Wear, 61 (1980) 21 - 30 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands

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THE THERMAL CONDITION OF THE TOOL CUTTING EDGE IN INTERMITTENT CUTTING

S. M. BHATIA, P. C. PANDEY and H. S. SHAN

Department of Mechanical and Zndustrial Engineering, University of Roorkee, Roorkee- 24 7 6 72 (India)

(Received October 9, 1978; in final form July 2,1979)

Summary

The tool/chip interface temperature in intermittent cutting was measured by a tool-work thermocouple method during the machining of mild steel plates with cemented carbide tools on a lathe. The tool/chip interface temperature builds up rapidly when the tool engages the work, retains a high level while cutting and falls off during idling. Both the maximum and minimum temperatures during a cycle depend on the cutting speed, feed and cutting time ratio. The measured average surface temperatures were used to compute the internal temperature distribution in carbide tools in order to estimate the nature of induced thermal stress.

1. Introduction

The development and propagation of microcracks and macrocracks is the most dominant mechanism controlling the wear and failure of cemented carbide tools in intermittent cutting. Formation of such cracks reduces the cutting ability of the tools. Studies of carbide tool failure in intermittent cutting have revealed that cyclic temperature variation during cutting is one of the important factors responsible for the formation of such cracks.

Okushima and Hoshi [ 1,2] Andreev [ 31 and Opitz and Frohlich [ 41 modelled tool performance under conditions of cutting with rapid temper- ature fluctuation by subjecting cemented carbide tools to cyclic heating and cooling and concluded that carbides possess poor resistance to thermal shock. Similar conclusions have been reported by other workers [ 5 - 71. A common feature observed in these tests was the presence of a network of cracks running perpendicular to the cutting edge which were termed thermal or comb cracks.

Zorev [ 81 established experimentally that the cutting time ratio is an important parameter governing the life of carbide tools executing intermittent cuts. He postulated that, owing to rapid cooling of the tool surface during idling, shrinkage tensile stresses developed on the tool face and these could

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lead to the formation of comb cracks. Okushima and Hoshi [ 21, Shinozaki [ 91 and Buda and Vasiko [lo] recommend heating the tool bits during non- cutting periods to minimize the incidence of comb crack formation.

It has been shown analytically [ 111 for a carbide tool undergoing periodic heating and cooling that the thermal stress during idling is tensile in nature and its magnitude is a function of the amplitude and frequency of the temper- ature fluctuation and the duration of the non-cutting period. This is in agree- ment with the results of Okushima and Hoshi [12] and Braiden and Dugdale [ 131. It has also been shown [ 141 that thermal cracks only occur at cutting speeds greater than a critical value.

Although thermal cycling appears to cause the comb cracks, in many cases it is not the sole reason for the failure of carbide tools executing inter- mittent cuts. Recent work [ 15 - 171 has suggested that in certain cases mech- anical impacts alone can bring about the failure of carbide tools. However, this normally occurs at feeds beyond a critical value [ 16,181 which depends on the cutting conditions employed.

In this paper the thermal state to which a carbide cutting edge is subjected during an intermittent cut is discussed. The average temperature at the surface of the carbide cutting edge was measured continuously during cutting by means of a natural (tool-work) thermocouple. The measured temp- eratures were used as boundary conditions to compute the internal temper- ature distribution. The influence of different parameters on temperature distribution in the interior of the tool tip was studied. Test conditions which minimized the influence of mechanical impacts were chosen.

2. Experimental

2.1. Procedure, equipment and materials Cutting tests were performed on an HMT H-22 lathe by machining mild

steel plates containing 0.25% carbon, 0.055% sulphur, 0.055% phosphorus and 0.2% copper to I&226/62 specification. The workpiece assembly was formed by clamping either two or four plates about 300 mm long and 20 mm thick in a specially designed slotted workpiece fixture [ 141. This arrangement allowed the plates to be pulled apart after making a cut so that the tests could be repeated at the same cutting speed and cutting time ratio. The tests were conducted with the workpiece fixture held between a four- jaw chuck and the tailstock centre (Fig. 1). Disposable tungsten carbide bits to IS0 classifications P-10, P-20, P-30 and P-40 were used. Details of the tool grades and cutting conditions used are listed in Tables 1 and 2.

Machining was conducted at different cutting speeds and feeds but a constant depth of cut was maintained. Tools were examined frequently for chipping, cracks etc.

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Fig. 1. Apparatus for carrying out intermittent cutting on a lathe.

TABLE 1

Tool specifications

Grade-IS0 standard K C p x 10-s px103 a! E x lo6

Sl-PlO 25.62 0.209 7.0 10.30 11.65 4.90 s2-P20 42.70 0.209 6.5 11.70 17.10 5.30 s4-P30 42.70 0.209 6.5 11.40 17.54 5.20 S6-P40 76.86 0.209 6.0 13.10 27.50 5.50

Tools, SPUN 12 03 08 and SPUN 12 03 12; tool clearance, 11” ;side and back rake, 6”.

TABLE 2

Cutting conditions

Speed Feed (m s -l) (mm rev-l)

Depth of cut (mm)

Remarks

1.50 - 4.5 0.04 - 0.12 1 .oo (constant) Dry cutting

2.2. Measurement of the tool/chip interface temperature The average temperature at the tool/chip interface was measured by the

tool-work thermocouple method to obtain a quick response [12,13,19, 201.

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Figure 2 shows a schematic diagram of the apparatus used for the measurement of the transient temperature during intermittent cutting. The temperature during the non-cutting period could not be measured by this method as the tool-work thermocouple circuit was broken during idling. However, the temperature at the end of the non-cutting period can be assumed to be equal to the temperature at the start of the next cut. The out- put of the thermocouple was fed to a d.c. preamplifier via a mercury terminal. The amplified output was fed to a Tektronix storage oscilloscope (Type 549) for the continuous recording and storage of the cutting temper- ature. Minimum and maximum cycle temperatures were found from the stored traces. Care was taken to allow a sufficient interval of time for the cutting process to stabilize before recording the temperature. Figure 3 shows a general view of the complete experimental set-up used.

Fig. 2. Schematic diagram of the method of temperature measurement during intermit- tent cutting: 1, hot junction; 2, cold junction; 3, machining plates; 4, insulation; 5, mercury contact; 6, thermocouple amplifier.

Fig. 3. General view of the experimental apparatus.

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3. Experimental findings

Figure 4 shows a typical oscilloscope record of the variation of the tool/chip interface temperature. The minimum and maximum temperatures at different cutting speeds and feeds during a cutting cycle are given in Fig. 5. As the cutting speed increases both the maximum and minimum temperatures obtained during a cycle increase. However, the maximum cyclic temperature rises at a faster rate than the minimum temperature. As a result the amplitude of the tool/chip interface temperature fluctuation is large at higher cutting speeds. A similar trend in cyclic temperature was observed when the feed was increased at a constant cutting speed (Fig. 5).

Fig. 4. A typical oscilloscope record of the transient cutting temperature.

The effect of cutting time ratio on the tool/chip interface temperature is shown in Fig. 6. The maximum temperature remains almost unaffected as the cutting time ratio is changed. However, the minimum cyclic temperature is lowered for a small cutting time ratio. This results in a larger amplitude of temperature fluctuation for a small cutting time ratio.

4. Internal temperature distribution

The measured temperature values (minimum and maximum) were used as boundary conditions to compute the internal temperature distribution in the tool tip under different machining conditions using one of the following equations [ 111 :

or

t x.7 = Tomax (2)

Figures 7 - 10 show the internal temperature variation with depth from the tool/chip interface at any given instant of time. Figures 7 and 8 are for a

800 r

600

Toot P-40

r = 0.1 *

/ *

9V-TJo Cutting Speed, m/s Cutting Speed, m/s

Fig. 5. Tool/chip interface temperature variation with cutting speed: tool, P-40; r = 0.1; broken curves, minimum temperature; solid curves, maximum temperature. Feed: 0, 0.06 mm rev-l;A, 0.10 mm rev-l; *, 0.12 mm rev-l.

Fig. 6. Effect of cutting time ratio on tool/chip interface temperatures: tool, P-40; feed, 0.12 mm rev-l ; broken curves, minimum temperature; solid curves, maximum temper- ature; 0, r = 0.1; 4 r = 0.2.

higher cutting time ratio (r = 0.2), while Figs. 9 and 10 correspond to r = 0.1. These temperature-distance graphs were plotted for distinct intervals of time, i.e. when the tool face is at maximum, minimum or average temper- atures. Comparison of Figs. 7 and 9 or Figs. 8 and 10 reveals that for a low cutting time ratio the tool face is subjected to a larger amplitude of temper- ature fluctuation and hence higher stresses.

Figures 7 - 10 show that compressive thermal stress occurs on the tool/ chip interface during the cutting period. However, the surface thermal stress is tensile during the non-cutting period as the surface temperature drops below the internal temperature. Thus a maximum range of thermal stress occurs on the surface where cyclic variation of the stress from tension to compression can lead to the formation of cracks.

Some representative results of thermal cracking are shown in Figs. 11 - 13. The cracks appear on the tool face almost perpendicular to the cutting edge and parallel to each other. A greater number of cracks are seen in a relatively short interval of time for a low cutting time ratio (Figs. 11 and 12).

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0 , I

b.4 0.6 I.2 1.6 20

Depth from Tool-Chip 16t6rf666B mm.

600-

700 -

#

600 -

SOOT

If 400 -

f

300-

200-

100- I I 110 I

0.6 I.2 I.6 2.0

Depth from Tool-Chip lnhrfocr, mm

Fig. 7. Internal temperature distribution when the tool face is maximum (solid curves) and minimum (broken curves) temperatures: tool, P-10; r = 0.2; feed, 0.10 mm rev-l. Cutting speed: A, 1.17 m s-l;*, 1.50 m s-‘;A, 2.0 m s-l; 0,3.34 m s-l.

Fig. 8. Internal temperature distribution when the tool face is at the mean temperature: tool, P-10 (four plates); r = 0.2; feed, 0.10 mm rev-‘. Cutting speed: A, 1.17 m s-l; 0, 1.50 m s-l ; A, 2.0 m s-l ; 0, 3.34 m s-l.

5. Discussion

If complete thermal constraint is assumed the thermal stress produced at the tool/chip interface by thermal loading is given by

where O(a) is a measure of temperature fluctuation during the heating- cooling cycle. When uY exceeds the yield strength of the tool material thermal cracking takes place.

Figure 5 shows that as the cutting speed is increased the peak cycle temperature increases. For a constant cutting time ratio one would expect the minimum cycle temperature to be greater at higher cutting speeds. This would be true if the surface heat transfer coefficient were held constant at different cutting speeds. However, in actual operation this may not be the case as convective heat transfer from the tool is likely to increase at higher cutting speeds. This results in 8 marginal increase in the minimum cyclic temperature as the cutting speed is increased. Consequently, the amplitude e(a) increases with cutting speed (Figs. 5, 7 and 9). Thus the chances of

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0 0.4 0.0 1.2 1.6 2.0 Depth From Tool Chit? lntWfac@, mm

8oo/ - -

200

I / I

IO 11/l I I I I I

0 o-4 0.8 I.2 I.6 2.0 Oapth from Tool-Chip Intarface,mm

Fig. 9. Internal temperature distribution when the tool face is at maximum (solid curves) and minimum (broken curves) temperatures: tool, P-10; r = 0.1; feed, 0.12 mm rev-l. Cuttingspeed:~,1.17ms~1;~,1.50ms~1;*,2.Oms~1;~,2.5ms~1;*,3.34ms~1.

Fig. 10. Internal temperature distribution when the tool face is at the mean temperature: tool, P-10; r = 0.1; feed, 0.12 mm rev-l. Cutting speed: A, 1.7 m 6-l; l ,1.50 m s-l ; *, 2.00 m 6-l; 0, 2.50 m s-l;A, 3.34 m 6-l.

thermal cracking increase with increasing speed. Figure 9 shows that the amplitude O(u) is greater for a low cutting time ratio under otherwise identical cutting conditions. Consequently thermal cracks are likely to occur more readily at high cutting speeds and low cutting time ratios. The cracking

Fig. 11. Comb cracks on the rake face: tool, P-20; cutting speed, 4.0 m 6-l; feed, 0.10 mm rev-l ; r = 0.1; duration of cut, 200 s.

Fig. 12. Comb cracks on the rake face: tool, P-20; cutting speed, 3.5 m 6-l; feed, 0.06 mm rev-l ; r = 0.2; duration of cut, 180 s.

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Fig. 13. Spalling due to thermal cracking: tool, P-20; cutting speed, 4.25 m 6-l; feed, 0.04 mm rev-l ; r = 0.2; duration of cut, 312 s.

behaviour shown in Figs. 11 and 12 can thus be explained by the above hypothesis. As the cutting speed is reduced, the parameter 8 (a) in eqn. (3) reduces, thus diminishing the chances of thermal cracking.

The characteristics of the cracks observed in the present investigation (Figs. 11 - 13) agree with those reported by other workers [l, 2, 5,6]. Once cracks are formed they act as stress raisers and cause further deterioration of the cutting edge by spalling (Fig. 13).

6. Conclusions

The following general conclusions can be drawn from this investigation. (1) The amplitude of the tool/chip interface temperature fluctuation

during intermittent cutting is mainly dependent on the cutting speed, feed and cutting time ratio.

(2) The magnitude of the maximum temperature during a cycle remains unaltered as the cutting time ratio is changed at a given speed.

(3) The amplitude of the tool/chip interface temperature fluctuation is large for a low cutting time ratio which accelerates the rate of comb crack formation and hence tool wear.

(4) The amplitude of the tool/chip interface temperature fluctuation increases with increasing cutting speed.

Nomenclature

c specific heat of the tool material, kJ kg-l K-l

; modulus of elasticity of the tool material, N mme2 frequency of the temperature wave, Hz

K thermal conductivity of the tool material, W m-l K-l r cutting time ratio, i.e. ratio of actual cutting time to total time during a cycle txn mean temperature of the to91 surface, “C t x,7 temperature at any instant T at a distance x from the tool face (see Fig. 6 for the

coordinate system), “C

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to.7 temperature of the tool surface at any time 7, “C T x,7 t x,7 - trill Tomax to, 0 - t,, maximum temperature amplitude of the surface above the mean

temperature level

; thermal diffusivity of tool material, mm2 s-l coefficient of linear expansion of the tool material, “C-l

P density of tool material, kg rnv3 V Poisson’s ratio

DY thermal stress in the y direction

References

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2 K. Okushima and T. Hoshi, Thermal cracks in the carbide face milling cutter (2nd Report), Bull. JSME, 6 (22) (1963) 317.

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