International Journal of Machine Tools & Manufacture 45 (2005) 1231–1245
www.elsevier.com/locate/ijmactool
Temperature measurement in high efficiency deep grinding
A.D. Batako*, W.B. Rowe, M.N. Morgan
Liverpool John Moores University, GERI, Advanced Manufacturing Research Laboratory, Byrom Street, Liverpool L3 3AF, UK
Received 25 November 2004; accepted 13 January 2005
Available online 7 April 2005
Abstract
Temperature measurements are employed for research into the mechanics of grinding and for process monitoring. Temperature
measurement in grinding presents a number of challenges particularly in High-Efficiency Deep Grinding (HEDG). High work-speeds
require fast response and deep cuts require measurement over a large temperature field. High-speed fluid delivery creates problems for all
types of temperature measurement. Electrical noise may require zero-shift filtering. Several techniques of temperature measurement are
described. The merits and problems using thermocouple techniques are discussed in detail. In particular, the effect of junction size and
shape are discussed. It is shown that the shape and size of the junction have a strong effect both on the reliability of the signal and on the
accuracy of the signal. Other problems discussed include improvement of signal to noise ratio, measurements under wet grinding
conditions and a technique for measuring contact surface temperatures when taking deep cuts in creep grinding and high-efficiency deep
grinding.
q 2005 Elsevier Ltd. All rights reserved.
Keywords: Grinding; Temperatures; HEDG
1. Introduction
Energy in grinding is mostly converted into heat.
Excessive heat leads to thermal damage, residual stress
and micro-structural changes in the surface layer. These
effects can be highly damaging to workpiece life and
component quality. Mathematical modelling of the grinding
process [1–10] has improved understanding of heat partition
between the elements of the system and the mechanisms of
heat transfer. Accurate analysis of temperature in grinding
requires a repeatable and stable measurement technique for
correlation with theory. Temperature predictions based on
power or force measurement need direct temperature
measurement for validation and calibration of heat convection factors.
Several techniques of temperature measurement are
commonly employed. Typical approaches include thermal
imaging and thermocouple measurements. Other techniques
involve use of heat-sensitive coatings, low melting-point
* Corresponding author. Tel.: C44 151 231 2126; fax: C44 151 231
2590.
E-mail address: a.d.batako@livjm.ac.uk (A.D. Batako).
0890-6955/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijmachtools.2005.01.013
coatings and fibre optics. This paper briefly reviews these
techniques and presents a detailed description of the
development of the single-pole thermocouple technique. A
method is suggested for reliable measurement of temperatures in high speed deep grinding using the single-pole
technique.
2. Non contact measurement
2.1. Optical techniques
Fig. 1a shows an example of a thermal imaging approach
to temperature measurement. A thermo-camera records the
temperature field distribution on the side of the workpiece. In
the example shown, results are compared with predictions
from FEM simulation. The technique provides a graphic
image of the whole temperature field including distances
well removed from the grinding zone. It is less useful in
accurately determining grinding contact temperatures.
Thermal imaging has the advantage that sub-surface
temperatures are provided in real-time. Direct readings
illustrate the temperature field throughout the grinding
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Fig. 1. Examples of non-contact temperature measurement.
process. A disadvantage with this approach is that the
camera records the heat radiated from a side-plane
perpendicular to the contact zone. The presence of fluid
can interfere with the radiation and hence affect the reading.
Also, the temperatures in the plane of measurement may not
be typical of temperatures central within the contact zone.
Heat convection to the grinding fluid takes place on the side
face of the workpiece, whereas in the central plane of the
workpiece the heat transfer is two-dimensional. These
factors introduce elements of uncertainty into the
measurement.
Fig. 1b illustrates the use of a phototransistor (4) that
records infrared radiation from the grinding zone through a
hole (2) made in the wheel. The current generated by the
phototransistor is proportional to the intensity of infrared
radiation. The signal is conditioned by an amplifier (6) and
sent to a recording device (7). This method provides an
almost noise free signal but has problems. The method only
works well in dry grinding. In wet grinding, fluid and chips
fill the measurement hole leading to loss of the output
signal. The hole in the wheel is difficult to make and may
require involvement of the wheel manufacturer. Control of
hole-size variation and angle of inclination is difficult. The
duration of the signal is a function of wheel speed and hole
diameter. The third problem is that the temperature is
recorded in a gap, where there is interference from swarf
and grinding debris.
2.2. Fibre optic techniques
Recent research in fibre optic sensing has made it
possible to develop a new temperature measurement device
based on fibre Bragg technique. The change in the sensing
grating due to the heat causes variation of the wavelength of
the reflected light. Wavelength division multiplexer
(WDM), coupler and two gratings method, and thin film
Fig. 2. Fibre optic technique; (a) system configuration; (b) output signal; [20].
A.D. Batako et al. / International Journal of Machine Tools & Manufacture 45 (2005) 1231–1245
dense wavelength division demultiplexer (DWDM) are the
techniques used to detect a change in the wavelength. Fig. 2
illustrates the DWDM technique, where Fig. 2a shows the
system configuration and Fig. 2b compares the output
voltage of the fibre optic sensor with a commercial double
pole thermocouple, where both sensors were located below
the grinding surface. The fibre optic produced a strong
signal virtually free from noise. However, this technique
requires measurement before and after cutting because the
sensor is located below the cutting surface. A safety layer is
required at the top of the sensor. The final temperature
values are then extrapolated; consequently this method does
not provide direct contact temperatures. The fibre optic
technique is relatively expensive and fragile to handle.
3. Direct contact measurement
3.1. The coating technique
The coating technique provides direct measurement of
maximum temperature experienced at a particular position.
The technique has the potential to provide better insight into
the process since temperature sensing takes place precisely
at the position of interest. Low melting point materials such
as indium, bismuth and heat sensitive paints have been used
to estimate temperatures in grinding. The method provides
temperatures below the contact surface with a simple set up
involving a split workpiece. Used together with metallurgical analysis the method gives an insight into the
temperatures experienced within a damaged layer.
Low melting point coatings provide a useful cross-check
on the order of magnitude of other methods of estimation.
3.2. Thermocouple techniques
There are various methods of setting up a thermocouple
(TC) within a workpiece to measure temperature in
the grinding contact zone. Some of these methods are
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reviewed in Ref. [13]. A relatively simple way of inserting a
thermocouple is by use of a split workpiece. There are three
main techniques. Conventional double-pole thermocouples
may be inserted below the workpiece surface. Alternatively,
a grindable double-pole thermocouple junction may be
formed at the grinding contact. A third possibility is a
grindable single-pole thermocouple formed at the grinding
contact. The single-pole thermocouple uses a conducting
metal dissimilar to the conducting workpiece material
inserted in the workpiece. The grinding action forms a
thermocouple junction at the surface.
3.2.1. Double-pole thermocouples
Double-pole thermocouples inserted below the workpiece surface have standard calibrations and usually provide
a good temperature signal. However, because these
thermocouples measure temperature below the surface, the
temperature measured is due to heat conducted through the
workpiece material and through the thermocouple insulation
(Fig. 4, right). The thermal properties of the insulation are
usually ignored and is a potential source of inaccuracy.
Since the temperature gradient near the surface may be steep
and non-linear, the accuracy of measurement relies on an
uncertain process of extrapolation. A further problem is that
the conventional thermocouple has a junction volume that is
large in relation to the temperature gradient. This means that
the temperature measured is an average value over the
junction volume. The junction volume lies at varying depths
below the surface.
The usual configuration of double-pole thermocouple is
shown in Fig. 3. Use of conventional double pole
thermocouples to measure temperature at the workpiece
contact surface requires accurate measurement of the
position of the measuring junction relative to the surface
before and after grinding in order to determine the average
location of temperature measurement. Usually, it is assumed
that the temperature is measured at the centre of the volume
of the measuring junction. It is important to know, where
this lies. The physical size of the junction, therefore, plays
Fig. 3. Double pole TC configuration for surface grinding.
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increases the size of the assembly and makes the process of
assembling the fragile foil elements and mica insulating
strips more difficult. It was therefore, decided to use singlepole thermocouples, wherever possible. The single-pole
configuration is relatively easy to assemble and provide
greater reliability.
Fig. 4. Configuration of grindable double pole thermocouple.
an important role in the temperature measured. If the
temperature gradient is high at the location of the measuring
junction, the reading will be an average of the temperatures
between the maximum and minimum temperature over
the measuring volume. A further problem is that if the
junction is not formed precisely in the plane of desired
measurement, there is a need to predict the temperature at
the desired location by extrapolation of an assumed
temperature gradient. Theoretical modelling, therefore,
plays an important role in extrapolation from a measurement within the body to a point on the surface.
Use of a conventional double-pole thermocouple for
contact surface measurement is time consuming because of
the number of measurements required. This is further
exasperated by the calculation to extrapolate temperatures
and the precision required to get the wheel close enough to
the thermocouple junction while avoiding cutting through it.
Grindable double-pole thermocouples as shown in Fig. 4
can be formed at the workpiece surface and overcome some
of the problems of the conventional double-pole thermocouple. The approach to assemble a grindable double-pole
thermocouple is similar to the assembly of a grindable
single-pole thermocouple as described below. The increased
number of layers involved in the double-pole arrangement
3.2.2. Single-pole thermocouples
Single-pole thermocouples are grindable and this is an
advantage compared to conventional double-pole thermocouples. In the single-pole technique, the measuring
junction is formed with the workpiece during grinding by
smearing the thermocouple material over the workpiece.
Thus the grinding temperature is measured at the contact
surface.
Single-pole thermocouples are based on an insulated foil
strip or wire inserted either in a grinding wheel or in a
workpiece. Usually, the thermocouple is inserted in the
workpiece rather than in the grinding wheel. This is a much
simpler and less expensive arrangement for experimental
purposes.
3.2.2.1. Insertion in a grinding wheel. Insertion of a
thermocouple in a grinding wheel allows continuous
monitoring of grinding temperature. The workpiece forms
the second pole of the thermocouple and is interchangeable;
consequently, this method is subject to calibration of the
temperature values and could be usable in a production
situation. The approach is illustrated in Fig. 5, where two
single-pole thermocouples (3) are inserted in a grinding
wheel. High-speed slip rings are required to extract the
signals and tend to introduce noise. The aggressive action of
the coolant and swarf also affect the quality of the signal.
The method requires fast signal recording equipment, since
each signal pulse characterising contact temperature has a
short duration, which depends on the process type (surface
or cylindrical grinding, shallow or deep cuts).
3.2.2.2. Insertion in a workpiece. In the single-pole
configuration, constantan is used to form a J-type
Fig. 5. Single-pole thermocouples inserted in a grinding wheel [12].
A.D. Batako et al. / International Journal of Machine Tools & Manufacture 45 (2005) 1231–1245
1235
Fig. 6. View of the Leg and Thin-End sections of a J or T-type TC.
thermocouple when grinding ferrous materials. Constantan
foil may be taken from a commercial J- or T-type
thermocouple and inserted in the workpiece. Fig. 6
illustrates a single-pole thermocouple. It shows a ‘ThinEnd’ and a ‘Leg’ of a thermocouple foil with typical
dimensions. The Thin-End allows measurement with depths
of cut with a range from 0 to 6 mm. The Leg can be used for
a greater range of depths. The Thin-End is suitable for
shallow grinding or when the increment of depth of cut is
not greater than 6 mm. For a depth of cut greater than 5 mm,
the cross section of the thermocouple begins to increase.
This leads to uncertainty in the calibration factor.
Fig. 7 illustrates the arrangement of a single-pole
thermocouple within a split workpiece, where configuration
is similar to the arrangement shown in Fig. 4. A typical
temperature signal is shown in Fig. 7b. This technique
provides good temperature readings, although problems
may be met particularly in wet grinding. The problem is to
maintain a good junction throughout the grinding contact. If
the junction breaks down the signal becomes difficult to
interpret. This problem becomes more serious in wet
grinding when there is a greater tendency for the junction
to break down. Problems can also arise if the insulation fails.
3.3. Tests on thermocouple performance
In High Efficiency Deep Grinding (HEDG), depths of
cut may exceed 5 mm. This makes the Thin-End
thermocouple unsuitable for temperature measurement.
Fig. 8 displays an arrangement used to investigate the
performance of single-pole thermocouples. Three singlepole thermocouples of different shape and size were
inserted side by side in a workpiece for comparative
tests. A constantan wire thermocouple of 25 mm in
diameter was installed in line with a narrow (Thin-End)
thermocouple (10!15 mm) and a wide (Leg) thermocouple
(50!750 mm). The sensors were assembled in a row to
allow the grinding wheel to travel across the measuring
junctions simultaneously. The thermocouples were therefore, tested under similar conditions. A series of grinding
tests were undertaken varying depth of cut. Analysis of
the results allowed the best configuration to be selected
for HEDG.
Under dry grinding conditions all three thermocouples
provided acceptable results. Under wet grinding conditions,
the results in Fig. 9 demonstrate that the wide Leg
thermocouple gave the most reliable temperature profile.
The narrow Thin-End thermocouple and the narrower Wire
thermocouple both produced poor signals. After grinding,
the junctions were examined under a microscope. Microphotographs of the junctions are shown in Fig. 10. It is
observed from these pictures that the wide Leg thermocouple provides a robust junction, whereas the narrow ThinEnd and Wire thermocouples possibly suffered some
distortion.
Fig. 11 compares temperature signals with hardware
filtering to remove noise and temperature spikes. The
experiment was repeated several times and the results were
found to be consistent. It can be seen that the use of a filter is
very helpful for interpretation of the signal. In principle, the
arrangement in Fig. 8 allows comparison of the speed of
response of different thermocouples. It is observed in Fig. 11
that both the wide Leg and the narrow Thin-End have
approximately the same speed of response as shown by the fast
rise of the temperature signal. However, a useful measure of
the quality of a thermocouple signal is the shape of the
Fig. 7. (a) Single pole TC configuration in split workpiece; (b) temperature signal.
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4. Calibration of thermocouples inserted in a workpiece
Fig. 8. Arrangement of three single pole thermocouples in workpiece.
temperature decay. The wide Leg displays a typical temperature decay as expected from heat transfer theory, whereas
the narrow Thin-End produced a poor signal decay shape.
It was observed from time-to-time that the wide Leg
junction was destroyed as soon as the wheel passed over
the measuring tip. It was considered that this was due to the
Leg being too stiff due to its greater thickness. This leads to
the Leg pulling back the smeared layer as the load is
reduced. The junction is not strong enough to restrain the
Leg. An example of loss of signal is shown in Fig. 12a.
This problem was investigated by using a standard
rolling machine to reduce the thickness of the thermocouple,
as shown in Fig. 12b. Several Legs were successfully
reduced in thickness. Thicknesses of 25, 38, 46 mm and the
original 50 mm were chosen for test. Fig. 12b shows the
configuration for rolling the thermocouples and the table
indicates the dimensions. Results using these thermocouples
are described below.
The thermocouples were assembled into the workpieces
and initial grinding was carried out to create the junctions.
An AD595 monolithic thermocouple amplifier was used to
amplify the signals. The thermocouples were calibrated
from zero to 150 8C. The calibration arrangement is shown
in Fig. 13a. Block (1) holding the workpiece is placed in the
middle of an oil tank. A hot plate heats the oil and a
magnetic stirrer distributes the heat uniformly within a
closed tank. The closed tank limits heat exchange with the
surrounding environment. A power controller uses the
feedback temperature (2) to maintain constant temperature.
A standard thermocouple (3) measures the temperature of
the oil. In addition a mercury thermometer (5) was
employed as a reference. For accurate calibration, it is
imperative to allow enough time for the assembly to heat up
and the oil temperature to stabilise before taking readings.
The readings from the thermocouples were studied for
several days at room temperature and compared with
a mercury thermometer. An average discrepancy of 0.5 8C
was observed. Fig. 13b depicts a view of the workpiece
assembly. Fig. 14 shows typical calibration graphs for the
rolled thermocouples during heating and cooling. It is seen
that the test thermocouple responses were in close
agreement with the standard thermocouple.
5. Grinding tests
A series of grinding tests at different depths were
undertaken on a surface grinder. Wheel speed and table
speed were maintained constant. This allowed temperature
readings to be compared for the thermocouples of different
thickness.
Fig. 9. Examples of temperature signals under wet grinding conditions (Leg, Thin-End, Wire).
A.D. Batako et al. / International Journal of Machine Tools & Manufacture 45 (2005) 1231–1245
1237
Fig. 10. Microphotograph of junctions.
It can be concluded from Fig. 15 that thermocouple
thicknesses within the range tested did not significantly
affect the temperature readings. A higher level of electrical
noise encountered during tests at depths of 24 and 31 mm
caused some discrepancy in temperature reading. However,
this problem was not caused by the thermocouple thickness.
It was also considered that the fluid application and
consequently the fluid convection factor might not have
been as consistent as desired throughout the grinding tests.
However, the results are reasonably consistent. It was found
that thermocouples with a thickness between 35 and 45 mm
agreed well with the reference thermometer. An average
thickness of 40 mm was judged to be optimal. Fig. 16
illustrates a temperature signal at a depth of cut 10 mm in dry
grinding of mild steel using an alumina wheel.
6. Analysis and validation of the results
Grinding forces were measured using a Kistler 3-axis
dynamometer. Table speed was measured using an acoustic
emission sensor that recorded the AE signal from the
grinding contact. Measured temperatures were validated
with the help of a thermal model [8].
6.1. Grinding temperature estimation
Power supplied to the grinding wheel is converted into
heat at the contact interface. Losses unaccounted for include
additional stresses locked into the workpiece and swarf
material during the deformation process. Energy locked into
the material does not reappear as heat within the time-span
of the grinding contact, neither is it recovered unlike elastic
energy. However, the energy locked into the material is very
small compared with the total energy dissipated in grinding
and may therefore be considered negligible. The total heat
flux is therefore equal to the grinding power per unit contact
area.
The total heat flux qt is partitioned into qw going into the
workpiece, qs transported by the wheel, qch taken by the chip
and qf carried out of the contact area by cooling fluid. The
total heat flux is the net grinding power P supplied to the
spindle per unit contact area blc.
qt Z
P
Fv
Z t s
blc
blc
(1)
where b-grinding width, lc contact length; Ft tangential
grinding force and vs is the linear speed of the grinding
Fig. 11. Effect of thermocouple width on temperature signal.
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Fig. 12. Arrangement for rolling a thermocouple. (a) Loss of function bond, (b) TC rolling arrangement.
wheel.
qch Z Tmp rcae
qt Z qw C qs C qch C qf
(2)
A heat flux may be expressed as a product of a heat
convection factor (h) and a temperature T.
qw Z hw Tmax ; qs Z hs Tmax ; qf Z hf Tmax ; qch Z hch Tmp
(3)
The heat into the workpiece can be expressed in terms of the
maximum temperature on the contact surface [7,8] as
sffiffiffiffiffiffi
bw
lc
T
qw Z
(4)
C
vw max
The workpiece convection factor is therefore hw
rffiffiffiffiffiffi
b w vw
Z
C
lc
(5)
C is a factor that depends on Peclet number [8,9] andpbffiffiffiffiffiffiffi
w is
the thermal property of the work material, bw Z krc,
where k is the conductivity, r is the density and c is the
specific heat capacity.
The heat generated in the chip raises the temperature to a
value somewhere between the softening temperature and the
melting point; it can be no higher than the melting point.
Thus taking the melting point/softening temperature Tmp as
1250 8C, the heat flux carried away by the chip is
vw
v
or qch Z ech ae w
lc
lc
(6)
The workpiece and abrasive grain contact may be
considered as a workpiece-wheel sub-system of the heat
partition system. The workpiece-wheel sub-system shares a
heat flux, qwsZqwCqs and the heat partition between the
workpiece and wheel is estimated using the Hahn concept of
a grain sliding on a workpiece [14,15]. Partition between
the workpiece and the wheel is given by
Rws Z
0:97kg K1
qw
hw
Z
Z 1 C pffiffiffiffiffiffiffiffi
qw C qs
hw C hs
bw r0 vs
(7)
where kg is the thermal conductivity of the abrasive grains
and r0 is the effective wear flat radius of the grains. Values
of Rws may be calculated from Eq. 7. Thus the wheel heat
convection factor obtained from Eq. 7 is
h s Z hw
1
K1
Rws
(8)
The maximum temperature Tmax in the grinding zone can
therefore be estimated from the values of convection factors
and Eqs. 1 and 2. The maximum temperature is therefore
given by
Fig. 13. Thermocouple calibration arrangement. (a) calibration settings, (b) wrokpiece assembly.
A.D. Batako et al. / International Journal of Machine Tools & Manufacture 45 (2005) 1231–1245
1239
Fig. 14. Typical calibration chart of the rolled TC.
Tmax Z
qt K qch
hw
Rws C hf
(9)
where hf is the heat convection factor for the fluid. In dry
and burnout conditions, the fluid convection factor is
effectively zero. As reported in [8,9,16,17] values of fluid
convection for water-based fluid vary broadly from 0 to
290,000 W/m2K depending on the effectiveness of fluid
delivery and the type of fluid. Fluid convection factor can
also be estimated very approximately assuming steady
conduction into a ‘solid’ fluid carried on the wheel surface
from
rffiffiffiffiffi
vs
(10)
hf Z 0:94bf
lc
Heat convection into the fluid estimated from Eq. 10 is
much lower than possible with a fluid surface on the wheel
since the fluid contact in practice is turbulent rather than
static. This increases the rate of heat transfer. However, the
Fig. 15. Temperature readings varying thermocouple thickness.
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In general the experimental results for measured
temperatures are in agreement with expected behaviour in
the transition region and with thermal modelling. During
trials, every single grinding test provided a good temperature measurement. On average 250 grinding tests were made
and 250 good signals were recorded. This demonstrates that
wide Leg thermocouples provide reliable temperature
readings in agreement with thermal modelling. The most
pleasing aspect was that taking temperature measurements
was a stable repeatable process.
7. The thermocouple size effect in HEDG and
temperature signal quality
Fig. 16. Temperature signal from a 40 mm thick TC in dry grinding at
10 mm depth of cut.
supply of fluid into the contact zone is often less than perfect
so that the convection factor for the fluid may over-estimate
the heat carried away. It is found that the heat convection
factor is strongly dependent on the effectiveness of fluid
delivery.
In practice, maximum temperature can be calculated for
two different conditions. These are for the case of effective
fluid delivery and for the case of fluid burn-out. This gives
two extremes for the calculation. With effective fluid
delivery, temperatures measured tend towards a value
predicted at the lower extreme. When burn-out occurs the
temperatures increase dramatically towards the other
extreme.
6.2. Experimental validation of grinding temperatures
Temperatures measured with rolled Leg thermocouples
were validated against predicted values using Eq. 1–10.
Fig. 17 illustrates results obtained in grinding three types of
steel. Fig. 17a demonstrates a typical sub-optimal fluid
delivery, where measured temperatures are close to
predicted values for the dry burn-out condition. In cast
iron grinding, Fig. 17b, a steady transition process appears
to take place. The first result at 6 mm depth of cut illustrates
an optimal fluid delivery but as the depth of cut increases
temperatures move into the transition zone between the two
extremes. These results suggest partial fluid burn-out. Since
the temperatures measured in this case are close to the burnout value measured with water-based emulsions, it suggests
a transition that commences at approximately 100 8C and is
complete at 200 8C. In Fig. 17c most of the results with the
hardened M50 tool steel (62HRC) indicate a dry burnout
condition and measured temperatures are close to values
predicted for the dry condition. It is observed that
temperatures at 4 and 7 mm depth of cut are within the
transition zone.
In the single-pole configuration, the speed of response of
a thermocouple is independent of the foil thickness. This is
because the thermocouple junction is created at the grinding
contact and the emf is generated equally in either a thick or a
thin thermocouple. The differences in thermocouple resistance are negligible if compared to the total resistance of
the workpiece and the connecting wires in the circuit. It is
interesting to speculate on effects on signal quality in a
single-pole thermocouple.
7.1. Causes of a poor temperature signal
In temperature measurement, using a single-pole thermocouple, signal quality is strongly affected by the quality
of the junction, which in turn is affected by the number of
abrasive grains simultaneously in contact with the thermocouple at a given time. Coolant has a detrimental effect on
the establishment of a secure bond between the thermocouple and the workpiece material. Fig. 18 illustrates
the effect of thermocouple size on the number of grains in
contact. In Fig. 18a it is observed that with a narrow
thermocouple a smaller number of grains is in contact at the
junction and there may be instants when no grain is in
contact with the thermocouple. In the presence of coolant, a
film can form between the smeared layer of the junction and
the workpiece surface so that the junction bond tends to
become intermittent and also the generation of emf. An
intermittent emf is frequently observed in experiments with
narrow thermocouples. This requires the experiment to be
repeated. With narrow thermocouples, the junctions are
fragile and it seems that often the smeared thermocouple
layer does not reach the workpiece body. This is shown in
Fig. 18b. A wide thermocouple has numerous grains in
contact at any given time and this increases the probability
of maintaining contact between the smeared layer and the
workpiece surface (Fig. 18c) resulting in a continuous
signal.
7.2. Measuring-source power
Apart from the causes underlined above, the power of
the emf source affects the temperature readings. In essence,
A.D. Batako et al. / International Journal of Machine Tools & Manufacture 45 (2005) 1231–1245
1241
Fig. 17. Comparison of experimental results with predicted values.
we measure the intensity of micro-current that flows
through the junction. Consequently, the electrical circuit
of the measuring device influences measured values
depending on the loading effect according to the Thévenin
theorem [18]. If the source is not powerful enough, the
dissipation due to internal resistance of the measuring
circuit affects the reading. Ideally the output impedance of
the thermocouple should match the input impedance of the
amplifier and the output impedance of the thermocouple
amplifier should match the input impedance of the data
logging system etc. However, in practice, this is difficult to
achieve.
Fig. 19 illustrates the importance of the volume of the
measuring junction. A Thin-End thermocouple has a small
junction volume, Fig. 19a; consequently the current
generated is small. Therefore, the drop in measured voltage
due to internal dissipation is a problem. The solution to
this problem is the use of a wide thermocouple (Fig. 19b)
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Fig. 18. Size effect in HEDG. (a) Number of grains in contact, (b) Thin-End junction, (c) Leg junction.
that generates sufficient current to minimise the effect
of losses in the internal circuit of the measuring device.
As the thermocouple stretches across the heat source
(wheel width), each grain generates a micro-junction as
illustrated in Fig. 19c. These micro-junctions are equivalent
to spot thermocouples connected in parallel and produce an
average reading over the junction.
8. Signal sampling, noise and interference
8.1. Signal sampling
The thermocouple response, the temperature signal
profile and signal magnitude are also affected by the
sampling rate and the processing speed of the data logging
equipment. The worktable speed affects the requirements
for these parameters. Fig. 20 shows the duration of
temperature signal as the wheel, that is the heat source
travels over a thermocouple. It is seen that in shallow cut
with thinner thermocouple, this time becomes very short. If
the work speed is 1 m/s and the depth of cut is 1 mm, the
maximum contact length is typically 15 mm. The temperature has to be measured over a period of less than 0.015 s.
If the sampling rate is 1000 Hz, which is one point per
millisecond there will be a maximum of 15 points to
characterise the temperature signal. This is insufficient. The
maximum temperature could be seriously underestimated.
Consequently, a high sampling rate is required. It is prudent
to over-sample the signal and filter the noise out.
A fast data acquisition system is paramount for accurate
temperature measurement. Unfortunately, with high
sampling rates the level of high frequency noise is
increased.
Hardware pre-acquisition filtering or post acquisition
software filtering can be used to recover the required
temperature signal.
8.2. Noise and interference
In the single-pole configuration, the workpiece is one
half of the thermocouple. This makes the single-pole
thermocouple signal vulnerable to environmental electrical
noise and interference. Noise is undesirable data added to
the measurement signal. Noise exists in the form of
Fig. 19. Effect of junction volume on source power. (a) Point resolution, (b) Measuring volume stretched across heat source, (c) Equivalent micro-junctions.
A.D. Batako et al. / International Journal of Machine Tools & Manufacture 45 (2005) 1231–1245
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between the TC and the workpiece to the worktable
(Fig. 21b). In the single-pole configuration as shown in
Fig. 21b, physical insulation of the workpiece is effective
for dry grinding.
Fig. 20. Duration of signal data logging.
interference from external electrical and magnetic fields
with the measurement system and random motion of
electrons and other charge carriers from components within
the system.
Fig. 21a [21] illustrates a signal recorded during a test to
identify adequate time and conditions for temperature
measurement. It clearly shows a high level of noise when
other machinery is operating nearby. This reflects the
importance of identifying sources of noise and defining
optimal conditions to minimise noise level.
8.2.1. Source of noise
Any operating machine is a source of noise but HF noise
is mainly generated by high frequency invertors and
electrical drives, hydraulic pumps, old CNC controllers
and signal monitoring kits (random noise and inductive
coupling). Noise is also generated by the mains power
supply system from nearby high-power machinery and the
earth (capacitive coupling, [18]). Also multiple earths can
cause noise due to difference in potential between earth
points. In grinding, fluid application drastically increases
noise interference, especially in the single-pole TC
configuration. This is because the coolant forms a loop
8.2.2. Noise minimisation
If the measurement is carried out in a workshop, where
other machinery is in operation, it is important to identify
the best time when the level of noise is a minimum.
Attention must also be paid to how extension cable and
sockets are laid out. If the measurement involves more than
one signal amplifier, it is advisable to supply power to them
from different main sockets, not from a single multiplesocket extension. Signal cables should not cross or lie close
to main power cables. This reduces inductive and capacitive
coupling. A single-point earth is important for noise
reduction.
Fig. 22 illustrates three signals recorded from surface
grinders in the same environment but under different noise
conditions. Fig. 22a shows an ideal clean temperature signal
recorded from a surface grinder with a single-speed motor
without speed control or an inverter. The signal was
sampled at 250 kHz. In this signal the superimposed grain
temperature spikes are observed above the background
temperature.
The signal in Fig. 22b was recorded on another surface
grinder in the location with the same data logging
equipment but the sampling rate was lowered to 150 kHz.
The machine has a variable speed controller with a high
frequency inverter. This result reflects noise generated by
the controller and the inverter, as no other machinery
operated at that instant. Fig. 22c illustrates a compound
effect of noise from numerous machines operating at the
instant when the signal was recorded at 150 kHz on the
same grinding machine tools as in Fig. 22b. The drastic
increase in noise amplitude makes it impossible to measure
maximum temperature accurately. A filtered signal is
always reduced in amplitude compared with the original
Fig. 21. Source of noise in recorded signal. (a) Environmental noise due to machinery [21], (b) Coolant application looping workpiece to table.
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A.D. Batako et al. / International Journal of Machine Tools & Manufacture 45 (2005) 1231–1245
Fig. 22. Temperature signal recorded from two types of surface grinder. (a) Single speed grinder no inverter, (b) Variable speed grinder & inverter on, (c)
inverter & machinery on.
signal. In creep-feed grinding, the workspeed is relatively
low and consequently, a lower sampling rate would cut
down the level of HF noise. However, in HEDG and in
shallow-cut grinding the table speed may reach 1 m/s thus
the signal sampling rate is critical.
Hardware and software filtering techniques are used to
recover the temperature signal. Hardware filtering has the
advantage that it can be incorporated before the amplifier
and it cuts off undesirable noise before it is amplified.
Hardware filters have a preset cut-off frequency and do not
have the flexibility of software filtering. Sometimes the
resulting signal from a hardware filter can be noisy due to
interference in the amplifiers and data acquisition devices.
When the sensor has a built-in amplifier it is better to use
software filtering as it offer more flexibility in the range of
cut-off frequencies and the type of filter to use. Depending
on the algorithm used a hardware filter may induce a shift in
the resulting signal. Software filtering allows use of the
zero-shift technique to reproduce the background temperature without shifting the result. Adopting the above
measures it is possible to obtain a signal clean enough that
may not need filtering. Fig. 23 shows a signal recorded using
the listed precautions. It also shows the difference between a
conventional Butterworth filter and the zero-shift technique
with the same filter. It seen that conventional filtering shifts
the signal to the right and also the moving average.
9. Conclusions on temperature measurement
The merits of different methods of temperature measurement have been thoroughly explored. The single-pole
grindable thermocouple technique was found to be the
simplest and most reliable technique. It has been shown that
it is important to optimise the thermocouple geometry to
achieve reliable signals under HEDG conditions.
Other factors that affect the accuracy of a measuring
system have been investigated and it has been shown that it
is possible to achieve reliable temperature measurements
under dry and wet grinding conditions. High-speed
sampling and zero-shift filtering reduce the problem of
interpreting the temperature signal and measuring maximum temperature.
Using this technique, temperatures measured have
demonstrated good agreement with predicted temperatures.
The work has highlighted the need for further work on the
effectiveness of fluid delivery.
Acknowledgements
The authors would like to express their gratitude to the
EPSRC for funding this project. (Grant GR/R68795) and to
the project partners: Cranfield University and to industrial
collaborators: Castrol Industrial, Element Six, Landis Lund,
Renold, Saint Gobain Abrasives, Stresstech–AST, Weston
EU. The authors would also like to thank Prof. O.
Mgaloblishvili for his valuable support and advice.
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Fig. 23. Resulting signal from adopted precautions.
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