Experimental evaluation of polycrystalline diamond tool geometries while drilling carbon fiber-reinforced plastics

Transcription

1 Int J Adv Mnuf Technol (14) 71: DOI 1.17/s ORIGINAL ARTICLE Experimentl evlution of polycrystlline dimond tool geometries while drilling cron fier-reinforced plstics Y. Krpt & B. Değer & O. Bhtiyr Received: 31 July 13 /Accepted: 19 Decemer 13 /Pulished online: 5 Jnury 14 # Springer-Verlg London 14 Astrct Polycrystlline dimond (PCD) drills re commonly employed in cron fier-reinforced plstic (CFRP) drilling to stisfy hole qulity conditions with n cceptle tool life nd productivity. Despite their common use in industry, only smll numer of studies hve een reported on drilling CFRPs with PCD drills. In this study, drilling performnces of three different PCD drill designs re investigted experimentlly using thrust force, torque, nd hole exit qulity mesurements. Results show tht work mteril properties, drilling conditions, nd drill design should ll e considered together during the selection of process prmeters, nd the reltionships mong these fctors re quite complex. Keywords Drilling. Cron fier-reinforced plstics. Polycrystlline dimond 1 Introduction The influences of gloliztion on interntionl trvel, including fuel prices nd pssenger demnd for cheper ticket prices, push mnufcturers to produce lighter ircrfts. Employing dvnced mterils such s luminum, titnium, nd nickel lloys hs contriuted significntly to the weight reduction of ircrfts. However, Bikin et l. [1] showed tht structurl efficiency (operting empty weight/mximum tkeoff weight) mong ll types of ircrfts produced etween 1955 nd decresed out 15 % since the weight reductions due to Y. Krpt (*) Deprtment of Industril Engineering, Bilkent University, Ankr 68, Turkey e-mil: ykrpt@ilkent.edu.tr B. Değer: O. Bhtiyr Turkish Aerospce Industries Inc., Ankr 698, Turkey employing dvnced mterils hve een offset y lrger engine sizes, modifictions in ircrft structures for etter erodynmics, nd pssenger requests such s personl entertinment systems. Recent developments in composite mterils nd their use in the erospce industry help increse the structurl efficiency of the ircrfts. In the new genertion of ircrfts, the use of composites such s cron fier-reinforced plstics (CFRP) in structurl prts hs reched 4 % of ll mterils. These CFRP prts hve produced ner net shpe, which mens rough mchining opertions re eliminted. However, drilling nd some milling opertions re still required to ring them to their finl shpes. Drilling, one of the most common mchining processes in ircrft mnufcturing, is the suject of this pper. CFRPs re known to e difficult to mchine due to the rsive nture of cron fiers nd low therml conductivity of the plstic mtrix. While mchining CFRPs, het generted t the cutting zone cnnot e trnsferred wy with chips s in metl cutting. Mteril removl is relized through series of rittle frctures tht depend on the fier direction of the lminte. An extensive review of issues relted to CFRP mchining is presented in [, 3]. Under these conditions, rpid tool wer is common while drilling CFRP lmintes even when polycrystlline dimond (PCD) [4] nd dimondcoted cride tools [5] re employed. Rpid tool wer results in incresed drilling forces, which leds to delmintion t the exit of the drilled holes. As n nisotropic nd lminted mteril, CFRPs re very sensitive to delmintion, which is considered to e the most importnt finished prt qulity mesure. Hole dimeter, nother importnt qulity mesure, is closely relted to the condition of the drill. As the drill wers out, its dimeter decreses, which in turn ffects the drilled hole dimeter. Tool wer my lso dmge the interior surfce of the drilled hole. In order to circumvent such prolems, the condition of the drilling tools nd drilling prmeters must e closely monitored during production.

2 196 Int J Adv Mnuf Technol (14) 71: Due to its populrity in the erospce industry, CFRP drilling hs lredy een the suject of mny studies. Most of these hve considered the influence of drilling conditions (drilling speed, feed, drill geometry) on delmintion, drilling forces, nd tool wer [6 13]. In these studies, it hs een shown tht feed nd tool geometry re the two most importnt input prmeters ffecting the qulity of drilled holes. It is recommended to set feed t low vlues, especilly when drilling unidirectionl (UD) CFRPs, in order to decrese thrust forces nd thus decrese the likelihood of delmintion. Drill geometries tht distriute thrust forces towrds the drill periphery re recommended, nd specil drill geometries hve een proposed to stisfy this requirement [11, 1]. Use of ck-up pltes, drilling with pilot hole, nd vrile feed rte drilling strtegy is lso recommended to decrese delmintion [13 15]. Mteril properties of CFRPs re lso known to e influentil. Fric woven type CFRP lmintes re known to e more resistnt to delmintion thn the unidirectionl type CFRPs. It must e noted tht mchining performnce nd mteril chrcteristics re directly relted, nd CFRP mteril properties my vry significntly depending on the cron fier dimeter, type of resin, volumetric rtio of cron fier, curing conditions, etc. PCD tool mteril comines high rsion resistnce, therml conductivity, hrdness, nd impct toughness. Shrp cutting edges cn e otined due to the fine grin size of the PCD; however, corse PCD grins help improve wer resistnce. Due to their fvorle properties, PCD drills re commonly employed in industry. However, there hve een reltively few studies on the influence of PCD drill geometry on drilling process outputs. This study considers drilling of UD- CFRPs with PCD drills. The performnces of three PCD drills with different geometries hve een investigted experimentlly. The pper is orgnized s follows: Firstly, the experimentl setup nd drilling conditions re explined. Secondly, thrust force nd torque mesurements during drilling with three different PCD drills re nlyzed nd the hole exit performnces of drills re compred. Thirdly, edge conditions of the cutting edges fter drilling hve een investigted. Finlly, Fig. 1 Doule point ngle PCD tool designs. T1. T. c T3

3 Int J Adv Mnuf Technol (14) 71: Tle Mteril properties of CFRP lmintes Mteril Fier volume (% v/v) Strength (MP) Modulus (GP) Density (g/cm 3 ) Intermedite modulus cron fier-reinforced epoxy resin unidirectionl tpe 59, Mechnicl properties re for tensile properties t room temperture Fig. CFRP plte used in drilling experiments with the luminum ck plte drilling chrcteristics of three different PCD drills re compred nd discussed. Experimentl setup Three different doule point ngle PCD drills with distinct designs re considered in the experiments s shown in Fig. 1 The tool tip imges were cptured using digitl microscope (Keyence VHX 1). All tools hve doule (primry nd secondry) tip ngles. The tool on the top (Fig. 1) isdesignted s T1 nd its secondry drilling edge is considerly lrger thn its primry drilling edge. The drill in the middle (Fig. 1) is designted s T, nd compred to T1, it hs different design where the primry drilling edge is considerly lrger thn the secondry drilling edge. The drill on the ottom (Fig. 1c) is designted s T3; it lso hs doule point ngle geometry, ut it is fricted in twist drill form with helix ngle of 3. The sme figure lso shows the chisel edge designs of the drills. T hs considerly smller chisel edge thn tht of T1 nd T3. All designs hve tertiry edge which does not contriute to thrust forces during drilling, ut which ffects drilling torques. Drilling experiments were conducted on five-xis precision CNC mchining center with mximum 4, rpm rottionl speed. The mchining center is equipped with the ir conditioning nd ir filtering systems required for CFRP mchining. Mchining ws performed under wet conditions, where emulsion type coolnt ws used. The test setup is shown in Fig.. A plte mde out of luminum lloy with 8-mm dimeter holes ws used s cking plte to drill 6.4-mm dimeter holes. The drilling forces nd torques were mesured y Kistler 913 rotting dynmometer nd its chrge mplifier. The collected dt were processed on personl computer. Tle 1 shows the rnge of drilling conditions used in the experiments. All drilling experiments were performed on the sme CFRP mteril. Rottionl speed is kept constnt t 5, rpm to otin n cceptle level of cutting speed without exciting the dynmics of the rottionl force dynmometer. A CFRP lminte of 1-mm thickness ws used in the experiments. The feed rte rnge covers 5 to 3 mm/min. Intermedite modulus UD-CFRP lmintes were produced for drilling tests. The CFRP lmintes hve.14-mm ply thickness, with UD-CFRP configurtion of /45 /9 / 45 / with n equl numer of plies in ech direction. Tle shows the mechnicl properties of the UD-CFRP lmintes. 3 Anlysis of thrust force nd torque mesurements during drilling with doule point ngle PCD drills A generlized doule point ngle drill geometry is shown in Fig. 3. In this figure, region O represents the chisel edge with length L, shows the primry drilling region, shows the secondry drilling region, nd BC shows the tertiry drilling edge. The drill design prmeters re the length (L) nd ngle (γ) of the chisel edge, length of (L ), length of BC (L BC ), nd the tool tip ngles α nd β. In the erospce industry, hole dimeters re stndrdized; therefore, the tool dimeter (D= Tle 1 Experimentl conditions Rottionl speed (rpm) Lminte thickness (mm) Feed per revolution (mm/rev) 5, Fig. 3 Doule tip point ngle PCD drill geometry

4 198 Int J Adv Mnuf Technol (14) 71: Tle 3 Drill geometry mesurements Tool Dimeter, D α (rb) (mm) ( ) T1 T T β L ( ) (μm) γ ra L L LBC ( ) (mm) (mm) (mm) (mm) 1 3 1, , rb) cn e ssumed s known. Tle 3 summrizes geometry mesurements for T1, T, nd T3 drill designs. Mesurements re otined through digitl microscope. For given tool dimeter (D=rB), known tool tip ngles (α nd β), primry edge length (L), chisel edge length (L), nd ngle (γ), Eq. 1 cn e used to clculte the length of the secondry drilling region (L) s dependent vrile. L 1 Lcosγ rb L sinα ¼ sinβ ð1þ The totl thrust force (Fz) nd torque (T) cting on the drill during hole entry cn e represented with Eq.. The first term (Fz-ch) represents the influence of the chisel edge, the second term represents the contriution of the primry drilling edge Fig. 4 Stges of drilling process with doule point ngle PCD drill. Thrust force nd torque mesurements for T1 t 5, rpm nd μm/rev feed

5 Int J Adv Mnuf Technol (14) 71: (Fz-), nd the lst term represents the role of the secondry drilling edge (Fz-) on the totl thrust force. The effect of the tertiry region (BC) on the thrust forces is neglected. A similr eqution cn lso e written for totl drilling torque (T). The chisel edge is known to crry smll mount of torque; therefore, it is neglected. X F ¼ F z ch þ F z þ F z X z ðþ T ¼ T þ T þ T BC 199 Due to the doule point ngle design of the drilling tool, the uncut chip thickness (t) is different in nd s shown in Fig. 3 nd cn e clculted with Eq. 3 where f is the feed vlue. t ¼ f = sinα t ¼ f = sinβ Fig. 5 Thrust force nd torque mesurements for T1, T, nd c T3 t 5, rpm ð3þ

6 13 Int J Adv Mnuf Technol (14) 71: c Fig. 5 continued. The uncut chip re under primry () nd secondry () edges (shown s htched res in Fig. 3) cn e pproximted y multiplying the length of the edge with the uncut chip thickness t ech region. It must e noted tht decresing the tool tip ngle β results in longer edge length due to constnt tool dimeter. The force nd torque mesurements otined during drilling revel importnt informtion out the influence of tool geometry nd drilling process prmeters on the CFRP drilling process. This informtion, long with hole qulity mesurements, cn e used to set process prmeters, to monitor tool wer, nd to improve drill geometries. Figure 4 illustrtes different phses (points 7) of the drilling process when doule point ngle PCD tool is employed. Figure 4 shows mesured thrust force nd torque vritions for drill T1 t 5, rpm nd μm/rev feed. The drilling process strts from position (point ) with enough distnce so tht the drill reches the required feed rte efore entering the hole; therefore, constnt feed rte is mintined until the end of the drilling process (point 7). The first point (1) on the thrust force curve corresponds to the loction where the chisel edge of the drill enters the mteril (1 in Fig. 4). The second point () corresponds to the loction where the primry cutting edge of the drill enters the hole. While thrust force increses drmticlly in this region, the increse in torque is quite smll. As the secondry drilling zone enters the hole (point 3), the rte of increse in thrust force decreses since drilling is performed with lower ngle (β) which decreses the uncut chip thickness. Pek thrust force is reched t the end of the secondry drilling zone (point 3). Thrust force slightly decreses towrds the hole exit due to the decresing numer of plies (point 4). It must e noted tht round this point, the torque reches its pek vlue. The torque difference etween points 3 nd 4 is due to contct etween the tertiry drilling edge nd the mteril. As soon s the chisel edge (point 5) nd primry drilling edge (point 6) leve the cut, thrust force decreses rpidly with rte similr to tht which is oserved t the hole entry (points 1 ). It must e noted tht the torque stys lmost the sme etween points 4 nd 6 since the secondry cutting edge, which crries most of the torque, is still in contct with the mteril. The thrust force nd torque decrese continuously etween points 4 nd 8 until the drill completely leves the cut. During retrction of the drill, the tertiry edge my contct the drilled hole, which corresponds to torque jump s oserved in Fig. 4. It must e noted tht for known drill geometry, lminte thickness, nd feed rte, the chrcteristic thrust force nd torque vlues through points 1 to 6 cn e otined from mesurements. Figure 5 shows the time history of thrust force nd torque mesurements s function of feed rtes otined during drilling for drills T1, T, nd T3. The thrust force nd torque vrition during hole entry (points 1 3) cn e used to compre the drilling chrcteristics of drills T1, T, nd T3. Distriution of thrust force nd torque mong chisel edge nd primry nd secondry drilling edges cn e otined s function of feed. It is cler from these mesurements (Fig. 5) tht drill geometry influences the progress of thrust forces nd torques during drilling, t oth entry nd exit points. From these mesurements, it cn e seen tht drill T yielded lrger thrust forces ut lower torque mesurements thn drills T1 nd T3.

7 Int J Adv Mnuf Technol (14) 71: Chisel Edge Thrust Force, Fz1(N) Feed per Revolution, f(mm/rev) Fig. 6 Chisel edge thrust force (F z1 ) mesurements s function of feed per revolution As mentioned erlier, using drill geometry mesurements, it is possile to otin chrcteristic force nd torque mesurements for ech drill from Fig. 5. The chisel edge thrust forces (point 1 in Fig. 4) re compred in Fig. 6 s function feed per revolution. Drill T yielded lower thrust forces, thnks to its smll chisel edge length. It must e noted tht T nd T3 exhiited similr thrust forces, while the chisel edge length of T3 is lmost twice s lrge s tht of T. Drill T1 yielded the lrgest thrust forces mong ll tested drills. The chisel edge geometry of T3, which employs rke ngle, is oviously more effective thn T1 sed on chisel edge thrust forces shown in Fig. 6. The chisel edge geometry of drill T1 s shown in Fig. 7 is different thn T1 T T3 T nd T3 in the sense tht it seems to form single cutting edge tht comines with the primry drilling edge s sper hed. In order to clculte nd compre the thrust force nd torques in the primry drilling edge, the chisel edge thrust forces nd torques corresponding to point 1 re sutrcted from the mesurements otined t point (F z_ =F z F z1 ). Similrly, the secondry drilling edge thrust force is clculted y sutrcting the primry edge force from the pek force (F z_ =F z3 F z ). The mesurements re given s thrust force per edge versus feed per edge (ll drills hve two drilling edges) s shown in Eq. 4. F z ¼ F z F z1 F z ¼ F z3 F z ð4þ In order to compre thrust force nd cutting force distriution mong drilling edges, clculted forces re divided y corresponding uncut chip re s function of feed. While clculting specific cutting force distriution, the midpoint distnce of ech drilling edge to the tool center is used to convert torque to cutting force [9, 16]. The necessry equtions re given in Eqs. 5, 6, nd 7. A ¼ r f ; A ¼ ðr OB r Þ f ð5þ Fig. 7 Chisel edge designs of drills T1 nd T3

8 13 Int J Adv Mnuf Technol (14) 71: K t ¼ F z A ; K t ¼ F z A ð6þ ð K c ¼ T T 1 Þ= ðt 3 T Þ= r ; K c ¼ r A þ r OB ð7þ A The results re shown in Figs. 8 nd 9.IndrillsTndT3, more homogenous distriution of thrust forces etween primry nd secondry drilling edges is otined, wheres drill T1 stress distriution on the chisel edge is lrger thn other edges. In terms of torque, for drill T1, lmost ll the torque is crried y the secondry drilling edge, wheres drills T nd T3 distriute the torque evenly etween the edges. It must e noted tht cutting force distriution etween edges chnges slightly s feed increses. Specific Thrust Force (N/mm^) Specific Thrust Force (N/mm^) Specific Thrust Force (N/mm^) Feed per Edge (mm) Feed per Edge (mm) c Feed per Edge (mm) Fig. 8 Specific thrust force vrition s function of feed for drills T1, T, nd c T3 Specific Cutting Force (N/mm^) Specific Cutting Force (N/mm^) Specific Cutting Force (N/mm^) Feed per Edge (mm) Feed per Edge (mm) Feed per Edge (mm) 4 Investigtion of hole exit qulity Delmintion is n importnt considertion in drilling CFRPs. The lod-crrying cpility of the lmintes significntly decreses due to seprtion of plies. The thrust forces t the lst ply of the lminte re considered to e responsile for delmintion. The underlying mechnism is elieved to e the indenttion effect cused y the drill chisel edge. The level of thrust force t the hole exit leding to delmintion is defined s criticl thrust force [11 15, 17, 18]. The influence of predrilling on delmintion ws considered, nd it ws shown tht pre-drilled pilot holes help reduce delmintion [14, 15]. Lchud [17] proposed n nlyticl model to predict the criticl thrust force sed on mteril properties of the composite mteril. Hocheng nd Tso [1] considered different drill geometries nd clculted criticl thrust force expressions for ech of them. They lso evluted the performnce of the drills sed on Tguchi nlysis [19]. They concluded tht drills tht distriute the lods evenly on the drill periphery perform etter nd lower feeds help reduce thrust forces t the c Fig. 9 Specific cutting force vrition s function of feed for drills T1, T, nd c T3

9 Int J Adv Mnuf Technol (14) 71: Feed T1 T T3 µm/rev 3 µm/rev 4 µm/rev 5 µm/rev 6 µm/rev Fig. 1 Hole exits otined with T1, T, nd T3 hole exit nd hence the likelihood of delmintion. Duro et l. [18] lso conducted experiments on different drill geometries nd lso showed tht low feed helps reduce the thrust forces t the hole exit ut lso increses the tool tempertures, which induces tool wer nd susequently leds to poor performnce of the drill. They found tht the twist drill, with 1 point ngle, yields the highest thrust forces t the hole exit, ut it exhiited the est performnce in terms of delmintion. Severl procedures hve een proposed for mesuring delmintion. One widely ccepted delmintion mesure considers the rtio of the imginry dimeter drwn round Fig. 11 Phses of the hole exit sequence. Mesured thrust forces for ech drill t point I Thrust Force Fz (N) T1 T T Feed, f(mm/rev)

10 134 Int J Adv Mnuf Technol (14) 71: the delminted zone to the originl hole dimeter [6 8]. In this study, visul exmintion of the hole exits re performed. Hole entry delmintion ws not oserved during drilling experiments. Figure 1 shows the hole exits otined fter drilling tests with T1, T, nd T3. New drills were used t the eginning of ech experiment. The performnce mesure is defined s the ility to drill delmintion-free holes t higher feeds, since drilling t higher feeds results in etter productivity. According to Fig. 1, holes with no significnt qulity defects cn e drilled t 4 μm/rev with T1 nd T3, wheres the limiting feed vlue for T is μm/rev. Therefore, sed on the defined performnce mesure, drills T1 nd T3 outperform T. Figure 11 shows the hole exit sequence of doule point ngle PCD drill. The process is divided into four phses. The point I corresponds to the loction of the chisel edge of the drill t the lst ply of the lminte. It is followed y point II, where the chisel edge leves the hole. Points III nd IV correspond to primry nd secondry drilling edges exiting the hole. Figure 11 shows the mesured thrust forces t point I. Bsed on the mesurements, the criticl thrust force for T cn e considered to e round 95 N t μm/rev feed vlue. However, for T1 nd T, criticl thrust forces cn e considered s 14 nd 86 N, respectively, t 5 μm/rev feed. The results show tht drill geometry nd feed ffect the vlue of criticl thrust force, ut low thrust forces do not gurntee delmintion-free holes. These results gree with the findings reported in [18]. Figure 11 lso shows the projected hole re. Mesured thrust forces (given in Fig. 5) t ech point cn e divided to the projected hole re to clculte norml stress vrition during hole exit. Eqution 8 cn e used to clculte the projected norml stresses. F z S I IV ¼ ð8þ π D 4 r As soon s the chisel edge leves the hole t point II, the re of the chisel edge is sutrcted from the projected hole re nd the norml stress cn e clculted. The sme clcultion is repeted for the cse where the primry drilling region exits the hole t point III. It must e noted tht drills hve different chisel nd primry nd secondry edge lengths; therefore, the influence of drill geometry comined with the mesured thrust forces is reflected on the norml stress clcultions. No norml stress is pplied to the workpiece when the secondry edge leves the hole t point IV. Figure 1 compres the norml stresses pplied to the workpiece during the hole exit sequence for T1, T, nd T3. It cn e clerly seen tht drills T1 nd T pply more norml stress thn T3 t the eginning of the hole exit Stress, S(N/mm^) Stress, S(N/mm^) Stress, S(N/mm^) Distnce from Tool Center, r (mm) Distnce From Tool Center, r (mm) Distnce From Tool Center, r (mm) c 1 mm/min 15 mm/min mm/min 3 mm/min 5 mm/min 1 mm/min 15 mm/min mm/min 5 mm/min 5 mm/min 1 mm/min 15 mm/min mm/min 5 mm/min Fig. 1 Comprison of norml stresses (S) on drilling edges clculted for T1, T, nd c T3 for vrious feed rtes sequence. Due to its lrge chisel edge, drill T1 concentrtes the norml stress t the hole center (r= in Eq. 8) nd its long secondry drilling edge helps decrese the norml stress steeply towrds the end of the process. Drill T3 exhiited the lowest norml stresses t the hole center, thnks to its chisel edge design, which includes rke ngle. Its norml stress distriution profile is similr to T ut with lower norml stress vlues. They oth employ the sme design intent, i.e., long primry drilling edge. Norml stresses pplied y the drill T re the lrgest s the secondry drilling edge leves the hole. The condition of the cutting edge is n importnt fctor on drilling qulity. In order to e le to further investigte the results, cutting edges of the drills were investigted fter experiments y using 3D lser scnning microscope

11 Int J Adv Mnuf Technol (14) 71: (Keyence VKX 11). Figure 13 shows the edge profiles of secondry drilling edges of T1, T, nd T3. Figure 13 revels tht the secondry drilling edge of T is significntly worn fter drilling the sme numer of holes s T1 nd T3. Under the drilling conditions tested, rsion is the min wer mechnism. The length of the wer lnd is round 1 μm. Drill T3 exhiited smller wer lnd compred to T, wheres the cutting edge of drill T1 seems to e intct. The edge rdius of the fresh drills is mesured to e round 1 15 μm. The reson why T exhiits lrger flnk wer lnd cn e relted to its design intent, which tries to minimize the thrust force t the hole exit with smll chisel edge, which increses the torque intensity crried on the secondry cutting edge. It must lso e noted tht while significnt wer exists on the secondry edge of T, the thrust force nd torque mesurements re still similr to T1 nd T3. In Fig. 1,forT,itcne seen tht the chisel edge length (L) is considerly smll, which lso llows smll we thickness. However, mjority of the lod hs to e crried y the secondry edge, which proly leds to fster wer on this edge. Drill T1 crries significnt portion of the thrust lod on the chisel edge, therey decresing the thrust force intensities on the primry nd secondry drilling edges. A short primry edge length results in longer secondry edge length, which llows for lower cutting force intensity on the secondry drilling edge. Drill T3 exhiits similr design intent s T, ut its twist drill form llows for Fig. 13 Profiles of (fresh nd worn) secondry drilling edges of T1, T, nd c T3

12 136 etter chip evcution nd its positive rke ngle on the secondry drilling edge helps to reduce drilling forces. It yields comprle hole qulity results to T1. Figure 14 shows the vrition of thrust force nd torque for T1 t 1 μm/rev feed nd 5, rpm (the CFRP lminte used in this prticulr experiment is 8-mm thick with sme mteril properties). The vrition of thrust forces etween 1st nd 3nd hole cn e seen in this figure. It must e noted tht tool wer tkes plce in secondry drilling zone s it is ovious from the progress of the thrust force nd torque mesurements when the loctions of point 3 of Fig. 4 re compred. Figure 14 shows the scnning electron microscope imge of the wer lnd on the secondry drilling edge, which is similr to the wer lnd of T s oserved in Fig. 13. Int J Adv Mnuf Technol (14) 71: It must e noted tht the influence of PCD mteril properties (grin size, percentge of colt/pcd, etc.) ws not studied in this pper. Since PCD drills from different mnufcturers were used in this study, some performnce differences cn lso e relted to the PCD mteril properties. Different drill geometries hving the sme PCD mteril properties fricted y the sme tool mnufcturer my yield more detiled informtion out the process. Nevertheless, the trde-offs in drill design re cler s result of the experimentl study presented in this pper. There is complex reltionship etween drill geometry design nd its delmintion performnce. Additionl experimentl reserch is required to drw definite conclusions out these drill geometries. The results re in fvor of Fig. 14 Effect of tool wer on thrust force nd torque. Scnning electron microscope imge of the wer lnd

13 Int J Adv Mnuf Technol (14) 71: long secondry drilling edge design, since lower tool wer ws oserved. However, it is importnt to note tht lrge secondry drilling edge increses the drilling time. Considering the totl numer of holes, longer drilling time my ecome n issue. In the cse of drilling thin lmintes, long secondry drilling edge would exit the hole without tertiry drilling edge strting to drill. This results in n rupt decrese in thrust force nd torque, which dversely ffects the qulity of the hole. Therefore, smll chisel edge nd short secondry edge my work etter in thin lmintes. 5 Conclusions In this study, drilling chrcteristics nd performnces of three PCD drills with different geometries were investigted. Experimentl oservtions on drilling thick CFRP lmintes showed tht drill geometry prmeters directly influence distriution of forces nd torques mong edges of the doule point ngle PCD drills. For the thick CFRP lminte used in this study: & & & Drill designs T1 nd T3 llowed drilling holes t higher feeds without delmintion compred to drill design T. Although drill T employed the smllest chisel edge design, it did not perform s well s T1 nd T3. In generl, smll chisel edge drill designs re recommended to decrese thrust forces t the hole exit to decrese the likelihood of delmintion. However, lrge chisel edge design my help to reduce thrust nd cutting force intensities on the secondry drilling edge, which results in mintining the shrpness of the cutting edge. Therefore, the reltionship etween secondry drilling edge tool wer nd delmintion must e investigted in more detil. Experimentl investigtions show tht criticl thrust force chnges s function of drill geometry, nd for ech drill geometry the criticl thrust force is different. Insted of merely monitoring thrust forces t the lst ply, investigtion of norml stresses t the hole exit sequence my yield more insight out the process since the condition of the edges is reflected on the clcultions. Acknowledgments The uthors would like to thnk Scientific nd Technicl Reserch Council of Turkey (TUBITAK) for their finncil support of this study. References 1. Bikin R, Lukchko SP, Witz IA () The historicl fuel efficiency chrcteristics of regionl ircrft from technologicl, opertionl, nd cost perspectives. J Air Trnsp Mng 8(6): Teti R () Mchining of composite mterils. CIRP Ann Mnuf Technol 51: Dndekr CR, Shin YC (1) Modeling of mchining of composite mterils: review. Int J Mch Tool Mnuf 57: Prk K, Bel A, Kim D, Kwon P, Lntrip PJ (11) Tool wer in drilling of composite/titnium stcks using cride nd polycrystlline dimond tools. Wer 71(11 1): Montoy M, Clmz M, Gehin D, Girot F (13) Evlution of the performnce of coted nd uncoted cride tools in drilling thick CFRP/luminum lloy stcks. Int J Adv Mnuf Technol. doi:1. 17/s Shyh IS, Aspinwll DK, Soo SL, Brdley S (9) Drill geometry nd operting effects when cutting smll dimeter holes in CFRP. Int J Mch Tool Mnuf 49(1 13): Schulze V, Becke C, Weidenmnn K, Dietrich S (1) Mchining strtegies for hole mking in composites with miniml workpiece dmge y directing the process forces inwrds. J Mter Process Technol 11(3): Piquet R, Ferret B, Lchud F, Swider P () Experimentl nlysis of drilling dmge in thin cron/epoxy plte using specil drills. Compos A 31: Lzr MB, Xirouchkis P (11) Experimentl nlysis of drilling fier reinforced composites. Int J Mch Tool Mnuf 51(1): Frz A, Biermnn D, Weinert K (9) Cutting edge rounding: n innovtive drill wer criterion in drilling CFRP composite lmintes. Int J Mch Tool Mnuf 49: Tso CC (8) Thrust force nd delmintion of core-sw drill during drilling of cron fier reinforced plstics (CFRP). Int J Adv Mnuf Technol 37(1 ): Hocheng H, Tso CC (6) Effects of specil drill its on drillinginduced delmintion of composite mterils. Int J Mch Tool Mnuf 46(1 13): Tso CC, Chen WC (1997) Prediction of the loction of delmintion in the drilling of composite lmintes. J Mter Process Technol 7: Tso CC, Hocheng H (3) The effect of chisel length nd ssocited pilot hole on delmintion when drilling composite mterils. Int J Mch Tools Mnuf 3(43): Won MS, Dhrn CHK () Chisel edge nd pilot hole effects in drilling composite lmintes. Trns ASME J Mnuf Sci Eng 14: Krpt Y, Değer B, Bhtiyr O (1) Drilling thick fric woven CFRP lmintes with doule point ngle drills. J Mter Process Technol 1(1): Lchud F, Piquet R, Collomet F, Surcin L (1) Drilling of composite structures. Compos Struct 5: Duro LMG, Gonçlves DJS, Tvres JMRS, de Aluquerque VHC, Vieir AA, Mrques AT (1) Drilling tool geometry evlution for reinforced composite lmintes. Compos Struct 9(7): Tso CC, Hocheng H (4) Tguchi nlysis of delmintion ssocited with vrious drill its in drilling of composite mteril. Int J Mch Tools Mnuf 4(44):185 19