Optimization of Cooling Effect in the Grinding with Mist Type Coolant

Optimization of Cooling Effect in the Grinding with Mist Type Coolant
H. Z. Choi, S. W. Lee, D. J. Kim

Korea Institute of Industrial Technology
35-3, HongChonRi, IbJangMyun, ChonAnSi, 330-825, Korea


ABSTRACT

Coolant generally contains the extreme pres sure additives such as chlorine, sulfur and phosphorus to
improve the grinding efficiency, but the severity of environmental pollution due to these additives always
has been proposed. Mist type coolant is one of the cooling materials developed for preventing heat
generation on the grinding point instead of coolant. It is so effective that it can increase the surface
integrity of workpiece, decrease considerable machining cost and in the end prevent the environmental
pollution.
This study focused on the methods for increasing the cooling effect in grinding using mist type coolant.
First, grinding characteristics in the grinding with mist type coolant were compared with that of coolant
and compressed cold air. And then mist type coolant supply and grinding conditions for increasing the
cooling with respect to workpiece quality were considered. As a result, it is possible to get the surface
integrity and cooling effect as much as coolant.

Key words : Extreme pressure additives, Grinding Efficiency, Environmental problem, Mist type coolant
Cooling material, Cooling effect, Surface integrity, Grinding temperature, Energy ratio

1. Introduction

Coolant has great performances such as lubrication, cooling and penetration. Especially, it contains the
extreme pressure additives such as chlorine, sulfur and phosphorus to improve the grinding efficiency.
But these chemical materials cause several environmental problems. The dry grinding with compressed
cold air was one of the grinding methods that have developed for solving these environmental problems.
It could prevent the environmental pollution through only using -10 ~ -40? of compressed air and also
decrease the considerable machining cost due to coolant. Nevertheless, it is not enough to get the
sufficient grinding performance and cooling effect because it has no lubricant material on the grinding
point. The semi -dry grinding with mist type coolant uses small amount of coolant for the lubrication on
the grinding point. These grinding methods not only can improve the surface integrity of workpiece but
also reduce the environmental pollution by minimizing the use of coolant. But it could not achieve the
cooling effect as much as coolant.
Therefore, in this study, the grinding experiments using coolant, mist type coolant were performed and
methods to increase the cooling effect of the grinding using mist type coolant were analyzed. First, the
surface integrity of workpiece such as surface roughness, roughness, residual stress and energy ratio into
workpiece were measured to analyze the grinding characteristic of mist type coolant. And then, the
grinding experiments according to the supply method of mist type coolant and grinding condition like
wheel speed were performed. Through these experiments, the cooling effect in grinding using mist type
coolant could be increased as much as that of coolant.

2. Experimentation

2.1 Fundamentals of Cooling Effect in Grinding

The main characteristic of grinding in comparison to other machining processes is the relatively large
contact area between the wheel and workpiece and the high friction between the abrasive grains and
workpiece surface. It causes high heat generation on the grinding point, resulting in a high risk of thermal

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damage to the workpiece surface layer. The semi -dry grinding using mist type coolant generally uses
5~300ml/min of coolant for cooling lubrication. This cooling lubrication largely depends on the proper
lubrication environment for preventing the film boiling on the grinding arc between wheel and workpiece.
The film boiling on the grinding arc can be escaped as the amount of mist type coolant increases and its
particle size become large. The mist type coolant also has to be supplied on the wheel surface prior to the
grinding point. In this experiment, the mist type coolant supply was fixed on the wheel surface prior to
the grinding point. Through observing the flow of mist type coolant with CCD CAMERA, 200ml/min of
mist type coolant was decided. Its particle size was 24µm.

2.2 Experimental Conditions

In this experiment, surface grinder and CNC cylindrical grinding machine, WA(White Alumina)
grinding wheel and SCM21 of crank shaft material were used. The experimental conditions are shown in
Table 1. The cooling methods, depth of cut, wheel speed were used as the variables for the experiment.

Table 1 Experimental Conditions

Surface grinder
Grinding machine
Cylindrical grinding machine
Grinding wheel
WA80I/J7V, ~Ø o305×25t×Ø i127
Workpiece
SCM21
Carburizing and quenching, HRC 58?60
Coolant
Emulsion (4%)
Grinding
Amount(ml/min)
100, 200
fluids
Mist
Temperature (?)
-30
Dresser
One-point diamo nd dresser
Dressing conditions
Depth of cut (ad, µm)
10
Feed rate (fd, mm/rev)
0.05
Depth of cut (a, µm)
20
Wheel speed (vc, m/sec)
20, 30
Surface grinding
Table feed speed
6
(vft, mm/sec)
Working conditions
Depth of cut (a, µm)
5, 10, 15, 20, 30
Wheel speed (vc, m/sec)
25, 30, 35, 40
Plunge grinding
Workpiece speed
18
(vw, m/min)


2.3 Experimental Device

The experimental equipment was composed of two components. That is, mist type coolant nozzle and
constant temperature water bathe. The nozzle makes coolant 10~24µ m of particles and constant
temperature water bathe controls the temperature of compressed cold air within ±0.5? over 5 hours.

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COLD AIR
WATER FILTER
CONSTANT
COLD AIR
TEMPERATURE
PRESSURE
WATER BATH
GAUGE
Mist
COLD AIR
TUBE2
Nozzle
AIR
Nozzle
DRYER
TUBE1
WHEEL
Mist
COLD AIR
Nozzle
Nozzle
VALVE
DUST
COLLECTOR
TAILSTOCK
AIR
WHEELHEAD
COMPRESSOR
WORKPIECE


Fig. 1 Schematic Diagram of Experimental Equipment

3. Experimental Results and Inquiries

3.1 Surface Integrity in the Grinding using Mist Type Coolant

Fig. 2 shows the surface roughness and roundness of workpiece after grinding according to cooling
methods. As sho wn in Fig. 2, the surface roughness was increased as much as coolant and roundness was
even better than that of coolant due to low pressure force on the grinding point between wheel and
workpiece. This results explain that small amount of mist type coolant only can increase the surface
integrity of workpiece with keeping the lubrication well on the grinding arc and decrease the
perpendicular pressure force (Fn) due to coolant.


3.5
4.0
Cold air
Coolant [Emulsion 4%]
2
3.0
3.5
Cold air [-30 ,
? 5kg /cm, v =80m/s, ??9.3mm]
f
air

)
Mist [100ml/min] + Cold air [-30
?]
Cold air
?
3.0
2.5

,


z

)
Coolant
Coolant
? 2.5
2.0
Mist+Cold air
s
s

(
e
n 2.0
Mist+Cold air
d

1.5
n
Plunge Grinding
u
Plunge Grinding
o 1.5
SCM21 WA80I/J7V
R
SCM21 WA80I/J7V
1.0
One Point Diamond Dresser
One Point Diamond Dresser
1.0
v =35m/sec, v =18m/min, s=0sec
v =35m/sec, v =18m/min, s=0sec
Coolant [Emulsion 4%]
c
w
c
w
Surface Roughness (R 0.5
2
a =10 ,
? f =0.05mm/rev
d
d
a =10 ,
? f =0.05mm/rev
Cold air [-30 ,
? 5kg /cm, v =80m/s, ??9.3mm]
f
air
0.5
d
d
Mist [100 ml/min] + Cold air [-30 ]
?
v '=50mm3/mm
v '=50mm3/mm
w
w
0.0
0.0
5
10
15
20
25
30
35
5
10
15
20
25
30
Depth of Cut (a ,
? /sec)
Depth of Cut (a ,
? /sec)
(a) Surface Roughness (b) Roundness

Fig. 2 Surface Integrity of grinding according to grinding methods

3.2 Cooling Effect in the Grinding using Mist Type Coolant
Fig. 3 shows the energy ratio conducted into workpiece among total grinding heat on the grinding point

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during grinding and residual stress on the workpiece surface after grinding. As shown in Fig. 3, the
energy ratio of coolant was 26.83%, mist 31.04% and compressed cold air 43.47%. These heat energy
conducted into workpiece generates the tensile residual stress in contrast to that by mechanical proces s.
At result, the residual stress in the grinding using coolant showed the compressed residual stress to depth
of cut 30µm, but mist type coolant showed the tensile stress over depth of cut 15µm.
300
Plunge Grinding
250
SCM21, WA80I/J7V
One Point Diamond Dresser
v =35m/sec, v =18m/min, s=0sec
200
c
w
a =10?m, f =0.05mm/rev
Cold Air
d
d
3
150
v '=50mm /mm
w
100
Mist+Cold Air

50
Depth of Cut (a , ?m /sec)
Coolant
0
Residual Stress (MPa)
5
10
15
20
25
30
-50

Coolant [Emulsion 4%]
-100
Cold air [-30oC, 5kg /cm2, v =80m/s, ??9.3mm]
f
air
o
-150
Mist [100ml/min] + Cold air [-30C ]

.
(a) Energy Ratio (b) Residual Stress
Fig. 3 Energy Ratio into Workpiece and Residual Stress

4. Conclusions

The experiments to optimize cooling effect in the grinding using mist type coolant were carried out.
The conclusions are drawn as follows:
1) The surface roughness in the grinding using mist type coolant was increased as much as coolant and
roundness was even better then coolant due to low pressure force generated on the grinding point.
2) The energy ratio into the workpiece in the grinding using coolant was 26.83%, but mist type coolant
is 31.04%. And, in case of residual stress, coolant showed the compressed residual stress to 30 µm of
depth of cut, but mist type coolant showed the tensile residual stress over 15µm of depth of cut.
Because it is not enough to get sufficient cooling effect on the grinding point
3) The supply method of mist type coolant on the peripheral surface of grinding wheel and approximate
200ml/min (particle size=24µm) of mist type coolant for optimizing the cooling effect were selected.
The grinding condition like wheel speed was also considered as a grinding variable.

5. Reference
1. H. Z. Choi, S. W. Lee, H. D. Jeong, “A comparison of the cooling effects of the compressed cold
air and coolant for cylindrical grinding with CBN wheel”, Proceeding of AFDM, pp 319~322,
1999
2. H. Z. Choi, S. W. Lee, J. S. Ahn, “A study on the surface integrity for the cylindrical grinding with
compressed cold air”, ISAAT, pp. 187~192, 1998.
3. H. Z. Choi, S. W. Lee, J. S. Ahn, “A comparision of the cooling effects of the compressed cold air
and coolant for the cylindrical grinding”, International euspen conference, pp. 416~419, 1999.
4. H. Z. Choi, S. W. Lee, D. J. Kim and H. D. Jeong, "Grinding of Spindle Shaft Material with Mist
Type Coolant", The 4th ISAAT, pp.279-284, 2000.
5. Toenshoff, H. K., Brinksmeier, E. Choi, H. Z., “Abrasive and their influence on force, temperatures
and surface", International grinding conference, SME Technical paper MR86-626, June 1986.
6. C. Guo, "Energy Partition and Cooling During Grinding", Proceeding of 3rd International
Machining & Grinding Conference, SME, Session 214, 1999

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