GENERATION OF QUALITY SURFACE IN EXTRUDE HONING

GENERATION OF QUALITY SURFACE IN EXTRUDE HONING
K. Narayanasamy, H.P.Raju
PEIL, Department of Mechanical Engineering
Indian Institute of Technology Madras, Chennai-600 036 India
E-mail: [email protected]
1.0 INTRODUCTION
Surface is a boundary that separates an object from another object or substance; it is either in contact
with some other surface or will be open to atmosphere. Hence the surface quality assumes greater
importance for proper functioning of any product with due importance to aesthetic aspect also. This
necessitates production of required surface texture to perform a specific function. Now a days surfaces
are being engineered to suit end application (1-2). Processes like traditional honing, lapping, grinding can
be used to regular shapes. But for lapping, a flexible process, others being rigid in nature, are of little/no
use to finish machining of irregular intricate shapes or contours like turbine blades. This has led to the
development of a flexible abrasive process called Extrude honing (EH) process.
Extrude honing is relatively a new non traditional machining process used to finish, deburr, radius, polish,
remove recast layer and to produce compressive residual surfacial stresses. This process removes
material in small quantities by forcing abrasive laden viscoelastic polymeric material normally known as
medium, which is the best candidate for finishing internal passages or holes which are inaccessible by
any other processes (3-4).
Finishing of I.C.Engine manifolds, hydraulic components, and bio medical components are few areas of
application. It is an alternative environmental friendly process for removal of protective aluminium nickel
coating applied to certain turbine engine blades. Removal of sharp corners enhances the edge quality. It
is a proven, reliable and accurate method that provides consistent results(5). Abrasive laden medium can
be thought of as a flowing flexible grinding wheel. The irregular cutting edge geometry of abrasive grits
and extremely small chips produced in this process makes it possible to concentrate the machining stress
at very local points on the work. It is this ability which makes finishing of difficult to machine material
possible. Large number of cutting edges with indefinite orientation and geometry makes the failure of any
grits insignificant and does not affect the overall process performance(6). Medium being a proprietary
item, is not easily available. In view of this, present study on extrude honing is concerned with an
alternative medium for abrasive carrier, its performance and consequent surface texture production in
extrude honing.
2.0 EXPERIMENTAL
Owing to their higher impact resistance and higher order fatigue life, SG Cast irons are very popular
material in many applications. Considering the cost efficiency, SG Cast iron of grade 600 is selected as
work material. Apart from rheological properties better grit retention, bond uniformity, and higher abrasive
exposure are few properties a material required to posses for satisfactory performance as a carrier
medium. A select grade of silicone polymeric medium is thoroughly mixed with a volume fraction of 25%
SiC abrasives of 36 grit size using a polymer mixing device. An experimental set-up was designed and
developed to implement extrude honing process. Details of work material, carrier medium, process
parameter used in the trials are presented in Table1.
Table1. Details of work material, carrier medium, process parameter
Work material
Carrier medium
SG Cast Iron
Silicone polymer
Process parameter
Carbon ( % )
3.61
Appearance
Translucent
Temperature
Ambient
Tensile Strength (N/mm2 )
614
Density (kg/m3)
1.13X103
Volume fraction
0.25
0.2% Proof Stress (N/mm2)
372
Viscosity(Pa.s)
20250
Velocity (m/min)
0.4.0.6,1.0,1.2
3.0 RESULTS AND DISCUSSION
3.1 Surface roughness
Typical observed parametric influence of surface characteristics of extrude honed SG Cast Iron bored
specimen is illustrated in Fig.1. It is seen that there is a visible / drastic reduction in Rt, Ra values during
early phase of honing, followed by progressive reduction with number of passes.Removal of
waviness/dominant asperities present over the bored surface during early phase of extrude honing results
V=0.4m/min
V=0.6m/min
V=1.0m/min
V=1.2m/min
Roughness parameter Ra, µm
4
3
2
1
0
Roughness parameter Rt, µm
in appreciable reduction in Rt, Ra values.
30
V=0.4m/min
V=0.6m/min
25
V=1.0m/min
V=1.2m/min
20
15
10
5
0
0
1
2
3
4
5
6
7
8
9
Number of passes
10 11
0
1
2
3
4
5
6
7
8
9 10 11
Number of passes
Fig.1. Typical observed parametrical influence of surface roughness characteristics
3.2 Bearing area characteristics
A surface texture can be better defined by referring to its bearing area fraction characteristics; a study on
bearing area characteristics will bring about typical characteristics of a surface texture in terms of its core
roughness, depth of profile allowing for crest flattening and valley fill up. It is observed from pass wise
monitored individual profile (Fig.2) that, with progressive honing crest flattening gets saturated, there after
improvement in core roughness, followed by change in valley depth.
Fig.2. Typical observed profiles of surface roughness and bearing area fraction
3.3 Acoustic emission signal
On-line monitoring of acoustic emission signal using a broad band acoustic emission sensor presented
mixed mode of emission during early phase indicating waviness correction/crest flattening. With
subsequent passes a steady rise in rms value of the monitored acoustic emission signal as well as
predominant peaks over lower frequency range 50-100 kHz were observed (Fig.3) indicative of
progressive honing. In subsequent passes, a reduction in rms value as well as low power high frequency
peaks were observed, indicative of glazing associated with localised spalling of material.
12
12
N=1pass
RMS=2.1606
10
4
8
R
E
W
O
P
6
4
2
2
100
150
200
FREQUENCY (kHz)
250
300
6
4
2
0
50
N=10passes
RMS=1.5463
10
8
R
E
W
O
P
6
0
N=4 passes
RMS=1.9865
10
8
R
E
W
O
P
12
50
100
150
200
FREQUENCY (kHz)
250
300
0
50
100
150
200
FREQUENCY (kHz)
250
300
Fig.3 Typical power spectrum at different stages of stages of extrude honing
3.4 Residual stress induced
Measurement of residual stress on honed surface indicates tensile residual stress during early phase of
honing, followed by compressive mode, with progressive honing and subsequent glazing (Fig.4). Impart
of compressive residual stress on the surface results in enhanced fatigue life of the component.
Residual stress
MPa
0
-100 0
-200
-300
-400
Number of passes
2
4
6
8
10
12
Fig. 4 Residual stress induced at the surface during extrude honing.
3.5 Scanning electron macrographs
For better understanding of the nature of texture produced by extrude honing, surfaces as bored and as
honed were observed (Fig.5) through scanning electron microscope.
(a) as bored
(b) after 5 passes
(c) after 10 passes
Fig.5 Scanning electron macrographs of surface at different stages of extrude honing
After extrusion honing fairly uniform lay pattern with localised folding of asperity can be seen. Further,
due to continuous honing, surface has experienced localised discrete glazed texture.
4.0 CONCLUSION
The major conclusions drawn based on the trials conducted are as follows.
1. A progressive improvement in surface roughness parameter and bearing area fraction is seen till
eighth pass, beyond which the surface starts deteriorating.
2. Higher order residual compressive stresses induced on the surface with number of passes.
3. With the increased surface finish, characteristics of acoustic emission spectrum changes, this
can be used for on line monitoring of the process.
5.0 REFERENCES
1. Chris J. Evans., James B. Bryan., “Structured”, “Textured” or “Engineered” Surfaces
Annals of the
CIRP, Vol. 48/2/1999 541-556
2. A.R.Jones., J.B.Hull., “Ultrasonic flow polishing”, Ultrasonics , 36 (1998) 97-101
3. L J.Rhoades., H. A.Clouser., “Abrasive Flow Machining”, ASM Metals Handbook,16 (1989) 514-519
4. R.E. Williams, K.P.Rajurkar, “Stochastic Modeling and of Analysis of Abrasive Flow Machining”,
Trans.ASME. Journal of Engineering for Industry 144 (1992) 74-81
5. J. Matechen, John., “Advantages of abrasive flow machining”, Abrasives, (1995) 20-21
6. I.Inasaki, H.K.Tonshoff, T.D.Howes, “Abrasive Machining in the Future”, Annals of the CIRP, Vol.
42/2/1993 723-732