Magnetic field assisted abrasive based micro-/nano-finishing

Abstract

Micro-/nano-machining (abbreviated as MNM) processes are classified mainly in two classes: traditional and advanced. Majority of the traditional MNM processes are embedded abrasive or fixed geometry cutting tool type processes. Conversely, majority of the advanced MNM processes are loose flowing abrasive based processes in which abrasive orientation and its geometry at the time of interaction with the workpiece is not fixed. There are some MNM processes which do not come under the abrasive based MNM category, for example, laser beam machining, electron beam machining, ion beam machining, and proton beam machining. This paper gives a comprehensive overview of various flowing abrasive based MNM processes only. It also proposes a generalized mechanism of material removal for these processes. The MNM processes discussed in this paper include: Abrasive Flow Finishing (AFF), Magnetic Abrasive Finishing (MAF), Magnetorheological Finishing, Magnetorheological Abrasive Flow Finishing, Elastic Emission Machining (EEM) and Magnetic Float Polishing. EEM results in surface finish of the order of sub-nanometer level by using the nanometer size abrasive particles with the precisely controlled forces. Except two (AFF and EEM), all other processes mentioned above use a medium whose properties can be controlled externally with the help of magnetic field. This permits to control the forces acting on an abrasive particle hence the amount of material removed is also controlled. This class of processes is capable to produce surface roughness value of 8 nm or lower. Using better force control and still finer abrasive particles, some of these processes may result in the sub-nanometer surface roughness value on the finished part. Understanding the mechanism of material removal and rotation of the abrasives in these processes will help in rationalization of some of the experimental observations which otherwise seem to be contradicting with the established theories. It also explains why a magnet used in MAF should have a slot in it even though the area under the slot has “non-machining” zone. It elaborates based on the experimental observations why to use pulse D.C. power supply in MAF in place of smooth D.C. power supply.

Introduction

Fabrication of products deals with the building of machines, structures or process equipment by cutting, shaping, welding and assembling of components made of the same or different materials. Fabrication can be classified into two main categories: macro-fabrication and micro-fabrication. The first one considers the process of fabrication of structures/parts/products/features that are measurable and observable by naked eye (≥1 mm in size) while the second category deals/considers the miniature structures/parts/products/features which are not easily visible with naked eye, and have dimensions smaller than 1 mm (1 μm ≤ dimension ≤ 999 μm). There are various methods/ways by which micro-fabrication of products can be achieved (Fig. 1). However, two of them are the most commonly used: (1) material deposition, and (2) material removal. This article deals with the material removal processes only. Some of the traditional material removal processes can be used for micro-fabrication. However, they have some constraints hence advanced material removal processes are more commonly used for this purpose. Fig. 2(a) shows a classification of advanced micro-/nano-machining (MNM) and micro-/nano-finishing (MNF) processes.

Majority of the advanced material removal processes can be employed for both, macro-machining as well as for micro-machining. While scaling down the applications of a process from macro- to micro (μ)-machining, the μ-machining process parameters have to be appropriately changed. Further, advanced micro-machining processes (abbreviated as AMMPs) are used for two main purposes: (i) shaping and sizing a part, (ii) finishing a part. For differentiating between these two classes, the first one is called as AMMPs and the second one as advanced micro-/nano-finishing processes (AMNFPs). The AMMPs can be further sub-categories as (i) those processes which use abrasive particles as tools for removing material from the work piece in the form of micro-/nano-chips, and (ii) those processes which use direct energy for removal of material by melting and/or vaporization, or electrochemical or chemical reaction (Fig. 2(a)). The AMMPs deal with fabrication of microstructures, generation of micro-features (say, micro-grooves, micro-cavity, micro-channels, etc. [Jain, 2009]).

This paper deals only with those processes which fall in the category of abrasive based micro-/nano-finishing processes. Some of the abrasive based advanced (micro-/nano-finishing) and traditional finishing processes are given in Fig. 2(b).