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Journal of Materials Processing Technology, 29 (1992) 3-13 3 Elsevier On-line tool-wear monitoring using a PC K.S. Lee, L.C. Lee and S.C. Teo Department of Mechanical and Production Engineering, National University of Singapore, Kent Ridge Crescent, Singapore 0511 Industrial Summary With the increasing use of computer numerical control, there is a growing need to ensure a reliable tool-wear monitoring system to optimise tool usage or tool wear. Findings have shown a good correlation between the dynamic tangential force and flank wear. This dynamic tangential force, when presented in a frequency spectrum, shows a characteristic peak which corresponds to the resonant frequency of the tool holder. Tracking of this peak during machining reveals a trend whereby the dynamic force increases as flank wear increases, and the dynamic tangential force declines preceding the onset of tool failure. A software program is being develop on a personal computer installed with a Fast Fourier Transform card. Signals collected during the cutting pro- cess are transformed of the FFT card to secure meaningful data. A pre-warning condition can be set in the software program to indicate the onset of tool failure when the dynamic force starts to decline. So far, the progress has been very satisfactory. Nomenclature OL 0 E r n kn E m l b N Back rake angle Side rake angle End relief angle Side relief angle End cutting edge angle Side cutting edge angle Nose radius Natural frequency of the tool holder Mode of vibration Parameter depending on the end condition Young's modulus of elasticity of the tool shank Second moment of area of the tool shank cross-section Mass per unit length of the tool shank Length of tool overhang Gradient of the least-squares line Number of acquired values 0924-0136/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

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Page 1: On-line tool-wear monitoring using a PC

Journal of Materials Processing Technology, 29 (1992) 3-13 3 Elsevier

On-line tool-wear monitoring using a PC

K.S. Lee, L.C. Lee and S.C. Teo Department of Mechanical and Production Engineering, National University of Singapore, Kent Ridge Crescent, Singapore 0511

Industrial Summary

With the increasing use of computer numerical control, there is a growing need to ensure a reliable tool-wear monitoring system to optimise tool usage or tool wear. Findings have shown a good correlation between the dynamic tangential force and flank wear. This dynamic tangential force, when presented in a frequency spectrum, shows a characteristic peak which corresponds to the resonant frequency of the tool holder. Tracking of this peak during machining reveals a trend whereby the dynamic force increases as flank wear increases, and the dynamic tangential force declines preceding the onset of tool failure. A software program is being develop on a personal computer installed with a Fast Fourier Transform card. Signals collected during the cutting pro- cess are transformed of the FFT card to secure meaningful data. A pre-warning condition can be set in the software program to indicate the onset of tool failure when the dynamic force starts to decline. So far, the progress has been very satisfactory.

Nomenclature

OL

0 E

r

n

kn E

m

l b N

Back rake angle Side rake angle End relief angle Side relief angle End cutting edge angle Side cutting edge angle Nose radius Natural frequency of the tool holder Mode of vibration Parameter depending on the end condition Young's modulus of elasticity of the tool shank Second moment of area of the tool shank cross-section Mass per unit length of the tool shank Length of tool overhang Gradient of the least-squares line Number of acquired values

0924-0136/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

Page 2: On-line tool-wear monitoring using a PC

~ X Sum of the acquiredX values Y Sum of the acquired Y values

~ X 2 Sum of the squares of the X values ~XY Sum of the products of the X and Y values VBmax Maximum flank wear

1. Introduct ion

Modern manufacturing equipment is using computer numerical control in- creasingly to aid in machining. However, tool-changing routines still have not changed dramatically. The progress in unmanned machining systems and the increase in use of flexible manufacturing systems have placed a greater demand for an automatic tool-monitoring strategy.

A cutting tool engaged in a machining operation is subjected to wear. Since tool wear leads to the deterioration of workpiece quality and possible damage, there is a need to change the tool in time to avoid catastrophes. Generally, a cutting tool fails either by a gradual and progressive wearing of the cutting edge, or it fails prematurely because of chipping or plastic deformation [1]. Therefore, the tool-wear monitoring system must be sensitive, reliable and respond quickly to both forms of tool failure.

The methods of monitoring tool wear can be classified as either direct or indirect. The direct method generally measures the volumetric loss of a cutting tool material, e.g. touch-trigger probes, optical devices. These methods are un- suitable for on-line tool-wear monitoring because of the need for the cutting process to be interrupted for measuring the tool wear. Furthermore, the detec- tion of premature tool failure is not possible while the tool is cutting. The indirect methods involve the measurements of cutting parameters, e.g. cutting temperature, cutting forces, vibration, acoustic emission. These methods are suitable for on-line tool wear monitoring as they permit uninterrupted cutting to take place and eliminate the problems that occur while using the direct methods.

Many research studies on the indirect methods of tool-wear monitoring have been carried out [2-4]. Some of the methods are less reliable as they lack in sensitivity. The effect caused by external disturbances, friction or energy losses may dilute the magnitude measured. Other methods may not respond fast enough to avoid tool failure. One parameter which appears promising for tool- wear monitoring is the cutting force. Martin [3 ] has claimed that the cutting force is one of the most sensitive methods as it is a direct effect of tool wear. The comparison of the sensitivity of indirect methods is shown in Fig. 1. Stud- ies on the cutting forces have so far been concentrated mainly on the static components. The problem with the static force approach is that the rate of increase with tool wear is rather insignificant.

Recently, some studies have been carried out on the dynamic components

Page 3: On-line tool-wear monitoring using a PC

Tool forces Acoustic emission

Bearing forces

Shaft torque (direct) Vibration

Shaft torque (by measurement of power and speed)

Power Motor current

A

increasing sensitivity

Fig. 1. Sensitivity of indirect methods of tool-wear monitoring (after Ref. [3 ] ).

of the cutting force [5-7]. Lay et al. [6] have reported that the detection of flank wear is possible by monitoring the dynamic component at a particular frequency: the rate of increase of the dynamic component is significantly high. Rao [ 7 ] has represented the relationship of the dynamic force amplitude with the flank wear in term of a wear index.

The present study examines the possibility of using a personal computer to perform on-line tool-failure sensing. Software is being developed currently for use on a personal computer installed with a Fast Fourier Transform (FFT) card which tracks the dynamic cutting force signals. The dynamic force fre- quency-spectrum output from the personal computer is compared with that generated by a spectrum analyser.

2. Experimental procedure

Figure 2 illustrates the experimental set-up. The experiments were con- ducted on a Colchester Mascot 1600 lathe. Intermittent cutting on the work- piece was carried out at 100 mm intervals. The depth of cut was 1.5 mm and the feed rate was 0.25 mm/rev. The workpiece materials used were AISI 4340 (BHN 320) and AISI 1148 (BHN 180) steel. The tool insert used was P30 tungsten-carbide insert of tool signature ~ = 6 °, c~ = 6 °, T = 5 °, fl= 5 °, 8 = 15 °, e = 15 ° and r = 0.8 mm. The cutting forces were measured with a Kistler piezo- electric dynamometer type 9263A. The force signals were recorded on a Nagara magnetic tape recorder, the tape recorded signals being run subsequently on the spectrum analyzer using the Fast Fourier Transform function to obtain the frequency spectrum. The force signals were input also to a 10 MHz 80286 per- sonal computer installed with a FFT card and a maths co-processor. The FFT card is a model 25 digital signal processor board based on the Texas Instru- ments TMS320C25. The board is equipped with a 12-bit 110 kHz analogue/ digital and digital/analogue converter for data acquisition and real-time sig- nal-processing applications. Tool wear was measured using a toolmaker's microscope.

Page 4: On-line tool-wear monitoring using a PC

Fig. 2. Experimental set-up for dynamic force monitoring.

3. Operating principle of the sensing device

During metal cutting, the tool is subjected to vibration, the origin of which can be attributed to the rubbing contact between the tool and the rotating workpiece, or to the shearing action of the chip. The force fluctuation gener- ated in metal cutting is registered by the dynamometer carrying the tool.

A study by Lay et al. [6] on dynamic cutting force concluded that there is no obvious relationship with the flank or crater wear in the time domain. How- ever, a prominent peak was observed when the dynamic cutting force was pre- sented in a frequency spectrum, the resonant frequency remaining notably constant throughout the life of the cutting tool. The amplitude of the resonant frequency, which is the dynamic force, increases as the flank wear increases. On approaching tool failure, the dynamic force exhibits a downward trend. Figure 3 shows the frequency spectra of the dynamic cutting force. A study [9 ] carried out to determine the origin of the peak frequency found it to be related to the natural frequency of the cutting tool.

Theoretically, the natural frequency of a tool-holder set-up can be repre- sented as a simple cantilever beam with a uniform cross-sectional area having a point load at the end.

(k~) 2 E~x fn-- ~ ~ / ~ (Hz)

There are four modes of vibration for a simple cantilever beam. Rao [7 ] has

Page 5: On-line tool-wear monitoring using a PC

20-

0

,

1. 328 mm

o.347 mm 1 o _ b.3~

- "' ' o . , 4 7

100.

o 8(>

60,

4o

0.416 mm

0.386 mm

0.367 mm

VBmax

Frequency (KHz)

Fig. 3. Frequency spectra of the dynamic tangential cutting force.

verified that the first mode, which is the weakest mode with the lowest damp- ing ratio, tends to dominate as the tool wears.

It can be deduced that when the cutting tool is new, the sharp edges of the tool minimise contact between the tool and the workpiece, which results in a small dynamic force. The flank wear-land develops as cutting progresses, which increases the contact area between the tool and the workpiece, resulting in the increase in the amplitude of the dynamic force.

On approaching tool failure, the force decreases because of thermal weak- ening at the edge of the cutting region of the tool. Plastic deformation of the tool material results in a damping effect, causing decrease in the dynamic force. This is one of the wear mechanisms affecting the tool life, identified by Wright [8]. Another contributory factor to the decline in the dynamic force is the corresponding increase in the normal rake angle. This is because the increase in the crater depth towards the later stage of the tool life increases the normal rake angle. Experimental work [9 ] has shown that the dynamic force decreases with an increase in the normal rake angle.

With some knowledge of the dynamic component of the cutting-force pat- tern, a software program using Turbo C language is currently being developed to monitor the dynamic force and give pre-warning of imminent tool failure.

Page 6: On-line tool-wear monitoring using a PC

The personal computer is equipped with a Fast Fourier Transform card to perform the necessary data acquisition and FFT processing.

Figure 4 illustrates briefly the flowchart of the program structure. The pro- gram starts at the RECORD sub-routine that instructs the computer to acquire the analogue input signals and convert them to digital signals at a sampling rate of 20 kHz. The FFT PROCESS sub-routine processes the digital signals into a frequency spectrum, an average of 128 frames of signals being used to smoothen out fluctuation. The TEST CONDITION sub-routine monitors the on- set of tool failure by testing it on two criteria. First, the percentage drop from the maximum amplitude of the dynamic force is calculated. If the percentage drop exceeds an analytically determined threshold value, the first criterion is reached. Second, the gradient of the dynamic force curve is calculated taking

I STnRT 1

] T~=O

'1 I SUBROIIT I HE

RECORD 1

ISUI~ROUTIHF 1 TEST COHI)TH

TI4S : 0 ~ ]'H$~: /

1 RE"TURH )

Fig. 4. Flowchart of the structure of the program.

Page 7: On-line tool-wear monitoring using a PC

several recently acquired values of peak frequency. If the gradient drops below an analytically determined threshold value, the second criterion is reached. The following least-squares equation calculates the gradient of the curve.

b = N ( Y~XY) - ( ~ X ) ( ~ Y) N ( ?:X 2) - ( ~ X ) 2

The pre-warning of tool failure is activated only when both of the criteria are reached.

4. Results and discussion

The 10 MHz personal computer installed with a maths co-processor requires about 7 seconds to acquire the signals at a sampling rate of 20 kHz, and a further 15 seconds to process and average 128 frames of signals. The dynamic tangential force output from the personal computer follows quite closely when compared to the output from the spectrum analyser as shown in Fig. 5. When an average of 64 frames is used, the acquisition and processing time of the personal computer are reduced. However, there are some fluctuations in the results when compared to the spectrum analyser output. With the use of a better-performance personal computer, the acquisition and processing time can be reduced.

Studies [ 10 ] have shown that either the feed or tangential component of the dynamic cutting force are suitable for monitoring tool wear. The amplitude of the dynamic feed force is small and any disturbance in the system can affect it. The dynamic tangential component is used for monitoring tool wear as it

=o 0

L L

m

==

c

ci

120~

100

80

60

40

20

~ Workplece material : AISI 4340 C;?!ltdl! ;:!p:ee/m~mT/~ ~ev~ • tme/nm ?: r b I d e P30

Depth of cut : 1.6 mm

Force (Computer) Force (FFT Analyeer} 1 1

0 - - I I I k I I k I I L _ _ _

0 2 4 6 8 10 12 14 16 18 20 22 Cutting time (rain)

Fig. 5. Results of the dynamic tangential force obtained from the computer and spectrum analyser.

Page 8: On-line tool-wear monitoring using a PC

10

has a higher amplitude. Figure 6 shows the experimental results of the dynamic tangential force against tool flank wear when cutting AIS14340 and AISI 1148 steels. From the various experimental runs under varying conditions, the trend of the dynamic tangential force exhibits a monotonic increase followed by a relatively sharp decline. Analysis of the results obtained from comprehensive experimental studies has shown that the absolute value of the dynamic force bears little relationship to the magnitude of the tool wear. However, a good relationship is found to exist between the trend of the dynamic force and the flank wear.

Figure 7 shows the experimental results of the dynamic tangential force against cutting time. Statistical analysis of the results indicates that the cut- ting process may be stopped prior to tool failure when the dynamic tangential force decreases below a certain percentage drop from its maximum amplitude and a certain negative gradient of the curve. The threshold values for the onset of tool failure were determined at 99% confidence level using a sample of 10 results for each type of workpiece. The threshold values of the percentage drop for AISI 4340 and AISI 1148 steels were calculated to be 36% in both cases.

Figure 8 shows the results of the gradients of the curve when using three, four, and five acquired values of the dynamic tangential force from the results of Fig. 7. The graphs of the gradient using three acquired values are not smooth because they are very sensitive to changes in the dynamic force. The graphs of the gradient using four and five acquired values fluctuate less, hence providing a better criterion for testing. The threshold values of the gradient using three, four and five acquired values for AIS14340 were calculated to be - 13.42, -9 .33

250 /

200 I

t

Graph Tool Cutting ~ / \ materiel epoed (A le l ) (m/rain)

qP • 4340 116

4340 36 I.I.

1 5 0 ~ 1148 186

I~" 1 0 0 Feed : 0.26 rnm/rev

0 I I I I r I

0 0.1 0.2 0.3 0.4 0.5 0.6 Maximum Flank Wear, VBmex (ram)

Fig. 6. Results showing the variation of the dynamic tangential force with maximum flank wear.

Page 9: On-line tool-wear monitoring using a PC

11

o o to. I

I -

C~

250

200

150

100

(a)

50

0 0 2

W o r k p l e c e m a t e r i a l : AI81 4340 • Tool m a t e r i a l ; Tungsten c a r b i d e P30

Cutting s p e e d : 116 m/rain . Feed : 0 , 2 6 m m l r e v

d' D e p t h of cut : 1.6 mm

L ~ Dyn Tang Force -4~- V B m a x 1

h i _ J _ _ l i i i _ _ i i

4 6 8 10 12 14 16 18 20

C u t t i n g t i m e (min)

i

22 24

0.6

' 0 .4

0.3

1 0.1

0 26

<

m x A 3

7o (b)

6 0

~ 5 0

o u_ - - 4 0

c

~, 2o

10

S ~ ~ ~"~" ~_~

0.7

6 .,

Z /

• : ~ f W o r k p l e c e m a t e r i a l : A I S 1 1 1 4 8

,~ Tool m a t e r i a l : Tungsten c a r b i d e P30 Cutting speed : 170 m/mln F e e d : 0.26 mm/rev D e p t h of cu t : 1.S mm

Dyn Tang Force ~ VBmax 1

i ~L , i i L _ _ 1 L i I

4 6 8 10 12 14 16 18 20

Cut t ing t ime (rain)

0 - - i i i i

0 2 22 24 26

0.6

0.5

0.4

0.3

0.2

0.1

0

<

3 )g

Fig. 7. Illustration of dynamic force and flank wear for AIS14340 and AISI 1148 steel.

and - 7.38 respectively, those for AISI 1148 being calculated to be - 1.7, - 1.36, - 1.3 respectively.

Some results showed a premature decrease in the dynamic force amplitude, which increased again before it finally descended. This was observed to occur when the wear notch appeared at the edge of the tool adjacent to the flank wear. It was observed also that when the amplitude of the dynamic force started to decrease, the edge of the cutting tool wore off at a faster rate. In this in- stance, plastic deformation had occurred at the cutting edge region and the

Page 10: On-line tool-wear monitoring using a PC

12

"o C~

2:l, al -20 i

-4O 1 !

-60 i ~ 3 P t e G r a d t

4 P t e Q r a d t

-80 ~ 6 P t s G r a d t

i i i

0 2 4 6 i [ J i i [ i i i I

8 10 12 14 16 18 20 22 24 26 Cutting time (mln)

"~ 0 0

=.. ~3 -2

-4

-6

-8

(b)

i i i i i k i i i

0 8 10 12 14 16 18 20 22 24 26 Cutting time (rain)

3 P t s Q r a d t

4 P t s Q r a d t

~ - 6 Pts Gradt i i i

2 4 6

Fig. 8. Illustration of dynamic force gradients for AIS14340 and AISI 1148 steel.

cutting edge became blunt. The blunt cutting edge caused additional heating as it rubbed against the workpiece, further weakening the material and leading to plastic collapse. This was more noticeable when cutting AISI 4340 steel, which has a high nickel content. Chipping of the cutting tool was observed to occur when severe crater wear had extended to the front of the cutting edge.

5. Conc lus ions

The amplitude of the dynamic force is found to increase monotonically with tool wear and to decrease prior to the onset of tool failure. However, the ab- solute value bears little relationship to the magnitude of the tool wear.

Page 11: On-line tool-wear monitoring using a PC

13

The use of a personal computer to monitor the tool wear and predict its failure is reliable and able to respond quickly to tool failure. The computer can be used to monitor the onset of tool failure by setting two criteria: the threshold values of the percentage drop of the dynamic tangential force from its maxi- mum and the gradient of the curve of the dynamic force with time.

The response time of the personal computer for on-line tool-wear monitor- ing can be improved further with a better performance computer.

References

1 N.H. Cook, Tool wear and tool life, J. Eng. Ind., 95 (1973) 931. 2 P.M. Lister and G. Barrow, Tool condition monitoring system, Proc. 26th Int. Machine Tool

Design and Research Conf., Macmillan, London, 1986, p. 271. 3 K.F. Martin, J.A. Brandon, R.I. Grosvenor and A. Owen, A comparison of in-process tool

wear measurement methods in turning, Proc. 26th Int. Machine Tool Design and Research Conf., Macmillan, London, 1986, p. 289.

4 J. Tlusty and G.C. Andrews, A critical review of sensors for unmanned machining, Ann. CIRP, 32(2) (1983) 563.

5 A. Kinnander, Choice of wear-criteria in fully automated turning, Proc. 22nd Int. Machine Tool Design and Research Conf., Macmillan, London, 1981, p. 255.

6 G.J. Lay et al., Detection of tool wear by dynamic component of cutting force, J. Jpn. Soc. Precis. Eng., 50(7) (1984) 1117.

7 S.B. Rao, Tool wear monitoring through the dynamics of stable turning, J. Eng. Ind., 108 (1986) 183.

8 P.K. Wright and A. Bagchi, Wear mechanisms that dominate tool life in machining, J. Appl. Met. Work., 1(4) (1984) 15.

9 L.C. Lee, K.S. Lee and C.S. Gan, On the correlation between dynamic cutting force and tool wear, Int. J. Mach. Tool Manuf., 29(3) (1989) 295.

10 L.C. Lee, K.S. Lee and K.G. Kwok, Effects of tool fracture on machining force dynamics, J. Mech. Work. Technol., 17 {1988) 205.