Magnetorheological Finishing Machine Research Articles

Estimation of magnetorheological fluid constituent’s concentration for efficient finishing process

ABSTRACT Efficacy and outcome of all magnetorheological finishing (MRF) processes depend on rheological properties of the magnetorheological (MR) fluid which consists of carbonyl iron particles (CIPs), abrasives and carrier fluid. Thus it is important to understand particle distribution in MR fluid which affects the rheological properties of the fluid under the influence of the applied magnetic field and hence the finishing process. A hexagonal microstructure model of MR fluid is considered and a modified model of abrasive doped MR fluid is developed for generation of estimation curve based on ratio of size of abrasive particles to CIP. MR fluid was prepared based on these estimated values and was used for finishing of N-BK7 glass on Ball End MRF (BEMRF) machine. MR fluid with four different abrasive size to CIP size ratios were prepared with CIP volume fraction of 32.9% and Cerium Oxide particles volume fraction of 12.5% with 54.6% water. Another experiment with size of cerium oxide particles to size of CIP ratio of 0.761 is taken for four different concentrations of abrasives and CIPs. Best results with 79.85% improvement in roughness and 0.672 µg/minute MRR were obtained for fluid prepared as per proposed estimation curve for efficient finishing process.

Magnetorheological finishing of an irregular-shaped small-bore complex component using a small ball-end permanent-magnet polishing head

A novel magnetorheological finishing (MRF) process using a small ball-end permanent-magnet polishing head is proposed, and a four-axes linkage dedicated MRF machine tool is fabricated to achieve the nanofinishing of an irregular ψ-shaped small-bore complex component with concave surfaces of a curvature radius less than 3 mm. The processing method of the complex component is introduced. Magnetostatic simulation during the entire finishing path is carried out to analyze the material removal characteristics. A typical ψ-shaped small-bore complex component is polished on the developed device, and a fine surface quality is obtained with surface roughness Ra of 0.0107 μm and surface accuracy of the finished spherical surfaces of 0.3320 μm (PV). These findings indicate that the proposed MRF process can perform the nanofinishing of a kind of small-bore complex component with small-curvature-radius concave surfaces.

Nanofinishing of 3D surfaces by automated five-axis CNC ball end magnetorheological finishing machine using customized controller

Ball end magnetorheological finishing (BEMRF) is a novel method for nanofinishing of ferromagnetic as well as nonmagnetic materials. With increasing demands of precision finished products in various markets, the quality of surface finish and production rate are of prime importance. For products having complex geometries, this need intensifies as the manual finishing methods are incapable of producing uniform finish throughout the complex geometry of the surface. This paper describes the finishing of 3D surfaces using five-axis CNC BEMRF process in a fully automated manner such that a uniform finish is obtained throughout the workpiece. This eliminates the labor-intensive efforts and the subjective errors involved in manual finishing operations and increases the production rate of the finished products. A customized five-axis CNC controller is developed that enables the sequential and precise control of motion as well as process parameters in the BEMRF machine. The magnetizing current which governs the forces responsible for finishing in BEMRF process is controlled in-process through CNC part program using a specially defined “E” code for magnetizing current. A dedicated graphical user interface is also developed in Visual C# for the control of BEMRF process, enabling the user to make use of BEMRF machine without going into the complexities of the system. Using the customized five-axis CNC controller, automated finishing of a freeform mild steel workpiece is done. The finishing results obtained by the automated five-axis BEMRF machine shows uniformly finished surface profile (up to order of nanometer) on the mild steel test specimen.

Polishing technique for potassium dihydrogen phosphate crystal based on magnetorheological finishing

Potassium dihydrogen phosphate (KDP) crystal is an extremely valuable and irreplaceable nonlinear optical crystal that is widely used as frequency converter and electric-optical switch in various areas such as inertial confinement fusion (ICF) laser systems. However, it is very difficult to machine KDP crystals because they are thermally sensitive, prone to fracture, hygroscopic and easily scratched. The typical process used for machining KDP crystals is single-point diamond turning (SPDT). Though ultra-precision flatness fly-cutting machines are employed to machine large-aperture KDPs, small-scale ripple errors persist, and the cutting accuracy is limited by machine tool imprecision. Magnetorheological finishing (MRF) has been developed for KDP crystal processing, but controlling the surface integrity of KDP crystal is a key associated problem, due to the low hardness of the crystal and water solubility. In this paper, a new type of oil-based magnetorheological fluid (MR fluid) is developed, and the removal mechanism for KDP crystals is studied. The roughness control and the defect-removing mechanism are also investigated. Using the MR fluid, surface integrity controlling techniques and other relevant technologies, a large-aperture KDP crystal is successfully polished. For a KDP crystal of 410 mm × 410 mm, PV value is greatly reduced from 4.36 λ to 1.32 λ, RMS from 0.698 λ to 0.138 λ and roughness is reduced to Rq 2 nm. Results show that large-aperture KDP crystal can be successfully polished by the MRF machine, indicating that the MR fluid designed has superior effectiveness, longer-term stability and an excellent surface integrity.

High-accuracy process based on the corrective calibration of removal function in the magnetorheological finishing

The corrective calibration of the removal function plays an important role in the magnetorheological finishing (MRF) high-accuracy process. This paper mainly investigates the asymmetrical characteristic of the MRF removal function shape and further analyzes its influence on the surface residual error by means of an iteration algorithm and simulations. By comparing the ripple errors and convergence ratios based on the ideal MRF tool function and the deflected tool function, the mathematical models for calibrating the deviation of horizontal and flowing directions are presented. Meanwhile, revised mathematical models for the coordinate transformation of an MRF machine is also established. Furthermore, a O140-mm fused silica plane and a O196 mm, f/1∶1, fused silica concave sphere samples are taken as the experiments. After two runs, the plane mirror final surface error reaches PV 17.7 nm, RMS 1.75 nm, and the polishing time is 16 min in total; after three runs, the sphere mirror final surfer error reaches RMS 2.7 nm and the polishing time is 70 min in total. The convergence ratios are 96.2% and 93.5%, respectively. The spherical simulation error and the polishing result are almost consistent, which fully validate the efficiency and feasibility of the calibration method of MRF removal function error using for the high-accuracy subaperture optical manufacturing.

Configuration design and accuracy analysis of a novel magnetorheological finishing machine tool for concave surfaces with small radius of curvature

Magnetorheological finishing (MRF) is a computer-controlled deterministic polishing technique that is widely used in the production of high-quality optics. In order to overcome the defects of existing MRF processes that are unable to achieve concave surfaces with small radius of curvature, a configuration method of a novel structured MRF machine tool using small ball-end permanent-magnet polishing head is proposed in this paper. The preliminary design focuses on the structural configuration of the machine, which includes the machine body, motion units and accessory equipment, and so on. Structural deformation and fabrication accuracy of the machine are analyzed theoretically, in which the reasonable structure sizes, manufacturing errors and assembly errors of main structural components are given for configuration optimization. Based on the theoretical analysis, a four-axes linkage MRF machine tool is developed. Preliminary experiments of spot polishing are carried out and the results indicate that the proposed MRF process can achieve stable polishing area which meets requirement of deterministic polishing. A typical small-bore complex component is polished on the developed device and fine surface quality is obtained with sphericity of the finished spherical surfaces 1.3 µm and surface roughness Ra less than 0.018 µm.

Design and fabrication of a novel magnetorheological finishing process for small concave surfaces using small ball-end permanent-magnet polishing head

In order to overcome the defects of conventional magnetorheological finishing (MRF) processes that are unable to finish small curvature radius concave surfaces of diameter within 10 mm, a novel precision MRF process using small ball-end permanent-magnet polishing head with a diameter of 4 mm is proposed in this paper. And experimental setup of a four-axis linkage MRF machine tool is fabricated. Magnetostatic simulation of the distribution of magnetic flux density indicates that the magnetic field uniformity of produced polishing head is 93.3 %. Magnetizing force analysis is done to analyze the formation of magnetorheological fluid in finishing region. In addition, the suitable range of C-axis angular position for finishing is generated to be 60°~70°. Finishing experiments on both nonmetallic and metallic specimens with different surface shapes have been conducted. The ability of the developed method of finishing process to improve surface characteristics of a workpiece is demonstrated to reach the surface roughness of finished surfaces below Ra 6 nm. It confirms that the present MRF process is capable of improving surface quality and performing the nanofinishing on small curvature radius concave surfaces.

Development of a Magnetorheological Finishing Machine for Small-Bore Part of Irregular Shape and Experimental Study

Magnetorheological finishing machine tool with small ball-end permanent-magnet polishing head is proposed as a method to solve the problem that small-bore parts of irregular shape with small curvature radius concave surface cannot be polished by traditional magnetorheological finishing machine tool. The four-axis linkage magnetorheological finishing machine tool with modular open numerical system and easy-operating man-machine interactive interface is developed. Then the machine tool is used to study the removal characteristics on machining parameters. It is indicated that the removal rate is not sensitive to processing time. When the angle between the polishing head and the workpiece is larger, the removal rate is higher. The surface roughness of workpiece can be less than 3 nm, which is machined by magnetorheological polishing with a small permanent magnet ball-end tool.

Compensating for velocity truncation during subaperture polishing by controllable and time-variant tool influence functions.

The velocity-varying regime used in deterministic subaperture polishing employs a time-invariant tool influence function (TIF) to figure localized surface errors by varying the transverse velocities of polishing tools. Desired transverse velocities have to be truncated if they exceed the maximal velocity of computer numerical control (CNC) machines, which induces excessive material removal and reduces figuring efficiency (FE). A time-variant (TV) TIF regime is presented, in which a TIF serves as a variable to compensate for excessive material removal when the transverse velocities are truncated. Compared with other methods, the TV-TIF regime exhibits better performance in terms of convergence rate, FE, and versatility; its operability can also be strengthened by a TIF library. Comparative experiments were conducted on a magnetorheological finishing machine to validate the effectiveness of the TV-TIF regime. Without a TV-TIF, the tool made an unwished dent (depth of 76 nm) at the center because of the velocity truncation problem. Through compensation with a TV-TIF, the dent was completely removed by the second figuring process, and a TV-TIF improved the FE from 0.029 to 0.066 mm(3)/h.

Identification and compensation for offset errors on the rotary axes of a multi-axis Magnetorheological Finishing machine tool

In this paper, offset errors in rotary axes of a multi-axis Magnetorheological Finishing (MRF) machine tool are identified using a “trial cut” method and compensated in postprocessor for MRF figuring process. Kinematic model considering the offset errors of the multi-axis MRF machine tool UPF700-7 is built, utilizing homogenous transformation matrices theory. Then, a trial cut method by acquiring removal functions (RFs) in given positions of a sphere workpiece is proposed to identify the offset errors, in which only one rotary axis and two translational axes move simultaneously to obtain a circular trajectory. The maximum material removal position of the RFs is defined as the actual tool tip position in MRF process. The difference between ideal and actual tip tool position is applied to determine the relation between actual tool tip position and offset errors. The relations between them are derived with a linear equation, and offset errors are calculated using the least squares method. Finally, figuring correction experiments are performed on an off-axis aspheric mirror to validate the feasibility of the improved technology in an integrated MRF figuring process, in which offset errors are compensated in a postprocessor.

Research of an Open CNC System for MRF Machine Tool Based on UMAC

In order to use magnetorheological finishing (MRF) to accomplish ultra-precision machining of complex curve surfaces, an open computer numerical control (CNC) system based on universal motion and automation controller (UMAC) has been developed. The paper introduces the hardware structure of the system, elucidates the control algorithm and parameters tuning of PID + velocity/acceleration feed-forward + notch filter, and explains the design of a friendly human-machine interface (HMI) to satisfy the need of actual manufacturing and complicated numerical control. With the open CNC system, a 4mm-diameter circle is polished on the MRF experimental prototype with the contour error of 1.67% and the surface roughness Ra of 1.4nm. Experimental results prove that the open CNC system for the MRF machine tool meets the requirements of ultra-precision machining.

Study on the Control System of Area-Contact Magnetorheological Finishing Machine

To achieve the goal of processing large diameter and ultra-smooth optical components, the control system of the area-contact magnetorheological finishing machine is introduced. The system includes motion control systems, structural adjustment systems, detection systems and alarm systems. PLC, the core of the system, is used to data collection, input and output control, process control and so on. The touch screen as the man-machine interface is simple and intuitive, easily to operate. This paper describes the algorithms of this control system and introduces the design of the hardware and software of the system. This control system have a stability performance, strong anti-interference ability. The result shows that the magnetorheological finishing machine could processφ300mm optical components the optical components, radius range of optical components is -∞≤R-30 or 30≤R≤∞, and the surface roughness less than 1nm.

Application of Taguchi Methods and ANOVA in Optimization of Process Parameters for Surface Roughness of Fused Silica in the Magnetorheological Finishing Processes

Surface roughness plays an important role on optical performances for optics in high-energy laser systems. In this study, optical surface of fused silica were polished by the Magnetorheological Finishing (MRF) processes. The polishing factors in term of Magnetorheological fluid (MR fluid) flow rate, polishing wheel rotational speed, electromagnet current, and polishing ribbon penetration depth, were carried out using an self-developed MRF machine to determine optimum conditions for surface roughness. The settings of the MRF processing parameters were determined by using Taguchi’s experimental design method. Taguchi’s orthogonal array, signal-to-noise (S/N) ratio and analysis of variance (ANOVA) were employed to investigate the optimal processing parameters. The experimental results indicate that surface with smaller roughness could be machined under the conditions with slower rotating speed and higher flow rate and current, and nearly independent of penetration depth.

Nano-Precision Finishing Technology Based on Magnetorheological Finishing

Surface roughness is normally regarded as an important criterion for assessing the quality of optic elements; surface roughness of a high-quality optic element is required to be less than RMS1nm. In this paper, a series of experiments has been conducted on the sample magnetorheological finishing machine by using self-prepared magnetorheological liquid as finishing liquid, to assess the removing efficiency of magnetorheological finishing. Optimization of technological parameters enables the authors to obtain a glass-ware with an ideal surface roughness of RMS0.56nm. Magnetorheological finishing (MRF) is an advanced technology for processing optic elements that has been developed in recent years. The technology polishes optic elements by using viscoplastic soft media produced by the MRF liquid under the variation of gradient magnetic field. Better than traditional polishing method in shape precision, surface roughness and inner surface destruction, MRF is an ideal technology for obtaining super-precision optic surface.

磁性流体研磨法によるELID研削面の高品位仕上げ(G13-1 機械加工および工作機械(1),G13 生産加工・工作機械部門)

In this paper, a new finishing method called magnetorheological finishing (MRF) of ELID grinding surface isintroduced. The concept of MRF machine and the surface characteristics by MRF is discussed.