STUDY OF PROFILE CHANGES IN MAGNETO-RHEOLOGICAL ABRASIVE HONING BY AN INGENIOUS RELOCATION TECHNIQUE

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1 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 214) December 12 th 14 th, 214, IIT Guwahati, Assam, India STUDY OF PROFILE CHANGES IN MAGNETO-RHEOLOGICAL ABRASIVE HONING BY AN INGENIOUS RELOCATION TECHNIQUE S. Chidambara Kumaran 1 and M.S. Shunmugam 2 1 Dept. of Mech. Engg., IIT Madras, Chennai-636, India 2 Dept. of Mech. Engg., IIT Madras, Chennai-636, India, shun@iitm.ac.in Abstract Magneto-rheological abrasive finishing is a non-traditional method of improving the surface finish of manufactured components. Performance of different variants of this finishing process in terms of surface roughness parameters and material removal with process variables have been reported in the literature. In this paper, an ingenious relocation profilometry is employed to study the changes in the surface during the process by tracing the same profile again and again. Analysis of the relocated profiles brings out the gradual changes in the profile and also the metal removal during the process unambiguously. The results of analysis are reported and discussed. Keywords: Magneto-rheological abrasive finishing, Relocation profilometry, Surface roughness, Material removal 1 Introduction Most of the finishing processes are pressure or force controlled, as against the surface or cylindrical grinding processes which are displacement controlled. In all cases, the process leaves its signature on the surface. Classical examples of pressure-controlled finishing processes are honing, lapping and super-finishing. In the last few decades, attention of researchers has turned towards non-traditional methods of finishing the surfaces. One such process is magneto-rheological abrasive finishing in which abrasive mixed with magneto-rheological (MR) fluid is used. Kordonski and Jacobs (1996) developed a setup in which magnetically stiffened magneto-rheological fluid mixed with abrasives is made to flow over a moving flat rigid wall and the polishing happens at a converging gap formed by the surface to be finished and a moving wall. Now it finds an interesting industrial application in polishing of optical lenses. The major advantage is that the abrasive medium exhibits higher yield strength when subjected to increased magnetic field. Apart from the ways of introducing the abrasive mixed MR fluid, the relative motions between the workpiece and abrasive medium are imparted in different ways; using reciprocation, rotation or combination of both. Reciprocation motion has been employed in magneto-rheological abrasive finishing by Jha and Jain (24). Seok, et al. (27) employed MR fluid alone for the fabrication and finishing of curved silicon-based micro-structures. While Sadiq and Shunmugam (29a,b) used both reciprocation and rotation, Dass et al. (21) later investigated the process with rotary motion imparted to the abrasive medium by rotating magnetic field. In general, researchers working with MR fluid based finishing processes have studied the improvement in surface finish with process variables such as abrasive size, abrasive concentration, magnetic strength, reciprocation speed, rotational speed and processing time, taking workpieces with different initial roughness. Many results have been published mainly on surface finish improvements in terms of R a and subsequently the study has been extended to include R max, R z, etc. It is interesting to note that these parameters, characterizing the profile in the height direction, are derived from the same profile and hence are correlated. These roughness parameters do not reveal the nature of the profile and the changes happening to it with time for a given set of process parameters. Studies on material removal during the magneto-rheological abrasive finishing have also been reported. The metal removal is measured by weighing the workpiece before and after processing. In studies relating to surface finish, the roughness parameters measured on the same specimen show a wide variation and hence the study of process behavior becomes more complex. One way is to identify a roughness parameter that does not show much variation. Average roughness value R a, for example, is statistically more consistent than peak-to-valley value R t. However, the response to process changes is very sluggish with R a. Therefore, attempts have been made to locate the same profile and study the changes due to process behavior. Relocation profilometry was first developed by Williamson and Hunt (1968) to study the progressive changes in the surface topography as the surface is subjected to different processing conditions. This requires that the workpiece is held in a fixture in same position with reference to a tracing stylus so that the same profile is traced again and again. For this purpose, a plate holding the specimen is provided with certain features to ensure kinematic constraints in the fixture. In the processes using MR fluid, introducing the plate 874-1

2 STUDY OF PROFILE CHANGES IN MAGNETO-RHEOLOGICAL ABRASIVE HONING BY AN INGENIOUS RELOCATION TECHNIQUE with specimen becomes difficult due to the process limitations and space constraints. Hence an ingenious relocation technique is proposed in this work. It is shown that the changes in the profile nature can be comprehensively brought out and the material removal can be derived from the profiles obtained using relocation profilometry. 2. Working Principle of MRAH In the present work, a combination of reciprocation and rotation is used and hence authors refer to it as magneto-rheological abrasive honing (MRAH). Working principle of MRAH setup is explained with reference to Fig. 1. MR fluid consists of a carrier medium like castor oil loaded with carbonyl iron particles (CIP) of appropriate size and concentration. Particles of abrasives such as silicon carbide are also mixed with the MR fluid. A surfactant is added to prevent the settling of the particles in the carrier medium and stirred thoroughly to achieve uniform mixing. The container filled with abrasive MR fluid is kept between the poles of a direct current electromagnet. The magnetic field is varied by changing current fed to the coil of the electromagnet. The abrasive mixed MR fluid is pushed up and down inside the container using a crank-driven piston at the bottom and a spring-loaded sliding disc at the top. A holder carrying the specimens is rotated within the abrasive MR fluid using a direct current motor. More details can be found in the paper published by Sadiq and Shunmugam (29a). Sliding disc N DC electroma gnet Spring Work piece Fig. 1 Schematic of MRAH setup In the absence of magnetic field, abrasive particles are free to move in the MR fluid (Fig. 2(a)). When magnetic field is applied, the carbonyl iron particles (CIP) align along the field direction and the abrasive particles are trapped between the CIPs as shown in Fig. 2(b). This S Abrasive mixed MR Fluid phenomenon prevents the free movement of abrasive particles and causes the abrasive mixed MR fluid to stiffen to different degree, depending on the magnetic field strength. The stiffened medium when rubs against the surface, material is removed in micron level and the surface finish is improved. With certain degree of flexibility in the MR fluid medium, the process has a potential to finish complex surfaces. Magnetic Pole N a) Without magnetic field N CIP Workpiece b) With magnetic field Fig. 2 Arrangement of CIP and abrasive particles 3. Experimental Details Base medium SiC CIP chain Field direction Workpiece Experiments were conducted on the MRAH set up described in Sec. 2. MR fluid required for experimentation was supplied by NPOL Kochi. MR fluid consisted of castor oil as carrier medium and the CIP particles of 8 µm size. Green silicon carbide abrasives were mixed with the MR fluid and stirred thoroughly to achieve a uniform mixture. Different abrasive sizes (8, 15, 25 µm) and different concentrations (5, 15, 25%) of abrasives were used in the experimentation. The specimen holder was rotated at 5, 7 and 9 rpm. From the study of Sadiq and Shunmugam (29a), a current of 2 A resulting in saturation magnetic field of.6 T is found to give better performance. 2 A current and 15 cycles/min frequency of piston reciprocation were maintained as for all the experiments. Stainless steel specimens were prepared in batches by cylindrical grinding using a special mandrel. Specimens with roughness values in a close ranges are selected for the trials. On each specimen, indentation was made using of a micro hardness tester (FM 7, Future-Tech, 874-2

3 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 214) December 12 th 14 th, 214, IIT Guwahati, Assam, India Japan). With a square-based pyramidal diamond indenter, a load of 5 gf was applied. Fig. 3 shows the specimen with indentation schematically. The indentation serves as a reference mark for measuring surface profile after different time intervals during the finishing process. The indentation was viewed using the optical system of Dektak 15 surface profiler (Vecco, USA) and the stylus was moved over it, keeping it to one end. A tracing length of 6 mm was used in this step. Out of the traces, the one showing the maximum depth of indentation was taken for further analyses. The depth of indentation may vary from one specimen to another, but it is unique for a given specimen. Due to space limitation, the results are presented here for a MR fluid having 3% CIP mixed with 25% Sic of 15 µm size. Indentation Fig. 3 Specimen showing indentation (not-to-scale) 4. Results and Discussion 4.1 Analysis of profile changes From the trace passing through the deepest point of the indentation, the roughness profile is obtained using a filter with.8 mm cut-off. Fig. 4 shows part of the profile taken on the initial surface and the same profile relocated after 1 min, 3 min and 5 min of processing carried out with a combination of MRAH parameters mentioned at the end of Sec. 3. The indentation mark can be easily seen in all the plots. For assessment of roughness parameter Ra, an assessment length of 4 mm is considered in Zone-B completely excluding the part containing indentation mark (Zone-A). Examination of profiles in Fig. 4 shows that the crest part of the profile undergoes changes during the magneto-rheological abrasive finishing process. Histograms of roughness profile ordinates are also shown in Fig. 5. The normal curve superimposed on the histograms shows that the ordinates corresponding to the crest are reduced in height. The skewness parameters also confirm this phenomenon. The profile measurements are taken at an interval of 5 min and Fig. 6 shows improvement in R a values as the process is continued. The improvement is not appreciable after a certain period as observed by many researchers. 4.2 Analysis of material removal It is seen from the literature that Chang (1998) used profilometry for assessing the metal removal in superfinishing. A step of.1 mm was provided at the end of a cylindrical specimen and the reduction in the step height was measured by profilometry. They also used an electronic balance to measure the weight loss. Estimates of material removal made by weight loss and change of step height measurements were seen to correlate very well with each other. In the present work, the deepest point of the indentation profile is taken as the reference point. It is shown from the previous analysis (Sec. 4.1) that the deepest point of the indentation profile remains unaffected by the finishing process and hence could serve as a reference point. The distance of this reference point from the centerline is an important process dimension. After each interval of time, the reduction in the process dimension can be taken directly as average thickness of material layer removed. However, it may also be noted in Fig. 4 that the valley is pulled up and a protrusion is seen next to indentation mark. This happens in the filtering process, when deep valley is present in the profile (Shunmugam and Whitehouse, 214). Unless this is taken care of, the measurement of the process dimension involving deepest point of the valley will not give a realistic value. In plateau honing also deep grooves are present and hence a double-step filtering is recommended; first step using a large cut-off and second step using a standard cut-off. In order to bring out the effect of filtering in the present case of MRAH, the raw profile as well as roughness profiles obtained with. 8 mm and 6 mm cut-off values are shown in Fig. 7. With longer cut-off, the distortion is minimized and hence the process dimension, i.e. depth of deepest point from the center line, is evaluated using maximum possible cut-off value (6. mm). This dimension will reduce with progress of time and it is measured at regular interval. In order to assess the material removal in the present work, the following relation is used, taking h as the reduction in the process dimension. The material loss w is computed from w = ρ[(2rθ)(l)] h (1) Value of θ is given by sin -1 (B/2R), where B is the width of the specimen (6 mm) and R is the radius (16 mm) on the upper surface. L is the length of the specimen (15 mm) as shown in Fig. 3 and ρ is the density of the material. As a cross-check, measurements are also taken using a calibrated electronic balance (Model GR-22, AND, Japan) with resolution of.1 mg. A quick calculation will show that the weight loss of.1 mg corresponds to 14.8 nm height reduction. The profilometer used in the present investigation has a resolution of 1 Å. The profilometry is, therefore, capable of providing more accurate estimates in material removal than the electronic balance. One should also consider repeatability, if uncertainty has to be estimated 874-3

4 STUDY OF PROFILE CHANGES IN MAGNETO-RHEOLOGICAL ABRASIVE HONING BY AN INGENIOUS RELOCATION TECHNIQUE A B a) Initial surface b) After 1 min c) After 3 min d) After 5 min Fig. 4 Roughness profile plots (cut-off:.8 mm) 874-4

5 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 214) December 12 th 14 th, 214, IIT Guwahati, Assam, India Frequency Height in micrometer a) Initial surface (Std. dev:.989, Skew: -.444) 8 Improvement in R a (µm) Time (min) Fig. 6 Improvement in surface finish with time at 7 rpm 7 F re q u e n c y Height in micrometers b) After 1 min (Std. dev:.827, Skew: -.657) F re que nc y Height in micrometers c) After 3 min (Std. dev:.7127, Skew: -1.15) Fre quency Height in micrometers d) After 5 min (Std. dev:.7171, Skew: -.973) Fig. 5 Histograms of roughness profiles (shown in Fig.4) Micrometer Micrometer Fig. 7 Determination of depth of indentation for both the techniques and this aspect is not included in the present paper. The material removal with finishing time is shown in Fig. 8. Interestingly, the trend also follows the trend shown in Fig. 6 for surface finish improvement. Further analyses are being carried out by the research group and the results will be reported shortly. Material removed (mg) Raw.8 mm Cutoff 6 mm Cutoff Time (min) Fig. 8 Material removal with time at 7 rpm 874-5

6 STUDY OF PROFILE CHANGES IN MAGNETO-RHEOLOGICAL ABRASIVE HONING BY AN INGENIOUS RELOCATION TECHNIQUE 5. Conclusion In this paper, an ingenious relocation profilometry is not only proposed, but its capabilities are also demonstrated. The relocation profilometry allows the same profile to be traced again and again, and hence the results throw more light on the process behavior. If the material removal alone has to be measured, the electronic balance provides simple and quick measurements. In the present work, it is obtained from one profile measurement. Of course, a different filtering of the profile with larger cut-off is necessary. The present system allows off-line processing of the profile, so that the system can be optimally used for measurements alone. References Chang, S.-H. (1998) Basic Study of Superfinishing of Hardened Steels, Ph.D. Thesis, 1998, Purdue University, USA. Jha, S. and Jain, V. K. (24) Design and development of the magnetorheological abrasive flow finishing (MRAFF) process, Int. J. Mach. Tools & Manufact, Vol. 44, No. 1, pp Kordonski, W. and Jacobs, S. D. (1996) Magneto- rheological finishing, Int. J. Modern Physics B, Vol , No. 1, pp Dass, M., Jain, V.K. and Ghoshdastidar, P.S. (21) Nano-finishing of stainless-steel tubes using rotational magnetorheological abrasive flow finishing process, Machining Sci. & Technol., Vol. 14, No. 3, Shunmugam; M.S. and Whitehouse, D.J. (213) Surfaces and surface metrology, Int J Precis Technol., Special Issue: Surfaces and Their Measurement, Pt 2, Vol. 3 No. 4, Sadiq, A. and Shunmugam, M.S. (29a) Investigation into Magnetorheological Abrasive Honing (MRAH). Int. J. Mach. Tools & Manufact, Vol. 49, No. 7 8, pp Sadiq, A. and Shunmugam, M.S. (29b) Magnetic field analysis and roughness prediction in magnetorheological abrasive honing (MRAH) Machining Sci. & Technol., Vol. 13, No. 2, pp Seok, J., Y. Kim, K. Jang, B. Min, S. J. Lee, (27) A study on the fabrication of curved surfaces using magnetorheological fluid finishing, Int. Journal of Machine Tools and Manufacture, Vol. 47, No. 14, pp Williamson, J.B.P and Hunt, R.T. (1968) Relocation profilometry, J. Phys. E: Sci. Instrum. Vol.1, No. 7, pp