Design and development of nanofinishing process for 3D surfaces using ball end MR finishing tool

International Journal of Machine Tools & Manufacture 51 (2011) 142–151

Contents lists available at ScienceDirect

International Journal of Machine Tools & Manufacture

0890-69

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/ijmactool

Design and development of nanofinishing process for 3D surfacesusing ball end MR finishing tool

Anant Kumar Singh, Sunil Jha n, Pulak M. Pandey

Department of Mechanical Engineering, Indian Institute of Technology, Delhi 110016, India

a r t i c l e i n f o

Article history:

Received 12 May 2010

Received in revised form

4 October 2010

Accepted 6 October 2010Available online 12 November 2010

Keywords:

Magnetorheological fluid

Nanofinishing

Magnetorheological (MR) finishing tool

55/$ - see front matter & 2010 Elsevier Ltd. A

016/j.ijmachtools.2010.10.002

esponding author. Tel.: +91 11 2659 1125; fa

ail address: [email protected] (S. Jha).

a b s t r a c t

A new precision finishing process for nanofinishing of 3D surfaces using ball end MR finishing tool is

developed. The newly developed finishing process is used to finish ferromagnetic as well as nonmagnetic

materials of 3D shapes using specially prepared magnetorheological polishing (MRP) fluid. The existing

MR finishing devices and methods are likely to incapable of finish 3D intricate surfaces such as grooves in

workpiece or complex in-depth profiles in the mold due to restriction on relative movement of finishing

medium and workpiece. In this newly developed finishing device, the ball end MR finishing tool is used for

finishing different kinds of 3D surfaces, as there is no limitation on relative movement of finishing

medium and workpiece. It can finish the work surfaces similarly as the machining of 3D surfaces by CNC

ball end milling cutter and open a new era of its applications in future. The developed process may have its

potential applications in aerospace, automotive and molds manufacturing industries. A computer

controlled experimental setup is designed and manufactured to study the process characteristics and

performance. The magnetostatic simulations were done on ferromagnetic as well as nonferromagnetic

materials of 3D surfaces to observe the ball end shape of magnetic field at the tip of the MR finishing tool.

The experiments were performed on flat EN31 and groove surface of copper workpieces in the developed

MR finishing setup to study the effect of finishing time on final surface roughness.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

The methods of finishing processes such as grinding, lapping,honing, etc. are labor intensive and comparatively less controllable forfinishing operations in the manufacturing of precision parts. Theseprocesses generally employ a rigid tool that subjects the workpiece tosubstantial normal stresses, which may cause micro-cracks and otherdefects resulting in reduced strength and reliability of the machinedparts. Among these, finishing of complex geometrical shapes andobtaining surface finish of nanometer order on advanced materials isstill a challenging task. In manual grinding and polishing, a high localpressure may lead to subsurface damage. To minimize the subsurfacedamage it is necessary to finish the materials under gentle conditions,i.e., by applying very low forces. The precise control of finishing forcesis an important consideration for fine finishing with close tolerancesand without damaging surface topography.

In the last decade, many advanced fine finishing processes havebeen developed to precisely control the abrading forces such asmagnetic abrasive finishing (MAF) [1], magnetic float polishing (MFP)[2], magnetorheological jet finishing (MRJF) [3], magnetorheologicalabrasive flow finishing (MRAFF) [4] and magnetorheological finishing

ll rights reserved.

x: +91 11 2658 2053.

(MRF) [5] in which the magnetic field is used to control the abradingforces. But the applications of these processes are limited to specificgeometries only such as concave, convex, flat and aspherical shapesdue to restriction on relative movement of finishing medium andworkpiece. These are incapable of finishing of 3D intricate shapedsurfaces.

To overcome the above mentioned limitation of aforesaidfinishing processes, a new precision finishing process for nanofin-ishing of 3D surfaces using ball end MR finishing tool is developed.The newly developed finishing process is useful to finish ferro-magnetic as well as nonmagnetic materials of 3D surfaces usingspecially prepared MRP-fluid. The MRP-fluid used is comprised ofcarbonyl iron powder particles and silicon carbide abrasivesdispersed in the viscoplastic base of grease and heavy paraffinliquid; it exhibits change in rheological behaviour in presence ofexternal magnetic field. This smart behaviour of MRP-fluid isutilized to precisely control the finishing forces, hence final surfacefinish. In this process a small hemi-spherical ball of MR polishingfluid is formed by specially designed tool and this ball is used toabrade/erode the material from work surface. The movement to theball is provided by computer controlled 3 axis motion controller.This MR finishing ball may be visualized similar to ball end of amilling cutter movement on a 3 axis vertical CNC machine.

In this newly developed finishing device, the ball end MRfinishing tool is used for finishing of 3D surfaces where there are

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Nomenclature

Ra center line average roughness value (mm)Fn normal forceFt shear force

FF finishing forceFreq force required to remove materialts shear strength of workpiece materialAp projected area of penetration

A. Kumar Singh et al. / International Journal of Machine Tools & Manufacture 51 (2011) 142–151 143

no limitations of relative movement of finishing medium andworkpiece. The MR finishing tool tip of stiffened ball end of MRpolishing fluid is likely to have very much flexibility to move overdifferent kinds of 3D surfaces and it can finish the work surfacessimilar to the machining of 3D surfaces by CNC ball end millingcutter and open a new era of its applications in future. Thedeveloped process may have its potential applications in aerospace,automotive, molds manufacturing industries, semiconductor andoptics machining, etc. A computer controlled experimental setup isdesigned to study the process characteristics and performance.

Key benefits of the present finishing process:

�
New precision finishing process for finishing of 3D surfacesmachined by CNC milling. � In-process control of tool geometry for controlled finishing
operation.
� No wear of cutting tool edges because polishing fluid is
continuously replenished at the tip.
� MRP-fluid carries heat and debris away from the polishing zone. � The finishing process is useful to finish ferromagnetic as well as
nonferromagnetic materials.
� Capability to finish up to nanometer.
2. Design and fabrication of experimental setup

The design of this setup is directed to improve methods anddevices for magnetorheological finishing of work surfaces. A newmagnetorheological finishing device for process of generatingmagnetically controlled ball end smart abrasive laden shape forfinishing of 3D surfaces is developed, which is automated and usedfor finishing in manufacturing systems.

A schematic elevation view of a magnetorheological (MR)finishing machine in accordance with the present method ofoperation is shown in Fig. 1. The device includes verticallyoriented MR finishing tool which comprises of concentricallyinner core, electromagnet coil and an outer core (Fig. 2). The MRfinishing tool is positioned vertically on a vertical Z-slide such thatthe tip of the MR finishing tool can approach the surface of aworkpiece and is driven by a servo motor. A motion controller isprovided to precisely control the rotational speed of MR finishingtool. The ball bearing, slip ring, timing pulley and rotary valve aremounted on the upper part of the MR finishing tool.

The means for holding the workpiece includes a platformmounted on X–Y linear movement slides. Three stepper motorsare used for controlling the linear motion of X–Y–Z directions,whereas the X–Y direction motion controllers are used for con-trolling horizontal linear motion of workpiece and Z motioncontroller is used for controlling vertical linear motion of MRfinishing tool.

The electromagnet coil is designed to obtain maximum mag-netic flux density of �0.8 T at the tip of the MR finishing tool.A comparative study of different coil design parameters such asnumber of turns, wire gauge, number of layers, geometricfactor and coil diameter was made before selecting the final coilparameters as: number of turns¼2000, maximum current¼4 A,copper wire gauge¼19 SWG, coil inner diameter¼20 mm, outer

diameter ¼ 80 mm and length ¼ 80 mm. The magnetically softiron is used as core materials. The electromagnet coil is placedbetween the inner and outer cores. The magnetic field flowsthrough inner core to outer core along tip of the MR finishingtool as shown in Fig. 2. A thin brass bush is used between two coresat the tip side of the MR finishing tool to concentrate linear flow ofmagnetic lines of forces towards the tip of the MR finishing tool.

MR fluid delivery system comprises of a storage tank (funnelshape) along with speed controlled stirrer, a delivery pump forsupplying MR fluid from the storage tank to the MR finishing tooland a suction pump for recollecting the MR fluid from a finishingzone to the fluid storage tank. Both pumps are peristaltic pumpsand an AC variable frequency drive (VFD) is used to vary the speedof peristaltic pump.

The MR finishing tool depicting the formation of a stiffen ballend shape of MR polishing fluid in the direction of magnetic field atthe tip of the tool is shown in Fig. 2. The pressurized MR fluid entersaxially from the upper end of the MR finishing tool. Once the MRP-fluid reaches the tip of the MR finishing tool, the magnetic carbonyliron particles of MRP-fluid are aligned and formed a chain likestructure along with magnetic field direction available at the tip ofthe MR finishing tool. Thus MRP-fluid forms a finishing spot at thetip of the MR finishing tool. The MRP-fluid becomes stiffened to aphysical texture like a wet clay ball end shape and apparentviscosity along the direction of magnetic field becomes veryhigh. This stiffened ball end of MRP-fluid may be visualizedsimilar to ball end of a milling cutter movement on a 3 axisvertical CNC machine. The magnetic field strength can becontrolled by controlling the magnetizing current in real timeand hence controls the stiffness of the MRP-fluid. The movement ofthe stiffened ball end of MRP-fluid over 3D work surface is enabledby means of the motion controller integrated with computer.

The schematic representation of microstructure of abrasive andmagnetic particles in the vicinity of workpiece surface during thepresent method of MR finishing process is shown in Fig. 3. Themagnetic force between iron particles encompassing abrasive grainprovides bonding strength to it. The nonmagnetic silicon carbideabrasive particles having cutting edges are tightly held betweenmagnetic iron particles over the surface of stiffened ball end. Whenthe stiffened ball end have relative motions with respect to thework surface during the rotation of MR finishing tool, the polishingspot formed by MR particle chains with abrasive particles shear thepeaks from the workpiece surface. The asperities on the surface areabraded due to plastic deformation at the tips of the asperities andhence appropriate finishing of 3D surfaces is performed. Theamount of material sheared from the peaks of the workpiecesurface by abrasive grains depends on the bonding strength ofstiffened ball end of MRP-fluid provided by field-induced structureof MR fluid.

The resultant finishing force due to bonding strength of MRP-fluid and rotation of MR finishing tool is given by FF as follows(Fig. 3)

FF ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiF2

nþF2t

qð1Þ

where, Fn is the normal force; Ft the shear force; FF thefinishing force.

P

Servo motor

MR finishing tool

MR P- fluid

Stiffened ball end of MRP- fluid

Work piece To D.C. Powersupply

Slip ring

XYZ Movement Table

Control

Steppermotor Rotary valve

Peristalticpump

XYZ movement table

Motion controllers

Belt drive

VFD

MRP- fluid circulation system Computer

Fig. 1. Schematic of the new MR finishing machine setup.

Pressurized MR polishing fluid

Carbonyl Iron ParticleCarrier or Base fluid

Abrasive particle

Stiffened Ball End of MRP-fluid

Outer core

Inner core

Electromagnet coil

Rotary valve

Flow of magnetic line of forces

Work surface

Tool rotation

Fig. 2. Mechanism of formation of the stiffen ball end of MR polishing fluid at the tip

of the tool.

Abrasive particle

Magnetic Carbonyl

Iron particles

Tool rotation

Workpiece

Fn

Ft

FF

Fig. 3. Microstructure of abrasive and magnetic particles in the vicinity of work-

piece surface.

A. Kumar Singh et al. / International Journal of Machine Tools & Manufacture 51 (2011) 142–151144

Depth of penetration depends on normal force Fn provided byfield-induced structure of MRP-fluid while the shear force Ft isresponsible for removal of material in the form of m-chips duringrotation of the MR finishing tool.

The resistance offered by the workpiece for removal of materialin the form of m-chip, is given by Freq as follows

Freq ¼ tsAp ð2Þ

where ts is the shear strength of workpiece material and Ap is theprojected area of penetration.

Workpiece

MR finishing tool

Abrasiveparticles Magnetically stiffened

unsheared ball end of MRP-fluid

Workpiece

MR finishing tool

Abrasiveparticles

Magneticallystiffened unsheared MRP-fluid

Sheared MRP- fluid

Magnetically stiffened unsheared ball end of MR fluid

Workpiece

MR finishing tool

Abrasiveparticles

Chip removed

Fig. 4. Finishing action in the presence of magnetic field at the tip of the MR finishing tool (a) abrasive grains of MR polishing fluid just approaching the roughness peak

(FF¼Freq), (b) abrasive grains slowly rolling over the roughness peak in absence of enough bonding forces (FFoFreq) and (c) abrasive grains takes a small cut on roughness peak

in the presence of enough bonding force (FF4Freq).

PressurizedMRP-fluid


During the MR finishing process, one of the following threeconditions may exist [6]:

flow

Tool rotation

Outer core

FFrFreq

FFoFreq

FF4Freq

Electromagnet coil

MRP- fluid flowpassage

MRP- fluid layer

Work surface

Inner core

Fig. 5. Electromagnetic model of MR finishing tool.

If FF¼Freq, then the material removal initiated and in this caseabrasive particles of stiffened ball end of MRP-fluid may lead toploughing over the work surface during the rotation of the tool(Fig. 4a).

If FFoFreq, then the material removal does not occur because theMRP-fluid may be sheared over the work surface during therotation of the MR finishing tool. The role of unsheared MRP-fluidparticipating in material removal is almost negligible while rub-bing and abrasion due to abrasive particles rolling in sheared fluidis predominant. In this case, the abrasives only roll over the worksurface without any material removal and may lead scratches onthe work surface (Fig. 4b).

If FF4Freq, then the material is removed in the form of m-chipdue to sufficient finishing force available to overcome the resistiveforce offered by the workpiece and in this case the MRP-fluid is instate of magnetically unsheared fluid. The material removal fromthe workpiece is maily contributed by unsheared of MRP-fluid withembedded abrasive particles on the surface (Fig. 4c).

3. Electromagnetic modeling for MR finishing tool

A 3D model of MR finishing tool along with workpiece andMRP-fluid has been developed in Maxwell student version software

is shown in Fig. 5. The current and number of turns to theelectromagnet are assigned as 2 A and 2000 turns. The materialfor electromagnet coil is copper of relative permeability 1. Theassignments of material for the inner core as well as outer core are


iron of relative permeability 4000.The relative permeability of MRfluid material properties is assigned as 4. A thin brass bush ofthickness 2 mm of relative permeability 1 is used between innerand outer cores at the tip side of the tool. The purpose to use thebrass bush is to concentrate linear flow of magnetic line of forcestowards the tip of the MR finishing tool. The workpiece materialsare used as ferromagnetic and nonferromagnetic of relativepermeability of 4000 and 1, (Table 1) respectively.

Fig. 6. (a) The shape of magnetic flux density generated at the tip of the MR finishing tool

interface between MR fluid and work surface

Table 1Assigned parameters to electromagnet model.

Parameter Material Relative

permeability

Current No. of

turns

Electromagnet coil Copper 1 2 A 2000

Inner core Iron 4000

Outer core Iron 4000

MR polishing fluid MR fluid 4

Thin bush Brass 1

Ferromagnetic Iron 4000

Nonferromagnetic Copper 1

4. Magnetostatic simulation for the model of MR finishing tool

The magnetostatic simulation has been done for the developedmodel (Fig. 5) using maxwell software to check the shape ofmagnetic field generation at the tip of MR finishing tool. The shapevariation of magnetic field generation at the tip of the MR finishingtool has been studied for the working gap 5 mm for the workmaterial of ferromagnetic as well as nonferromagnetic.

4.1. Effect of ferromagnetic work material on the variation of

magnetic flux density at the tool tip

The simulation results for ferromagnetic work material for theworking gap of 5 mm is shown in Fig. 6. The variation of magnetic fluxdensity at the tip of the MR finishing tool is visualized similar to ballend cutter of a CNC milling machine.This ball end of MRP-fluid is usedas finishing tool when it rotatates over the workpiece surface.

4.2. Effect of nonferromagnetic work material on variation of

magnetic flux density at the tool tip

The simulation results of variation of magnetic flux density withnonferromagnetic work material for working gap of 5 mm are

Magnetic flux density at the

interface of MRP- fluid and

work surface

5mm

MR finishing tool

I = 2A

N = 2000 turns

Ball end shape of MRP-

fluid at tool tip

Ferromagnetic

workpiece

for ferromagnetic work material when the working gap is set to 5 mm and (b) at the

I = 2AN = 2000 turns

MR finishing tool

Magnetic flux density at tool tip

5mm

Non ferromagneticworkpiece

Fig. 7. The shape of magnetic flux density generated at the tip of the MR finishing

tool for nonferromagnetic work material when the working gap is set to 5 mm.

Table 3Properties of carbonyl iron powder of CS grade (BASF).

Physical Composition (%) Particle size distribution

diameter (mm) of particles

10% 50% 90%

Mechanically soft Iron499.5 3.5 6.0 18.0

Carbono0.05

Oxygen 0.2

Nitrogeno0.01

Table 2Composition of synthesized MR polishing fluid.

Constituent % volume concentration

Carbonyl iron powder of CS grade 20

Silicon carbide of mesh size 600 20

Base fluid medium 60

Table 4Experimental parameters and conditions.


shown in Fig. 7. The magnetic lines of forces are not attractedtowards the workpiece due to nonferromagnetic and it does notmake a very good shape of ball end of MR polishing fluid. This isbecause, almost all the magnetic line of forces are diverted frominner core to outer core at the tip of the tool and MR fluid becomesstiffened along these lines of magnetic forces only. Therefore, thepresent methods and devices are excellent for finishing of magneticmaterials as compared to the nonmagnetic materials of worksurfaces. It may take a few more number of cycles for finishingof nonmagnetic work surfaces as compared to ferromagnetic workmaterials.

Parameter Conditions for

EN31 workpiece 3D copper workpiece

Total finishing time (min) 100 60

Tool rotation speed (rpm) 100 600

Working gap distance (mm) 2 2

SiC abrasive (20 vol%) (mesh) 600 800

DC power supply to coil 30 V, 2.6 A 30 V, 2.6 A

Magnetic field used at tool tip (T) 0.2 0.2

5. Experimentation

To examine the performance of the newly developed finishingdevice, the preliminary experiments were conducted to study theeffect of finishing time on the surface finish for both ferromagneticas well as nonferromagnetic work material.

5.1. Preparation of MR polishing fluid

MRP-fluid is prepared with 20 vol% carbonyl iron powder(CIP) of grade CS from BASF of average particle size 18 mm,20 vol% silicon carbide abrasive powder and 60 vol% of viscoplasticbase medium (20 wt% AP3 grease and 80 wt% heavy paraffin liquid(Tables 2 and 3)). First the base fluid medium is prepared by mixingof AP3 grease and heavy paraffin liquid and then the suspension isprepared mixing abrasive and iron particles into the base mediumand stirring with the help of multiple blade stirrer in the funnel.This results in uniform dispersion of iron and abrasive particles inthe base medium.

5.1.1. Preparation of base fluid

Total sample of base fluid was prepared as 2 kg¼2000 gm.In which, the heavy paraffin liquid is 80% by weight¼1600 gm.And AP3 grease is 20% by weight¼400 gm.Both paraffin liquid and AP3 grease were mixed together for

getting final base fluid, the density of base fluid was found byknowing the weight of base fluid for 100 cm3 volume. It was found63.8 gm for 100 cm3 volume of base fluid.

Therefore, the density of base fluid¼63.8/100¼0.638 gm/cm3.

5.1.2. MRP-fluid

Total sample of MRP-fluid was prepared as 1 l¼1000 cm3.In which, the CIP is 20% by volume¼200 cm3 and by weight¼

200 cm3�7.8 gm/cm3 (density of CIP)¼1560 gm¼1.56 kg.

The silicon carbide abrasive powder is 20% by volume¼200 cm3

and by weight¼200 cm3�3.22 gm /cm3 (density of silicon carbide

abrasive powder)¼644 gm.

The base fluid is 60% by volume¼1000 cm3�60%¼600 cm3

and by weight¼600 cm3�0.638 gm/cm3 (density of base fluid)¼

382.8 gmThese three components of MRP-fluid in above mentioned

propotion was mixed and stirred in funnel.Thus requiredMRP-fluid has been prepared for conducting the experiments.

5.2. Experimental conditions for both ferromagnetic and

nonferromagnetic work materials

DC power of 30 V and 2.6 A is supplied to the electromagnet coilvia slip ring. The magnetic field is produced around 0.2 T at the tipof the MR finishing tool. Experiments are conducted at differentfinishing times. The variation of surface roughness of workpiecewith finishing time is measured at every 20 min during MRfinishing. The rotational speed of the MR finishing tool is kept at100 rpm and 600 rpm for EN31 and groove surface of copperworkpieces, respectively. The working gap is set at 2 mm betweenthe tool tip and work surface. The MRP-fluid with 600 mesh sizesilicon carbide abrasive powder is used for ferromagnetic workmaterial and 800 mesh size silicon carbide abrasive powder is usedfor nonferromagnetic work material. Since initial surface rough-ness of copper workpiece is less as compared to EN31 and alsocopper workpiece is soft material, therefore bigger mesh size ofabrasive particles of 800 was used. To clearly understand thecharacteristics of material removal during actual polishing, theexperiment is carried out at a fixed location on the workpiecesunder the present conditions (Table 4).

Fig. 9. Initial surface of an EN31 workpiece (a) SEM micrograph at 1000� and (b) surface roughness profile.

414.1 nm

317.2 nm

292.3 nm

149.7 nm

108.0 nm

70.0 nm

450

400

350

300

250

200

150

100

50

0

Surf

ace

Rou

ghne

ss (

nm)

0 20 40 60 80 100 120

Finishing Time (min)

Fig. 8. Effect of finishing time on surface roughness value of a workpiece.



5.3. Experimentation on ferromagnetic workpiece

The finishing experiments were conducted on flat EN31 work-piece of size 50�30�5 mm. The workpiece was hold by precisionvice on X –Y movement linear slides. The initial centre line average(CLA) surface roughness Ra of approximately 414 nm was obtainedby grinding the workpiece on surface grinder.

5.4. Experimentation on nonferromagnetic workpiece

The finishing experiments were conducted on groove surfaces ofnonferromagnetic copper workpice as shown in Fig. 11. A suitableworkpiece holder of die steel material was designed to hold theworkpiece as shown in Fig. 12 and the workpiece holder was holdby precision vice on X–Y movement linear slides. The initial surfacecharacteristic of groove surface of copper workpiece was observed

Fig. 10. Finished surface of an EN31 workpiece with 100 min of finishing time

(a) SEM micrograph at 1000� and (b) surface roughness profile.

Groove surfaceto be finished

Fig. 11. (a) 3D and (b) 2D view of non

using scanning electron microscopy (SEM) at 1000� and the initialcentre line average (CLA) surface roughness Ra of approximately336 nm was measured by using Taylor Hobson Talysurf as shown inthe Fig. 13.

6. Result and discussion

The experiments were performed to examine the effect offinishing time on final surface roughness value (Ra) of a workpiece.The experimental result for variation in surface roughness withfinishing time for ferromagnetic workpiece is shown in Fig. 8 andfor nonferromagnetic finished groove, the surface characteristicand roughness profile of a copper workpiece are shown in Fig. 14.The experiments are conducted at magnetic field around 0.2 T atthe tip of the MR finishing tool for which stiffened ball end finishingspot is formed. The surface roughness profiles were obtained byTaylor Hobson Talysurf for all the experiments and to understandthe finished surface characteristics and texture, the scanningelectron microscope (SEM) was used.

6.1. Observations and discussion for finishing of ferromagnetic

workpiece

Fig. 8 shows the effect of finishing time on surface roughnessvalue of a workpiece measured at every 20 min during the MRfinishing. As shown in this figure, it can be seen that the surfaceroughness of the workpiece is reduced gradually from 414.1 nmand reaches to 70 nm with finishing time of 100 min. Thereforenewly developed ball end MR finishing tool is having ability toreduce surface roughness of the workpiece and it is clearlydemonstrated by studying the surface characteristic of aworkpiece using scanning electron microscopy (SEM) and thechange in the surface roughness profiles before and after thefinishing as shown in Figs. 9 and 10. This conforms that the presentdeveloped method of finishing process is capable of performing thefinishing action.

Fig. 9 shows the initial surface roughness profile of a workpiece,which was obtained by grinding the workpiece on surface grinder.The first experiment was conducted for the finishing time of 20 minand it can be seen that surface roughness of a workpiece wasdecreased from initial Ra value 414.1 nm to 317.2 nm as shown inFig. 8. This change in surface roughness value demonstrates thefinishing capability of rotating stiffened ball end MR finishing toolover the work surface. The silicon carbide abrasive particles havingcutting edges hold by carbonyl iron chains move relative to theworkpiece surface during the rotation of the stiffened ball end ofMR finishing tool and shear the peaks from the workpiece surface.The amount of material sheared from the peaks of the workpiecesurface by abrasive grains depends on the bonding strength ofstiffened ball end of MR finishing tool provided by field-induced

5

ferromagnetic copper workpiece.

Workpiece

Upper part of the fixture

Lower part of the fixture

Fig. 12. 3D view of assembled fixture along with copper workpiece.

Fig. 13. Initial groove surface of a copper workpiece (a) SEM micrograph at 1000�

and (b) surface roughness profile.


structure of MRP-fluid. In this way magnetic field strength controlsthe extent of abrasion of peaks by abrasives. The schematicrepresentation of microstructure of abrasive and magneticparticles in the vicinity of workpiece surface during the presentmethod of MR finishing process is shown in Fig. 3.

The next experiment was performed for the further finishingtime of 20 min and the surface roughness of the workpiece wasfound further decreased to 292.3 nm as shown in Fig. 8. In this run,the change in surface roughness value was found less as comparedto first run of experiment. This is because of finishing action takenplace without conditioning the MR polishing fluid at the tip of

the MR finishing tool. The same stiffened ball end of MR polishingfluid was used in this run as used in the preceding run ofexperiment. The conditioning of MR polishing fluid at the tip ofthe MR finishing tool can be done by replenishing fresh MRfluid. This is achieved by switching OFF the DC power supplyand getting fresh MR fluid at the tip of the MR finishing tool byswitching ON the delivery peristaltic pump. Once the fresh MRpolishing fluid reached at the tip of the MR finishing tool then DCpower supply is switched ON to get magnetic field at the tip of thetool, which will provide formation of stiffened ball end of MRpolishing fluid at the tip of tool and then delivery peristaltic pumpis switched OFF.

After conducting the finishing experiment for the next 20 min,the surface roughness of workpiece was found decreased to149.7 nm as shown in Fig. 8. In this run, the change in surfaceroughness value was found more as compared to preceding runof experiment. This is because of finishing action taken placewith conditioned MR polishing fluid at the tip of the finishing toolafter the end of previous experiment. This result confirmed thatconditioning the MR polishing fluid is must at the tip of the MRfinishing tool after certain period of finishing time. The reason forthis improvement is that the cutting edges of abrasive particle in MRpolishing fluid may be blunted after certain period of finishing timeand further it becomes less effective to remove the peaks of theworkpiece.

Further experiments were conducted for next 20 and 40 min,the surface roughness of workpiece was found decreased to 108and 70 nm, respectively, as shown in Fig. 8.

The surface characteristic of an EN31 workpiece by usingscanning electron microscopy (SEM) at 1000� was measuredfor initial and final surface of a workpiece as shown in Fig. 9 andFig. 10, respectively, and it can be seen that there is betterimprovement in surface characteristic of finished surface ascompared to initial surface of a workpiece. This conforms thatthe present developed method of finishing process is capable ofremoving the peaks of roughness of a workpiece. It was alsoobserved that there are left over grinding marks as well as somelight abrasive grain marks are seen on the finished surface. The leftover grinding marks can be removed for further finishing of aworkpiece. The reason for the abrasive grain marks is that theabrasive mesh size 600 was used for entire finishing time, which isnot fine abrasive. The abrasive grain marks can be removed usingfine abrasive grains after certain period of finishing. This means atthe beginning, for better material removal, coarse grain abrasivescan be used but at the end of the finishing time a fine abrasivegrains are used to avoid abrasive grain marks.

Fig. 14. Finished groove surface of a copper workpiece with 60 min of finishing time

(a) SEM micrograph at 1000� and (b) surface roughness profile.


6.2. Observations and discussion for finishing of nonferromagnetic

workpiece

Fig. 14 shows the effect of total finishing time on groove surfacecharacteristic and surface roughness of a copper workpiece wasmeasured for 60 min during the MR finishing. As shown in thisfigure, it can be seen that the surface roughness of the groovesurface of workpiece is reduced from 336.8 to 102 nm withfinishing time of 60 min and surface characteristic of theworkpiece is also improved. The improvement in surfacecharacteristic and reduction in surface roughness demonstratesthe finishing capability of rotating stiffened ball end MR finishing

tool over the groove surface. Therefore newly developed ball endMR finishing tool is having capability to reduce groove surfaceroughness of a nonferromagnetic material and improve the surfacecharacteristic of the workpiece, which is clearly demonstrated bystudying the change in the surface characteristic and surfaceroughness profile.

7. Conclusion

A new precision finishing process for 3D surfaces using ball endMR finishing tool is designed and developed. Magnetostaticsimulation of the variation of magnetic flux density in the finishingregion indicates clear formation of ball end finishing surface. Thesmart behaviour of MR polishing fluid is utilized to preciselycontrol the finishing forces, hence final surface finish. It has beenseen that the surface roughness of the ferromagnetic workpiece isreduced gradually from 414.1 nm and reaches to 70 nm withfinishing time of 100 min. Also for nonferromagnetic workpiece,it has been found that there is better improvement in the groovesurface characteristic of finished surface as compared to initialsurface of a workpiece and the surface roughness of the groovesurface is reduced from 336.8 to 102 nm with finishing time of60 min. The ability of newly developed ball end MR finishing tool toreduce surface roughness and improve the surface characteristicsof a workpiece is demonstrated and this confirms that the presentdeveloped method of finishing process is capable of performingthe nanofinishing action on plane and 3D groove surfaces offerromagnetic as well as nonferromagnetic work materials.

References

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[2] R. Komanduri, On material removal mechanisms in finishing of advancedceramics and glasses, Annals of CIPR 45/1 (1996) 509–514.

[3] W. Kordonski, A. Shorey, Magnetorheological (MR) jet finishing technology,Journal of Intelligent Material Systems and structures 18 (2007) 1127–1130.

[4] S Jha, V.K Jain, Design and development of the magnetorheological abrasive flowfinishing (MRAFF) process, International Journal of Machine Tools and Manu-facture 44 (2004) 1019–1029.

[5] W.I Kordonski, S.D Jacobs, Magnetorheological finishing, International Journal ofModern Physics B 10 (1996) 2857–2865.

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Design and development of nanofinishing process for 3D surfaces using ball end MR finishing tool

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