- Email: [email protected]
Identification of Effective Zones for High Pressure Coolant in Milling
Identification of Effective Zones for High Pressure Coolant in Milling M. Rahman (2), A. Senthil Kumar, M.R. Choudhury Department of Mechanical and Production Engineering, National University of Singapore, Singapore Received on January 3,2000
Abstract
Effective zones of high pressure coolant have been identified in milling performing a wide range of machining operations on ASSAB 718 mould steel using un-coated tungsten carbide inserts. The effects of high pressure coolant are evaluated in terms of machining parameters by comparing with those of conventional coolant and dry cut. Chipping and catastrophic failure are the dominant factors of insert rejection for dry cut and conventional coolant, whereas progressive flank wear is observed for high pressure coolant within its effective zones at lower depth of cut, lower feed rate and higher cutting speed. It is found that the cutting force is reduced, surface finish is improved, chip width is narrowed and cooling effect is better with the use of high pressure coolant. Keywords: High pressure coolant, Tool wear, Cutting force
I
INTRODUCTION
Machining of hardened steel and other difficult to cut materials requires instant heat transfer from the cutting edge of tool to aid tool life. Supply of high volume and high pressure coolant often provides the best answer. Conventional coolant does not reach the real cutting area near the cutting edge of the tool and vaporises before it reaches the cutting area as the heat is very intense during machining. The use of high pressure coolant to improve tool life was started in 1952 when Pigott and Colwell [ I ] carried out turning on SAE 3150 at pressure range of 1.72 to 41.4 bar. The results showed improved tool life, surface finish and also eliminated built-up edge upto a critical pressure of 27.6 bar. Similar experiments by Sharma et al. [2] on SAE1040 steel at a pressure of 640 bar showed significant reduction of friction in the region between tool face and chip and chip coil diameter was reduced approximately 4 to 8 times as compared to dry cut. Negpal et al. [3] studied the effect for a pressure range of 3.4 to 34.3 bar in turning and found that the optimum pressure for water-soluble coolant is a strong function of feed, whereas it is independent of feed for straight mineral, chlorinated, and sulpho-chlorinatedoils. Kurimoto and Barrow [4] found accelerated groove wear on the trailing edge particularly at small feeds primarily due to corrosiveness of fluids and hence the fluids were found to be detrimental to tool life. Wertheim et al. [5] studied the effect on grooving and found that higher fluid pressure (15-25 bar) increases tool life proportionally, both for un-coated and coated tools. Turning operation by Ezugwu et al. [6]at 140 bar on nickel based super-alloy rendered lower tool lives as compared to conventional coolant. Similar tests carried out by Machado et al. [7, 81 on Ti6A14V and lnconel 901 showed improved tool life when machining the titaniumbase alloy and shorter tool life when machining the nickelbase alloy with high pressure coolant. A study at 700 bar by Rasch and Vigeland [9] showed that even with difficult to break materials, short and easy to handle chips can be obtained through the method of hydraulic chip breaking.
Annals of the ClRP Vol. 49/7/2000
Kovacevic et al. [10,11.12] found that in face milling, high pressure coolant increased the tool life significantly, improved the surface finish drastically, eliminated built-up edge, decreased co-efficient of friction, reduced cutting force and improved chip shape. Most of the research works done so far as discussed above, have compared the performance of high pressure coolant either with dry cut or conventional coolant. They have not made a general comparison among the three conditions. Moreover, none of the above studies have shown the distinct effective zones of high pressure coolant as compared to conventional coolant and dry cut. Consequently, an attempt has been made in this study to establish the distinct effective zones of high pressure coolant. 2
EXPERIMENTAL SETUP AND PROCEDURE
A schematic diagram of experimental setup is shown in Figure 1. The set-up is composed of mainly four parts: (a) high pressure cooling system, (b) 5 axis CNC milling machine equipped with cutter and work-piece assembly,
I
-
pressure coolant
I
i2 1
I
I
1
10
Figure 1: Schematic diagram of experimental set-up 1. Coolant tank, 2. High pressure pump, 3. Spindle, 4. Too holder, 5. Cutter, 6. Cutting insert, 7. Balancing insert, 8. Work-piece, 9. Dynamometer, 10. Machine table.
47
(c) cutting force data acquisition system, (d) temperature measuring system. The experiment is carried out in a five-axis CNC milling machine of 10 kW motor power. This machine provides the supply of through spindle high pressure coolant of 17 bar. A schematic diagram of the cutting force data acquisition system IS shown in Figure 2 which consists of a 3component piezo-electric dynamometer, charge amplifier, data recorder and a data acquisition software. Temperature measuring system consists of a work-piece thermocouple assembly, connectors, extension wires, mechanical multiplexor, data logger and computer equipped with software. A schematic diagram of thermocouple arrangement inside work-piece during machining is shown in Figure 3. Full immersion milling through the center of the work-piece has been performed on ASSAB 718 mould steel workpieces of dimensions 206mmX43mmX104mm using uncoated tungsten carbide inserts mounted on a cutter of diameter 32mm. Tool wear is measured by a tool makers microscope and surface roughness is measured by a portable surface roughness tester at different machining intervals. Detailed analysis of chips and failure of inserts are done by Scanning Electron Microscope (SEM).
3
RESULTS AND DISCUSSION
The experiments are conducted over a wide range of cutting speed (~50-225m/min), feed rate (fi=0.02O.lmm/tooth) and depth of cut (aa=0.15-1.0mm). A minimum of three inserts is used for each data point. In certain experiments when the results are not consistent additional inserts are used to verify.
3.1 Tool wear and tool life A typical example of flank wear against cutting time within effective zone of high pressure coolant (to be discussed later) is shown in Figure 4 where it can be seen that the wear rate is minimum for high pressure coolant. However, for dry cut due to chipping flank wear increases drastically at higher cutting speed. The end of tool life is considered at 0.3 mm uniform flank wear or chipping whichever is earlier. But some experiments are continued after 0.3 mm flank wear to observe failure mode of inserts for comparison. Figures 5(a) to 5(c) show tool life and failure mode of carbide inserts with variation of cutting speed, feed rate, and depth of cut respectively. From these figures, high pressure coolant is found to be most effective for the cutting condition of aa=0.35mm, fz=0.05mm/tooth and v=l50m/min, which is considered as optimum cutting condition for this high pressure system. Experiments have been carried out by varying speed, feed and depth of cut, however, one parameter is varied while other two are kept constant at optimum value and thus, effective zones of high pressure coolant are identified. At lower speed (Figure 5(a)) below 100m/min, the use of high pressure coolant is not advisable and at very low speed of 50 mlmin the use of coolant is found to be detrimental and the inserts fail because of chipping for both conventional and high pressure coolant. However, only progressive flank wear is observed for dry cut. At higher speeds, inserts fail due to chipping and excessive flank wear for dry cut and conventional coolant whereas only progressive flank wear is observed for high pressure coolant. High pressure coolant is found to be effective at higher cutting speeds (125-200m/min) and it also suppresses the premature fracture of inserts even at 225mlmin whereas at this speed catastrophic failure is observed for dry cut only after 35 sec of machining. High pressure coolant is also found to be very effective at lower feed rate (0.02-0.05mm/tooth), suppressing the chipping of inserts, which is observed for both dry cut and conventional coolant.
48
QLl Interface
U
Printer (optional)
Figure 2: Cutting force data acquisition system I
Work-piece
L
LLJ-
Thermocouple tips
..............
. . . . :.I.: . . . ............ ..........1.:............:. ................................. , . . .. ....I
.
!
I
Figure 3: Thermocouple arrangement inside work-piece At lower depth of cut (0.15-0.5mm), high pressure coolant of 17 bar is found to be effective and above 0.5mm the use of coolant, both conventional and high pressure, is found to be detrimental. Several reasons may be attributed to the fact. During machining, at lower depth of cut, the generation of heat is low and accordingly the temperature rise in insert is also low and when the coolant is used, all modes of heat transfer from the insert to coolant including phase change take place. First film boiling starts and persists for very short time. Then vigorous nucleate boiling starts and then forced convection takes place. At lower depth of cut, pressure is high enough to remove the heat from the cutting zone and cyclic thermal shock does not take place. But when the depth of cut is high, the heat generation is high enough and accordingly the temperature rise in the insert is also high. However when coolant is used, film boiling persists for relatively longer time and nucleate boiling may not take place. Consequently no phase change takes place to cause sharp temperature drop. For this reason, although cooling is done by film boiling at the depth of cut area, the heat comes from other places and increases the temperature causing severe cyclic thermal shock. So at the beginning of machining, micro-chipping is observed for both conventional and high pressure coolant and inserts fail because of chipping at the depth of cut region. However only progressive flank wear is observed for dry cut. 1 h
E L
0.75
m
Y
- . -8. - .C.Coolant +HP Coolant
fz=.05mmltooth
0.5
C
m LL
as
0.25
01 0
I 5 10 15 Machining Time (min)
20
Figure 4: Relationship between average flank wear and machining time
35
Effective Zone of C T Coolant & Drv cut
q =zone of High Pressure Coolant
1
I
a.=0.35mm J:=0.05mnlitooth
30
f 25 1 j
>
14
20
-
I
FW
C ._
Effective Zone of High Pressure Coolant
Effective Zone of Conventional Coolant
12 10
E ..--a
15
P = 0
6
10
z
4
2
5
0
0 50
200
.10
Cutting Speed (nJnin)
Feed Rate (mmhooth) (b) Variation with feed rate
(a) Variation with speed 12
I( Effective zone of Drv cut
Effective Zone of High Pressure coolant
.
II
F d
f p 0 05mm/toolh
Fwl
! e l 50 m/mm
FW= Flank wear C = Chipping
Conventional coolant
"
.I5
.20
.25
.35
~
.50
1
.55
57;
1.o
Depth of Cut (mm) (c) Variation with depth of cut Figure 5: Tool life and failure mode of carbide inserts Figures 6(a) and 6(b) show 2-0 and 3D SEM photographs of worn inserts respectively at optimum cutting condition. It is clear from the SEM pictures that inserts fail because of nose chipping for both dry cut and conventional coolant and largest wear zone is observed for dry cut, followed by conventional coolant. No chipping is observed for high pressure coolant and wear zone is also the smallest. Figures 7(a) and 7(b) show 3-D SEM photograph of worn inserts at a higher speed (225mimin) and a higher depth of cut (1.OOmm) respectively. At higher speed, catastrophic failure of inserts is observed for both dry cut and conventional coolant whereas only progressive flank wear is seen for high pressure coolant as shown in Figure 7(a). At higher depth of cut inserts fail because of chipping at depth of cut area for both conventional and high pressure coolant whereas no chipping is observed for dry cut as shown in Fig 7(b). 3.2 Cutting forces The cutting forces with change of depth of cut are plotted in Figure 8 for conventional coolant and high pressure coolant at lower depths of cut. Within the effective zone of high pressure coolant i.e. at lower depths of cut (0.150.5mm), the cutting forces with high pressure coolant are always less than that of conventional coolant. At higher depth of cut (0.55-1.OOmm), the cutting forces with conventional coolant are always less than that of high pressure coolant. The influence of feed rate on the cutting forces for the two different cooling conditions is depicted in Figure 9. Within the effective zone of high pressure coolant i.e. at lower
feed rate (0.02-0.05mm/tooth), all the components of cutting forces are higher with conventional coolant as compared to high pressure coolant. But at higher feed rate, z-component of cutting force is lower with conventional coolant as compared to high pressure coolant. Comparative study of dynamic cutting forces during total machining time between conventional and high pressure coolant is shown in Figure 10. When the insert is new, the cutting force is steady for a while during machining and then it starts fluctuating between high and low values. This fluctuation becomes more prominent with further increase of tool wear. If the inserts suffer any breakage or chipping, the cutting force jumps to very high magnitude and stays for a while and then drops, which is observed for conventional coolant after approximately 13 minutes of machining as shown in Figure 10. But for high pressure coolant, no sudden increase of cutting force is observed and during full duration of machining all components of cutting forces with high pressure coolant are less compared to conventional coolant. Within the effective zone of high pressure coolant, the reduction in the cutting force with high pressure coolant could be due to several reasons. High pressure coolant jet strikes the chip at a point very close to the cutting edge of insert, removing the serrated portion of the chip, resulting in the reduction of the width of the chip (SEM analysis of chips is discussed in section 3.4). Consequently the toolchip contact area on the rake face is reduced. which reduces the friction between insert and chip and eventually reduces cutting force.
49
Dry cut
Conventional coolant
High pressure coolant
Figure 7(b): 3-D SEM photographs of worn inserts ( aa=l.OOmm, fz=0.05mm/tooth,v=150m/min) 3.3 Surface Finish
Figure 11 shows the variation of surface roughness with machining time for optimum cutting condition. In all the cases, within the effective zone of high pressure coolant, best surface finish is obtained with high pressure coolant, followed by conventional coolant and then dry cut. It is very encouraging to note that the surface finish obtained with high pressure coolant in milling is within the range of (Ra=0.5-0.7pm)which is very close to finishing operations of grinding. Sometimes roughness of surface produced by conventional coolant is better than high pressure coolant. When chipping occurs, the surface obtained by dry cut is the worst and shows very high value of average surface roughness because of melting of chips on the work-piece surface. The improvement of surface finish with the use of coolant is attributed to the fact of better lubrication. Moreover, because of effective cooling with high pressure coolant within the effective zone tool wear rate becomes
50
slower. This phenomenon may also be the cause of improved surface finish. 3.4 Chip Shape The type of chip produced during machining depends on the material being machined, the tool, the cutting conditions, and the existence or absence of coolant. Figure12 (a) shows the SEM photograph of chips for dry cut, conventional coolant and high pressure coolant. For most of the cutting conditions, the chip shape is short and tubular and generally the chips produced by dry cut are dark blue color and at higher depth of cut, the color is yellowish. The dark blue color of chips, produced by dry cut, indicates the extreme heat generation at the tool-chip interface, which results in burnt chips. The chips produced by conventional coolant are black, which also indicates high heat generation at tool-chip interface; certain portion of which is carried away by chips and conventional coolant does not provide proper cooling effect at the tool-chip
225
250 X-Component
g 8 '0
0 LL
cn
125
5
Y-Component
150
250
9cn 200
p loo .-c -.---..C.COOlant
C .-5
..... .. c
550
2 150
Coolant
0 025
03
100
015 02 025 0 3 035 0 4 045 0 5
035 0 4 045 0 5
-HP.
Coolalt
i=/
015 0 2 025
Depth of cut(mn)
Depth of Cut(mn)
..c. COOlalt
0
H P Coolant
015 0 2
__--
3
LL
C
300
0 3 035 0 4 045 0 5
Depth of Cut(mn)
Figure 8: Variation of cutting force with depth of cut for conventional and high pressure coolant Cutting condition: fz=0.05mm/tooth, v=l50m/min
-t 3
210
,
190
a
X -Component
170
0 LL 150
cn c
3
300 250
f
Y-Component
130
- . ~.- . - C . Coolant
110
2
9
150
2
.g
130
.-c
200
. - . ~.. . C
3
5
Coolant
0 110
008
-HP
100
002
01
004
Feed Rate (Mtooth)
008
006
_ _ _ . ._ _._ _ _
. ~.- .. .c Coolant
0
P Coolant
-H
006
--
150
90 004
2-Component
cn
90 002
I
I
01
Coolant
I
I
002
004
Feed Rate ( M t o o t h )
006
008
01
Feed Rate ( M t o o t h )
Figure 9: The influence of feed rate on cutting forces for two different cooling Cutting condition: aa=0.35mm, v=l50m/min 875 775 275
. - - . . - .C. Coolant
_.... - .c.Coolant
225 H -
.
P Coolant
_..
H.P
.--, ...-
*
175
125 0.25
. ,**---
*
*
6 .
Coolant
- _.
.-
- - ,
('
0)
C
125 3
575
85
2 - C o m p o nent
11 25
Total cutting time(rrin)
14
025
475
175 3
575
85
11.25
14
0.25
Total Cutting lim (nr'n)
3
575
8.5
11 25
14
Total cutting time(nr'n)
Figure 10: Comparative study of cutting forces during total machining time between conventional and high 0 pressure coolant. Cutting condition: aa=0.35mm, fz=0.05mm/tooth, ~ 1 5 m/min interface. Whereas the chips produced by high pressure coolant supply have silver color (may be due to insufficient supply of oxygen) indicating that the chips are not burnt and high pressure coolant provides necessary cooling effect. Figure 12(b) depicts SEM photographs of width of chips. The width of chips produced by high pressure coolant is smaller as compared with conventional coolant and dry cut, which reduces the tool-chip contact area and may reduce frictional force at tool-chip interface. The chips produced by conventional coolant are serrated; indicating the self-tendency of breakage, which is absent in the case of dry cut chips. When the high pressure coolant is used, high velocity of coolant breaks the serrated portion and reduces the width of chips. 3.5 Cooling Effect Figure 13 shows the typical temperature profiles of workpiece with machining time for dry cut, conventional coolant and high pressure coolant respectively. Temperature profiles for dry cut show gradual increase of temperature in the work-piece with machining time and reach peaks when the milling cutter passes the thermocouple locations and then gradually decrease. Temperature increases sharply when the milling cutter passes the thermocouple locations for conventional coolant and then decreases sharply. This increase and decrease of temperature is sharper for high pressure coolant. Maximum temperature rise in work-piece for dry cut is 75OC, for conventional coolant is 37.5OC, and
for high pressure coolant is 27.5OC for the optimum cutting condition. High pressure coolant decreases the work-piece temperature by 26% in comparison with conventional coolant and by 63% in comparison with dry cut indicating better cooling effect. Above results clearly indicate the effectiveness of high pressure coolant in reducing the temperature.
-A-
-e-
aa=0.35rnm fz=.05mm/tooth ~150rnlrnin
CCoolant HP Coolant
I
0
5
10
15
20
Machining lime(m'n) Figure 11: Variation of average surface roughness with machining time 4
CONCLUSIONS
1.
High pressure coolant is effective in terms of tool wear and tool life at higher speed (125-225m/min), lower depth of cut (0.15-0.50mm) and lower feed rate (0.02-
51
Conventional coolant
oa 60
Maximm Tenp=37S0C
High pressure coolant
(a) Magnification = I 8 (b) Magnification =I00 Figure 12: SEM photographs of chips
2.
3.
4
5
6.
7.
5
0.05mm1tooth). At 17 bar, the optimum speed, feed and depth of cut are 150 mlmin, 0.05 mm1tooth and 0.35mm respectively. Moreover, high pressure coolant supply suppresses premature fracture and catastrophic failure of inserts at high cutting speed of 225 m/min. Within the effective zones of high pressure coolant, inserts fail mostly because of chipping for dry cut and conventional coolant, whereas progressive flank wear is observed for high pressure coolant supply. At higher depths of cut, the use of coolant is found to be detrimental to tool wear and tool life because of severe cyclic thermal shock at the depth of cut region. Inserts fail because of chipping at the depth of cut region for both conventional and high pressure coolant supply, whereas only progressive flank wear is observed for dry cut. Within the effective zones of high pressure coolant, there is a significant reduction of the cutting forces with the application of high pressure coolant as compared to conventional coolant. Surface finish obtained with the use of high pressure coolant is much better than those obtained for conventional coolant and dry cut for most of the cutting conditions. Under certain cutting conditions surface finish obtained with the use of high pressure coolant is almost comparable to that obtained in grinding operation. Worst surface finish is obtained during dry cut at higher speed because of chipping of inserts and melting of chips on work-piece because of high temperature rise at the cutting zone. The serration produced due to the application of coolant is removed by high pressure coolant causing the reduction of the width of chip, which reduces toolchip contact area. High pressure coolant jet reduces the work-piece temperature by effective forced convection, showing better cooling effect. REFERENCES ~~
[ I ] Pigott, R. J. S., Colwell, A. T., 1952, Hi-Jet System for Increasing Tool Life, SAE Q. Trans., 613547466, [2] Sharma3c. w.B.8 R.l 19712Some Effects of Injecting Cutting Fluids Directly into the Chip-Tool Interface, J. of Engng Ind, Trans. ASME, 931441444.
52
0
25
50
75
100 125 150 175
Machining time (sec) Figure 13: Typical temperature profiles of workpiece with machining time Nagpal, B. K., Sharma, C. S., 1973, Cutting Fluid Performance---Part I---Optimization of Pressure for Hi-Jet Method of Cutting Fluid Application, J. of Engng Ind, Trans. ASME, 95881-889. [4] Kurimoto, T., Barrow, G., 1982, The Influence of Aqueous Fluid on the Wear Characteristics and Life of Carbide Cutting Tool, Annals of the CIRP. 31/1:1923. [5] Wertheim, R., Rotberg, J., Ber, A,, 1992, Influence of High-pressure Flushing Through the Rake Face of the Cutting Tool, Annals of the CIRP, 41/1:101-106. [6] Ezugwu, E. O., Machado, A. R., Pasby, I. R., Wallbank, J., 1990, The Effect of High-pressure Coolant Supply When Machining a Heat Resistant Nickel-Based Superalloy, J. of Society of Trib. & Lub. Engineers, 471 9:751-757. [7] Machado, A. R., Wallbank, J., 1994, The Effects of a High Pressure Coolant Jet on Machining, J. of Engng Manufacture, 208:29-38. [8] Machado, A. R., Wallbank, J., Pasby, I. R., Ezugwu, E. O., 1998, Tool Performance and Chip Control when Machining Ti6A14V and lnconel 901 Using High Pressure Coolant Supply, Machining Science and Technilogy, 211:I-12. [9] Rasch, F.O., Vigeland, T., 1981, Hydraulic Chip Breaking, Annals of the CIRP, 30/1:333-335. [ l o ] Kovacevic, R., Mohon, R., Cherukuthota, C., 1993, High Pressure Waterjet as a CoolanffLubricant in Milling Operation, Manufacturing Science and Engng, PED, ASME Winter Annual Meeting, 64:733-748. [I I] Kovacevic, R., Cherukuthota, C., Mazurkiewicz, M., 1995, High Pressure Waterjet Cooling/Lubrication to Improve Machining Efficiency in Milling, International J. of Machine Tools and Manufacture, 35/10:14591473. [I21 Kovacevic, R., Cherukuthota, C., Mohon, R., 1995, Improving Milling Performance with High Pressure Waterjet Assisted CoolinglLubrication, J. of Engng Ind, Trans. ASME, 117/3:331-339.
[3]
Abstract
Effective zones of high pressure coolant have been identified in milling performing a wide range of machining operations on ASSAB 718 mould steel using un-coated tungsten carbide inserts. The effects of high pressure coolant are evaluated in terms of machining parameters by comparing with those of conventional coolant and dry cut. Chipping and catastrophic failure are the dominant factors of insert rejection for dry cut and conventional coolant, whereas progressive flank wear is observed for high pressure coolant within its effective zones at lower depth of cut, lower feed rate and higher cutting speed. It is found that the cutting force is reduced, surface finish is improved, chip width is narrowed and cooling effect is better with the use of high pressure coolant. Keywords: High pressure coolant, Tool wear, Cutting force
I
INTRODUCTION
Machining of hardened steel and other difficult to cut materials requires instant heat transfer from the cutting edge of tool to aid tool life. Supply of high volume and high pressure coolant often provides the best answer. Conventional coolant does not reach the real cutting area near the cutting edge of the tool and vaporises before it reaches the cutting area as the heat is very intense during machining. The use of high pressure coolant to improve tool life was started in 1952 when Pigott and Colwell [ I ] carried out turning on SAE 3150 at pressure range of 1.72 to 41.4 bar. The results showed improved tool life, surface finish and also eliminated built-up edge upto a critical pressure of 27.6 bar. Similar experiments by Sharma et al. [2] on SAE1040 steel at a pressure of 640 bar showed significant reduction of friction in the region between tool face and chip and chip coil diameter was reduced approximately 4 to 8 times as compared to dry cut. Negpal et al. [3] studied the effect for a pressure range of 3.4 to 34.3 bar in turning and found that the optimum pressure for water-soluble coolant is a strong function of feed, whereas it is independent of feed for straight mineral, chlorinated, and sulpho-chlorinatedoils. Kurimoto and Barrow [4] found accelerated groove wear on the trailing edge particularly at small feeds primarily due to corrosiveness of fluids and hence the fluids were found to be detrimental to tool life. Wertheim et al. [5] studied the effect on grooving and found that higher fluid pressure (15-25 bar) increases tool life proportionally, both for un-coated and coated tools. Turning operation by Ezugwu et al. [6]at 140 bar on nickel based super-alloy rendered lower tool lives as compared to conventional coolant. Similar tests carried out by Machado et al. [7, 81 on Ti6A14V and lnconel 901 showed improved tool life when machining the titaniumbase alloy and shorter tool life when machining the nickelbase alloy with high pressure coolant. A study at 700 bar by Rasch and Vigeland [9] showed that even with difficult to break materials, short and easy to handle chips can be obtained through the method of hydraulic chip breaking.
Annals of the ClRP Vol. 49/7/2000
Kovacevic et al. [10,11.12] found that in face milling, high pressure coolant increased the tool life significantly, improved the surface finish drastically, eliminated built-up edge, decreased co-efficient of friction, reduced cutting force and improved chip shape. Most of the research works done so far as discussed above, have compared the performance of high pressure coolant either with dry cut or conventional coolant. They have not made a general comparison among the three conditions. Moreover, none of the above studies have shown the distinct effective zones of high pressure coolant as compared to conventional coolant and dry cut. Consequently, an attempt has been made in this study to establish the distinct effective zones of high pressure coolant. 2
EXPERIMENTAL SETUP AND PROCEDURE
A schematic diagram of experimental setup is shown in Figure 1. The set-up is composed of mainly four parts: (a) high pressure cooling system, (b) 5 axis CNC milling machine equipped with cutter and work-piece assembly,
I
-
pressure coolant
I
i2 1
I
I
1
10
Figure 1: Schematic diagram of experimental set-up 1. Coolant tank, 2. High pressure pump, 3. Spindle, 4. Too holder, 5. Cutter, 6. Cutting insert, 7. Balancing insert, 8. Work-piece, 9. Dynamometer, 10. Machine table.
47
(c) cutting force data acquisition system, (d) temperature measuring system. The experiment is carried out in a five-axis CNC milling machine of 10 kW motor power. This machine provides the supply of through spindle high pressure coolant of 17 bar. A schematic diagram of the cutting force data acquisition system IS shown in Figure 2 which consists of a 3component piezo-electric dynamometer, charge amplifier, data recorder and a data acquisition software. Temperature measuring system consists of a work-piece thermocouple assembly, connectors, extension wires, mechanical multiplexor, data logger and computer equipped with software. A schematic diagram of thermocouple arrangement inside work-piece during machining is shown in Figure 3. Full immersion milling through the center of the work-piece has been performed on ASSAB 718 mould steel workpieces of dimensions 206mmX43mmX104mm using uncoated tungsten carbide inserts mounted on a cutter of diameter 32mm. Tool wear is measured by a tool makers microscope and surface roughness is measured by a portable surface roughness tester at different machining intervals. Detailed analysis of chips and failure of inserts are done by Scanning Electron Microscope (SEM).
3
RESULTS AND DISCUSSION
The experiments are conducted over a wide range of cutting speed (~50-225m/min), feed rate (fi=0.02O.lmm/tooth) and depth of cut (aa=0.15-1.0mm). A minimum of three inserts is used for each data point. In certain experiments when the results are not consistent additional inserts are used to verify.
3.1 Tool wear and tool life A typical example of flank wear against cutting time within effective zone of high pressure coolant (to be discussed later) is shown in Figure 4 where it can be seen that the wear rate is minimum for high pressure coolant. However, for dry cut due to chipping flank wear increases drastically at higher cutting speed. The end of tool life is considered at 0.3 mm uniform flank wear or chipping whichever is earlier. But some experiments are continued after 0.3 mm flank wear to observe failure mode of inserts for comparison. Figures 5(a) to 5(c) show tool life and failure mode of carbide inserts with variation of cutting speed, feed rate, and depth of cut respectively. From these figures, high pressure coolant is found to be most effective for the cutting condition of aa=0.35mm, fz=0.05mm/tooth and v=l50m/min, which is considered as optimum cutting condition for this high pressure system. Experiments have been carried out by varying speed, feed and depth of cut, however, one parameter is varied while other two are kept constant at optimum value and thus, effective zones of high pressure coolant are identified. At lower speed (Figure 5(a)) below 100m/min, the use of high pressure coolant is not advisable and at very low speed of 50 mlmin the use of coolant is found to be detrimental and the inserts fail because of chipping for both conventional and high pressure coolant. However, only progressive flank wear is observed for dry cut. At higher speeds, inserts fail due to chipping and excessive flank wear for dry cut and conventional coolant whereas only progressive flank wear is observed for high pressure coolant. High pressure coolant is found to be effective at higher cutting speeds (125-200m/min) and it also suppresses the premature fracture of inserts even at 225mlmin whereas at this speed catastrophic failure is observed for dry cut only after 35 sec of machining. High pressure coolant is also found to be very effective at lower feed rate (0.02-0.05mm/tooth), suppressing the chipping of inserts, which is observed for both dry cut and conventional coolant.
48
QLl Interface
U
Printer (optional)
Figure 2: Cutting force data acquisition system I
Work-piece
L
LLJ-
Thermocouple tips
..............
. . . . :.I.: . . . ............ ..........1.:............:. ................................. , . . .. ....I
.
!
I
Figure 3: Thermocouple arrangement inside work-piece At lower depth of cut (0.15-0.5mm), high pressure coolant of 17 bar is found to be effective and above 0.5mm the use of coolant, both conventional and high pressure, is found to be detrimental. Several reasons may be attributed to the fact. During machining, at lower depth of cut, the generation of heat is low and accordingly the temperature rise in insert is also low and when the coolant is used, all modes of heat transfer from the insert to coolant including phase change take place. First film boiling starts and persists for very short time. Then vigorous nucleate boiling starts and then forced convection takes place. At lower depth of cut, pressure is high enough to remove the heat from the cutting zone and cyclic thermal shock does not take place. But when the depth of cut is high, the heat generation is high enough and accordingly the temperature rise in the insert is also high. However when coolant is used, film boiling persists for relatively longer time and nucleate boiling may not take place. Consequently no phase change takes place to cause sharp temperature drop. For this reason, although cooling is done by film boiling at the depth of cut area, the heat comes from other places and increases the temperature causing severe cyclic thermal shock. So at the beginning of machining, micro-chipping is observed for both conventional and high pressure coolant and inserts fail because of chipping at the depth of cut region. However only progressive flank wear is observed for dry cut. 1 h
E L
0.75
m
Y
- . -8. - .C.Coolant +HP Coolant
fz=.05mmltooth
0.5
C
m LL
as
0.25
01 0
I 5 10 15 Machining Time (min)
20
Figure 4: Relationship between average flank wear and machining time
35
Effective Zone of C T Coolant & Drv cut
q =zone of High Pressure Coolant
1
I
a.=0.35mm J:=0.05mnlitooth
30
f 25 1 j
>
14
20
-
I
FW
C ._
Effective Zone of High Pressure Coolant
Effective Zone of Conventional Coolant
12 10
E ..--a
15
P = 0
6
10
z
4
2
5
0
0 50
200
.10
Cutting Speed (nJnin)
Feed Rate (mmhooth) (b) Variation with feed rate
(a) Variation with speed 12
I( Effective zone of Drv cut
Effective Zone of High Pressure coolant
.
II
F d
f p 0 05mm/toolh
Fwl
! e l 50 m/mm
FW= Flank wear C = Chipping
Conventional coolant
"
.I5
.20
.25
.35
~
.50
1
.55
57;
1.o
Depth of Cut (mm) (c) Variation with depth of cut Figure 5: Tool life and failure mode of carbide inserts Figures 6(a) and 6(b) show 2-0 and 3D SEM photographs of worn inserts respectively at optimum cutting condition. It is clear from the SEM pictures that inserts fail because of nose chipping for both dry cut and conventional coolant and largest wear zone is observed for dry cut, followed by conventional coolant. No chipping is observed for high pressure coolant and wear zone is also the smallest. Figures 7(a) and 7(b) show 3-D SEM photograph of worn inserts at a higher speed (225mimin) and a higher depth of cut (1.OOmm) respectively. At higher speed, catastrophic failure of inserts is observed for both dry cut and conventional coolant whereas only progressive flank wear is seen for high pressure coolant as shown in Figure 7(a). At higher depth of cut inserts fail because of chipping at depth of cut area for both conventional and high pressure coolant whereas no chipping is observed for dry cut as shown in Fig 7(b). 3.2 Cutting forces The cutting forces with change of depth of cut are plotted in Figure 8 for conventional coolant and high pressure coolant at lower depths of cut. Within the effective zone of high pressure coolant i.e. at lower depths of cut (0.150.5mm), the cutting forces with high pressure coolant are always less than that of conventional coolant. At higher depth of cut (0.55-1.OOmm), the cutting forces with conventional coolant are always less than that of high pressure coolant. The influence of feed rate on the cutting forces for the two different cooling conditions is depicted in Figure 9. Within the effective zone of high pressure coolant i.e. at lower
feed rate (0.02-0.05mm/tooth), all the components of cutting forces are higher with conventional coolant as compared to high pressure coolant. But at higher feed rate, z-component of cutting force is lower with conventional coolant as compared to high pressure coolant. Comparative study of dynamic cutting forces during total machining time between conventional and high pressure coolant is shown in Figure 10. When the insert is new, the cutting force is steady for a while during machining and then it starts fluctuating between high and low values. This fluctuation becomes more prominent with further increase of tool wear. If the inserts suffer any breakage or chipping, the cutting force jumps to very high magnitude and stays for a while and then drops, which is observed for conventional coolant after approximately 13 minutes of machining as shown in Figure 10. But for high pressure coolant, no sudden increase of cutting force is observed and during full duration of machining all components of cutting forces with high pressure coolant are less compared to conventional coolant. Within the effective zone of high pressure coolant, the reduction in the cutting force with high pressure coolant could be due to several reasons. High pressure coolant jet strikes the chip at a point very close to the cutting edge of insert, removing the serrated portion of the chip, resulting in the reduction of the width of the chip (SEM analysis of chips is discussed in section 3.4). Consequently the toolchip contact area on the rake face is reduced. which reduces the friction between insert and chip and eventually reduces cutting force.
49
Dry cut
Conventional coolant
High pressure coolant
Figure 7(b): 3-D SEM photographs of worn inserts ( aa=l.OOmm, fz=0.05mm/tooth,v=150m/min) 3.3 Surface Finish
Figure 11 shows the variation of surface roughness with machining time for optimum cutting condition. In all the cases, within the effective zone of high pressure coolant, best surface finish is obtained with high pressure coolant, followed by conventional coolant and then dry cut. It is very encouraging to note that the surface finish obtained with high pressure coolant in milling is within the range of (Ra=0.5-0.7pm)which is very close to finishing operations of grinding. Sometimes roughness of surface produced by conventional coolant is better than high pressure coolant. When chipping occurs, the surface obtained by dry cut is the worst and shows very high value of average surface roughness because of melting of chips on the work-piece surface. The improvement of surface finish with the use of coolant is attributed to the fact of better lubrication. Moreover, because of effective cooling with high pressure coolant within the effective zone tool wear rate becomes
50
slower. This phenomenon may also be the cause of improved surface finish. 3.4 Chip Shape The type of chip produced during machining depends on the material being machined, the tool, the cutting conditions, and the existence or absence of coolant. Figure12 (a) shows the SEM photograph of chips for dry cut, conventional coolant and high pressure coolant. For most of the cutting conditions, the chip shape is short and tubular and generally the chips produced by dry cut are dark blue color and at higher depth of cut, the color is yellowish. The dark blue color of chips, produced by dry cut, indicates the extreme heat generation at the tool-chip interface, which results in burnt chips. The chips produced by conventional coolant are black, which also indicates high heat generation at tool-chip interface; certain portion of which is carried away by chips and conventional coolant does not provide proper cooling effect at the tool-chip
225
250 X-Component
g 8 '0
0 LL
cn
125
5
Y-Component
150
250
9cn 200
p loo .-c -.---..C.COOlant
C .-5
..... .. c
550
2 150
Coolant
0 025
03
100
015 02 025 0 3 035 0 4 045 0 5
035 0 4 045 0 5
-HP.
Coolalt
i=/
015 0 2 025
Depth of cut(mn)
Depth of Cut(mn)
..c. COOlalt
0
H P Coolant
015 0 2
__--
3
LL
C
300
0 3 035 0 4 045 0 5
Depth of Cut(mn)
Figure 8: Variation of cutting force with depth of cut for conventional and high pressure coolant Cutting condition: fz=0.05mm/tooth, v=l50m/min
-t 3
210
,
190
a
X -Component
170
0 LL 150
cn c
3
300 250
f
Y-Component
130
- . ~.- . - C . Coolant
110
2
9
150
2
.g
130
.-c
200
. - . ~.. . C
3
5
Coolant
0 110
008
-HP
100
002
01
004
Feed Rate (Mtooth)
008
006
_ _ _ . ._ _._ _ _
. ~.- .. .c Coolant
0
P Coolant
-H
006
--
150
90 004
2-Component
cn
90 002
I
I
01
Coolant
I
I
002
004
Feed Rate ( M t o o t h )
006
008
01
Feed Rate ( M t o o t h )
Figure 9: The influence of feed rate on cutting forces for two different cooling Cutting condition: aa=0.35mm, v=l50m/min 875 775 275
. - - . . - .C. Coolant
_.... - .c.Coolant
225 H -
.
P Coolant
_..
H.P
.--, ...-
*
175
125 0.25
. ,**---
*
*
6 .
Coolant
- _.
.-
- - ,
('
0)
C
125 3
575
85
2 - C o m p o nent
11 25
Total cutting time(rrin)
14
025
475
175 3
575
85
11.25
14
0.25
Total Cutting lim (nr'n)
3
575
8.5
11 25
14
Total cutting time(nr'n)
Figure 10: Comparative study of cutting forces during total machining time between conventional and high 0 pressure coolant. Cutting condition: aa=0.35mm, fz=0.05mm/tooth, ~ 1 5 m/min interface. Whereas the chips produced by high pressure coolant supply have silver color (may be due to insufficient supply of oxygen) indicating that the chips are not burnt and high pressure coolant provides necessary cooling effect. Figure 12(b) depicts SEM photographs of width of chips. The width of chips produced by high pressure coolant is smaller as compared with conventional coolant and dry cut, which reduces the tool-chip contact area and may reduce frictional force at tool-chip interface. The chips produced by conventional coolant are serrated; indicating the self-tendency of breakage, which is absent in the case of dry cut chips. When the high pressure coolant is used, high velocity of coolant breaks the serrated portion and reduces the width of chips. 3.5 Cooling Effect Figure 13 shows the typical temperature profiles of workpiece with machining time for dry cut, conventional coolant and high pressure coolant respectively. Temperature profiles for dry cut show gradual increase of temperature in the work-piece with machining time and reach peaks when the milling cutter passes the thermocouple locations and then gradually decrease. Temperature increases sharply when the milling cutter passes the thermocouple locations for conventional coolant and then decreases sharply. This increase and decrease of temperature is sharper for high pressure coolant. Maximum temperature rise in work-piece for dry cut is 75OC, for conventional coolant is 37.5OC, and
for high pressure coolant is 27.5OC for the optimum cutting condition. High pressure coolant decreases the work-piece temperature by 26% in comparison with conventional coolant and by 63% in comparison with dry cut indicating better cooling effect. Above results clearly indicate the effectiveness of high pressure coolant in reducing the temperature.
-A-
-e-
aa=0.35rnm fz=.05mm/tooth ~150rnlrnin
CCoolant HP Coolant
I
0
5
10
15
20
Machining lime(m'n) Figure 11: Variation of average surface roughness with machining time 4
CONCLUSIONS
1.
High pressure coolant is effective in terms of tool wear and tool life at higher speed (125-225m/min), lower depth of cut (0.15-0.50mm) and lower feed rate (0.02-
51
Conventional coolant
oa 60
Maximm Tenp=37S0C
High pressure coolant
(a) Magnification = I 8 (b) Magnification =I00 Figure 12: SEM photographs of chips
2.
3.
4
5
6.
7.
5
0.05mm1tooth). At 17 bar, the optimum speed, feed and depth of cut are 150 mlmin, 0.05 mm1tooth and 0.35mm respectively. Moreover, high pressure coolant supply suppresses premature fracture and catastrophic failure of inserts at high cutting speed of 225 m/min. Within the effective zones of high pressure coolant, inserts fail mostly because of chipping for dry cut and conventional coolant, whereas progressive flank wear is observed for high pressure coolant supply. At higher depths of cut, the use of coolant is found to be detrimental to tool wear and tool life because of severe cyclic thermal shock at the depth of cut region. Inserts fail because of chipping at the depth of cut region for both conventional and high pressure coolant supply, whereas only progressive flank wear is observed for dry cut. Within the effective zones of high pressure coolant, there is a significant reduction of the cutting forces with the application of high pressure coolant as compared to conventional coolant. Surface finish obtained with the use of high pressure coolant is much better than those obtained for conventional coolant and dry cut for most of the cutting conditions. Under certain cutting conditions surface finish obtained with the use of high pressure coolant is almost comparable to that obtained in grinding operation. Worst surface finish is obtained during dry cut at higher speed because of chipping of inserts and melting of chips on work-piece because of high temperature rise at the cutting zone. The serration produced due to the application of coolant is removed by high pressure coolant causing the reduction of the width of chip, which reduces toolchip contact area. High pressure coolant jet reduces the work-piece temperature by effective forced convection, showing better cooling effect. REFERENCES ~~
[ I ] Pigott, R. J. S., Colwell, A. T., 1952, Hi-Jet System for Increasing Tool Life, SAE Q. Trans., 613547466, [2] Sharma3c. w.B.8 R.l 19712Some Effects of Injecting Cutting Fluids Directly into the Chip-Tool Interface, J. of Engng Ind, Trans. ASME, 931441444.
52
0
25
50
75
100 125 150 175
Machining time (sec) Figure 13: Typical temperature profiles of workpiece with machining time Nagpal, B. K., Sharma, C. S., 1973, Cutting Fluid Performance---Part I---Optimization of Pressure for Hi-Jet Method of Cutting Fluid Application, J. of Engng Ind, Trans. ASME, 95881-889. [4] Kurimoto, T., Barrow, G., 1982, The Influence of Aqueous Fluid on the Wear Characteristics and Life of Carbide Cutting Tool, Annals of the CIRP. 31/1:1923. [5] Wertheim, R., Rotberg, J., Ber, A,, 1992, Influence of High-pressure Flushing Through the Rake Face of the Cutting Tool, Annals of the CIRP, 41/1:101-106. [6] Ezugwu, E. O., Machado, A. R., Pasby, I. R., Wallbank, J., 1990, The Effect of High-pressure Coolant Supply When Machining a Heat Resistant Nickel-Based Superalloy, J. of Society of Trib. & Lub. Engineers, 471 9:751-757. [7] Machado, A. R., Wallbank, J., 1994, The Effects of a High Pressure Coolant Jet on Machining, J. of Engng Manufacture, 208:29-38. [8] Machado, A. R., Wallbank, J., Pasby, I. R., Ezugwu, E. O., 1998, Tool Performance and Chip Control when Machining Ti6A14V and lnconel 901 Using High Pressure Coolant Supply, Machining Science and Technilogy, 211:I-12. [9] Rasch, F.O., Vigeland, T., 1981, Hydraulic Chip Breaking, Annals of the CIRP, 30/1:333-335. [ l o ] Kovacevic, R., Mohon, R., Cherukuthota, C., 1993, High Pressure Waterjet as a CoolanffLubricant in Milling Operation, Manufacturing Science and Engng, PED, ASME Winter Annual Meeting, 64:733-748. [I I] Kovacevic, R., Cherukuthota, C., Mazurkiewicz, M., 1995, High Pressure Waterjet Cooling/Lubrication to Improve Machining Efficiency in Milling, International J. of Machine Tools and Manufacture, 35/10:14591473. [I21 Kovacevic, R., Cherukuthota, C., Mohon, R., 1995, Improving Milling Performance with High Pressure Waterjet Assisted CoolinglLubrication, J. of Engng Ind, Trans. ASME, 117/3:331-339.
[3]