Device for Ultrasonic Assisted Magnetic Abrasive Finishing Process and process thereof
Field of Invention:
The present invention relates to Ultrasonic Assisted Magnetic Abrasive Finishing device for improved surface finish and reduced finishing time and process thereof.
Background and Prior Art:
Traditional machining involving a single process cannot satisfy the present demand for both high quality and high efficiency simultaneously. Thus, a compound or hybrid machining process has to be developed that integrates two or more than two processes to meet the current demands in the industries for various finishing processes. The requirements of high finish, accuracy, and minimal surface defects, such as micro-cracks have necessitated the development of an alternate finishing technology, namely, Magnetic Abrasive Finishing [MAF] [1,2].
There are several existing non-conventional processes like Magneto Rheological Finishing Process (MRF), Abrasive Flow machine (AFM) and variations of Magnetic Abrasive Furnishing (MAF) that can obtain nano-finished surfaces on work pieces. MAF is a process in which surface is smoothened by removing the material in the form of micro chips by magnetic abrasive particles in the presence of magnetic field [2, 3]. The working gap between the workpiece and the magnet is filled with magnetic abrasive particles, composed of ferromagnetic particles and abrasive powder. These particles form a flexible magnetic abrasive brush (FMAB) which does not require dressing [3-5]. The traditional magnetic abrasive finishing (MAF) process has intrinsically superior finishing characteristics [5, 6].
The MAF apparatus was thought to be useful because it neither needs a very precise worktable nor a very stiff structure since its cutting tool is a unique flexible magnetic brush. Nevertheless, a mirror like refined surface of high quality can be obtained easily. Finished surface neither showed a deteriorated layer nor micro-cracks. MAF yielded better surfaces, especially of complex shapes. However, it has the disadvantage of
low efficiency in terms of surface finish improvement when applied to hard materials [5, 6]. Important literature in the field of MAF and related areas has been described as under.
Kim et al. [7] proposed a magneto-electrolytic-abrasive polishing system, which included a magnetic field and an electrolytic abrasive polishing system. Addition of the magnetic field increased the finishing efficiency. However, many difficulties arose when using the process to polish a surface of complex shape.
Yan et al. [5] performed electrolyte magnetic abrasive finishing (EMAF) which involved traditional MAF and an electrochemical machining process. They observed that increase in the electrolytic current and the rate of workpiece rotation increased finishing efficiency, and the surface finish improved rapidly. However, high electrolytic current severely disturbed the distribution of the existing magnetic field, facilitating the extraction of the steel grit from the working gap.
El-Taweel [6] integrated electrochemical turning (ECT) process and MAF to improve MRR and to reduce surface roughness of 6061 AI/AI2O3 (10 %wt) composite. The results demonstrated that assisting ECT with MAF led to an increase in machining efficiency (147.6%) and improvement in surface quality (33%), as compared to that achieved with the traditional ECT.
Below is some of additional non-patent literature which has been referred:
[1] T. Shinmura, K. Takazawa, E. Hatano, M. Matsunaga, Study on Magnetic Abrasive
Finishing, Annals of the CIRP 39 (1)1990 325-328. [2] V. K. Jain, Advanced Machining Processes, Allied Publisher Pvt. Ltd., New Delhi,
2004. [3] D.K. Singh, V.K. Jain, V. Raghuram, Experimental investigations into forces acting
during a magnetic abrasive finishing process, International Journal of Advanced
Manufacturing Technology 30 (2006) 652-662. [4] B. Girma, S.S. Joshi, M.V.G.S. Raghuram, R. Balasubramaniam, An experimental
analysis of magnetic abrasives finishing of plane surfaces, Machining Science and
Technology 10 (3) 2006 323-340. [5] B. H. Yan., G. W. Chang, T. J. Cheng, R. T. Hsu, Electrolytic magnetic abrasive
finishing, International Journal of Machine Tools and Manufacture 43 (2003)
1355-1366.
[6] T. A. El-Taweel, Modelling, Modelling and analysis of hybrid electrochemical turning magnetic abrasive finishing of 6061 A1/A1203 composite, International Journal of Advanced Manufacturing Technology 37 (2008) 705-714.
[7] J. Kim, M. Choi, Development of the magnetoelectrolytic abrasive polishing system (MEAPS) and finishing characteristics of a Cr-coated roller, International Journal of Machine Tools and Manufacture 37((1997) 997-1006.
[8] ASTM A 295/A 295 M-05.
[9] G.K. Grover, S.P. Nigam, Mechanical vibrations, Nemchand and Bros., Roorkee, India (2001). [10] G.K. Lai, Introduction to machining science, New Age International (P) Limited,
New Delhi, India (2003). [11] E.P. Degarmo, J.T. Black, R. A. Kohser, Materials and Processes in Manufacturing, 9th ed. John Wiley and Sons (Asia) Pte Ltd., Singapore (2003).
Literature review presented above reveals that previous attempts [5, 6, 7] for improving the performance of MAF were made for cylindrical work-pieces by applying electrolyte in the machining zone. However, the problem while using electrolyte, is its toxic nature which is hazardous for environment in the era of global warming. Also, at increased current and RPM values, surface finish has been reported to be deteriorating [5, 6,7].
Therefore, looking at the shortcomings and drawbacks of the above prior arts / literature, it was planned to use ultrasonic vibrations with MAF to enhance machining performance like improving surface finish and reducing finishing time by way of a novel concept described herein as Ultrasonic Assisted Magnetic Abrasive Finishing (UAMAF).
The Applicants have further attempted to overcome the disadvantages of the above prior arts by designing an electromagnet to develop circumferential magnetic lines of forces and used to develop experimental setup for MAF and UAMAF.
The disadvantage of the known prior art shows the limitation of the conventional surface finishing process namely the MAF process and deliberates the need to find a solution for improved surface finish and reduced finishing time.
Thus, the present invention overcomes the above stated problems by providing a device comprising of ultrasonic vibrator generator unit in the MAF setup having a specially designed workpiece fixture. The present invention also relates to a process for utilizing the said device towards improved surface finish and reduced finishing time.
The present invention is described herein along with working examples, said examples not limiting the scope and essence of the invention.
OBJECTS OF THE INVENTION:
The principle object of the present invention relates to a device comprising of ultrasonic vibrator generator unit in the Magnetic Abrasive Finishing (MAF) setup having a specially designed workpiece fixture.
Another object of the present invention is to provide a process for utilizing the said device towards improved surface finish and reduced finishing time.
Yet another object of the present invention is to provide a device to determine the effect of process variables on the surface finish obtainable during the UAMAF process.
Still another object of the present invention is to provide a process employing environmental friendly, non-toxic ultrasonic vibrations with the MAF process to improve the surface finish and reduce the finishing time.
The objective of the present invention relates to the design and fabrication of a device to determine the effect of process variables on the surface finish obtainable during the UAMAF process. Another objective of the present invention is to carry out performance evaluation of the UAMAF process by performing experiments with important process parameters such as voltage, abrasive mesh number, rotation of magnet (RPM), abrasive weight percentage, and pulse on time (Ton) of ultrasonic vibrations for plain work piece of hardened AISI 51200 steel. Experimental study using MAF process has also been performed to prove efficacy of UAMAF process over existing MAF process.
A further objective of the present invention is to design an electromagnet to develop circumferential magnetic lines of forces and used to develop experimental setup for MAF and UAMAF, wherein a special work piece fixture is employed so that work piece can be excited using ultrasonic vibrations.
SUMMARY OF THE INVENTION:
The present invention relates to a device for improved surface finish of workpiece and reduced finishing time, said device comprising of
a) Magnetic abrasive finishing set-up consisting of an electromagnet having four poles,
arranged alternatively to generate circular magnetic lines of forces, ferromagnetic abrasive
brush formed of unbonded magnetic abrasive particles along with workpiece fixture; and
b) Ultrasonic vibration generator unit for generating longitudinal movement of the
workpiece.
The present invention also relates to a process for utilizing the said device towards improved surface finish and reduced finishing time.
STATEMENT OF INVENTION:
The present invention relates to ultrasonic assisted magnetic abrasive finishing device for improved surface finish and reduced finishing time and process thereof.
DESCRIPTION OF THE INVENTION:
The present invention relates to relates to a device comprising of ultrasonic vibrator generator unit in the MAF setup having a specially designed workpiece fixture.
The present invention also provides a process for utilizing the said device towards improved surface finish and reduced finishing time.
The present invention provides a device comprising of ultrasonic vibrator generator unit in
the MAF setup wherein an electromagnet has been designed to develop circumferential
magnetic lines of forces , and used to develop experimental setup for MAF and UAMAF.
A special workpiece fixture is utilized so that workpiece can be excited using ultrasonic
vibrations. The forces resulted during UAMAF were measured by mounting the workpiece
fixture on a dynamotor (Kistler-9272, Switzerland). The electromagnet and workpiece
fixture design, selection of process parameters, and experimental procedure is described
herein in detail.
Fig. 1(a). Arrangement of poles in electromagnet
Fig. 1(b). Schematic of annular electromagnet used in previous attempts [3, 4]. Fig.2. Variation offerees with time, (a) MAF (b) UAMAF at input voltage=60 V, rotation
of magnet=224 RPM, abrasive weight=15%, working gap=2mm, SiC abrasive mesh
no.=800, Ton=4 s, and T0ff=2 s (in case of UAMAF only).
Fig. 3. Change in flux density along axial distance from the center of pole towards outer
pole at different voltages. Working gap =1.5 mm.
Fig. 4. Schematic of UAMAF set-up
Fig. 5. Magnification factor v/s frequency ratio for different amounts of damping [10]
Fig.6. The experimental setup of UAMAF. (a) Setup details (b) Horn and workpiece
interface.
Fig. 7. Change in surface roughness with time. Voltage=80 V, rotation of magnet=280
RPM, MAP=Fe powder (75%) + Sic abrasive (25%) by weight, Mesh size: Fe-300 and
SiC-800. Ultrasonic supply power=720 W, and Ton=3 s (in case of UAMAF only).
Fig. 8. Effect of voltage on surface roughness for different RPM (SiC mesh no.=800, wt =
25% and Ton=3 s are constant).
Fig. 9. Effect of SiC abrasive mesh number on surface roughness improvement for
different weight percentage of SiC abrasives (Rotation of magnet=280 RPM, voltage= 70
V, and Ton=3 s are constant).
Fig.10. Effect of percentage weight on surface roughness improvement for different RPM
(Voltage=70 V, SiC abrasive mesh number=800 and Ton=3 s are constant).
Fig. 11. Effect of voltage and mesh number on improvement in surface roughness
(Rotation of magnet=280 RPM, SiC abrasive weight=25%, and Ton=3 s are constant).
Fig. 12. Schematic of forces acting on a MAP during MAF
Fig. 13. Effect of voltage and pulse on time (Ton) on % improvement in surface roughness
(ΔRa). Rotation of magnet=280 RPM, %wt=25, and mesh number=800 are constant.
Fig. 14. Effect of percentage weight and mesh number on improvement in surface roughness (Voltage=70 V, rotation of magnet=280 RPM, and Ton=3 s are constant). Fig. 15. Effect of percentage weight and pulse on time (Ton) on improvement in surface roughness (Voltage=70 V, rotation of magnet=280 RPM, and mesh number=800 are constant).
Fig. 16 (a) Surface roughness profiles: (a) before UAMAF;
Fig. 16 (b) after UAMAF at input voltage=70V, rotation of magnet = 280 RPM, mesh no. =800, wt=25%, Ton=3 s; (c) before UAMAF; (d) after UAMAF at input voltage=70V, rotation of magnet = 280 RPM, mesh no.=400, wt =25%, Ton=3 s. Fig. 17 (a) SEM micrographs (a) Ground surface (Ra=0.1024 fim)
Fig. 17(b) After MAF (Ra=0.0606 fim), voltage= 70 V, Mesh no. = 400, magnet rotation= 280 RPM, % wt= 25. (Ground surface Ra=0.1082/wi)A/?a= 43.99 % (c) After UAMAF (Ra=0.0219 fim), at input voltage=70V, rotation of magnet=280 RPM, mesh no. =800, wt=25%, Ton=3 s; ΔRa = 79.76%.
Fig. 18 (a) SEM micrographs (a) Ground surface (Ra=0.1072fim)
Fig. 18 (b) After UAMAF (Ra=0.0342fim), at input voltage=70V, rotation of magnet=280 RPM, mesh no.=1200, %wt=25, Ton=3 s; ΔRa= 68.10%. Fig. 19 (a) . AFM images ; Ground surface,
Fig. 19 (b) After UAMAF. Voltage= 70 V, Mesh no. =800, magnet rotation= 280 RPM Fig. 19(c) Results
Electromagnet design
The present invention provides an electromagnet designed with four poles which are arranged alternately as shown in Fig. 1. The windings of the coils have been done on each of the pole with number of turns as 480 with maximum current rating of 1A to generate magnetic field intensity in a range of 0-0.2 Tesla at bottom of the tool. The advantage of this kind of design of electromagnet tool is to generate approximately circular magnetic lines of forces at the bottom of the tool as shown in Fig. 1(a).
The direction of cutting forces (Fc) in this design of magnet (Fig. 1(a)) is along the magnetic line of forces but in annular magnet used in various previous attempts [4, 5], cutting forces (Fc) is perpendicular to the magnetic lines of forces (Fig. 1(b)) and thereforz
resulting in less finishing efficiency. The magnetic lines of forces obtained in new design of electromagnet (Fig. 1(a)) are capable of holding abrasives along the direction of cutting (tangential direction) with an ability to hold FMAB to finish even very hard work surfaces at low magnetic force values [6]. Fig. 2 shows the variation of normal force (F:) required to hold FMAB together and the resulting torque during finishing process in MAF and UAMAF respectively. The normal force is within 12-16 N range, and the average torque value is within 2-8 N-m for UAMAF.
The normal forces and cutting torque during finishing (Fig. 2) for the designed electromagnet show that this design of magnet is able to finish the workpiece with lower forces as compared to previous attempt [3]. The increase in torque in UAMAF (Fig. 4(b)) is due to ultrasonic vibrations. The forces were in the range of 30-100 N as reported by Singh et al. [3] and overall range of forces for various process parameters was less than reported Singh et al. Thus, the new designed electromagnet finishes the workpiece surface with very gentle forces.
Upon energizing, the magnetic poles form a flexible brush of magnetic abrasive particles (MAPs) around them. The magnetic flux density in the working gap is varied by changing input voltage to the electromagnet, and it is measured by using a digital Gauss meter (model DGM-102, range 0- 2.0 T) probe in the working gap between pole and workpiece. The measurement of the flux density has been carried out by moving probe of the Gauss meter perpendicular to the direction of magnetic field, from center of magnet to the end along the north pole, in the vicinity of steel workpiece as shown in Fig. 3.
Workpiece fixture design
Fig. 4 illustrates schematically the experimental setup of UAMAF. The workpiece was held in the specially designed fixture. Along with holding the workpiece, the main aim of fixture is to facilitate small longitudinal movement to the workpiece. The small longitudinal movement may promote the relative motion of abrasives against the peaks of the workpiece surface needed for the finishing operation in addition to rotary motion of FMAB. The abrasives cutting edges may scratch the workpiece material in many directions in intermittent cutting mode. Hence, the small longitudinal movement of the workpiece is very important to get the effect of ultrasonic vibrations in UAMAF to finish the workpiece surface effectively. The workpiece was mounted on the support of four steel balls (6 mm diameter) to give minimum friction at workpiece and balls interface in the fixture.
Fig. 5 shows the relationship between magnification factor and frequency ratio (ω/ωn) for different damping coefficients ()[9]. It can be seen from Fig. 5, that the highest amplitude is possible only at resonance where the ratio of excitation frequency ()and natural frequency of the system {ωn) is one. But, because of high excitation frequency (20 KHz) it is impossible to have resonance condition as natural frequency of system (which is spring and workpiece in this case) is much below the ultrasonic frequency. However, the curve helps in finding out the amplitude of vibration of workpiece. Therefore, the spring design in the fixture was important. The spring should not be so rigid that it would resist the deflection and thus resulting very small amplitude of vibration. Also, to get the spring of particular stiffness at smaller length and diameter is not possible. Hence, based on trail and judgment spring was selected to give suitable amplitude to the workpiece. In this work, the ω is 125600 rad/s and the ωn is 47.11 rad/s. The stiffness of the spring used in the fixture is 1223 N/mm.
Experimental Setup
The UAMAF device of the present invention consists of an ultrasonic vibration generator unit (Table 1), and a special designed workpiece fixture. The ultrasonic vibration generator unit consists of a power supply, piezoelectric transducer and a concentrator or horn. The ultrasonic power supply generates high frequency electrical signals, which are supplied to the piezoelectric crystals within the transducer. The high frequency electrical signals of 20 KHz are converted to mechanical vibrations by the transducer. The amplitude achievable from the transducer does not exceed 3 to 5 microns. Hence, the amplitude is amplified by the concentrator or horn and then transmitted to the workpiece which is connected to the horn. Table 1 Details of ultrasonic vibration generator unit
(Table Removed)
The amplitude of vibration available at the workpiece end, held in the fixture varies from 5 to 12 urn depending upon input power supply. The amplitude of vibration at the workpiece end is measured by a dial indicator having least count of 1 µm. The concentrator
transmits the ultrasonic vibration to the workpiece in the longitudinal direction as shown in Fig. 4. The amplitude of the ultrasonic vibration can be adjusted by changing the input power to transducer through power supply. The experimental setup used to study UAMAF is shown in Fig. 6. The entire setup (Fig. 6(a)) is mounted on the table of milling machine (BFW, India). The Fig. 6(b) shows the interface of the workpiece and the ultrasonic horn. The workpiece is in direct contact of horn and ultrasonic vibrations are transmitted to the workpiece through horn.
Selection of process parameters
The process parameters and ranges used for experimentation are listed in Table 2. They have been selected based on the literature survey [3-7] and setup constraints. The initial surface roughness of ground work pieces is not identical for all the work pieces and it is in the range of 0.10 to 0.45 µm. To take into account the variation in the initial surface roughness, a ratio of improvement of surface roughness to the initial roughness has been considered as the response during this experimentation and is given by eq. 1.
(Equation Removed)
Table 2 Process parameters to study improvement in surface finish
(Table Removed)
Experimental procedure
In the present invention, the mixture of unbonded magnetic abrasive particles was prepared in the different ratios of percentage weight as mentioned in Table 2. The working gap was maintained at 1.5 mm owing to difficulties of machine used for experimentation. The total MAPs weight was 8 gm in which iron particles (mesh no. 300) and SiC particles
were mixed by percentage weight. Other examples of abrasives suitable to be utilized in the present invention may be AI2O3 and diamond. The influence of the abrasive grain diameter on material removal has been found comparatively small, while it is remarkably large on surface roughness as reported by Shinmura et al. [1]. Therefore, in the present study the mesh number of iron powder is kept constant for all experiments. The workpiece material considered in the present work was high carbon anti friction bearing steel with hardness 61 HRC. Hence, SiC abrasives were used for the experimentation [10, 11]. The composition of workpiece material is given in Table 3 below [8].
Table 3 Workpiece composition (AISI 52100)
(Table Removed)
Initially all workpieces were ground to get surface roughness (Ra) values in suitable range (varied between 0.10 to 0.45 urn Ra). Ra values were measured at three different positions to get average roughness value. The Ra values were measured by using Talysurf 6 (Taylor Hobson, U.K., resolution 16 nm) with a cut-off evaluation length of 0.8 mm. The measurement of surface roughness was carried out in the same area before and after MAF or UAMAF.
Based on the initial trail experiments, finishing time was finalized for all the experiments. The feed value was kept zero i.e. no feed was given during experimentation. Fig. 7 clearly shows that after 80 and 120 seconds of finishing time surface roughness starts increasing for UAMAF and MAF respectively. This may be because abrasive powder becomes blunt after a certain period of time and needs replacement. Hence, all experiments were conducted for 80 seconds in case of UAMAF and 120 seconds in case of MAF. The input power supply to transducer unit was kept constant at 720 W (60%). It can be seen from Fig. 7 that best possible surface finish achievable by UAMAF is much better than MAF and in almost 33% less time.
4. Results and Discussion
4.1. Comparison of MAF and UAMAF for surface roughness improvement
The comparison of surface roughness improvement (ΔRa) in MAF and UAMAF with respect to different process parameters has been presented in Table 4. The Table 4 clearly shows that UAMAF has better ability to improve surface finish than MAF. The improvement ratio ranges from 27 to 124% depending upon the process parameters used for finishing. The effect of various process parameters on the performance of MAF and UAMAF along with comparison of the two processes has been discussed below. Table 4 Comparison of surface roughness improvement for MAF and UAMAF (Table Removed)
"Improvement ratio (%) = (ΔRaVAMAF - ΔRaMAF)/ΔRaMAF
The effect of voltage on A/to for MAF and UAMAF is shown in Fig. 8. It clearly shows that with increase in voltage or magnetic flux density (Fig. 3), A/to increases. However, the effect of the voltage is more dominant in case of UAMAF than MAF. Increased flux density enhances the strength of the brush leading to increase in normal forces acting on abrasives. This results in indentation of magnetic abrasive particles (MAPs) on the workpiece surface to get improved surface finish or mors ΔRa. In UAMAF, as ultrasonic vibrations are transmitted to the workpiece, the MAPs strike to workpiece surface with higher kinetic energy. The abrasive tends to deviate from the circumferential tracks due to vibration. The abrasive particles of FMAB make impact on the peaks of surface texture of the workpiece. This results in rapid removal of peaks from workpiece surface and enhances surface quality.
It can be seen from Fig. 9 that ΔRa decreases with increase in mesh number in MAF as well as in UAMAF. This may be because of higher mesh number (lesser is the average grain diameter of abrasives), more number of abrasives get accommodated in FMAB and reduce the strength of flexible magnetic abrasive brush (FMAB). Hence, there is overall reduction in the improvement in surface roughness in case of both the processes.
Increasing percentage weight of SiC abrasives results in increase in A/to and it decreases with increasing RPM for both the processes (Fig. 10). This is because of more cutting edges has been available with increased percentage weight, which will cut the peaks of workpiece top surface effectively to give good improvement in surface roughness. In case of UAMAF, ΔRa starts decreasing after 27% weight (approximately) because of impact of more cutting edges associated with increased percentage weight which will end up in deteriorating surface finish.
It can be seen form Fig. 10 that up to 27% weight abrasive weight (%) dominates the UAMAF process and after that ultrasonic vibrations dominate the finishing process. In case of MAF, reduction in ΔRa was reported for higher mesh numbers (or less abrasive grain diameter) with high percentage weight of abrasives [4]. Therefore, it is concluded that UAMAF has better ability than MAF to finish the workpiece surface for the various process parameters.
4.2. Effect of process parameters on the performance of UAMAF
The effect of RPM and voltage on the surface roughness improvement (ΔRa) is shown in Fig. 11. The figure shows that with increase in voltage (magnetic flux density), ΔRa increases. Increased flux density enhances the strength of the brush leading to sufficient indentation on the peaks of the surface texture of the workpiece surface, and results in increased ΔRa.
It has been felt that at higher speeds, centripetal force (Cp) acting on MAPs would not be sufficient to hold MAPs together (Fig. 12). At higher speeds abrasives start jumbling irregularly increasing the frictional force [3]. Also, due to ultrasonic vibrations high kinetic energy is induced in MAPs. The tangential force (F,) necessary for cutting peaks obtained in grinding may not act due to insufficient centripetal force. Hence, the MAP grains would tend to move outward and consequently, may not cause effective material removal. Therefore, at higher spindle speeds, the machined surface could be rougher even if supply voltage is more (Fig. 11).
ΔRa increases with increase in voltage and pulse on time (Fig. 13). Striking of MAPs on the workpiece surface would be for more time duration with increase in Ton. So, there would be more interaction of abrasives with peaks on the workpiece surface and
abrasives would be making impact on the peaks. This results in cutting/shearing off the peaks of workpiece surface to get improvement in surface finish.
As percentage weight increases, more cutting edges are available (in case of smaller mesh number grain or larger size grain diameter of abrasives). This will cut the peaks of workpiece top surface effectively to give good improvement in surface roughness. However, at smaller diameter of abrasives (or larger mesh number), number of abrasives available per unit area may be more and hence reduce the total normal magnetic force available for producing the indentation required. Also, more number of cutting edges of abrasives available in smaller area will deteriorate the surface finish [4].
The effect of percentage weight and SiC abrasive mesh number on surface roughness improvement is shown in Fig. 14. It can be seen from Fig. 14 that there is reduction in improvement in surface roughness at higher values of percentage weight and mesh number of abrasives. As weight of abrasive is increased, cutting edges available for cutting the peaks of surface texture will be more. MAPs strike on workpiece surface with ultrasonic vibrations, furthermore increasing percentage weight and mesh number may deteriorate surface finish as ultrasonic vibrations dominate the finishing process.
Increasing percentage weight of abrasives, ΔRa increases initially and starts reducing further increasing weight beyond 27% approximately (Fig. 15). As Ton and percentage weight are increased, higher the kinetic energy with which the abrasive strike the work surface. Hence, the greater the material removal rate per active grain will be more due to the high frequency interaction of active grains on the AISI 51200 steel workpiece. Furthermore, increasing Ton may deteriorate the surface finish due to continuous interaction of high energy impacts of abrasives on workpiece surface.
Study of surface integrity generated by UAMAF
The surface roughness profiles obtained from Talysurf 6 (Taylor Hobson, U.K., resolution-16 nm) for various conditions of UAMAF of AISI 51200 steel are shown in Fig. 16. It can be seen that the maximum peak to valley height has been reduced to approximately half due to finishing. Fig. 16(b) shows that the peaks and valleys are leveled and almost less than 0.1 urn giving surface roughness of the order of 22 nm.
The surface roughness profiles alone do not reflect the interaction of cutting edges of the abrasives with workpiece material during UAMAF. Therefore scanning electron microscopy (SEM) and atomic force microscopy (AFM) have been performed. Fig. 17 and 18 show SEM micrographs of the ground surface and UAMAFed surfaces obtained for different processing conditions. The SEM micrographs and roughness profiles together show the mechanism of material removal involved in UAMAF process. Grinding marks, pits and digs can be seen in Fig. 17(a) and 18(a). They disappear after UAMAF as shown in Fig. 17(c) and 18(b) for AISI 51200 workpiece.
Fine scratches produced by the relative motion of abrasives against workpiece surface appear on the surface finished by UAMAF. Along with the rotational motion of the electromagnet, the abrasive particles in the FMAB impact the machined surface at different places. Surface profiles obtained during grinding seems to be sheared off and the new surface profiles containing fine scratch marks have been produced during UAMAF process. These fine scratches are due to high energy impacts of abrasives on the workpiece surface. The SEM micrographs, at a magnification of 2000, indicate that the finished surface is quite smooth with fine scratches distributed randomly. The lays obtained after MAF as shown in Fig. 17(b) disappear after UAMAF.
The finished surface is observed using an atomic force microscope (AFM). Fig. 19 shows the AFM images before and after UAMAF. The surface topography shows that the high peaks and deep valleys of the ground surface (Fig. 19(a)) are flattened, leaving behind fine scratches resulting due to high energy impacts of abrasives under the influence of ultrasonic vibrations, which can be seen in Fig. 19(b). The roughness profile ranges from -200 nm to +200nm at a particular section as shown in Fig. 19(a). These profiles are almost flattened and ranging from -15 to +15 nm (approximately) by UAMAF (Fig. 19(b)). It is because of the rotational motion of the FMAB and the longitudinal movement of the workpiece due to ultrasonic vibrations, the scratches of the abrasive particles and micro-grooves on the finished surface are observed (Fig. 19(b)).
Advantages:
1. In the present invention an UAMAF setup has been designed and fabricated. High carbon antifriction bearing steel workpiece (AISI 52100) having hardness value of 61 HRC has been successfully finished using unbonded MAPs. The surface finish
roughness of the order of 22 nm could be obtained within 80 seconds. Thus, it can eliminate use of hazardous chemicals causing pollution problems in the environment.
2. The present invention also relates to an electromagnet which has been designed to give better surface finish at lower values of magnetic forces.
3. The present invention facilitates Improvement in surface finish can be obtained as high as 79% and surface profiles obtained after UAMAF are almost flat.
4. At higher RPM values, lesser improvement in surface finish has been observed as centripetal force necessary to hold FMAB may not be sufficient at high RPM values. Increasing supply voltage leads to increase in surface finish improvement.
5. The surface finish of a ground AISI 52100 steel workpiece with initial average Ra of the order of 100 nm can be improved up to 22-31 nm within 80 seconds of time using the developed process. Hence, there is overall 33% reduction in time required for finishing when compared with MAF.
6. Though, ultrasonic vibration has been induced in the workpiece, there has been no tool wear unlike ultrasonic machining. The ultrasonic vibrations helped in eliminating the abrasive shear tracks which might have been resulted if only MAF would have been performed.

We Claim:
1. A device for improved surface finish of workpiece and reduced finishing time, said
device comprising of
a) Magnetic abrasive finishing set-up consisting of an electromagnet having four poles,
arranged alternatively to generate circular magnetic lines of forces, ferromagnetic abrasive
brush formed of unbonded magnetic abrasive particles along with workpiece fixture; and
b) Ultrasonic vibration generator unit for generating horizontal movement of the
workpiece.
2. A process for improved surface finish of workpiece employing the device as claimed in
claim 1, said process comprising of the steps of:
a) Applying electromagnetic field in a range of 0-0.2 Tesla, on the magnetic abrasive particles;
b) providing ultrasonic vibrations to the workpiece;
c) actuating longitudinal motion of the workpiece fixture caused by ultrasonic vibrations
d) generating relative motion of abrasives against the peaks of work piece surface by
longitudinal motion of step c)
3. A process as claimed in claim 2 wherein variable process parameters namely voltage in
the range of 50-90 V, Abrasive mesh number of 400-1200, Rotation of magnet of 180-450
RPM, percent weight of SiC abrasives in the range of 15-35% wt., Pulse on time of
ultrasonic vibrations of 1-5 seconds on are employed.
4. A device as claimed in claim 1 substantially as herein described with reference to the
accompanying drawings.