A study on modification of endrills for finishing holes

Journal of Materials Processing Technology, 28 ( 1991 ) 83-92 Elsevier

A study on modification of endrills" for finishing holes S. Wang ~, V. C. Venkatesh b and W. Xue c

~ i s i t i n g Professor, bProfessor, cPh.D Student Center for Manufacturing Research, Tennessee Technological University Cookeville, TN 38505, USA

Abstract High speed drilling has gathered momentum through the use of indexable carbides for hole diameters of 16 to 38 ram. These drills which resemble end mills are also known as endrills. The drills have an outbound and an inbound insert usually of the same shape, square or triangular; one manufacturer even uses a diamond shaped outer insert and a square inner insert. The configuration of these inserts is such that a positive side cutting edge angle, and a negative point angle are obtained. The latter reduces extrusion action at the center thus lowering thrust. Modifications such as introduction of chip splitting grooves, change in point angle, cutting edge angle and relief angle have improved surface roughness and roundness and reduced vibration at the same time.

1. INTRODUCTION

How to drill higher-quality holes is an interesting and practical problem which has been investigated for a long time. MFD's (Multifacet Drills), based on traditional twist drill, were introduced in the 1950s [1]. Although MFD's could obtain higher-quality holes, the geometrical shape of these drills are very complicated, and it is very difficult to grind them. Mathematical models, methods of grinding, computer plotting and drill wandering motion about MFD's were not analyzed until the 1980s [2,3]. Based on MFD's, several new types of drills for obtaining very high quality holes were introduced several years ago by Wang[4,5]. However, all of them are made of HSS and cannot drill at high speeds and high feedrates. J u s t a few years ago, endrills which use carbide inserts were successfully introduced into industry. Several different shapes of carbide inserts such as diamond outer insert combining with square inner insert, square inner insert with square outer insert, and triangular inner insert with triangular outer insert have appeared. There are also different drill holders with different flutes such as spiral flutes and linear flutes. The performance of one of them has been 0924-0136/91/$03.50 © 1991 ElsevierSciencePublishers B.V. All rights reserved.

84

evaluated by Venkatesh e t a t [6], and ideal qualities of holes without any "walking phenomenon" were observed and reported. Modified endrills which can be applied for both drilling and enlarging with a pilot hole to obtain very good quality holes are presented in this paper. In the study, the performance of the endrill A having

(a)

(b)

(c)

Figure 1. Schematic diagram of endrills. (a) endrill A. (b) Endrill B. (c) Endrill C. diamond outer insert and square inner insert combined with spiral flutes, endrill B having triangular outer and inner inserts with linear flutes, and the endrill C having triangular outer insert and inner insert with spiral flutes, before and after modifying was studied. Fig. 1 shows the structures of these three endrills.

2. ANALYSIS OF SHAPES OF INSERTS

Although endrills with uncoated inserts produced the ideal holes with out-of roundness error in the order of 10-25 pm and surface roughness of 6-10 IJm for medium carbon steel at certain machining conditions [6], they could not meet conditions that require much higher hole qualities. To improve the ability of finishing holes for endrills, chip-splitting grooves, clearance on flank surface, and modified side cutting edge angle at point of outer insert were used. 2.1 Chip-splitting g r o o v e s To improve the performance of endrills, chip-splitting grooves were introduced. There are three reasons for applying the chip-splitting grooves for the endrills. Firstly, it can split the chips while the feedrate is low, or feedrate is equal to or less than the depth of the groove. Secondly, if the feedrate is high, or the feedrate is higher than the depth of the splitting groves, the grooves can improve chip deformation. Fig. 2 shows the chips machined by the inserts with and without the chip-splitting grooves. Thirdly, the grooves produce several protruding rings on

85 the bottom surface of the drilled hole, which eliminate the radial vibration of drills during drilling, thus improving the out-of-roundness error of the hole. Generally, the deeper the depth of the grooves, the better the quality of the holes obtained. But the depth of the grooves is limited by the strength of the inserts, as the strength would be reduced if the depth of the grooves is too large. 2.2 R e l i e f o n flank s u r f a c e There is only clearance on flank surface for almost all endrills considering that endrills are applied for drilling coarse or semi-finished hol~s. However, if they are used for drilling finished holes, the relief on flank surface is necessary, because relief on flank surface can reduce the vibration and thus improve the hole quality. In addition, the life of the inserts will increase with the relief on the flank surface. The width of the clearance is only 1.5 mm, with a 8° - 10 ° slope.

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Figure 2. Photograph of the chips machined by original inserts and modified inserts (at v=59.82, f=0.08 mm/r). (a) Chips machined by original inserts. (b) Chips. machined by modified inserts. (The left machined by inner insert and the right by outer insert).

2.3 M o d i f i e d c u t t i n g e d g e angle IG at out p o i n t of the o u t e r i n s e r t When endrills are used as enlarging holes to obtain very good quality holes, the out point of the insert cuts as shown in Fig. 3(a). In this case the drill could be regarded as a cantilever beam and there would a deflection of the drill when it is subjected to a cutting force. For several endrills such as ones with the diamondouter insert, the cutting edge angle K~ is larger than 90 °, and the cutting force resulting from this cutting edge angle could be resolved into a radial force F r and an axial force F a as shown in Fig. 3(b). The force Fr would force the cutting edge into the drilled surface and thus damage it. The surface finish of the drilled hole will be worse. On the other hand, the height EF 1 of the remaining area (triangle

86 EBC), shown in Fig. 3(d) would be bigger, and the surface roughness of the hole would be also worse. In order to avoid this situation, the cutting edge angle should be less than 90 °, and then the radial force resolved from the cutting force is in the centripetal direction as shown in Fig. 3(c). In this case, there is a deflection on the end of the drill due to the centripetal force. If the angle is less than but close to 90 °, the remaining area is triangle ABC (shown in Fig. 3(d)), and the height AF2 of this triangle is large. If the cutting edge angle is smaller, the remaining area is a triangle DBC, and the height BF 3 of this triangle is less. Thus, the lower the value of the side cutting edge angle, the better is the surface roughness of the hole. Fig. 3(d) shows the principle of the effect of the cutting edge angle on surface finish. However, if the angle is too small, the centripetal force would be so large that the magnitude of the drill deflection would be considerable, and result in

Finished hole surface

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Figure 3. Effect of cutting edge angle on hole quality while endrills are used as finishing drills. (a) Schematic diagram of cutting edge angle. (b) and (c) Principle of effect of cutting edge angle on deflection of the drill. (d) Principle of the effect of cutting edge angle on surface finish.

87 radial vibrations, if the quality of the pilot holes drilled were not good enough, which is usually the case. From our experimental experience, the side cutting edge angle I~ should be 15-20 °. The length of the cutting edge at out point of the outer insert should be short. If it is too long, the radial force would be unbalanced due to the component of the radial force resulting from the change of the angle when the modified endrills are used to drill without a pilot hole. The above modification were not necessary for drills B and C as the cutting edge K~ is 15°. However the negative point angle represented-by AG in Fig. 3(d) is ideal for drill A but not for B and C.

3. D R I L L I N G E X P E R I M E N T

A schematic diagram of the experimental set-up for drilling is shown in Fig. 4. The workpiece (AISI 1026, ¢ 30 x 30) was clamped by a 3-jaw chuck which was fixed on a KISTLER 4-component piezo-electric dynamometer (Type 9273 and charge amplifier Type 5004). The signals of the drilling forces were processed by

Wo~plece

Figure 4. Experimental set-up for drilling

a Macintosh computer. The dynamometer was fixed on the table ofa Fadal 5-axis CNC machining center. Two piezo-electric accelerometers were used. One was fixed in axial direction, and another in the radial direction of the drills. The vibration signals were processed by DATA 6000. In the experiment, mainly uncoated tungsten carbide inserts and TiN coated carbide inserts were used in the

88 endrills. Surface roughness was measured by a Mitutoyo Surftest 402 Surface Roughness Tester, and out-of-roundness was measured by a Mitutoyo RA-211 Roundness Measuring Machine.

4. E X P E R I M E N T A L R E S U L T S AND ANALYSIS

Both drilling and enlarging of holes have been carried out in this study with original and modified endrills.

4.1 S t u d y o f drilling The quality of the holes drilled improved with modified endrills. Table. 1 shows the comparison of the surface roughness and the roundness of the holes. The magnitude of vibrations measured during drilling, shown in Fig. 4, was reduced considerably. The results illustrate that the analysis of the shape of endrills is correct. Fig. 5 shows that drilling forces, thrust and torque, have not changed significantly. Table 1 Comparison of performance of modified and original endrill ( diameter of the endrills are ¢ 19.05 mm) Machining Condition Inserts

Hole Quality Roughness (Ra: ~m)

Roundness (~m)

V (m/min)

f (mm/r)

Original Insert A

59.82

0.04

5.6

16.0

Modified Insert A

59.82

0.04

3.7

9.0

Original Insert B

59.82

0.04

6.7

19.4

Modified Insert B

59.82

0.04

5.1

11.3

Original Insert C

59.82

0.04

4.3

15.9

Modified Insert C

59.82

0.04

3.8

9.9

4.2 S t u d y o f e n l a r g i n g w i t h a pilot h o l e Sometimes, when very high-quality roundness is required, then enlarging with a pilot hole is necessary. Although endrills can be used to drill ideal quality holes, their good performance of them as enlarging drills are not expected. Modified endrills can be used as enlarging drills to obtain the high quality holes. Thus the application of the endrills has been extended. Fig. 7. shows that the surface roughness and out-of-roundness error of the holes versus the drilling s~eed. The

89 curves show that the quality of the holes drilled by endrill B has not improved, because the cutting edge angle at out point of this triangular outer insert is already an ideal angle. These results also support our analysis of the cutting edge angle at the out point of the outer insert is correct. Fig. 8. shows typical records of roundness and surface roughness drilled by endrill A. Fig. 9 shows that there is almost no plastic deformation near the hole surface, and therefore much less residual stress. This is an advantage for further machining operations such as honing of strain hardening materials.

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(d)

Figure 5. Magnitude of vibration measured by piezo accelerometer (at v=59.82 m/min, f=0.05 mm/r). (a) Vibration magnitude in Z-direction by original insert A. (b) Vibration magnitude in X-direction by original insert A. (c) Vibration magnitude in Z-direction by modified insert A. (d) Vibration magnitude in Xdirection by modified insert A.

5. C O N C L U S I O N S In this study, the modified endrills have been introduced and hole quality obtained by them have been compared with that by original type of endrills. The results have shown that the modified endrills can be used for both drilling and enlarging. High-quality holes can be drilled by modified endrills and very highquality holes can also be enlarged with a pilot hole, especially by some endrills.

90

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Figure 6. Drilling forces measured by dynamometer (at v=59.82 m/min, f=0.05 ram/r). (a) Fz, drilled by original insert A. (b) Mz, drilled by original insert A. (c) F~, drilled by modified insert A. (d) M~, drilled by modified insert A.

In this study, we have also found that there are limitations of existing endrills, if they are expected to be applied for both drilling and enlarging to obtain much higher-quality holes. However, it is possible to change the structure of the endrills to make 'drilling conditions more stable.

91

LEGEND O InSertS A (A~O o Inserts A ( 0 ) A Inserts B • Inserts B CO)

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LEGEND o Inserts A o Inserts A A inserts B • Inserts B

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Figure 7. Plot of surface roughness and roundness versus drilling speed using modified and original endrills. (a) Surface roughness vs speed (at f=0.008 ram/r). (b) Roundness vs speed (at f=0.008 mm/r).

(a)

(b)

(c)

(d)

Figure 8. Records of roundness and surface roughness of holes enlarged with a pilot hole by endrill A. (a) Roundness = 21.3 pm, by original endrill. (b) Roundness = 4.1 pro, by modified endrill. (c) Surface roughness = 5.2 pm, by original endvill. (d) Surface roughness = 0.6 pm, by modified endrill.

92

(a)

(b)

Figure 9. Microstructure of hole surface. (a) Machined by original endrill. (b) Machined by modified endrill.

6. R E F E R E N C E S

1. "The 'Ni Zhi-Fu' Drill," China Machine Press, (in Chinese), 1964. 2. Chen, L. H., and Wu, S. M., "Further Investigation of Multifacet Drills (MFD's) Mathematical Models, Methods of Grinding, and Computer Plotting." ASME Journal of Engineering for Industry. Vol. 106, Nov. 1984, pp. 313-324. 3. Lee, S. J., Eman, K. F., and Wu, S. M., "An Analysis of the Drill Wandering Motion," ASME Journal of Engineering for Industry, Vol. 109, Nov. 1987, pp. 297-305. 4. Wang, S., Zhang, M., Shen, Z., and Liu, C., "Cutting Experiment of New Drills," Journal of Xi'an Petroleum Institute, Vol. 2, No. 1 Jul. 1987, pp. 25-32 (in Chinese). 5. Wang, S., Liu, Z., and Peng, H., "Cutting Experiment and Analysis of New Drills for Finishing Holes," Tool Technique, May 1989, pp. 23-29 (in Chinese). 6. Venkatesh, V. C., et al, "Performance Evaluation of Endrills," International Journal of Machine Tools and Manufacture, Vol. 28, No. 4, 1988, pp. 341-349.