The Effect of Process Variables on Surface Grinding of SUS304 Stainless Steel

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The Effect of Process Variables on Surface
Grinding of SUS304 Stainless Steel
S. Y. Lin, Professor Department of Mechanical
Manufacturing Engineering National Formosa
University, Taiwan
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Abstract
This study performs an experiment to investigate
the effect of process variables such as grain
size of abrasive particles, rotational cutting
speeds of the wheel and grinding depth of cut on
surface roughness and the fluctuations of
grinding forces for SUS304 stainless steel.
STP-1623 ADC surface grinding machine, grinding
wheel with aluminum oxide (Al2O3) material and
SUS304 stainless steel workpiece are used in the
experiment. The roughness of the grinding surface
was measured by the roughness measuring
instruments and the fluctuations of grinding
forces were measured through dynamometer after
each surface layer ground from the workpiece in
the experiment.
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The grinding performance can be ascertained from
the signal fluctuations phenomena of the grinding
forces both along normal and tangential
directions, which may also be utilized as an
index for the quality of surface finish judgment.
The results show that excellent surface quality
being always consistent with the stable grinding
force fluctuations and can be obtained under the
conditions of small grain size of abrasive
particles, high revolutions of the wheel and
shallow depth of cut. Keywords surface
grinding, surface roughness, grinding forces.
Continued
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1.Introduction
Grinding is a chip removal process, and the
cutting tool is an individual abrasive grain.
Individual grains have irregular shapes and are
spaced randomly along the periphery of the wheel.
The average rake angle of the grains is highly
negative, and consequently grinding chips undergo
much larger deformation than in other cutting
processes. The grinding process can be
distinguished into three phases, including
rubbing, plowing and cutting as shown in Figure
1. When the grain engages with the workpiece in
up-cut grinding, the grain slides without cutting
on the workpiece surface due to the elastic
deformation of the system.
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This is the rubbing phase. As the stress between
the grain and workpiece is increased beyond the
elastic limit, plastic deformation occurs. This
is the plowing phase. The workpiece material
piles up to the front and to the sides of the
grain to form a groove. A chip is formed when the
workpiece material can no longer withstand the
tearing stress. The chip formation stage is the
cutting phase.
Continued
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Grinding of metals is a complex material removal
operation involving rubbing, plowing and cutting
between the abrasive grains and the work material
depending on the extent of interaction between
the abrasive grains and the workpiece under the
conditions of grinding. Grinding is a very
complex machining process with a large number of
characteristic parameters that influence each
other. Grinding can also be considered as an
interactive process where the grains of the
grinding wheel interact with the workpiece at
high speed and under high pressure.
Continued
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In order to investigate the effects of varying
process variables related to grinding operation
on the grinding performance, surface grinding
experiments were performed by accounting the
grain size of abrasive particles, rotational
cutting speeds of the wheel and grinding depth of
cut in this study. The grinding performance can
be ascertained from the fluctuations of the
grinding forces both along normal and tangential
directions, and the distributions of ground
surface integrity. The results show that
excellent surface quality being always consistent
with the stable grinding force fluctuations and
can be obtained under the conditions of small
grain size of abrasive particles, high
revolutions of the wheel and shallow depth of
cut.
Continued
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2.Experiments Planning
2.1 Grinding Conditions Under a constant table
speed, three process variables related to surface
grinding, i.e. grain size of abrasive particles,
rotational cutting speeds of the wheel and
grinding depth of cut are selected in this study.
Each of these variables was set at three levels
and there are totally 27 (333) combinations of
grinding conditions, and are shown in Table1. The
number denoted for grain size is determined from
the sieve and 46 grits number represents the
abrasive particle may go through a sieve with
4646 holes per unit square inch area.
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Table 1 Various combinations of grinding process
variables and the corresponding results measured
from the experiments
Continued
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3.Results and Discussions
The signal chart of the normal grinding force
component,, sampled from the experiments is shown
in Figure 3, which build a square wave shape when
the chip was removed from the workpiece. The
fluctuations phenomena exhibited in the signal
chart are attributed to the toughness properties
of the workpiece material of stainless steel. The
relationships, between tangential and normal
grinding force components and rotational cutting
speeds, for different grits numbers under various
fixed depths of cut are shown in Figure 4 and 5,
respectively. The grinding force components are
decreased as the rotational cutting speed is
increased.
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2.2 Experiment Set-up Surface grinding experiment
set-up and its apparatus arrangement are shown in
Figure 2. Here, grinding forces are measured with
a piezoelectric type dynamometer and surface
roughness left on ground surface are measured by
the roughness measuring instruments. The rotation
balance of the wheel was calibrated and the
dressing of wheel surface was undertaken with
dressing diamond tool before each experiment of
grinding condition set indicated in Table 1.
Continued
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mentioned above, the smaller number are the
grits, the coarse grain size is the abrasive
particles. Hence, the structure of the particles
packing in wheel is not dense in the smaller grit
number. It has much particles emerged out on the
wheel surface, which increases the real contact
area participating the grinding
processes. Similarly, the relationships, between
tangential and normal grinding force components
and rotational cutting speeds, for different
depths of grinding under various fixed grain
sizes are shown in Figure 6 and 7, respectively.
As expected, the grinding force is proportional
to the grinding depth of cut. It is due to a
large depth of wheel indentation and hence the
loading applied to the abrasive particles getting
larger. While shallow engagement between wheel
and worjkpiece resulting in a light loading acted
on the workpiece.
Continued
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Lower grinding force can be obtained in higher
surface speed of the wheel, The grinding wheel
passing very fast over the workpiece surface as
the high revolutions of the wheel is set. As a
result, light loading being applied to the
abrasive particles in contacting with workpiece
and a lower summation load is deduced.
Furthermore, the force component along the
tangential direction is less than that in the
normal contact direction due to the high pressure
as the wheel engaged with the workpiece in
surface grinding operation. Generally, the ratio
of thrust force to cutting force is about two for
frictional rubbing contact grinding. The forces
required for abrasive particles in grinding wheel
with coarse grain size are greater for that with
fine grain size.
Continued
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The relationships between surface roughness and
rotational cutting speeds for different grits
numbers under a fixed depth of cut, and for
different depths of cut under a fixed grain size
are shown in Figure 8 and 9, respectively. The
surface roughness is reduced as the surface speed
of the grinding wheel is increased, while surface
roughness is increased when the size of abrasive
particles in the wheel is coarse and the depth of
cut is increased. Higher surface speed of the
wheel results in lower grinding forces and a flat
ground surface. Large grit number of the grain
size owns a dense structure of the particles
packing and a lower surface roughness thus
induced by the smaller spacing between abrasive
grains. While large depth of wheel indentation
corresponding to deep grinding depth of cut owns
greater grinding forces which is easier to make a
ground surface being not flat.
Continued
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Conclusions
The effects of the variations of the process
variables relating to grinding operation on the
grinding performance are investigated in this
study. From the above analyses, the following
conclusions can be drawn   1. The smaller number
are the grits, the coarse grain size is the
abrasive particles. Hence, the structure of the
particles packing in wheel is not dense in the
smaller grit number. It has much particles
emerged out on the wheel surface, which increases
the real contact area participating the grinding
processes. 2.Large grit number of the grain size
owns a dense structure of the particles packing
and a lower surface roughness thus induced by the
smaller spacing between abrasive grains. While
large depth of wheel indentation corresponding to
deep grinding depth of cut owns greater grinding
forces which is easier to make a ground surface
being not flat.
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rubbing
chip formation
Figure 1 Three stages involved during surface
grinding processes
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Figure 2 Surface grinding experiment set-up and
its apparatus arrangement
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Figure 3 The signals of normal grinding force
component sampled from the experiment by
dynamometer under the condition of n900rpm,
46grits and d0.05mm
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Figure 4 The relationship between tangential
force component and rotational cutting speed for
different grain sizes of abrasive particles and a
fixed depth of cut d 0.01mm
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Figure 5 The relationship between tangential
force component and rotational cutting speed for
different depths of cut and a fixed grain size
46grits
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Figure 6 The relationship between normal force
component and rotational cutting speed for
different depths of cut and a fixed grain size 80
grits
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Figure 7 The relationship between normal force
component and rotational cutting speed for
different grain sizes of abrasive particles and a
fixed depth of cut d0.05mm
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Figure 8 The relationship between surface
roughness and rotational cutting speed for
different depths of cut and a fixed grain size
60grits
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Figure 9 The relationship between surface
roughness and rotational cutting speed for
different grain sizes of abrasive particles and a
fixed depth of cut d0.03mm
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The End
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