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.

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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.

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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.

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.

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2.Experiments Planning2.1 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.

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Table 1 Various combinations of grinding process variables and the corresponding results measured from the experiments

3.Results and DiscussionsThe 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.

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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.

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.

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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.

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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.

Figure 1 Three stages involved during surface grinding processes

chip formation rubbing

hm

d

plowing

Vs

d

workpieceVw

ngrinding wheel

Ft

Fn

grit

Daq view/2000

Ft

Fn

charge amplifier

spindle

grinding wheel

workpiece

vise

dynamometer

work table

PCsignal A/D converter

Figure 2 Surface grinding experiment set-up and its apparatus arrangement

0 200 400 600 800

data num ber

-200

0

200

400

600

Fn

318

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

800 1000 1200 1400 1600rotational cutting speed (rpm )

8 0

9 0

1 0 0

1 1 0

1 2 0

1 3 0

Ft

(N)

46 grits

60 grits

80 grits

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

800 1000 1200 1400 1600ro tational cutting speed (rpm )

8 0

1 2 0

1 6 0

2 0 0

2 4 0

2 8 0

F (N

)t d=0.01m m

d=0.03m m

d=0.05m m

Figure 5 The relationship between tangential force component and rotational cutting speed for different depths of cut and a fixed grain size 46grits

800 1000 1200 1400 1600ro ta tional cutting speed (rpm )

8 0

1 2 0

1 6 0

2 0 0

2 4 0

F (N

)n

d=0.01m m

d=0.03m m

d=0.05m m

Figure 6 The relationship between normal force component and rotational cutting speed for different depths of cut and a fixed grain size 80 grits

800 1000 1200 1400 1600ro tational cutting speed (rpm )

220

240

260

280

300

320

F (N

)

46 grits

60 grits

80 grits

n

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

800 1000 1200 1400 1600ro tational cutting speed (rpm )

0.12

0.16

0.20

0.24

0.28

surf

ace

ro

ug

hn

ess

Ra

(u

m)

d=0.01m m

d=0.03m m

d=0.05m m

Figure 8 The relationship between surface roughness and rotational cutting speed for different depths of cut and a fixed grain size 60grits

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

800 1000 1200 1400 1600ro tational cutting speed (rpm )

0.16

0.20

0.24

0.28

0.32

surface

roughness

Ra (um

)

46 grits

60 grits

80 grits

~~The End~~

Thank you for your attention