of Ti 6Al 4V in aerospace industry. However, its poor machinability and formation of long chips in conventional turning are main limitations for its wide-spread application. Therefore, a new alloy based on Ti 6Al 2Sn 4Zr 6Mo, namely Ti 6Al 7Zr 6Mo 0.9La, was developed; it shows enhanced machinability generating short chips during metal cutting, which prevents entanglement with cutting tools improving productivity. To further enhance the machinability of this material, a novel hybrid machining technique called ultrasonically assisted turning (UAT) was used. Experimental investigations were carried out to study the machinability, chip shapes, cutting forces, temperature in the process zone and surface roughness for conventional and ultrasonically assisted turning of both alloys. UAT shows improved machinability with reduced nominal cutting forces, improved surface roughness of the machined workpiece and generation of shorter chips when compared to conventional machining conditions.     

Modeling of dynamic characteristic of the aerostatic bearing spindle in an ultra-precision fly cutting machine

Axis orientation stability of aerostatic bearing spindles has great influence on machining precision of ultra-precision fly cutting machines used for processing ultra-precision optical components of large diameter. Mid-spatial frequency errors (amplitude<0.1 μm, wavelength about 100 nm) always existed on the machined surfaces along feeding direction. Generally, the waviness errors on processed surfaces will impact the performance of workpiece used as optical components greatly, and the tilting motions of spindles were believed to be the main source which produced the waviness errors. In this paper, to study the tilting motions of spindles, the Euler dynamic equations of angular displacements of spindles were proposed, and analytic solutions of the equations were also presented. At the same time, the 3D surface profile simulations of workpieces based on analytic solutions of Euler equations were achieved. The simulation results have been verified by lots of experiments on an ultra-precision fly cutting machine. At last, the inertia tensor criterion which can decrease the waviness errors of machining surface was represented, and it can be applied to instruct the structure design of aerostatic bearing spindles.     

Examination of wire electrical discharge machining of Al2O3p/6061Al composites

Alumina particle reinforced 6061 aluminum matrix composites (Al₂O₃p/6061Al) have excellent physical and chemical properties than those of a traditional metal; however, their poor machinability lead to worse surface quality and serious cutting tool wear. In this study, wire electrical discharge machining (WEDM) is adopted in machining Al₂O₃p/6061Al composite. In the experiments, machining parameters of pulse-on time were changed to explore their effects on machining performance, including the cutting speed, the width of slit and surface roughness. Moreover, the wire electrode is easily broken during the machining Al₂O₃p/6061Al composite, so this work comprehensively investigates into the locations of the broken wire and the reason of wire breaking.The experimental results indicate that the cutting speed (material removal rate), the surface roughness and the width of the slit of cutting test material significantly depend on volume fraction of reinforcement (Al₂O₃ particles). Furthermore, bands on the machined surface for cutting 20 vol.% Al₂O₃p/6061Al composite are easily formed, basically due to some embedded reinforcing Al₂O₃ particles on the surface of 6061 aluminum matrix, interrupt the machining process. Test results reveal that in machining Al₂O₃p/6061Al composites a very low wire tension, a high flushing rate and a high wire speed are required to prevent wire breakage; an appropriate servo voltage, a short pulse-on time, and a short pulse-off time, which are normally associated with a high cutting speed, have little effect on the surface roughness.     

Characterisation of nanosurface generation in single-point diamond turning

This paper describes a parametric analysis of nanosurface generation in single-point diamond turning (SPDT). The properties of the surface roughness profiles were extracted and analysed using the power spectrum analysis method. A series of face cutting experiments was undertaken on an aluminium alloy under various cutting conditions. The results indicate that the power spectrum of a surface roughness profile is basically composed of several periodical components that can be correlated to different process parameters and mechanisms of surface generation. Moreover, it is found that the tool feed, tool geometry, spindle error motions and relative vibration between the tool and the workpiece are not the only dominant components contributing to the surface generation in SPDT. Materials swelling and tool interference are other important factors. Based on these findings, relationships are proposed to explain the influence of tool interference on the variation of the spectral components and process parameters. The implications of these findings on the optimisation of the surface quality in SPDT are also discussed.     

超声振动磨削放电复合加工SiCp／Al试验研究

针对SiCp／Al的加工,提出一种超声振动磨削放电复合加工的方法．从加工效率、加工稳定性及表面质量等方面与电火花加工进行了对比试验研究。分析了两种加工方法的脉冲宽度和峰值电流对加工速度和表面粗糙度的影响,结果表明：电火花加工的表面粗糙度平均值为尺04．5μm,超声振动磨削放电复合加工的表面粗糙度平均值为Ra2μm：超声振动磨削放电复合加工的稳定性比电火花加工好,但加工速度较低。通过扫描电镜对两种加工方法下零件表面形貌和重熔层进行了观测,对试件表面进行了X射线衍射分析,表明采用超声振动磨削放电复合加工SiCp／Al复合材料可获得较好的表面质量。     

Modeling of chip–tool interface friction to predict cutting forces in machining of Al/SiCp composites

In Al/SiCp metal matrix composites, in addition to machine, tool and process-related parameters, a change in composition (size and volume fraction of reinforcement) has a influence on machining force components. In the analytical models in the literature, the effect of abrasive reinforcement particles, which affects the coefficient of friction and consequently the friction angle, has not been considered while predicting cutting forces in machining of MMCs. In this paper, chip–tool interface friction in machining of Al/SiCp composites has been considered to involve two-body abrasion and three-body rolling caused due to presence of reinforcements in composites. The model evaluates resulting coefficient of friction to predict the cutting forces during machining of Al/SiCp composites using theory of oblique cutting. Further, the model considers various frictional forces on the wiper geometry on the cutting edge that has been found to improve the integrity of machined surface on composites. The predicted cutting force values were found to agree well with the corresponding experimental values for finer reinforcements composites with the assumption that 40% of the reinforcement particles contribute to the abrasion at chip–tool interface. However, for the coarser reinforcement composites, assumption that the 60% of the particles contribute to the abrasion yields better results.     

碳纤维/树脂基复合材料铣削表面粗糙度及表面形貌研究

目的研究了CFRP材料铣削加工过程中,部分主要工艺对CFRP材料加工表面质量的影响规律,为工艺参数优化,提高此类零件的表面质量提供依据。方法设计了CFRP材料铣削中的切削参数、刀具结构、加工方法与加工表面粗糙度及表面形貌之间的单因素试验。通过单调改变一个切削参数而其余切削参数不变,得到了工件表面粗糙度和表面形貌随切削参数、刀具结构、加工方法的变化规律。结果当铣削速度增大时,工件的表面粗糙度变化不大,表面微坑缺陷的数量却有所增加,但变小、变浅。当进给速度增大时,工件表面粗糙度呈上升趋势,表面缺陷也随之增加。无涂层多齿刀具铣削后的工件表面粗糙度最大,其次是金刚石涂层多齿刀具铣削的工件,最小的是金刚石涂层交错齿刀具铣削的工件。多齿刀具加工后的表面有较多的微坑缺陷,但普遍深度较浅且面积较小。交错齿刀具对分层缺陷的抑制作用最明显,但在左旋和右旋刀齿交错处容易出现较严重的加工缺陷。与普通机械加工方法相比,超声振动加工方法得到的工件表面质量较好,可以有效减少表面微坑缺陷,改善CFRP铣削加工表面质量。结论 CFRP材料铣削加工时,为了获得较好的加工表面质量,切削参数应选用较高的切削速度和较低的进给速度,切削刀具宜选用多齿带涂层刀具。和普通机械加工方法相比,超声振动铣削加工方法更为有利于获得好的表面质量。     

Investigation on the influence of tool-tip vibration on surface roughness and its representative measurement in ultra-precision diamond turning

High frequency tool-tip vibration, in the cutting force direction, is an intrinsic feature of ultra-precision single point diamond turning (SPDT). This paper is dedicated to a study of the influence of the tool-tip vibration on surface roughness. The resulting periodic fluctuation of the surface profile is identified in a particular spatial frequency range by a tangential measurement method. The ISO standard provides merely a minimal, not an optimal requirement for surface measurement. Thus, in this paper, a more representative measurement method is proposed to better characterise the machined surface in SPDT. The conventional radial and aerial surface measurements yield a relatively biased result on surface roughness which cannot adequately reflect the detrimental effect of tool-tip vibration. Representative measurement takes account of the sample area ratios and is able to be used to objectively study the discrepancies in surface measurement. The proposed model for surface generation and representative measurement are applicable to the problems in surface generation in ultra-precision SPDT, such as the spiral turning marks and spatial errors in the radial profile measurement.     

A theoretical and experimental investigation of surface roughness formation in ultra-precision diamond turning

In this paper, a model-based simulation system is presented for the analysis of surface roughness generation in ultra-precision diamond turning. The system is based on a surface roughness model which takes into account the effect of tool geometry, process parameters and relative tool-work vibration. It is evaluated through a series of cutting experiments. The results indicate that the system can predict well the surface roughness profile and the roughness parameters of a diamond turned surface under various cutting conditions. With the use of the spectrum analysis techniques, the system can also help to analyze the effect of vibration on the surface quality of workpiece and to diagnose the machine faults. The potential application of the system in process optimization is also discussed in the text.     

端面超精密车削中的面型误差辨识技术研究

为分析超精密车削过程中典型干扰因素对面型精度的影响机制以及这些干扰因素所造成面型误差的特性，基于国产超精密机床Nanosys600对Al6061进行单点金刚石车削实验。通过切削-面型轮廓测量-面型轮廓滤波-分析的迭代过程，辨识和排查了加工过程中的误差来源。在进行环境温度调控和更换导轨工作区域后，滤波后的面型轮廓波峰波谷差值由1 510 nm降低到203 nm。     

预测数控加工表面粗糙度的解析模型研究

解析模型是基于刀具切削刃包络面形成的原理来研究零件表面形貌的形成.在解析模型的基础上研究球头刀铣削过程的零件表面生成机理、分析影响加工表面粗糙度大小的因素以及表面粗糙度的趋势,进而预测表面粗糙度,有助于数控加工条件的最优化.本文利用计算机图形学算法进行建模,该模型能够仿真已加工表面轮廓的形成和表面形貌的可视化、预测表面粗糙度和评估加工过程参数的合理性.     

Study of the correlation between surface roughness and cutting vibrations to develop an on-line roughness measuring technique in hard turning

Surface roughness is one of the important factors in all areas of tribology and in evaluating the quality of a machining operation. In order to achieve computer controlled machining in a flexible manufacturing system, a technique is needed for the on-line, real-time monitoring of each machining process parameter that affects surface roughness. As the first step in developing such an algorithm, research on the correlation between surface roughness and cutting vibration is reported in this paper. The algorithm utilizes the relative cutting vibrations between tool and workpiece, which are measured through an inductance pickup and filtered by an analog bandpass filter for a specific vibrational component. The cutting vibration signals of a specific :frequency are superimposed onto the kinematic roughness, which is calculated by the tool edge radius and feed rate. Experimental results show good correlation between the simulated roughness obtained using the proposed algorithm and the roughness actually measured with a surface profilometer     

Statistical analysis of surface roughness and cutting forces using response surface methodology in hard turning of AISI 52100 bearing steel with CBN tool

The present work concerns an experimental study of hard turning with CBN tool of AISI 52100 bearing steel, hardened at 64 HRC. The main objectives are firstly focused on delimiting the hard turning domain and investigating tool wear and forces behaviour evolution versus variations of workpiece hardness and cutting speed. Secondly, the relationship between cutting parameters (cutting speed, feed rate and depth of cut) and machining output variables (surface roughness, cutting forces) through the response surface methodology (RSM) are analysed and modeled. The combined effects of the cutting parameters on machining output variables are investigated while employing the analysis of variance (ANOVA). The quadratic model of RSM associated with response optimization technique and composite desirability was used to find optimum values of machining parameters with respect to objectives (surface roughness and cutting force values). Results show how much surface roughness is mainly influenced by feed rate and cutting speed. Also, it is underlined that the thrust force is the highest of cutting force components, and it is highly sensitive to workpiece hardness, negative rake angle and tool wear evolution. Finally, the depth of cut exhibits maximum influence on cutting forces as compared to the feed rate and cutting speed.     

A theoretical and experimental investigation into five-DOF dynamic characteristics of an aerostatic bearing spindle in ultra-precision diamond turning

In ultra-precision diamond turning (UPDT), spindle vibration has great influence on machining precision of high precision optical components. However, the spindle-vibration mechanism has not been fully understood. In this study, mathematical solutions for a proposed five-degree-of-freedom (FDOF) dynamic model of an aerostatic bearing spindle are derived to explore natural mechanisms of spindle vibration. Thus, the potential benefits of the solutions are to be applied for the prediction and optimization of the effects of spindle vibration on surface generation. Its dynamic characteristics possess three translational frequencies along the radial and axial directions, a spindle rotational frequency (SRF), and a pair of coupled tilting frequencies (CTFs) around the radial directions influenced the SRF. The theoretical results are identified by the frequency characteristics of thrust cutting forces, and the periodic, concentric, spiral, radial and two-fold patterns (PCSRPs) of the machined and simulated surface topographies, respectively.