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.     

Development of monitoring system on the diamond tool wear

Recently, ultra-precision machining using a single crystal diamond tool has been developing very rapidly, especially in the fields of production processes for optical or magnetic parts such as magnetic discs, laser mirrors, polygon mirrors and copier drums. As a result, it has been successfully extended to machine various soft materials, generating mirror-like surfaces to sub-micron geometric accuracy with the ultra-precision CNC machine and the single crystal diamond tool. With the real cutting operation, the geometric accuracy and the surface finish attainable in machined surfaces are mainly determined by both of the sharpness of a cutting tool and stability of the machine vibration. In this study, for monitoring the progress of machining state for assuring the machining accuracy and the surface quality, a new monitoring method of machining states in face-cutting with diamond tool is proposed, using the frequency response of multi-sensors signal, which includes wear state of tool in terms of the energy within the specific frequency band. A magnetic disc is machined on the ultra-precision lathe.     

基于展开轮廓三轴代码转换的轴类零件四轴加工研究

以具备复杂展开轮廓外圆柱面的轴类零件为研究对象，针对复杂展开轮廓外圆柱面四轴数控加工代码生成的不稳定、耗时长及效率低的问题，研究其展开轮廓三轴代码转换四轴代码的方法。基于四轴数控机床结构及运动控制机制，分析UG软件中复杂展开轮廓外圆柱面的数控代码生成方法。研究复杂展开轮廓三轴代码的刀位坐标与四轴代码的数据转换关系，开发轴类零件外圆柱面展开轮廓的四轴代码转换软件。通过UG及所开发代码转换软件，生成轴类零件复杂展开轮廓外圆柱面的四轴数控加工代码，并于Vericut软件中仿真验证。测试结果表明：所研究方法可直接实现轴类零件复杂展开轮廓外圆柱面的四轴数控代码生成，具备稳定性与速度效率的优势。     

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。     

Research Progress of Diamond Tool Machining Technology for Ferrous MetalsEI北大核心CSCD

金刚石刀具是超精密加工最理想的刀具之一,但在黑色金属超精密加工领域“石墨化”导致刀具快速磨损,其应用极大地受到了限制。首先,针对金刚石刀具的磨损机理进行介绍。然后,综述金刚石刀具切削黑色金属的几种常见方法,如刀具表面改性、工件表面改性、低温辅助切削、超声振动辅助切削等,通过研究实例来分析各方法的应用效果和存在问题,并从技术层面分析影响金刚石刀具在黑色金属加工领域发展的关键因素。最后,对金刚石刀具切削黑色金属未来的发展趋势进行探讨。总结金刚石刀具在黑色金属领域的加工方法,分析加工黑色金属时抑制金刚石刀具磨损的核心技术,对黑色金属的精密超精密加工具有重要的引领和推动作用。     

Ultra-precision machining of Fresnel microstructure on die steel using single crystal diamond tool

With the increasing demand for the replication of structured optical elements such as Fresnel lenses and prism arrays, more attention is being paid to the development of ultra-precision diamond machining technology for the fabrication of die steel molds. However, the machining process would be a catastrophic failure because of rapid and excessive tool wear if a diamond tool is used to machine die steel. In the present paper, a micromachining method for fabricating microstructures on die steel using single crystal diamond tool is presented. The presented technology is based on a thermochemical technique that uses plasma nitriding treatment to suppress the rapid and excessive tool wear in the diamond machining of steel. Experimental findings revealed that severe chemical tool wear, which is the main wear mechanism in the diamond machining of steel, was reduced significantly after plasma nitriding treatment, and a mirror-quality surface with an average surface roughness of 20 nm root-mean-square (RMS) was achieved over a cutting distance of approximately 5.4 km. Furthermore, a Fresnel microstructure with surface roughness RMS better than 40 nm was precisely fabricated on AISI 4140 die steel using single crystal diamond tool.     

Elliptical Vibration Cutting of Tungsten Alloy Molds for Optical Glass Parts

Elliptical vibration cutting is applied to ultra-precision machining of tungsten alloy molds for optical glass parts in the present research. The tungsten alloy is expected as a new mold material instead of conventional ones such as sintered tungsten carbide and CVD-silicon carbide. However, it cannot be finished precisely by ordinary cutting because of rapid tool wear, brittle fracture and adhesion to the tool. Therefore, the ultrasonic elliptical vibration cutting is applied to the ultra-precision machining of tungsten alloy. Practical ultra-precision molds are obtained by the elliptical vibration cutting, and they are applied to glass molding successfully.     

Spiral cutting operation strategy for machining of sculptured surfaces by conformal map approach

Although compound surfaces and polyhedral models are widely used in manufacturing industry, the tool path planning strategies are very limited for such surfaces in five-axis machining and high speed machining. In this paper, a novel conformal map based and planar spiral guided spiral tool path generation method is described for NC machining of complex surfaces. The method uses conformal map to establish a relationship between 3D physical surface and planar circular region. This enables NC operation to be performed as if the surface is plat. Then through inversely mapping a planar spiral defined by a mathematical function into 3D physical space, the spiral cutter contact paths are derived without inheriting any corners on the boundary in the subsequent interior paths. The main advantage of the proposed method is that a smoother, longer and boundary conformed spiral topography tool path is developed. Therefore, the machined surface can be cut continuously with minimum tool retractions during the cutting operations. And it allows both compound surfaces and triangular surfaces can be machined at high speed. Finally, experimental results are given to testify the proposed approach.     

Ultra-precision grinding

Ultra-precision grinding is primarily used to generate high quality and functional parts usually made from hard and difficult to machine materials. The objective of ultra-precision grinding is to generate parts with high surface finish, high form accuracy and surface integrity for the electronic and optical industries as well as for astronomical applications. This keynote paper introduces general aspects of ultra-precision grinding techniques and point out the essential features of ultra-precision grinding. In particular, the keynote paper reviews the state-of-the-art regarding applied grinding tools, ultra-precision machine tools and grinding processes. Finally, selected examples of advanced ultra-precision grinding processes are presented.     

Tool wear mechanisms and tool life enhancement in ultra-precision machining of titanium

Titanium and its alloys are generally considered as difficult-to-machine materials due to their poor thermal conductivity and high strength, which is maintained at elevated temperatures. This paper examines the tool wear mechanisms involved in ultra-precision machining of titanium. In this study single-crystal diamond tools were used to machine commercial pure titanium (CP-Ti) and Ti-6Al-4V alloy. Industrial expectations for surface quality and tool life based on optical grade applications are presented. Results obtained from the characterization of the tool, chip and workpiece led to the identification of graphitization as the mechanism that initiates tool wear. As the cutting edge rounds-off due to graphitization the rate of adhesion of the workpiece material onto the tool increased, which caused the quality of the surface finish to deteriorate. To reduce this wear mechanism a protective barrier made of Perfluoropolyether (PFPE) polymer, was explored. Tribometer studies with PFPE coated diamond tools and titanium pins showed a reduction in the coefficient of friction (COF). Subsequent machining tests using PFPE coated diamond tools showed promising results in extending the tool life and enhancing the surface quality to a point where Ti can now be considered as a viable option for applications involving optical grade surfaces.     

Surface characterization in ultra-precision machining of Al/SiC metal matrix composites using data dependent systems analysis

This paper presents a data dependent systems (DDSs) method for the analysis of surface generation in ultra-precision machining of Al/SiC metal matrix composites (MMCs). The DDS analysis provides a component by component wavelength decomposition of the surface roughness profile of the machined surface. A series of face cutting experiments was done on Al6061/15SiC_p MMCs under different cutting conditions. The cutting results indicate that the characteristics of the wavelength components analyzed by the DDS analysis method are correlated well with the surface generation mechanisms. Since the relative powers of the wavelength components are used to measure the contributions of the cutting mechanisms to the total roughness, this resolves the shortcomings of the conventional spectrum analysis method in characterizing the surface properties such as pits and cracks in ultra-precision machining of MMCs.     

A study of the diamond tool wear suppression mechanism in vibration-assisted machining of steel

Inability of machining steel strongly inhibits the application of diamond machining in manufacturing industry, especially in the fields of ultra-precision and micro machining. In recent years, vibration-assisted machining (VAM) has been proved to be capable of efficiently suppressing the diamond tool wear in cutting steel. Currently, the prevailing speculation claimed by most researchers for such suppression is that the tool–workpiece flash temperature was reduced in VAM, which would slow the chemical reaction between iron on steel and carbon on diamond. However, the correctness of this speculation has not been proved by any experimental or theoretical research. In this paper, in order to understand the true wear suppression mechanism of diamond tools in VAM of steel, a study is conducted by measuring the workpiece temperatures and modeling the cutting energy consumption in both VAM and conventional cutting (CC). Based on the comparison results, it is concluded that the cutting temperature and energy consumption in VAM are not smaller than in CC, and hence the reduced diamond tool wear in VAM should not be caused by the claimed reduced temperature, especially when the material removal rate is very small. Finally, based on the EDS analysis and the comparison of experimental results under different air pressure, two probable reasons are proposed for the significantly reduced diamond tool wear in VAM of steel: (i) increase of gas pressure at the tool–workpiece interface and (ii) generation of an oxide layer on the freshly machined surface.     

Tool path generation for 4-axis contour EDM rough machining

Contour or CNC EDM machining of free-form surfaces requires tool paths that are different from those used in mechanical milling although in geometry both processes are described by the similar model of intersection between the rotating tool and the workpiece. In this paper, special requirements on tool paths demanded by contour EDM machining are studied and a two-phase tool path generation method for 4-axis contour EDM rough milling with a cylindrical electrode is developed. In the first phase of the method, initial tool paths for virtual 3-axis milling are generated in a commercial CAD/CAM system—Unigraphics, which provides users with plenty of options in choosing suitable tool path patterns. From these tool paths, cutter contact (CC) points between electrode and workpiece are reversely calculated. In the second phase, considering the special requirements of EDM machining, which include discharging gap compensation, electrode wear compensation, DC arcing prevention, etc., the electrode is adjusted to an optimized interference-free orientation by rotating it around the CC points obtained in the previous phase. This new orientation together with the reference point of electrode is output as new tool path. The whole algorithm has been integrated into Unigraphics, machining simulations and tests have been conducted for 4-axis contour EDM rough machining.     

微圆弧金刚石刀具修磨技术研究

微圆弧金刚石刀具是超精密加工中的重要应用工具,基于机械研磨法开展微圆弧金刚石刀具修磨技术研究。通过分析微圆弧金刚石刀具制备技术现状和制备难点,设计一种微圆弧金刚石刀具的修磨技术路线。通过修磨实验解决难磨、易磨方向定位,刀尖容易崩刃及精修刀尖圆弧等技术难点,制备刀尖半径R 8.2μm的微圆弧金刚石刀具。利用原子力显微镜和高倍光学显微镜进行观测,结果表明:前刀面粗糙度R_a=1.50nm,刀尖轮廓波纹度优于0.1μm,与进口高精度刀具指标相当。将其应用在微结构的超精密切削中,取得良好效果。      

High-speed five-axis milling of hardened tool steel

This paper investigates critical issues related to high-speed five-axis milling of hardened D2 tool steel (hardness HRc 63). A forging die cavity was designed to represent the typical features in dies and molds and to simulate several effects resulting from complex tool path generation. Cutting tool materials used were coated carbide for the roughing and semi-finishing processes and polycrystalline cubic boron nitride (PCBN) for the finishing process. The effects of complex tool paths on several critical machining issues such as chip morphology, cutting forces, tool wear mechanisms, tool life and surface integrity were also investigated. The main tool failure mode was chipping due to the machine tool dynamics. A five-axis analytical force model that includes the cutter location (CL) data file for computing the chip load has been developed. The effect of instantaneous tilt angle variation on the forces was also included. Verification of the force model has been performed and adopted as a basis for explaining the difficulties involved with high-speed five-axis milling of D2 tool steel.     

Effect of micro-textured tools on machining of Ti–6Al–4V alloy: An experimental and numerical approach

In this work, an attempt is made to reduce the detrimental effects that occurred during machining of Ti–6Al–4V by employing surface textures on the rake faces of the cutting tools. Numerical simulation of machining of Ti–6Al–4V alloy with surface textured tools was employed, taking the work piece as elasto-plastic material and the tool as rigid body. Deform 3D software with updated Lagrangian formulation was used for numerical simulation of machining process. Coupled thermo-mechanical analysis was carried out using Johnson-cook material model to predict the temperature distribution, machining forces, tool wear and chip morphology during machining. Turning experiments on Ti–6Al–4V alloy were carried out using surface textured tungsten carbide tools with micro-scaled grooves in preferred orientation such as, parallel, perpendicular and cross pattern to that of chip flow. A mixture of molybdenum disulfide with SAE 40 oil (80:20) was used as semi-solid lubricant during machining process. Temperature distribution at tool–chip interface was measured using an infrared thermal imager camera. Feed, thrust and cutting forces were measured by a three component-dynamometer. Tool wear and chip morphology were captured and analyzed using optical microscopic images. Experimental results such as cutting temperature, machining forces and chip morphology were used for validating numerical simulation results. Cutting tools with surface textures produced in a direction perpendicular to that of chip flow exhibit a larger reduction in cutting force, temperature generation and reduced tool wear.     

Development of ultra-precision machining system with unique wire EDM tool fabrication system for micro/nano-machining

Aiming at quality machining of very hard materials with nanometer level surface quality and 0.1 μm dimensional accuracy, an ultra-precision machining system has been developed. The machine has feed axes accompanied by the counter motion mechanism driven at the center of gravity to eliminate the vibration caused by high acceleration/deceleration. A custom tool fabrication system consists of a 6-axis wire EDM machine and dedicated custom tool CAM system. High quality machining of sophisticated three-dimensional (3D) products made of tungsten carbide has successfully been demonstrated on the developed machine tool with custom-made PCD cutters fabricated by the tool fabrication system.     

光学微结构超精密多轴铣削加工技术

采用单点金刚石飞刀加工可以直接加工出具有纳米级的表面粗糙度和亚微米级形状精度的光学微结构元件而不需要后续处理。通过超精密飞刀加工微V沟槽的实验,分析了主轴转速、进给速度、切削深度和切削行间距对微V沟槽加工精度的影响,并对切削参数进行优化。最后,利用优化后的切削参数加工出微V沟槽结构。实验结果显示,超精密飞刀加工微V沟槽可达到满足光学微结构加工精度的要求。     

6-Axis control ultraprecision microgrooving on sculptured surfaces with non-rotational cutting tool

New optical devices with multi-functions such as diffraction and focusing have been recently required to miniaturize optical systems and to decrease cost. However, their shape is generally very complicated. Typical example is a curved microgroove on a three-dimensional complex surface. To machine the shape, a non-rotational cutting tool is employed in the study. In case of machining microgrooves on sculptured surface with a non-rotational cutting tool, 6-axis control is inevitable to control tool attitude and tool direction. The study deals with the development of CAM system for 6-axis control ultraprecision microgrooving.     

Tool path generation and tolerance analysis for free-form surfaces

This paper focuses on developing algorithms that generate tool paths for free-form surfaces based on the accuracy of a desired manufactured part. A manufacturing part is represented by mathematical curves and surfaces. Using the mathematical representation of the manufacturing part, we generate reliable and near-optimal tool paths as well as cutter location data file for post-processing. This algorithm includes two components. First is the forward-step function that determines the maximum distance, called forward step, between two cutter contact (CC) points with a given tolerance. This function is independent of the surface type and is applicable to all continuous parametric surfaces that are twice differentiable. The second component is the side step function which determines the maximum distance, called side step, between two adjacent tool paths with a given scallop height. This algorithm reduces manufacturing and computing time as well as the CC points while keeping the given tolerance and scallop height in the tool paths. Several parts, for which the CC points are generated using the proposed algorithm, are machined using a three-axes milling machine. As part of the validation process, the tool paths generated during machining are analyzed to compare the machined part and the desired part.