Transcript The Effect of Process Variables on Surface Grinding of SUS304 Stainless Steel S.

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
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.
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
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.
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
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
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
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 (3×3×3) 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 46×46
holes per unit square inch area.
Table 1 Various combinations of grinding process variables and the corresponding results
measured from the experiments
Continued
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.
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
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
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
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
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.
grinding wheel
n
d
Vw
Ft
workpiece
Fn
hm
Vs
grit
d
chip formation
plowing
rubbing
Figure 1 Three stages involved during surface grinding processes
spindle
grinding wheel
workpiece
vise
dynamometer
Ft
work table
Fn
charge amplifier
signal A/D converter
Daq view/2000
PC
Figure 2 Surface grinding experiment set-up and its apparatus arrangement
600
Fn
400
318
200
0
-200
0
200
400
600
800
data number
Figure 3 The signals of normal grinding force component sampled
from the experiment by dynamometer under the condition of
n=900rpm, 46grits and d=0.05mm
130
46 grits
60 grits
120
80 grits
Ft (N)
110
100
90
80
800
1000
1200
1400
1600
rotational cutting speed (rpm)
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
280
240
F (N)
200
t
d=0.01mm
d=0.03mm
160
d=0.05mm
120
80
800
1000
1200
1400
1600
rotational cutting speed (rpm)
Figure 5 The relationship between tangential force component and
rotational cutting speed for different depths of cut and a fixed grain
size 46grits
240
d=0.01mm
200
d=0.03mm
Fn (N)
d=0.05mm
160
120
80
800
1000
1200
1400
rotational cutting speed (rpm)
Figure 6 The relationship between normal force
component and rotational cutting speed for different depths
of cut and a fixed grain size 80 grits
1600
320
46 grits
60 grits
300
80 grits
Fn (N)
280
260
240
220
800
1000
1200
1400
1600
rotational cutting speed (rpm)
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 d=0.05mm
0.28
d=0.01mm
d=0.03mm
d=0.05mm
surface roughness Ra (um)
0.24
0.20
0.16
0.12
800
1000
1200
1400
1600
rotational cutting speed (rpm)
Figure 8 The relationship between surface roughness and
rotational cutting speed for different depths of cut and a
fixed grain size 60grits
0.32
46 grits
60 grits
80 grits
surface roughness Ra (um)
0.28
0.24
0.20
0.16
800
1000
1200
1400
1600
rotational cutting speed (rpm)
Figure 9 The relationship between surface roughness and rotational
cutting speed for different grain sizes of abrasive particles and a fixed
depth of cut d=0.03mm
~~The End~~
Thank you for your attention