International Journal of Machine Tools & Manufacture 47 (2007) 63–74 High-throughput drilling of titanium alloys Rui Li a , Parag Hegde b , Albert J. Shih a, a Mechanical Engineering, University of Michigan, Ann Arbor, MI 48109, USA b Global Machining Technology and Technical Center, Kennametal, Latrobe, PA 15650, USA Received 23 January 2006; received in revised form 21 February 2006; accepted 24 February 2006 Available online 18 April 2006 Abstract Experiments of high-throughput drilling of Ti–6Al–4V at 183 m/min cutting speed and 156 mm 3 /s material removal rate (MRR) using a 4 mm diameter WC–Co spiral point drill were conducted. The tool material and geometry and drilling process parameters, including cutting speed, feed, and fluid supply, were studied to evaluate the effect on drill life, thrust force, torque, energy, and burr formation. The tool wear mechanism, hole surface roughness, and chip light emission and morphology for high-throughput drilling were investigated. Supplying the cutting fluid via through-the-drill holes has proven to be a critical factor for drill life, which can be increased by 10 times compared to that of dry drilling at 183 m/min cutting speed and 0.051 mm/rev feed. Under the same MRR of 156 mm 3 /s with a doubled feed of 0.102 mm/rev (91 m/min cutting speed), over 200 holes can be drilled. The balance of cutting speed and feed is essential to achieve long drill life and good hole surface roughness. This study demonstrates that, using proper drilling process parameters, spiral point drill geometry, and fine-grained WC–Co tool material, the high-throughput drilling of Ti alloy is technically feasible. r 2006 Elsevier Ltd. All rights reserved. Keywords: Drilling; High throughput; Material removal rate; Titanium alloys 1. Introduction Drilling is a widely used machining process and has considerable economical importance because it is usually among final steps in the fabrication of mechanical components. The tool geometry and material deformation in drilling are complicated. In the drill center, so-called chisel edge, cutting speed is close to zero and work material is plowed under a high negative rake angle. Along the drill cutting edge, the cutting speed and rake angle both vary with respect to the distance from the drill center. The outmost point on the cutting edge has the highest cutting speed, called the peripheral cutting speed. In drilling, high material removal rate (MRR) and long drill life are essential to increase productivity and reduce cost. To achieve the desired MRR, high cutting speed and feed per revolution of drill are required [1]. This research investi- gates the process parameters, tool material, and drill geometry to achieve the high-throughput drilling of Ti–6Al–4V, a material widely used in aerospace, medical, and defense industry. Feed and cutting speed are two important process parameters to achieve the desired MRR and productivity in drilling. The use of better tool material with higher strength and hot hardness [1] and better drill geometry design can enable larger feed in drilling. The effect of feed in drilling is an area that has not been studied extensively, so it becomes one of the research goals of this paper. The feed for turning of Al alloys can be 1.00 and 2.05 mm/rev using high-speed steel (HSS) and tungsten carbide in cobalt matrix (WC–Co) tools, respectively [2]. Gray cast iron can be turned at 0.75, 0.65, and 1.00 mm/rev with HSS, ceramic, and WC–Co tools [2]. In comparison, the recommended maximum feed is small for Ti–6Al–4V, 0.40 mm/rev, using a WC–Co tool [2]. In drilling, the maximum recommended feed depends on the drill diameter [2]. For drilling using a WC–Co twist drill with diameter less than 6 mm, the feeds for Al alloys, gray cast iron, and Ti–6Al–4V are 0.18, 0.10, and 0.05 mm/rev, respectively [2]. It is obvious that the recommended feed for machining Ti is small. ARTICLE IN PRESS www.elsevier.com/locate/ijmactool 0890-6955/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2006.02.012 Corresponding author. Tel.: +1 734 647 1766; fax: +1 734 936 0363. E-mail address: [email protected] (A.J. Shih).
Transcript
International Journal of Machine Tools & Manufacture 47 (2007) 63–74
High-throughput drilling of titanium alloys
Rui Lia, Parag Hegdeb, Albert J. Shiha,�
aMechanical Engineering, University of Michigan, Ann Arbor, MI 48109, USAbGlobal Machining Technology and Technical Center, Kennametal, Latrobe, PA 15650, USA
Received 23 January 2006; received in revised form 21 February 2006; accepted 24 February 2006
Available online 18 April 2006
Abstract
Experiments of high-throughput drilling of Ti–6Al–4V at 183m/min cutting speed and 156mm3/s material removal rate (MRR) using
a 4mm diameter WC–Co spiral point drill were conducted. The tool material and geometry and drilling process parameters, including
cutting speed, feed, and fluid supply, were studied to evaluate the effect on drill life, thrust force, torque, energy, and burr formation. The
tool wear mechanism, hole surface roughness, and chip light emission and morphology for high-throughput drilling were investigated.
Supplying the cutting fluid via through-the-drill holes has proven to be a critical factor for drill life, which can be increased by 10 times
compared to that of dry drilling at 183m/min cutting speed and 0.051mm/rev feed. Under the same MRR of 156mm3/s with a doubled
feed of 0.102mm/rev (91m/min cutting speed), over 200 holes can be drilled. The balance of cutting speed and feed is essential to achieve
long drill life and good hole surface roughness. This study demonstrates that, using proper drilling process parameters, spiral point drill
geometry, and fine-grained WC–Co tool material, the high-throughput drilling of Ti alloy is technically feasible.
r 2006 Elsevier Ltd. All rights reserved.
Keywords: Drilling; High throughput; Material removal rate; Titanium alloys
1. Introduction
Drilling is a widely used machining process and hasconsiderable economical importance because it is usuallyamong final steps in the fabrication of mechanicalcomponents. The tool geometry and material deformationin drilling are complicated. In the drill center, so-calledchisel edge, cutting speed is close to zero and work materialis plowed under a high negative rake angle. Along the drillcutting edge, the cutting speed and rake angle both varywith respect to the distance from the drill center. Theoutmost point on the cutting edge has the highest cuttingspeed, called the peripheral cutting speed. In drilling, highmaterial removal rate (MRR) and long drill life areessential to increase productivity and reduce cost. Toachieve the desired MRR, high cutting speed and feed perrevolution of drill are required [1]. This research investi-gates the process parameters, tool material, and drillgeometry to achieve the high-throughput drilling of
Ti–6Al–4V, a material widely used in aerospace, medical,and defense industry.Feed and cutting speed are two important process
parameters to achieve the desired MRR and productivityin drilling. The use of better tool material with higherstrength and hot hardness [1] and better drill geometrydesign can enable larger feed in drilling. The effect of feedin drilling is an area that has not been studied extensively,so it becomes one of the research goals of this paper. Thefeed for turning of Al alloys can be 1.00 and 2.05mm/revusing high-speed steel (HSS) and tungsten carbide in cobaltmatrix (WC–Co) tools, respectively [2]. Gray cast iron canbe turned at 0.75, 0.65, and 1.00mm/rev with HSS,ceramic, and WC–Co tools [2]. In comparison, therecommended maximum feed is small for Ti–6Al–4V,0.40mm/rev, using a WC–Co tool [2]. In drilling, themaximum recommended feed depends on the drill diameter[2]. For drilling using a WC–Co twist drill with diameterless than 6mm, the feeds for Al alloys, gray cast iron, andTi–6Al–4V are 0.18, 0.10, and 0.05mm/rev, respectively [2].It is obvious that the recommended feed for machining Tiis small.
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Besides feed, cutting speed can also be increased toimprove the MRR. Depending on the work and toolmaterials, the cutting speed in machining may varyconsiderably [3]. In turning, Al alloys can be machined at300m/min using HSS and 600m/min using WC–Co toolmaterials [4]. For gray cast iron, typical maximum cuttingspeeds are 40, 120, and 450m/min, using HSS, WC–Co,and ceramic cutting tools, respectively [2]. Machining Tialloys at high speed are difficult because of the inherentmaterial properties, particularly the low thermal conduc-tivity, which results in high tool temperature at increasedcutting speed. High tool temperature accelerates tool wearand limits drill life [5]. For Ti alloys, over 100m/min can beconsidered as a high speed in turning [6,7]. The cuttingspeed of drilling is typically much lower than that ofturning. Using a HSS twist drill with diameter less than6mm, the maximum recommended peripheral cuttingspeeds for Al alloys, gray cast iron, and Ti–6Al–4V are105, 49, and 11m/min, respectively [2]. With internalcutting fluid supply, the cutting speeds are increased to 245,90, and 30m/min using a WC–Co twist drill with diameterless than 6mm, for Al alloys, gray cast iron, andTi–6Al–4V, respectively [2]. The recommended cuttingspeed for drilling Ti alloys is very low. This has hinderedthe productivity and increased the cost of machining Ti forindustrial applications.
Machining of Ti–6Al–4V at high cutting speed has beenstudied. In turning, Lei and Liu [8] achieved 480m/mincutting speed at 0.15mm/rev feed using a rotary WC–Cotool. Zareena et al. [7] conducted face milling at 400m/mincutting speed and 0.1mm/tooth feed using a polycrystallinecubic boron nitride (PCBN) tool insert. Balkrishna andShin [9] using a polycrystalline diamond tool in face millingreached 180m/min cutting speed and 0.051mm/rev feed.Norihiko [10] was able to use a WC–Co tool for end millingat 283m/min cutting speed and 0.05mm/tooth feed.Machining of other Ti alloys with high cutting speed hasalso been investigated: Kitagawa et al. [6] studied the endmilling of Ti–6Al–6V–2Sn at a cutting speed up to 628m/min and 0.06mm/tooth feed using a WC–Co tool andHirosaki et al. [11] investigated the turning ofTi–6Al–2Nb–1Ta at 252m/min cutting speed and0.15mm/rev feed and milling at 498m/min cutting speedand 0.05mm/tooth feed using a binder-less PCBN tool. Itis noticed that all these studies were limited in eitherturning or milling. The research of high-throughput drillingof Ti alloys is still lacking.
High-throughput drilling has been studied by Lin and Ni[12] using coated WC–Co drills with specially designed drillgeometry. For cast Al and gray cast iron, 533 and 235m/min peripheral cutting speed and 0.50 and 0.36mm/revfeed, respectively, have been achieved [12]. In the pastdecade, drills with advanced geometry and new fine-grained WC–Co tool material have become commerciallyavailable. The research presented in this paper is asystematic study of the performance of conventional HSStwist drills and advanced WC–Co web thinned and spiral
point drills for high-throughput drilling of Ti–6Al–4V. Thethrust force, torque, drill life, burr formation, and chipmorphology are investigated. The experimental setup anddesign is first introduced in Section 2. Drilling results forHSS drill are presented in Section 3. A performancecomparison of three types of drill at the same peripheralcutting speed and feed is summarized in Section 4. InSections 5 and 6, the spiral point drill for high-throughputdrilling is discussed. The light emission in chip formationand chip morphology are elaborated in Section 7. The toolwear is analyzed in Section 8.
2. Experimental setup and design
2.1. Drill geometry and tool material
Three types of drills, as shown in Fig. 1, were studied. Alldrills were double-flute and 4mm in diameter.
1. HSS twist drill (Greenfield Industries 44210): This M2HSS twist drill, denoted as HSS Twist, was a conven-tional drill used as the baseline for Ti drilling tests. Thedrill had 1181 point angle, 301 helix angle, and 0.7mmweb thickness.
2. WC–Co web thinned point twist drill (KennametalKWCD00344): This uncoated drill, denoted as WC–CoTwist, also had 1181 point angle and 301 helix angle. Theweb was thinned to 0.45mm. A notch was ground at thechisel edge corner for web thinning [1]. The tool materialwas Kennametal grade K600 WC with 10% Co.
3. WC–Co spiral point drill (Kennametal K285A01563):This uncoated drill, denoted as WC–Co Spiral, hadadvanced drill geometry design. The drill point with S-shaped chisel edge had lower negative rake angle thanthat of the conventional twist drill [5,13]. The lownegative rake angle was expected to generate low thrustforce. The tool material was Kennametal grade K715WC with 9.5% Co. The drill had 1351 point angle, 301helix angle, and 0.72mm web thickness.
2.2. Experiment design
Cutting fluid and delivery method are important toreduce the tool and workpiece temperatures and enhancethe drill life [4]. Two ways to supply the cutting fluid,internally from two through-the-drill holes and externallyfrom outside the drill, were studied. The cutting fluid was5% CIMTECH 500, a synthetic metalworking fluid.The workpiece material was 6.35mm thick Ti–6Al–4V
plate with Knoop hardness of 4.34GPa. The thrust forceand torque during drilling were measured using a Kistler9272 dynamometer.Five sets of experiments, marked as Exps. I, II, III, IV,
and V, were conducted. Table 1 summarizes the type ofdrill, cutting fluid supply, peripheral cutting speed, andfeed in each drilling experiment. Exp. I was conducted
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using the HSS Twist drill at three peripheral cuttingspeeds, 9.1, 13.7, and 18.3m/min and a constant feed of0.051mm per revolution or 0.025mm feed per tooth,without cutting fluid. To study the effect of cuttingfluid, Exp. II was conducted with external cutting fluidsupply, HSS Twist drill, higher peripheral cutting speedsat 13.7, 18.3, and 27.4m/min, and 0.051mm/rev feed.Exp. III was conducted to compare the thrust force,torque, energy, and burr formation of three drills(HSS Twist, WC–Co Twist, and WC–Co Spiral) at18.3m/min peripheral cutting speed and 0.051mm/rev feedwithout cutting fluid. Exp. IV studied the high-speeddrilling of Ti using WC–Co Spiral drills. By graduallyincreasing the drill rotational speed and maintaining thesame feed at 0.051mm/rev, drilling at 183m/min peripheralcutting speed, which corresponded to 14700 rpm of thedrill, was possible under the dry drilling condition. Threesets of drilling under dry and external and internal(through-the-drill) cutting fluid supply conditions wereconducted to evaluate the drill life and drilling perfor-mance. Using the same WC–Co Spiral drill with internalcutting fluid supply, Exp. V studied the balance of feed andcutting speed on drill life in high-throughput drilling of Tiunder the same MRR. The feed was gradually increasedfrom 0.051 to 0.076, 0.102, and 0.152mm/rev at corre-sponding drill rotational speed to maintain the same0.75m/min feed rate and 156mm3/s MRR. The drill life,hole surface roughness, chip morphology, thrust force,torque, and energy were studied. For all five sets of drillingexperiments, chips were collected, and drills and work-pieces were cooled by free convection in air by at least 60 safter drilling each hole. The drill life was defined as thenumber of holes drilled before tool breakage.
Fig. 2 illustrates the three stages, marked as A, B,and C, in drilling. Stage A occurs when the drill hastraveled a distance d, which is called the drill point length,from the initial contact. For the HSS Twist, WC–Co Twist,and WC–Co Spiral drills used in this study, d is equal to1.24, 1.19, and 0.74mm, respectively. Neglecting thedeformation of the workpiece, the drill cutting edgebecomes fully engaged with the workpiece at this stage.When the drill tip reaches the back surface of the platewithout considering material deformation, as shown in Fig.2, it is marked as Stage B. Stage C is defined when the drillcutting edge is disengaged with the workpiece withoutconsidering the deformation of the workpiece and burrformation.The torque and force changed throughout the drilling
process. Values of thrust force and torque at Stage A,denoted as FA and TA, and maximum thrust force andtorque during drilling, denoted as Fm and Tm, respectively,were compared to quantify different drilling conditions.
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Fig. 1. Top and side view of three types of drill: (a) HSS Twist, (b) WC–Co Twist, and (c) WC–Co Spiral.
Stage CStage BStage A
d
Drill
Workpiece
Fig. 2. Illustration of three stages A, B, and C in drilling.
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Besides thrust force and torque, another variableindicating the effect of drilling condition is the energy E
required to drill a hole. Drilling energy E can be expressedas
E ¼
Zl
F dl þ
Zl
2pT
fdl, (1)
where l is the depth of drilling, F is the thrust force, T is thetorque, and f is the feed per revolution. E was found to bedominated by the torque term, which accounted for over99% of the E, based on the experimental data of all drillingtests in this research.
2.3. Tool wear observation and hole surface roughness
A Hitachi S-4700 scanning electron microscope (SEM)was used to examine the tool wear. Energy dispersivespectroscopy (EDS) method was applied to further studythe tool wear mechanism.The test pieces were cut into two, along the hole axis to
measure the roughness of hole surface using a TaylorHobson Talysurf Model 120 stylus surface profilometerwith 0.8mm cutoff length and 100:1 bandwidth ratio. Foreach drilling test in Exp. V, the first three holes weremeasured. For each hole, three roughness readings weretaken, approximately 451 apart circumferentially.
3. HSS twist drill
The performance of HSS Twist drill under dry condition(Exp. I) and with external cutting fluid supply condition(Exp. II) were investigated.
3.1. Dry drilling (Exp. I)
Fig. 3 shows the thrust force and torque as a function ofdrilling depth using the HSS Twist drill without cuttingfluid. The time to reach Stage C, i.e., the drill penetrating6.35mm thick workpiece, was 12.2, 8.1, and 6.1 s at 9.1,13.7, and 18.3m/min peripheral cutting speeds and0.051mm/rev feed, respectively. This time is labeled inFig. 3. Cutting speed has a significant effect on the drillperformance. No obvious wear was observed on the HSSTwist drills after drilling three holes at 9.1 and 13.7m/mincutting speeds. At 18.3mm/min cutting speed, the drill tipmelted in the second hole due to high temperature. TheHSS Twist drill, a low-cost, conventional tool, has limitedcutting speed and low MRR in drilling Ti.By increasing the cutting speed from 9.1 to 13.7m/min,
the thrust force rise by about 50N or 12%. This is becausethe strain-rate hardening outweighs thermal softening [4].At both speeds, the thrust force decreased slightly afterStage A. Between Stages B and C, the thrust force rapidlyreduced because the chisel and cutting edges graduallydisengaged with the workpiece. The torque increased,instead of decreasing as in the thrust force, after Stage A,because the resistance to chip ejection [14] had a more
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significant effect on torque. Torque started to decreaseafter Stage B at 9.1m/min cutting speed. At 13.7m/mincutting speed, the torque started to peak after Stage B, i.e.,the drill chisel edge had already penetrated the workpiece.The difficulty for chip ejection resulting from the chipwelding to the drill flute near the cutting edge was the likelycause. It was also confirmed by the difficulty to remove thechip from the drill after each drilling. At 18.3m/mincutting speed, the curves of thrust force and torque lastlonger past Stage C due to the melting of drill tip.
3.2. Drilling with external cutting fluid supply (Exp. II)
Fig. 4 shows the thrust force and torque at 13.7, 18.3,and 27.4m/min peripheral cutting speeds and 0.051mm/rev feed with external cutting fluid supply. At 13.7m/mincutting speed, compared to dry drilling, the thrust forceand torque were both reduced. At 18.3m/min, themaximum cutting speed in dry drilling, the drill lastedmore than three holes with no obvious wear on cuttingedges. This is a significant improvement of drill life fromthe dry drilling condition. At both cutting speeds, thebenefit of cutting fluid is obvious. When the peripheralcutting speed further increased to 27.4m/min, which wasstill low for effective material removal, the drill tip melted
in two holes. This illustrates the limited cutting speed andMRR of the conventional HSS twist drill in Ti machining.The TA, FA, Tm, Fm, and E for all drilling tests are
summarized in Table 1. For HSS Twist drill, as shown inExps. I and II, at 13.7m/min peripheral cutting speed, thesupply of cutting fluid decreased Tm by about 30%, Fm byabout 10%, FA by about 10%, and E by 15%. With thecombination of low energy input and the cooling effect ofcutting fluid, the drill life was improved.
4. Drill performance comparison (Exp. III)
Fig. 5 shows the thrust force and torque for WC–CoTwist and WC–Co Spiral drills at the same 18.3m/minperipheral cutting speed and 0.051mm/rev feed with nocutting fluid. By comparing with the results of HSS Twistunder the same drilling condition in Fig. 3, the advantageof drill material can be identified. The WC–Co Spiral drillhad the lowest thrust force and torque among all threedrills used in this paper. The torque curve for WC–CoSpiral drill is particularly stable, maintaining at 0.4Nmafter Stage A. The chip ejection was not a problem forWC–Co Spiral drill at this cutting speed. In comparison,the torque for WC–Co Twist drill continuously increasedfrom about 0.3Nm in Stage A to 0.8Nm in Stage B. Asshown in Table 1, comparing with the WC–Co Twist drill,
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Fig. 3. Exp. I—thrust force and torque of HSS Twist drill without cutting fluid.
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the WC–Co Spiral drill also had lower thrust force(166–181 vs. 233–257N) and energy (351–375 vs.603–628 J). The better drill geometry design is the mainreason of this difference. For the HSS Twist drill,compared to the WC–Co Twist drill, the first hole neededmuch higher thrust force (390N) and about the same peak
torque (0.8Nm). This shows the benefit of web-thinning toreduce the thrust force in drill geometry design.Burr formation is another advantage of the WC–Co
Spiral drill. Exit burr in Ti drilling is a problem and hasbeen studied [15,16]. Fig. 6 shows pictures of the exit burrformation of three drills at 18.3m/min peripheral cutting
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Fig. 4. Exp. II—thrust force and torque of HSS Twist drill with external cutting fluid supply.
Fig. 5. Exp. III—thrust force and torque of WC–Co Twist and WC–Co Spiral at 18.3m/min peripheral cutting speed (1470 rpm) and 0.051mm/rev feed.
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speed and 0.051mm/rev feed. Due to the low thrustforce in the S-shaped web, the short drill point length, andlarge point angle of WC–Co Spiral drill, a cap wasformed between Stages B and C at the exit. As a result ofthe efficient cutting on the periphery of the cutting edge asthe drill exits the workpiece, the residual burr was verysmall, compared to the crown and flower-like exit burrgenerated by the HSS Twist and WC–Co Twist drills,respectively.
5. WC–Co spiral high-throughput drilling—effect of cutting
fluid supply (Exp. IV)
The WC–Co Spiral drill was selected for high-through-put Ti drilling. At a constant feed (0.051mm/rev), testswere conducted by gradually increasing the drill rotationalspeed. The drill could perform at 183m/min peripheralcutting speed (14,700 rpm) under dry drilling condition. Atthis high speed, the feed rate was also high, 0.75m/min,which was more than 5 times the value commonly used inindustrial practice [2]. It took only 0.57 s at 156mm3/sMRR to drill a hole in the 6.35mm thick Ti plate.
5.1. Dry drilling
As shown in the summary of drill life in Table 1, underdry condition, the WC–Co Spiral drill lasted about 10 holesat 183m/min peripheral cutting speed and 0.051mm/revfeed. Fig. 7 shows the thrust force and torque before drill
breakage in dry drilling. The thrust force was about150–200N after Stage A. This was the same level of thrustforce for the same drill at 18.3m/min cutting speed, asshown in Fig. 5. The torque started to peak after Stage B,as a result of difficulty in chip ejection due to the chipwelding to the drill. The same phenomenon was observedin HSS Twist drill at 13.7m/min cutting speed, as shown inFig. 3. A significant increase in torque (Tm increased byabout 30%) but not the thrust force was observed in thelast hole before the drill broke catastrophically duringdrilling. It indicated that the wear was concentrated on theperiphery of the cutting edges, which had been confirmedby visual observation of this drill wear pattern during thetest.
5.2. Drilling with external cutting fluid supply
Using external flood cutting fluid supply, the drill lifeonly increased slightly to about 14 holes under the samecutting speed and feed as in dry drilling. The high drillrotational speed generated high centrifugal force andprevented the cutting fluid from reaching the cuttingregion. As shown in Fig. 7, the thrust force and torquehave similar values and patterns as in dry drilling. Theincrease in torque near the end of drilling also exists. This isanother indication that the externally supplied cutting fluiddid not help drilling at high speed. The chip ejectioncontinued to be a problem for drilling with external cuttingfluid supply.
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Fig. 6. Exit burr formation of three types of drill at 18.3m/min (1470 rpm) peripheral cutting speed and 0.051mm/rev feed.
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5.3. Drilling with internal cutting fluid supply
The internal cutting fluid supply significantly increasedthe drilling life to 101 holes at the 183m/min cutting speedand 0.051mm/rev feed. The internal cutting fluid suppliedthrough machine spindle and drill could go directly to thetool–chip interface for effective cooling, lubrication, and,more importantly, assistance of chip ejection. The torquecurve does not have the high peak between Stages B and C.The peak torque ranged from 1.02 to 1.48Nm and waslocated before reaching Stage B. The thrust force wasslightly higher, about 200–240N, likely caused by thehydrodynamic force. As shown in the energy E in Table 1,drilling with internal cutting fluid supply required moreenergy (762–995 vs. 497–709 J for dry and 504–574 J forexternal fluid supply) because of the more uniform torquecurve after Stage A.
6. WC–Co spiral high-throughput drilling—effect of feed
(Exp. V)
While maintaining the same MRR (156mm3/s) asdrilling under 183m/min cutting speed and 0.051mm/revfeed in Exp. IV, the feed was increased to 0.076, 0.102, and0.152mm/rev, which, as shown in Table 1, matched withthe 122, 91, and 61mm/rev feed and 153, 205, and 164holes drill life, respectively. The time required to penetratethe 6.35mm Ti–6Al–4V plate, 0.57 s, was the same as inExp. IV. The best drill life of 205 holes was achieved at
91m/min cutting speed and 0.102mm/rev feed, which islarger than the recommended feed for Ti drilling.Adjusting the feed has a significant impact on drill life.
Feed is a process parameter which has been overlooked bydrilling researchers. Using the conventional twist drill, therange of feed is limited. For the 4mm drill, therecommended feed is 0.051mm/rev. Advanced spiral pointdrill geometry design and new tool material enable the drillto take a higher cutting force and feed. As shown in Figs. 7and 8, as the feed increases from 0.051 to 0.076mm/rev, thethrust force stays at about 200N. Increasing the feed to0.102mm/rev only raised the thrust force to about 260Nfor the new drill and 330N at the end of drill life (205holes). New drill geometry design can withstand suchincrease in cutting forces. The lower cutting speed at higherfeed helps to reduce the drill temperature, which isbeneficial to the drill life.This study also found that there was a limit of drill life
associated with the increase of feed. As the feed increasedto 0.152mm/rev, the drill life (164 holes) started to reducedue to the high cutting force. In summary, Exp. V leads tothe conclusion that feed is an important process parameterin high-throughput drilling of Ti. The suggested values forfeed are low for advanced drill design to achieve optimaldrill life.The drill which had good drill life also produced holes
with lower surface roughness. Fig. 9 shows the hole surfaceroughness vs. drill life (number of holes) for high-throughput drilling tests in Exps. IV and V. All tests hadthe same MRR, 156mm3/s. The range of upper and lower
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Fig. 7. Exp. IV—thrust force and torque of WC–Co Spiral drill for high-throughput drilling at 183m/min peripheral cutting speed and 0.051mm/rev feed
under the dry and internal and external cutting fluid supply conditions.
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surface roughness of the first three holes is presented. InExp. IV, the internal cutting fluid supply helped to improvethe surface finish. Combined data in Exps. IV and V showthe long drill life positively correlates to the good initialhole surface finish.
7. Light emission in chip formation and chip morphology
Chip light emission or sparks could be observed in someTi drilling tests. Due to the low thermal conductivity of Ti,the chip temperature is high and subsequently creates
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Fig. 8. Exp. V—thrust force and torque of WC–Co Spiral drill for high-throughput drilling at 0.75m/min feed rate and 156mm3/s material removal rate
Fig. 9. Hole surface roughness under high-throughput drilling with the same material removal rate (156mm3/s) using WC–Co Spiral drill in Exps. VI and
V.
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oxidation or burning [17–19]. For HSS Twist drill, the chiplight emission was observed at 13.7 and 27.4m/min cuttingspeed for drilling under dry and external cutting fluidsupply condition, respectively. For WC–Co Spiral drillingat 0.051mm/rev feed, no light emission occurred at cuttingspeed below 82.3m/min. For the same WC–Co Spiral drillat 183m/min cutting speed and 0.051mm/rev feed, chipssparked in almost every hole drilled in dry drilling anddrilling with external cutting fluid supply. When the cuttingfluid was supplied internally through the drill at the samecutting speed and feed, no chip light emission could beobserved initially. In the 80th hole (out of 101 holesdrilled), when the drill wear reached a level, the chip lightemission started to occur. For three drilling conditions inExp. V, the chip light emission occurred in the 120th,125th, and 140th hole at 122, 91, and 61m/min cuttingspeed, respectively. This shows that drilling at the highestand lowest feed under the same MRR in Exp. V, the drillfailed quickly after the start of chip light emission. Thislight emission was usually associated with the chip welding,which could be identified by the difficulty to remove thechip in the flute near the drill tip.
After drilling, the Ti chip could be entangled around twoflutes of the drill and bent by the tool holder. This is calledchip entanglement, which is due to the difficulty for smoothchip ejection. Chip entanglement occurred in all testsexcept the drilling with internal cutting fluid supply beforethe drill wear reached a threshold level. In Exps. IV and Vwith internal cutting fluid supply, chip entanglement
occurred after the 95th, 130th, 150th, and 150th holes at183, 122, 91, and 61m/min cutting speeds, respectively.The same chip morphology, a continuous chip with three
regions of initial spiral cone followed by the steady-statespiral cone and folded long ribbon chip, could be seen in allTi drilling tests in this study. An example of the chipgenerated by WC–Co Spiral drill at 18.3m/min cuttingspeed and 0.051mm/rev feed in dry drilling is shown in Fig.10(a). The close-up view of the initial spiral cone, generatedat the start of drilling from the beginning of contact toStage A, is illustrated in Fig. 10(b). After Stage A, thesteady-state spiral cone chip morphology, as shown in Fig.10(c), was generated. Due to the increased resistance toeject the chip, the spiral cone was changed to folded ribbonchip morphology. Close-up view of the chip transitionregion and the folded ribbon chip are shown in Figs 10(d)and (e), respectively.The length of spiral cone chip can be considered as a
scale to evaluate the difficulty for chip evacuation indrilling. As shown in Table 1, for HSS Twist drill, the chiplength of spiral cone was about 12–35 and 14–24mm in drydrilling and drilling with cutting fluid, respectively. At highcutting speed, the chip was generated at a high rate, thechip ejection was more difficult, the transition from steady-state spiral cone to folded long ribbon chip occurred early,and the length of spiral cone was short. In dry drillingusing WC–Co Spiral drill, the length of spiral cone chipdecreased from 48–54 to 0–5mm when the cutting speedincreased from 18.3 and 183m/min. At 183m/min cutting
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Fig. 10. Chip morphology of Ti–6Al–4V generated by WC–Co Spiral drill at 18.3m/min cutting speed and 0.051mm/rev feed in dry drilling: (a) whole
chip, (b) initial spiral cone (c) steady-state spiral cone, (d) transition between spiral cone and folded long ribbon, and (e) steady-state folded long ribbon.
R. Li et al. / International Journal of Machine Tools & Manufacture 47 (2007) 63–7472
speed, the use of internal cutting fluid supply could assistthe chip ejection and increase the length of spiral cone chipto 18–25mm. In Exp. V, at the same MRR, the drillingcondition with the longest drill life also had the longestspiral cone chip length, indicating the improved chipejection under such drilling condition.
8. Tool wear
SEM micrographs of the progressive wear of a WC–CoSpiral drill for high-throughput drilling at 61m/min cuttingspeed and 0.152mm/rev feed with internal cutting fluidsupply in Exp. V are presented in Fig. 11. The drill wasremoved from the machine and examined after drilling 25,100, and 164 holes. The drill broke in the drill body whiledrilling the 164th hole and left the drill tip in the workpiece.It enabled the observation of the drill wear at the end of drilllife. The same mode of drill breakage was observed in allhigh-throughput Ti drilling tests using WC–Co Spiral drills.
Four SEM micrographs, the overview of drill point andthe close-up views of the chisel edge, middle of cutting edge,and intersection of the margin and cutting edge, as shown byboxes in the overview of the drill tip, are presented afterdrilling 25, 100, and 164 holes. After drilling 25 holes,minimal drill wear could be recognized. The chisel edge wassharp with slight work-material buildup. The middle ofcutting had slight material buildup, as marked by A2.Relatively more significant, but not severe, material buildupwas observed at the intersection of cutting edge near themargin due to the relatively high cutting speed in this area.The EDS X-ray of areas A1, A2, and A3 showed identicaloutcome in elemental analysis. It had Ti, Al, and V, thecomposition of the Ti–6Al–4V alloy, which indicated theadhesion of work material on the drill. It also had W and C,the composition of the tool material, and O, representing theoxidation at high drilling temperatures.After drilling 100 holes, as shown in Fig. 11, the tool wear
was obvious. On the chisel edge, residual Ti work-material
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Fig. 11. SEM micrographs of tool wear of the WC–Co Spiral drill at 61m/min cutting speed and 0.152mm/rev feed with internal cutting fluid supply.
R. Li et al. / International Journal of Machine Tools & Manufacture 47 (2007) 63–74 73
buildup on the surface became more apparent. Similarmaterial buildup was observed in the middle and outsidecorner of the cutting edge. The corner of the drill marginand cutting edge remained sharp, indicating the drill was stillcapable of effective drilling.
The drill tip at the time of drill breakage at 164 holes wasnot melted but had fractured significantly. Close-up viewsshow that the chisel edge and middle cutting edge wereboth fractured. The corner of drill margin and cutting edgewas also severely fractured and no longer sharp. The drillhad lost the shape for effective drilling. The cutting forceswere expected to be high and a broken drill was expected.
9. Conclusions
This study demonstrated the feasibility of high-through-put drilling of Ti–6Al–4V and the importance of feed andinternal cutting fluid supply to improve the drill life. HighMRR of 156mm3/s with peripheral cutting speed up to183m/min was achieved using a commercially available4mm diameter WC–Co spiral point drill. In comparison, aconventional HSS twist drill could only work below27.4m/min cutting speed and 23.3mm3/s MRR. The spiralpoint drill design showed advantages of low thrust force,torque, energy, and burr size. The balance of cutting speedand feed was demonstrated to be critical in high-throughput drilling of Ti. Under the same MRR(156mm3/s), the best drill life and surface finish resultswere achieved at 91m/min peripheral cutting speed and0.102mm/rev feed using the WC–Co spiral point drill.
The internal cutting fluid supply through the drill at0.2MPa was critical for long drill life. It also helped toimprove the chip ejection, as demonstrated by the increasedlength of steady-state spiral cone chip region. As a futureresearch work, high-pressure fluid delivery systems [20]may further improve the tool life for high-throughputdrilling of Ti. Chip light emission in high-throughputdrilling of Ti was observed. It was particularly prominentfor the worn drill.
Modeling of the drilling process based on the finiteelement method [21] is an important future research directionto understand the work-material deformation and tempera-ture distribution in high-throughput drilling. Drilling tests inthis study were time consuming and expensive. Drillingprocess modeling can potentially narrow the range ofexperiments carried out and reduce the time and effortrequired to find optimal drilling process conditions.
Acknowledgments
A portion of this research was sponsored by the HeavyVehicle Propulsion Systems Materials Program, Office ofTransportation Technologies, US Department of Energy,and by the High Temperature Materials Laboratory UserProgram, Oak Ridge National Laboratory, managed byUT-Battelle, LLC for the US Department of Energy. Theassistance from Drs. Paul Becker, Peter Blau, Ray
Johnson, and Jun Qu of Oak Ridge National Laboratoryare greatly appreciated.
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