Correlation between Acoustic Emission and Torque in Drilling

Martín P. GOMEZ1,2

, José RUZZANTE1,2

1Center of Nondestructive and Structural Testing, National Technological University, Delta Regional Faculty,

San Martín 1171, Campana (2804), Buenos Aires, Argentina.

Phone: +54 11 67727990, Fax: +54 11 67727134, e-mail: mpgomez@cnea.gov.ar, 2Elastic Waves Group, Constituyentes Atomic Center, National Comission of Atomic Energy, Av. Gral. Paz

1499, San Martín (1650), Buenos Aires, Argentina.

Abstract

This work involves the search of predictive parameters for assessing the condition of cutting tools in metal ma-

chining. The indirect method chosen is based on the acoustic emission generated during the cutting process. The

aim of this research is to shorten the way for utilization of control systems for possible application in industrial

machinery. A large number of hexagonal cylindrical steel samples with good homogeneity were prepared and

consecutively drilled at their centers with drill bits having different levels of damage at the edges and at the outer

corners. The wear of these tools was obtained artificially by spark erosion, but also mechanically. By scanning

electron microscopy the evolution of the damage was monitored. During the machining process, the features of

acoustic emission and torque were recorded. For different conditions of the drill bits, the acoustic emission pa-

rameters and torque were correlated. The "Mean MARSE Power", defined as MARSE energy divided by dura-

tion of the hit, showed a linear correlation with torque.

Keywords: Acoustic Emission (AE), drilling, torque, AE features, wear, Mean MARSE Power

1. Introduction

During cutting in drilling, the drill gives work to the piece, and activates various mechanisms

that generate elastic waves, with frequencies in the range of ultrasound [1,2]. The Acoustic

Emission (AE) method allows the study of these elastic waves. Along with other indirect

methods, the aim of AE is to get information of the cutting process in real time [3,4]. AE sig-

nals can be transient (burst) or continuous depending on the type of process in the emitter

source. The occurrence of both types of signal is determined by the mechanisms involved in

each specific case of cut [5,6]. Chip fracture, breaking of the tool edge, and the chip hitting on

workpiece and tool are AE burst sources. Otherwise, plastic deformation of the workpiece,

and the chip and friction between tool, workpiece and chip are sources of continuous AE [6]. These signals come from stochastic processes [7] and can be studied from the characteristic

parameters or waveforms themselves. It should be noted that the stochastic nature of the cut-

ting process may also be related to material properties such as strength and hardness that can

cause random fluctuations in the forces associated with the cut, as also in AE features [8]. In

previous works, some authors [9] observed the good qualities of the AE method for detection

of chip breakage, compared with the poor results obtained for the signals of cutting forces.

The classical AE parameters for transient signals are: hit time, duration, rise time, ring down

counts, MARSE energy, amplitude, root mean square (RMS), average frequency and others

[10]. Also the MARSE mean power (MP) and its variance can be defined as signal features

[11]. The last four mentioned parameters are not dependent on signal duration and for that

reason they can be applied to all types of AE signals.

This work evaluates the effectiveness of different AE parameters to represent the condition of

cutting tools and process in metal machining. The aim is to find one that simplifies the subse-

quent steps to extract the needed information of the signal, trying to avoid sophisticated math-

ematical algorithms. In that way, the AE features will be related with a mechanical parameter,

the torque.

The torque produced by the drill bit on the workpiece during drilling has shown, in the litera-

ture, to be a good parameter to obtain information about the dynamics of the process. Differ-

30th European Conference on Acoustic Emission Testing & 7th International Conference on Acoustic Emission University of Granada, 12-15 September 2012

www.ndt.net/EWGAE-ICAE2012/

ent models have evolved how the characteristics of the drill bit material and the cutting pro-

cess can be related with the torque [12-15]. Also the torque together with the feed force has

been widely used as a reference parameter to characterize rotating tool wear [2].

Then, the information obtained by the AE produced in drilling for different conditions of the

drill bits could be correlated to the torque, to compare both results and give a mechanical

frame of reference to the AE parameters of the measured signals. Following previous works

of the authors [16,17], the MARSE mean power was evaluated as a good parameter to get

information about the tool condition and the cutting process.

2. Experimental Method

The aim of this study was to characterize tool wear and cutting process in a set of tests, in

which AE features and torque were measured. For carrying out the tests, the workpiece, the

drill bit and the machining parameters were fixed as follows.

2.1. Hardware

2.1.1 Workpiece specimens

Workpiece specimens were produced by cutting 95-mm long pieces from an SAE 1040 steel

drawn hexagonal bar 14-mm wide (Figure 1). The material of the workpiece showed homo-

geneity and low content of inclusions in the zone to be drilled. It was assessed by different

studies. Optical metallography showed a 7-8 mean grain size accordingly with ASTM E112

standard (Figure 2). In microstructures, inclusions and precipitates were not observed in quan-

tity and size that could affect the state of the drill bits or the rate of emission of AE signals.

Rockwell B average hardness was found to be 98.5 ± 1.5 for ten randomly selected speci-

mens. These features were essential to ensure the repeatability of the tests and results. The

chemical composition of the steel given by the manufacturer was: C: 0.40, Mn: 0.72, Si: 0.29,

P: 0.011, S: 0.012.

Figure 1. Hexagonal workpiece with pilot hole Figure 2. Microestructure of steel SAE 1040

For every specimen, a pilot hole 10-mm depth and 1.5-mm in diameter was drilled on an end

face along the longitudinal axis. This pilot hole allows to study two situations during drilling,

one avoiding the contribution of the chisel edge to the cutting process while the drill bit works

in the pilot hole, and the other when the chisel works in drilling deeper than the pilot hole.

2.1.2 Drill bits

Cutting tools used in the tests for the present experiment were commercial HSS high-speed

steel drill bits of 5-mm diameter. Defects were produced in a controlled manner on the cutting

edges and the outer corners of the flute edges to simulate different degrees of wear of the drill

bits. The artificial wear was produced by mechanical procedures and by spark-erosion. HSS

was the material used in drill bits to avoid large wear time in a future correlation between the

action of drills artificially worn and drills with real wear.

Table 1 shows the condition and nomenclature of the eight drill bits used in this work. The

condition of the drills were: New with sharp edges, used with sharp edges, new with defective

edges, spark erosion dented cutting edges, spark erosion blunted cutting edges, mechanically

dented cutting edges and mechanically worn flute edges. The former two drills correspond to

no damage state.

Table 1. Drill bit nomenclature and condition.

Drill

bit

Name Condition

1 NE1 New, with sharp cutting edges

2 NE2 Used, with sharp cutting edges

3 NE3 New, with defective cutting edges

4 SDE Spark erosion dents on cutting edges (small craters)

5 BE1 Spark erosion blunted cutting edges (case 1)

6 BE2 Spark erosion blunted cutting edges (case 2)

7 MDE Mechanically dented cutting edges (big craters)

8 WF Mechanically worn flute edges (worn outer corner)

Spark-erosion or EDM is the process of particle removal from a workpiece by an electric arc

formed between electrode and workpiece, which are separated by dielectric liquid. The spark-

erosion was produced on drill bits by an EDM machine. Different electrodes were used for

cases of blunted and dented edges. In the first case a flat shaped electrode was located with

different angles to achieve the condition of blunted edges BE1 and BE2. Those conditions

were adopted as an intermediate condition between sharp cutting edges and excessive worn

edges (as the mechanically dented cutting edges or MDE condition), altering the effective

cutting angle and producing a larger contact surface between the tool edge and the workpiece.

For the second case, a cylindrical electrode (a wire, 0.3 mm diameter) was used to obtain

dents (or craters) of 500-µm diameter on the middle of the cutting edges. About the mechani-

cal wear, the extreme condition MDE of big craters on the edges was obtained by regular

drilling in very low-quality steel with hard inclusions. Also in WF condition, the flute edges

(outer corners) were mechanically worn with a drill sharpener to simulate other typical kind

of tool wear.

Figure 3. SEM images of damaged drill bits

The evolution of the drill bits condition was assessed by means of scanning electron micros-

copy (SEM). Figure 3 shows a qualitative example of SEM images of damaged drill bits. The

advantage of this method of observation, beyond the possibility of obtaining a higher magni-

fication, is its excellent focal field, very useful for viewing a body of overall size and macro-

scopic three-dimensional shape such as a drill bit, and in particular its tip which is the object

of study.

2.1.3 Drilling operation

The drilling operation was performed in a FIRST LC 50RS vertical mill, in continuous drive

mode. Each test was performed at the same cutting conditions. The rotational speed was (530

± 8) RPM and the feed rate was (0.353 ± 0.005) mm/s. The tests were carried out using con-

tinuous lubrication with Kansaco machining soluble oil at a dilution of 20 parts of water per

lubricant part. The lubricant flow rate was 250 ml/min. The depth of drilled holes was 20 mm,

with 10 mm pilot hole.

2.1.4 AE and torque measurement

Elastic waves produced by AE sources during metal machining were detected using a PAC

WD broadband AE sensor. A broadband CISE preamplifier of 40 dB gain (100 x) conditioned

the sensor signal and a PAC PCI-2 AE system with 18-bit amplitude resolution was used to

acquire, digitize, parameterize and store the AE measurements. AE sampling rate was 5 Ms/s.

The torque was measured using an Instron torsion cell and a CNEA signal conditioner (source

and amplifier). This torque signal was measured as an external parameter by the PCI-2 AE

system and it was digitized with a resolution of 16 bits in amplitude at a sampling rate of 10

ks/s. The typical AE features and torque were acquired simultaneously and recorded in the

same data files. The AE signal acquisition was triggered at a threshold of 27 dBAE in order to

minimize the influence of background noise. Figure 4 shows the block diagram of the AE

system. Figure 5 shows the drilling machine, the mounting of the workpiece in the torque cell

and a sketch of the drilling path. Several thousand hits, and their corresponding parameters

(and waveforms), were recorded for each test.

Figure 4. AE system block diagram in a drilling test

Amplitude, RMS, counts, MARSE energy and duration of the AE signals and torque were

recorded for each AE hit. The duration of the AE hits was up to 20 ms (defined by the max.

duration time) for the AE signals measured in stationary drilling. MARSE energy and dura-

tion were used to calculate the MARSE mean power or MP, defined as MARSE energy di-

vided by duration. Counts and duration were used to calculate the average frequency (AF),

defined as counts divided by duration.

Before each test, the correct operation of the sensor and its acoustic coupling to the probe

were verified by means of the Hsu-Nielsen method. Also for each session of measurements,

the torque cell was calibrated.

a. b. c.

Figure 5. a. Drilling machine. b. Mounting of the workpiece in the torque cell. c. Sketch of the drilling path

2.2 Performing drilling tests

The cutting conditions, defined by parameters such as speed of rotation and feed rate, or addi-

tional procedures such as lubrication, were adjusted to replicate a typical situation of drilling.

In the literature some studies have chosen typical cut and feed rates close to those used in this

work [18-20].

All tests were performed on many sessions, maintaining the same cutting conditions, to en-

hance the statistical independence of sampling experiments. The meaning in this work of the

word “test” will be defined to one drilling operation over one workpiece sample with a drill

bit with a particular condition. Then 32 tests were performed in which the tool condition was

varied, with 8 cases or states of the edges of the drill bits used. The details of the feed rate and

rotation speeds, and lubrication were given in Section 2.1.3.

Drilling procedure

For each of the specimens, a single hole was drilled. Each hole was drilled with a drill repre-

senting a specific condition of the edges. At least three trials were performed for each condi-

tion of wear to obtain representative results. Each test drilling was divided into stages in

agreement to the guidelines detailed in literature [17,22]. The description of each of these

stages is described as follows:

First stage: approach of the drill bit to the sample, free rotation.

Second stage: contact between the drill bit and the piece, slip.

Third stage: start of the drill bit cutting, until its tip enters fully into the hole. The chis-

el of the drill does not enter in contact with the workpiece as it rotates inside the pilot

hole.

Fourth stage: the wall of the hole contains the outer corners. The drill goes into a qua-

si-stationary regime of cut (after the initial transient) in the region with pilot hole.

Fifth stage: the chisel contacts the bottom of the pilot hole.

Sixth stage: the cutting edges restart the action of cut (transitional).

Seventh stage: quasi-stationary regime of drilling deeper than the pilot hole.

At the end of the seventh stage, the drilling ends abruptly with the rapid removal of the drill

out of the hole.

2.3 Signal analysis

The aim of the signal analysis was to find a correlation between typical AE parameters, repre-

senting stress waves released during drilling, and torque representing a mechanical parameter

in that process.

2.3.1 Average of the AE features and torque for quasi-stationary drilling

For each of 30 drilling tests, for eight drill bit conditions, the AE and torque signals were av-

eraged in a time window of approximately 20 seconds, corresponding to all the hits measured

in the fourth stage of cutting of quasi-stationary drilling with pilot hole, avoiding the chisel

action. After taking each average, the results for tests with the same tool condition was

grouped and averaged again, to represent in a graph the studied parameters as a function of

each drill bit condition. Then the average of amplitude, RMS, MP and AF of AE were plotted

in terms of the drill bit condition. The behavior of each AE feature on the evolution of the

degree and the type of wear of the tools was evaluated. Also the torque was computed and

plotted with identical methodology.

2.3.2 Linear correlation between AE parameters and torque

After finding a strong correlation between torque and MP the study was continued in that di-

rection. In the literature, torque has shown to be an old and good indicator of the mechanics of

the cutting, having been used in many works as predictive parameter of the process status for

rotating tools (such as drilling or milling) [2,21,23]. Therefore, the study of the linear correla-

tion between torque and the two main AE features for continuous AE signals as MP and RMS

was proposed. Then for the measured signals, each AE feature and torque were correlated in a

time window of approximately 60 seconds, corresponding to each whole drilling test for each

case performed at similar cutting conditions (velocities, lubricant, drill diameter, etc.). Both

the torque and the RMS are lower frequency signals than the MP, then before correlation the

rapid variation of the MP was filtered applying a moving average to MP. The moving aver-

aged mean power of AE (MAMP) was calculated with a moving window of 10 neighboring

points and acted as low-pass filter.

Then, the correlation between torque and MAMP was calculated and the results were plotted

and compared with those for RMS to find the best AE feature to evaluate mechanical charac-

teristics of drilling.

3. Results and discussion

The AE signals and parameters measured in drilling tests with different tool condition were

performed at the same cutting conditions. The acquired data was studied in two parts, one in

which the average of the torque and the AE features were computed in the stage of quasi-

stationary drilling, and the second in which the correlation between the torque and the AE

parameters (RMS and MAMP) were evaluated for each complete test.

3.1 Average of AE features vs. drill condition

Figures 6, 7, 8 and 9 show the behavior of the AE features evaluated for the different states of

wear of the drill bits. It should be remembered that this part of the study does not take into

account the action of the chisel edge of the tip of the drill, as the analyzed signals correspond

to the stationary cutting with pilot hole.

In Figure 6, the average of amplitude of the hits of AE for different drill bit conditions is plot-

ted. The graph showed a growth of this parameter as a function of the wear of the cutting edg-

es. The averaged amplitude for the condition of worn outer corners (WF) showed a lower

level than in the most worn cutting edges.

In Figure 7 the evolution of the average of the RMS value of the AE plotted as a function of

the different tool condition was shown. In this figure, a similarity with the Amplitude in low

and high wear was observed.

NE1 NE2 NE3 SDE BE1 BE2 MDE WF

0.0

0.5

1.0

1.5

2.0

Am

plit

ude

(V

)

Drill bit condition Figure 6. Averaged Amplitude vs. drill bit condition.

NE1 NE2 NE3 SDE BE1 BE2 MDE WF

0.00

0.02

0.04

0.06

0.08

0.10

0.12

RM

S (

V)

Drill bit condition

Figure 7. Averaged RMS vs. drill bit condition.

NE1 NE2 NE3 SDE BE1 BE2 MDE WF120

180

240

AF

(co

unts

/ms)

Drill bit condition Figure 8. Averaged AF vs. drill bit condition.

The average of the AF plotted in Figure 8 also shows a behavior in which there are three

zones, one starting with the new bit, an intermediate with less damaged bits, and a third with

the most damaged bits, the last two in the form of two closely spaced plateaus, within which

one cannot distinguish the different cases belonging to these. This would indicate that in the

test conditions, the AF would not be a good parameter to classify the degree of wear.

In Figure 9, the average of the "MARSE mean power" (MP) showed three different behaviors

depending on the progress of the variable "drill bit condition". First, for new drill bits (NE1)

the averaged MP showed a very low value. Second, the next three conditions with little dam-

age or wear (NE2, NE3 and MDE) showed a plateau, with no difference in MP between drills

with little use, poor completion of manufacture, and edges with indentations by EDM. The

third set corresponds to the case of further deterioration (FE1, FE2, MDE and WF), for which

a monotonically increasing approximately linear behavior can be observed. This linear behav-

ior could be used to discriminate the degree of damage. In this analysis, the drill bit with worn

outer corner (WF) can be separated from the drill bits with damage in cutting edges. For this

particular case, the averaged MP showed a larger value than that obtained for the drill condi-

tion MDE, with the greatest damage on their cutting edges (highly cratered mechanically).

Thus, there is a relationship between the averaged RMS and MP for cases with damage in the

edges. For the case of the worn outer corner, mostly the behavior is similar to what happened

to the study of the amplitude of AE.

NE1 NE2 NE3 SDE BE1 BE2 MDE WF0

20

40

60

80

MP

(E

counts

/ms)

Drill bit condition

Figure 9. Averaged MP vs. drill bit condition.

NE1 NE2 NE3 SDE BE1 BE2 MDE WF

4

8

12

16

Torq

ue (

kg.c

m)

Drill bit condition

Figure 10. Averaged Torque vs. drill bit condition

3.2 Average of torque vs. drill condition

Figure 10 shows the mean torque for each drill bit condition with the corresponding standard

deviation. Analogous to the behavior of the MP plotted in Figure 9, the torque showed little

variation for drilling with low wear drill bits (less than for the three cases of the parameters of

the AE) and a marked variation in cases of major damage to the edges and wear on the outer

corner. For lower wear, torque occurring in a plateau of indistinguishable values, and for

higher wear the mean torque is increased with the degree of damage of the drill bit (in both

edges and in the outer corner). For this second case, it would be possible to evaluate the de-

gree of wear of the drill bit analyzing the measured torque during drilling. The values ob-

tained for the torque in this work are in agreement with other studies measured in similar

conditions [21].

3.3 Relationship between averaged Torque and AE features

Comparing torque and MP as a function of drill bit conditions in Figures 9 and 10, the same

behavior can be adjusted to both parameters, neglecting the first point representing the new

bit condition. A linear ratio with torque is obtained applying to the MP a multiplicative con-

stant and a shift with respect to zero. Figure 11 shows the fitting of torque and MP, adjusted

from the equation (1).

cmkgEcounts

scmkgMPTorque .26.2

..10188.0* 3

(1)

For the conditions of this study, we can conclude that a proportionality or linear relationship

between the results of mean torque and averaged MP has been found.

Both parameters were calculated as averages over a large number of AE hits (the torque was

sampled simultaneously to each AE hit), which were studied in steady cutting regime, drilling

with pilot hole. The linearity observed for the four cases of major wear would allow estimat-

ing the torque applied by the drill bit to the workpiece, from the MP of AE.

NE1 NE2 NE3 SDE BE1 BE2 MDE WF

2

4

6

8

10

12

14

16

C2=2.26 kg.cm

Drill bit condition

MP

*C1 +

C2 (

kg.c

m)

Torq

ue (

kg.c

m)

Torque

MP*C1 + C

2

C1=0.188 10

-3 kg.cm.s/counts E

Figure 11. Fitting of torque and PM vs. drill bit condition.

3.4 Correlation between torque and AE features

Another point of view of the relationship between AE features and torque was obtained by

means of the study of the linear correlation between some AE features and torque as time

functions. In Figure 12, the correlation coefficients were plotted as a function of the drill bit

condition. The plotted conditions were: new (NE1, NE2 and NE3), edges worn by EDM (BE1

and BE2), worn outer corners (WF) and mechanically dented edges (MDE). The selected AE

features for this evaluation were MAMP and RMS, based in a previous work [17]. In the

graph, the red circles represented the correlation between torque and MAMP and the brown

circles the correlation between torque and RMS. This figure shows clearly that MAMP have

the best correlation with torque as a function of time. The correlation coefficient for the

MAMP is higher than that for the RMS. RMS is the most used parameter in the applications

of AE in metal machining and one of the recommended in the literature [24]. Although the

mathematical procedure for both parameters is comparable, MAMP analysis showed better

results than the RMS.

NE1 NE1 NE2 NE2 NE2 NE3 NE3 NE3 BE1 BE1 BE1 BE1 BE1 BE2 BE2 BE2 BE2 BE2 MDEMDEMDE WF WF WF

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Corr

ela

tio

n w

ith t

orq

ue

Drill bit condition

RMS

MAMP

Figure 12. Correlation of MAMP (red circles) and RMS (black squares) with torque

0 10 20 30 40 50 600

2

4

6

8

10

12

14

Time (sec)

To

rqu

e (

kg

.cm

)

0 10 20 30 40 50 600

10

20

30

40

50

Time (sec)

MA

MP

(E

co

un

ts/m

s)

Figure 13. MAMP and torque vs. time, for a test of drilling for BE1 condition

The good correlation between torque and MAMP, specified in Figure 12, is evidenced in Fig-

ure 13 in which are shown MAMP and torque versus time for a new drill bit. Both graphs,

torque and MAMP show the different stages of the cutting process, which were previously

detailed. For example, watching the MAMP AE signal the drilling machine operator can infer

if the tool is entering in the material (second stage), drilling through the pilot hole (fourth

stage) or through the region without a pilot hole (sixth stage). Irregularities related to abnor-

malities of the machining process could be observed in the signals. In earlier works, Carpenter

[25] showed the same high correlation behavior between AE RMS and frictional force, and

Marinescu and Axinte [26] correlating the MARSE and the cutting force.

The energy of the rotating drill is transmitted to the sample through the friction of the chisel

with the end of the hole and the flute with the wall, and in proper cutting of the material (plas-

tic deformation). This is actually measured by the torque. It is outstanding how accurately the

AE follows these mechanical changes giving place to an important correlation.

For higher sampling frequencies, the AE MP should show greater sensitivity compared to the

dynamic response of torque, due to its higher bandwidth for detection of fast transients, as the

elastic waves emission of the chip breaking. Another advantage of the AE in the sensing of

the cutting process could be looking at a simpler implementation and lower cost. Regarding

the use of the MAMP, it is based on the stochastic nature of AE, which according to the wear

of the drill bit varies the dispersion in the MP. Filtering the higher frequency, the dispersion

of the remaining variations is coincident with the detected torque [6]. The moving average

acts as a low-pass filter to reveal information related to mechanical processes of lower charac-

teristic frequency.

4. Conclusions

For the drilling conditions of this study, a proportionality or linear relationship between the

results of the mean torque and the averaged MP has been found. This correspondence could

be useful to estimate torque values from the MP of AE signals.

Analogous to the behavior of the averaged MP, the mean torque showed little variation for

drilling with low wear drill bits (less than for the three cases of the parameters of the AE) and

a marked variation for major wear on the edges and wear on the outer corner. For these four

conditions it would be possible to evaluate the degree of wear of the drill bit analyzing the

measured torque during drilling.

The analysis of the linear correlation between torque and AE features as function of time

showed that the correlation coefficient for the MAMP is higher than that for the RMS. The

moving average acts over the MP as a low-pass filter to reveal information related to the me-

chanical processes of lower characteristic frequency, as that detected by the torque.

Other findings include:

The averaged Amplitude of the AE showed a growth behavior depending on the wear of

the cutting edges. This parameter for the condition of worn outer corners (WF) showed a

lower level than in the most worn cutting edges.

The evolution of the averaged RMS value of the AE as a function of the tool wear showed

a similar behavior to the averaged Amplitude.

The averaged frequency (AF) of the AE showed no qualities to classify the degree of wear

of the drill bit.

The averaged MP showed three different behaviors depending on the progress of damage

in the cutting edges. For new drills the MP showed a low value. The next three conditions

with little wear showed a plateau, with no difference in MP. The third set corresponds to

the case of further deterioration on cutting edges, for which the averaged MP manifested a

monotonically increasing linear behavior, which could be used to discriminate the degree

of damage. The condition WF showed a higher value than that obtained for the condition

MDE, with the greatest damage on their cutting edges (highly cratered mechanically).

Acknowledgements

To Julio Migliori, Alfredo Hey, Rosa Piotrkowski, Edgardo Cabanillas, Elena Forlerer, UTN

and CNEA.

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