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Cutting performance of end mills with different helix angles
Int. J. Mach. Tools Manufact. Printed in Great Britain
Vol. 29, No. 2. pp.217--227, 1989.
CUTTING
0890-6955/8953.00 + .00 Pergamon Press plc
PERFORMANCE OF END MILLS DIFFERENT HELIX ANGLES
WITH
S. EMA* a n d R . DAVlESt
(Received 17 June 1988) Abstract--In this paper an investigation of the cutting performance of several end mills with right and left hand helix angles is reported. Cutting tests were performed on aluminum alloy L65 for three milling processes in which cutting force, surface roughness and concavity of a machined plane surface were measured. The cutting performance of the end mills was assessed using variance analysis. The investigation showed that end mills with left hand helix angles are generally less effective than those with right hand helix angles, except for the case where the helix angle is small. The effects of spindle speed, depth of cut and feed rate on the cutting force and the surface roughness are generally significant. There is no significant difference between down milling and up milling with regard to the cutting force, although the difference between them regarding the surface roughness was large.
1. INTRODUCTION
ONE OF the most common metal removal operations used in industry is the end milling process. In the past much work has been done to improve the cutting performance of end mills [1-4], but the effects of helix angle have received little attention. No study of end mills with left hand helix angle has been reported. In this investigation, five end mills with right and left hand helix angles were used. Cutting tests were carried out in order to investigate the effects of cutting parameters on cutting performance of each end mill and also to make a comparison of the cutting performance of the end mills. The cutting tests were performed on aluminum alloy L65 for three milling operations, i.e. side milling, slot milling and face milling in which cutting force, surface roughness and concavity of a plane machined surface were measured. The cutting performance of the end mills was assessed using variance analysis. 2.
CU'ITING PERFORMANCE INDICES AND EQUIPMENT
In this investigation, cutting force, surface roughness of machined workpieces, concavity of a plane machined surface and observations of cutting stability were taken as indices for the assessment of the cutting performance of end mills with different helix angles. Cutting tests were carried out on a Hayes milling machine with a vertical milling head. The cutting force was measured by a piezo-electric three-component dynamometer via a charge amplifier with a calibration factor of 500 N V-1. A block diagram of the experimental set-up is shown in Fig. 1. The surface roughness of the machined workpiece and the concavity of a plane machined surface were measured using a Taylor-Hobson Talysurf 10. Cutting stability, i.e. occurrence of chatter vibration, was assessed from observations of variations of the cutting force together with chatter marks on the machined workpiece surface. 3.
END MILLS AND WORKPIECE
As shown in Fig. 2, five end mills with different helix angles were used. From the left side in the figure, three tools have right hand helix angles of 45, 25 and 5°. The *Department of Education, Gifu University, Japan; Honorary Research Fellow, Department of Mechanical Engineering, University of Birmingham, Birmingham, U.K. eDepartment of Mechanical Engineering, University of Birmingham, Birmingham. U.K. 217
218
S. EMA and R. DAVIES
~
SPINDLE END MILL
I..... ':
MILLING MACHINE
r~
i I ~.',~.IFI~I n .....
I
I
FIG. 1. Milling test and experimental set-up.
FIG. 2. End mills (45 R H H tool, 25 R H H tool, 5 RHH tool, 25 LHH and 45 LHH tool).
remainder have left hand helix angles of 25 and 45 ° . In order to simplify the description of the tools, the one with a right hand helix angle of 45 ° for example, is simply coded as 45 R H H and a tool with a left hand helix angle of 45 ° is coded as 45 LHH. The other tools are coded in the same manner. All the tools had two blade inserts of cemented carbide, grade K20, which are brazed on to a high speed steel shank. The dimensions of the tools are 25 mm diameter, 114 mm overall length and 45 mm flute length. The cutting angles of the end mills used are indicated in Fig. 3. Two end mills of each type were used. In the cutting tests, blocks measuring 150 x 90 × 45 mm in aluminum alloy L65 were machined. The hardness of the material was 146 HB. All tests were carried out without coolant. 4.
E X P E R I M E N T A L METHOD
In order to investigate the effect of cutting parameters on the cutting performance of each end mill and also to make a comparison of the cutting performance of the end mills, cutting tests were carried out for three types of operation: (a) side milling, (b) slot milling and (c) face milling as shown in Fig. 4. Details are given below.
4.1.
Side milling
The cutting performance of the five end mills were investigated at various spindle speeds, depths of cut, feed rates and two types of milling operation, i.e. down milling or up milling. Down milling corresponds to a cutting operation in which the workpiece
Cutting ,Performance of End Mills
219
A
SECTION A - A PRIN/~Y CLEARANCEANGLE: SECONDARY CLEARANCEANGLE: HOOK ANGLE: PRINARY CLEARANCEANGLE: SECONDARY CLEARANCE ANGLE:
K Ib "r $ 8
-
5° [
14 ° • 55° • 10°
1
E~O TOOTH
FIG. 3. Cutting angles. is f e d in t h e s a m e d i r e c t i o n as t h e r o t a t i o n o f t h e c u t t e r [6]. I n u p m i l l i n g , t h e w o r k p i e c e is f e d in t h e o p p o s i t e d i r e c t i o n t o t h e r o t a t i o n o f t h e c u t t e r . T h e f o u r v a r i a b l e s a r e s u m m a r i z e d in T a b l e 1. S i n c e c u t t i n g t e s t s a r e t i m e c o n s u m i n g a n d e x p e n s i v e in t e r m s o f m a t e r i a l u s e d , a 24 factorial design of experiment for each end mill was used which consisted of sixteen d i f f e r e n t c o m b i n a t i o n s o f v a r i a b l e s as g i v e n in T a b l e 2a. T h e s e t t i n g s u s e d in v a r i o u s TABLE 1.
Designation
Variables
Values of different levels
Description
Low ( - )
High (+)
Down milling 1500 12 200
Up milling 3000 24 400
A B C D
Milling operation Spindle speed (rpm) Depth of cut (mm) Feed rate (mm min-~)
TABLE 2. (a) Factorial design Test no. A B 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
VALUES OF TEST VARIABLES
+ +
+ +
÷ +
+ +
+ +
+ +
÷ +
+ +
C
+ + + + + + + +
D E S I G N MATRIX OF A
24
FACTORIAL DESIGN
D
(b) Variable settings A B (rpm)
+ + + + + + + +
Down milling Up milling Down milling Up milling Down milling Up milling Down milling Up milling Down milling Up milling Down milling Up milling Down milling Up milling Down milling Up milling
1500 1500 3000 3000 1500 1500 3000 3000 1500 1500 3000 3000 1500 1200 3000 3000
C D (ram) (ram rain -1) 12 12 12 12 24 24 24 24 12 12 12 12 24 24 24 24
200 200 200 200 200 200 200 200 400 400 400 400 400 400 400 400
220
S. EMA and R. DAvms
tests are listed in Table 2b. For all side milling tests the width of cut was fixed at 5 mm. In this series of tests, three components of the cutting forces, i.e. the normal component Fn, the feed component FI and the vertical component Fv as illustrated in Fig. 4a, were measured. For each cut, the average of the maximum peak values of each component was obtained. Since the variance analysis would become very complicated if the effects of cutting parameters were assessed for each individual cutting force component, the analysis was carried out using the specific cutting force Fs obtained from the following equation
(a)
oe .lu.
' ; ~ I ")
(b)
(c)
FIG. 4. Milling process. (a) Side milling. (b) Slot milling. (c) Face milling.
Although the specific cutting force Fs does not represent the instantaneous resultant cutting force, it is useful and effective for investigating the effects of the cutting performance of each end mill and for comparisons between different end mills. After each cut, the side surface roughness and the bottom surface roughness of the machined workpiece were measured using the Ra value. The side and bottom surface roughnesses were obtained by averaging the measured values at a minimum of six points located along the central line of the surface. 4.2. Slot milling In these tests illustrated in Fig. 4b, the cutting performances of the 45 RHH tool, the 25 R H H tool and the 25 LHH tool were investigated at various feed rates and depths of cut. The 5 RHH tool was not used because of chatter and heavy tool loading. Build-up of material in the clearance space prevented use of the 45 LHH tool. For each cut, the three components of the cutting force and the bottom surface roughness were measured. 4.3. Face milling The plane surface illustrated in Fig. 4c was produced with an overlap of 1 mm between neighbouring cutter passes. After producing each surface, the concavity of the plane along the line A-B, which is perpendicular to the tool pass, was measured. Values of the concavity were obtained at various feed rates and depths of cut. The same tool were used as for slot milling. 5,
5.1.
EXPERIMENTAL RESULTS AND DISCUSSIONS
Side milling
The experimental results obtained from the use of the 45 RHH tool, the 25 RHH tool, the 5 R H H tool and the 25 LHH tool are summarized in Tables 3, 4, 5 and 6 respectively. In each table, the values of F,, Fr and Fv are listed together with the side surface roughness and the bottom surface roughness. These data are the average values of two replicants.
Cutting Performance of End Mills
221
TABLE 3. SUMMARY OF EXPERIMENTAL RESULTS FOR THE 45 R H H TOOL
Test no.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Normal component
Feed component
Vertical component
Fn (N)
Fr (N)
F, (N)
720 110 430 100 760 110 530 90 970 170 570 110 860 170 670 160
320 720 170 600 350 920 180 670 540 1070 220 610 520 1220 260 930
190 200 160 120 220 260 130 150 350 350 190 170 280 300 200 270
Side surface roughness
Bottom surface roughness
(o.m)
(gin)
0.64 0.56 0.40 1.27 0.50 0.95 0.43 1.59 0.54 0.50 0.45 1.08 1.10 1.20 0.41 0.89
0.76 0.82 0.93 1.06 0.87 1.31 0.79 1.25 1.41 1.71 0.75 1.13 1.75 2.13 0.95 1.61
TABLE 4. SUMMARY OF EXPERIMENTAL RESULTS FOR THE 25 R H H TOOL
Test no.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Normal component
Feed component
Vertical component
F~ (N)
F~ (N)
F,. (N)
880 180 580 130 1290 210 860 100 1340 290 770 180 1780 280 940 120
380 910 210 550 590 1410 360 900 620 1420 310 770 1010 2060 430 1060
140 130 80 70 190 200 130 110 250 240 140 130 350 330 170 160
Side surface roughness
Bottom surface roughness
(l~m)
(l~m)
0.41 0.30 0.26 0.64 0.49 0.46 0.33 0.55 0.47 0.41 0.34 0.44 0.69 0.86 0.48 0.87
0.42 0.42 0.32 0.35 0.49 0.58 0.34 0.44 0.50 0.78 0.46 0.56 0.69 0.84 0.50 0.57
A dash in Table 5 for the 5 RHH tool shows that the experimental result for this cutting condition could not be obtained because of chatter vibration and overloading of the tool. No experimental results were obtained from the 45 LHH tool because of build-up of machined metal in the clearance spaces at the tool end, which caused severe overloading. This is shown in Fig. 5 and was due to metal being forced downward along the helix angle. The procedure for variance analysis is shown in Table 7, in which an analysis of the specific cutting force Fs obtained from the use of the 25 RHH tool was carried out. The effects of the milling operation, spindle speed, depth of cut and feed rate on the specific cutting force are shown. The values of the variance ratios marked with an asterisk in Table 7 are significant, the remainder being insignificant. The variance ratios of specific setting forces obtained from the use of the 45 RHH tool, the 25 RHH tool and the 25 LHH tool are summarized in Table 8. The ratio for the 5 RHH tool could not be obtained because all the experiments required for the 24 factorial design were not performed as mentioned in Table 5. These variance analyses show that the effects of the spindle speed, the depth of cut and the feed rate on the specific cutting force are significant at a level of 0.05. The results show that the slower the slaindle speed, the deeper the depth of cut and the faster the feed rate, the greater
222
S. Er,tA and R. DAVIES TASTE 5. SUMMARY OF EXPERIMENTAL RESULTS FOR THE
Test no.
Normal component F~ (N)
1 2 3 4 5 6 7
1580 . 970 . 1900
8
.
Feed component F/(N)
430 . 240 .
. 730
14
15 16
400 .
2320 . 1390 . 2730 . 2320 .
70 . 50 . 70 -80
.
1610
9 10 11 12 13
Vertical component Fv (N)
.
360 . 1240 .
1.25
0.61
1.02
0.38
0.83 -0.76
0.65 -0.38
0.86
0.88
0.94
0.49'
2.02
3.81
0.87
0.60
.
. .
.
.
580 .
Bottom surface roughness (~m)
.
100 . 80 . 120
.
.
Side surface roughness (~,m)
.
710 .
5 R H H TOOL
110 .
.
.
TABLE 6. S U M M A R Y OF EXPERIMENTAL RESULTS FOR THE 25 L H H
Test no.
Normal component F. (N)
Feed component Ff (N)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
880 200 540 120 1260 260 780 200 1290 300 710 180 1560 330 990 230
450 950 240 500 600 1460 300 850 740 1490 360 750 840 2140 460 1220
Vertical component Fv (N)
TOOL
Side surface roughness (p.m)
Bottom surface roughness (Ixm)
0.59 0.90 0.35 1.51 0.59 2.08 0.37 1.15 0.72 0.89 0.49 0.89 1.21 2.06 0.55 0.89
0.75 0.84 0.36 0.53 0.76 1.32 0.41 0.46 1.66 1.64 0.69 0.62 1.90 2.34 0.60 0.71
-200 -230 - 100 - 110 -240 -300 -110 - 180 -370 -440 - 170 - 200 -370 -470 - 180 -280
TABLE 7. VAaL~NCE ANALYSIS FOR SPECIFIC CUTTING FORCE FOR THE 25 R H H TOOL
Source of variance
Sum of squares
Milling operation Spindle speed Depth of cut Feed rate Residual error
1.309x los 2.611x 10~ 1.106x107 9.586x l0 s 4.031x los
*F(1,11)0.05 = 4.84.
Degree of freedom 1 1 1 1 11
Mean squares
Variance ratio
1.309x los 2.611x 107 1.106x10 ~ 9.586x los 3.665x 10s
0.04 71.24 30.17 26.16
Significance
* * *
Cutting Performance of End Mills
223
~iii~i~¸~¸
FIG. 5. Build-up of machined metal (45 LHH tool). TABLE 8. VARIANCE RATIOS OF SPECIFIC CuT'rING FORCE
Source of variance Milling operation Spindle speed Depth of cut Feed rate
45 R H H tool 4.06 53.30* 5.84* 26.49*
25 RHH tool 0.04 71.24" 30.17" 26.16"
25 LHH tool 1.83 112.53" 42.49* 45.84*
*F(1,11)0.05 = 4.84.
is the specific cutting force Fs. The effect of the milling operation, i.e. down milling or up milling, on the specific cutting force is insignificant. Tables 9 and 10 show the variance ratios of side surface roughness and bottom surface roughness which were obtained from the use of the 45 R H H tool, the 25 R H H tool and the 25 LHH tool. The milling operation tends to be significant for both side and bottom roughness. The side surface roughness is less affected by the other variables, while the bottom surface roughness is markedly affected by the cutting conditions. From the variance analyses described above, the cutting performance of each end mill has been determined. The milling operation is not significant for the specific cutting force but is significant for the surface roughness. Therefore in order to simplify the cutting performance comparison between the end mills, an assessment has been made using only the experimental results obtained from down milling. Figure 6 shows the average values of the normal component, the feed component, the vertical component and the specific cutting force which were obtained by arithmetically
S. EMA and R. DAVIES
224
TABLE 9. VARIANCE RATIOS OF SIDE SURFACE ROUGHNESS Source of variance
45 R H H tool
25 R H H tool
Milling operation Spindle speed Depth of cut Feed rate
7.83* 0.17 1.63 0.02
3.58 0.10 6.79* 4.00
25 LHH tool 13.73" 3.64 2.98 0.01
*F(1,11)0.05 = 4.84.
TABLE 10. VARIANCE RATIOS OF BO'FI'OM SURFACE ROUGHNESS
Source of variance
45 R H H tool
25 RHH tool
Milling operation Spindle speed Depth of cut Fe~d rate
7.70* 5.12" 4.26 13.00"
12.67" 26.23* 7.72* 44.67*
25 LHH tool 1.36 35.85* 1.53 17.20"
*F(1,11)0.05 -- 4.84.. 2OOO
2.~HH - 4.TdiHH 17.21 t") 2sx. sH se.69 (*)
I~ 19"~1
I'~ I I
Fn
0
4SRtlH
25RHH TYPEOF'~
SRHH MILL
25L,HH
FIG. 6. Average cutting force components and specific force.
averaging all the values in each column for down milling. For the 45 RHH tool, the 25 R H H tool and the 5 RHH tool, the average vertical component decreases with decreasing helix angle, although the remaining forces show a significant increase. The cutting performance of the 25 LHH tool is similar to that of the 25 RHH tool. These results can be attributed to the fact that the helix angle, and an effective rake angle related to it, alter the magnitude and direction of the resultant cutting force. In order to confirm statistically the differences in the cutting performance between the tools, the variance analysis wascarried out in which tool type was substituted for the milling operation in the test variables described in Table 1, i.e. setting the 25 RHH tool at a low level and another tool at a high level. By this method, the specific cutting force of any tool can be compared directly with that of the 25 RHH tool. The variance ratios for the tool type are shown in the table attached to Fig. 6. It is clear that the difference between the 25 RHH and 45 RHH tool and between the 25 RHH and 5 RHH tool, is significant, while the difference between the 25 RHH and 25 LHH tool is insignificant. The same process as described for the cutting force has also been carried out for side surface and bottom surface roughness. Figure 7 shows the average surface roughness values and the table in the figure shows the variance ratios for the tool type. It is clear that the smoothest surface is produced using the 25 RHH tool, while the roughest surface is produced using the 5 R H H tool. From the results obtained for side milling, it was found that the 45 RHH tool had the lowest cutting force but the 25 RHH tool produced the lowest value of surface
Cutting Performance of End Mills
225
1.5 SIDE BOTTOM 3.53 29.12 (*) 25RHH - 5,qHH 19.76 (m) 1,83 25R1.l.I - 2SI.HH 8.18 (*) 8.17 (*)
E
2SRHH- 45R1~
~1.0
O.S
dqC
45RHH
25RHH
SRI.IH
2S1.~
TYPEOF ENDMILL FIG. 7. Average side and bottom surface roughness.
roughness. The 5 R H H tool was the least effective, being more prone to chatter and to producing high impact loads due to the almost instantaneous engagement of the full length of the insert with the workpiece at the beginning of chip formation [7]. 5.2.
Slot milling
The tests for slot milling were carried out using the 45 RHH, 25 RHH and 25 LHH tools. The 25 LHH tool however could not cut effectively due to the build-up of machined metal in the tool clearance spaces as described in Fig. 5. The 5 RHH tool was not used in following preliminary tests which showed that it was equally ineffective for slot milling as it was for side milling. Figure 8 shows the relationships between the feed rate and three components of the cutting force. They tend to increase with depth of cut and feed rate. For depths of cut of 4 mm and 6 mm, the normal and feed components of the 25 R H H tool are similar to those of the 45 R H H tool, although those of the former are greater when the depth of cut is 8 ram. The results for bottom surface roughness are shown in Fig. 9. For both end mills the surface roughness increases with the feed rate but is not affected by the depth of cut except for the depth of cut of 8 mm with the 25 R H H tool. These results coincide with those for side milling which were indicated by the variance ratios for the depth of cut and the feed rate in Table 10. The surface roughness produced by the 45 R H H tool was much greater than that produced by the 25 RHH tool. This discrepancy is probably due to the difference in the angle ~ which is shown in Fig. 3. This angle significantly affects theoretical surface roughness and consequently, the bottom surface roughness may be improved by reducing the front clearance angle of the end of the inserts. 5.3.
Face milling
The tests for face milling were carried out using the 45 R H H tool and the 25 RHH tool. Figure 10 shows the method for measuring the concavity of a plane. The concavity was evaluated using the value of the maximum depth indicated in the figure. This concavity results from deflection of the tool due to the action of the cutting force. Figure 11 shows the relationship between the depth of the concavity and the feed rate. As would be expected the depth is in proportion to the feed rate and to the depth of cut, although the depth for the 45 RHH tool is slightly greater than that for the 25 RHH tool. This result can be attributed to the difference in bending rigidity between the end mills used, the 45 RHH tool being less rigid than the 25 RHH tool.
226
S. E M A and R. DAVIES
:1
+,+1+ 300
500 PATE
700 (mm/~n)
300
500 100 reED I~TE {mm/mln)
300
500 FEED P A T E
FIG. 8. Cutting force components (spindle speed 2100
100 (mm/mln)
rpm).
5
°---" =
:
25RflH 4SRHH
J
4
~
+,
~
.
:::+::,
I 500
0
300
v
400
600
700
FEEDRATE (mmlmin) FIG. 9. Bottom surface roughness (spindle speed 2100 rpm). Depth of concavity
IlO/~m
5 mm I I
FIG. 10. Concavity of a machined surface and its evaluation. 6.
CONCLUSIONS
The cutting force, the surface roughness and the concavity of a machined surface have been measured using five end mills having various helix angles. The effects of the cutting parameters on each end mill and the comparison of cutting performance of the end mills have been investigated using variance analysis. The following conclusions can be made. (1) The effects of spindle speed, depth of cut and feed rate on the cutting force and the surface roughness are significant. There is no difference between down milling and up milling with respect to the cutting force, but there is a significant difference between them with respect to surface roughness, down milling giving the smoother surface. (2) End mills with small helix angles develop the greatest cutting force and surface roughness due to chatter and reduction of the period when the cutting inserts engage
Cutting Performance of End Mills 70
227
I
~ - - - - * 25RHH : 45RHH
DEPTH OF
4 mm
200
350
500 I~F'D R A T E
650 (mmlmin)
800
FIG. 11. Depth of concavity (spindle speed 2100 rpm). with the workpiece material. Furthermore, end mills with left hand helix angles are relatively ineffective. (3) E n d mills with right hand helix angles of 45 and 25~ are very effective and would be r e c o m m e n d e d for the types of milling o p e r a t i o n detailgd in this p a p e r for type 1.65 a l u m i n u m alloy. O n the grounds of tool manufacturing costs, the 25 R H H tool would be the m o s t economical. N o a t t e m p t has been m a d e to evaluate tool life for the various types of end mill, although this would be necessary in o r d e r to provide a c o m p l e t e assessment. Acknowledgements--The authors gratefully acknowledge the help of Mr D. H. Wale of Marwin Cutting
Tools Ltd in supplying the cutting tools and thank the members of the Laboratory of Machine Tool Vibration in the University of Birmingham. The first author also wishes to thank the Ministry of Education of Japan for giving him the opportunity to study in the United Kingdom. REFERENCES [1] F. KOEN1GSBERGERand A. J. P. SABBERWAL,Int. J. Mach. Tool Des. Res. 1, 15 (1961). [2] J. TLUSTYand E; ELeESTAWl,Ann. CIRP 28, 253 (1979). [3] L. S. ZHANG,M. M. SADEKand D. H. WALE, TraM. ASME J. Engng Ind. 107, 153 (1985). [4] J. W. SUTHERLANDand R. E. DEVOR, Trans. ASME J. Engng Ind. 108, 269 (1986). [5] C. CHATFELD,Statistics for Technology. Chapman and Hall, London (1983). [6] A. J. P. SABBERWAL,Int. J. Mach. Tool Des. Res. 2, 27 (1962). [7] J. TLUSTYand P. MAcNEIL,Ann. CIRP 24, 21 (1975).
Vol. 29, No. 2. pp.217--227, 1989.
CUTTING
0890-6955/8953.00 + .00 Pergamon Press plc
PERFORMANCE OF END MILLS DIFFERENT HELIX ANGLES
WITH
S. EMA* a n d R . DAVlESt
(Received 17 June 1988) Abstract--In this paper an investigation of the cutting performance of several end mills with right and left hand helix angles is reported. Cutting tests were performed on aluminum alloy L65 for three milling processes in which cutting force, surface roughness and concavity of a machined plane surface were measured. The cutting performance of the end mills was assessed using variance analysis. The investigation showed that end mills with left hand helix angles are generally less effective than those with right hand helix angles, except for the case where the helix angle is small. The effects of spindle speed, depth of cut and feed rate on the cutting force and the surface roughness are generally significant. There is no significant difference between down milling and up milling with regard to the cutting force, although the difference between them regarding the surface roughness was large.
1. INTRODUCTION
ONE OF the most common metal removal operations used in industry is the end milling process. In the past much work has been done to improve the cutting performance of end mills [1-4], but the effects of helix angle have received little attention. No study of end mills with left hand helix angle has been reported. In this investigation, five end mills with right and left hand helix angles were used. Cutting tests were carried out in order to investigate the effects of cutting parameters on cutting performance of each end mill and also to make a comparison of the cutting performance of the end mills. The cutting tests were performed on aluminum alloy L65 for three milling operations, i.e. side milling, slot milling and face milling in which cutting force, surface roughness and concavity of a plane machined surface were measured. The cutting performance of the end mills was assessed using variance analysis. 2.
CU'ITING PERFORMANCE INDICES AND EQUIPMENT
In this investigation, cutting force, surface roughness of machined workpieces, concavity of a plane machined surface and observations of cutting stability were taken as indices for the assessment of the cutting performance of end mills with different helix angles. Cutting tests were carried out on a Hayes milling machine with a vertical milling head. The cutting force was measured by a piezo-electric three-component dynamometer via a charge amplifier with a calibration factor of 500 N V-1. A block diagram of the experimental set-up is shown in Fig. 1. The surface roughness of the machined workpiece and the concavity of a plane machined surface were measured using a Taylor-Hobson Talysurf 10. Cutting stability, i.e. occurrence of chatter vibration, was assessed from observations of variations of the cutting force together with chatter marks on the machined workpiece surface. 3.
END MILLS AND WORKPIECE
As shown in Fig. 2, five end mills with different helix angles were used. From the left side in the figure, three tools have right hand helix angles of 45, 25 and 5°. The *Department of Education, Gifu University, Japan; Honorary Research Fellow, Department of Mechanical Engineering, University of Birmingham, Birmingham, U.K. eDepartment of Mechanical Engineering, University of Birmingham, Birmingham. U.K. 217
218
S. EMA and R. DAVIES
~
SPINDLE END MILL
I..... ':
MILLING MACHINE
r~
i I ~.',~.IFI~I n .....
I
I
FIG. 1. Milling test and experimental set-up.
FIG. 2. End mills (45 R H H tool, 25 R H H tool, 5 RHH tool, 25 LHH and 45 LHH tool).
remainder have left hand helix angles of 25 and 45 ° . In order to simplify the description of the tools, the one with a right hand helix angle of 45 ° for example, is simply coded as 45 R H H and a tool with a left hand helix angle of 45 ° is coded as 45 LHH. The other tools are coded in the same manner. All the tools had two blade inserts of cemented carbide, grade K20, which are brazed on to a high speed steel shank. The dimensions of the tools are 25 mm diameter, 114 mm overall length and 45 mm flute length. The cutting angles of the end mills used are indicated in Fig. 3. Two end mills of each type were used. In the cutting tests, blocks measuring 150 x 90 × 45 mm in aluminum alloy L65 were machined. The hardness of the material was 146 HB. All tests were carried out without coolant. 4.
E X P E R I M E N T A L METHOD
In order to investigate the effect of cutting parameters on the cutting performance of each end mill and also to make a comparison of the cutting performance of the end mills, cutting tests were carried out for three types of operation: (a) side milling, (b) slot milling and (c) face milling as shown in Fig. 4. Details are given below.
4.1.
Side milling
The cutting performance of the five end mills were investigated at various spindle speeds, depths of cut, feed rates and two types of milling operation, i.e. down milling or up milling. Down milling corresponds to a cutting operation in which the workpiece
Cutting ,Performance of End Mills
219
A
SECTION A - A PRIN/~Y CLEARANCEANGLE: SECONDARY CLEARANCEANGLE: HOOK ANGLE: PRINARY CLEARANCEANGLE: SECONDARY CLEARANCE ANGLE:
K Ib "r $ 8
-
5° [
14 ° • 55° • 10°
1
E~O TOOTH
FIG. 3. Cutting angles. is f e d in t h e s a m e d i r e c t i o n as t h e r o t a t i o n o f t h e c u t t e r [6]. I n u p m i l l i n g , t h e w o r k p i e c e is f e d in t h e o p p o s i t e d i r e c t i o n t o t h e r o t a t i o n o f t h e c u t t e r . T h e f o u r v a r i a b l e s a r e s u m m a r i z e d in T a b l e 1. S i n c e c u t t i n g t e s t s a r e t i m e c o n s u m i n g a n d e x p e n s i v e in t e r m s o f m a t e r i a l u s e d , a 24 factorial design of experiment for each end mill was used which consisted of sixteen d i f f e r e n t c o m b i n a t i o n s o f v a r i a b l e s as g i v e n in T a b l e 2a. T h e s e t t i n g s u s e d in v a r i o u s TABLE 1.
Designation
Variables
Values of different levels
Description
Low ( - )
High (+)
Down milling 1500 12 200
Up milling 3000 24 400
A B C D
Milling operation Spindle speed (rpm) Depth of cut (mm) Feed rate (mm min-~)
TABLE 2. (a) Factorial design Test no. A B 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
VALUES OF TEST VARIABLES
+ +
+ +
÷ +
+ +
+ +
+ +
÷ +
+ +
C
+ + + + + + + +
D E S I G N MATRIX OF A
24
FACTORIAL DESIGN
D
(b) Variable settings A B (rpm)
+ + + + + + + +
Down milling Up milling Down milling Up milling Down milling Up milling Down milling Up milling Down milling Up milling Down milling Up milling Down milling Up milling Down milling Up milling
1500 1500 3000 3000 1500 1500 3000 3000 1500 1500 3000 3000 1500 1200 3000 3000
C D (ram) (ram rain -1) 12 12 12 12 24 24 24 24 12 12 12 12 24 24 24 24
200 200 200 200 200 200 200 200 400 400 400 400 400 400 400 400
220
S. EMA and R. DAvms
tests are listed in Table 2b. For all side milling tests the width of cut was fixed at 5 mm. In this series of tests, three components of the cutting forces, i.e. the normal component Fn, the feed component FI and the vertical component Fv as illustrated in Fig. 4a, were measured. For each cut, the average of the maximum peak values of each component was obtained. Since the variance analysis would become very complicated if the effects of cutting parameters were assessed for each individual cutting force component, the analysis was carried out using the specific cutting force Fs obtained from the following equation
(a)
oe .lu.
' ; ~ I ")
(b)
(c)
FIG. 4. Milling process. (a) Side milling. (b) Slot milling. (c) Face milling.
Although the specific cutting force Fs does not represent the instantaneous resultant cutting force, it is useful and effective for investigating the effects of the cutting performance of each end mill and for comparisons between different end mills. After each cut, the side surface roughness and the bottom surface roughness of the machined workpiece were measured using the Ra value. The side and bottom surface roughnesses were obtained by averaging the measured values at a minimum of six points located along the central line of the surface. 4.2. Slot milling In these tests illustrated in Fig. 4b, the cutting performances of the 45 RHH tool, the 25 R H H tool and the 25 LHH tool were investigated at various feed rates and depths of cut. The 5 RHH tool was not used because of chatter and heavy tool loading. Build-up of material in the clearance space prevented use of the 45 LHH tool. For each cut, the three components of the cutting force and the bottom surface roughness were measured. 4.3. Face milling The plane surface illustrated in Fig. 4c was produced with an overlap of 1 mm between neighbouring cutter passes. After producing each surface, the concavity of the plane along the line A-B, which is perpendicular to the tool pass, was measured. Values of the concavity were obtained at various feed rates and depths of cut. The same tool were used as for slot milling. 5,
5.1.
EXPERIMENTAL RESULTS AND DISCUSSIONS
Side milling
The experimental results obtained from the use of the 45 RHH tool, the 25 RHH tool, the 5 R H H tool and the 25 LHH tool are summarized in Tables 3, 4, 5 and 6 respectively. In each table, the values of F,, Fr and Fv are listed together with the side surface roughness and the bottom surface roughness. These data are the average values of two replicants.
Cutting Performance of End Mills
221
TABLE 3. SUMMARY OF EXPERIMENTAL RESULTS FOR THE 45 R H H TOOL
Test no.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Normal component
Feed component
Vertical component
Fn (N)
Fr (N)
F, (N)
720 110 430 100 760 110 530 90 970 170 570 110 860 170 670 160
320 720 170 600 350 920 180 670 540 1070 220 610 520 1220 260 930
190 200 160 120 220 260 130 150 350 350 190 170 280 300 200 270
Side surface roughness
Bottom surface roughness
(o.m)
(gin)
0.64 0.56 0.40 1.27 0.50 0.95 0.43 1.59 0.54 0.50 0.45 1.08 1.10 1.20 0.41 0.89
0.76 0.82 0.93 1.06 0.87 1.31 0.79 1.25 1.41 1.71 0.75 1.13 1.75 2.13 0.95 1.61
TABLE 4. SUMMARY OF EXPERIMENTAL RESULTS FOR THE 25 R H H TOOL
Test no.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Normal component
Feed component
Vertical component
F~ (N)
F~ (N)
F,. (N)
880 180 580 130 1290 210 860 100 1340 290 770 180 1780 280 940 120
380 910 210 550 590 1410 360 900 620 1420 310 770 1010 2060 430 1060
140 130 80 70 190 200 130 110 250 240 140 130 350 330 170 160
Side surface roughness
Bottom surface roughness
(l~m)
(l~m)
0.41 0.30 0.26 0.64 0.49 0.46 0.33 0.55 0.47 0.41 0.34 0.44 0.69 0.86 0.48 0.87
0.42 0.42 0.32 0.35 0.49 0.58 0.34 0.44 0.50 0.78 0.46 0.56 0.69 0.84 0.50 0.57
A dash in Table 5 for the 5 RHH tool shows that the experimental result for this cutting condition could not be obtained because of chatter vibration and overloading of the tool. No experimental results were obtained from the 45 LHH tool because of build-up of machined metal in the clearance spaces at the tool end, which caused severe overloading. This is shown in Fig. 5 and was due to metal being forced downward along the helix angle. The procedure for variance analysis is shown in Table 7, in which an analysis of the specific cutting force Fs obtained from the use of the 25 RHH tool was carried out. The effects of the milling operation, spindle speed, depth of cut and feed rate on the specific cutting force are shown. The values of the variance ratios marked with an asterisk in Table 7 are significant, the remainder being insignificant. The variance ratios of specific setting forces obtained from the use of the 45 RHH tool, the 25 RHH tool and the 25 LHH tool are summarized in Table 8. The ratio for the 5 RHH tool could not be obtained because all the experiments required for the 24 factorial design were not performed as mentioned in Table 5. These variance analyses show that the effects of the spindle speed, the depth of cut and the feed rate on the specific cutting force are significant at a level of 0.05. The results show that the slower the slaindle speed, the deeper the depth of cut and the faster the feed rate, the greater
222
S. Er,tA and R. DAVIES TASTE 5. SUMMARY OF EXPERIMENTAL RESULTS FOR THE
Test no.
Normal component F~ (N)
1 2 3 4 5 6 7
1580 . 970 . 1900
8
.
Feed component F/(N)
430 . 240 .
. 730
14
15 16
400 .
2320 . 1390 . 2730 . 2320 .
70 . 50 . 70 -80
.
1610
9 10 11 12 13
Vertical component Fv (N)
.
360 . 1240 .
1.25
0.61
1.02
0.38
0.83 -0.76
0.65 -0.38
0.86
0.88
0.94
0.49'
2.02
3.81
0.87
0.60
.
. .
.
.
580 .
Bottom surface roughness (~m)
.
100 . 80 . 120
.
.
Side surface roughness (~,m)
.
710 .
5 R H H TOOL
110 .
.
.
TABLE 6. S U M M A R Y OF EXPERIMENTAL RESULTS FOR THE 25 L H H
Test no.
Normal component F. (N)
Feed component Ff (N)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
880 200 540 120 1260 260 780 200 1290 300 710 180 1560 330 990 230
450 950 240 500 600 1460 300 850 740 1490 360 750 840 2140 460 1220
Vertical component Fv (N)
TOOL
Side surface roughness (p.m)
Bottom surface roughness (Ixm)
0.59 0.90 0.35 1.51 0.59 2.08 0.37 1.15 0.72 0.89 0.49 0.89 1.21 2.06 0.55 0.89
0.75 0.84 0.36 0.53 0.76 1.32 0.41 0.46 1.66 1.64 0.69 0.62 1.90 2.34 0.60 0.71
-200 -230 - 100 - 110 -240 -300 -110 - 180 -370 -440 - 170 - 200 -370 -470 - 180 -280
TABLE 7. VAaL~NCE ANALYSIS FOR SPECIFIC CUTTING FORCE FOR THE 25 R H H TOOL
Source of variance
Sum of squares
Milling operation Spindle speed Depth of cut Feed rate Residual error
1.309x los 2.611x 10~ 1.106x107 9.586x l0 s 4.031x los
*F(1,11)0.05 = 4.84.
Degree of freedom 1 1 1 1 11
Mean squares
Variance ratio
1.309x los 2.611x 107 1.106x10 ~ 9.586x los 3.665x 10s
0.04 71.24 30.17 26.16
Significance
* * *
Cutting Performance of End Mills
223
~iii~i~¸~¸
FIG. 5. Build-up of machined metal (45 LHH tool). TABLE 8. VARIANCE RATIOS OF SPECIFIC CuT'rING FORCE
Source of variance Milling operation Spindle speed Depth of cut Feed rate
45 R H H tool 4.06 53.30* 5.84* 26.49*
25 RHH tool 0.04 71.24" 30.17" 26.16"
25 LHH tool 1.83 112.53" 42.49* 45.84*
*F(1,11)0.05 = 4.84.
is the specific cutting force Fs. The effect of the milling operation, i.e. down milling or up milling, on the specific cutting force is insignificant. Tables 9 and 10 show the variance ratios of side surface roughness and bottom surface roughness which were obtained from the use of the 45 R H H tool, the 25 R H H tool and the 25 LHH tool. The milling operation tends to be significant for both side and bottom roughness. The side surface roughness is less affected by the other variables, while the bottom surface roughness is markedly affected by the cutting conditions. From the variance analyses described above, the cutting performance of each end mill has been determined. The milling operation is not significant for the specific cutting force but is significant for the surface roughness. Therefore in order to simplify the cutting performance comparison between the end mills, an assessment has been made using only the experimental results obtained from down milling. Figure 6 shows the average values of the normal component, the feed component, the vertical component and the specific cutting force which were obtained by arithmetically
S. EMA and R. DAVIES
224
TABLE 9. VARIANCE RATIOS OF SIDE SURFACE ROUGHNESS Source of variance
45 R H H tool
25 R H H tool
Milling operation Spindle speed Depth of cut Feed rate
7.83* 0.17 1.63 0.02
3.58 0.10 6.79* 4.00
25 LHH tool 13.73" 3.64 2.98 0.01
*F(1,11)0.05 = 4.84.
TABLE 10. VARIANCE RATIOS OF BO'FI'OM SURFACE ROUGHNESS
Source of variance
45 R H H tool
25 RHH tool
Milling operation Spindle speed Depth of cut Fe~d rate
7.70* 5.12" 4.26 13.00"
12.67" 26.23* 7.72* 44.67*
25 LHH tool 1.36 35.85* 1.53 17.20"
*F(1,11)0.05 -- 4.84.. 2OOO
2.~HH - 4.TdiHH 17.21 t") 2sx. sH se.69 (*)
I~ 19"~1
I'~ I I
Fn
0
4SRtlH
25RHH TYPEOF'~
SRHH MILL
25L,HH
FIG. 6. Average cutting force components and specific force.
averaging all the values in each column for down milling. For the 45 RHH tool, the 25 R H H tool and the 5 RHH tool, the average vertical component decreases with decreasing helix angle, although the remaining forces show a significant increase. The cutting performance of the 25 LHH tool is similar to that of the 25 RHH tool. These results can be attributed to the fact that the helix angle, and an effective rake angle related to it, alter the magnitude and direction of the resultant cutting force. In order to confirm statistically the differences in the cutting performance between the tools, the variance analysis wascarried out in which tool type was substituted for the milling operation in the test variables described in Table 1, i.e. setting the 25 RHH tool at a low level and another tool at a high level. By this method, the specific cutting force of any tool can be compared directly with that of the 25 RHH tool. The variance ratios for the tool type are shown in the table attached to Fig. 6. It is clear that the difference between the 25 RHH and 45 RHH tool and between the 25 RHH and 5 RHH tool, is significant, while the difference between the 25 RHH and 25 LHH tool is insignificant. The same process as described for the cutting force has also been carried out for side surface and bottom surface roughness. Figure 7 shows the average surface roughness values and the table in the figure shows the variance ratios for the tool type. It is clear that the smoothest surface is produced using the 25 RHH tool, while the roughest surface is produced using the 5 R H H tool. From the results obtained for side milling, it was found that the 45 RHH tool had the lowest cutting force but the 25 RHH tool produced the lowest value of surface
Cutting Performance of End Mills
225
1.5 SIDE BOTTOM 3.53 29.12 (*) 25RHH - 5,qHH 19.76 (m) 1,83 25R1.l.I - 2SI.HH 8.18 (*) 8.17 (*)
E
2SRHH- 45R1~
~1.0
O.S
dqC
45RHH
25RHH
SRI.IH
2S1.~
TYPEOF ENDMILL FIG. 7. Average side and bottom surface roughness.
roughness. The 5 R H H tool was the least effective, being more prone to chatter and to producing high impact loads due to the almost instantaneous engagement of the full length of the insert with the workpiece at the beginning of chip formation [7]. 5.2.
Slot milling
The tests for slot milling were carried out using the 45 RHH, 25 RHH and 25 LHH tools. The 25 LHH tool however could not cut effectively due to the build-up of machined metal in the tool clearance spaces as described in Fig. 5. The 5 RHH tool was not used in following preliminary tests which showed that it was equally ineffective for slot milling as it was for side milling. Figure 8 shows the relationships between the feed rate and three components of the cutting force. They tend to increase with depth of cut and feed rate. For depths of cut of 4 mm and 6 mm, the normal and feed components of the 25 R H H tool are similar to those of the 45 R H H tool, although those of the former are greater when the depth of cut is 8 ram. The results for bottom surface roughness are shown in Fig. 9. For both end mills the surface roughness increases with the feed rate but is not affected by the depth of cut except for the depth of cut of 8 mm with the 25 R H H tool. These results coincide with those for side milling which were indicated by the variance ratios for the depth of cut and the feed rate in Table 10. The surface roughness produced by the 45 R H H tool was much greater than that produced by the 25 RHH tool. This discrepancy is probably due to the difference in the angle ~ which is shown in Fig. 3. This angle significantly affects theoretical surface roughness and consequently, the bottom surface roughness may be improved by reducing the front clearance angle of the end of the inserts. 5.3.
Face milling
The tests for face milling were carried out using the 45 R H H tool and the 25 RHH tool. Figure 10 shows the method for measuring the concavity of a plane. The concavity was evaluated using the value of the maximum depth indicated in the figure. This concavity results from deflection of the tool due to the action of the cutting force. Figure 11 shows the relationship between the depth of the concavity and the feed rate. As would be expected the depth is in proportion to the feed rate and to the depth of cut, although the depth for the 45 RHH tool is slightly greater than that for the 25 RHH tool. This result can be attributed to the difference in bending rigidity between the end mills used, the 45 RHH tool being less rigid than the 25 RHH tool.
226
S. E M A and R. DAVIES
:1
+,+1+ 300
500 PATE
700 (mm/~n)
300
500 100 reED I~TE {mm/mln)
300
500 FEED P A T E
FIG. 8. Cutting force components (spindle speed 2100
100 (mm/mln)
rpm).
5
°---" =
:
25RflH 4SRHH
J
4
~
+,
~
.
:::+::,
I 500
0
300
v
400
600
700
FEEDRATE (mmlmin) FIG. 9. Bottom surface roughness (spindle speed 2100 rpm). Depth of concavity
IlO/~m
5 mm I I
FIG. 10. Concavity of a machined surface and its evaluation. 6.
CONCLUSIONS
The cutting force, the surface roughness and the concavity of a machined surface have been measured using five end mills having various helix angles. The effects of the cutting parameters on each end mill and the comparison of cutting performance of the end mills have been investigated using variance analysis. The following conclusions can be made. (1) The effects of spindle speed, depth of cut and feed rate on the cutting force and the surface roughness are significant. There is no difference between down milling and up milling with respect to the cutting force, but there is a significant difference between them with respect to surface roughness, down milling giving the smoother surface. (2) End mills with small helix angles develop the greatest cutting force and surface roughness due to chatter and reduction of the period when the cutting inserts engage
Cutting Performance of End Mills 70
227
I
~ - - - - * 25RHH : 45RHH
DEPTH OF
4 mm
200
350
500 I~F'D R A T E
650 (mmlmin)
800
FIG. 11. Depth of concavity (spindle speed 2100 rpm). with the workpiece material. Furthermore, end mills with left hand helix angles are relatively ineffective. (3) E n d mills with right hand helix angles of 45 and 25~ are very effective and would be r e c o m m e n d e d for the types of milling o p e r a t i o n detailgd in this p a p e r for type 1.65 a l u m i n u m alloy. O n the grounds of tool manufacturing costs, the 25 R H H tool would be the m o s t economical. N o a t t e m p t has been m a d e to evaluate tool life for the various types of end mill, although this would be necessary in o r d e r to provide a c o m p l e t e assessment. Acknowledgements--The authors gratefully acknowledge the help of Mr D. H. Wale of Marwin Cutting
Tools Ltd in supplying the cutting tools and thank the members of the Laboratory of Machine Tool Vibration in the University of Birmingham. The first author also wishes to thank the Ministry of Education of Japan for giving him the opportunity to study in the United Kingdom. REFERENCES [1] F. KOEN1GSBERGERand A. J. P. SABBERWAL,Int. J. Mach. Tool Des. Res. 1, 15 (1961). [2] J. TLUSTYand E; ELeESTAWl,Ann. CIRP 28, 253 (1979). [3] L. S. ZHANG,M. M. SADEKand D. H. WALE, TraM. ASME J. Engng Ind. 107, 153 (1985). [4] J. W. SUTHERLANDand R. E. DEVOR, Trans. ASME J. Engng Ind. 108, 269 (1986). [5] C. CHATFELD,Statistics for Technology. Chapman and Hall, London (1983). [6] A. J. P. SABBERWAL,Int. J. Mach. Tool Des. Res. 2, 27 (1962). [7] J. TLUSTYand P. MAcNEIL,Ann. CIRP 24, 21 (1975).