Poly(Methyl Methacrylate) Coating of Titanium Workpieces to Reduce Burrs in Micro-Drilling

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Article Poly(methyl methacrylate) Coating of Titanium Workpieces to Reduce Burrs in Micro-drilling

Luca Giorleo Department of Mechanical and Industrial Engineering, University of Brescia, 25136 Brescia, Italy; [email protected]

Received: 31 October 2019; Accepted: 28 November 2019; Published: 30 November 2019

Abstract: A technique to reduce burr height in titanium micro-drilling is presented: a poly (methyl methacrylate) coating was applied before machining on the upper and lower surfaces of a titanium specimen (0.5-mm thick). After drilling, a cleaning process (acetone bath) was executed to eliminate the coating, and holes with less burr were obtained. The coating process was executed with a spin-coating machine. To test the efficacy of the technique, two different coating thicknesses (7.9 and 5.4 µm) and two drill bits (0.25- and 0.5-mm diameter) were evaluated. Qualitative and quantitative analyses of the holes obtained were performed with scanning electron microscopy and three-dimensional microscopy, respectively. The results highlight the efficacy of the technique to reduce the burr height by 70% in coated titanium relative to that in an uncoated titanium sheet.

Keywords: micro-drilling; burr reduction method; spin coating

1. Introduction Manufacturing technologies for material removal have been constantly evolving to fulfill consumer requirements. Currently, downscaling of processes such as machining, injection molding, and electric discharge machining (EDM) is needed to produce microscale workpieces that are used in fields ranging from aeronautical to biomedical applications [1,2]. Despite the existence of several competing technologies, drilling is the most common and still in high demand. Frequently, the manufacturing of a through bore-hole is characterized by the formation of entrance and exit burrs. The dimension of the exit burr exceeds that of the entrance burr considerably. Owing to the hangover on the drill exit surface, the quality of the workpiece is reduced significantly. Generally, the dimensions of the burr are affected by the process parameters [3,4], and the height of the burr increases with the dimension of the drill bit diameter. Despite this, the problems associated with burrs became more critical when the environment shifts from macro- to microscale. Neugebauer, et al. (Figure 16 in Ref. [5]) reports the trend of the absolute and relative height (evaluated as the ratio between the burr height and hole diameter) as a function of the drill bit diameter. As can be observed, the path of the absolute burr height shows an increase of the burr height in the range from 0.1 mm d 7 mm. Afterwards, the burr process mechanism becomes unstable ≤ ≤ for a change of burr geometry (from crown to ring). On the opposite, in the case of the micro drill bit with a diameter less than 2 mm, Figure 16 in Ref. [5] shows how the relative burr height exponentially increases [5]. In particular, when the drill diameter is lower than 1 mm, the relative burr height becomes higher than 1 which means the absolute burr height is higher than the hole realized. In micromachining, this trend leads to a high amount of burr that surrounds the hole and affects its function. The exit burr degrades the quality of parts and is problematic for assembly, so a deburring process is necessary. Problems arise during manual deburring, which is often performed at a conventional size level and is difficult to apply. A chemical deburring process is often unsuitable because it can distort the shape of the hole. Moreover, burr generation is critical not only to industry

Micromachines 2019, 10, 838; doi:10.3390/mi10120838 www.mdpi.com/journal/micromachines Micromachines 2019, 10, 838 2 of 12 but also to scientific applications. In micro-drilling, the dimensions of important output parameters are affected, such as hole diameter and roundness. These parameters are fundamental in tool wear analysis, but the burr makes it difficult to use a machine tool interferometer. In a previous study, the author tested a method to avoid this problem by choosing brass as a reference material to reduce burr interference in hole diameter measurement [6]. However, processes able to reduce burr are always preferred. In the literature, several solutions have been studied to minimize burr. Referring to the process parameter, a linear correlation between the burr height and feed per tooth was found [7], in particular, benefits have been achieved imposing a feed variation model based on a sinusoidal function [8]. Gariani et al. reduced burr height controlling the cutting fluid quantity as a function of the heat generated [9]. Drill bit geometry still influences the burr generated and a direct correlation between burr height and cutting edge radius still exists [10]. da Silva et al. showed a considerable reduction in burr height when cutting tools with lower exit angles are used [11] and Sugita et al. proposed a new drill bit design to reduce burr in composite materials drilling [12]. Assisted machining is another solution able to reduce burr problem: Takeyama et al. demonstrated the efficacy of ultrasonic vibration during aluminum micro [13] and Hussein et al. found a relationship between high-frequency and drilling parameters to reduce burr height by 86% in Ti6Al4V drilling [14]. Pilny et al. used a vacuum clamping set up to obtain a reduction of approximately 50% in exit burr formation [15]. Despite the important results achieved in these studies, the proposed approaches to reduce burr formation by imposing new drill bit geometry, hybrid processing, or constraints such as feed rate or spindle speed are sometimes difficult to realize in industrial environments. In particular, the author wants to underline that the mentioned approaches as examples of ultrasonic-assisted machining or customized drill geometry have been tested with success but in a laboratory environment, and a real possibility to apply these methods in an industrial environment could be critical or impossible for production or plant constraints. On the contrary, strategies that could be applied in the pre-processing step could be investigated because they do not modify the drilling process. Another approach that could be followed to reduce burr height is to apply an auxiliary material on the top and the bottom of the workpiece. Based on chip mechanism formation, burrs are formed at the entrance and exit surfaces of the workpiece and are due to tearing and bending actions followed by shearing or lateral extrusion; so the possibility to apply on these surfaces a material that could be removed after playing a significative role in protecting the surface form the press foot, driving the drill bit and improving the accuracy and in particular to reduce the burr height [16]. This technique is usually applied to improve position accuracy and reduce burr height in the drilling of printed circuit boards where an entry board and backboard are commonly used [17]. However, despite the benefits, 1 the entry and backboard solution found application only in the case of 2 2 D application because the auxiliary material is solid. Based on this idea, Kim et al. applied a soft material (cyanoacrylate adhesive) at the exit surface of a sheet to prevent the exit burr and achieved burr reduction in the drilling of low-hardness materials, such as aluminum or copper, but the technique proved to be ineffective for hard materials, such as AISI 304 stainless steel [18]. Starting from the last works to improve technique application, liquid coating was tested by the author; in particular, a preliminary study of a burr reduction technique to reduce burr height in titanium drilling is presented in which a biocompatible coating (PMMA) is applied to the upper and lower surfaces of a titanium sheet To experimentally evaluate the proposed method, two thicknesses of coating and two different drills diameter were tested. The efficacy of the method in terms of burr homogeneity and height was analyzed with scanning electron microscope (SEM) and 3D digital microscopy, respectively. The results highlight some benefits and critical aspects of the proposed approach. Of interest is the suitability of the method for application in biomedical device production. Titanium is strong, biocompatible, and completely resistant to bodily fluids, making it the material of choice for the manufacture of a myriad of medical and dental devices, such as artificial valves in the heart [19]; stents in blood vessels [20]; and replacement implants in shoulders, knees, hips, elbows [21] Micromachines 2019, 10, 838 3 of 12

Micromachinesears, and dental 2019, 10 structures, x [22]. These devices often require machining, including micro-drilling.3 of 11 However, if the drilling process generates a significant amount of burr that cannot be removed, it may [21]have ears, an adverse and dental influence structures on the [22]. survival These ofdevices the implant often require and trigger machining, adverse including side effects micro-drilling. in recipients; However,therefore, methodsif the drilling to reduce process burr heightgenerates play a a signi fundamentalficant amount role in of medical burr that device cannot production, be removed, yet they it maymust have be developed an adverse with influence the constraint on the of survival using materials of the implant that do notand contaminate trigger adverse the biocompatibility side effects in recipients;of the workpiece. therefore, methods to reduce burr height play a fundamental role in medical device production, yet they must be developed with the constraint of using materials that do not contaminate2. Materials andthe biocompatibility Methods of the workpiece.

2.2.1. Materials Coating Processand Methods The top and bottom of the part to be machined are first covered with a film (step A), as shown 2.1. Coating Process in Figure1. After machining, a cleaning process is executed to eliminate the coating, while avoiding surfaceThe microstructure top and bottom modifications. of the part to Finally, be machined a hole withare first a reduced covered top with and a bottomfilm (step burr A), is as obtained. shown Thein Figure film should1. After be machining, thick enough a cleaning to ensure process that theis executed burr is derived to eliminate partially the coating, or completely while avoiding from the surfacedeposited microstructure material (step modifications. B). The coating Finally, material a ho isle removed with a reduced by a cleaning top and process bottom (step burr C)is obtained. to obtain veryThe film precise should holes. be Steps thick Aenough and C areto ensure crucial that to the the feasibility burr is derived of a precise partially machining or completely process; therefore,from the parametersdeposited material must be (step carefully B). The identified coating to:material is removed by a cleaning process (step C) to obtain very precise holes. Steps A and C are crucial to the feasibility of a precise machining process; ensure a low and controlled film deposition that contributes a negligible amount to the therefore,• parameters must be carefully identified to: • workpieceensure thickness;a low and controlled film deposition that contributes a negligible amount to the guarantee good adhesion between the substrate and the film to avoid detachment during the • workpiece thickness; • micro-drillingguarantee good process; adhesion between the substrate and the film to avoid detachment during the usemicro-drilling a material that process; can be easily removed, avoiding any modification to the machined surface. • • use a material that can be easily removed, avoiding any modification to the machined surface.

Figure 1. Scheme of of the proposed method: coating coating step step to to apply apply extra extra material material on on the the surface surface ( (AA),), drilling operation andand relativerelative burrburr formation formation ( B(B),), and and chemical chemical cleaning cleaning to to remove remove coating coating and and part part of ofburr burr (C ).(C).

Based on these assumptions, spin-coating was selectedselected as the deposition technique. Spin coating is widely used in science and engineering to deposit uniform coatings of organic materials and to uniformly distribute distribute particulate particulate matter matter on on a aflat flat surface surface [23,24]. [23,24 In]. this In this process, process, the thefilm film thickness thickness is a functionis a function of the of thematerial material (in (interms terms of ofsolids solids cont content)ent) and and the the revolution revolution speed. The spin-coating technique depositsdeposits aa thinthin andand uniformuniform coatingcoating (in (in the the range range of of 0.1–10 0.1–10µ m).μm). The The thickness thickness of of a filma filmh ish isgiven given by by Equation Equation (1). (1). KC2 h = KC 2 (1) h = f (1) f where K is a constant characteristic of the solvent-polymer system used, and C and f denote the concentration of the solution and the rotational speed (rpm) of the machine, respectively [25]. The coating selection focused on choosing a material that has not only good mechanical characteristics to

Micromachines 2019, 10, 838 4 of 12 where K is a constant characteristic of the solvent-polymer system used, and C and f denote the

Micromachinesconcentration 2019 of, 10 the, x solution and the rotational speed (rpm) of the machine, respectively4 of [25 11]. The coating selection focused on choosing a material that has not only good mechanical characteristics supportto support the the machining machining process process but but also also low low chemic chemicalal resistance resistance for for easy easy removal. removal. We We chose chose poly (methyl methacrylate),methacrylate), or PMMA,PMMA, asas thethe coating.coating. PMMA PMMA swells swells and and dissolves in many organic solvents, it also has poor poor resistance resistance to to many many other other chemicals chemicals due due to to its its easily easily hydrolyzed hydrolyzed ester ester groups. groups. Nevertheless, its its environmental environmental stability stability is issuperior superior to tomany many other other plastics, plastics, such such as polystyrene as polystyrene and polyethylene.and polyethylene. Moreover, Moreover, PMMA PMMA is extremely is extremely bioc biocompatibleompatible and and resistant resistant to to long long exposure exposure to temperatures, chemistry, and cellularcellular activitiesactivities in humanhuman tissue.tissue. In In orthopedic orthopedic surgery, surgery, PMMA is known asas “bone“bone cement”cement” andand isis used used to to fill fill spaces spaces between between implants implants and and bone bone because, because, in in addition addition to toits its biocompatibility, biocompatibility, PMMA PMMA is easy is easy to make to make and manipulateand manipulate in a hospital in a hospital environment. environment. In this In study, this study,a commercial a commercial Positive Positive PMMA 950PMMA k E-Beam 950 k Resists E-Beam AR-P Resists 630–670 AR-P series 630–670 was usedseries for was the used experiments, for the experiments,and PMMA was and diluted PMMA in was a chlorobenzene diluted in a solutionchlorobenzene (9%) to solution produce (9%) the coating to produce polymer. the Ancoating SCS polymer.P-6700 top-precision An SCS P-6700 spin-coating top-precision system spin-coating was used to system realize was the used coating to realize (Figure the2). Thecoating film (Figure thickness 2). Theof PMMA film thickness as a function of PMMA of the as rotational a function speed of the is shownrotational in Figure speed3 .is Theshown film in thickness Figure 3. decreases The film thicknessrapidly using decreases speeds uprapidly to 1000 using rpm speeds until a thicknessup to 1000 of 4rpmµm. until To avoid a thickness depositing of a 4 film μm. that To was avoid too depositingthin, the authors a film that tested was the too film thin, deposition the authors on test twoed sheet the film metal deposition coatings on at two rotational sheet metal speeds coatings of 250 atand rotational 500 rpm andspeeds obtained of 250 films and that 500 were rpm 7.9 and and obtained 5.4 µm thick, films respectively. that were Using7.9 and the 5.4 same μm process thick, parameters,respectively. Ti Using grade the 2sheets same process were coated parameters, on the top Ti grade and bottom 2 sheets surfaces were coated as specimens on the top to evaluateand bottom the surfacesburr reduction as specimens method. to The evaluate sheet metalthe burr thickness reduction was method. 0.5 mm. The Specimen sheet metal parameters thickness are was summarized 0.5 mm. Specimenin Table1. parameters are summarized in Table 1.

Figure 2. Spin-coating machine.

2 film thickness [µm] thickness film film thickness [µm] thickness film 0 0123456 Rotational speed * 103 [rpm][rpm]

Figure 3. PMMA film ﬁlm thickness growth.

Table 1. Sheet metal-coated specimens.

Material Composition (Wt %) Dimension (mm) f (rpm) h (μm) C: Max 0.1 250 7.9 Fe: Max 0.3 H: Max 0.015 Length and width: 3 Ti grade 2 N: Max 0.03 Thickness: 0.5 500 5.4 O: Max 0.25 Ti: 99.2

2.2. Micro-drilling Process

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Table 1. Sheet metal-coated specimens.

Material Composition (Wt %) Dimension (mm) f (rpm) h (µm) C: Max 0.1 250 7.9 Fe: Max 0.3 Length and width: 3 Ti grade 2 H: Max 0.015 500 5.4 N: Max 0.03 Thickness: 0.5 O: Max 0.25 Ti: 99.2 Micromachines 2019, 10, x 5 of 11 2.2. Micro-drilling Process Drilled holes were realized with a Kern Pyramid Nano, a five-axis ultra-precision CNC machine (Kern,Drilled Murnau, holes Germany), were realized as shown with in a KernFigure Pyramid 4. The clamping Nano, a ﬁve-axis system shown ultra-precision in Figure CNC 5 was machine created (Kern,to avoid Murnau, sheet movement Germany), during as shown the in drilling Figure 4op. Theeration, clamping and a system gap of shown 5 mm inwas Figure designed5 was between created tothe avoid bottom sheet of movement the sheet duringand the the clamping drilling operation,system to and allow a gap for of bottom 5 mm wasburr designed generation between without the bottominterference. of the sheet and the clamping system to allow for bottom burr generation without interference.

Figure 4.4. Kern Pyramid Nano.

Figure 5.5. Clamping system.

Two didifferentfferent diametersdiameters (0.5(0.5 andand 0.270.27 mm)mm) ofof HSSHSS DINDIN 1899 drill bits (Sphinks, Derendingen, Switzerland) werewere used used in thein experimentalthe experimental campaign. campaign. The choice The ofchoice the two of drillthe diameterstwo drill considereddiameters that,considered when movingthat, when from moving macro- from to microscale macro- to processing, microscale the processing, drill’s core the in drill’s proportion core in to proportion the diameter to isthe larger diameter for smaller is larger diameter for smaller tools in diameter order to tools give themin order strength, to give which them causes strength, the chisel which edge’s causes length the tochisel increase edge’s and length influences to increase the micro-drilling and influences process the reactionsmicro-drilling [26,27 ].process In other reactions words, when[26,27]. the In drill other bit diameterwords, when becomes the drill small, bit the diameter process becomes moresmall, like the a process deep drawing becomes process. more Iflike the a hole deep generation drawing processprocess. is If di thefferent, hole thegeneration associated process burr willis differe also bent, generated the associated with diburrfferent will dynamics. also be generated The selection with ofdifferent process dynamics. parameters The was selection set according of process to technicalparameters catalogs. was set To according compare to the technical effectiveness catalogs. of theTo proposedcompare the method, effectiveness three holesof the onproposed the uncoated method, and three coated holessheet on the were uncoated executed and coated for each sheet drill were bit diameterexecuted forfor aeach total drill of 18 bit holes diameter (two diameters, for a total threeof 18 coatings,holes (two and diameters, three replicas). three Tablecoatings,2 summarizes and three thereplicas). process Table parameters 2 summarizes for each the test process specimen. parameters for each test specimen.

Table 2. Process parameters used for the experimental campaign.

Drill Diameter Test Drill Tool Spindle Speed (rpm) Feed (mm/min) Coating Thickness (μm) ϕ (mm) 1 0.27 1705 5.11 0 2 0.27 1705 5.11 5.4 3 0.27 1705 5.11 7.9 4 0.5 2228 8.9 0 5 0.5 2228 8.9 5.4 6 0.5 2228 8.9 7.9

After drilling, the PMMA was removed by ultrasonic cleaning in an acetone bath for 10 min. The acetone chemically reacts with the PMMA and avoids any metal microstructure modification. Because it is possible for the ultrasound process alone to reduce the burr, uncoated sheets were also cleaned. As an example, images of a titanium sheet drilled with a 0.5-mm drill bit diameter are shown in Figure 6. After the cleaning treatment, a more precise hole is obtained.

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Table 2. Process parameters used for the experimental campaign.

Test Drill Diameter φ (mm) Drill Tool Spindle Speed (rpm) Feed (mm/min) Coating Thickness (µm) 1 0.27 1705 5.11 0 2 0.27 1705 5.11 5.4 3 0.27 1705 5.11 7.9 4 0.5 2228 8.9 0 5 0.5 2228 8.9 5.4 6 0.5 2228 8.9 7.9

After drilling, the PMMA was removed by ultrasonic cleaning in an acetone bath for 10 min. The acetone chemically reacts with the PMMA and avoids any metal microstructure modiﬁcation. Because it is possible for the ultrasound process alone to reduce the burr, uncoated sheets were also cleaned. As an example, images of a titanium sheet drilled with a 0.5-mm drill bit diameter are shown Micromachines 2019,, 10,, xx 6 of 11 in Figure6. After the cleaning treatment, a more precise hole is obtained.

((a)) ( (b))

FigureFigure 6.6. Drilled holes before and afterafter cleaning.cleaning. ( a)) After After step step B; B; ( (b)) After After step step CC

3.3. Results ToTo analyzeanalyze burrburr homogeneity, homogeneity, images images of of the the drilled drilled holes holes before before and and after after an an ultrasonic ultrasonic cleaning cleaning in aninin anan acetone acetoneacetone bath bathbath were werewere acquired acquiredacquired by byby SEM SEMSEM with withwith a tungsten aa tungstentungsten filament. filament.filament. The TheThe SEM SEMSEM analysis analysisanalysis made mademade it possible itit possiblepossible to establishtoto establishestablish the the presencethe presencepresence or absence oror absenceabsence of PMMA ofof PMMAPMMA on the onon sheet thethe after sheetsheet the afterafter cleaning thethe cleaningcleaning operation operationoperation due to adi dueduefference toto aa indifference electron in beam electron absorption beam absorption of PMMA that of PMMA causes th localizedat causes darker localized areas darker to appear areas (arrows to appear in Figure (arrows7). Theinin FigureFigure results 7).7). confirm TheThe resultsresults the absence confirmconfirm of thethe PMMA absenceabsence from of PMMA each cleaned from each titanium cleaned sheet. titanium sheet.

((a)) ((b))

FigureFigure 7. Localized presence or absence of PMMA PMMA (darker (darker area) area) before before and after after the the cleaning cleaning process, process, respectively.respectively. ((aa))) PresencePresencePresence ofofof PMMA;PMMA;PMMA; (((bb))) AbsenceAbsenceAbsence ofofof PMMA.PMMA.PMMA.

The analysis of replicas reveals good repeatability of the experiments; a representative example is The analysis of replicas reveals good repeatability of the experiments; a representative example shown in Figure8. isis shownshown inin FigureFigure 8.8.

((a)) ((b)) ((c))

Figure 8. Three replicas of Test 6-ϕ 0.5-HI-Bottom0.5-HI-Bottom view.view. ((a)) firstfirst ((b)) secondsecond ((c)) third.third.

The experimental campaign results are shown in Figure 9. Images are labeled according to the followingfollowing criteria:criteria: testtest number;number; drdrillill diameter;diameter; coatingcoating ththickness;ickness; andand burrburr location.location. ToTo betterbetter analyzeanalyze thethe results,results, thethe coatingcoating thicknessthickness isis classifiedclassified asas nono thicknessthickness (NO),(NO), aa thicknessthickness equalequal toto 5.45.4 μm (LOW), and a thickness equal to 7.8 μm (HI); the burr is classified as either top or bottom.

Micromachines 2019, 10, x 6 of 11

(a) (b)

Figure 6. Drilled holes before and after cleaning. (a) After step B; (b) After step C

3. Results To analyze burr homogeneity, images of the drilled holes before and after an ultrasonic cleaning in an acetone bath were acquired by SEM with a tungsten filament. The SEM analysis made it possible to establish the presence or absence of PMMA on the sheet after the cleaning operation due to a difference in electron beam absorption of PMMA that causes localized darker areas to appear (arrows in Figure 7). The results confirm the absence of PMMA from each cleaned titanium sheet.

(a) (b)

Figure 7. Localized presence or absence of PMMA (darker area) before and after the cleaning process, respectively. (a) Presence of PMMA; (b) Absence of PMMA.

MicromachinesThe analysis2019, 10 of, 838 replicas reveals good repeatability of the experiments; a representative example7 of 12 is shown in Figure 8.

(a) (b) (c)

FigureFigure 8. ThreeThree replicas replicas of of Test Test 6- ϕφ 0.5-HI-Bottom view. ( a) first ﬁrst (b) second (c) third.

The experimental campaign results are shown in Figure 99.. ImagesImages areare labeledlabeled accordingaccording toto thethe following criteria: test test number; number; dr drillill diameter; diameter; coating coating th thickness;ickness; and burr location. To To better better analyze the results, the coating thickness is classified classiﬁed as no no thickness thickness (NO), (NO), a a thickness thickness equal equal to to 5.4 5.4 μµmm (LOW), (LOW), and a thickness equal to 7.8 µm (HI); the burr is classiﬁed as either top or bottom. andMicromachines a thickness 2019 equal, 10, x to 7.8 μm (HI); the burr is classified as either top or bottom. 7 of 11

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

(j) (k) (l)

FigureFigure 9. 9. ResultsResults of of qualitative qualitative analysis. analysis. (a) Test (a) 1- Testϕ 0.27-NO-Top; 1-φ 0.27-NO-Top; (b) Test (2-bϕ) Test0.27-LOW-Top; 2-φ 0.27-LOW-Top; (c) Test (c)3- Testϕ 0.27-HI-Top; 3-φ 0.27-HI-Top; (d) Test (d 1-)ϕ Test 0.27-NO-Bottom; 1-φ 0.27-NO-Bottom; (e) Test 2- (eϕ) Test0.27-LOW-Bottom; 2-φ 0.27-LOW-Bottom; (f) Test 3-ϕ ( f0.27-HI-) Test 3-φ 0.27-HI-Bottom;Bottom; (g) Test (g )4- Testϕ 0.5-NO-Top; 4-φ 0.5-NO-Top; (h) Test (h 5-) Testϕ 0.5-LOW-Top; 5-φ 0.5-LOW-Top; (i)Test (6-i)Testϕ 0.5-HI-Top; 6-φ 0.5-HI-Top; (j) Test ( j4-) Testϕ 0.5- 4-φ 0.5-NO-Bottom;NO-Bottom; (k ()k Test) Test 5- 5-ϕ φ0.5-LOW-Bottom;0.5-LOW-Bottom; (l) (Testl) Test 6-ϕ 6- 0.5-HI-Bottom.φ 0.5-HI-Bottom.

Different assertions can be made from these results. First, for the 0.5-mm holes, there is a significant burr reduction between the coated and uncoated (Test 4 vs. Tests 5 and 6). Moreover, according to previous research [4] that established basic burr types in conventional and micro- drilling, there is a generation of transient burr for uncoated holes, while a uniform burr is generated for HI holes (Tests 1 and 2 vs. Tests 3 and 6). Finally, superior burr reduction is achieved for 0.5-mm holes (Tests 2 and 3 vs. Tests 5 and 6); the reason is attributed to the different drill bit geometry, such as chisel edge length, that affects the chip removal mechanism. After SEM analysis, the burr height for each test was measured by 3D digital microscopy (Hirox). A 3D model of each hole was captured for each test, and eight height measurements were acquired. Figure 10 plots the results, and Table 3 summarizes the mean values of 288 burr height measurements (eight measurements, six tests, three replicas, top, and bottom). A statistical approach was executed to analyze the results: a two-way ANOVA to verify the significance of coating and replicas and a Tukey’s range test to demonstrate whether the burr heights were significantly different from each other. The results of these statistical analyses are shown in Table 4 and Figure 11.

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Different assertions can be made from these results. First, for the 0.5-mm holes, there is a significant burr reduction between the coated and uncoated (Test 4 vs. Tests 5 and 6). Moreover, according to previous research [4] that established basic burr types in conventional and micro-drilling, there is a generation of transient burr for uncoated holes, while a uniform burr is generated for HI holes (Tests 1 and 2 vs. Tests 3 and 6). Finally, superior burr reduction is achieved for 0.5-mm holes (Tests 2 and 3 vs. Tests 5 and 6); the reason is attributed to the different drill bit geometry, such as chisel edge length, that affects the chip removal mechanism. After SEM analysis, the burr height for each test was measured by 3D digital microscopy (Hirox). A 3D model of each hole was captured for each test, and eight height measurements were acquired. Figure 10 plots the results, and Table3 summarizes the mean values of 288 burr height measurements (eight measurements, six tests, three replicas, top, and bottom). Micromachines 2019, 10, x 8 of 11

(a) (b)

Figure 10. BurrBurr height height measurements measurements as as a a fu functionnction of the drill diameter ( φϕ)) and on hole position (Top(Top/Bottom)./Bottom). (a) ϕφ 0.27-Top; ((bb)) ϕφ 0.27-Bottom; (cc)) ϕφ 0.50 Top; ((dd)) ϕφ 0.500.50 Bottom.Bottom.

Table 3. Mean burr height measurements. Table 3. Mean burr height measurements. ϕ ϕ ϕ ϕ DrillDrill Test test Top-φ == 0.27 mmmm Bottom- Bottom-φ == 0.270.27 mm mm Top- Top-φ = =0.500.50 mm mm Bottom- Bottom-φ = 0.50= 0.50 mm mm ReplicaReplica HIHI LOW LOW NO NO HI HI LOW LOW NO HI HI LOW LOW NO NO HI HI LOW LOW NO NO 11 5.95.9 6.5 6.5 9.9 9.9 10.0 10.0 10.4 10.4 23.9 3.3 3.3 5.6 5.6 14.6 14.6 6.0 6.0 9.9 9.9 22.2 22.2 22 6.46.4 6.9 6.9 11.2 11.2 7.7 7.7 10.4 10.4 26.0 3.3 3.3 4.6 4.6 14.8 14.8 7.5 7.5 7.1 7.1 22.7 22.7 33 4.24.2 6.3 6.3 10.8 10.8 11.0 11.0 10.8 10.8 26.2 3.3 3.3 5.3 5.3 15.6 15.6 6.8 6.8 10.4 10.4 21.9 21.9

A statistical approach was executedTable 4. Results to analyze of two-way the results: ANOVA. a two-way ANOVA to verify the significance of coating and replicas and a Tukey’s range test top-value demonstrate whether the burr heights were significantly different fromParameter each other. The resultsφ 0.27 of these statisticalφanalyses 0.50 are shown in Table4 and Figure 11. Top Bottom Top Bottom Coating 0.001 0.001 0.001 0.001 Replica 0.36 0.157 0.876 0.931 Interaction 0.645 0.173 0.97 0.7

(a) (b)

Figure 11. Results of Tukey’s range test. (a) ϕ = 0.27 mm-Top; (b) ϕ = 0.27 mm-Bottom; (c) ϕ = 0.50 mm-Top; (d) ϕ = 0.50 mm-Bottom.

The ANOVA results confirm the efficacy of the technique (p-value of coating < 0.05) and the robustness of the experimental design (differences among replicas are not significant). The results of

Micromachines 2019, 10, x 8 of 11

(a) (b)

Figure 10. Burr height measurements as a function of the drill diameter (φ) and on hole position (Top/Bottom). (a) φ 0.27-Top; (b) φ 0.27-Bottom; (c) φ 0.50 Top; (d) φ 0.50 Bottom.

Table 3. Mean burr height measurements.

Drill test Top-φ = 0.27 mm Bottom-φ = 0.27 mm Top-φ = 0.50 mm Bottom-φ = 0.50 mm Replica HI LOW NO HI LOW NO HI LOW NO HI LOW NO 1 5.9 6.5 9.9 10.0 10.4 23.9 3.3 5.6 14.6 6.0 9.9 22.2 Micromachines2 20196.4, 10 , 8386.9 11.2 7.7 10.4 26.0 3.3 4.6 14.8 7.5 7.1 22.79 of 12 3 4.2 6.3 10.8 11.0 10.8 26.2 3.3 5.3 15.6 6.8 10.4 21.9

Table 4. Results of two-way ANOVA. Table 4. Results of two-way ANOVA. p-Value p-value ParameterParameter ϕ 0.27 φ 0.27 ϕ 0.50φ 0.50 TopTop Bottom Bottom Top Top BottomBottom Coating 0.001 0.001 0.001 0.001 Coating 0.001 0.001 0.001 0.001 ReplicaReplica 0.36 0.36 0.157 0.157 0.876 0.876 0.931 0.931 InteractionInteraction 0.6450.645 0.173 0.173 0.970.97 0.7 0.7

(a) (b)

Figure 11. Results of Tukey’s rangerange test.test. (a)) φϕ == 0.27 mm-Top; ((bb)) φϕ == 0.270.27 mm-Bottom;mm-Bottom; (cc)) φϕ == 0.50 mm-Top; ((dd)) φϕ == 0.50 mm-Bottom.

The ANOVA results confirmconfirm the eefficacyfficacy of the technique (p-value (p-value of coating < 0.05) and the robustnessMicromachines of 2019 the the, 10 experimental experimental, x design design (differences (differences among among replicas replicas are are not not significant). significant). The The results results9 of of11 of the Tukey’s range test show that an improvement in burr reduction is achieved when a coating is the Tukey’s range test show that an improvement in burr reduction is achieved when a coating is applied, but the thicknesses of coatings tested yielded comparable burr heights. To better highlight applied, but the thicknesses of coatings tested yielded comparable burr heights. To better highlight the efficacy of the method, the mean height reduction and histograms with standard deviation are the efficacy of the method, the mean height reduction and histograms with standard deviation are reported in Table5 and Figure 12, respectively. reported in Table 5 and Figure 12, respectively. Table 5. Mean percentage of burr height reduction. Table 5. Mean percentage of burr height reduction. Hole Position ϕ LOW HI Hole position φ LOW HI ϕ 0.27 38% 48% Top φ 0.27 38% 48% Top ϕ 0.50φ 0.50 65% 65% 78%78% ϕ 0.27 59% 62% Bottom φ 0.27 59% 62% Bottom ϕ 0.50 59% 69% φ 0.50 59% 69%

Figure 12. Mean burr heights relative to co coatingating with standard deviations.

The following results are noted in Table 4 and Figure 12: • Superior burr reduction is obtained for 0.50-mm holes compared with 0.27-mm holes. In particular, as reported in Table 4, the burrs generated on the top surface with the 0.5 drill diameter are reduced by 65% and 78% for LOW and HI coating thickness; on the contrary, a reduction of 38% and 48% is obtained with the 0.27 drill. Regarding the bottom surface, the burr height reduction is similar for both drills tested. • The efficacy of the technique reduces bottom burr height by more than 50% in all tests, the minimum burr height recorded for top burr height is 38% (Table 4). • A reduction in standard deviation is seen between coated and uncoated holes indicating that a more uniform burr is produced. This results is more evident in the case of bottom burr generated by the 0.5 drill diameter were a reduction of 50% of standard deviation is achieved (Figure 12). In terms of the burr formation process, different considerations can be pointed out based on the qualitative and quantitative analysis. Figure 9 highlights that the coating affects the type of burr generated, in particular, in agreement with [28], an irregular burr is observed in holes realized without coating (Test 1 and 4). The effect of coating is to reduce the irregular trend when a low thickness of PMMA is coated during the drilling process (Test 2 and 5) and burr type becomes uniform in the case of high thickness (Test 3 and 6). Regarding the relative burr height problem that affects micro drilling, the authors also found in the case of titanium drilling results similar to what [5] reported in the case of steel; indeed, as reported in Table 3, the height burr is similar in the case of holes generated with a 0.27- or 0.5-mm drill diameter so that on the contrary the relative burr height is almost double that of the 0.27 drill bit. Finally, Figure 12 shows benefits in burr height reduction but the titanium burr is also still present in the coated sheets. As is known, the burr is formed by the plastic flow of the material and the role of the coating applied is to reduce/absorb this flow in order to only have a PMMA burr. The results highlight that the mechanical properties of PMMA coating are not able to fully compensate the pressure lift due to the titanium deformation so that a part of titanium burr is still generated.

Micromachines 2019, 10, 838 10 of 12

The following results are noted in Table4 and Figure 12:

Superior burr reduction is obtained for 0.50-mm holes compared with 0.27-mm holes. In particular, • as reported in Table4, the burrs generated on the top surface with the 0.5 drill diameter are reduced by 65% and 78% for LOW and HI coating thickness; on the contrary, a reduction of 38% and 48% is obtained with the 0.27 drill. Regarding the bottom surface, the burr height reduction is similar for both drills tested. The eﬃcacy of the technique reduces bottom burr height by more than 50% in all tests, the • minimum burr height recorded for top burr height is 38% (Table4). A reduction in standard deviation is seen between coated and uncoated holes indicating that a • more uniform burr is produced. This results is more evident in the case of bottom burr generated by the 0.5 drill diameter were a reduction of 50% of standard deviation is achieved (Figure 12).

In terms of the burr formation process, different considerations can be pointed out based on the qualitative and quantitative analysis. Figure9 highlights that the coating a ffects the type of burr generated, in particular, in agreement with [28], an irregular burr is observed in holes realized without coating (Test 1 and 4). The effect of coating is to reduce the irregular trend when a low thickness of PMMA is coated during the drilling process (Test 2 and 5) and burr type becomes uniform in the case of high thickness (Test 3 and 6). Regarding the relative burr height problem that affects micro drilling, the authors also found in the case of titanium drilling results similar to what [5] reported in the case of steel; indeed, as reported in Table3, the height burr is similar in the case of holes generated with a 0.27- or 0.5-mm drill diameter so that on the contrary the relative burr height is almost double that of the 0.27 drill bit. Finally, Figure 12 shows benefits in burr height reduction but the titanium burr is also still present in the coated sheets. As is known, the burr is formed by the plastic flow of the material and the role of the coating applied is to reduce/absorb this flow in order to only have a PMMA burr. The results highlight that the mechanical properties of PMMA coating are not able to fully compensate the pressure lift due to the titanium deformation so that a part of titanium burr is still generated.

4. Conclusions In this work, a new technique to reduce burr generation in titanium micro-drilling was tested. The technique consists of the deposition of a thin layer of PMMA on the top and bottom of a surface before the chip removal process. After, acetone cleaning was realized to remove the PMMA deposited. Spin coating was selected to realize uniform and thin coating. An experimental campaign was executed to measure the effectiveness of the technique with different drill diameters and coating layer thicknesses. The results highlight the efficacy of the technique, and burr height reductions between 40% and 80% were achieved. The founded results open the possibility to improve the surface quality of typical titanium medical devices (as such stents in blood, artificial knees, hips, elbows, dental applications) after a machining process without affecting their biocompatibility. In particular, nowadays different medical devices are produced using additive manufacturing processes that lead to the possibility of customizing the part design; however, additive process need post process machining operations to improve surface roughness or to realize micro features so that a technique able to reduce burr formation is needed. Future studies will be designed to understand the correlation between hole quality and coating thickness better.

Funding: This research received no external funding. Acknowledgments: The authors wish to thank Soldano and SENSOR Lab (Nanotechnologies for Sensing and Advanced Applications) for their effective contributions in sheet coating. Conflicts of Interest: The author declares no conflict of interest. Micromachines 2019, 10, 838 11 of 12

References

1. Masuzawa, T. State of the art of micromachining. Cirp Ann. 2000, 49, 473–488. [CrossRef] 2. Ramsden, J.J.; Allen, D.M.; Stephenson, D.J.; Alcock, J.R.; Peggs, G.N.; Fuller, G.; Goch, G. The design and manufacture of biomedical surfaces. Cirp Ann. 2007, 56, 687–711. [CrossRef] 3. Stirn, B.; Lee, K.; Dornfeld, D. Burr Formation in Micro Drilling. In Proceedings of the Sixteenth Annual Meeting of the American Society for Precision Engineering, Arlington, VA, USA, 2001; pp. 473–476. 4. Stein, J.M.; Dornfeld, D. Burr formation in drilling miniature holes. Cirp Ann. 1997, 46, 63–67. [CrossRef] 5. Neugebauer, R.; Schmidt, G.; Dix, M. Size Effect in Drilling Burr Formation. In Burrs—Analysis, Control and Removal; Springer: Berlin, Germany, 2010. 6. Giorleo, L.; Ceretti, E.; Giardini, C. ALD coated tools in micro drilling of Ti Sheet. Cirp Ann. 2011, 60, 595–598. [CrossRef] 7. Lee, K.; Dornfeld, D. Micro-burr formation and minimization through process control. Precis. Eng. 2005, 29, 246–252. [CrossRef] 8. 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] 9. Gariani, S.; Shyha, I.; Inam, F.; Huo, D. Evaluation of a novel controlled cutting fluid impinging supply system when machining titanium alloys. Appl. Sci. 2017, 7, 560. [CrossRef] 10. Wyen, C.F.; Jaeger, D.; Wegener, K. Influence of cutting edge radius on surface integrity and burr formation in milling titanium. Int. J. Adv. Manuf. Technol. 2013, 67, 589–599. [CrossRef] 11. da Silva, L.C.; da Mota, P.R.; da Silva, M.B.; Sales, W.F.; Machado, Á.R.; Jackson, M.J. Burr height minimization using the response surface methodology in milling of PH 13-8 Mo stainless steel. Int. J. Adv. Manuf. Technol. 2016, 87, 3485–3496. [CrossRef] 12. 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. 2019, 68, 89–92. [CrossRef] 13. Takeyama, H.; Kato, S. Burrless drilling by means of ultrasonic vibration. Cirp Ann. 1991, 40, 83–86. [CrossRef] 14. Hussein, R.; Sadek, A.; Elbestawi, M.A.; Attia, M.H. Elimination of delamination and burr formation using high-frequency vibration-assisted drilling of hybrid CFRP/Ti6Al4V stacked material. Int. J. Adv. Manuf. Technol. 2019, in press. [CrossRef] 15. Pilny, L.; De Chiffre, L.; Pìska, M.; Villumsen, M.F. Hole quality and burr reduction in drilling aluminium sheets. Cirp J. Manuf. Sci. Technol. 2012, 5, 102–107. [CrossRef] 16. Huang, X.; Zheng, L.; Wang, C.; Wang, L.; Lin, D.; Liao, B.; Zhang, L. Drilling Characteristic of Entry Board and the Influence on PCB Microdrilling Process. In Proceedings of the 2016 11th International Microsystems, Packaging, Assembly and Circuits Technology Conference (IMPACT), Taipei, Taiwan, 26–28 October 2016. 17. Drilling and Routing Subcommittee (4–12) of the Fabrication Processes Committee (4-10) of IPC. IPC-DR-572A Standard: Drilling Guidelines for Printed Boards; Association Connecting Electronics Industries: Bannockburn, IL, USA, 2007. 18. Kim, D.W.; Lee, Y.S.; Oh, Y.T.; Chu, C.N. Prevention of exit burr in microdrilling of metal foils by using a cyanoacrylate adhesive. In. J. Adv. Manuf. Technol. 2006, 27, 1071–1076. [CrossRef] 19. Zhang, L.-C.; Chen, L.-Y. A review on biomedical titanium alloys: Recent progress and prospect. Adv. Eng. Mater. 2019, 21, 1801215. [CrossRef] 20. Geetha, M.; Singh, A.K.; Asokamani, R.; Gogia, A.K. Ti based biomaterials, the ultimate choice for orthopaedic implants—A review. Prog. Mater. Sci. 2009, 54, 397–425. [CrossRef] 21. Long, M.M.; Rack, H.J. Titanium alloys in total joint replacement—A materials science perspective. Biomaterials 1998, 19, 1621–1639. [CrossRef] 22. Gepreel, M.A.H.; Niinomi, M.S. Biocompatibility of Ti-alloys for long-term implantation. J. Mech. Behav. Biomed. Mater. 2013, 20, 407–415. [CrossRef] 23. Burroughs, J.H.; Bradley, D.D.C.; Brown, A.R.; Marks, R.N.; Mackay, K.; Friend, R.H.; Burns, P.L.; Holmes, A.B. Light-emitting diodes based on conjugated polymers. Nature 1990, 347, 539–541. [CrossRef] 24. Mihi, A.; Ocana, M.; Miguez, H. Oriented colloidal-crystal thin films by spin-coating microspheres dispersed in volatile media. Adv. Mater. 2006, 18, 2244–2249. [CrossRef] Micromachines 2019, 10, 838 12 of 12

25. Damon, G.F. The Eﬀect of Whirler Acceleration on the Properties of the Photoresist Film. In Proceedings of the Kodak Seminar on Microemulsion, Rochester, NY, USA, 1967. 26. Klocke, F.; Gerschwiler, K.; Abouridouane, M. Size eﬀects of micro drilling in steel. Prod. Eng. 2009, 3, 69–72. [CrossRef] 27. Min, S.; Kim, J.; Dornfeld, D. Development of a drilling burr control chart for low alloy steel AISI 4118. J. Mater. Proc. Technol. 2001, 113, 4–9. [CrossRef] 28. Kim, J.; Min, S.; Dornfeld, D.A. Optimization and control of drilling burr formation of AISI 304L and AISI 4118 based on drilling burr control charts. Int. J. Mach. Tools Manuf. 2001, 41, 923–936. [CrossRef]

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