Magnetic Field Assisted Abrasive Polishing

Abstract

Surface roughness of both machined and additively manufactured components is a critical characteristic that ensures both functional requirements and efficient performance in many high-precision applications requiring precise surface morphologies such as those used in the aerospace and medical industry. Due to uncertainties and the complex nonlinear nature of hundreds of manufacturing process parameters ranging from melt pool temperature to velocity of the cutting tool, the surface roughness of the manufactured product is not controllable and hence requires post-processing.

In this study, a system to remove material and reduce the surface roughness of physically hard-to-reach external and internal surfaces of a non-magnetic workpieces is designed, developed and tested for efficacy and flexibility. Unlike conventional Magnetic Abrasive Finishing (MAF), this technique does not involve any moving parts to impart rotating motion to either the electromagnet or the workpiece. The system works on the principle of generating Rotating Magnetic Fields (RMF) to manipulate the motion of Magnetic Abrasive Particles (MAPs) on the surface of a workpiece for material removal in the form of micro and nano scale chips. An optimal configuration of stationary electromagnets is activated using an FPGA control unit coupled with digital servomotor drives and a DC power supply to generate the required magnetic flux density in the working region. The magnetic field gradient can be significantly magnified by selecting an appropriate core tip shape from a selection of eleven different tip shapes resulting in reduced leakage of magnetic flux. The modular nature of the system allows the operator to change the core tip shapes, the orientation of coils, current, amplitude, frequency, and phase difference at any instant. Moreover, the entire setup is mounted on four caster wheels that can be easily moved and safely secured at any location.

Finite element parametric optimization in Ansys Maxwell is used to design and fabricate a novel tapered electromagnet geometry optimized for maximum gradient of magnetic field on both axial and off-axial locations. The performance of the optimized geometry is validated via both analytical and experimental results with less than an average error of 10%. The novel electromagnet shape is designed to reduce the distance between adjacent coils and maintain a uniform distribution of magnetic field in the workspace.

To quantify the effectiveness of this technique, different optimal electromagnet configurations are implemented on aluminum specimens subjected to six different abrasives and tested under a Laser scanning confocal microscope. The average surface roughness (Ra) is improved from 93% to 36% depending on the input process parameters, namely current, frequency and cycle time.