Acoustic emission true RMS signals used to indicate wear of a high speed ceramic insert

Journal of Mechanical Working Technology, 20 (1989) 79-91 Elsevier Science Publishers B.V., Amsterdam- Printed in The Netherlands

79

ACOUSTIC EMISSION TRUE RMS SIGNALS USED TO INDICATE WEAR OF A HIGH SPEED CERAMIC INSERT CHANG-FEI YANGI and d. RICHARDHOUGHTON2 1Graduate Research Assistant, Mechanical Engineering Department, Tennessee Technological University, Box 5772, Cookeville, TN 38505 2professor of Mechanical Engineering, Center for Manufacturing Research, Tennessee Technological University, Box 5104, Cookeville, TN 38505 SUMMARY Tool wear measurements are a significant factor in the operation of an automation machine tool i n s t a l l a t i o n . T h i s paper reports four methods for detection of tool w e a r : cutting force, chip direction, temperature rise and acoustic emission signal. The simple combined measurement method w i l l be used to indicate ceramic tool wear or breakage. Ceramic inserts on a lathe single point cutting were used in the tests at high cutting speed. The true root-mean-square, RMS, of the acoustic emission signal measured at the base of the insert is shown to be the most sensitive to tool wear. INTRODUCTION Advanced cutting-tool

materials are being widely used.

Ceramic tools

have found wide and economic applications in high speed cutting to achieve higher metal removal rates.

The on-line sensing of tool wear, an essential

part of any r e a l i s t i c adaptive control optimization system, is particularly important

in e f f i c i e n t scheduling of machine down time for

and for tool f a i l u r e detection.

tool

changing

The need to sense tool condition in-process

complicates the problem considerably.

Cutting force, vibration, temperature,

acoustic emission, power, etc. are basic signals for development of a model which can be used as a

b a s i s for

in-process

tool

sensing.

Different

combinations of these signals can be used to link the desired decision with the metalcutting process.

Set point, signature and pattern recognition are

the three most common analysis methods [1]. The set-point technique assumes that the desired value can be based on the violation of a predetermined boundary by the measured signal.

For the

signature method, a deviation of the signature beyond the allowable deviation boundaries indicates a process change which is interpreted as a worn tool condition.

The pattern recognition method analyzes key process parameters

and tries to match them to a sequence of events known to be uniquely indicative of a problem. During Machining, the f a i l u r e of a cutting tool 0378-3804/89/$03.50

© 1989 Elsevier Science Publishers B.V.

is caused by wear due

80 to the interactions between the tool and the workpiece (flank wear) and between the tool and the chip (crater wear). revealed that

the

Machining tests at higher speeds have

sliding wear of ceramic tools

fracturing in addition to plastic deformation.

is

accelerated by local

Ceramic tools also wear by

reaction diffusion which of course is a slower process where iron, FeD and A1203 interdiffuse [2].

We w i l l

use the flank wear and crater wear as the

significant measure of deterioration. METHODS Experimental Set-Up A sketch of the experimental equipment being assembled for the research is shown in Figure I.

An acoustic emission pin transducer was attached to

the bottom of the tool insert.

Force measurements were made using a three

component piezoelectric Kistler

dynamometer, model 9257A, attached to

MSRNR tool holder.

an

The outputs of the dynamometer were converted to voltage

signals using Kistler Dual Mode charge amplifiers, model 5004.

The three

force components were digitized along with a pulse signal using a 13-bits (including sign-bit) analog-to-digital converter, model 98640A, and a HewlettPackard 9000 Series 200 computer. A chromel-alumel thermocouple has been embedded in a shim to contact the bottom of the cutting tool.

A transient calibration was used to study the

temperature lag from the cutting edge to the thermocouple measurement location.

WORKPIECE

1300 AE PIN TRANSDUCER I

I I_! .... !l-1 cl:.....i ' ...~._J_.L~I

PRE

IAMPLIFIERJ I ,,.[-,

COUPLE IILOW'PASSl

FILTER /

f.) 1100o

.oo~

700500' 300

I

HP 9 8 1 6 _ _ _

40

80

120

160

200 240

Bottom T~l~r~ure ( ° C ) Figure 1

Figure 2.

Top & Bottom Temperature

Relationship Line

81 Transient

experiments were conducted over a two minute period

to measure

the response of the thermocouple to unknown temperature at the top and bottom of the cutting insert mounted in a tool holder.

A temperature at the top

of the cutting tool was from contact through a metal rod with a propane torch for

120 seconds and then the heat source was removed.

The corresponding

temperature relationship at the bottom and the top of the tool insert is shown in Figure 2.

Using these relationships, estimation of the temperature

of the cutting t i p were made. A 35mm camera was placed with i t s face.

axis normal to the tool insert rake

The photographic method was used to measure the chip flow angle and

chip shape. DoALL ceramic (DO-80) throw away inserts were used. The insert nose radius was different between SNG432 and SNG433.

The set of tool wear measurements

were obtained at feedrates of 0.006, 0.008 and 0.012 ipr, depth of cut of 1/32 and 3/64 inch, cutting speed 700 to 1400 fpm and material hardness 207 BHN.

The workpiece material comprised 6 inch diameter by 8.2 inch long AISI

4340 steel bar. Cutting Force Results The measurement of cutting forces as well

as detecting total

tool

has been used for tracking tool wear

breakage and microbreakage on the insert

surface or on the cutting edge has been investigated by numerous researchers

300

300'

270

270"

240"

240"

210-

210"

180-

A 180.Q m '~" 150"

150" a)

o I.I.

12o-

120" 0 U,.

90"

[] Feed Force 0 Radial Force Cutting Force

60" 30" 0 0

.~3 .o~ .0~9 .o~2.0lS Flank

Figure 3.

W e a r (in)

90" 60-

[] Feed Force 0 Radial Force z~ Cutting Force

300

0

.oh3 .oh6 .oi~ .o~2 .o1: Flank

Wear

(in)

C u t t i n g Force vs. Flank Wear, L e f t (SNG432), Right (SNG433)

82 [3-5].

It

is

generally concluded that the tool f a i l u r e is well detected

by measuring the component as well

ratio

of

the feed force component to

the radial

force

as the changes of the individual force components.

general, a change in

In

the level of the cutting force components indicates

the fracture or chipping of a cutting tool.

The tool force component increases

during machining both due to an increase in the feed rate and tool wear. Measuring the increase in this

force component, the increase in tool wear

during the process can be determined at a particular feed rate, and a criterion for tool wear can be applied. Figure 3 shows the force component values against flank wear width of tools at a cutting speed of 1300 fpm, feed rate of 0.008 and depth of cut of 3/64 inch.

The observation of the tool force aspect of wear is that the

positive slope of

the force measurement is

tool wear progression.

a direct indication of steady

The detail work of the slope and deviation of the

force measurement are planned for a future study.

Figure 3 shows cutting

forces vary only s l i g h t l y with flank wear. Toshiaki Ohtani, Kenryo Fujise and Hiroshi

Yokogawa have indicated Fr/Ff

(radial force/feed force) increased l i n e a r l y with respect to tool flank wear. Our results shown in Figure 4 indicate that the relationship was not a linear function.

Thus, we observed that

the

force

ratio

was not

a

reliable

measurement of the tool wear.

Fr: radial force, Ff: feed force, Fc: cutting force

Fr: radial force, Ff: feed force, Fc: cutting force

[] Fr/Ff 0 Fc/Ff Z~ FrlFc

2.5'

o

It.

0

LL

IJ,.

2..

U.

IJ. o 14.

1.5-

I.I.

2.-

U.

13 Fr/Ff

0 Fc/Ff Fr/Fc

1.5q,,,,

,#.,.

I.i. "~ ,t

~

2.5"

c

1-

I,.1.

,~

Z,,,,

.5-

.5 .0

l-

M.

.0

o

.oh .o 6 .o 9 .o 2 .o15 Flank Wear

Figure 4.

(in)

0

.0~13 .006 .0()9 .0~12 .015 Flank Wear

(in)

Force Ratio vs. Flank Near, Left (5NG432), Right (5NG433)

83 Temperature Rise Results When high strength materials

are machined, the temperature rises with

the speed and the corresponding tool strength decreases, leading to a faster wear and f a i l u r e . considered to

Thus, temperature rise during machining has always been

be very important.

quantitatively u n t i l concentrated reliable

on measuring cutting

methods of

temperature [6].

measuring the

interaction zone. Therefore, is

T o o l temperatures could not be treated

the 1920s. Much of the work in laboratories has been

frequently used to

in

the

no simple chip/tool

an analytical approach has been developed and

obtain

respect to metal cutting.

There are

temperature f i e l d

important

thermal dependent properties with

For our experiments, a thermocouple was embedded

in the shim and was used to measure i n d i r e c t l y the cutting t i p temperature. The results were in good agreement with published analytical methods. This thermocouple technique is r e l a t i v e l y simple to apply. Figure 5 shows the actual cutting temperature diagram. Using calibration relationship, the top tool

t i p was estimated to be 1100 degree centigrade. TIME

DOMAIN

PLOT

Slope Value = 2.778E-05 Avg. Value = 0.0033 Data Points = 5999 Standard Dev. = 1.589E-03 Max. Value = 0.0055 Volts

o -.4~ -.8~ -1

0

1;

3'2

48

64

Seconds

Figure 5.

Temperature Signals

The relationship of maximum cutting temperature with respect to tool is shown in Figure 6 and 7.

wear

As shown in these figures, the variations of

the values against tool wear were unpredictable.

The present testing shows

that a small nose radius insert caused higher temperatures than big nose radius insert. Chip Direction Results A wide variety of chip control the years

[7-8].

techniques have been investigated over

Increasing cutting

speeds and cutting temperature lead

to chips which have inconvenient chip form.

These chips may entangle with

A

(.)

1200'

1000"

1150-

950"

0

"" 1100" .=

0

900"

"-"

850"

0

1050-

.= :3

Boo. Q.

E Q

I--

[] Flank Wear 0 Crater Wear

900 I 0

, .006

, .012

TOOl W e a r F i g u r e 6.

l/

750'

[] Flank Wear o Crater Wear

700.

.018 .024

.03

the tooling or the workpiece,

,

,

.01

.02

Tool

Wear

0

(in) Figure

SN8432 Temperature v s . Wear

f

7.

.(}3

.014

,05

(in)

SNG433 Temperature

vs. Wear

damaging the surface f i n i s h of the part or

interfering with the workpiece or tooling changes. As C. Y. Jiang, Y. Z. Zhang(1), Z. J. Chi have observed, depth of cut, feed rate, corner radius, cutting edge angle, normal rake angle and cutting edge inclination have influences on chip flow angle.

The chip flow angle

is defined as the angle between the chip flow direction and the normal to the main cutting edge on the plane of the rake face.

C h i p flow angle was

calculated as follows:J8] v=O.208,d -0.744 , f 0.424 *(r+0.45) 0.682 *(x-16) 1.28 *0.988 n + 0.62s where v:

chip flow angle, degree

d:

depth of cut, mm

f:

feed rate, mm/rev

r:

corner radius, mm

x:

cutting edge angle, degree

n:

normal rake angle, degree

s:

cutting edge i n c l i n a t i o n , degree

A sharp new tool insert has the correct nose radius.

A worn tool insert

does not have a good nose radius and i t frequently becomes larger.

The formula

stated above showed that the chip flow angle changed when nose radius changed. Keeping the same cutting condition can s t i l l cause d i f f e r e n t chip shapes and chip flow angles.

One apparent reason is tool

the photograph of the chip flow.

wear.

Figure 8 shows

Figure 9 and 10 show the chip flow angle

85

Figure 8 versus the flank and crater wear width. Figure 11.

The chipped tool is shown in the

When our insert becomes chipped on the surface i t behaves similar

to the new tool insert with chip-breaking groove. the chip from tangled type to short comma type.

It

changed the shape of

Stabler also observed that

a crater on the insert increases the tendency of the chip to curl observation w i l l

be very useful

[9].

This

when one uses computer vision to mornitor

tool wear. When the short comma chip occures, the tangent of the chip at the cutting edge should be considered as the chip flow angle.

It

is sometimes hard to

measure the correct chip flow angles for these cases.

3o

32

t

<

G)

m 28i ,<

~ 24 o It. ," 0

30-

G)

o

c

Q.

22 [] Flank Wear 0 CraterWear 20

0

.006 .012

.018 .024

.03

T o o l Wear (in) Figure 9.

26"

,'r

SNG432 Chip Flow Angle vs. ~ear

T(J

l

J

24" [] Flank Wear J 0 Crater Wear

l

22' 0

~1

~2

Tool

Wear

Figure 10.

~3

o4

(in)

SNG433 Chip Flow Angle vs. Wear

os

86

Figure 11 Acoustic Emission Results The acoustic emission technique has been successfully used in the fields of nondestructive testing, material testing, and the estimation of structural integrity.

Acoustic emission is the elastic stress wave generated as a result

of the rapid release of strain energy within a solid material in association with the deformation, fracture or phase change of the material.

I t is reported

that high level of acoustic emission signals are observed when the f a i l u r e of the cutting tool takes place.

The level of acoustic emission signal

closely related to the size of the fracture surface of the tool

is

[10-11].

Acoustic emission analysis has been recently applied to the study of tool wear and tool fracture. detection technique is

One major advantage of an acoustic emission based that the acoustic emission signal is

generated and

measured during the tool fracture event and is very sensitive to the fracture characteristics [5, 12-15]. The sensing technology of developing.

the

tool

failure

in

metalcutting is

still

The gap between the research in the laboratories and the practical

application of the technology seems to be quite large at present.

Toshimichi

Moriwaki has sta~ed, simple and yet r e l i a b l e sensors and sensing technologies must be developed so that the past achievements in research and development a c t i v i t i e s are u t i l i z e d widely in

practice

[3].

Justification of

sensor

i n s t a l l a t i o n should be made by evaluating economic considerations, weighing the cost of the sensors against expected economic returns. Parallel developments on another project in the use of acoustic emission at Tennessee Technological University have led to miniature transducers that can f i t

in

the v i c i n i t y of

a cutting

tool

insert.

A miniature acoustic

emission pin transducer was located as close to the tool-work interaction zone as possible.

The acoustic emission signal

is

amplified and f i l t e r e d

87 TIME

DOMAIN

PLOT

Slope Value = 1.027E-03 Avg. Value = 0.7936

Data Points = 5999 Standard Dev. = 1.363 Max. Value = 4.2048 Volts

°.

1 , 0

1'2

2'4

~

4~

Seconds

Figure 12. by a f i l t e r

AE Signals

with cut-off frequencies of 40KHz and 1MHz for the high pass

and low pass f i l t e r i n g

respectively.

The total

system cost is

less than

two hundred dollars. Some of the signature analysis methods used to analyze acoustic emission signal

are

count and count

rate,

amplitude

spectral analysis and RMS signal analysis.

distribution

analysis,

power

We have settled on the true RMS

signal analysis method. Cutting test of AISI 4340 steel

using DO-80 SNG433 was conducted, with

cutting speed 1300 fpm, feed rate 0.008 and depth of cut 3/64 inch.

Figure

12 to 14 display the typical acoustic emission signal under this metal cutting condition.

Figure 12 shows the raw total time domain acoustic emission signal. TIME

,,,,,,

~

~,

r

o -5 0

.0002

PLOT

•

.0004

--~

Bt

Seconds

_

.o.~

.o~o2

.o~o4

1"2

Seconds Figure 13.

DOMAIN

Slope Value = -7.081 E-02 Avg. Value = 1.4579 Data Points = 5999 Standard Dev. = 5.953E-01 Max. Value = 3.429 Volts

AE Signals & True RMS Values

2'4

3'6

48

Seconds Figure 14.

AE True ~

Signals

$8 5E-5-

V^2/Hz

0

-5E-5. o

.2;=5 .4~=5 .8~5 .sEs ~,~s ~.2'ES Hertz

~>--5

V^2/Hz o

-5Eo5t

0

.2E5 .4E5 .6E5 .81=5 1E5' 1.2E5 Hertz

5E'5 1

t

-5E-5

o

:;=5 .4;=s o6,;s .8~s Hertz

l~s 1.2'E5

Figure 15. AE Power Spectrum of Before, At, After Tool Chipping

89 Top of the Figure 13 shows short zoomed region of the time domain results. Bottom of the Figure 13 shows the corresponding true RMS values. amplitude of the signals shows high energy release rate.

The high

The abrupt change

in the geometry of the cutting tool due to fracture results in some change in the amplitude of these signals. true root-mean-square results.

Figure 14 displays the total time domain

The observation

of the RMS values is that

the average and deviation of the RMS measurement is a direct indication of tool wear [15].

Figure 15(a), 15(b), 15(c) show the power spectrum of acoustic

emission signals generated before,

at,

and after

the tool

chipping.

The

dominant signal lies in the frequency range of 15-85 KHz. The high amplitude of the power spectrum at chipping is seen in this frequency range. Figure 16 displays the mean RMS acoustic emission versus tool

flank wear,

Note

that the higher frequency components remain as a signature after the tool chipping has occurred in 15(c). E. Kannatey-Asibu, Jr. distribution

for

statistically. wear.

the Figure

and D. A. Dornfeld have proposed to assume Beta

RMS acoustic emission signal

to

analyze tool

wear

17 and 18 show the skew and kurtosis versus flank

The results were in agreement with E. Kannatey-Asibu, Jr.

and D. A.

Dornfeld [15].

2.5,

[] FlankWear ,~ ,38

>

~" 2.2

.36-

:~ o= >

U)

1.9

09 n-

.34 -

1.6" G)

l=.

I-

.32 [] FlankWear 0 CraterWear .3

,

,

.0 6 .0 2 .018 .024 .03 T o o l Wear (in) Figure 16.

I-

1,3"

1

0

.;1 Tool Wear (in)

AE RHS Values vs. Wear, L e f t (SNG432), Right (SN6433)

.os

90

-0.05 b~

.15~

0

G) Or) .1"

-0.1-

.05, -0.15

.o

.o63

0

.ob .o;2.0lS

Flank

Wear

.o 9 .o;2 .ol.

Flank

(in) Figure 18.

Skew vs. Flank Wear

Figure 17.

.o63

Wear

(in)

Kurtosis vs. Flank Wear

SUMMARY AND CONCLUSIONS The true RMS energy of acoustic emission signal decrease.

level may increase or

The variation depends upon the characteristics of insert fracture

or breakage.

The acoustic emission signal is less sensitive to the change

of the insert geometry since the fracture area is the significant fracture characteristic.

The insert

fracture

may cause rise

or

drop in

force,

temperature and chip flow angle, the fracture event always generates a burst of acoustic emission signal.

The results at different measurement methods

due to tool wear were shown in different figures.

Acoustic emission true

RMS signal showed more significance than the others.

More research is needed

and is continuing at Tennessee Technological University to find a general rule for

the acoustic

emission signature

analysis method as a measure of

tool insert wear. This experimental setup for acoustic emission signal sensing is very cheap and simple.

I t w i l l lead to an economical and competitive sensor for untended

metal cutting, when the physics of the acoustic emission sources is f i n a l l y understood. ACKNOWLEDGMENTS The authors wish to acknowledge the help of Mr. Bradley K. Herman for the design of the acoustic emission signal f i l t e r c i r c u i t , and the financial support from the Center for Manufacturing Research at Tennessee Technological University for conducting this research work.

91 REFERENCES i 2 3 4 5

6 7 8 9 i0 11 12 13 14

15

Charles R. Brown, "Monitor Tools as They Cut," Machine and Tool Blue Book, February 1988. A. K. Chattopadhyay and A. B. Chattopadhyay, "Wear Characteristics of Ceramic Cutting Tools in Machining Steel," Wear, 93 (1984), pp. 347-359. Toshimichi Moriwaki, "Sensing and Prediction of Cutting Tool F a i l u r e , " B u l l . Japan Soc. of Prec. Engg., Vol. 18, No. 2 (June 1984), pp. 90-96. Toshiaki Ohtani, Kenryo Fujise and Hiroshi Yokogawa, "Cutting Force Characteristics in Machining of Hardened Steel," B u l l . Japan Soc. of Prec. Engg., VoI. 20, No. 2 (June 1986), pp. 127-129. Erdal Emel and E l i j a h Kannatey-Asibu, J r . , "Acoustic Emission and Force Sensor Fusion f o r Mornitoring the Cutting Process," Third International Conference on Computer-Aided Production Engineering, U n i v e r s i t y of Michigan, Ann Arbor, June 1988, pp. 171-187. Milton C. Shaw, "Metal Cutting P r i n c i p l e s , " Oxford Science Publications, 1984. W. K l u f t , W. Konig, C. A. van L u t t e r v e l t , K. Nakayama and A. J. Pekelharing, "Present Knowledge of Chip Control," Annals of the CIRP, Vol. 28(2), 1979, pp. 441-455. C. Y. Jiang, Y. Z. Zhang and Z. J. Chi, "Experimental Research of the Chip Flow Direction and i t s Application to the Chip Control," Annals of the ClRP, Vol. 33, January 1984, pp. 81-84. G. V. Stabler, "The Fundamental Geometry of Cutting Tools," Proc. I. Mech. E., Vol. 165, 1951, pp. 14-21. D. A. Dornfeld and E. Kannatey-Asibu, "Acoustic Emission During Orthogonal Metal C u t t i n g , " I n t . J. Mech. S c i . , Vol. 22, 1980, pp. 285-296. E. Kannatey-Asibu, Jr. and D. A. Dornfeld, "Quantitative Relationships f o r Acoustic Emission from Orthogonal Metal C u t t i n g , " Transactions of the ASME, Vol. 103, August 1981, pp. 330-340. M. S. Lan and D. A. Dornfeld, "In-Process Tool Fracture Detection," Transactions of the ASME, Vol. 106, A p r i l 1984, pp. 111-118. Shuhei Aida, I c h i r o Inasaki and S h i n i c h i r o Fukuoka, "Monitoring System f o r Cutting Tool Failure Using Acoustic Emission Sensor," Nihon Kikaigakkai Rombunsheu, Vol. 52(481), September 1986, pp. 2563-2569. Yoshiaki Kakino, Hirotsugu Suizu, Michiaki Hashitani, Takuro Yamada, Hajime Yoshioka and Akihiko Fujiwara, "In-Process Detection of Thermal Crack of Cutting Tool by Making Use of Acoustic Emission," B u l l . Japan Soc. of Prec. Engg., Vol. 17, No. 4 (1983), pp. 241-246. E. Kannatey-Asibu, Jr. and D. A. Dornfeld, "A Study of Tool Wear Using S t a t i s t i c a l Analysis of Metal Cutting Acoustic Emission," Wear, 76 (1982), pp. 247-261.