Effect of the components of Magnetic Compound Fluid (MCF) slurry on polishing characteristics in aspheric-surface finishing with the doughnut-shaped MCF tool

For the aspheric optical surface finished with a doughnut-shaped MCF (magnetic compound fluid) polishing tool, the performance of the polishing tool depends mainly on the properties of the MCF slurry. Therefore, understanding the effect of each MCF slurry component on the polishing characteristics is crucial to developing novel polishing techniques. In this paper, the polishing principle was depicted and the corresponding polishing jig was constructed with a six-degree-of-freedom manipulator. The conical surfaces, which were considered as special aspheric surfaces, were experimentally polished under proper polishing conditions to examine the effects of the carbonyl iron particles (CIPs) concentration and the sizes of the abrasive particles (APs) on the polishing ability to remove material/tool marks and improve work surface qualities. Theoretical analyses were also performed to gain a more comprehensive understanding of the behaviors of CIPs and APs in the magnetic field. The results were shown as follows: (1) The CIPs concentration affected positively the magnetization of the MCF slurry, leading to better performance in the removal rate of material/tool marks when a higher CIPs concentration of was applied. The best surface quality was attained with a CIPs concentration of 45 wt%. (2) Larger APs were beneficial for obtaining higher removal rates of material/tool marks. The APs with 1 μm in diameter were preferred for achieving a better surface quality. (3) Ferric clusters were formed along the magnetic line of force and their orientations changed periodically to stir the APs with the magnet revolution. (4) The Aps, at a given working gap, can squeeze the work-surface. The squeezing action was much more intense when larger APs and the MCF slurry with a higher magnetization were employed. (5) The material removal model suggested that the material was removed due to the APs and the relative motion between the work-surface and APs.     

A novel polishing method for single-crystal silicon using the cavitation rotary abrasive flow

Traditional low-pressure abrasive flow polishing can produce highly smooth surfaces, but the efficiency of this method is too low for polishing of hard-brittle materials parts. This paper proposes a novel cavitation rotary abrasive flow polishing (CRAFP) method. The energy generated from the cavitation bubble collapse is used to increase the kinetic energy of the abrasive particles in the low-pressure abrasive flow and the motion randomness of the abrasive particles near the wall; thereby, the efficiency and quality of low-pressure abrasive flow polishing are improved. The CRAFP mechanism was first introduced, and then the characteristics of the CRAFP process were investigated using computational fluid dynamics (CFD)-based abrasive flow simulation. Subsequently, a single-crystal silicon wafer polishing test was carried outperformed to verify the validity of the CRAFP method. The polishing results were compared with those of the traditional low-pressure abrasive flow polishing method. After 8 h of polishing using the CRAFP method and the traditional low-pressure abrasive flow polishing method, the surface roughness of the workpiece decreased to7.87 nm and 10.53 nm, respectively. Furthermore, by starting at similar initial roughness values, the polishing time required to reduce the roughness to 12 nm was 3.5 h and 6 h, respectively. The experimental results demonstrated that CRAFP can satisfy the surface requirements of single-crystal silicon (R_a < 12 nm) and exhibit high polishing efficiency and good quality.     

A study on the magneto-assisted spiral polishing on the inner wall of the bore with magnetic hot melt adhesive particles (MHMA particles)

The spiral polishing mechanism employed a fast turning screw rod to drive the abrasive for workpiece surface polishing. In this study, the powerful ring magnet installed around the workpiece would attract the self-developed magnetic hot melt adhesive particles (MHMA particles) during the process of polishing, driving the SiC particles against the workpiece, the inner wall of the bore. At the same time, the flexibility of MHMA particles helped improve the surface quality of the bore by preventing the SiC particles from heavily scratching it. The effects of magnetic flux density, size and concentration of SiC particles, concentration of MHMA particles, viscosity of silicone oil, revolution speed of the spindle as well as machining time and machining gap on operation temperature, slurry viscosity, surface roughness, and material removal were discussed and the best parameter combination was identified based on the results of the experiment. The effects of each machining parameter on the finished surface topography of the workpiece were also examined. Both analysis of variance and F-test indicated that magnetic flux density and the concentration of MHMA particles were the two most important variables affecting the surface roughness. In other words, magnetic force helped improve spiral polishing. Furthermore, the results showed that adding new MHMA particles to the slurry greatly improved the surface quality, at a rate of 90 %, and reduced the workpiece surface roughness from 0.9 μm down to 0.094 μm.     

立、卧式磁性复合流体抛光对比实验研究

针对磁性复合流体(MCF)抛光的两种抛光头,进行MCF抛光特性对比实验研究。开展了黄铜H26的平面抛光实验,调查立式和卧式抛光头分别对工件材料去除率、表面形貌与粗糙度以及MCF水分损耗等抛光特性的影响。实验结果表明,卧式MCF抛光能够较快地降低表面粗糙度,获得较高的材料去除率,MCF水分损失相对较快;立式MCF抛光能够在相对较长时间内获得稳定的材料去除率和表面粗糙度,MCF水分损失也较为平稳。     

Fundamental investigation on nano-precision surface finishing of electroless Ni–P-plated STAVAX steel using magnetic compound fluid slurry

Electroless nickel–phosphorus (Ni–P) plating used in a range of hot embossing metal molds/dies and injection metal molds/dies must be manufactured to nano-precision roughness for proper operation of the molds/dies. We therefore developed a novel polishing technique for mirror surface finishing of this kind of magnetic material using a magnetic compound fluid (MCF) slurry. The effects of the magnetic and gravitational forces acting on the carbonyl iron particles (CIPs) and abrasive particles (APs) within the MCF slurry were studied first, and the behaviors of the CIPs and APs in the presence of an external magnetic field were predicted. Then, experiments were performed to confirm the predictions by investigating the distribution of the CIPs and APs on the working surface of the MCF slurry. Finally, four MCF slurries containing CIPs and APs with different diameters were employed to finish the Ni–P-plated STAVAX steel specimen at different working gaps. The results revealed that for the magnetic workpiece, the resultant vertical force attracted CIPs towards the work surface, whereas APs were pushed away from the work surface. However, the CIPs and APs showed opposite behaviors with the non-magnetic workpiece. The percentage of APs distributed on the working surface increased and the distribution became more even as either the diameter of the CIPs or the working gap increased, whereas that of CIPs had the opposite tendency. The MCF slurry containing bigger CIPs and smaller APs should be employed and the working gap should be set at a smaller value in order to perform mirror surface finishing of a magnetic Ni–P-plated surface. Under the experimental conditions in this work, the Ni–P-plated surface quality improved significantly, and a mirror surface roughness (Ra) of 4 nm was successfully achieved without leaving scratches or particle adhesion when using an MCF slurry containing CIPs 7 μm in diameter and APs 1 μm in diameter, showing that MCF slurries containing commercial CIPs are applicable to the nano-precision finishing of magnetic materials.     

Modeling of the static tool influence function of bonnet polishing based on FEA

The purpose of this paper is to investigate the model of the static tool influence function (sTIF) of bonnet polishing (BP). Three kinds of sTIF are mathematically modeled, which are static tool influence function of tilted polishing (sTIF_t), static tool influence function of discrete precession polishing (sTIF_d), and static tool influence function of continuous precession (sTIF_c), respectively. Pressure distribution in the contact area is confirmed based on finite element analysis (FEA) technology. A group of experiments to extract the polishing spots have been conducted to verify the accuracy of the sTIF model. Meanwhile, the difference between sTIF_d and sTIF_c is studied. It turns out that the removal depths of sTIF_d and sTIF_c are almost the same, and the continuous precession polishing can be replaced by discrete precession polishing to ease control in practical polishing process especially for the aspheric surfaces polishing.     

Multi-objective optimization of circular magnetic abrasive polishing of SUS304 and Cu materials

In this paper, a Multi-objective particle swarm optimization algorithm (MOPSOA) is applied to optimize surface roughness of workpiece after circular magnetic abrasive polishing. The most important parameters of polishing model, namely current, gap between pole and workpiece, spindle speed and polishing time, were considered in this approach. The objective functions of the MOPSOA depend on the quality of surface roughness of polishing materials with both simultaneous surfaces (Ra1, Ra2), which are determined by means of experimental approach with the aid of circular magnetic field. Finally, the effectiveness of the approach is compared between the optimal results with the experimental data. The results show that the new proposed polishing optimization method is more feasible.

     

Improved measure of workpiece surface deterioration during turning using non-contact vision method

The surface finish quality of a machined workpiece is commonly measured using the average roughness parameter, R_a. This parameter, however, is insensitive to the lateral changes undergone by the surface in the feed direction as a consequence of tool wear. In this work, the effectiveness of four methods of workpiece surface analysis, namely autocorrelation, cross-correlation, and two new methods, called lateral material shift (LMS) ratio and profile slope ratio (PSR) analyses are investigated. Dry machining experiments were carried out on 316 stainless steel. Images of tool nose and workpiece profiles were captured using digital camera, and the edges were extracted using sub-pixel edge detection. In the autocorrelation approach, each workpiece profile was correlated with a shifted version of the same profile. In the cross-correlation approach, the workpiece profiles at different stages of machining were correlated with a reference profile generated using the unworn tool edge. In the LMS ratio method, the material shift ratios were determined from each waveform on the workpiece profile at various stages of tool wear, while in the PSR method the slopes at the right and left part of the waveform were compared. Among the four methods, the LMS ratio method produced the best correlation with tool flank wear with the maximum R-squared value of 0.9461, while average roughness R_a showed no correlation at all with both major and nose flank wear.     

Using specially designed high-stiffness burnishing tool to achieve high-quality surface finish

This paper is focused on the process of ball burnishing. The influence of tool stiffness on surface roughness parameters was considered theoretically, while experimental investigation was conducted to establish the influence of initial surface roughness (previous machining) on the effects of ball burnishing as the finishing process. Experimental investigations were conducted over a wide interval of most influential process parameters (burnishing forces, burnishing feed, and number of burnishing passes). The material used in the experiments was aluminum alloy EN AW-6082 (AlMgSi1) T651. Burnishing was performed using a specially designed tool of high stiffness. Statistical analysis of experimental data revealed strong correlation between roughness, R _a, and burnishing force, burnishing feed, and number of passes for the three surfaces, each with different roughness parameters. Particular combinations of process parameters yielded very low surface roughness, R _a, equivalent to polishing. It is worth noting that high surface quality can be achieved with relatively small burnishing forces, which differs from the investigations published so far. Contrary to conventional approaches, which are based on elastic tool systems, the authors propose the burnishing process to be conducted with high-stiffness tools. Further investigation shall be focused on optimization of burnishing process parameters in order to achieve surface finish equivalent to high polish.     

NC polishing of aspheric surfaces under control of constant pressure using a magnetorheological torque servo

In this paper, a method of maintaining a constant polishing pressure is proposed for a NC polishing system by controlling the polishing force during the polishing process. First, the NC polishing system is developed to resolve the force–position coupling problem encountered in common polishing processes. It mainly consists of a force control subsystem based on a magnetorheological torque servo to provide a controllable torque to polishing tool to generate the polishing force and a position control subsystem based on a general CNC lathe to control the position of the polishing tool. Second, a constant polishing pressure model is established by controlling the polishing force according to the variation of the curvature of the aspheric surfaces, and the polishing parameters for model are planned. Then, the control model of the polishing system is proposed, and a PID controller is designed for torque tracking with the actual torque feedback from a torque sensor. Finally, polishing experiments are conducted with constant force and constant pressure, respectively. Experimental results show that the surface roughness is greatly improved, the aspheric surfaces can be polished more uniformly with constant pressure than with constant force, and the PID controller can meet the requirements for the polishing force control.     

光学非球面磁性复合流体抛光运动控制及误差分析

针对精密光学系统中对高精度光学非球面元件的加工需求,设计磁性复合流体抛光的直线光栅式运动轨迹,并通过运动轨迹和非球面方程计算出各抛光加工点坐标。根据工件表面形貌和抛光头运动姿态设计了抛光加工路径,建立各抛光加工点间的弓高误差模型,通过模型对工件表面弓高误差变化规律进行仿真分析。仿真结果表明,弓高误差会随着Y轴上步长的增大而增大。这对非球面超精密加工具产生了深远的影响,促进了光学元件超精密高效制造技术的发展。     

Performance of laser polishing in finishing of metallic surfaces

Laser polishing is presently regarded as one of the enabling technologies hoped to eventually replace the need for time-consuming and error-prone manual polishing operations which are often required by metallic surfaces. During laser polishing, a thin layer of material is being melted as a result of laser irradiation. Since molten metal is characterized by increased relocation capabilities, laser polishing is generally accompanied by a more or less significant decrease in the surface roughness. The primary objective of this study is to present a comprehensive snapshot of the advancements made over more than one decade with respect to theoretical and experimental investigation of laser polishing technology. However, in addition to the usual review of the state-of-the-art in the field, the study places an increased emphasis on the finishing performance of the process, defined through the perspective of pre- and postpolishing surface roughness. The implementation of this metric with strong practical implications has revealed that under appropriate process parameters, certain classes of metallic materials can reduce their average surface roughness by more than 80 %, possibly to R _a ?=?5 nm. Nonetheless, a more rigorous and fundamental understanding of the intrinsic mechanisms underlying laser polishing remains one of the currently unfulfilled premises toward a wider industrial adoption of the process.     

小型非球面数控抛光技术的研究

用计算机控制抛光的方法对小型非球面数控抛光技术进行了研究。对计算机控制小磨头抛光的材料去除作用进行了计算机模拟;依据计算机模拟结果,调整驻留时间函数,进行抛光补偿;最后,在自行研制的三轴联动非球面数控抛光原理样机上高效地完成了70 mm左右非球面的抛光,各项指标达到了中等精度要求,表面粗糙度为2.687 nm,面形精度为0.45 μm,且重复精度良好。结果表明,该技术有效提高了小型非球面光学零件的批量生产效率。     

介电泳效应磨粒流抛光中的电极形态

针对薄壁陶瓷工件内表面抛光,提出一种基于介电泳效应的磨粒流抛光方法。将非均匀电场布置于陶瓷工件外表面,极化磨粒,实现陶瓷工件内表面高效抛光。仿真分析发现:电极间隙比为2时,SiC磨粒具有最好的介电泳效应,参与抛光的磨粒最多。陶瓷工件初始内表面粗糙度值Ra为(208±5)nm时,抛光10 h后,无介电泳效应的磨粒流抛光工件内表面粗糙度值Ra为51 nm,有介电泳效应的磨粒流抛光工件内表面粗糙度值Ra为23 nm。     

Effect of material microstructure and tool geometry on surface generation in single point diamond turning

There is a strong desire in industry to improve surface finish when performing ultra-precision, single point diamond turning (SPDT) to reduce the amount of post process polishing required to meet final product specifications. However there are well known factors in SPDT which limit achievable surface finish. This paper focuses on the role of material microstructure, including grain boundary density and the presence of inclusions, as well as tool design on surface roughness using the concept of size effect. Size effect can be described as an interplay between the material microstructure dimension and the relative size of the uncut chip thickness with respect to the cutting edge radius. Since one of the controllable parameters in size effect is grain size and dislocation density, controlled studies were performed on samples whose microstructure was refined by mechanical strain hardening through rolling and a friction stir process (FSP). The use of the ultra-fine grained workpiece prepared using an FSP was observed to reduce side flow as well as grain boundary and inclusion induced roughness. The role of tool geometry on material induced roughness was investigated using a tool with a rounded primary cutting edge and a flat secondary edge. The use of the flat secondary edge was observed to improve surface finish when machining a flat surface. This improvement was primarily attributed to a reduction in side flow and material microstructural effects. By combining these approaches an average surface roughness R_a value of 0.685 nm was achieved when SPDT a flat surface. Furthermore the custom tool has the potential to significantly improve the productivity of SPDT by allowing for a much higher feed rate while still achieving a high quality surface finish.     

On finishing of pressing die surfaces using machine hammer peening

Machine hammer peening (MHP) is a new method for finishing of surfaces. With this method, the workpiece surface is hammered with a spherical carbide tool. The main objective of the study was to evaluate whether the MHP method could become a plausible substitute for manual polishing in pressing die manufacturing where nodular cast iron is a common workpiece material. To do this, sample nodular cast iron surfaces were hammered and evaluated. Changes to the surfaces were evaluated using surface roughness measurements, hardness measurements and optical images. First of all, the workpiece surface was smoothened. Secondly, the surface hardness was increased significantly. Thirdly, the nodules on the workpiece surface were affected. They appeared to be smaller and not as visible. This effect would likely create a die surface less prone to galling since the cavities would not be filled with sheet metal to the same extent in a forming operation. In addition, with MHP, the amount of polishing needed to manufacture a die surface can be reduced because of the smoothening effect.     

Roughness and wear characteristics of spark-eroded surfaces

Measurements of surface roughness H_r.m.s were made on spark-eroded samples of tungsten carbide, high speed tool steel, high carbon steel and mild steel machined with graphite, copper and tungsten tools. The roughness increased with increasing pulse energy for particular work-tool combinations. The surface finish improved as the melting point of the work material increased for a particular tool material and as the work function of the tool material decreased for a particular work material.The surface roughness profile records were graphically analysed to obtain the average and maximum peak-to-valley height and the free polishing depth. The heterogeneity of the surface, expressed as the ratio of the range of microirregularities to its average value, was between 1.1 and 1.5. Reduction in pulse energy, which resulted in a better surface finish, correlated with reduction in the free polishing depth. The average diameter of the crater increased with increasing pulse energy.     

A study on the mirror surface machining by using a micro-energy EDM and the electrophoretic deposition polishing

In order to achieve a mirror-like workpiece surface by EDM, a micro-energy EDM or a manual lapping is normally used. However, both methods are time consuming and economically unattractive. To have better performance, a sinking EDM machine was first used in this research on SKD 61 for a micro-energy discharge process followed by the electrophoretic deposition (EPD) process to coat Al₂O₃ particles uniformly on a rotation electrode. The precision polishing process was then applied on the discharged workpiece with suitable parameters including the voltage, electrode rotating speed, pH value of electrolyte, and abrasive concentration. After the EDM process, the surface roughness and the recast layer thickness of a workpiece could also be reduced. The experimental data showed that when 0.3 μm of Al₂O₃ particles was used for the EPD polishing process, the initial roughness of a discharge surface could be improved from 0.52 μm Ra (6.50 μm Rt) to a mirror-like surface of 0.068 μm Ra (0.742 μm Rt). Also, the total working time could be reduced significantly to within the range of 5 to 10 min.     

Research on unbounded abrasive polishing process with assisted ultrasonic vibration of workpiece

Ultrasonic-assisted machining was an effective method to improve the material removal quality especially to difficult-to-cut metal materials. The ultrasonic vibration was usually superimposed on the machining tool but seldom on the workpiece, although the ultrasonic vibration of workpiece could improve the processability of material more effectively. In this paper, a rectangle hexahedron ultrasonic sonotrode with optimized slots was developed as a platform to realize the assisted ultrasonic vibration of workpiece and the ultrasonic-assisted polishing process of austenitic stainless steel was also studied. The unbounded abrasive was selected as polishing medium, and the path compensation strategy of soft polishing tool was carried out for getting uniform polishing force. The orthogonal experiments were designed to study the optimization of ultrasonic polishing parameters and the relation between different types of ultrasonic polishing path and polishing quality. The results appear that the horizontal ultrasonic vibration of workpiece can reduce polishing force and improve polished surface roughness, which can also reinforce the proportion of plastic shear effect in the material removal process. The ultrasonic polishing path keeping consistent with workpiece vibration direction can get more uniform polishing force and better surface roughness. And the 45° oblique crossing ultrasonic path can get the maximum average polishing force reduction by 75.2 %.     

Single-point diamond turning of CaF2 for nanometric surface

Single-crystal CaF₂ is an important optical material. In this work, single-point diamond turning experiments were performed to investigate the nanometric machining characteristics of CaF₂. The effects of tool feed, tool rake angle, workpiece crystal orientation and cutting fluid were examined. It was found that two major types of microfracturing differing in mechanism limited the possibility of ductile regime machining. The critical conditions for microfracturing depend strongly on the tool rake angle and the type of cutting fluid. The results also indicate that one type of the microfractures is caused by thermal effect, and can be completely eliminated by using a sufficiently small undeformed chip thickness and an appropriate negative rake angle under dry cutting conditions. Continuous chips and ductile-cut surfaces with nanometric roughness were generated.