SOAP MIST JET COOLING OF GRINDING PROCESSES
D.M. Babic, A.A.Torrance and D.B.Murray
Department of Mechanical and Manufacturing Engineering
Trinity College Dublin, Ireland
Keywords: Grinding, cooling, air jets, water mist, soap.
Abstract. Grinding, like other abrasive processes, may generate high local temperatures along the
arc of cut. These can cause various forms of surface damage in the most sensitive finishing phase of
the manufacturing cycle. Traditional cooling methods based on large amounts of water-oil
emulsions can be both ineffective and environmentally unacceptable. A new approach to this
problem has been devised utilizing the high penetrative power of fast air jets combined with a
water/soap mist to greatly improve convective cooling and lubrication along the arc of cut. The
results obtained offer striking improvements compared to traditional liquid coolants especially if the
relative simplicity of the method is considered.
Introduction
Grinding, in its primitive form, is one of the oldest manufacturing processes know to man. It is
typically used as a finishing operation intended to achieve high dimensional accuracy and excellent
surface quality. Despite for long being considered as just a finishing operation with little metal
removal involved, new grinding methods are, more and more, examples of an “all in one” type of
machining operation. Procedures such as HEDG (High Efficiency Deep Grinding) and HSPG (High
Speed Peel Grinding) allow for both major metal removal and surface finish of workpieces with
complex geometry to be completed in a single operation on a single machine [1]. Grinding still
accounts for about 25% of all of machining swarf in the industrialized world [2].
The specific energy in grinding is usually much higher than in traditional cutting processes such as
turning or milling. This is because the process of metal removal in grinding, as in all abrasive
methods, is fundamentally different to that of traditional metal cutting. In turning, for example, a
cutting tool with defined geometry and, typically, a positive rake angle removes the swarf by a
process of concentrated shear. There is little friction, and consequently a smaller part of the total
energy is turned into heat, whilst chip size is usually measured in millimeters. In grinding, on the
other hand, the cutting tool (grinding wheel) consist of a large number of cutting grits with no
defined cutting geometry. On average, rake angles are highly negative which leads to significantly
higher frictional work. Metal removal in grinding also involves a large amount of redundant plastic
work [3]. Depending on the grinding parameters, chip size can vary, but in the micron size range,
which leads to an intrinsically greater comminution energy than in traditional cutting processes.
Specific energies for the turning of steel range from 3 – 9 J/mm3 [4], compared with 14 J/mm3 in
stock removal grinding (SRG) and 69 J/mm3 in finish form grinding (FFG) [1]. Such high energy
input is almost completely turned into heat, and can cause high temperatures in the cutting zone.
There are several possible ways for the heat generated in grinding to dissipate. It has long been
acknowledged that in shallow surface grinding with a short (~ 1mm) arc of cut, around 5% of the
total energy input is taken away with the chips or coolant [1]. The remaining heat (up to 95%) is
distributed between grinding wheel and workpiece. The exact partition ratio depends on the thermal
properties of both bodies. If low-conductivity wheels (alumina for example) are used typically 7090% of the heat generated will end up in the workpiece. Such a large heat input is the main cause of
overheating and damage of the workpiece surface.
As grinding was for a long time considered to be just another cutting operation the cooling methods
applied were (and often still are) more or less the same as in turning or milling. Usually the
workpiece is simply flooded with large amount of liquid coolant (typically an emulsion of water
and a small amount of mineral oil). This has long been proven to be of limited effect for several
reasons – the cutting speed in grinding is much higher than in turning meaning that just a fraction of
the coolant applied actually reaches the cutting zone to remove the heat. Moreover process
temperatures in grinding may be much higher which can cause film boiling of a liquid coolant,
dramatically diminishing its cooling potential [5]. In addition to this, there is the cost of mineral oils
used with liquid coolants to be considered. They are expensive both initially and in their disposal
stage.
It is obvious that a better cooling solution is required: one which will satisfy several conditions – to
be effective, cheap and environmentally friendly. Many suggestion have been put forward Cryogenic cooling using liquid nitrogen [6]; using graphite as a lubricant [7]; intermittent grinding
by slotted wheels [8]; on-line ultrasonic cleaning of the wheel surface [9]; and the use of CBN
wheels [10]. Each of these methods offers significant advantages in dealing with the initial problem,
but has downsides somewhere else, either in cost, complexity of use, or difficulties in the control of
the entire process.
Recently a new approach has been suggested: high-speed (close to mach 1) air jets directed at the
arc of cut [11]. These jets with a speed an order of magnitude greater than that of the grinding wheel
can penetrate the stiff boundary layer around the wheel and reach the cutting zone. The results
achieved suggest that this technique is at least as effective as liquid cooling under the conditions
studied. Further improvements were made by introducing a small amount of water droplets into the
air jets [12] to increase their heat transfer potential and their ability to clean the wheel. Although
encouraging results were obtained it was felt that further improvements are still possible. Water is
not a good grinding lubricant, and the addition of some substance with good lubricity would be an
advantage. Liquid soap, which does not degrade the environment was thought to be a suitable
choice. This paper discusses the results of an investigation into the feasibility of using soap mist jets
to cool the grinding process.
Experimental set-up
Experiments were performed on a standard Jones & Shipman 540P surface grinding machine. The
wheel rotates, nominally, at 3000 rpm and, with a total wheelhead power of 3.5 kW, wheel speed
can be maintained constant over all the depths of cut used here. The speed of the grinding wheel
was checked from time to time using an optical tachometer. The maximum table speed is 0.2 m/s. In
all experiments just one alumina-grit grinding wheel of 186 mm diameter was used (Norton 77A
601H LNAA). The machine was equipped
with two nozzles of 2.6 mm exit diameter,
connected to a supply of compressed air via a
mass flow-meter (Omega FMA – 1600). The
exit diameter was chosen to allow air speeds
of Mach 1 to be attained easily. Both nozzles
are mounted on sleeve-like supports allowing
adjustment of their incidence angle and
distance to the cutting zone. In these tests, the
Figure 1: Grinding set up
incidence angle was 15° and the distance
between the nozzle exit and the centre line of the grinding wheel touching the workpiece was 40
mm, the practical minimum. The whole arrangement can be seen in Fig. 1.
Figure 2: Water injection system
To inject small amounts of liquid (water/soap
mixture) into the flow a special arrangement
was used. This consisted of a vessel of liquid
which was pressurized by the same compressed
air used to supply the nozzles. A small pressure
differential was generated to force liquid out of
the vessel through a flowmeter equipped with a
Valve which controlled the rate at which liquid
was injected into the air stream.
The
arrangement is shown in figure 2.
The workpiece was mounted on top of a piezo-electric dynamometer (Kistler 9011B) to record the
tangential and normal forces experienced during the grinding process. The dynamometer was
connected to a PC data acquisition system via two charge amplifiers. An LVDT was mounted on
the worktable to monitor its speed. The temperature of the cutting zone was accurately measured by
the technique known as a single-pole thermocouple [13]. This technique has been accepted as the
most reliable method available for measuring grinding temperature. One half of a standard
thermocouple (in this case a constantan part) is separated from the other (the workpiece), insulated
with two layers of mica, “sandwiched” between two halves of a split workpiece (Fig. 3). With the
passage of the grinding wheel over the junction, constantan is smeared over the surrounding steel
thus making a hot junction. In dry grinding, and with mist jets, the signal obtained is of high quality
and due to the thermocouple's small size, its response
is rapid making it possible to record the temperatures
caused by individual grits (temperature spikes). It was
necessary to calibrate the new thermocouple –
workpiece assembly and this was done by submerging
it in a hot oil bath, and simultaneously recording the
temperature of the oil and the voltage output of the
thermocouple. It was found that the calibration
characteristic is almost an exact match to that of a
standard J – type thermocouple (iron – constantan). A
typical sampling rate for this arrangement would be
Figure 3: Thermocouple assembly
about 80 kHz.
Results and discussion
The tests were performed using AISI 1020 mild steel as the workpiece material. In addition to the
test performed using different concentrations of soap in water, one series of grinding tests was
performed using a traditional liquid coolant (5% HOCULT B60CB in water, 0.92 l/min) for
comparison. Water/soap was injected into the air at a constant flow of 1 cm3/s, and the liquid soap
concentration was increased in three steps – 5%, 15% and 25%. Air flow was also constant at 6 g/s
per nozzle. The workpiece width was 12.7 mm, the wheel diameter 186 mm and the wheel speed
3034 rpm. Four downfeeds were used: 7.5, 10.0, 15.0 and 20.0 microns. The soap used in this
investigation was a typical hand-cleansing liquid. The results obtained for several key grinding
parameters will be now presented. Fig. 4 and Fig. 5 show the results for tangential force and
grinding temperature.
Tangential force in grinding
Workpiece temperature in grinding
60.00
330.00
310.00
50.00
290.00
270.00
Temperature (°C)
Force (N)
40.00
30.00
Liquid coolant
Air + water/soap mist (5%)
20.00
Air + water/soap mist (15%)
250.00
230.00
Liquid coolant
210.00
Air + water/soap mist (5%)
Air + water/soap mist (25%)
Air + water/soap mist (15%)
190.00
Air + water/soap mist (25%)
10.00
170.00
0.00
6.00
8.00
10.00
12.00
14.00
16.00
18.00
Depth of cut (microns)
Figure. 4: Tangential force
20.00
22.00
150.00
6.00
8.00
10.00
12.00
14.00
16.00
18.00
20.00
22.00
Depth of cut (microns)
Figure. 5: Grinding temperature
As can be seen from the above figures, the fall in grinding temperatures roughly follows that of
tangential force and this improvement increases with soap concentration. The tangential force in
grinding is highly dependant on friction conditions between the wheel and the workpiece. Better
lubrication tends to reduce friction (but only to a certain point) and a drop in tangential force values
is a natural consequence. However, there are some interesting details to note: the tangential force
with the highest soap concentration can be 30-40% less than with liquid cooling, depending on the
downfeed. The specific energies also fall in a similar way. At the same time, the temperature drop
(if only the actual temperature rise is taken into account) is about 21-23%.
Every parameter observed improved with increased soap concentration in the water/soap mist. The
tangential force at 5% soap concentration is roughly the same as for liquid cooling but the
temperature rise is less, indicating better convective cooling. Adding more soap to the mist does not
change the overall liquid flow, and may degrade its thermal properties, so the better results with
higher soap concentrations are almost certainly due to better lubrication and lower tangential forces.
This can be verified by calculating the partition ratio, R, for each test condition. This can be done by
calculating the temperature rise (T* ) to be expected if all the heat enters the workpiece, and
comparing this with the measured temperature rise (T). The theoretical temperature rise was
calculated using Jaeger’s [14] equation for the maximum temperature rise beneath a moving heat
source (Eq. 1). R is then given by Eq. 2.
T * 
R
2  Ft  v s
  lc
,

k  lc  b
  vw
T
,
T *
(1)
(2)
Where Ft (N) is the measured tangential force, vs and vw (m/s) are wheel and workpiece speed
respectively, k (W/mK) is the workpiece thermal conductivity,  (m2/s) is the workpiece thermal
diffusivity, lc (m) is the length of the arc of cut, and b (m) is workpiece width. The required thermal
properties of the workpiece are taken from [15] and are as follows: k = 48.5 W/mK,  = 1.2  10-5
(m2/s).
The results for specific energy of grinding and partition ratios under different cooling conditions are
given in Fig. 6 and Fig. 7.
Specific energy of grinding
Partition ratio
35.00
1.00
33.00
Liquid coolant
Air + water/soap mist (5%)
0.90
Air + water/soap mist (15%)
29.00
Air + water/soap mist (25%)
27.00
0.80
25.00
R
Specific energy (J/mm 3)
31.00
23.00
0.70
Liquid coolant
Air + w ater/soap mist (5%)
Air + w ater/soap mist (15%)
Air + w ater/soap mist (25%)
Linear (Air + w ater/soap mist (25%))
Linear (Air + w ater/soap mist (15%))
Linear (Air + w ater/soap mist (5%))
Linear (Liquid coolant)
21.00
19.00
17.00
0.60
15.00
6
8
10
12
14
16
Depth of cut (microns)
18
Figure 6: Specific energy of grinding
20
22
0.50
6.00
8.00
10.00
12.00
14.00
16.00
18.00
20.00
22.00
Depth of cut (microns)
Figure 7: Partition ratio
It is, at first, surprising to see that, with mist jets, the specific energy tends to rise as the depth of cut
increases, whereas it usually tends to go down. This may be due to the ability of the jet to inject
droplets into the centre of the arc of cut, which will clearly decrease as the arc of cut becomes
longer with rising depths of cut. We may therefore expect tests at lower depths of cut to be better
lubricated, which can explain the trends seen in specific energy.
Figure 7. shows that the partition ratios go up as the amount of soap in the mist jet is increased,
which indicates that the addition of soap to the water is diminishing the fraction of heat dissipated
by convection. However, the measured temperature rise (fig 5.) is significantly lower when soap
concentrations are high. This shows clearly that the temperature reduction is entirely due to the
better lubrication and cleaning of the wheel, which the addition of soap provides. The higher value
of R with higher soap concentrations may be partly due to the liquid droplets having a lower latent
heat of evaporation, and partly to a lower temperature difference between the workpiece surface and
the mist.
Summary




A new approach to the old problem of cooling in grinding has been presented. Mist jet
cooling with a water/soap mist cools and lubricate the cutting zone to such an extent that
significant reductions in tangential forces and grinding temperatures have been achieved.
The tangential force reduction is greater at lower depths of cut.
The specific grinding energy increases with depth of cut.
The method presented is surprisingly simple and all the fluids used are cheap and clean
References
[1] M.C. Shaw: Principles of abrasive processing, Oxford Science Publications, (1996).
[2] S. Malkin: Grinding technology: theory and applications of machining with abrasives, Ellis
Horwood, Chichester, (1989).
[3] A.A. Torrance, J.A. Badger: The relation between the traverse dressing of vitrified grinding
wheels and their performance, International Journal of Machine Tool Manufacture Design and
Research Vol. 40 (2000), pp. 1787-1811.
[4] S. Kalpakjian, S.R. Schmid: Manufacturing processes for engineering materials, Prentice
Hall, New Jersey, (2003).
[5] C. Andrew, T.D. Howes, T.R.A. Pearce: Creep feed grinding (1985).
[6] S. Paul, A.B. Chattopadhyay: Effects of cryogenic cooling by liquid nitrogen jet on forces,
temperature and surface residual stresses in grinding steels, Cryogenics Vol. 35 (1995).
[7] S. Shaji, V. Radhakrishnan: An investigation on surface grinding using graphite as lubricant,
International Journal of Machine Tools & Manufacture, Vol. 42, (2002) pp. 733-740.
[8] M.C. Shaw: Interrupted grinding principle, Instn Engrs (India), Journal of Production Engng
Vol. 66, (1985), pp. 29.
[9] T. Akoyama, I. Inasaki: Suppression of temperature rise in creep feed grinding, Proc. 5th
ICPE, Tokyo, (1984) p. 46.
[10] E. Pecherer, S. Malkin: Grinding of steels with CBN, Ann CIRP 33 (1), (1984), 211.
[11] D.M. Babic, D.B. Murray, A.A Torrance: Control of grinding temperature by high speed air
jets, Proc. ASME-ZSIS International Thermal Science Seminar II, Bled Slovenia, (2004), pp.
399-406.
[12] D.M. Babic, D.B. Murray, A.A Torrance: Mist jet cooling, to be published in International
Journal of Machine Tool Manufacture Design and Research 2005.
[13] S.C.E. Black, W.B. Rowe, B. Mills, H.S. Qi: Temperature measurement in grinding,
Proceedings of the Matador Conference, (1995), pp. 409 – 413.
[14] J.C. Jaeger: Moving Sources of Heat and the Temperature at Sliding Contacts, Proceedings of
the Royal Society of New South Wales, 76 (1942), pp. 203-204.
[15] J. Woolman, R.A. Mottram: The mechanical and physical properties of the British Standard
En steels (B.S. 970-1955), compiled by Steel User Section, British Iron and Steel Research
Association. Oxford, Pergamon Press 1964 – 1969.