Design and fabrication of a novel polishing tool for finishing freeform surfaces in magnetic field assisted finishing (MFAF) process

Surface finish is a critical requirement for different applications in industries and research areas. Freeform surfaces are widely used in medical, aerospace, and automobile sectors. Magnetic field assisted finishing process can be used very efficiently to finish freeform surfaces. In this process, magnetorheological fluid is used as the polishing medium and permanent magnet is used to control its rheological properties to generate finishing force during polishing. To avail sufficient magnetic field in the finishing zone, it is necessary to design an optimum polishing tool. In the present study, a specially designed polishing tool is designed using a finite element based software package (Ansys Maxwell^®) based on Maxwell equations. At first, dimension of the permanent magnet is determined for designing optimum tool geometry. After that, dimension and configuration of the magnet fixture are optimized. A special type of metal named mu-metal which is a nickel-iron based alloy is selected for magnet fixture due to its magnetic-field shielding property. Mu-metal directs the magnetic flux lines in such a way that in the finishing zone the magnetic flux can be concentrated on the workpiece surface required for finishing. Also, the Mu-metal magnet fixture shields the magnetic field from outside environment so that MR fluid as well as any surrounding magnetic materials do not stick to the polishing tool. Experiments are carried out to validate the Maxwell simulation results to compare the magnetic flux distribution on the workpiece surface which shows good agreement between them. Also, finishing of flat titanium workpieces are carried out and it is found that the novel polishing tool has the capability to finish the workpieces in the nanometer range.     

MAGNETIC FIELD ANALYSIS AND ROUGHNESS PREDICTION IN MAGNETORHEOLOGICAL ABRASIVE HONING (MRAH)

Magnetorheological Abrasive Honing (MRAH) is a recently developed process to finish engineering surfaces. The process makes use of a magnetically stiffened abrasive-mixed magnetorheological fluid as the flexible tool and rotation-cum-reciprocation movements between the finishing medium and the workpiece surface for providing finishing action. In the present work, a finite element analysis with Mechanical/Emag module of ANSYS is performed to understand the nature of magnetic field developed in the process and verification is done with actual measurements. Considering the simulated magnetic field, a model to predict final roughness value (R _a ) is developed. The model, when applied for different work materials and various process parameters, such as magnetic flux density, process duration and workpiece rotation, yields results that are in good agreement with experimental results.     

集群磁流变变间隙动压平坦化加工试验研究

为了提高光电晶片集群磁流变平坦化加工效果,提出集群磁流变变间隙动压平坦化加工方法,探究各工艺参数对加工效果的影响规律。以蓝宝石晶片为研究对象开展了集群磁流变变间隙动压平坦化加工和集群磁流变抛光对比试验,通过检测加工表面粗糙度、材料去除率,观测加工表面形貌、集群磁流变抛光垫中磁链串受动态挤压前后形态变化,研究挤压幅值、工件盘转速、挤压频率以及最小加工间隙等工艺参数对加工效果的影响规律。试验结果表明：集群磁流变平坦化加工在施加工件轴向微幅低频振动后,集群磁流变抛光垫中形成的磁链串更粗壮,不但使其沿工件的径向流动实现磨粒动态更新、促使加工界面内有效磨粒数增多,而且在工件与抛光盘之间的加工间隙产生动态抛光压力、使磨粒与加工表面划擦过程柔和微量化,形成了提高材料去除效率、降低加工表面粗糙度的机制。对于2英寸蓝宝石晶电（1英寸=2.54 cm）集群磁流变变间隙动压平坦化加工与集群磁流变抛光加工效果相比,材料去除率提高19.5%,表面粗糙度降低了42.96%,在挤压振动频率1 Hz、最小加工间隙1 mm、挤压幅值0.5 mm、工件盘转速500 r/min的工艺参数下进行抛光可获得表面粗糙度为Ra0.45 nm的超光滑表面,材料去除率达到3.28 nm/min。证明了集群磁流变变间隙动压平坦化加工方法可行有效。     

ABRASIVE-BASED NANO-FINISHING TECHNIQUES: AN OVERVIEW

The surface finishing techniques can be divided into two categories: traditional and advanced. To overcome some of the problems of traditional finishing techniques, hybridized processes have been evolved by the researchers. Some of the advanced finishing processes that have been reviewed are abrasive flow machining (AFM), magnetorheological finishing (MRF), magnetorheological abrasive flow finishing (MRAFF), magnetic abrasive finishing (MAF), chemo mechanical polishing (CMP), etc. Most of these processes have been developed in the recent past and they can be employed to produce optical, mechanical, and electronic components with micrometer or sub-micrometer form accuracy and surface roughness within nanometer range with hardly any surface defects. However for large size flat components, MAF seems to be the most suitable finishing process. In MAF, DC power supply is given to the electromagnet hence intermixing of ferromagnetic abrasive particles during the process does not take place and the worn out cutting edges keep interacting with the workpiece surface. As a result, the finishing rate is quite low. The use of pulsed DC power supply to the electromagnet results in pulsating flexible magnetic abrasive brush (P-FMAB), which substantially enhances the finishing rate. The on-line measurement of the forces has helped in understanding the mechanism of material removal during Static-FMAB (S-FMAB) and Pulsating-FMAB. The magnitude of normal magnetic force (originating indentations) in P-FMAB has been found to be dynamic in nature and substantially high in magnitude as compared to S-FMAB.     

Effect of fluid composition on nanofinishing of single-crystal silicon by magnetic field-assisted finishing process

Magnetic field-assisted finishing is a deterministic process particularly used for finishing optical materials. The main component of this process is magnetorheological fluid which consists of magnetic particles, abrasive particles, carrier fluid such as water or oil, and some additives to impart stability. Under the influence of magnetic field (generated by either permanent magnet or electromagnet), magnetic particles form chain-like structure and support many abrasive particles to perform finishing of workpiece surface. Selection of abrasive and carrier fluid in this process is one of the major concerns which play vital role on finishing mechanism and surface quality. In the present experimental investigation, aluminum oxide and cerium oxide are chosen as abrasives while deionized water and paraffin oil are selected as carrier fluids. A set of experiments are carried out to study chemical interactions of abrasive and carrier fluid on the silicon surface. A rheological study is carried out to study behavior of magnetorheological fluid fluids under magnetic field.     

Differential finishing of freeform surfaces (knee joint) using R-MRAFF process and negative replica of workpiece as a fixture

It is difficult and challenging to achieve uniform nanoscale surface finish in the contact zone, particularly on freeform (or sculptured) surfaces having different curvatures at different locations. Femoral (or, Knee joint component) is one of such biomedical freeform component which has complex profile along its curvature. Surface conditions of a femoral decide the life of the implant and they play a crucial role in its functionality. The variation in surface roughness of the femoral should be minimum in the contact zone. For this purpose, a special tooling is being proposed for rotational magnetorheological abrasive flow finishing (R-MRAFF) process. A negative replica of the workpiece (knee joint) as a tool (or a fixture) is used so that the medium flow velocity in the fluid flow channel is almost constant (or minimum possible variations) along the medium flow direction. It is able to do differential finishing also along the curvature. In addition, pulsating magnetic field has been used to generate vibrations in the medium in the finishing zone so that the possibility of fresh abrasive particles interacting with the surface of femoral is high. The surface finish has been achieved ranging from 26 nm to 62 nm using the proposed finishing technique and negative replica of the workpiece (femoral) as a fixture.     

Effect of extrusion pressure and number of finishing cycles on surface roughness in magnetorheological abrasive flow finishing (MRAFF) process

Magnetorheological abrasive flow finishing (MRAFF) was developed as a new precision finishing process for complicated geometries using smart magnetorheological polishing fluid. This process introduces determinism and in-process controllability of rheological behaviour of abrasive laden medium used for finishing intricate shapes. Magnetorheological polishing (MRP) fluid is comprised of carbonyl iron powder and silicon carbide abrasives dispersed in a viscoplastic base of grease and mineral oil and exhibits change in rheological behaviour in presence of external magnetic field. This smart behaviour of MRP fluid is utilized to precisely control finishing forces. The process performance in terms of surface roughness reduction depends on process variables like hydraulic extrusion pressure, magnetic flux density in the finishing zone, number of finishing cycles, and composition of MRP fluid. In the present work, experiments were conducted on a hydraulically powered MRAFF experimental setup to study the effect of extrusion pressure and number of finishing cycles on the change in surface roughness of stainless steel grounded workpieces. A new observation of “illusive polishing” action with the initial increase in number of finishing cycles is reported. The actual finishing action is possible only after removal of initial loosely held material remaining after grinding.     

Stepwise approach to reduce the costs and environmental impacts of grinding processes

Grinding is a common finishing process to meet specific technological requirements; however, it is energy, resource and time consuming. Thus, the improvement of grinding processes should not only consider the technological requirements but also environmental and economic impacts. There are a number of factors involved in grinding processes. Besides the process parameters and workpiece properties, there are three enabling factors for improvement opportunities, such as tool, cutting fluid and machine tool. However, in practice, not all factors can be changed or modified easily at the same time. To support process improvement, this paper proposes a stepwise approach to compare alternative enabling factors in conjunction with the process parameters in order to reduce the costs and environmental impacts of a grinding process under consideration of technological requirements. The proposed approach is demonstrated by means of an internal cylindrical grinding process and applications of different tools, cutting fluid and machine tools.     

Some aspects of fabrication of micro devices by electrochemical micromachining (ECMM) and its finishing by magnetorheological fluid

Electrochemical micromachining (ECMM) is an advanced machining process for machining of electrically conducting materials. In the present work, a micro nozzle and a fluidic mixer having complex structures are made using masking technique by ECMM process. Mask is made of 50-μm transparent sheet and the micro nozzle and micro mixer are fabricated on an 800-μm thick copper sheet. The resulting rough inner walls and bottom surface of micro nozzle are finished using magnetorheological fluid-based finishing process. Surface finish of the nozzle is significantly improved after finishing. A comparison of width, depth, and surface roughness of the micro nozzle is also carried out before and after finishing. The mixing behavior of two fluids is visualized by microscope in micro mixer. The rough inner walls of the mixer’s channels act as obstructions and result in zigzag path of flowing fluid. Hence, mixing occurs at microscopic level because of rapid molecular diffusion.     

Effect of working gap and circumferential speed on the performance of magnetic abrasive finishing process

Magnetic abrasive finishing (MAF) is one of the advanced finishing processes in which workpiece is kept between two magnets, and cutting force is controlled by working gap and magnetic field between the two magnets. MAF setup is designed for finishing cylindrical workpieces and it is mounted on lathe machine. The loosely bounded powder is prepared for experimentation by homogeneous mixing of magnetic powder (Fe powder of 300 mesh size (51.4 μm)), abrasive powder (Al₂O₃ of 600 mesh size (25.7 μm), and lubricant called servospin-12 oil. To investigate the effects of working gap and circumferential speed on material removal, change in surface finish and percent improvement in surface finish, a series of experiments have been conducted using in-house fabricated setup. Based upon the results, in general, material removal decreases by increasing working gap or decreasing circumferential speed of the workpiece. Change in surface finish increases by increasing circumferential speed of the workpiece.     

Effect of temperature on the magnetic abrasive finishing process of Mg alloy bars

Experiments were conducted to evaluate the effect of temperature during magnetic abrasive finishing of Mg alloy bars. A magnetic abrasive finishing process is an unconventional finishing technique that has been used to achieve high-quality surfaces with dimensional accuracy. In this study, a Mg alloy bar, which is widely used in automobiles, aircraft, IT, and the defense industry, was chosen as a cylindrical workpiece. The workpiece was then finished with a magnetic abrasive finishing process at three different temperatures, i.e., a cryogenic temperature, room temperature, and high temperature. In the cryogenic temperature condition, liquid nitrogen and argon gas were used as the cryogenic cooling gases in the finishing process; the results from this treatment were compared with those obtained at room temperature and high temperature conditions. At the room temperature condition, the finishing process of the cylindrical workpiece was performed at 24 °C. To carry out the high temperature condition, a hot air dryer was used to maintain a finishing temperature of 112 °C. The experimental results show that the room and cryogenic temperatures could yield excellent performance in terms of the surface roughness. However, in terms of the removal weight and change in diameter, the high temperature condition was found to be superior. In the present research, the improvements of the surface roughness (Ra) at room temperature (24 °C) and cryogenic temperature (-120 °C) conditions were 84.21 % and 55 %, respectively.

     

新型的复杂曲面磁粒光整加工机床

介绍了一种复杂曲面光整加工机械--三坐标数字化磁粒光整加工机床.该机床通过对复杂曲面的数字化测量和自动编程,得到磁极与工件保持一定间隙沿工件表面运行的加工轨迹,利用吸附在工具磁极上面的磁性磨料为工具,通过磁极带动磁粒沿工件表面作高速旋转,从而实施对工件表面的光整加工.该机床的研制成功实现了复杂曲面光整加工的自动化.     

基于ANSYS的磁流变阀磁场分析

阐述了磁流变流体的磁流变特性及其Bingham模型,依据磁流变流体在磁场作用下的可逆、可控特性设计了磁流变阀。采用ANSYS有限元分析软件对设计的磁流变阀3种结构模型进行了磁场分析。研究磁流变阀的结构与磁流变流体的磁感应强度和磁流变阀性能的关系。分析结果表明:根据磁力线分布和磁感应强度大小合理设计磁流变阀结构,可使磁流变流体在相同外加电流条件下获得更高磁感应强度,提高磁流变阀的性能。     

Nanofinishing of freeform surfaces (knee joint implant) by rotational-magnetorheological abrasive flow finishing (R-MRAFF) process

Freeform complex surfaces have become an inevitable part of many devices to perform specific functions. Some of these components require nanolevel surface roughness value to meet the desired requirements in their applications. Finishing of freeform surfaces to nanometer surface roughness value is always difficult for any process. Rotational-magnetorheological abrasive flow finishing (R-MRAFF) process has been applied so far for finishing internal surfaces of relatively simple geometry. In this work, an attempt has been made to improve external topography of freeform surfaces using this process. Large hydrodynamic pressure coupled with magnetic fluid is the principal idea behind these experiments. A smooth mirror like finished surface is achieved with improved finishing rate (nanometer/min) by controlling two motions (axial and rotational) simultaneously on stainless steel workpiece similar to knee joint implant. Magnetorheological polishing fluid with different mesh sizes of abrasive particles and at different extrusion pressures is used to reduce final surface roughness value, to increase uniformity of surface finish on the freeform surface and to enhance finishing rate. Surface roughness ranging from 35 to 78 nm is achieved at various locations as compared to larger variation in Ra value obtained in the earlier research work.     

磁流变抛光工艺参数的研究

磁流变抛光是一种新型的光学零件加工方法,它不仅可以精确控制抛光后光学零件的面型,还能得到较高的表面质量和较高的加工效率。本文在介绍磁流变抛光基本原理的基础上,重点分析了磁流变液与工件相对速度对磁流变抛光最终效果的影响规律,在一定速度大小范围内,随着相对速度的提高,抛光加工的效率也会随之提高,光学零件的表面粗糙度会随之降低,并且还提出在抛光过程中保持相对速度一致的必要性和保持相对速度一致的方法,通过实验验证了该方法的正确性。     

太钢为核电工业攻克了世界性的技术难题

太钢不锈钢钢管公司成功攻克了一项世界性技术难题，研发出全球第一支使用挤压机生产的W型钢，产品主要用于核电裂变反应堆的管道支架。为全面提高不锈钢管市场占有率和应用范围，W型钢作为AP1000第三代核电站汽轮机发电余热排出换热系统的支撑件用钢，一直是由热轧加焊接工艺最终成W型钢，一方面热轧成本较高，另一方面对厚壁材料也增大了焊接难度，且所承担的风险远远大于无缝件。     

Analysis of magnetorheological abrasive flow finishing (MRAFF) process

A new precision finishing process called magnetorheological abrasive flow finishing (MRAFF), which is basically a combination of abrasive flow machining (AFM) and magnetorheological finishing (MRF), has been developed for nano-finishing of parts even with complicated geometry for a wide range of industrial applications. In this paper microstructure of the mixture of magnetic and abrasive particles in magnetorheological polishing fluid (MRPF) has been proposed, and normal force on the abrasive particles is calculated from the applied magnetic field. A model for the prediction of material removal and surface roughness achieved has also been presented. And, finally theoretical results are compared with the experimental data available in the literature, and they are found to agree well.     

Experimental investigations into forces acting during a magnetic abrasive finishing process

A magnetic abrasive finishing (MAF) process is the one in which material is removed in such a way that surface finishing and deburring are performed simultaneously with the applied magnetic field in the finishing zone. Knowledge of forces acting during MAF is important to understand the mechanism of material removal. Forces have direct influence on the generation of a finished surface and accuracy of the workpiece. This paper reports the experimental findings about the forces acting during MAF and provides correlation between the surface finish and the forces. The resistance type force transducer (ring dynamometer) has been designed and fabricated. It is used to measure the normal magnetic force component responsible for microindentation into the workpiece and tangential cutting force component producing microchips. The force data have been recorded on-line by making use of virtual instruments (using Lab-View software). It is concluded that forces and change in surface roughness (ΔRa) increase with increase in current to the electromagnet (or magnetic flux density) and decrease in the working gap.On deputation from M.M.M.Engg. College, Gorakhpur (UP) India     

Development of a new magnetic abrasive finishing process with renewable abrasive particles using the circulatory system

In order to improve the finishing efficiency of the Magnetic Abrasive Finishing process, we proposed a new MAF process with renewable abrasive particles using compound magnetic finishing fluid circulatory system in this paper. This new finishing process has a circulating system that uses a conveyor belt to renew the mixed abrasive particles. This not only maintains the stability of the finishing but also ensures that the processing does not need to be interrupted. In this study, we investigated the magnetic field distribution, finishing force, and finishing behavior of the processing area. Furthermore, we designed experimental device to finish the sus304 stainless steel plate, to verify the feasibility of this process and understand its characteristics through processing experiments. Moreover, the influence of important process parameters, including magnetic particles, abrasive particles, conveyor belt line speed and working gap, on the surface quality of the workpiece is studied through the experiment. The experimental results indicate that the present process can achieve stable processing of the material surface without interruption, and the surface roughness of the sus304 stainless steel plate has been improved from 273 nm to 23 nm through this process.     

NANO-FINISHING OF STAINLESS-STEEL TUBES USING ROTATIONAL MAGNETORHEOLOGICAL ABRASIVE FLOW FINISHING PROCESS

A new polishing method called Rotational (R)-Magnetorheological Abrasive Flow Finishing (MRAFF) process has been proposed by rotating a magnetic field applied to the Magnetorheological polishing (MRP) medium in addition to the reciprocating motion provided by the hydraulic unit to finish internal surface of cylindrical stainless steel (non-magnetic) workpiece. By intelligently controlling these two motions uniform smooth mirror-like finished surface in the range of nm has been achieved. For parametric analysis of the process, the experiments have been planned using design of experiments technique and response surface regression analysis is performed to analyze the effects of process parameters on finishing performance. Analysis of Variance (ANOVA) is conducted and contribution of each model term affecting percent improvement in surface finish is calculated. The experimental results are discussed and optimum finishing conditions are identified from optimization study. The present study shows that rotational speed of the magnet has most significant effect on output response (percentage improvement in surface roughness, %ΔR _a ). The best surface finish obtained on stainless steel workpiece with R-MRAFF process is 16 nm.