Pergamon Int. J. Mich. Tools Manufact. Vol. 37, No. 10, pp. 1475-1484, 1997

© 1997 Elsevier Scicr~e Ltd Pnnted in Great Britain. All fights reserved

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PII: S0890-6955(96)00091-0

C A L I B R A T E D M E T H O D F O R U L T R A S O N I C O N - L I N E

M O N I T O R I N G O F G R A D U A L W E A R D U R I N G T U R N I N G

O P E R A T I O N S

NIDAL H. ABU-ZAHRAt and TAYSIR H. NAYFEHI":I:

(Received 15 December 1995; in final form 3 October 1996)

Abstract---On-line tool condition monitoring is essential for modern machining systems, especially in the case of precision and unmanned machining. Knowledge of the condition and the expected life of the tool are very important inputs for determining the optimal machining parameters. Previous efforts have indicated that ultrasonic gaging methods can be used to directly measure in-process gradual tool wear during turning operations. Good correlation was shown between the level of gradual wear and the ultrasonic signals. However, the correlation was tool dependent. This was mainly attributed to variations in the tool materials and inconsistent coupling of the transducer to the tools. This paper describes a robust method for on-line gradual wear monitoring using normalized ultrasonic signals. A consistent calibration mark, cut in the lower corner of the tool nose, is used to generate a calibration echo. The calibration echo is affected by the same variations as that of the gradual wear and is used to normalize the nose and flank echoes. Experiments under various cutting conditions showed that the gradual wear measurements can be made tool independent by normalizing the measurements with the calibration mark. In addition, the variations in the signals which were previously reported are also eliminated. © 1997 Elsevier Science Ltd

I. INTRODUCTION

Appropriate "in process" tool-wear monitoring is becoming more important, especially in modem machining systems in which many computerized numerically controlled machines are operated in flexible and unmanned systems. Knowledge of the condition of the tool in-process is of crucial importance to optimizing the operation through adaptive control [1, 2]. To this end, several monitoring methods have been developed during the last few decades by many researchers. These methods may be classified into two groups: direct and indirect [3]. Direct methods are based upon direct measurements of the worn area of the tool using optical sensors [4, 5], vision systems [6], micro probes [7], etc. These methods have the advantage of high measurement accuracy, but cannot be easily adopted for on-line applications mainly because of the interruption of coolant and chips [8, 9].

Various indirect methods have also been developed in which the state of the wear is estimated from measurable parameters such as the cutting forces [10-14], vibration [15], acoustic emission [16-24], cutting temperature and surface roughness [25]. However, few reliable indirect methods have yet been established for industrial use. This is mainly due to the complexity of the machining process and the uncertainty in the correlation between the process parameters and tool wear [26, 27].

Gradual wear develops on the tool in two locations: the major flank and the face. Flank wear mainly results from the cutting action and to a lesser extent is due to friction between the newly machined surface and the tool flank. Wear on the face of the tool is termed crater wear and is located at the tool-chip interface. Small amounts of material are gradu- ally removed from the tool face in part due to the friction generated between the flowing chips and the tool face. In time, a crater is generated in the face of the tool, hence the name crater wear. In general and under normal cutting conditions, flank wear develops much faster than crater wear. As such, tools tend to fail due to flank wear long before

flndustrial Engineering Department/Advanced Manufacturing Center, Cleveland State University, Euclid Avenue at East 24th Street, Cleveland, OH 44115, U.S.A. :[:Author to whom correspondence should be addressed.

1475

1476 N. H, Abu-Zahral" and T. H. Nayteht¢.

crater wear becomes significant. Therefore, flank wear (wear land height) is the more common measure of tool life [28] under normal cutting conditions.

An ultrasonic system for on-line measuring of gradual wear of the flank during turning operations was developed by Nayfeh [29, 30]. The method relies on inducing ultrasonic waves in the tool which propagate the length of the tool and are reflected by the nose and flanks. The amount of energy reflected is a function of several parameters, among which are the areas and the orientation of the reflecting surfaces. It was shown that linear corre- lation exists between the level of the reflected ultrasonic energy and the wear land height. However, for every tool tested the correlation was shown to be tool dependent. This in part was attributed to the variations in the tools material which causes changes in the acoustic response of the individual tools. Other potential sources of measurement errors were observed to be the repeatability of the acoustic coupling between the transducers and tools, transducer efficiency and changes in the overall energy transmitted from the transducer to the tool caused by changes in the tool temperature.

A normalization procedure is presented in this work which eliminates the variations previously encountered. The ultrasonic signal is normalized to an intrinsic marker cut on each tool. This procedure greatly reduces the effects of tool material variations and elimin- ates other acoustic property changes resulting from changes in the tool and transducer temperature. In addition, the inconsistencies resulting from the solid coupling of the trans- ducer to the tool are eliminated by using a fluid delay line.

2. EXPERIMENTAL SETUP

The experimental setup is shown schematically in Fig. 1. The hardware used is available commercially; consequently certain limitations are imposed on the capabilities and per- formance of the experimental setup. The shape of the ultrasonic transducers selected for this research dictated the cutting inserts' geometry. The transducer used is a Panametrics (V-203-RM) flat face contact transducer operating at frequency of l0 MHz. The active element is 3.175 mm in diameter and has an overall cylindrical package geometry of 4.76 mm in diameter. Hydraulic fluid (300 hydraulic I.C. oil) is pumped through the tool holder to serve as the coupling medium of the transducer to the tool. In addition, slow circulation of the fluid cools the transducer during cutting.

An ultrasonic pulser-receiver operated in burst mode, i.e. short duration spikes, activates the transducer. A 100 MHz digitizing oscilloscope (Tektronics TDS 420) is used to digitize the reflected analog signal up to 15 bit resolution. The sampling rate in real-time mode is 100 MHz; however, a 50 GHz sampling rate is available in equivalent time-sampling mode (ETS). A general purpose interface board (IEEE 488 GPIB) is used to communicate between the oscilloscope buffer and the data acquisition computer, in which the data is also processed.

10 MHz <~Tronsducer

r~

Tool Insert (TNU334)

COmp uter I 2q EEE 488 GPIB|

to Processor 9

ULTRASONIC Pulser/Receiver

Goln 40 dB

TDS420 Olgitol~ Oscilloscope J 15 Bit A/D l

100MHz J Ir, OMS/s j

Computer ~1 1") ONIX-AD100 / 8 Bit A/O /

I OOMHz J

Fig. I. Schematic representation of the system.

On-line monitoring of wear during turning operations 1477

The tool holder depicted in Fig. 2 is specifically designed to accommodate the transducer assembly. The tool clamp is shifted to the right side of the normal position in order to accommodate a hole drilled through the entire length of the tool holder. The transducer and a solid handle are partially inserted into the hole, leaving a 5 mm gap between the face of the transducer and the back of the tool. Hydraulic fluid is slowly circulated into the cavity between the transducer and the tool. The fluid serves to couple the ultrasonic waves to the tool and cools the transducer. The direction of the fluid feed and drain is from the bottom to the top in order to reduce the presence of air bubbles and maintain a hydrostatic head for better transducer-tool coupling.

The tools used in the experiments are (TNU334 KC910) negative triangular carbide inserts, 4.76 mm thick and 1.587 mm nose radius. The tool holder geometry provides a 6 ° nose and main cutting edge tool clearance along with 6 ° negative back and side rake angles. A calibration mark in the form of a square cut at the lower comer of the nose is machined with an EDM on all tools, as shown in Fig. 3. The dimensions of the cut are 1.27 mm deep and 1.27 mm thick.

3. THE ULTRASONIC METHODOLOGY

Gradual tool wear manifests itself in two locations, which are the primary and secondary flanks and the crater area. The current transducer-tool orientation only allows the detection of the flank wear. Detecting the crater wear requires a different transducer-tool orientation.

In the present configuration, a discrete ultrasonic wave in the form of a pulse is gener-

Tool Holder Clamp - - ~

Calibration - J Mark Transducer J

Element

2 ~ - ~ OIL OUT

I _

Handle Cable

Fig. 2. Tool holder and transducer assembly.

Colibrotion Mark

, Flonk Signol Nose & Mork Sgnols

Nose & Flank Signals

Mark Signal

Fig. 3. Geometry of the calibration mark.

1478 N.H. Abu-Zahrat and T. H. Nayfeh?:~

ated by the transducer in the transmit mode. The wave travels through the fluid cavity before reaching the back of the tool. Part of the energy of the wave is transmitted into the tool and the rest is reflected back. The wave which is transmitted into the tool travels through the length of the tool and is reflected back from the nose, flanks and the mark, as shown in Fig. 3. The reflected wave is then received by the same transducer which is now switched to listening mode.

The contents of the wave returning from the nose and the flanks contain three wave packets as illustrated by Fig. 4. The first is the direct reflection off the calibration mark which is consistent among all the tools used. The second part of the signal is the reflection off the edge nose of the tool and the surrounding areas of the flanks. The extent of the contribution of the flanks at the 10 MHz frequency is limited to the area encompassed by a distance of 0.254 mm from the absolute nose. This area behaves as a flat reflector at the current ultrasonic wave's frequency. The third part is an internally reflected wave, which corresponds to the energy that strikes the flanks at the point of the start of the nose curvature. The wave, which strikes either flank, is internally reflected to the opposite flank, which is then reflected back along a different path to the transducer. In the case of the third reflection, the travel path is the longest compared to the other two signals, which is manifested by the longest time of flight (TOF), shown in Fig. 4, where the calibration signal travels the shortest distance, and hence appears the earliest in time.

In the course of cutting and due to wear, a fiat spot begins to develop at the tool nose and the flanks. This change in the geometry of the tool serves to change the total amount of the reflected ultrasonic energy. In this case the flat area is a more favorable reflector. As such, the total amount of reflected ultrasonic energy increases with gradual wear for both components of the wave. In the ideal case the increase in the amount of the reflected energy obeys the square law. In the case of turning the principle does not strictly hold since the reflecting surfaces are marred and are at off-angles from the normal to the trans- ducer, thus resulting in complex wave interactions. In spite of the complex interactions, an overall increase in the energy content develops with wear.

4. ULTRASONIC SIGNAL CALIBRATION

Previous efforts [30] at measuring gradual wear with ultrasonics did not incorporate means for reducing the signal variations inherent in such a measurement. As such, although a correlation was found to exist between the actual level of gradual wear and the net change in the absolute value of the reflected energy, the correlation was shown to be tool dependent. This was attributed to variations in the tool materials, transducer-tool coupling and transducer efficiency. In addition, the measurements had to be made at a nominal tool temperature to eliminate inconsistent temperature effects on the acquired signals.

Flank Signal Nose Signal

Mark Signal

Fig. 4. Wave form of the tool's echo.

On-line monitoring of wear during turning operations 1479

The amount of energy transferred from the transducer to the tool is a function of the acoustic impedance mismatch between the transducer and the tool. Furthermore, the tool's acoustic impedance changes with variations in its temperature, i.e. the signal level changes as a result of the cutting temperature. Results of static heating and cooling cycles of the tool nose and crater area indicate that in general the temperature will be uniformly distributed in the tool after a few seconds of an idle state. A propane torch was used to locally heat the tool nose and crater area. The tool was heated up to 600 °F while echo signals were recorded at a speed of 2.2 s wave -I. The echo signals were divided into two main parts: the calibration component and the wear component. The ratio between the integrals of the absolute energy of the two signal components was plotted at different temperatures, and the relationship was found to be constant at all temperatures as shown in Fig. 5. Thus, normalizing the nose and flank signals to the calibration mark eliminates the variations in the measurement due to temperature.

Additional signal variations were encountered while machining with low-grade carbide tools. This was attributed to the fact that these tools were not fully sintered during the manufacturing phase; therefore exposing the tools to the high cutting-temperature results in secondary sintering. The change in the state of the tool during cutting is manifested by changes in the amplitude of the ultrasonic signal.

High- and low-grade carbide tools were tested against the effect of sintering on the acoustic properties of the cutting tool material. The high grade carbide tools tested were Kennametal (TNU334 KC910).

Both tool sets were heated using a propane torch and cooled with cutting fluid repeat- edly. Ultrasonic echo signals for both set of tools were recorded after each test at close temperatures. The ultrasonic signals were compared for each tool with the original signal. It was observed that there exists a significant change in the echo signals for the low-grade carbide tools, as shown in Fig. 6, compared to negligible effects on the high-grade tools shown in Fig. 7.

Thus, all the following tests were conducted with the high-grade tools (TNU334 KC910) in order to eliminate any secondary sintering effect.

5. EXPERIMENTAL PROCEDURE

Gradual wear of the nose and flanks of the tool is a comparatively slow process, on the order of minutes or perhaps hours in some cases. Determining gradual wear requires

2.2

"~ 2

1.8

._~ ~ 1.6

,'r

1.4 o

Z

.o_ 1.2 ca

C ~ -.~ ~ ~, 0 ~ ~ ~ ~ ~ ~

100 200 300 400 500

Tool Tempcraturt (F)

Fig. 5. Temperature calibration of the echo signal.

600 700

1480 N.H. Abu-Zahmt and T. H. Nayfeht~t

Variation in the echo signals duc to thermal cycles

Fig. 6. Heat effect on low-grade carbide tools.

A-A AA Fig. 7. Heat effect on high-grade carbide tools.

A A

comparing the integral of the absolute value of the wave form to that of the new tool (base wave-form).

Several tool wear tests were conducted to evaluate gradual wear measurement of the nose and the flanks. The work material and the cutting parameters were:

work material---untreated H13 tool steel (Rockwell hardness 52) bars; bar geometry--12.7 cm in diameter, 50.8 cm in length; cutting speed--106 m min- ' ; feed rate---0.356 mm rev- '; depth of cut-- l .016 mm.

Cutting fluid was applied during all machining operations. In all cases the wear varied randomly at different measurement points, from uniform to somewhat irregular. Tools

On-line monitoring of wear during turning operations 1481

were tested beyond the standard tool life to verify the behavior of the correlation. Maximum wear land height VB was in the range of 0.5-0.76 mm.

No effort was made to isolate the nose from flank wear, since they are directly related to each other. In addition, isolating the individual wear contributions to the individual ultrasonic echoes is not possible at frequency signals of 10 MHz. The nominal wave length in carbide at this frequency is 0.5 ram, i.e. no area less than this can be individually isolated. However, the sum of both parts (i.e. flank and nose) is believed to be representa- tive of the gradual wear.

As was previously noted, the amount of energy reflected back from the nose and flanks of the tool is a function of the acoustic impedance of the medium which is directly in contact with it. In addition, it is also a function of the intimacy of the contact.

During turning operations, and while the tool is engaged, the acoustic impedance of the workpiece determines in part the amount of reflected energy. In addition, the intimacy of the contact between the tool and the workpiece is also changing due to small or large levels of vibration and/or chatter. Therefore, in order to make absolute measurements of the amount of reflected ultrasonic energy, the tool must be disengaged from the work for approximately 1/1000 of a second, during which time a wave form can be acquired and fully processed.

However, additional dead time is needed (2-3 s) in order to ensure that the tool tempera- ture has equalized as was previously discussed.

The gradual wear measurement procedure was conducted as follows:

(1) A new tool insert was mounted on the tool holder and an ultrasonic wave form was recorded. The recorded wave was considered as a base (reference) wave--no tool wear was developed at this stage. (2) The tool was then engaged in cutting action for 2-3 min before it was disengaged for the first wear measurement. (3) After disengaging the tool, the tool was left to cool for 2-3 s, and an ultrasonic wave form was recorded. The tool holder was then taken off the lathe, and the wear land height and width were measured with a tool maker's microscope while the tool remained in the holder. (4) The tool holder was then mounted back, and the tool was engaged in cutting for 20--100 s. Steps 3 and 4 were repeated until the wear land height reached 0.5-0.76 ram.

The integrals of the absolute energy of the two parts of the signal were calculated for each recorded signal. The first part was obtained by summing the absolute Y-values of the data points in the "nose" and "flank" portions of the ultrasonic waves as shown in Fig. 4, for each recorded signal. This part was referred to as the "wear value".

The second part was obtained by summing the absolute Y-values of the data points in the "mark" portion of the ultrasonic waves, shown in Fig. 4, for each recorded signal. This part was referred to as the "mark value".

The "wear value" was then divided by the "mark value" to normalize for the temperature and material effects. This ratio was referred to as the "calibrated wear value". The "cali- brated wear value" was calculated for each recorded signal. The absolute difference between each successive values was then calculated and the cumulative difference was plotted against the measured value of the flank wear (considering VBm~).

Since the measurements were taken at random intervals, the total number of measure- ment points per tool varied from 9 to 13 points. All wave forms were recorded at 25 GHz sampling rate and an equivalent 15 bit A/D resolution.

The above procedure indicates that all wear measurements were conducted off-line, since the tool was pulled off the work piece. However, the time required to stabilize the temperature of the cutting tool, at the tool nose, does not exceed few seconds, and the wear measurement can still be performed without removing the tool holder. Therefore this method is still very valuable, since the cutting operation will be interrupted for few seconds only (2-3 s) during hours of machining.

1482 N. H. Abu-Zahra't and T. H. Nayfeht~:

6. RESULTS AND DISCUSSION

Results of the calibrated wear measurements from the seven tool life tests conducted are presented in Fig. 8, in which the ultrasonic wear measurements are plotted versus the corresponding bench top optical measurements. The results indicate that for all tools tested there is an overall correlation between the two types of measurements which is not tool dependent. The individual tool correlation curves (see Fig. 9) previously developed by Nayfeh [30] collapsed into a single curve with moderate scatter (see Fig. 10). It is clear that, in the current results, all the tested tools have exhibited consistent correlation between the ultrasonic measurements and the actual measured wear land height.

Although the gradual wear curves show some scatter among the data points, it is believed that the scatter is mainly due to the experimental procedure. The optical tool wear measurements were acquired by taking the tool holder off the machine and measuring the wear land height under the microscope. It is difficult to assure that the tool holder is retumed exactly to its original place. Thus, any change in the tool holder's orientation will cause a change in the geometry of the progressive wear during future cuts. It is believed that the ultrasonic wear measurements will have much less scatter if the optical measurements are conducted while the tool holder remains in place.

0.45

0.4

~= 0.25

0.15

0.!

0 , I J i

0 5 10 15 20 25 30 35

Wear l..aad Height (mils)

Fig. 8. Correlation curves of gradual wear measurement.

0.45

I JL Tool1 .-B.-Tool2 -..e--Tool3 --e--Tool4 t Tool5 -.N--Tool6J

0.4

0.35

i 03

0.2 015

01 t 0.05

OI 0 2 4 6 8 10 12 14

M l e m ~ Wmr Land Hqht( ln 1/1000")

Fig. 9. Previous results obtained by Nayfeh [30].

16 18

On-line monitoring of wear during turning operations 1483

0.4

E

~E

0.35

0.3

0.25

0.2

0.15

0.1

0.05

0

y = -O.O002x 2 + O.OlSx O

R 2 = 0 . 9 7 8 7 ~

o o

o o

1 i I i

0 5 10 15 20 25 30 wear Land Height (mils)

Fig. 10. Quadratic regression of the correlation curve.

It is apparent that most of the variations in the ultrasonic signal encountered in the previous efforts have been reduced or eliminated. The fluid transducer-tool coupling solved many of the problems previously encountered. Circulating the hydraulic fluid cools the transducer during cutting operation and reduces the noise level by attenuating both the fired and the reflected echo signals. In addition, the fluid coupling insures intimate full contact between the transducer and the tool, which is difficult to achieve with solid coupling.

Finally, the machined calibration marks on the tools provide total internal normalization of the measurement. Variations due to tool material discrepancy, tool temperature and others caused by the mechanics and/or the electronics of the setup are effectively elimin- ated.

7. CONCLUSIONS

The calibrated ultrasonic method has shown that there exists an excellent correlation between the ultrasonic measurement of the gradual wear and the true state of wear. The fluid coupling method had successfully eliminated the inconsistencies resulting from the use of solid delay lines. In addition, variation in the tool 's temperature and its effect on the ultrasonic echo signal was eliminated by a uniformly machined calibration mark at all tested tools. It was also shown that secondary sintering of the inserts may take place during cutting at high speeds, especially for low-grade carbide tools.

Application of the current system is limited to turning operations with negative inserts and the ultrasonic transducer used in this work. However, in order for the system to be utilized commercially, a more versatile tool-transducer coupling system needs to be developed to accommodate various tool holder designs and insert geometry.

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1484 N. H. Atm-Zahra1" and T. H. Nayfeht:l:

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