Study on cylindrical magnetic abrasive finishing using unbounded magnetic abrasives

9 Pages·0.612 MB·English

by  Chang G., Yan B., Hsu R.

International JournalofMachineTools &Manufacture 42(2002) 575–583
Study on cylindrical magnetic abrasive finishing using unbonded
magnetic abrasives
Geeng-Wei Chang a, Biing-Hwa Yan a,*, Rong-Tzong Hsu b
a DepartmentofMechanical Engineering, NationalCentral University,Chung-Li, Taiwan
bDepartment ofMechanical Engineering,Lee-mingInstituteofTechnology, Tai-Shan,Taipei, Taiwan
Received20 April2001;received inrevisedform12 October2001;accepted 19October 2001
Abstract
The process principle and the finishing characteristics of unbonded magnetic abrasive within cylindrical magnetic abrasive fin-
ishing are described in this study. The unbonded magnetic abrasive is a mechanical mixture of SiC abrasive and ferromagnetic
particles with a SAE30 lubricant. Iron grit and steel grit, for which three various particle sizes were prepared for both, were used
asferromagneticparticles, eachofthem beingmixed with1.2 and5.5µmSiC abrasive,respectively.Also, thefinishing character-
istics on surface roughness and material removal as well as their mechanisms were investigated. Experimental results indicate that
steelgritismoresuitableformagneticabrasivefinishingbecauseof itssuperiorhardnessand thepolyhedronshape. Thevariations
of material characteristics on the work surface both before and after finishing were also investigated. Si content was increased
obviously, however its corrosion resistibility decreased on a surface that was finished via steel grit mixed with SiC abrasive.
 2002 Elsevier Science Ltd. All rights reserved.
Keywords:Magnetic abrasive finishing; Finishing characteristics; Material removal; Surface roughness; Ferromagnetic particle; Unbonded mag-
neticabrasives
1. Introduction als [5]. For a workpiece with superior hardness, SiC or
Al O abrasiveshouldbe mixedwith ferromagneticpar-
2 3
Magnetic abrasive finishing (MAF) can be employed ticle (FP). Furthermore, the magnetic abrasive can be
to finish most mechanical parts with few shape limi- used in the form of abrasives held in a ferromagnetic
tations. Via MAF, the initial surface roughness of 0.25 matrixformedbysintering,chemicalorothertechniques
µm Ra can be improved easily to 0.05 µm Ra within a [6,7]. The unbonded magnetic abrasive (UMA) applied
few minutes, and produces neither a deteriorated layer in this study is a mechanical mixture of FP and SiC
nor micro-cracks on the finished surface. In addition, abrasive (SA), so the SA is not physically bonded to
MAFpossessesmanyattractiveadvantages,suchasself- ferromagnetic material. Within a magnetic field, the SA
sharpening, self-adaptability, controllability, and the fin- can move around freely within the constraints of the
ishing tool requires neither compensation nor dressing. adjacent FP. Fox et al. presented a brief description on
It has been shown that this technique can be applied to finishing non-magnetic stainless steel using UMA [8].
numerous products [1–3]. Notably, they achieved a worse surface roughness after
The finishing characteristics of MAF are closely finishing, which differed significantly from our empiri-
related to the magnetic abrasive that is employed [4]. cal results.
Magnetic abrasives, such as hard iron alloys, are gener- Owing to its excellent finishing effects on MAF, the
ally only applied to finish soft alloys or nonferrous met- sintered magnetic abrasive has been applied by most
researchers[9,10].However,sinteringrequiresbothhigh
temperatureandpressurewithinaninertgasatmosphere.
Subsequently, the sintered material should be crushed
* Corresponding author. Tel.: [phone]; fax: +886-3-
mechanically and then sieving is required to sort it into
4254501.
E-mail address:[email protected] (B.-H. Yan). a specific particle size. Due to this complex production
0890-6955/02/$-seefrontmatter2002ElsevierScienceLtd.Allrightsreserved.
PII:S0890-6955(01)00153-5
576 G.-W.Changetal./InternationalJournalofMachineTools&Manufacture42(2002)575–583
process, the sintered magnetic abrasive is expensive. gradient. The magnetic forces represented in Eq. (1) not
Hence,theproductioncostwillcertainlybereducedsub- only concentrate the FP in the working gap where mag-
stantially if UMA can be employed in place of the sin- netic field strength is superior, but also prevent the FP
tered magnetic abrasive. In this study, it will be shown from splashing due to workpiece rotation. The congre-
that the characteristics of UMA that are produced by gated FP form a magnetic brush along the line of mag-
mixing steel grit with SA are as good as those of a sin- netic force within the working gap, which causes press-
tered magnetic abrasive. ure P on the work surface. This pressure will act on the
SA beneath the FP, to generate abrasion. The SA can
not perform the cutting task unless it obtains abrasion
2. Fundamental principles pressure from the FP. Eq. (2) represents pressure, P, as
follows [11]:
Fig. 1 shows a two dimensional magnetic field distri-
P(cid:1)[m H2(1(cid:2)1/m )]/2 (2)
bution and magnetic force acting on a FP within cylin- o m
drical MAF. The magnetic forces affect the FP at pos- wherem istherelativemagneticpermeabilityoftheFP.
m
ition “A” outside the working gap as follows [11]: During finishing, the forces acting on a FP near the
(cid:1) (cid:2)
work surface are in position “B” (Fig. 1). Owing to the
∂H
F (cid:1)Vc m H (1) rotation of the workpiece, a cutting resistance, Rt, will
x m o ∂x act on the FP in the tangential direction of the rotational
(cid:1) (cid:2)
∂H motion. Moreover, due to the magnetic field strength
F (cid:1)Vc m H gradients in the working gap, the FP exerts a normal
y m o ∂y
force, Rn, to the work surface, while simultaneously, a
where x is the direction of the line of magnetic force, magnetic force, Fm, will act on the FP in the anti-direc-
y is the direction of the magnetic equipotential line, tion of Rt. Fm will prevent FP from flowing or disper-
c is susceptibility of the FP, m is permeability of vac- sing out of the working gap, which ensures that the fin-
m o
uum, V is volume of the FP, H is the magnetic field ishing process will be successful.
strengthatpoint“A”,and∂H/∂xand∂H/∂yaregradients Within UMA, SA performs the primary cutting task
of magnetic field strength in the x and y directions, and FP performs the secondary one. However, the SiC
respectively. does not contain the ferromagnetic property, therefore
From Eq. (1), it is evident that the magnetic forces the pressure required for the SA to abrade must be
Fxand Fy areproportionalto boththesusceptibility and derived from the contiguous FP. Accordingly, the
the volume of FP, the magnetic field strength and its motionbehaviorofFPwillsignificantlyaffectthefinish.
When Fm is larger than Rt (Fig. 1), the FP will execute
a regular cutting state on the work surface and will also
transmit abrasion pressure to the SA beneath it. How-
ever, ifthe workpiece hasa higher hardness, the FP cut-
ting effect decreases, and will slide on the work surface.
Also, when Fm is smaller than Rt, the FP will roll on
the work surface. However, when Fm is very small, the
FP will splash out of the working gap.
OnceFPbeginstoroll,thepressuretotheSAbeneath
it will disappear. Consequently, the finished quality will
be decreased. Therefore, for abrasive treatment to be
effective, the FP must be prevented from not only
splashingoutoftheworkinggap,butalsorollingtherein.
3. Experimental setup and test conditions
Fig. 2 shows the experimental apparatus used. A
brushless DC motor was designed to rotate the work-
piece, and the axialvibration wasactuated viaan eccen-
tric cam mechanism, which was driven by an induction
motor combined with a frequency converter. A series
connection of two magnetically excited coils, around
which copper wire of f1.0 diameter was wound 2150
Fig. 1. Schematic of magnetic field distribution and magnetic force
actingona ferromagnetic particle. turns, produced the magnetic field. Soft iron with high
G.-W.Changetal./InternationalJournalofMachineTools&Manufacture42(2002)575–583 577
Table 1
Particle sizeandhardness offerromagneticparticles
Average
Ferromagnetic Range of
particle Hardness
particle particle size
size
(µm) (µm)
Steelgrit& Steelgrit:HV
53(cid:1)105 80
irongrit 780(cid:1)940
Irongrit:HV
105(cid:1)150 130
110(cid:1)125
150(cid:1)210 180
Table 2
Experimentalconditions
Workpiece SKD11, HRC55(HV600)
f15mm×80mmL
Workingclearance 1 mm
Circumferential speedofworkpiece 0.4m/s
Axialvibration ofworkpiece Frequency: 5Hz
Amplitude: 5mm
Magneticfluxdensity 0.65 T
Excitingcurrent 0.3A
Mixedweight ofunbondedmagnetic Ferromagneticparticles: 4
abrasives g
SiC abrasives:1 g
Machiningfluid SiC abrasiveslurry
Distilled water:SiC=20:1
Mixedratio ofslurry
Fig.2. Threedimensionalviewofexperimental apparatus. (by wt)
Flowrateofslurry 2.4cc/min
relative magnetic permeability was used as both the Lubricant SAE30 oil,0.6g
magnetic core and poles material. To improve the fin-
ishing quality, the abrasive slurry, which consisted of
SA mixed with distilled water, was introduced during cates that the 180 µm iron particle mixed with 1.2 µm
finishing. In addition to cooling and lubricating, the SA provides a better surface roughness, however it can
abrasive slurry would supplement the SA. The abrasive only beimproved from0.25to0.13µmRa,afterhaving
slurry was supplied through an electric stirrer and a been processed for 30 min. Fig. 4 presents the finishing
micro tube pump. results of steel grit mixed with 5.5 and 1.2 µm SA,
The UMA used in this study was a mixture of FP and respectively.Fig.4(b)showsthatthebestsurfacerough-
SA withSAE30 lubricant. After the muddy mixture was ness is obtained by using 180 µm steel grit mixed with
stirred uniformly, it was poured into the working gap, 1.2 µm SA. It requires only 5 min to improve the work
thenthefinishingexperimentwasconducted.Inaddition surface to 0.06 µm Ra, and after 15 min it settles to a
to lubricating, via its viscosity, the primary function of saturated level of 0.042 µm Ra. When 5.5 µm SA is
SAE30 was to bind the SA to the FP. Notably, this pre- mixed with 180 µm steel grit, the highest material
vents SA from scattering into the air during finishing. removal is obtained but the saturated roughness is only
#8000and#2500SAwereused,whichhadanaverage 0.09 µm Ra.
particle size of 1.2 and 5.5 µm, respectively. Iron grit By comparing Fig. 3 with Fig. 4, it is evident that a
and steel grit, of three distinct particle sizes were used finer finish isobtained more rapidly with steel than with
as FP. Table 1 lists the particle size and hardness of FP. iron grit but the patterns of change are similar. This
Table 2 presents the experimental conditions. phenomenon can be summarized into the following two
points. Firstly, withthe same FP particlesize,a superior
surface roughness will be obtained if 1.2 µm SA is
4. Experimental results and discussion
mixed.If5.5µmSAismixed,morematerialisremoved,
4.1. Finishing characteristics of unbonded magnetic however there is greater roughness. As the amount of
abrasive 1.2µmSAbeneaththeFPexceedsthatof5.5µmwithin
the same FP particle size, the average abrasion pressure
Fig. 3 shows the finishing results of iron grit mixed produced from FP for each 1.2 µm SA particle is
with 5.5 and 1.2 µm SA, respectively. The figure indi- smaller. Thus, when 1.2 µm SA is mixed, superior sur-
578 G.-W.Changetal./InternationalJournalofMachineTools&Manufacture42(2002)575–583
Fig.3. Variationofthesurfaceroughnessandmaterialremovalwith Fig.4. Variationofthesurfaceroughnessandmaterialremovalwith
finishing time for iron grit mixed with SiC abrasive. (a) Effects of finishing time for steel grit mixed with SiC abrasive. (a) Effects of
particlesizeofirongritmixedwith5.5µm SiCabrasive.(b)Effects particlesizeofsteelgritmixedwith5.5µmSiCabrasive.(b)Effects
ofparticlesizeofirongrit mixedwith 1.2µmSiCabrasive. ofparticle sizeofsteelgrit mixedwith 1.2µmSiCabrasive.
face roughnesscanbeobtained. Alternately,astheaver- magnetic force, the average abrasion pressure of each
age abrasion pressure for each 5.5 µm SA particle is SA particle beneath the FP is smaller. That is why the
larger, the abrasion scratches are deeper, therefore, larger FP particle size will produce not only higher
although more material is removed, the roughness is material removal but also better surface roughness.
worse. Secondly, regardless of the size of SA mixed, if However, due to the shape of FP, the pre-described
the larger FP particle size is used, not only will more phenomenon will reach a limit. When the limit is
material beremoved, butalso a bettersurface roughness exceeded,theaverageabrasionpressureofeachSiCpar-
isobtained.AsmagneticforceisproportionaltoFP vol- ticlebeneaththeFPdoesnotdecreaseasFPparticlesize
ume, a larger FP particle size produces greater magnetic is increased. Accordingly, the FP with a particle size
force. Furthermore, the greater the magnetic force, the that exceeds that of the limit will cause higher material
lower the FP rolling probability. in addition, due to the removal, but worse surface roughness.
larger vacancies within the larger FP, more SA particles Under the same finishing conditions, steel grit pro-
are carried. Accordingly, for two kinds of FP with equal duces finishing results that are superior to those of iron
weight but distinct particle sizes, the larger FP has a grit for the following reasons:
greater total volume of SA particles. Therefore, the
larger FP together with the carried SA has a broader 1. The hardness of steel grit and iron grit are HV
contact area on the work surface. Despite the slow 780(cid:1)940 and HV110(cid:1)125, respectively. Clearly the
migrationofFPwithintheworkinggapduringfinishing, steel grit possesses the cutting ability on the
this enables more SA particles to exist beneath the FP. HRC55(HV600) workpiece because of its superior
Hence, the larger amount of SA will continuously hardness.
receive abrasion pressure from the FP. Therefore, 2. The similar spherical surface of iron grit, as shown
although the larger particle size of FP results in larger in Fig. 5(a), results in rolling during finishing.
G.-W.Changetal./InternationalJournalofMachineTools&Manufacture42(2002)575–583 579
Fig. 6. Effects of particle size of steel grit without the added SiC
abrasive on surface roughness and material removal with finishing
time.
obtained with 180 µm steel grit on both surface rough-
ness and material removal. Fig. 7 presents the effects of
180 µm steel grit. Fig.8 displays the SEM micrographs,
photographs and surface roughness profiles of the work-
piecebeforeandafterfinishing.Fig.7alsoindicatesthat
theamountofmaterialthatisremovedviasteelgritonly
increased initially, and increased even further than by
mixing it with 1.2 µm SA. However, after the surface
roughness is saturated, the increase of material removal
graduallyslows.Thatis,thesteelgritcutstheworkpiece
and due to the magnetic field effect, all of the steel grit
gathers closely within the working gap. Thus, not only
does the magnetic force prevent the steel grit from
splashing, but also reduces its rolling probability. How-
ever, when the 1.2 µm SA was mixed, the steel grit was
Fig. 5. SEM micrographs of ferromagnetic particles. (a) Iron grit surroundedandseparatedbySiCparticles.Thisloosened
(180 µm in average particle size). (b) Steel grit (180 µm in average the packing density of the steel grit. Consequently, both
particlesize). parts of the steel grit and SA will be extracted from the
working gap by the cutting resistance easily. The
3. Thesteelgritispolyhedroninshape,asshowninFig.
5(b). Both the edges and vertices for each steel par-
ticle form a sharp cutting edge. Additionally, the
polyhedron plane surface causes plane distribution of
abrasion pressure to be applied to the SA beneath,
thus resulting in a broader abrasion area and uniform
abrasion pressure.
Toprovethecuttingeffectsofsteelgrit alone,several
experiments were performed on the HRC55 workpiece
via steel grit without the added SA and under identical
finishing conditions. Fig. 6 presents the experimental
results, which confirm that the surface roughness
obtained via 180 µm or 130 µm steel grit alone is equal
tothatobtainedbymixing1.2µmSAwith180µmsteel
grit. Hence, steel grit can be used alone to finish the
Fig. 7. Effects of particle size of SiC abrasive mixed with 180 µm
workpiece with lower hardness.
steel grit on surface roughness and material removal with finishing
The above discussion reveals that the best effects are time.Note: “Fnone”representsthecase ofusingsteel grit alone.
580 G.-W.Changetal./InternationalJournalofMachineTools&Manufacture42(2002)575–583
Fig. 8. Photographs and surface roughness profiles of workpiece before and after finishing (the workpiece was finished by using 180 µm steel
gritmixedwith 1.2µmSiCabrasive).
extracted steel grit and SA will congregate at the exit of ened to HRC61(HV720). Subsequently, some experi-
the working gap and will roll with the rotating work- ments were conducted via 180 µm steel grit, which was
piece, as shown in Fig. 9. This phenomenon resulted mixed with 1.2 µm and 5.5 µm SA, respectively.
from an interaction of the magnetic field and the cutting Notably, the steel grit only was also used, under the
resistance. Such a finishing mechanism reduces both the samefinishingconditions.Fig.10demonstratesthatonly
abrasion area and the cutting relative motion. Further- alimitedamountofmaterialwasremovedwhen180µm
more, due to the small particle size of 1.2 µm SA, the steel grit only was employed. However, as in the pre-
cutting that occurs on each SA is extremely limited. vious experiments, the work surface improved to 0.05
Therefore, the amount of material that is removed via µm Ra after having been processed for 25 min. Adding
steelgritaloneismuchlargerthanthatobtainedbymix- 1.2 µm SA produces the same excellent surface rough-
ing it with 1.2 µm SA. ness as before. When 5.5 µm SA is used, the surface
roughness that is produced is superior to that obtained
4.2. Influence of workpiece hardness on finishing onthe HRC55workpiece. Thereason isthat this cutting
characteristics depth is not as deep as that of the HRC55 workpiece.
By comparing Fig. 10 with Fig. 7, it was found that
To investigate the finishing performance of steel grit when 5.5 µm SA is added, double the removal rate was
ona harder workpiece, the SKD11 workpiece was hard- obtained on the HRC61 workpiece. However, the
G.-W.Changetal./InternationalJournalofMachineTools&Manufacture42(2002)575–583 581
material removal increased approximately 60% when a
1.2 µm SAmixture wasused.As thesteel gritpossesses
theabilitytocuttheHRC55workpiece,occasionallythe
larger cutting resistance will cause the steel grit to roll.
Alternatively, for the HRC61 workpiece, the cutting
depth was very shallow, hence the cutting resistance is
so limited that most of the steel grit does not roll. How-
ever,itdidslide,trappingalargeamountofSiCparticles
beneath it and applying continuous pressure to it. Also,
due to the limited cutting resistance, the magnetic force
islargeenoughtoretainthesteelgritintheworkinggap
and prevent it from splashing. Therefore, the abrasion
specialties of SA can be fully developed, and superior
surfaceroughnessandmaterialremovalwillbeobtained.
4.3. Micro-Vickers hardness on the work surface
The Micro-Vickers hardness of the surface both
beforeand afterfinishingwasmeasured. Table3depicts
thatregardlessoftheexistenceofSA,theMicro-Vickers
hardness of the surface both before and after finishing
is almost identical. This indicates that the MAF using
UMA does not change the micro hardness on the work
surface. Furthermore, almost nodeteriorated layer exists
on the skin surface, whose cross-section was observed
from both the secondary electron image and back scat-
tered electron image via EPMA apparatus. The deterio-
rated layer is characterized herein as a damaged layer,
the material structure of which is distinct from that of
the matrix. However, it is always produced on the work
surface after machining and will reduce the mechanical
properties of the workpiece.
Fig. 9. Schematic (a) and external (b) view showing the unbonded
magneticabrasivesrollingon theworksurfaceduring finishing.
4.4. Composition analysis and corrosion resistibility
of the finished surface
To determine whether the composition of the finished
surface was altered, both qualitative and quantitative
analyses, aimed at the compositions of Si and C, were
conducted using EPMA. As shown in Fig. 11, the inten-
sity of Si and C are represented in different columns on
the Y-axis. Owing to the lower atomic number of C, an
accurate measurement is difficult to obtain. Hence, this
discussion is omitted here. The content of Si was
Table 3
Micro-Vickers hardness ofsurfacesbeforeand afterfinishing
Typesofunbonded Micro-Vickershardness
magneticabrasives
before after
Fig.10. EffectsofparticlesizeofSiCabrasivemixedwith180µm 180µmsteel gritmixed HV580(cid:1)615 HV586(cid:1)615
steel grit on surface roughness and material removal with finishing with1.2µmSiC abrasive
time.Workpiece:SKD11,HRC61;otherconditions:seeTable2;note: Steelgritalone HV580(cid:1)615 HV585(cid:1)610
“Fnone”representsthecaseofusing steel gritalone.
582 G.-W.Changetal./InternationalJournalofMachineTools&Manufacture42(2002)575–583
of 1800, reveals that it barely exists, thus it is thought
that the layer of insertion is extremely thin.
To investigate the corrosion resistibility of the sur-
faces before and after finishing, the specimens were
immersed in aqua regia solution and the corrosion rate
wasmeasured.Fig.12showsthatthecorrosionratevari-
ation was not obvious for the surface that was finished
via steel grit only, however, it had obviously increased
for the surface that was finished by mixing with SA.
Thus, the insertion of SiC fragments caused an increase
of lattice grain micro-distortion on the skin surface,
which caused slightly increased residual compression
stress. The presence of residual compression stress is
coincident with the results of researchers [12,13]. The
residual stress caused higher energy at highly stressed
regions, where it acted as an anode [14]. Accordingly,
the stress corrosion was accelerated on the surface and
corrosion resistibility was decreased.
5. Conclusions
The results obtained in this research can be summar-
ized as follows.
1. Owingtoitssuperiorhardnessand polyhedronshape,
steel grit is better suited to magnetic abrasive fin-
ishing.
2. If the FP particle size does not exceed that of the
limit, the larger FP particle size will obtain not only
greater material removal but also superior surface
roughness. To obtain better surface roughness, a
smaller SA particle size should be used.
Fig. 11. EPMA qualitative analysis of workpiece before and after 3. Due to the smaller particle size of 1.2 µm SiC abras-
finishing(theworkpiecewasfinishedbyusing180µmsteelgritmixed ive, the best surface roughness of 0.042 µm Ra can
with1.2µmSiC abrasive).(a) Beforefinishing,(b)after finishing.
be obtained if the 180 µm steel grit is mixed. How-
increasedonlywhenSAwasusedinthemix.Moreover,
the quantitative analysis shows the Si content on the
initial surface was only 0.2% and it increased to
1.0(cid:1)2.35% when 1.2 µm SA was added.
Duetothepresenceofabrasiveslurry,thetemperature
of the work surface was so low that it could be touched
with a hand during finishing. Thus, it was deduced that
at low finishing temperatures, an increase in Si content
by diffusion is impossible. Visual inspection of finished
surfaces showed that regardless of the existence of SA,
mirrorlike surfaces were produced. However, their color
obviously differed. When only steel grit was employed,
thesurfacebeforeandafterfinishingwasthesamecolor.
However, after SA was added, it changed to dark gray.
Itissuggestedthatsub-micronfragmentsofcrushedSiC
particles were impressed into the surface of the work-
Fig.12. Corrosionrateofsurfacesafterfinishingincomparisonwith
piece duringfinishing, causing an increasein Sicontent.
the initial surface. (The specimens were immersed in aqua regia
However, its cross-section image, with a magnification solution.)
G.-W.Changetal./InternationalJournalofMachineTools&Manufacture42(2002)575–583 583
ever, owing to the deeper cutting depth for each 5.5 process applying magnetic abrasive machining, Transactions of
µm SiC particle, although more material is removed, the Japan Society of Mechanical Engineers Part C 62 (600)
(1996) 3313–3319.
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