coatings

Article Experimental and Simulation Study on Life Models in of Brass Using Uncoated-WC and AlCrN Coated-WC

Manit Timata and Charnnarong Saikaew *

Department of Industrial , Faculty of Engineering, Khon Kaen University, Khon Kaen 40002, Thailand; [email protected] * Correspondence: [email protected]

 Received: 30 October 2019; Accepted: 11 December 2019; Published: 13 December 2019 

Abstract: is an undesirable phenomenon occurring in drilling operation which is one of the essential operations in the industry since it is directly influencing the operating costs. Exit burr height (EBH) values as the function of drilling time during drilling the specific holes of the forging brass used for producing the water-valve components with the uncoated- tungsten carbide (WC) and the AlCrN coated-WC were discussed. The data sets of drilling time, corresponding to EBH values at the appropriate criterion were used to develop the tool life models in terms of cutting speed and feed rate using Taylor’s equation. Monte Carlo simulation was adopted to study the uncertainty of cutting speed and feed rate on tool life predictions for sensitivity analysis. The results showed that drilling with a low feed rate decreased the averages of EBH. The predicted tool life values of the AlCrN coated-WC drills were higher than those of the uncoated-WC ones based on the results of tool life predictions. The appropriate operating condition of the cutting speed of 60 m/min and the feed rate of 0.2 mm/rev was recommended for manufacturers in the drilling of the forging brass workpieces using the AlCrN coated-WC drills. Moreover, the predicted tool life values for the uncoated-WC and the AlCrN coated-WC drills were about 600 and 800 min, respectively. This indicated that the AlCrN coated-WC increased tool life by 30%.

Keywords: drilling; tool life; burr height; Taylor’s model; Monte Carlo

1. Introduction Tool life is very important in the machining industry to improve the quality of a as well as a component. If tool life expires, the reliability of the tool affects quality of the machined parts and . It is one of the important five criteria (tool life, material remove rate, cutting forces, surface finish and chip shape) used for studying the material and cutting tool behaviours during machining [1]. The tool life model can be effectively used to predict and length of cutting time that the tool can be operated before changing a new tool. Tool life criteria have been adopted for performing tool life models. Generally, tool life criteria used in the cutting operations can be observed from failure of the tool, occurrences of maximum width of the flank wear, the depth and width of crater wear, sound of the tool, type of chip, degradation of machined surface [2]. There have been previous researches on drilling tool life based on various tool life criteria. de Paiva et al. [3] compared the performance of coatings deposited on drills in drilling compacted graphite based on tool life criterion of flank wear. They found that the Cr-based coatings (AlCrN and TiSiN/AlCrN) had better performance than other ones. Wu et al. [4] also used the flank wear of the drill cutting edge as a tool life criterion in drilling compacted graphite iron with coated carbide drill at the feed rate of 26.5 mm/s.

Coatings 2019, 9, 853; doi:10.3390/coatings9120853 www.mdpi.com/journal/coatings Coatings 2019, 9, 853 2 of 14

Burr height is another criterion used to develop a tool life model. It is noted that burr is a critical problem of various manufacturing processes, especially drilling operation which is one of the most important operations in the machining industry since it is directly influencing the operating costs. Burr significantly influences productivity. Gillespie [5] stated that burr elimination can account for 30% of the total operating cost. The deburring procedure after a drilling operation of thousands drilled holes requires a tremendous operating cost to various industries [6]. Drilling operation accounts for approximately 33% of all cutting operations [7]. Consequently, machined part should be used without the process of deburring. Costa et al. [8] used the burr height of the drill coated with TiAlN as the criterion in drilling of the steel DIN 38MnS6 with two cutting speeds (45, 60 m/min). They used the average burr height as a function of the drill life levels (in percentage). They found that the average burr height increased with the wear of the drills. The average burr heights for the cutting speed of 45 m/min were twice than those for the cutting speed of 60 m/min. Some researchers have applied Taylor’s equation for the tool life modelling of drilling tools to investigate the effect of drilling parameters on drilled holes. Lukic and Djapic [9] proposed a model relationship between cutting speed and tool life for optimization of the cutting parameter in drilling of various workpieces based on Taylor’s extended equation. Shen et al. [10] developed the tool life of spade drills in drilling of ductile materials by varying cutting speeds, feed rates and drill point angles. They showed that the extended Taylor’s equation could be used to predict qualitatively and quantitatively the tool life of spade drills with variations of drilling parameters and drill geometry. Astakhov et al. [11] proposed the logarithmic coordinate transformations of the regression equation to investigate the effect of cutting speed, feed rate and cutting fluid flow rate on the drilled holes roughness and roundness, the tool life and the cutting force in deep holes drilling. However, there has been no previous study regarding tool life modelling using Taylor’s equation with the burr height criterion in drilling of forging brass. Timata and Saikaew [12] attempted to systematically investigate the effect of different speeds and feed rates on EBH and drilled diameter during drilling of forging yellow brass with the special uncoated carbide drill. It was found that spindle speed and feed rate on the exit burr height (EBH) and drilled diameter were statistically significant at the level of significance of 0.05. In addition, the appropriate operating condition of the drilling parameters was obtained based on the results of analysis of variance and response surface methodology. In this work, CNC drilling machine was employed to carry out the experiments under dry condition for making the specific of the forging brass workpieces for producing the water-valve components using the uncoated-WC and the AlCrN coated-WC drills. It is noted that AlCrN was considered to be an alternative coating material for drilling tools due to its excellent properties such as low thermal conductivity, better wear resistance, low friction coefficient, high hardness, elastic modulus, H/E ratio, and bonding condition between coating and the unique carbide substrate [13–15]. Tool life models of both drills were developed in terms of cutting speed and feed rate using Taylor’s equation with a maximum EBH value of 0.16 mm for the tool life criterion based on manufacturer’s specifications and customer’s requirement. In addition, Monte Carlo simulation was used to investigate the influence of cutting speed and feed rate on tool life predictions for sensitivity analysis and studying the uncertainty of drilling factors on EBH and tool life. Hence, the novel research contributed to the systematic understanding of the sensitivities of tool life predictions for uncertainty assessment of the drilling factors within the 95% confidence interval due to the inherent variation of the drilling process. Only a few experimental runs were required to formulate the tool life model. Furthermore, this study was simplified by the fact that the running time for the Monte Carlo simulation was less time consuming using the Minitab® software.

2. Materials and Methods Brass is an important material used for various manufacturing purposes. It is used to produce a water-valve component by the forging manufacturing process under real working conditions in a company located in a province of Thailand. Figure1 shows a water-valve component and its Coatings 2019, 9, x FOR PEER REVIEW 3 of 13 company located in a province of Thailand. Figure 1 shows a water-valve component and its dimensions including the specific holes with the diameters of 14.8 ± 0.15 mm and 24.5 ± 0.15 mm. The water-valve component is made from yellow brass (model JIS-C3771). The chemical compositions in wt.% of the brass consisted of 59.157% Cu, 38.35% Zn, 2.232% Pb, 0.131% Fe, 0.077% Sn, 0.031% Ni, 0.022% Sb, 0.010% As, 0.003% Mn, 0.0013% Mg. Figure 2 shows a schematic drawing of a drill. The drilling tool was designed and manufactured for drilling the specific holes of the water-valve component. The CNC drilling machine (Number Five: model DR-8P) was used for all experiments in this study. A special drill is made of tungsten carbide (WC) and prepared by sintering process (class K20, DIN: DK 255F). The special drills were also coated with AlCrN by magnetron sputtering, filtered cathodic arc (FCA) system equipped with two material targets and the proprietary straight duct filters (Kitagawa (Chon CoatingsBuri, Thailand) 2019, 9, 853x FOR and PEER Nanoshield REVIEW Co., Ltd.). The drills were cleaned by detergent and sonicated3 withof 13 14 alcohol for 30 min and then placed into the FCA chamber followed by evacuation to a base pressure companyof 5 × 10− 3 locatedPa. The drillsin a provincewere heated of toThailand. 350 °C and Figure bombarded 1 shows with a water-valvemetal ions produced component by aand filtered its dimensions including the specific holes with the diameters of 14.8 0.15 mm and 24.5 0.15 mm. dimensionscathodic arc including under a thesubstrate specific bias holes of with1000 the V fordiameters 10 min of in 14.8 order ± 0.15 to mmenhance and 24.5the adhesion± 0.15 mm. of The water-valve component is made from yellow brass (model JIS-C3771). The chemical compositions Thedeposited water-valve AlCrN filmcomponent and remove is made residual from surfac yelle owimpurities. brass (modelThe thickness JIS-C3771). of the TheAlCrN chemical coating in wt.% of the brass consisted of 59.157% Cu, 38.35% Zn, 2.232% Pb, 0.131% Fe, 0.077% Sn, 0.031% Ni, compositionsmeasured by in a wt.% commercial of the brass stylus consisted profiler of 59.157%(Bruker Cu,model 38.35% Dektak Zn, ®2.232%, Karlsruhe, Pb, 0.131% Germany) Fe, 0.077% was 0.022% Sb, 0.010% As, 0.003% Mn, 0.0013% Mg. Sn,approximately 0.031% Ni, 0.022% 3 μm. Sb, 0.010% As, 0.003% Mn, 0.0013% Mg. Figure 2 shows a schematic drawing of a drill. The drilling tool was designed and manufactured for drilling the specific holes of the water-valve component. The CNC drilling machine (Number Five: model DR-8P) was used for all experiments in this study. A special drill is made of tungsten carbide (WC) and prepared by sintering process (class K20, DIN: DK 255F). The special drills were also coated with AlCrN by magnetron sputtering, filtered cathodic arc (FCA) system equipped with two material targets and the proprietary straight duct filters (Kitagawa (Chon Buri, Thailand) and Nanoshield Co., Ltd.). The drills were cleaned by detergent and sonicated with alcohol for 30 min and then placed into the FCA chamber followed by evacuation to a base pressure of 5 × 10−3 Pa. The drills were heated to 350 °C and bombarded with metal ions produced by a filtered cathodic arc under a substrate(a) bias of 1000 V for 10 min in( border) to enhance the adhesion of deposited AlCrN film and remove residual surface impurities. The thickness of the AlCrN coating measured by aFigure Figurecommercial 1.1. ((aa)) PhotographPhotograph stylus profiler ofof aa water-valvewater-valve (Bruker componentcomponent model Dektak andand ((bb)®) its,its Karlsruhe, dimensions.dimensions. Germany) was approximately 3 μm. Figure2 shows a schematic drawing of a drill. The drilling tool was designed and manufactured for drilling the specific holes of the water-valve component. The CNC drilling machine (Number Five: model DR-8P) was used for all experiments in this study. A special drill is made of tungsten carbide (WC) and prepared by sintering process (class K20, DIN: DK 255F). The special drills were also coated with AlCrN by magnetron sputtering, filtered cathodic arc (FCA) system equipped with two material targets and the proprietary straight duct filters (Kitagawa (Chon Buri, Thailand) and Nanoshield Co., Ltd.). The drills were cleaned by detergent and sonicated with alcohol for 30 min and then placed into the FCA chamber followed by evacuation to a base pressure of 5 10 3 Pa. The drills × − were heated to 350 ◦C and bombarded with metal ions produced by a filtered cathodic arc under a substrate bias of 1000 V for 10 minFigure in order 2. Schematic to enhance drawing the of adhesion a drill. of deposited AlCrN film and remove residual surface impurities.(a) The thickness of the AlCrN(b) coating measured by a commercial ® stylusThere profiler has (Bruker Figurebeen no 1. model (previousa) Photograph Dektak study of, Karlsruhe, ofa water-valvethe effect Germany) of component cutting was speed and approximately ( band) its feeddimensions. rate 3 µonm. burr formation and tool life in drilling of forging brass with AlCrN coated-WC and other coated drills. However, in this study, the cutting speed and feed rate, and their levels were selected from the tool manufacturer’s recommendation and based on the real production process as well as some literature reviews. Some researchers studied the effects of different feed rates and cutting speeds on burr formation [16], drilled hole surface roughness [11,17], tool life [18,19], and tool wear [20,21] during drilling of various materials. Lin and Shyu [18] improved the cutting performance of drilling with coated drills by varying feed rates while Lin [17] studied the effect of cutting speed and feed rate on burr height and surface roughness and found that cutting speed and feed rate were superior to constant feed rate regarding tool life, burr height and surface roughness. Karnik et al. [16] investigated the effect of Figure 2. Schematic drawing of a drill.

There has been no previous study of the effect effect of cutting speed and feed rate on burr formation and tool life in drilling of forging brass with Al AlCrNCrN coated-WC and other coated drills. However, in this study,study, the the cutting cutting speed speed and feedand rate,feed and rate, their and levels their were levels selected were from selected the tool manufacturer’sfrom the tool manufacturer’srecommendation recommendation and based on the and real based production on the real process production as well process as some as literature well as some reviews. literature Some reviews.researchers Some studied researchers the effects studied of diff erentthe effects feed rates of different and cutting feed speeds rates onand burr cutting formation speeds [16 ],on drilled burr formationhole surface [16], roughness drilled [hole11,17 surface], tool life roughness [18,19], and [11,17], tool wear tool [life20,21 [18,19],] during and drilling tool wear of various [20,21] materials. during drillingLin of and various Shyu materials. [18] improved the cutting performance of drilling stainless steel with coated drills by varyingLin and feed Shyu rates [18] while improved Lin [ 17the] studiedcutting performance the effect ofcutting of drilling speed stainless and feed steel rate with on coated burr height drills byand varying surface feed roughness rates while and Lin found [17] that studied cutting the speedeffect andof cutting feed ratespeed were and superior feed rate to on constant burr height feed andrate regardingsurface roughness tool life, and burr found height that and cutting surface speed roughness. and feed Karnik rate etwere al. [16superior] investigated to constant the efeedffect rate regarding tool life, burr height and surface roughness. Karnik et al. [16] investigated the effect of

Coatings 2019, 9, 853 4 of 14 Coatings 2019, 9, x FOR PEER REVIEW 4 of 13 ofcutting cutting speeds speeds (8, (8,12, 12, 1616 m/min) m/min) and and feed feed rates rates (0.04, (0.04, 0.08, 0.08, 0.12 0.12 mm/rev) mm/rev) on on burr burr height height and burr thickness during the drilling of AISI 316L stainlessstainless steel. Wang Wang et al. [19] [19] selected the two cutting speeds (80 and 120 mm/min)/min) and two feed rates (0.14 and 0.18 mmmm/rev)/rev) in drilling of 42CrMo steel with TiN-coated finefine graingrain cementedcemented carbidecarbide twisttwist drills.drills. Polli and Cardoso [20] [20] carried out the deep drilling experiments on the forged and heat-treated SAE 4144M steel using solid cemented carbide twist drills with cutting speeds speeds of of 50, 50, 65 65 and and 70 70 m/mi m/minn and and feed feed rates rates of of 0.05, 0.05, 0.12 0.12 and and 0.15 0.15 mm/rev. mm/rev. CCardosoardoso et al. [[21]21] selected two cutting speeds and two feed rates in drilling of SAE4144M steel with TiAlN andand AlCrNAlCrN coatedcoated drills.drills. They stated that AlCrN coated drill had better performance than TiAlN coatedcoated drilldrill basedbased onon aa significantsignificant reductionreduction ofof adheredadhered materialmaterial onon thethe drill.drill. Table1 1 shows shows threethree cuttingcutting speedsspeeds (40,(40, 5050 andand 6060 mm/min)/min) and three feed rates (0.15, 0.20 and 0.25 mmmm/rev)/rev) for carrying out the drilling experimentsexperiments using the uncoated-WC and the AlCrN coated-WC drills. Table 1. Drilling factors and their levels for carrying out the drilling experiments using the uncoated-WC andTable AlCrN 1. Drilling coated-WC factors drills. and their levels for carrying out the drilling experiments using the uncoated-WC and AlCrN coated-WC drills. Level Factor Level Factor Low MiddleMiddle HighHigh Cutting speed (m(m/min)/min) 40 50 50 60 60 Feed rate (mm(mm/rev)/rev) 0.15 0.20 0.20 0.25 0.25

The CNC drilling machine produced approximately 1000 drilled workpieces for each drilling interval time of 72 min.min. At this drilling interval time, the drill was taken ooffff thethe CNCCNC machine.machine. One hundred samples of drilled workpieces were randomlyrandomly selected from all drilled workpieces. Then, EBH valuesvalues of of each each sample sample were were measured measured at di ffaterent different three positions three positions using a burrusing height a burr measuring height instrumentmeasuring (Bruker:instrument model (Bruker: Dektak model XT) as Dektak shown inXT) Figure as shown3. An average in Figure EBH 3. wasAn calculatedaverage EBH from was the threecalculated EBH measurementsfrom the three of EBH each measurements sample and an of average each sample EBH of and one hundredan average samples EBH wasof one determined. hundred Thesamples averages was determined. EBH of each The of one averages hundred EBH samples of each were of one collected hundred at everysamples drilling were intervalcollected time at every of 72 mindrilling until interval reaching time the EBH of criterion72 min ofuntil 0.16 reaching mm. After the EBH EBH determinations criterion of were 0.16 obtained, mm. After other EBH new drillsdeterminations were installed were in obtained, the CNC machineother new to continuedrills were drilling installed of the in workpieces. the CNC machine to continue drilling of the workpieces.

probe tip

Figure 3. Burr height height measuring measuring instrument instrument and and schematic schematic diagram diagram of of burr burr height height measurement measurement on on a aworkpiece. .

The drilling performances of both drills were investigated by the Taylor’s model fitting for tool The drilling performances of both drills were investigated by the Taylor’s model fitting for tool life equations of both drills using the EBH data sets. The Taylor’s model is expressed as: life equations of both drills using the EBH data sets. The Taylor’s model is expressed as: n m S(ST()Tn()f)m(f =) c = c (1)

Hence,Hence, nlogT + mlogf + logS = logc (2) nlogT + mlogf + logS = logc (2)

Coatings 2019, 9, 853 5 of 14 where S denotes cutting speed, T represents tool life, f indicates feed rate. The constants of n, m and c are determined using Taylor’s model fitting to the observations during drilling of the water-valve workpieces with both drills. Monte Carlo simulation is a sampling experiment whose objective is to estimate the distribution of an outcome factor that depends on several probabilistic input factors [22]. Monte Carlo simulation was used to assess the uncertainties of various applications such as a life prediction model of a fishing net weaving machine component [23], rotary axis error calibration of five-axis machine tool [24], measurement uncertainty of an H-drive stage with air in the semiconductor manufacturing industry [25], uncertainty on each of the eight link errors of a five-axis machine tools [26], verification of the effectiveness of the robust optimization approach for improving the machining accuracy retainability of the five-axis NC machine tool [27]. In this study, the Monte Carlo simulation technique was used to evaluate the tool life prediction uncertainty of the two drills. This technique involved normal probability distribution based on the Taylor’s model. This technique was employed to perform random sampling from normal probability distribution by varying cutting speeds and feed rates within 5% of ± each factor for inputs at a confidence level of 95%. The five cutting speeds (40, 45, 50, 55 and 60 m/min) and four feed rates (0.20, 0.22, 0.24 and 0.26 mm/rev) were assigned as the inputs to generate random data sets for each factor by specifying mean for each factor and standard deviation for the variation of each factor based on the tool life models of the uncoated-WC and AlCrN coated-WC drills. Each factor generated a random data set of 10,000 that followed the normal distribution. After drilling reaching the criterion of maximum EBH value of 0.16 mm, the surfaces of the two drills were examined by scanning electron (SEM: Hitachi Model S-3400N) at an accelerating voltage of 20 kV.

3. Results Figure4 presents averages EBH as functions of drilling time at the three cutting speeds and feed rates during drilling of the water-valve workpieces with the uncoated-WC drills. Figure4a shows that the averages EBH upon drilling of the workpieces increased monotonically with increasing drilling time for all cutting speeds. The averages EBH at the three cutting speeds reached the criterion of 0.16 mm at the drilling time approximately 640 min. On the other hand, Figure4b illustrates that the averages EBH increased rapidly with increasing drilling time within the range of approximately 200–500 min when drilling at a feed rate of 0.25 mm/rev by keeping the cutting speed constant at 50 m/min. The averages EBH increased very slightly with increasing time within the range of approximately 70–650 min when drilling at a feed rate of 0.15 mm/rev by keeping the cutting speed constant at 50 m/min and increasing very rapidly after 650 min. The result also showed that at the criterion of 0.16 mm, the drilling time increased two times when drilling at the feed rate of 0.15 mm/rev compared to drilling at the feed rate of 0.25 mm/rev by keeping the cutting speed constant at 50 m/min. The results indicated that drilling at a low feed rate decreased the averages EBH. Figure4c illustrates a relationship between averages EBH and drilling time at the three cutting speeds (40, 50 and 60 m/min) by keeping the feed rate constant at 0.2 mm/rev throughout the experiments during drilling of the workpieces with the AlCrN coated-WC drills. The results showed that averages EBH increased monotonically with increasing drilling time for all cutting speeds. The averages EBH at the three cutting speeds reached the criterion of 0.16 mm at the drilling time approximately 800 min. This indicated that the average drilling time was reduced by approximately 25% during drilling of the workpieces until reaching the EBH criterion when using AlCrN coated-WC drills by varying the cutting speeds and keeping the feed rate constant at 0.2 mm/rev. Coatings 2019, 9, x FOR PEER REVIEW 6 of 13 cutting speed constant at 50 m/min. The averages EBH increased very expeditiously with increasing time within the range of approximately 500–800 min when drilling at a feed rate of 0.2 mm/rev and increasing very rapidly after 650 min. On the other hand, the averages EBH increased very slightly with increasing time within the range of approximately 70–700 min when drilling at a feed rate of 0.15 mm/rev and increasing rapidly after 700 min. The results also illustrated that the drilling time increased two times when drilling at the feed rate of 0.15 mm/rev by holding the cutting speed constant at 50 m/min compared to drilling at the feed rate of 0.25 mm/rev by evaluating at the

Coatingscriterion2019 of, 9 ,0.16 853 mm. The results also indicated that drilling with a low feed rate decreased6 ofthe 14 average EBH.

0.20 0.20

0.18 0.18

0.16 0 0.16 0

0.14 0.14

0.12 0.12

0.10 0.10

0.08 0.08 Exit burr height (mm) Exit burr height (mm) burr height Exit 0.06 0.06

0.04 0.04 40 m/min 0.02 0.02 0.25 mm/rev 50 m/min 0.20 mm/rev 60 m/min 0.00 0.00 0.15 mm/rev 72 . 216 . 360 . 504 . 648 72 . 216 . 360 . 504 . 648 . 792 . 936 . Time (min) Time (min) (a) (b)

0.20 0.20

0.18 0.18

0.16 0 0.16 0

0.14 0.14

0.12 0.12

0.10 0.10

0.08 0.08 Exit burr height (mm) Exit burr height (mm) burr height Exit 0.06 0.06

0.04 0.04 40 m/min 0.25 mm/rev 0.02 0.02 50 m/min 0.20 mm/rev 0.00 60 m/min 0.00 0.15 mm/rev

72 . 216 . 360 . 504 . 648 . 792 . 72 . 216 . 360 . 504 . 648 . 792 . 936 . 1080 . 1224 . Time (min) Time (min) (c) (d)

Figure 4.4. Average exit burr height versus drilling time using the uncoated-WC drills by varying ( a) cutting speeds (keeping the feed rate constant at 0.2 mmmm/rev)/rev) and (b)) feedfeed ratesrates (keeping(keeping thethe cuttingcutting speed constant at 50 mm/min)/min) and using AlCrN coated-WC drills by varying ( c) cutting speeds and ( d) feed rates.rates.

FigureAccording4d shows to the a relationship results of variations between averages in the averages EBH and drillingEBH at time the atthree the threedifferent feed ratesfeed (0.15,rates 0.20versus and drilling 0.25 mm time/rev) using by holding the two the cuttingdifferent speed drills, constant the drilling at 50 m time/min increased all over the approximately experiments. The1.5 resultstimes when showed drilling a dramatical with the variation AlCrN incoated-WC the averages drills EBH compared at the diff toerent drilling feed rateswithversus the uncoated drilling time.ones Thebefore averages reaching EBH the increased EHB criterion rapidly of with0.16 mm. increasing This could drilling be timeprobably within attributed the range to of the approximately reduction in 200–600cutting temperature, min when drilling prevention at a feed of the rate different of 0.25 mm wear/rev occurring by keeping at thethe cutting tool surface, speed constantincreasing at 50hardness m/min. and The wear averages resistance EBH increasedand decreasing very expeditiouslycoefficient of friction with increasing by applying time the within thin thefilm range coating of approximatelyon the cutting tools 500–800 [28,29]. min when drilling at a feed rate of 0.2 mm/rev and increasing very rapidly after 650 min. On the other hand, the averages EBH increased very slightly with increasing time within the range of approximately 70–700 min when drilling at a feed rate of 0.15 mm/rev and increasing rapidly after 700 min. The results also illustrated that the drilling time increased two times when drilling at the feed rate of 0.15 mm/rev by holding the cutting speed constant at 50 m/min compared to drilling at the feed rate of 0.25 mm/rev by evaluating at the criterion of 0.16 mm. The results also indicated that drilling with a low feed rate decreased the average EBH. According to the results of variations in the averages EBH at the three different feed rates versus drilling time using the two different drills, the drilling time increased approximately 1.5 times when Coatings 2019, 9, 853 7 of 14 drilling with the AlCrN coated-WC drills compared to drilling with the uncoated ones before reaching the EHB criterion of 0.16 mm. This could be probably attributed to the reduction in cutting temperature, prevention of the different wear occurring at the tool surface, increasing hardness and wear resistance and decreasing coefficient of friction by applying the thin film coating on the cutting tools [28,29]. Tables2 and3 show the results of drilling time at the three cutting speeds and feed rates corresponding to the EBH criterion for carrying out the drilling experiments using the uncoated-WC and the AlCrN coated-WC drills, respectively.

Table 2. Drilling time at the three cutting speeds and feed rates corresponding to the EBH criterion for carrying out the drilling experiments using the uncoated WC drills.

Factor Level Drilling Time (min) 40 625 Cutting speed (m/min) 50 620 (Feed rate was fixed at 0.2 mm/rev) 60 600 Feed rate (mm/rev) 0.15 1010 (Cutting speed was hold constant 0.20 620 at 50 m/min) 0.25 450

Table 3. Drilling time at the three cutting speeds and feed rates corresponding to the EBH criterion for carrying out the drilling experiments using the AlCrN coated WC drills.

Factor Level Drilling Time (min) 40 857 Cutting speed (m/min) 50 850 (Feed rate was fixed at 0.2 mm/rev) 60 803 Feed rate (mm/rev) 0.15 1289 (Cutting speed was hold constant 0.20 850 at 50 m/min) 0.25 641

The drilling time data sets at the EBH criterion of 0.16 mm were used to develop the tool life models of both drills by Taylor’s equation. The tool life models in terms of cutting speed and feed rate were investigated based on Equation (2). The unknown constants of the models were calculated based on the Taylor’s equation and the results in Tables2 and3. According to the results from Table2, the unknown constant (n) of the model for the uncoated-WC drills was obtained as follows:

nlog625 + mlog0.2 + log40 = logc (3)

and nlog600 + mlog0.2 + log60 = logc (4)

According to Equations (3) and (4), the constant of n was 9.9325. The unknown constant (m) of the model for the uncoated-WC drills was estimated as follows:

nlog1010 + mlog0.15 + log50 = logc (5)

and nlog450 + mlog0.25 + log50 = logc (6)

According to Equations (5) and (6), the constant of m was 7.1855. Accordingly, the constant c was 2.2375 1024. Thus, the tool life model for the uncoated-WC drill was defined as: × S(T )9.9325(f )7.1855 = 2.2375 1024 (7) 1 × Coatings 2019, 9, 853 8 of 14

where T1 represents tool life for the uncoated-WC drill. Hence Equation (7) could be expressed as:

282.8264 T1 = (8) S0.1007 f 0.7234

Similarly, according to the results from Table3, the tool life model in terms of cutting speed and feed rate for the AlCrN coated-WC drill was defined as:

S(T )6.2299(f )8.3155 = 1.1537 1014 (9) 2 × Equation (9) is represented as: 180.7912 T2 = (10) S0.1605 f 1.3348 where T2 represents tool life of the AlCrN coated-WC drill. The tool life models of both drills were used to investigate the effects of cutting speed and feed rate on average tool life values. These models were valid for the value of EBH lower than 0.16 mm. Equations (8) and (10) were used to predict tool life values during drilling of the forging brass workpieces with the uncoated-WC and the AlCrN coated-WC drills, respectively. Figure5 shows the relationships between predicted tool life values and the cutting speed and feed rate during drilling of the workpieces with both drills. Figure5a illustrates the predicted values of tool life as a function of cutting speed. It could be seen that predicted tool life values of both drills varied almost linearly with cutting speed. Furthermore, predicted tool life values of both drills remained almost unchanged with increased in cutting speed by keeping the feed rate constant at 0.2 mm/rev. Figure5b depicts the predicted values of tool life as a function of feed rate. It could be seen that predicted tool life values of both drills varied sharply with feed rate. In addition, predicted tool life values of both drills decreased sharply with increased in feed rate by keeping the cutting speed constant at 50 m/min for both drills. This indicated that feed rate had a stronger significant influence on predicted tool life than the cutting speed. It was also found that predicted tool life values of the AlCrN coated-WC drills were higher (approximately 1.5 times) than those of the uncoated-WC ones. The longer tool life of the AlCrN coated-WC drill was attributed to its synergistic hardness, strengthening and toughness mechanisms induced by the WC particles and CrN particles. Fox-Rabinovich et al. [30] reported that the presence of Cr and Al improves thermal stability that prevents particles removal by oxidation and abrasion. In addition, tool life of the coated drill could be enhanced by increasing tool surface hardness and segregation of the tool cutting edge from the severe cutting environment (high temperature and pressure) leading to the resistance of abrasive wear and oxidation of the hard-coated cutting tool [14,31,32]. Based on Equations (8) and (10), Monte Carlo simulation technique was used to understand the impact of uncertainty in the prediction of tool life values when varying the cutting speeds and feed rates within 5% of each factor. Figure6 illustrates an example of output results from Monte Carlo ± simulation based on the tool life model of an uncoated-WC drill when varying the cutting speeds and feed rates within 5% of each factor using the software Minitab® Version 16 [33]. The random data ± set of 10000 was generated from the normal distribution. The mean of the tool life was 596.59 min with 95% confidence intervals of 596.52 and 596.66 min while simulating tool life values at the cutting speed of 50 m/min and the feed rate of 0.2 mm/rev. The standard deviation was 3.68 min with 95% confidence intervals of 3.63 and 3.73 min. Coatings 2019, 9, x FOR PEER REVIEW 8 of 13

S(T2)6.2299(f)8.3155 = 1.1537 × 1014 (9)

Equation (9) is represented as: = 180.7912 T2 (10) S 0.1605 f 1.3348 where T2 represents tool life of the AlCrN coated-WC drill. The tool life models of both drills were used to investigate the effects of cutting speed and feed rate on average tool life values. These models were valid for the value of EBH lower than 0.16 mm. Equations (8) and (10) were used to predict tool life values during drilling of the forging brass workpieces with the uncoated-WC and the AlCrN coated-WC drills, respectively. Figure 5 shows the relationships between predicted tool life values and the cutting speed and feed rate during drilling of the workpieces with both drills. Figure 5a illustrates the predicted values of tool life as a function of cutting speed. It could be seen that predicted tool life values of both drills varied almost linearly with cutting speed. Furthermore, predicted tool life values of both drills remained almost unchanged with increased in cutting speed by keeping the feed rate constant at 0.2 mm/rev. Figure 5b depicts the predicted values of tool life as a function of feed rate. It could be seen that predicted tool life values of both drills varied sharply with feed rate. In addition, predicted tool life values of both drills decreased sharply with increased in feed rate by keeping the cutting speed constant at 50 m/min for both drills. This indicated that feed rate had a stronger significant influence on predicted tool life than the cutting speed. It was also found that predicted tool life values of the AlCrN coated-WC drills were higher (approximately 1.5 times) than those of the uncoated-WC ones. The longer tool life of the AlCrN coated-WC drill was attributed to its synergistic hardness, strengthening and toughness mechanisms induced by the WC particles and CrN particles. Fox-Rabinovich et al. [30] reported that the presence of Cr and Al improves thermal stability that prevents particles removal by oxidation and abrasion. In addition, tool life of the coated drill could be enhanced by increasing tool surface hardness and segregation of the tool cutting edge from the severe cutting environment (high temperature and pressure) leading to the resistance of abrasive wear and oxidation of the hard-coated cutting tool [14,31,32]. Based on Equations (8) and (10), Monte Carlo simulation technique was used to understand the impact of uncertainty in the prediction of tool life values when varying the cutting speeds and feed rates within ±5% of each factor. Figure 6 illustrates an example of output results from Monte Carlo simulation based on the tool life model of an uncoated-WC drill when varying the cutting speeds and feed rates within ±5% of each factor using the software Minitab® Version 16 [33]. The random data set of 10000 was generated from the normal distribution. The mean of the tool life was 596.59 min with 95% confidence intervals of 596.52 and 596.66 min while simulating tool life values at the Coatings 2019, 9, 853 9 of 14 cutting speed of 50 m/min and the feed rate of 0.2 mm/rev. The standard deviation was 3.68 min with 95% confidence intervals of 3.63 and 3.73 min.

Coatings 2019, 9, x FOR PEER REVIEW 9 of 13 (a) (b) Figure 5. Plots of the relationships between predicted tool life values and (a) cutting speed (keeping theFigure feed 5. ratePlots constant of the relationships at 0.2 mm/rev) between and ( predictedb) feed rate tool (keeping life values the and cutting (a) cutting speed speedconstant (keeping at 50 m/min)the feed during rate constant drilling at of 0.2 the mm workpieces/rev) and ( bwith) feed both rate drills. (keeping the cutting speed constant at 50 m/min) during drilling of the workpieces with both drills.

Anderson-Darling Normality Test A-Squared 0.78 P-Value 0.043

Mean 596.59 StDev 3.68 Variance 13.51 Skewness 0.0503028 Kurtosis 0.0051690 N 10000

Minimum 580.91 585 590 595 600 605 610 1st Quartile 594.10 Median 596.53 3rd Quartile 599.06 Maximum 614.05 95% Confidence Interval for Mean 596.52 596.66 95% Confidence Intervals 95% Confidence Interval for Median Mean 596.45 596.61 Median 95% Confidence Interval for StDev 3.63 3.73 596.45 596.50 596.55 596.60 596.65 Figure 6. Output results from Monte Carlo simulation based on the tool life model of an Figure 6. Output results from Monte Carlo simulation based on the tool life model of an uncoated-WC drill. uncoated-WC drill. According to the Monte Carlo simulation results from the tool life models of both drills, According to the Monte Carlo simulation results from the tool life models of both drills, Figures Figures7 and8 graphically display the effects of cutting speed and feed rate on tool life for both drills 7 and 8 graphically display the effects of cutting speed and feed rate on tool life for both drills when when varying the cutting speeds and feed rates within 5% of each factor. Figure7 shows that the varying the cutting speeds and feed rates within ±5% of ±each factor. Figure 7 shows that the AlCrN AlCrN coated-WC drills had longer tool life than the uncoated-WC ones at most of the feed rates. coated-WC drills had longer tool life than the uncoated-WC ones at most of the feed rates. Moreover, Moreover, variations of tool life values at the feed rate of 0.2 mm/rev for both drills were larger than variations of tool life values at the feed rate of 0.2 mm/rev for both drills were larger than those of those of tool life values at the rest levels. Figure8 reveals that most of the tool life values of the tool life values at the rest levels. Figure 8 reveals that most of the tool life values of the AlCrN AlCrN coated-WC drills were significantly higher than those of the uncoated-WC ones at feed rate in coated-WC drills were significantly higher than those of the uncoated-WC ones at feed rate in the the range of 0.2–0.26 mm/rev. On the other hand, variations of tool life values at all cutting speeds range of 0.2–0.26 mm/rev. On the other hand, variations of tool life values at all cutting speeds corresponding to each feed rate were not different for both drills. This indicated that there were no corresponding to each feed rate were not different for both drills. This indicated that there were no statistically significant differences of tool life values while drilling of forging brass at the cutting speed statistically significant differences of tool life values while drilling of forging brass at the cutting in the range of 40–60 m/min corresponding to each feed rate (overlapping of the intervals of tool life speed in the range of 40–60 m/min corresponding to each feed rate (overlapping of the intervals of values for the cutting speed in the range of 40–60 m/min at each feed rate). tool life values for the cutting speed in the range of 40–60 m/min at each feed rate). Based on the EBH criterion and simulation results, the highest acceptable tool life values were obtained at the lowest feed rate (0.2 mm/rev) and highest cutting speed (60 m/min) during drilling of forging brass using the AlCrN coated-WC drill. At the high cutting speed and low feed rate, it was apparent for chips to evacuate along the hole axis due to shortened chip and coated drill contact based on low friction and high wear resistance of the AlCrN coated-WC drill. This result agreed with the previous study found in [15]. Although drilling at higher cutting speeds gave more productivity than drilling at lower cutting speeds, it resulted in accelerated tool wear or failure and higher EBH formation. Boopathi et al. [34] stated that higher cutting speeds could increase the frictional heat transfer between the drill and the workpiece, which in turn increased the temperature of the brass attaining the maximum value of 468 °C. It was also observed that higher feed rates required higher thrust force for the penetration of the workpiece. On the other hand, the thrust force decreased at the higher cutting speeds for constant feed rate. Hence this operating condition was recommended for manufacturers in drilling manufacturing industry based on the machine performance and its ability to produce a very large number of drilled parts. At this operating condition, the predicted tool life values of the uncoated-WC and the AlCrN coated-WC drills were

Coatings 2019, 9, x FOR PEER REVIEW 10 of 13 Coatings 2019, 9, x FOR PEER REVIEW 10 of 13 approximately 600 and 800 min, respectively based on estimates from the tool life models. This Coatings 2019, 9, 853 10 of 14 indicatedapproximately that the 600 coated and 800drill min, increased respectively tool life based by 30%. on estimates from the tool life models. This indicated that the coated drill increased toolF eed life0.20 by 30%. F eed 0.22 850 850 F eed 0.240.20 800 F eed 0.26 F eed 0.22 800 850 850 F eed 0.24 750 800 F eed 0.26 750 800 700 750 700 750 650 700 650 700 600 650 600 Tool life (min) Tool life (min) life Tool 650 550 600 550 600 Tool life (min) 500 (min) life Tool 550 500 550 Feed 0.20 450 500 450 Feed 0.22 500 Feed 0.24 400 Feed 0.20 450 400 Feed 0.26 450 Feed 0.22 Feed 0.24 400 40 45 50 55 60 40 45 50 55 60 C utting speed (m/min) 400 C utting speed (m/min) Feed 0.26 40 45 50 55 60 40 45 50 55 60 C utting( aspeed) (m/min) C utting(b speed) (m/min)

Figure 7. Effect of cutting (speeda) on tool life for (a) the uncoated-WC and(b )( b) the AlCrN coated-WC Figuredrills. 7.7. EEffectffect of of cutting cutting speed speed on on tool tool life life for for (a) the(a) uncoated-WCthe uncoated-WC and (andb) the (b AlCrN) the AlCrN coated-WC coated-WC drills. drills. C utting speed 40 C utting speed 40 850 C utting speed 45 C utting speed 45 850 C utting speed 5040 C utting speed 5040 C utting speed 55 C utting speed 55 800850 C utting speed 45 C utting speed 45 800850 C utting speed 6050 C utting speed 6050 750800 C utting speed 55 C utting speed 55 750800 C utting speed 60 C utting speed 60 700750 700750 650 700 650 700 600 650 600 Tool life (min) Tool life (min) 650

550600 550600 Tool life (min) Tool life (min) 500550 500550 450 500 450 500 400450 400450

400 0.20 0.22 0.24 0.26 0.20 0.22 0.24 0.26 400 F eed rate (mm/rev ) F eed rate (mm/rev ) 0.20 0.22(a) 0.24 0.26 0.20 0.22(b) 0.24 0.26 F eed rate (mm/rev ) F eed rate (mm/rev ) (a) (b) Figure 8. EffectEffect of feed rate on tool life for (a) the uncoated-WC andand ((b)) thethe AlCrNAlCrN coated-WCcoated-WC drills.drills. Figure 8. Effect of feed rate on tool life for (a) the uncoated-WC and (b) the AlCrN coated-WC drills. BasedFigure on9 theshows EBH SEM criterion micrographs and simulation of an uncoated-WC results, the highest drill and acceptable an AlCrN tool lifecoated-WC values were one obtainedbeforeFigure and at after the9 shows lowestdrilling SEM feed of themicrographs rate workpieces (0.2 mm /rev)of until an and reachinguncoated-WC highest the cutting criterion drill speed and of EBH (60an AlCrN m of/min) 0.16 coated-WCmm. during According drilling one oftobefore forgingFigure and brass9b,after the drilling using main the ofwear AlCrNthe workpiecesmechanisms coated-WC until were drill. reaching At the theand high criterion outer cutting corner of EBH speed wear. of and 0.16 The lowmm. feedchisel According rate, wear it wasoccurredto Figure apparent in9b, the forthe region chips main toof wear evacuate the chiselmechanisms along edge the because holewere axis chiselof duehigh and to stress shortened outer in cornerthe chip flow andwear. zone coated The of the drillchisel drill contact wear and basedworkpieceoccurred on lowin interface. the friction region The and of highouter the wearchisel corner resistance edge wear because was of theth eof AlCrNcritical high coated-WCstressfailure in mode the drill. flow because This zone result the of thermalthe agreed drill withloadand thewasworkpiece previous the highest. interface. study The found The tool outer in wear [15 ].corner rapidly Although wear increased drilling was the atleading critical higher failureto cutting chipping mode speeds occurringbecause gave more the on thermal productivity the cutting load thanedgewas the drillingof anhighest. uncoated-WC at lower The cuttingtool drill.wear speeds, Cutting rapidly it resulted force,increased ther in acceleratedmal leading fatigue, to toolchipping torque wear and oroccurring failurestress andoccurred on higherthe cuttingon EBH the formation.drilledge whenof an ituncoated-WC Boopathi moved etalong al. drill. [ 34the] statedCuttinghole axis that force, to higher remove ther cuttingmal chips. fatigue, speeds Chipping torque could on and increase the stress tool the occurredcould frictional affect on heat the transferqualitydrill when of between drilled it moved the holes drillalong and and the productivity the hole workpiece, axis todegradation. remove which inchips. turn In addition, Chipping increased the theon worn the temperature toolarea could of cutting of affect the brassedge the attainingproducedquality of the highdrilled maximum roughness holes valueand and productivity of burr 468 ◦heightC. It wasdegradation. of alsothe drilled observed In holesaddition, that of higher the the workpieces. worn feed rates area required ofHowever, cutting higher edge the thrustchippingproduced force occurringhigh for theroughness penetration on the andcutting of burr the edge workpiece.height can of be the Onresolved drilled the other andholes hand, reduced of thethe thrustbyworkpieces. using force the decreased However, coated drills. at the higherCoatingchipping cutting materials occurring speeds can on for reducethe constant cutting the feedmagnitude edge rate. canHence be of resolvedcu thistting operating forces, and reduced torque condition byand using wasstresses. recommendedthe coatedSince AlCrN drills. for manufacturerscoatingCoating has materials low in thermal drilling can reduce conductivity, manufacturing the magnitude friction industry ofcoe cu basedfficienttting on forces, and the high machine torque thermal performanceand stabilitystresses. and[14,30],Since its AlCrN ability it can toreducecoating produce temperaturehas a low very thermal large between number conductivity, coated of drilled drill friction parts.and coechip Atfficient thisresulting operating and cuttinghigh condition, thermal force andstability the predictedstress [14,30], reductions. tool it can life valuesAccordingreduce oftemperature the to uncoated-WCFigure 9d, between there and were coated the no AlCrN drillvisible and coated-WC crac chipk and resulting chipping drills were cutting formations approximately force on and the stress 600surface and reductions. of 800 AlCrN min, respectivelycoated-WCAccording to drill based Figure during on 9d, estimates theredrilling were from of theno the visibleforging tool life crac brass models.k and workpieces chipping This indicated formationsuntil reaching that the on coatedthethe surfaceEBH drill criterion increasedof AlCrN of tool0.16coated-WC lifemm. by The 30%. drill AlCrN during coating drilling deposited of the forging using brass magn workpiecesetron sputtering, until reaching filtered thecathodic EBH criterion arc (FCA) of reduced0.16 Figuremm. the The9 showsformation AlCrN SEM coatingof micrographs the droplet deposited ofphase an using uncoated-WC and magn voidsetron as drillwell sputtering, and as provided an AlCrN filtered high coated-WC cathodic adhesive onearc strength before(FCA) andbetweenreduced after thethe drilling formationcoating of the and of workpieces the the droplet carbide until phase substrate reaching and voids [3 the5,36]. criterionas wellThe asAlCrN of provided EBH coating of 0.16high mm.exhibited adhesive According thestrength best to Figure9b, the main wear mechanisms were chisel and outer corner wear. The chisel wear occurred in between the coating and the carbide substrate [35,36]. The AlCrN coating exhibited the best

Coatings 2019, 9, 853 11 of 14 the region of the chisel edge because of high stress in the flow zone of the drill and workpiece interface. The outer corner wear was the critical failure mode because the thermal load was the highest. The tool wear rapidly increased leading to chipping occurring on the cutting edge of an uncoated-WC drill. Cutting force, thermal fatigue, torque and stress occurred on the drill when it moved along the hole axis to remove chips. Chipping on the tool could affect the quality of drilled holes and productivity degradation. In addition, the worn area of cutting edge produced high roughness and burr height of the drilled holes of the workpieces. However, the chipping occurring on the cutting edge can be resolved and reduced by using the coated drills. Coating materials can reduce the magnitude of cutting forces, torque and stresses. Since AlCrN coating has low thermal conductivity, friction coefficient and high thermal stability [14,30], it can reduce temperature between coated drill and chip resulting cutting force and stress reductions. According to Figure9d, there were no visible crack and chipping formations on the surface of AlCrN coated-WC drill during drilling of the forging brass workpieces until reaching the EBH criterion of 0.16 mm. The AlCrN coating deposited using magnetron sputtering, filtered cathodic arc (FCA) reduced the formation of the droplet phase and voids as well as provided high adhesive strength between the coating and the carbide substrate [35,36]. The AlCrN coating exhibited theCoatings best 2019 properties, 9, x FOR PEER of REVIEW abrasive, spalling, and adhesive wear resistances [37–39].11 However,of 13 the adhesive wear could be caused by rubbing of a cutting edge of the drill against the workpiece surface properties of abrasive, spalling, and adhesive wear resistances [37–39]. However, the adhesive wear by fast movingcould be chip, caused as by well rubbing as friction of a cutting generated edge of atthe a drill contact against interface the workpiece between surface the by chips fast and the drill. The cuttingmoving chip, edge as was well coveredas friction withgenerated a large at a contact amount interface of asperities between the of workpiecechips and the surfacedrill. The that could generate adheredcutting edge layer. was covered This result with a caused large amount an undesirable of asperities of increase workpiece of surface the EBH that could with generate the specification limit of 0.16adhered mm. layer. This result caused an undesirable increase of the EBH with the specification limit of 0.16 mm.

500 μm (a) (b)

500 μm

(c) (d)

Figure 9. SEMFigure images 9. SEM images of (a) anof ( uncoated-WCa) an uncoated-WC drill drill before before andand ( (bb) )after after drilling drilling of the of workpieces the workpieces (c) (c) an AlCrN coated-WCan AlCrN coated-WC drill before drill and before (d )and after (d) drillingafter drilling of theof the workpieces. workpieces.

4. Conclusions4. Conclusion Tool life modelling based on Taylor’s equation and Monte Carlo simulation technique has been Tool lifesuccessfully modelling employed based toon investigate Taylor’s the equation effect and and uncertainty Monte of Carlo cutting simulation speed and feed technique rate on has been successfullyaverages employed EBH and to investigatetool life predictions the eff ectwhile and drilling uncertainty of forging of brass cutting workpieces speed with and feedthe rate on averages EBHuncoated-WC and tool and life AlCrN predictions coated-WC while drills. drilling The conclusions of forging could brass be drawn workpieces as follows: with the uncoated-WC and AlCrN• coated-WCFeed rate affected drills. averages The conclusions EBH more strongly could than be cutting drawn speed. as follows: • The AlCrN coated-WC drill had better performance than the uncoated-WC one. • Tool life of the AlCrN coated-WC drill at the criterion of EBH of 0.16 mm was higher (about 1.5 times) than that of the uncoated-WC one. • The average drilling time was reduced by about 25% during drilling of the workpieces until reaching the EBH criterion when using the AlCrN coated-WC drills by varying the cutting speeds and keeping the feed rate constant at 0.2 mm/rev. • Chipping occurred on the cutting edge of an uncoated-WC drill. • The cutting speed of 60 m/min and the feed rate of 0.2 mm/rev were recommended for manufacturers in drilling of forging brass using the AlCrN coated-WC drill. • The predicted tool life values of the uncoated-WC and the AlCrN coated-WC drills were approximately 600 and 800 min, respectively at the recommended operating condition.

Coatings 2019, 9, 853 12 of 14

Feed rate affected averages EBH more strongly than cutting speed. • The AlCrN coated-WC drill had better performance than the uncoated-WC one. • Tool life of the AlCrN coated-WC drill at the criterion of EBH of 0.16 mm was higher (about 1.5 • times) than that of the uncoated-WC one. The average drilling time was reduced by about 25% during drilling of the workpieces until • reaching the EBH criterion when using the AlCrN coated-WC drills by varying the cutting speeds and keeping the feed rate constant at 0.2 mm/rev. Chipping occurred on the cutting edge of an uncoated-WC drill. • The cutting speed of 60 m/min and the feed rate of 0.2 mm/rev were recommended for • manufacturers in drilling of forging brass using the AlCrN coated-WC drill. The predicted tool life values of the uncoated-WC and the AlCrN coated-WC drills were • approximately 600 and 800 min, respectively at the recommended operating condition. The coated drill increased tool life by 30% during drilling of the forging brass workpieces at the • cutting speed of 60 m/min and the feed rate of 0.2 mm/rev.

Although using coated-WC drills can reduce burr height and increase tool life, drilling tool design can reduce burr height and prolong tool life leading to quality of drilled workpieces and productivity improvement. Sugita et al. [40] proposed a dedicated drill design of tool shape and nicked shape to reduce burr height and delamination and to increase the accuracy drilled holes during the drilling of composite materials. Hence, further research should be focused on tool design for burr height reduction and quality improvement of the drilling process for forging brass.

Author Contributions: M.T. carried out the experiments, collected the data sets, analyzed the data and wrote the manuscript; C.S. analyzed the data, wrote, edited and reviewed the manuscript. Funding: This research was funded by the Faculty of Engineering, Khon Kaen University. Acknowledgments: The authors gratefully acknowledge the Faculty of Engineering, Khon Kaen University for the financial supports of this research. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Wright, P.K.; Trent, E. Metals Cutting; Butterworth-Heinemann: Oxford, UK, 2000; ISBN 9780750670692. 2. Viktor, P.A. Geometry of Single-Point Tools and Drills Fundamentals and Practical Applications; Springer: Berlin/Heidelberg, Germany, 2010. 3. de Paiva, J.M.F.; Amorim, F.L.; Soares, P.; Torres, R.D. Evaluation of hard coating performance in drilling compacted graphite iron (CGI). J. Mater. Eng. Perf. 2013, 22, 3155–3160. [CrossRef] 4. Wu, W.; Kuzu, A.; Stephenson, D.; Hong, J.; Bakkal, M.; Shih, A. Dry and minimum quantity lubrication high-throughput drilling of compacted graphite iron. Mach. Sci. Technol. 2018, 22, 652–670. [CrossRef] 5. Gillespie, L.K. The $2 billion deburring bill. Manuf. Eng. Manag. 1975, 74, 20–21. 6. Schafer, F. Deburring: Theory, Processes, Systems; Krausskopf Verlag: Mainz, Germany, 1975. 7. Toenshoff, H.K.; Spintig, W.; Koenig, W.; Neisses, A. Machining of holes developments in drilling . CIRP Ann. Manuf. Technol. 1994, 43, 551–561. [CrossRef] 8. Costa, E.S.; da Silva, M.B.; Machado, A.R. Burr produced on the drilling process as a function of tool wear and lubricant-coolant conditions. J. Braz. Soc. Mech. Sci. Eng. 2009, 31, 57–63. [CrossRef] 9. Lukic, L.; Djapic, M. Machinability parameters for optimal cutting process in flexible manufacturing systems. Metal. Int. 2012, 17, 10–16. 10. Shen, Q.; Lee, T.C.; Lau, W.S. Wear characteristics and flank wear model of the main cutting edges in spade drilling of ductile metals. Wear 1997, 208, 73–80. [CrossRef] 11. Astakhov, V.P.; Osman, M.O.M.; AlAta, M. Statistical design of experiments in metal cutting 2 Applications. J. Test. Eval. 1997, 25, 328–336. 12. Timata, M.; Saikaew, C. Influences of spindle speed and feed rate on exit burr height and workpiece diameter in drilling forging brass. Solid State Phenom. 2018, 279, 67–71. [CrossRef] Coatings 2019, 9, 853 13 of 14

13. Gong, M.; Chen, J.; Deng, X.; Wu, S. Sliding wear behavior of TiAlN and AlCrN coatings on a unique cemented carbide substrate. Int. Refract. Met. Hard Mater. 2017, 69, 209–214. [CrossRef] 14. Mo, J.L.; Zhu, M.H.; Lei, B.; Leng, Y.X.; Huang, N. Comparison of tribological behaviours of AlCrN and TiAlN coatings-deposited by physical vapor deposition. Wear 2007, 263, 1423–1429. [CrossRef] 15. Tekaüt, I.;˙ Demir, H.; ¸Seker, U. Experimental analysis and theoretical modelling of cutting parameters in the drilling of AISI H13 steel with coated and uncoated drills. Tran. FAMENA 2018, 42, 83–96. [CrossRef] 16. Karnik, S.R.; Gaitonde, V.N.; Davim, J.P. A comparative study of the ANN and RSM modeling approaches for predicting burr size in drilling. Int. J. Adv. Manuf. Technol. 2008, 38, 868–883. [CrossRef] 17. Lin, T.R. Cutting behaviour using variable feed and variable speed when drilling stainless steel with TiN-coated carbide drills. Int. J. Adv. Manuf. Technol. 2002, 19, 629–636. [CrossRef] 18. Lin, T.R.; Shyu, R.F. Improvement of tool life and exit burr using variable feeds when drilling stainless steel with coated drills. Int. J. Adv. Manuf. Technol. 2000, 16, 308–313. [CrossRef] 19. Wang, X.; Huang, C.; Zou, B.; Liu, H.; Wang, J. Effects of geometric structure of twist drill bits and cutting condition on tool life in drilling 42CrMo ultrahigh-strength steel. Int. J. Adv. Manuf. Technol. 2013, 64, 41–47. [CrossRef] 20. Polli, M.L.; Cardoso, M.J. Effects of process parameters and drill point geometry in deep drilling of SAE 4144M under MQL. J. Braz. Soc. Mech. Sci. Eng. 2018, 40, 137. [CrossRef] 21. Cardoso, M.J.; Polli, M.L.; Pintaude, G. Wear analysis of PVD-coated twist drills under MQL. Ind. Lubr. Tribol. 2018, 70, 1664–1669. [CrossRef] 22. Evans, J.R.; Olson, D.L. Introduction to Simulation and Risk Analysis; Prentice Hall: Upper Saddle River, NJ, USA, 2002. 23. Prengela, H.G.; Jindal, P.C.; Wendt, K.H.; Santhanam, A.T.; Hegde, P.L.; Penich, R.M. A new class of high performance PVD coatings for carbide cutting tools. Surf. Coat. Technol. 2011, 139, 25–34. [CrossRef] 24. Ibaraki, S.; Ota, Y. A machining test to calibrate rotary axis error motions of five-axis machine tools and its application to thermal deformation test. Int. Mach. Tool Manuf. 2014, 86, 81–88. [CrossRef] 25. Liu, Y.; Yuan, M.; Cao, J.; Cui, J.; Tan, J. Evaluation of measurement uncertainty in H-drive stage during high acceleration based on Monte Carlo method. Int. Mach. Tool Manuf. 2015, 93, 1–9. [CrossRef] 26. Andolfatto, L.; Mayer, J.R.R.; Lavernhe, S. Adaptive Monte Carlo applied to uncertainty estimation in five axis machine tool link errors identification with thermal disturbance. Int. Mach. Tool Manuf. 2011, 57, 618–627. [CrossRef] 27. Cai, L.C.; Zhang, Z.; Cheng, Q.; Liu, Z.; Gu, P.; Qi, Y. An approach to optimize the machining accuracy retainability of multi-axis NC machine tool based on robust design. Precis. Eng. 2016, 43, 370–386. [CrossRef] 28. Lin, Y.J.; Agrawal, A.; Fang, Y. Wear progressions and tool life enhancement with AlCrN coated inserts in high-speed dry and wet steel lathing. Wear 2008, 264, 226–234. [CrossRef] 29. Srisattayakul, P.; Saikaew, C. Effects of hard and MoN-coated stainless steel on wear behavior and tool life model under two-body abrasion wear testing. Metalurgija 2017, 56, 371–374. 30. Fox-Rabinovich, G.S.; Beake, B.D.; Endrino, J.L.; Veldhuis, S.V.; Parkinson, R.; Shuster, L.S.; Migranov, M.S. Effect of mechanical properties measured at room and elevated temperatures on the wear resistance of cutting tools with TiAlN and AlCrN coatings. Surf. Coat. Technol. 2006, 200, 5738–5742. [CrossRef] 31. Bourhis, E.L.; Goudeau, P.; Staia, M.H.; Puchi-Cabrera, E.S. Mechanical properties of hard AlCrN-based coated substrates. Surf. Coat. Technol. 2009, 203, 2961–2968. [CrossRef] 32. Endrino, J.L.; Fox-Rabinovich, G.S.; Reiter, A.; Veldhuis, S.V.; Galindo, R.E.; Albella, J.M.; Marco, J.F. Oxidation tuning in AlCrN coatings. Surf. Coat. Technol. 2007, 201, 4505–4511. [CrossRef] 33. Minitab®Version 16.2.4; Minitab Inc.: State College, PA, USA, 2013. 34. Boopathi, M.; Shankar, S.; Manikandakumar, S.; Ramesh, R. Experimental investigation of friction drilling on brass, aluminium and stainless steel. Procedia Eng. 2013, 64, 1219–1226. [CrossRef] 35. Vereschaka, A.A.; Grigoriev, S.N. Study of cracking mechanisms in multi-layered composite nano-structured coatings. Wear 2017, 378, 43–57. [CrossRef] 36. Vereschaka, A.A.; Grigoriev, S.N.; Sitnikov, N.N.; Batako, A.D. Delamination and longitudinal cracking in multi-layered composite nanostructured coatings and their influence on cutting tool life. Wear 2017, 390, 209–219. [CrossRef] 37. Aihua, L.; Jianxin, D.; Haibing, C.; Yangyang, C.; Jun, Z. Friction and wear properties of TiN, TiAlN, AlTiN and CrAlN PVD nitride coatings. Int. J. Refract. Met. Hard Mater. 2012, 31, 82–88. [CrossRef] Coatings 2019, 9, 853 14 of 14

38. Mo, J.L.; Zhu, M.H.; Leyland, A.; Matthews, A. Impact wear and abrasion resistance of CrN, AlCrN and AlTiN PVD coatings. Surf. Coat. Technol. 2013, 215, 170–177. [CrossRef] 39. Birol, Y. Sliding wear of CrN, AlCrN and AlTiN coated AISIH13 hot work tool steels in aluminium extrusion. Tribol. Int. 2013, 57, 101–106. [CrossRef] 40. Sugita, N.; Shu, L.; Kimura, K.; Arai, G.; Arai, K. Dedicated drill design for reduction in burr and delamination during the drilling of composite materials. CIRP Ann. Manuf. Technol. 2019, 68, 89–92. [CrossRef]

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