%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\chapter{Experimental Methodology} \label{chap:method}


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
In order to study the generation of acoustic emission (AE) in metal cutting, a systematic
set of experiments were performed on a CNC turning centre, and data from different sensors
were collected. This chapter describes the methodology adopted for the experiments. In the
first part, the experimental apparatus utilized is described, and, in the second part, the
experimental procedure adopted is illustrated.


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Apparatus} \label{sec:apparatus}

This section describes the turning machine, tool, and work materials selected for the
experiments, and also the instrumentation system conceived for the acquisition of data.


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Machine, Tool, and Materials} \label{sec:mach_tool_mat}

\begin{figure}[h]
   \center{\includegraphics{turning_center.eps}}
   \caption{Image of the turning centre used for the experiments.}
   \label{fig:turning_center}
\end{figure}
All turning tests were performed on a MT50~CNC Turning Centre from MHP~Machines
(Figure~\ref{fig:turning_center}). The chuck of the machine can be rotated up to a
maximum of 4000~rpm and can provide a constant power of 34~kW between 1000 and
3000~rpm. The maximum resolution of the main cutting parameters are: cutting speed,
1~m$/$min; feed rate, 0.001~mm$/$rev; width of cut, 0.001~mm.

Continuous chip formation and semi-orthogonal machining were the primary motivations
for the choice of the tooling system. A tool holder, ISO code SCLCL~2020K12, was used
in combination with an ungrooved and uncoated carbide insert, ISO code CCMW~120404,
grade K10 (Figure~\ref{fig:holder_insert}). This combination resulted in the
following tool geometry:\footnote{The definition and nomenclature of a
three-dimensional tool geometry can be found, for example, in
reference~\cite{oxley89}.} side cutting edge angle, $-5^\circ$; end cutting edge
angle, $5^\circ$; inclination angle, $0^\circ$; normal rake angle, $0^\circ$;
clearance angle, $7^\circ$.
\begin{figure}[h]
   \center{\includegraphics{holder_insert.eps}}
   \caption[Tool holder and insert.]
   {Tool holder and insert (front-top view).}
   \label{fig:holder_insert}
\end{figure}

Four different work materials were used: a low and a medium carbon steel, an austenitic
stainless steel, and an aluminium alloy. These materials are described next in detail:
\begin{description}

   \item [080A15 (BS 970)] This low carbon steel was supplied in round
   bars of 90~mm diameter. The chemical composition (provided
   by the supplier) was 0.15\%~C, 0.17\%~Si, 0.78\%~Mn, 0.015\%~P,
   0.027\%~Ni, 0.021\%~Cr, 0.003\%~Mo, 0.018\%~S, 0.049\%~Cu,
   0.002\%~Sn, and 0.001\%~Al.

   \item [080M40 (BS 970)] This medium carbon steel was supplied in round
   bars of 90~mm diameter. The chemical composition (provided
   by the supplier) was 0.44\%~C, 0.24\%~Si, 0.72\%~Mn, 0.024\%~P,
   and 0.009\%~S\@.

   \item [304S15 (BS 970)] This austenitic stainless steel was supplied in
   round bars of 76.2~mm diameter. The chemical composition
   (provided by the supplier) was 0.023\%~C, 0.35\%~Si, 1.67\%~Mn,
   0.023\%~P, 10.09\%~Ni, 18.13\%~Cr, 0.36\%~Mo, 0.029\%~S,
   0.2\%~Cu, 0.066\%~N, and 0.14\%~Co.

   \item [6082-T6 (BS 1474)] This medium strength aluminium alloy, in the
   T6 condition, was supplied in round bars of
   88.9~mm. The chemical composition was not provided by the
   supplier, but the typical composition is
   0.7--1.3\%~Si, 0.4--1\%~Mn, 0.1\% max.~Ni, 0.25\% max.~Cr,
   0.1\% max.~Cu, 0.5\% max.~Fe, 0.6--1.2\%~Mg, 0.2\% max.~Zn, and
   0.1\% max.~Ti.

\end{description}


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Instrumentation} \label{sec:instrument}

In conjunction with the measurement of AE, other signals were measured such as the cutting
forces, vibrations, and temperatures. This section describes the sensors, analog signal
conditioning, and signal acquisition. Figure~\ref{fig:instrument} shows a diagram of the
overall measurement system.

\begin{figure}[h]
   \center{\includegraphics{instrumentation_diagram.eps}}
   \caption{Instrumentation system.}
   \label{fig:instrument}
\end{figure}

\subsubsection{Acoustic Emission Measurement}

The main criterion for the selection of the AE transducers was based on the flatness
of the sensors response in the \mbox{100--1000 kHz} bandwidth. Two piezoelectric
sensors were selected from Physical Acoustics Corporation:
\begin{description}

   \item [MICRO~80S] This sensor is 10~mm in diameter and 12~mm in height, and the
   operating frequency range is \mbox{175--1000 kHz}.
   Figure~\ref{fig:ae_sensors_calib.eps}a shows the frequency response of the sensor as
   obtained from its calibration certificate.

   \item [PICO] This sensor is 5~mm in diameter and 4~mm in height, and the operating
   frequency range is \mbox{200--750 kHz}. Figure~\ref{fig:ae_sensors_calib.eps}b shows
   the frequency response of the sensor as obtained from its calibration certificate.

\end{description}
\begin{figure}[h]
   \center{\includegraphics{ae_sensors_calib.eps}}
   \caption[Frequency response calibration of AE transducers.]
   {Frequency response calibration of AE transducers.
   (a)~MICRO~80S\@. (b)~PICO\@.}
   \label{fig:ae_sensors_calib.eps}
\end{figure}
The two sensors were mounted at different locations
(Figure~\ref{fig:ae_sensors_location}) on the tool holder, and grease was used to
couple the sensors to the surfaces of the tool holder. As shown in
Figure~\ref{fig:ae_sensors_location}, the PICO was mounted on the bottom surface of
the tool holder, underneath the cutting tip, whereas the MICRO~80S was mounted at the
rear of the tool holder.
\begin{figure}[h]
   \center{\includegraphics{ae_sensors_location.eps}}
   \caption[Position of AE sensors on tool holder.]
   {Position of AE sensors on tool holder (back-bottom view).}
   \label{fig:ae_sensors_location}
\end{figure}

The two sensor signals were then amplified and band-pass filtered with two 1220A
preamplifiers from Physical Acoustics Corporation. The amplifier gain can be selected
to be either 40 or 60 dB, and the band-pass filter operates in the 100--1200~kHz
range. Furthermore, two electronic circuits, using integrated circuit AD536AJQ from
Analog Devices (Figure~\ref{fig:rms_circuit}), were used to perform the on-line RMS
computation of the AE signals, allowing the choice of two RMS time constants: 0.25
and 2.5~ms.

\subsubsection{Cutting Force Measurement}

For the measurement of the tangential force ($F_\mr{X}$) and feed force ($F_\mr{Y}$),
two sets of four strain gauges were attached to the tool holder
(Figure~\ref{fig:strain_gauges}) and were arranged to form two full Wheatstone
bridges\footnote{Reference~\cite{shaw84} can be consulted for more information on
strain gauge dynamometry.} (Figure~\ref{fig:strain_gauge_bridges}).
\begin{figure}[h]
   \center{\includegraphics{strain_gauges.eps}}
   \caption[Position of strain gauges on tool holder.]
   {Position of strain gauges on tool holder (front-top view).}
   \label{fig:strain_gauges}
\end{figure}
The strain gauges, model PP/350/PC11/A, were supplied by TSM and were bound to the tool
holder with an heat curing adhesive (TSM300 adhesive kit).

Two strain gauge amplifiers, using integrated circuit \mbox{846-171} and printed
circuit board \mbox{435-692} from RS Components, were used to amplify the signals from
both strain gauge bridges. Furthermore, two low-pass filters with a 10~kHz cut-off
frequency (Figure~\ref{fig:filter_circuit}) were used to avoid aliasing.\footnote{All
signals must be band-limited to less than half the sampling rate of the sampling
system.}

Static calibration of the output signals from both strain gauges was conducted by applying
the gravitational forces of dead weights in the $F_\mr{X}$ and $F_\mr{Y}$ directions
(Figure~\ref{fig:force_calib}).
\begin{figure}[h]
   \center{\includegraphics{force_calib.eps}}
   \caption[Calibration of cutting force signals.]
   {Calibration of cutting force signals. (a)~Tangential force.
   (b)~Feed force.}
   \label{fig:force_calib}
\end{figure}

\subsubsection{Vibration Measurement}

An accelerometer from Kistler Instruments was used to monitor the vibrations from the
cutting process. The accelerometor was an industrial model, type 8752A50, operating in the
\mbox{0.6--5 kHz} frequency range. Figure~\ref{fig:accel_calib} shows the frequency
response of the accelerometer as obtained from its calibration certificate.
\begin{figure}[h]
   \center{\includegraphics{accel_calib.eps}}
   \caption{Frequency response of accelerometer.}
   \label{fig:accel_calib}
\end{figure}

A coupler, model 5108 from Kistler Instruments, was used to provide power to the
accelerometer and read the signal from the sensor. The output from the coupler was then
fed into a low-pass anti-aliasing filter with 8~kHz cut-off frequency
(Figure~\ref{fig:filter_circuit}).

\subsubsection{Temperature Measurement}

In order to assess the temperatures near the cutting zone, two grooves were machined
onto the opposite surfaces of the shim,\footnote{The shim is a plate placed between
the tool and the insert.} so that two thermocouples (thermocouples 1 and 2, 5 and
7~mm below the top of the cutting tool, respectively) could be  installed under the
nose of the cutting insert (Figure~\ref{fig:thermocouples}). The thermocouples, model
\mbox{5TC-TT-K-36-36}, type~K, were firmly attached in the grooves with a CC high
temperature cement (both products provided by OMEGA Engineering).
\begin{figure}[h]
   \center{\includegraphics{thermocouples_location.eps}}
   \caption[Location of thermocouples.]
   {Location of thermocouples. (a)~View of tool holder and grooved
   shim without the cutting insert. (b)~Shim and
   embedded thermocouples.}
   \label{fig:thermocouples}
\end{figure}

The thermocouple signals were amplified with the integrated circuit AD595AQ from Analog
Devices (Figure~\ref{fig:thermocouple_circuit}). The amplifier produced an output of 10
mV$/^\circ$C\@. The temperatures measured with thermocouples 1 and~2 were named $T_1$
and $T_2$, respectively.

\subsubsection{Data Acquisition System}

For the acquisition and storage of data collected from the sensors described above, three
acquisition boards were mounted into a personal computer:
\begin{description}

   \item [AT-MIO-16H]
   This 12-bit National Instruments board has a maximum of eight sampling channels
   available in differential mode and a maximum sampling rate of
   100~kS$/$s.\footnote{Unit S stands for number of samples.} This board was used to
   acquire the output signals from the two strain gauges, accelerometer, and AE from the
   RMS converter.

   \item [Lab-PC+]
   This 12-bit National Instruments board has a maximum of four channels in differential
   mode and a maximum sampling rate of 83~kS$/$s. This board was used to acquire the
   output signals from the two thermocouples.

   \item [CS1250]
   This 12-bit, two-channel board, supplied by Gage Applied Sciences, was used to acquire
   the raw AE signals. Although the sampling frequency can go as high as 50~MS$/$s, only
   rates of 5 and 10~MS$/$s were utilized.

\end{description}


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Experimental Procedure} \label{sec:procedure}

This section describes the experiments that were carried out in order to study the
generation of AE from metal cutting. Two different types of experiments were
accomplished: in the first type (Test~A), the cutting conditions were varied over a
wide range, and, in the second type (Test~B ), the workpieces were preheated before
cutting.


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Test A---Variation of Cutting Conditions and Materials}
\label{sec:procedure_test_A}

In this first set of tests, the variation of AE, cutting forces, vibration, and
temperature were assessed for a wide range of cutting conditions and for the four work
materials listed above (080A15, 080M40, 304S15, and \mbox{6082-T6}).
Table~\ref{tab:speed_feed} shows the conditions used in the tests with varying cutting
speed ($V$) and feed rate ($t_1$), and Table~\ref{tab:width} shows the conditions used in
the tests with varying width of cut ($w$).
\begin{table}[p]
   \caption{Variation of cutting speed and feed rate for constant width of cut.}
   \label{tab:speed_feed}
   \center{\begin{tabular}{cccc}
      \hline
      test & $V$, m$/$min & $t_1$, mm$/$rev  & $w$, mm \\
      \hline
      1    & 75           & 0.01             & 1.2     \\
      2    & 75           & 0.02             & 1.2     \\
      3    & 75           & 0.04             & 1.2     \\
      4    & 75           & 0.06             & 1.2     \\
      5    & 75           & 0.1              & 1.2     \\
      6    & 75           & 0.2              & 1.2     \\
      7    & 200          & 0.01             & 1.2     \\
      8    & 200          & 0.02             & 1.2     \\
      9    & 200          & 0.04             & 1.2     \\
      10   & 200          & 0.06             & 1.2     \\
      11   & 200          & 0.1              & 1.2     \\
      12   & 200          & 0.2              & 1.2     \\
      13   & 50           & 0.02             & 1.2     \\
      14   & 100          & 0.02             & 1.2     \\
      15   & 150          & 0.02             & 1.2     \\
      16   & 300          & 0.02             & 1.2     \\
      17   & 50           & 0.1              & 1.2     \\
      18   & 100          & 0.1              & 1.2     \\
      19   & 150          & 0.1              & 1.2     \\
      20   & 300          & 0.1              & 1.2     \\
      \hline
   \end{tabular}}
\end{table}
\begin{table}[p]
   \caption{Variation of width of cut for constant cutting speed and feed rate.}
   \label{tab:width}
   \center{\begin{tabular}{cccc}
      \hline
      test & $V$, m$/$min & $t_1$, mm$/$rev  & $w$, mm \\
      \hline
      1    & 75           & 0.02             & 0.1     \\
      2    & 75           & 0.02             & 0.3     \\
      3    & 75           & 0.02             & 0.6     \\
      4    & 75           & 0.02             & 2.4     \\
      5    & 75           & 0.1              & 0.1     \\
      6    & 75           & 0.1              & 0.3     \\
      7    & 75           & 0.1              & 0.6     \\
      8    & 75           & 0.1              & 2.4     \\
      9    & 200          & 0.02             & 0.1     \\
      10   & 200          & 0.02             & 0.3     \\
      11   & 200          & 0.02             & 0.6     \\
      12   & 200          & 0.02             & 2.4     \\
      13   & 200          & 0.1              & 0.1     \\
      14   & 200          & 0.1              & 0.3     \\
      15   & 200          & 0.1              & 0.6     \\
      16   & 200          & 0.1              & 2.4     \\
      \hline
   \end{tabular}}
\end{table}

From the output signals shown in Figure~\ref{fig:instrument}, only
the following signals were used:
\begin{itemize}
   \item Raw AE from MICRO~80S
   \item Tangential force
   \item Feed Force
   \item Accelerometer
   \item Thermocouple 1
\end{itemize}

A computer programme was produced in LabVIEW (computer package from National Instruments)
to manage the acquisition of the various sensor signals into the computer
(Figure~\ref{fig:acq_program_1}).
\begin{figure}[h]
   \center{\includegraphics{acq_program_1.eps}}
   \caption{Screen shot of acquisition programme used in Test~A\@.}
   \label{fig:acq_program_1}
\end{figure}
The \mbox{AT-MIO-16H} board was used to acquire sets of 16384 samples of the signals
from the tangential force, feed force, and accelerometer at a sampling rate of
25~kS$/$s. The \mbox{Lab-PC+} was used to acquire sets of 200 samples from the upper
temperature sensor ($T_1$) at a sampling rate of 50~kS$/$s. The CS1250 board was used
to acquire sets of 8388608 samples from the AE sensor at a sampling rate of 10~MS$/$s.
The acquisition of both \mbox{AT-MIO-16H} and CS1250 boards was initiated at exactly
the same instant, since the commencement of the acquisitions was triggered by means of
a common electrical impulse. For each of the four work materials, each cutting
condition was run at least twice, during which three sequential sets of readings were
acquired.  The results from Test~A are presented in Section~\ref{sec:results_test_A}.


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Test B---Preset Workpiece Temperature} \label{sec:procedure_test_B}

The aim of these tests was to observe the variation of AE with cutting temperature.
The work material was firstly preheated in an oven up to a temperature of
approximately $500^\circ$C\@. Thereafter, the material was rapidly removed from the
furnace, placed in the turning machine, the temperature of the workpiece surface
($T_\mr{WS}$) was measured with a handheld thermocouple, and the machining test was
immediately initiated. This procedure was repeated for different values of
$T_\mr{WS}$, as the workpiece was allowed to cool down between tests. Two 080A15
workpieces were prepared for testing with cutting speeds of 200 and 75~m$/$min,
whereas the remaining work materials (080M40, 304S15, and 6082-T6) were tested only
for speeds of 200~m$/$min. For all tests, the feed rate and width of cut were set to
0.1~mm$/$rev and 1.2~mm, respectively.
Tables~\ref{tab:temperature1}--\ref{tab:temperature4} show the different initial
temperatures from the handheld thermocouple measured before each test.
\begin{table}[h!]
   \caption{Preset workpiece temperature tests for the 080A15.}
   \label{tab:temperature1}
   \center{\begin{tabular}{ccccc}
      \multicolumn{2}{c}{75~m$/$min} & &
      \multicolumn{2}{c}{200~m$/$min} \\
      \cline{1-2} \cline{4-5}
      test & $T_\mr{WS}$, $^\circ$C & &
      test & $T_\mr{WS}$, $^\circ$C \\
      \cline{1-2} \cline{4-5}
      1 & 212 & & 1 & 284 \\
      2 & 148 & & 2 & 194 \\
      3 & 162 & & 3 & 141 \\
      4 & 146 & & 4 & 120 \\
      5 & 127 & & 5 & 95  \\
      6 & 122 & & 6 & 83  \\
      7 & 107 & & 7 & 82  \\
      8 & 94  & & 8 & 72  \\
      \cline{1-2} \cline{4-5}
   \end{tabular}}
\end{table}
\begin{table}[h!]
   \caption{Preset workpiece temperature tests for the 080M40.}
   \label{tab:temperature2}
   \center{\begin{tabular}{ccccc}
      \cline{1-2}
      test & $T_\mr{WS}$, $^\circ$C \\
      \cline{1-2}
      1  & 256 \\
      2  & 235 \\
      3  & 218 \\
      4  & 185 \\
      5  & 163 \\
      6  & 156 \\
      7  & 142 \\
      8  & 125 \\
      9  & 122 \\
      10 & 115 \\
      11 & 96  \\
      12 & 99  \\
      13 & 88  \\
      14 & 85  \\
      15 & 80  \\
      16 & 75  \\
      17 & 31  \\
      \cline{1-2}
   \end{tabular}}
\end{table}
\begin{table}[h!]
   \caption{Preset workpiece temperature tests for the 304S15.}
   \label{tab:temperature3}
   \center{\begin{tabular}{ccccc}
      \cline{1-2}
      test & $T_\mr{WS}$, $^\circ$C \\
      \cline{1-2}
      1  & 303 \\
      2  & 283 \\
      3  & 230 \\
      4  & 238 \\
      5  & 222 \\
      6  & 160 \\
      7  & 160 \\
      8  & 117 \\
      9  & 91  \\
      10 & 93  \\
      11 & 83  \\
      12 & 83  \\
      13 & 75  \\
      14 & 31  \\
      \cline{1-2}
   \end{tabular}}
\end{table}
\begin{table}[h!]
   \caption{Preset workpiece temperature tests for the \mbox{6082-T6}.}
   \label{tab:temperature4}
   \center{\begin{tabular}{ccccc}
      \cline{1-2}
      test & $T_\mr{WS}$, $^\circ$C \\
      \cline{1-2}
      1  & 213 \\
      2  & 163 \\
      3  & 130 \\
      4  & 99  \\
      5  & 83  \\
      6  & 88  \\
      7  & 78  \\
      8  & 75  \\
      9  & 71  \\
      10 & 57  \\
      11 & 53  \\
      12 & 31  \\
      \cline{1-2}
   \end{tabular}}
\end{table}

From the output signals shown in Figure~\ref{fig:instrument}, the following signals
were used:
\begin{itemize}
   \item Raw AE from MICRO~80S
   \item Raw AE from PICO
   \item RMS of AE from PICO
   \item Tangential force
   \item Feed Force
   \item Accelerometer
   \item Thermocouple 1
   \item Thermocouple 2
\end{itemize}

As the raw AE signals were detected with two sensors (\mbox{MICRO 80S} and \mbox{PICO})
positioned at different locations (Figure~\ref{fig:ae_sensors_location}), it was also
possible to analyse the difference in the AE emissions arriving at different locations
and detected with two different AE sensors.

Another computer programme was written in LabVIEW to manage the acquisition of the
various sensors into the computer (Figure~\ref{fig:acq_program_2}).
\begin{figure}[h]
   \center{\includegraphics{acq_program_2.eps}}
   \caption{Screen shot of acquisition programme used in Test~B\@.}
   \label{fig:acq_program_2}
\end{figure}
The \mbox{AT-MIO-16H} board was used to acquire the RMS of AE, the two cutting forces,
and the accelerometer signals in a continuous process at a sampling rate of 25~kS$/$s.
During this process, at every 500~ms, the CS1250 board was used to acquire sets of
65536 samples from the two raw AE signals at a sampling rate of 5~MS$/$s. Also, at
every 500~ms, the \mbox{Lab-PC+} was used to acquire 100 samples from the thermocouple
signals at a sampling rate of 20~kS$/$s. The results from Test~B are shown in
Section~\ref{sec:results_test_B}.

Other two types of experiments, using the same acquisition program and the same sensor
signals as Test~B, were performed: feed rate variation and width of cut variation
tests. In the first case, the feed rate was varied quasi-continuously from
0.24~mm$/$rev down to a very low value of 0.005~mm$/$rev and then increased again to
the initial value. For the width of cut variation tests, the width of cut was varied
linearly from 3 to 0~mm and also reversely from 0 to 3~mm. The aim of these tests was
to study the generation of AE when both feed rate and width of cut vary continuously
and assume very low values. However, as the results from these two types of tests were
observed to be quite affected by random noise signals, these results are not shown in
Chapter~\ref{chap:results}, but a small discussion on these results is presented in
Section~\ref{sec:feed_width_variation_tests}.
