Magnetic Abrasive Finishing Process

Contents S NoMajorPage No 1Introduction1 2Process details1 3Mechanism of Material removal5 4Process Parameters Analysis6 5Conclusions7 6Advantages8 References9 Magnetic Abrasive Finishing (MAF) Process Harry P. Coats first patented MAF in 1938. Although US originate this idea, most of later period development is done by USSR + Bulgaria. Japanese explore the technology for polishing purpose. Other countries in this field are: India, CIS, England, France, and Germany etc. Process details: •In MAP, w/p is kept between the two magnets & the air gap in-between the w/p & the magnet is filled with Magnetic Abrasive Particles (MAPs).

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The MAPs joined to each other, along the lines of magnetic force and form a Flexible Magnetic Abrasive Brush. The brush behaves like a multi-point cutting tool for finishing process. The vibrational, rotary & axial motion is imparted to the w/p to enhance the performance of finishing operation •For demonstration, there are three setups commonly used. These are: 1. External contour of cylindrical workpiece: the systematic diagram are as follow: The process principle of magnetic abrasive finishing is shown in fig.

The magnetic abrasives are joined to each other; magnetically in-between magnetic poles ‘N’ & ‘S’ along the lines of magnetic forces, this imparts flexibility of magnetic brushes. Flexibility to brush means it ability to modify itself as per workpiece contours. When a cylindrical workpiece; with rotatory and translatory motions; is inserted, the surface and edge finishing are performed by these magnetic abrasives brushes. The magnetic field polishing showing the two-dimensional magnetic field distribution in the finishing zone of the process.

The magnetic abrasive particles form a brush around the workpiece linking the N & s poles. The magnetic flux density is stronger around the nonmagnetic workpiece (along the magnetic brushes) than through the workpiece. The magnetic abrasive at position “A” in fig. is affected by the magnetic forces represented by the following equations: Fx= VCH *? H/? X FY= VCH *? H/? Y Where V=volume of magnetic abrasive particle, C= susceptibility of the particle H= magnitude of magnetic field strength at point “A” X & Y are coordinate points fixed at A, nd ? H/? X, ? H/? Y are gradient of magnetic field strength in X and Y directions. From above equations it is evident that the magnetic forces Fx, FY are proportional to the volume of the magnetic abrasive particles, the susceptibility of the particle, the magnetic field strength and its gradient. If the gradients are not equal to zero then magnetic abrasive particles are pushed toward the work surface. The magnetic force FY actuates the magnetic abrasive particles to take part in the surface finishing of the workpiece.

In addition, the force Fx is acting on the abrasive grain in the rotating tangential direction of the work surface by cutting and frictional action. The runoff of the abrasive grains from the working zone is prevented by the FY. From the above formula if X is non-zero the magnetic force will not act, if ? H/? X, ? H/? Y is equal to zero. Thus both susceptibility and magnetic field gradients are important in this operation. The larger values of magnetic strength gradients; forces the abrasive grains to move towards the working zone thus preventing separation and splashing of the abrasive grains from the working zone.

The magnetic flux density is stronger around the non magnetic workpiece ( along the magnetic brushes) than through the workpiece 2. Internal contour of hollow cylindrical workpiece: 3. Flat surfaces: Let’s have a look on the basic functional elements of MAF. These are: 1. Magnetic Abrasives Particles (MAPs): MAPs are made up of ferromagnetic material (e. g.. iron powder=300 mesh size i. e. 51. 4? m) and abrasive grains (e. g. Al2O3 =600 mesh size=25. 7? m, SiC, Diamond powder). Generally ferromagnetic particles size is kept larger than abrasive size. The abrasives are the small size cutting tools.

MAPs can be un-bonded & bonded: •Bounded abrasives are prepared by sintering of ferromagnetic iron powder (forms the matrix) + abrasive powder + small amount of lubricant (to add holding strength); at a very high pressure & temperature; in an inert gas atmosphere. Theses results in better surface finish. •Unbonded abrasives are the mechanical mixture of ferromagnetic iron powder + abrasive particles without any lubricant. These yields higher Material Removal Rate (MRR) because of the availability of free abrasives that can scratch much deeper that the bonded. . 2.

Workpiece material: The workpiece can be ferromagnetic or non-ferromagnetic. It is widely used for the tubes & for cylindrical workpiece (external + internal surface) and the flat workpiece. 3. Magnetic Poles: ‘N’& ‘S’ Poles, are Electromagnetic in nature, and is of a good quality. Here we assume ideal poles. Magnetic brush: the iron particle being responding to magnetic field gets in alignment along the magnetic flux line & the abrasives particle in between, forming a stable brush, they retains this shape, the brush is rigid, during the operation.

The brush gets stick to contour of workpiece; the electro magnetically generated field is providing required pressing pressure. The workpiece can be given rotary, axial and vibratory motion Assumptions: The magnetic field is induced by electromagnetic phenomenon & the gradient of the magnetic field in the air-gap causes the required machining pressure. It is assumed that: •The system does not permit the leakage of magnetic field. •Magnetic core is saturated, uniformly throughout the section. •Magnetic field force on the abrasive grain powder, the coefficient of friction, and the frictional force are constant.

Mechanism of material removal: •The work piece to be machined is located between the two magnetic poles and the gap in-between the workpiece and the poles are filled with magnetic abrasives. The magnetic field retains the powder in the gaps, ferromagnetic particles acts as a binder, which causes the powder to be pressed against the surface to be machined. Cooling fluid is supplied into the gaps if needed and the required cutting movement (work piece rotates and oscillates, or the work piece rotates and the poles oscillate) are imparted by known electromechanical drives. The working gap is kept constant during the process. MAF is a machining process with free abrasives. It is convenient to deal with MAF in two stages: before the cutting or static stage and during the cutting or dynamic stage. 1) In the first stage the gaps between the cylindrical workpiece and the poles are filled with magnetic abrasive powder. The magnetic field retains the powder in the gaps, forms two abrasive cutting tools called magnetic brush, and presses them to the nearest layer of the surface to be machined. The grains of the powder tend to smoothens the surface by entering into the valleys of the unevenness of surface.

The results of electron microscopic investigation of micro-profiles and sub-micro-profiles of the abrasive grains show that the grains surfaces of the powder match the unevenness of the workpiece surface. In other words, the peaks of the grains enter into the valleys of the workpiece. 2)In the second stage, when the workpiece is rotated and oscillated, the grains are displaced to the exits of the gaps and are compacted. When the workpiece is rotated, the grains that are held by magnetic field do not rotate. The grain moves within the limits of the shoes (poles).

The force of the magnetic field acts on the grains preventing them from contacting the poles and pressing them against the workpiece. This way is commonly found in fixed abrasive process e. g. grinding, polishing, lapping etc. It is possible to have the following cases of grains contact with the workpiece: •A peak of the grains enters into the valley of matching unevenness of the workpiece. •Peaks of the grains contact the peaks of the uneven surface of the workpiece. Process parameter: The influences of various parameters are: 1.

Effect of vibrating amplitude/ frequency: •With increase in vibrations the stock removal improves hence surface roughness will decreases. The increase is due to increase of finishing distance per unit time & Multi-directional finishing effect. 2. Effect of working clearance on stock removed / surface roughness: •With increase of clearance roughness increases hence stock removal decreases this is due to decrease in magnetic field density across the gap. 3. Effect of magnetic flux density (T) on stock removal/surface roughness: •As the ‘T’ increases (up to 1. 2-1. T) stock removal increases rapidly due to more rigidity of brush and due to increase of magnetic pressure. 4. Effect of cutting fluid: •It improved stock removal and surface finish, because the grain cutting traces becomes deep by the addition of machining fluid, also results in stirring action of magnetic abrasive yield. It also brings down the temperature rise. E. g. Zinc-stearate, stearic acid, servopin-12 etc. 5. Effect of size of abrasives on stock removal/surface roughness: •For a given size, the roughness improves first, but then becomes constant after ue period of time. •Lager the particle size rougher the surface but higher stock removal or radius decreases. 6. Effect of rotational speed and axial vibration on stock removal/surface roughness: •Without axial vibrations and with only rotation, circumferential grooves will form and stock removal rate is lowest. •By introducing axial vibration the resultant velocity will be changed which will results in cross-hatching also removal rate increases. Hence by varying both motions different types of hatched patterns are obtained. 7.

Effect of abrasive composition on stock removal/surface roughness: •For a given size, as the iron % increases the stock removal or machining depth increases; and reaches to a highest value then decreases; because then effective cutting tools will decreases. But surface roughness first decreases then constant then increases. •But as iron-particle size increases machined depth decreases but surface finish improve. 8. Effect of magnetic flux density (T) on magnetic pressure : •Pressure exerted by magnetic abrasive decreases as clearance increases provided that the filling density of magnetic abrasive grains as the gap remains constant. Pressure acting on work surface increases as flux density of magnetic abrasive grains Increases for a given clearance. 9. Effect of finishing time on stock removal/surface roughness: •As the finishing time increases the stock removal improves but there is decrease in roughness as time elapsed. Conclusions: The research conclusion of magnetic abrasive machining are: •The magnetic flux density in the air gap is affected greatly by the length of the air gap; it increases as the gap length decreases. The machining pressure between the magnetic brush and the work piece has its maximum value at about B=1. 2T. •OOR Error (out of roundness error) it can be corrected: by research it was found that cutting tool (i. e. abrasive grains) tends to occupy stable position during their contact with the cylindrical surface when the part is rotated. A rotation of the parts leads to idealize the form of the cross section of the body of revolution. •Magnetic field assisted polishing apparatus that can be incorporated on a conventional lathe and can be used for finishing non-magnetic stainless steels rollers.

Consequently this process can be economical and cost effective. •The surface finish of a grounded rod can be finished to about 10nm. •While unbonded magnetic abrasive are found to yield higher removal rates, bonded magnetic abrasive are found to give better finish. •Increasing the magnetic flux density was found to increase the rate of finishing Run out is measured by setting up work piece datum axis (established by work piece centre) in inspection equipments centers that are parallel to the surface plate.

Using a gauge indicator the entire profile of the work piece must lie with two cylindrical tolerance zones 0. 001 inch apart. MAP is insensitive to run out is capable of reducing the OOR error even when run out is present. The amount of OOR error depends on machining conditions such as the number of magnetic poles, the speed of oscillation o the work piece or the poles, the width of the gap between the poles and the work piece, the magnetic flux density, the speed of cutting, and others. Advantages: •Internal & external polishing of tubes can easily achieve rather than other methods. Since magnetic force is used for creating magnetic pressure, the process is controllable because magnetic pressure is can be easily controlled by input current to the coil of solenoid. •For achieving better machining efficiency the surface roughness is predicted as a function of finish time, hence volume of material removed per unit time, these data can be future utilized for any surface roughness model for introducing automation to the system. •Two characteristics i. e. small chip size and self-sharpening of the tool.

These characteristics give the process a stability of tool sharpness and a level of surface finish that is difficult to attain by any other process. It is also applicable for machining hard and difficult to machine (DTM) materials that currently cannot be machined by any other process. • The work piece tolerance is constrained by the machine tools / work piece system relationship for given machining process. The MAP is capable of achieving a surface roughness Ra=0. 04 ? m and an out of roundness (OOR) of 0. 5 ? m. •MAF results in extremely small sized chips and the self-sharpening of tool. MAF is classified as a resilient tool. •MAF can be used for correcting OOR error. •Enhances surface integrity by applying a residual compressive stress. References: 1. Jain R. K & Jain V. K. “Abrasive Fine Finishing Process-A Review”. Vol. 2 No. 1, 1999. 2. Jain V. K, Kumar Prashant, Behera P. K, Jayswal S. C. “Effect of working gap & circumferential speed on the performance of magnetic abrasive finishing process”. Wear 250 (2001) 384-390. 3. Kremen G. Z, Elsayed E. A, Ribeiro J. L. “Machining time estimation for magnetic abrasive processes”. Int. J. Prod. ,1994,vol. 32,No. 12,2817-2825 4.

Kremen G. Z, Elsayed E. A, Rafalovich V. L. “Machining time estimation for magnetic abrasive processes”. “Mechanism of material removal in the magnetic abrasive process and the accuracy of machining”. Int. J. Prod. , 1994, vol. 32, No. 12, 2829-2838. 5. M. Fox, Agrawal, T. Shinmura, R. Komanduri(1),Oklahoma State University,Stillwater,OK,USA. “Magnetic Abrasive Finishing of Rollers”. Jan 11, 1994. 6. Takeo Shinmura, Toshio Aizawa. “Study on a New Finishing Process of Fine Ceramics by Magnetic Abrasives Machining”. Int. H. Japan Soc. Pre. Eng. , Vol. – 28, No. -2, June 1994.

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