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.     

The prediction of dimensional error for sculptured surface productions using the ball-end milling process. Part 2: Surface generation model and experimental verification

This paper presents a surface generation model for sculptured surface productions using the ball-end milling process. In this model, machining errors caused by tool deflections are studied. As shown in Part 1 of this paper, instantaneous horizontal cutting forces can be evaluated from the cutting geometries using mechanistic force models. In this paper, a tool deflection model is developed to calculate the corresponding horizontal tool deflection at the surface generation points on the cutter. The sensitivity of the machining errors to tool deflections, both in magnitude and direction, has been analyzed via the deflection sensitivity of the surface geometry. Machining errors are then determined from the tool deflection and the deflection sensitivity of the designed surface. The ability of this model in predicting dimensional errors for sculptured surfaces produced by the ball-end milling process has been verified by a machining experiment. In addition to providing a means to predict dimensional accuracy prior to actual cutting, this surface generation model can also be used as a tool for quality control and machining planning.     

Tool path generation for ultra-precision machining of free-form surfaces

The generation of tool paths for ultra-precision machining is still a limiting factor in the manufacturing of parts with complex optical surfaces. In conventional machining as well as in complex five axes machining the application of CAD- and CAM-software for the generation of tool paths is state of the art. But these software solutions are not able to generate tool paths according to the high requirements of ultra-precision machining. This paper describes possible ways to generate tool paths for ultra-precision machining when the optical surface can be analytically described or when the surface data is derived from optical design software. Ultra-precision milling experiments with different tool paths have been carried out and the quality of the machined geometry has been evaluated concerning the achievable form accuracy.     

The prediction of dimensional error for sculptured surface productions using the ball-end milling process. Part 1: Chip geometry analysis and cutting force prediction

The study of machining errors caused by tool deflection in the balkend milling process involves four issues, namely the chip geometry, the cutting force, the tool deflection and the deflection sensitivity of the surface geometry. In this paper, chip geometry and cutting force are investigated. The study on chip geometry includes the undeformed radial chip thickness, the chip engagement surface and the relationship between feed boundary and feed angle. For cutting force prediction, a rigid force model and a flexible force model are developed. Instantaneous cutting forces of a machining experiment for two 2D sculptured surfaces produced by the ball-end milling process are simulated using these force models and are verified by force measurements. This information is used in Part 2 of this paper, together with a tool deflection model and the deflection sensitivity of the surface geometry, to predict the machining errors of the machined sculptured surfaces.     

Active integration of tool deflection effects in end milling. Part 2. Compensation of tool deflection

This study presents a compensation method in milling machining in order to take into account tool deflection during tool-path generation. Tool deflection that occurs during machining, and especially when flexible tools such as end mills are used, can result in dimensional errors on workpieces. The study presented here is part two of a two-part paper. In part one the cutting force models and the surface prediction method have been presented.Here the focus is on tool deflection effects' integration during the generation of the tool path. A strategy is proposed that modifies the nominal tool trajectory, compensates for the machining errors due to tool deflection, without degrading the production performance and the machined accuracy. The methodology allows optimization of the tool path trajectory in order to achieved a specified tolerance. Some experimental results are presented.     

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.     

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.     

Cutting force estimation in sculptured surface milling

Cutting force milling models developed up to now are mostly used for planar milling using end-mills. Only a reduced number of models applying ball-end mills have been developed. Furthermore these models usually only consider horizontal surface machining, even though the main application of ball-end mills is sculptured surface machining. This article proposes a model that is able to estimate the cutting forces in inclined surfaces machined both up-milling and down-milling. For this purpose a semi-mechanistic model has been developed that calculates the cutting forces based on a set of coefficients which depend on the material, the tool, the cutting conditions, the machining direction and the slope of the surface.A coordinate transformation has been included in order to consider the slope milling case with different cutting directions.The model has been tested on two materials, an aluminum alloy Al7075-T6 and a 52 HRC tool steel AISI H13. Validation tests have been carried out on inclined planes using different slopes and different machining directions.The results provide errors below 10% in most of the cases and both the value and shape of the predicted forces adjust the measured cutting force.     

The effects of spindle vibration on surface generation in ultra-precision raster milling

Spindle vibration has a significant influence on surface quality of ultra-precision-machined components. However, relatively few studies on the particular spindle vibration under the excitation of intermittent cutting forces in ultra-precision raster milling (UPRM) have been reported. In this study, a specialized model for an aerostatic bearing spindle under the impulsive excitation from intermittent cutting forces of UPRM is developed and its derived mathematical solutions reveal that the spindle vibration is impulsive response. The theoretical and experimental results signify that the impulsive spindle vibration produces inhomogeneous scallops forming ribbon-stripe patterns and irregular patterns like run-out on a surface of UPRM. The potential benefits for UPRM are the theoretical supports for optimization and prediction of surface generation through the optimal selection of spindle speed.     

Integrated planning for precision machining of complex surfaces. Part 1: Cutting-path and feedrate optimization

In order to achieve higher productivity and product quality simultaneously for sculptured surface productions, two advanced strategies are proposed for machining planning, namely a cutting-path-adaptive-feedrate strategy and a control surface strategy. In the cutting-path-adaptive-feedrate strategy, machining time is reduced by cutting along low-force-low-error machining directions and by maximizing feedrates. In the control surface strategy, machining errors are minimized by using a compensated control surface based on predicted machining errors. In part 1 of this paper, the cutting-path-adaptive-feedrate strategy, which improves the productivity of sculptured surface machining when subjected to both force and dimensional constraints, is described. In this proposed strategy, a new machining-planning aid called a maximum feedrate map is developed. In this map, the maximum allowable feedrates, subjected to the specified constraints, at each control point along various machining directions, are determined using a surface generation model. These local maximum-feedrate boundaries indicate the acceptable range of feedrates that a part programmer can use in the NC programming. In addition, the maximum feedrate map also provides the part programmer an important aid in selecting the cutting directions. In order to illustrate the application of the maximum feedrate map and to examine the capability of the proposed cutting-path-adaptive-feedrate strategy in improving the productivity of sculptured surface machining, simulation studies of a two-dimensional curved surface are performed and the results are presented in this paper. The applications of the proposed strategy to real three-dimensional complex surfaces (e.g. a turbine blade die) along with experimental verifications are presented in part 2 of this paper. In part 3 of this paper the control surface strategy and its applications to the finish milling of three-dimensional complex surfaces are discussed.     

Increasing productivity in sculpture surface machining via off-line piecewise variable feedrate scheduling based on the force system model

Sculpture surface machining is a critical process commonly used in various industries such as the automobile, aerospace, die/mold industries. Since there is a lack of scientific tools in practical process planning stages, feedrates for CNC machining are selected based on the trial errors and previous experiences. In the selections of the process parameters, production-planning engineers are conservative in order to avoid undesirable results such as chipping, cutter breakage or over-cut due to excessive cutter deflection. Currently, commonly used CAD/CAM programs use only the geometric and volumetric analysis, but not the physics of the processes, and rely on experience based cutting tool database and users’ inputs for selection of the process parameters such as feed and speed. Usually, the feeds and cutting speeds are set individual constant values all along the roughing, semi-finishing, and finishing processes. Being too conservative and setting feedrate constant all along the tool path in machining of sculpture surfaces can be quite costly for the manufacturers. However, a force model based on the physics of the cutting process will be greatly beneficial for varying the feedrate piecewise along the tool path.The model presented here is the first stage in order to integrate the physics of the ball-end milling process into the selection of the feeds during the sculpture surface machining. Therefore, in this paper, an enhanced mathematical model is presented for the prediction of cutting force system in ball end milling of sculpture surfaces. This physical force model is used for selecting varying and ‘appropriate’ feed values along the tool path in order to decrease the cycle time in sculpture surface machining. The model is tested under various machining conditions, and some of the results are also presented in the paper.     

Improving Cycle Time in Sculptured Surface Machining Through Force Modeling

In this paper, an enhanced mathematical model is presented for the prediction of cutting force system in ball end milling of sculptured surfaces. This force model is also used as the basis for off-line feed rate scheduling along the tool path in order to decrease the cycle time in sculptured surface machining. As an alternative for setting a constant feed rate all along the tool path in rough machining of sculptured surfaces, resultant cutting forces are aimed to be kept under a pre-set threshold value along the tool path by off-line scheduled piecewise variable feed rates. In this paper, it is shown that machining time, depending on complexity of sculptured surfaces, can be decreased significantly by scheduling feed rate along the tool path. The model is tested under various cutting conditions and some of the results are also presented and discussed in the paper.     

Five-axis tool orientation smoothing using quaternion interpolation algorithm

In five-axis machining, abrupt changes of tool axes may deteriorate the part surface, hence smoothing of tool orientations becomes an important issue. In the present tool path generation procedure, the tool orientation smoothing method (TOS method) is coupled with the cutting error improvement method (CEI method). TOS method is used to smooth tool orientations in order to reduce cutting errors and increase the machining efficiency. CEI method modifies existing cutter location data (CL data) so that final cutting errors are kept within the required tolerance. Experimental results show that the tool paths generated by the present procedure have better machining efficiency, better surface quality, and no interferences between the tool and part surfaces.     

Tribological behavior of micro structured surfaces for micro forming tools

Mechanical micro machining processes, like milling and grinding are appropriate technologies for the flexible production of precise molds with complex shapes for metal forming processes. In most cases machining strategies are orientated towards form accuracy of the desired forming tool only. Thus, the generation of tribologically advantageous surfaces is often carried out in subsequent machining steps like honing. In micro scale the subsequent treatment of complex surfaces is very difficult. For that reason it is desirable to create the shape and a suitable surface texture with one tool in one step.This paper is focusing on the comparison of the tribological behavior of polished surfaces with structured surfaces machined by micro milling and micro grinding processes. Micro milling tools and grinding pins with ballend shape are used to create micro structured surfaces. The machining strategy (tool path and line pitch) was varied for both tool types in the same manner. The experiments were carried out on hardened cold working steel using tungsten carbide micro cutters with TiAlN coating and micro grinding pins with an abrasive diamond layer. White light interferometry was used to characterize the machined surfaces and determine the surface parameters. Moreover, a strip drawing test was set up to investigate the tribological behavior of the system consisting of the machined surfaces and thin sheet metals. The results of the strip drawing test suggest a relationship between micro structure and tribological behavior. Finally, the dependencies between machining technology, surface parameters and tribological behavior will be discussed.     

Dynamic rotating-tool turning of micro lens arrays

Diamond micro milling of high-quality micro lens arrays suffers from low machining efficiency, due to the inevitable milling marks along tangential feed direction and the slow spiral tool path interpolated by multiple linear axes. In this article, an advanced cutting process is proposed, namely dynamic rotating-tool (DRT) turning, in which a U-axis attachment on a rotary stage is developed to enable synchronous cutter rotation and radial feed motions of a diamond turning tool. This method is experimentally verified and compared with milling, with significantly enhanced surface quality and machining efficiency, thus bringing a new perspective into ultra-precision machining.     

Influence of radial and axial runouts on surface roughness in face milling with round insert cutting tools

In face milling processes, the surface quality of the machined part depends on many factors, including feed, cutting tool geometry and tool errors. In this work, a numerical model for predicting the surface profile and surface roughness as a function of these factors is presented, incorporating a random values generation algorithm that makes it possible to determine the variation in surface roughness from the values that can be adopted by tool errors. This work is focused on round insert cutting tools and the influence of tool errors such as radial and axial runouts. The results that correspond to a number of teeth equal to 4, insert diameter of 12 mm, depth of cut of 0.5 mm, cutting speed of 120 m/min and feed of 0.4–1.4 mm/rev are analysed. Milling experiments are made to verify the validity of the model and the discrepancies between the experimental and theoretical surface profiles are assumed to be a consequence of different factors such as the variation in undeformed chip thickness along the surface profile.     

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.     

Toolpath selection based on the minimum deflection cutting forces in the programming of complex surfaces milling

In this paper, a new methodology for the selection of the milling toolpaths on complex surfaces that minimize dimensional errors due to tool defection is presented. In this way, an improvement on the accuracy of milled surfaces is achieved. The methodology can be applied to both three and five axes milling. In the three axes case, it is based on the calculation of the minimum cutting force component that is related with the tool deflection. This component has been previously defined as that perpendicular to the tool axis and contained on the plane defined by the tool axis and the normal vector to the workpiece surface.Cutting forces are calculated for each 15° sense on the tangent plane to the milled surface, in a grid of control points defined by the user, both for dowmilling and upmilling. With this information there are two possibilities. First, select a general toolpath direction that minimizes the mean value of the tool deflection force on the surface, and bearing this in mind, the CAM operator can produce a CNC program which leads to an accuracy improvement. The second option is the selection of different milling directions at each control point. Joining these points, the minimum force lines are defined on the workpiece surface. These can be used as the master guides for the toolpath programming of a complete surface.In the case of five axes milling, the approach is different, because in this case the tool-axis orientation with respect to the workpiece surface may be changed using the two rotary axes. Therefore, for each workpiece area both tool-axis orientation and machining direction can be selected to keep tool deflection force below a threshold value.Some case studies of both techniques and in-deep discussion of results are presented. Applying this approach, in three axes milling dimensional errors fall down from 30 μm to below 4 μm. In five axes milling errors can be kept below 15 μm in most of the cases.     

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.     

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.