Application of Ultra-Small Micro Grinding and Micro Milling Tools: Possibilities and Limitations

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Article Application of Ultra-Small Micro Grinding and Micro Milling Tools: Possibilities and Limitations

Benjamin Kirsch *, Martin Bohley, Peter A. Arrabiyeh and Jan C. Aurich Institute for Manufacturing Technology and Production Systems, University of Kaiserslautern, P.O. Box 3049, 67653 Kaiserslautern, Germany; [email protected] (M.B.); [email protected] (P.A.A.); [email protected] (J.C.A.) * Correspondence: [email protected]; Tel.: +49-631-205-3770

Received: 18 July 2017; Accepted: 7 August 2017; Published: 24 August 2017

Abstract: Current demands for flexible, individual microstructures in high quality result in high requirements for micro tools. As the tool size defines the minimum structure size, ultra-small tools are needed. To achieve tool diameters of 50 µm and lower, we investigate the complete manufacturing chain of micro machining. From the development of the machine tools and components needed to produce and apply the micro tools, the micro tools themselves, as well as the micro machining processes. Machine tools are developed with the possibility of producing the micro geometry (cutting edge design) of micro tools as well as plating processes to produce super abrasive micro grinding tools. Applying these setups, we are able to produce ultra-small micro grinding and micro milling tools with typical diameters of 50 µm and down to 4 µm. However, the application of such tools is very challenging. The article presents possibilities and limitations in manufacturing the micro tools themselves as well as microstructures made with these tools. A special emphasis will be on the influence of the tool substrate in micro milling and grain sizes in micro grinding.

Keywords: micro pencil grinding tools; micro end mills; electroless plating

1. Introduction A component’s surface characteristics are crucial for its functionality, durability, and the quality at which it performs the task it is designed for. The surface of a component determines how the component interacts with its environment [1]. This interaction can be demonstrated by changing the corrosive nature of a steel workpiece by adding alloying elements or coating its surface [2]. Heat treatment can increase a components hardness, change its metallurgical microstructure or reduce residual stresses [3]. Another way to influence the mechanical and physical properties of a component is manufacturing geometrically defined microstructures on its surface [4]. The wear of components such as chain joints can be drastically reduced by machining microstructures into their chain pins. The microstructures on the pins can hold lubricant, increase its wettability, and in turn reduce the joint friction between the two components [5]. Micro structures can also influence the mechanical properties of micro components. A study conducted by Godart et al., showed that 50 µm wide micro milled microstructures with a depth of 10–20 µm could increase tensile strength and decrease the fracture elongation in commercially pure-titanium workpieces [6]. Micro components with functionally optimized surfaces are used in bioreactors to increase bacterial adhesion and increase bacterial growth [7] or in medical applications to increase the lifetime of implants or cardiac pacemakers [8]. The precision industry takes advantage of these properties to manufacture lighter and smaller micro parts with more functions for given product size [9]. To meet the demand of precision industries such as the biomedical, telecommunication, aerospace, and electronics industries, processes are modified or new processes developed [10]. With regard to economic success, both technological advantages as well as a competitive price level are necessary. Processes like etching,

Micromachines 2017, 8, 261; doi:10.3390/mi8090261 www.mdpi.com/journal/micromachines Micromachines 2017, 8, 261 2 of 18 micro molding and Focused Ion Beam (FIB) are used to mass produce micro parts and components [11]. Due to the high accuracy, low achievable surface roughness, and very high geometrical flexibility of produced microstructures, machining processes like micro milling [12] and micro grinding are much more suitable for small batch production on the micron scale compared to mass production processes [13]. Micro pencil grinding tools (MPGTs) and single edged micro end mills made of cemented carbide and manufactured via precision grinding are available in various geometrical shapes and can be scaled down to 4 µm for micro pencil grinding tools [14] and 10 µm for micro end mills [15]. Micro pencil grinding tools consist of a cemented carbide basic body and an abrasive body. The abrasive body consists of superabrasive grits (diamond or cubic boron nitride (cBN)) and the bond; in this case the bond is a nickel coating. Micro grinding uses much lower feed rates than micro milling, but covers harder and brittle materials [11]. No references exist that evaluate the influence of the cemented carbide grain size on the performance of micro end mills. This paper will deal with the influence of the tungsten carbide grain size on the applicability of micro end mills with effective milling cutter diameters of 50 µm. Concerning micro grinding, the electroless plating method to produce the abrasive body will be evaluated. While the plating method itself is not new, only little reference can be found where it was applied to micro grinding tools [16]. The influence of the size of the abrasive grits on the performance of MPGTs and the boundaries of electroless plating will be evaluated.

2. Materials and Methods

2.1. Micro Milling Center The manufacturing and the application of the ultra-small micro end mills (USM-mills) [17] was conducted on a precision three axes machine tool. This machine tool, called micro milling center (MMC) [18], was developed at our lab. The key aspect of this machine tool is that it enables to produce and use a micro end mill without reclamping. Thus, the run-out error is reduced by the reclamping error. The design of the machine tool follows the idea of a small machine tool for the production of small parts and structures. The whole machine tool is desktop sized covering an installation space of only 760 mm × 675 mm × 500 mm. Due to this small size, small axes with low travel and low moving masses could be implemented, resulting in high velocities and low energy consumption. The machine tool consists of three functional units: the main spindle unit, the tool grinding unit, and the application unit (Figure1). The main spindle unit is mounted on a precision linear axis with a travel of 560 mm connecting the tool grinding unit and the application unit. The main spindle itself is mounted on an Aerotech ANT130-110-L-Plus (Aerotech Inc, Pittsburgh, PA, USA) axis with a travel of 110 mm, a resolution of 1 nm and a positioning accuracy of ± 0.25 µm. This axis conducts the precise Z-movement (in axial spindle/tool direction). The main spindle is an ABL MM125 (Air Bearings Ltd, Poole, UK)air bearing spindle with a maximum rotational speed of 125,000 rpm and a run-out of 2–4 µm in a range of 10,000 to 45,000 rpm. The spindle as well as the spindle mount is liquid cooled with a temperature stability of 0.1 K. For manufacturing the tools (compare Section 2.3), the spindle can be precisely positioned (rotatory) with a stepping motor and a belt drive. The tool grinding unit is built on a granite linear stage with a travel of 150 mm, a resolution of 1 nm and an accuracy of ±0.3 µm. On this granite stage, an air bearing rotation stage is mounted which can carry up to four hydrodynamic spindle motors with diamond grinding wheels for tool manufacturing. These spindle motors are powered by 6.4 W brushless DC motors (BLDC) and have a maximum rotational speed of 12,500 rpm and a run-out error of <0.8 µm peak-to-peak. The application unit is mounted on two Aerotech ANT130-110-L Plus axes. As the Y-axis moves in a vertical direction, it needs to be ﬁtted with Airpel (Airpot Corporation, Norwalk, CT, USA) air bearing counterweights to limit the needed motor power and to reduce heat generation for a higher Micromachines 2017, 8, 261 3 of 18 machining precision. Both axes have a travel of 110 mm, a resolution of 1 nm, and a positioning ± µ accuracyMicromachines of 20170.375, 8, 261m. 3 of 18

FigureFigure 1. Micro milling center (MMC) after after [18]. [18].

2.2. Precision 4-Axes Machine The precision 4-axes machine tool was used for the application of the micro pencil grinding tools (MPGTs), Figure 2. This machine tool is built up on a massive granite bed with a moving table design. This table consists of two air bearing axes in X- and Y-direction. They are powered by stepper motors

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2.2. Precision 4-Axes Machine The precision 4-axes machine tool was used for the application of the micro pencil grinding tools (MPGTs),Micromachines Figure 2017, 28., 261 This machine tool is built up on a massive granite bed with a moving table design.4 of 18 This table consists of two air bearing axes in X- and Y-direction. They are powered by stepper motors andand ballball screwsscrews atat a resolution of 2.54 nm. With th thisis configuration, conﬁguration, a travel travel of of 100 100 mm mm in in each each direction direction withwith aa positioning accuracy accuracy of of <1 <1 µmµm is is possible. possible. The The Z-axis Z-axis carries carries the therotational rotational axis axis as well as well as the as theair bearing air bearing main main spindle. spindle. This ThisZ-axis Z-axis is cross is crossroller rollerbearing bearing guided guided and driven and drivenwith a withstepper a stepper motor motorin combination in combination with awith ball ascrew. ball screw. The travel The travelis 100 ismm 100 with mm a with positioning a positioning accuracy accuracy of <1 ofµm. <1 Theµm. Therotational rotational axis is axis a harmonic is a harmonic drive servo drive system servo system with a resolution with a resolution of 0.00045°. of 0.00045 With this◦. Withaxis, the this main axis, thespindle main can spindle be tilted can bein tilteda range in aof range ±30°. ofThe± 30air◦ .bearing The air bearingmain spindle main spindlehas a maximum has a maximum spindle spindle speed speedof 54,000 of 54,000rpm. The rpm. run-out The run-out is rising is from rising 3 µm from at 3theµm minimal at the minimal spindle spindlespeed of speed5000 rpm of 5000 to 6 rpmµm at to 654,000µm at rpm 54,000 [19]. rpm [19].

Figure 2. Precision 4-axes machine after [19]. [19].

2.3.2.3. ManufacturingManufacturing ofof Micro End Mils (MEMs) TheThe micromicro toolstools werewere manufacturedmanufactured via grinding. Advantages of of grinding grinding are are the the short short process process time,time, thethe excellentexcellent achievableachievable quality,quality, andand smallsmall diametersdiameters of the micro tools. The The manufacturing manufacturing was was donedone accordingaccording toto thethe methodmethod proposed in [[15],15], comparecompare Figure 33.. InIn SectionSection I, a cylindrical tungsten carbidecarbide tooltool blankblank isis ground with a 4040°◦ cone at th thee top top on on a a conventional conventional tool tool grinding grinding machine machine tool. tool. AllAll subsequentsubsequent steps are are conducted conducted on on the the MMC MMC descri describedbed in inSection Section 2.1. 2.1 The. The pre-grinding pre-grinding of the of tip the tipcylinder cylinder (Section (Section II) II)is isconducted conducted with with a adiam diamondond grinding grinding wheel wheel (mesh (mesh #800, #800, width width 250 250 µm,µm, diameterdiameter 5858 mm).mm). The tip cylinder of the pre-grinding process is is adapted adapted to to the the desired desired tool tool diameter diameter inin thisthis casecase 5050 µµm.m. With a fine-grainedfine-grained diamond grinding wheel (mesh (mesh #4800, #4800, width width 50 50 µm,µm, diameter diameter 5858 mm),mm), thethe finalfinal singlesingle edgeedge micro end mill is fabricated (Section III) III).. In this step, step, the the actual actual cutting cutting tooltool geometry,geometry, namelynamely thethe cuttingcutting edge geometry, thethe flank, flank, and the the rake rake face, face, etc., etc., are are manufactured. manufactured. TheThe facetingfaceting ofof thethe flankflank faceface isis adjusted to the in intendedtended feed per tooth and and the the run-out run-out of of the the spindle spindle toto assureassure thatthat thethe flankflank faceface doesdoes notnot contactcontact thethe workpiece.workpiece.

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Figure 3. Schematic of the Tool grinding process and nomenclature after [15,17]. [15,17].

2.4. Manufacturing of Micro Pencil Grinding Tools (MPGTs) 2.4. Manufacturing of Micro Pencil Grinding Tools (MPGTs) MPGTs, just like common high performance grinding wheels, consist of a basic body and the MPGTs, just like common high performance grinding wheels, consist of a basic body and abrasive body; the abrasive body consist of the abrasive grains or grits and the metallic bond [20]. A the abrasive body; the abrasive body consist of the abrasive grains or grits and the metallic bond [20]. number of methods have been used to bind these abrasive grits to the basic body. One of the more A number of methods have been used to bind these abrasive grits to the basic body. One of the more commonly used ones is a sintering method that uses high temperatures and pressures up to 20 MPA commonly used ones is a sintering method that uses high temperatures and pressures up to 20 MPA to to manufacture abrasive layers that bind the abrasive grits with a metallic matrix (commonly bronze). manufacture abrasive layers that bind the abrasive grits with a metallic matrix (commonly bronze). The metallic matrix covers the abrasive grits completely. Thus, a dressing process is required to create The metallic matrix covers the abrasive grits completely. Thus, a dressing process is required to create the required grit protrusion [21]. The process can manufacture micro pencil grinding tools down to the required grit protrusion [21]. The process can manufacture micro pencil grinding tools down to 180 µm [22]. 180 µm [22]. Gäbler et al. used an chemical vapor deposition method (CVD) to manufacture an abrasive layer Gäbler et al. used an chemical vapor deposition method (CVD) to manufacture an abrasive for micro grinding tools by depositioning a synthetic, polycrystalline diamond layer from a gas layer for micro grinding tools by depositioning a synthetic, polycrystalline diamond layer from a gas atmosphere [23]. This method can produce tools with much smaller diameters, Hoffmeister et al. atmosphere [23]. This method can produce tools with much smaller diameters, Hoffmeister et al. manufactured micro pencil grinding tools with diameters down to 50 µm. In addition, the method manufactured micro pencil grinding tools with diameters down to 50 µm. In addition, the method can can be applied to diverse basic body geometries. The diamond crystals are fine grained and have besharp applied cutting to diverseedges that basic cannot body be geometries. produces using The diamond other methods crystals [24]. are However, fine grained this and leads have to rapid sharp cuttingtool clogging edges and that in cannot turn to be tool produces breakage using [25]. other methods [24]. However, this leads to rapid tool cloggingAnother and inoption turn tofor tool manufacturing breakage [25 micro]. pencil grinding tools can be achieved by binding the abrasiveAnother grits option to the forabrasive manufacturing with an electrolyt micro pencilic nickel grinding layer. tools Two can methods be achieved have bybeen binding used theto abrasivemanufacture grits these to the tools; abrasive an electrop withlating an electrolytic method [26] nickel and an layer. electroless Two methods plating method have been [16]. usedIn both to manufacturemethods the thesetool is tools; immersed an electroplating into a plating method solution [26 containing] and an electroless nickel ions plating (Ni+2 method). The surface [16]. In of both the 2 methodsbasic body the needs tool is to immersed provide with into athe plating missing solution electrons containing in order nickel to manufact ions (Ni+ure). a Thenickel surface layer of[27]. the basicElectroplating body needs connects to provide the basic with body the missing to an electr electronsic cycle in in order which to manufacturethe tool functions a nickel as a layer cathode [27]. Electroplatingand a nickel resource connects that the is basic immersed body to in an the electric plating cycle solution in which functions the tool as functionsan anode. as Once a cathode a current and ais nickel provided, resource an electrical that is immersedfield is produced, in the platingcausing solutionthe nickel functions ions to gravitate as an anode. towards Once the basic a current body. is provided,In addition, an the electrical anode fieldprovides is produced, electrons causing to the ba thesic nickel body ionswhile to replenishing gravitate towards the plating the basic solution body. Inwith addition, new nickel the anodeions [28]. provides Electrol electronsess plating to provides the basic uses body an while reducing replenishing agent to provide the plating nickel solution affine withsurfaces new with nickel the ions missing [28]. Electrolesselectrons [16]. plating The providesadjustable uses grit an protrusion reducing agentfor both to providemethods nickel eliminates affine

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Micromachines 2017, 8, 261 6 of 18 surfaces with the missing electrons [16]. The adjustable grit protrusion for both methods eliminates trueingtrueing andand dressingdressing stepssteps thatthat are necessarynecessary for other plating methods like like sintering. sintering. However, However, for for thisthis paperpaper thethe electrolesselectroless platingplating methodmethod was chosenchosen because it generally produces produces more more uniform uniform abrasiveabrasive layerslayers withwith aa higherhigher binderbinder hardness [[27].27]. ForFor thisthis paper,paper, thethe basicbasic bodybody introducedintroduced in Figure 33 (cylindrical(cylindrical tungstentungsten carbidecarbide tool blank) is used.used. TheThe MPGTsMPGTs undergoundergo thethe same same procedure procedure as as the the MEMs MEMs up up to to Section Section II II (compare (compare Section Section 2.3 2.3)) to ato ﬁnal a final length length of the of tipthe cylinder tip cylinder of 150 ofµ 150m. Theµm. diameter The diameter of the of tip the cylinder tip cylinder is adjusted is adjusted to theprocess to the andprocess the gritand sizethe ofgrit the size MPGT. of the For MPGT. instance, For when instan ace, ﬁnal when diameter a final of 50diameterµm is toof be50 manufactured µm is to be withmanufactured an average with grit an size average of 2 µm grit and size a monolayer of 2 µm and is toa monolayer be achieved, is ato diameter be achieved, of 46 aµ mdiameter is ground of 46 on theµm tipis ground cylinder on (see the Figure tip cylinder4a). (see Figure 4a).

FigureFigure 4. (a) Schematic of electroless plated MPGT tip; tip; ( (b)) schematic schematic of of electroless electroless plating plating setup. setup.

TheThe abrasiveabrasive body is manufactured via electroless plating. plating. This This is is done done in in the the plating plating bath, bath, containingcontaining all ingredients ingredients of of the the abrasive abrasive body body to tobe bemanufactured. manufactured. Before Before electroless electroless plating, plating, the thesubstrate substrate is degreased is degreased at a at temperature a temperature of of80 80°C◦ inC ina 200 a 200 g/L g/L sodium sodium hydroxide hydroxide solution, solution, etched etched in ina a1.5 1.5 g/L g/L hydrochloric hydrochloric acid acid solution solution for for cleaning, cleaning, and and then then electroplated electroplated with with a a thin thin adhesive adhesive nickel nickel layer in a solution made of 250 g/L nickel chloride and 10 g/L hydrochloric acid. The thin nickel layer layer in a solution made of 250 g/L nickel chloride and 10 g/L hydrochloric acid. The thin nickel layer is necessary for the tungsten carbide surface to respond quicker to the following electroless plating. is necessary for the tungsten carbide surface to respond quicker to the following electroless plating. Sodium hypophosphite is used to reduce the metal ions in the plating solution to enable the metal Sodium hypophosphite is used to reduce the metal ions in the plating solution to enable the metal deposition onto the substrate. A detailed description of the chemical background can be found in [27]. deposition onto the substrate. A detailed description of the chemical background can be found in [27]. Table 1 lists the components of the plating solution and Figure 4b shows the experimental setup for Table1 lists the components of the plating solution and Figure4b shows the experimental setup for plating. The grits are whirled up using a magnetic stirrer with a rotation speed of 60 rpm. The grits plating. The grits are whirled up using a magnetic stirrer with a rotation speed of 60 rpm. The grits adhere to the substrate surface and are coated with a nickel layer. The substrate rotates at 1–3 rpm adhere to the substrate surface and are coated with a nickel layer. The substrate rotates at 1–3 rpm (lower values for larger grit sizes) to gather grits on the whole circumference [27]. (lower values for larger grit sizes) to gather grits on the whole circumference [27].

Table 1. Composition of electroless plating solution. Table 1. Composition of electroless plating solution. Component Concentration (g/L) Nickel sulfateComponent (NiSO4·6H2O) Concentration 30 (g/L) Sodium hypophosphite (NaH2PO2) 20 Nickel sulfate (NiSO4·6H2O) 30 2 3 2 SodiumSodium hypophosphite acetate (C H NaO (NaH) 2PO2) 20 20 SodiumThiourea acetate (CH (C4N22HS)3 NaO2) 20 0.0004 HydrochloricThiourea acid (CH 4(HCl)N2S) Adapted 0.0004 to pH-value HydrochloricAbrasive grits acid (HCl) Adapted to pH-value 0–50 Abrasive grits 0–50 There are two characteristic values of the abrasive body: the grit concentration (grits per area) and the grit protrusion (to assure chip space and space for the metal working fluid). The grit concentration is controlled by the amount/concentration of grits in the plating bath, and the grit protrusion by the

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MicromachinesThere are 2017 two, 8, 261 characteristic values of the abrasive body: the grit concentration (grits per area)7 of and 18 the grit protrusion (to assure chip space and space for the metal working ﬂuid). The grit concentration embedding time. Using the components listed in Table 1, a nickel growth of 21 µm/h was measured. is controlled by the amount/concentration of grits in the plating bath, and the grit protrusion by the The embedding time can hence be calculated to reach the desired plating thickness and grit embedding time. Using the components listed in Table1, a nickel growth of 21 µm/h was measured. protrusion. Common values taken from macro grinding wheels are plating thicknesses of 50–70% of The embedding time can hence be calculated to reach the desired plating thickness and grit protrusion. the average grit size. This value is a balance between grain retention forces and sufficient chip space. Common values taken from macro grinding wheels are plating thicknesses of 50–70% of the average Plating times and grit concentrations are listed in Section 3.2. The plating time consists of the plating grit size. This value is a balance between grain retention forces and sufﬁcient chip space. Plating times time needed to gather the grits for the tool (main plating time) and the time needed to adjust the grit and grit concentrations are listed in Section 3.2. The plating time consists of the plating time needed protrusion (embedding time). During the embedding step, the stirring motion in the plating solution tois turned gather off the to grits stop for the the grit tool motion (main in the plating beaker time), so no and further the time grains needed adhere to to adjust the substrate. the grit protrusion (embedding time). During the embedding step, the stirring motion in the plating solution is turned off to3. stopResults the grit motion in the beaker, so no further grains adhere to the substrate.

3. Results 3.1. Electroless Plating of Micro Pencil Grinding Tools 3.1. ElectrolessMPGTs with Plating different of Micro grit Pencil sizes Grinding were produced Tools . The substrates (tool blanks) were made of tungstenMPGTs carbide with different(WC content: grit sizes 92%, were Co content: produced. 8% The, grain substrates size: 0.2(tool µm). blanks) Figure were5 shows made MPGTs of tungsten with different grit sizes; Table 2 lists the plating parameters used to manufacture these tools. For each carbide (WC content: 92%, Co content: 8%, grain size: 0.2 µm). Figure5 shows MPGTs with different nominal grit size, the diameter of the substrates was prepared to reach an effective diameter of ~50 grit sizes; Table2 lists the plating parameters used to manufacture these tools. For each nominal grit µm after plating. size, the diameter of the substrates was prepared to reach an effective diameter of ~50 µm after plating.

FigureFigure 5.5. Electroless plated MPGTs with grit sizes sizes ( (aa)) 1–2 1–2 µm;µm; ( (bb)) 2–4 2–4 µm;µm; ( (cc)) 3–6 3–6 µm;µm; ( (dd)) 5–10 5–10 µm;µm; ((ee)) 6–126–12 µµm,m, andand ((ff)) 8–168–16 µµm.m.

Table 2. Plating parameters for MPGTs with a 50 µµmm tool.

Grit Size Main Plating Time Embedding Time Grit Concentration (Solution) CaseCase Grit Size (µm) Main Plating Time (Min) Embedding Time (Min) Grit Concentration (Solution) (g/L) (a) 1–2(µm) (Min) 20 ± 3 (Min) 0.5–1 (g/L) 0.5–1 (b)(a) 2–4 1–2 20 20 ±± 33 0.5–1 1.5–2 0.5–1 4 (c)(b) 3–6 2–4 20 19 ±± 33 1.5–2 2–3 4 8 (d) 5–10 19 ± 3 3–4 15 (c) 3–6 19 ± 3 2–3 8 (e) 6–12 18 ± 3 4 20 (f)(d) 8–16 5–10 19 18 ±± 33 3–4 5 15 23 (e) 6–12 18 ± 3 4 20 (f) 8–16 18 ± 3 5 23

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The electroless plating process is limited by the solution lifetime. We observed that a solution The electroless plating process is limited by the solution lifetime. We observed that a solution can can spontaneously decompose after 60–120 minutes, completely depleting the nickel ions in the spontaneously decompose after 60–120 minutes, completely depleting the nickel ions in the solution. solution. This time frame can be influenced by the thiourea concentration in the solution. It was This time frame can be influenced by the thiourea concentration in the solution. It was documented documented that a larger thiourea concentration increases the lifetime of the solution, but negatively that a larger thiourea concentration increases the lifetime of the solution, but negatively influences influences the plating quality and the nickel growth. A too high thiourea concentration can inhibit the plating quality and the nickel growth. A too high thiourea concentration can inhibit the nickel the nickel growth completely [27]. This time limitation complicates the plating process for bigger grit growth completely [27]. This time limitation complicates the plating process for bigger grit sizes, sizes, as those necessitate higher embedding times. as those necessitate higher embedding times. Generally speaking, the grit size is the greatest limitation to the plating process. Plating tools Generally speaking, the grit size is the greatest limitation to the plating process. Plating tools with with smaller grits is good to control; the grits have a larger area to populate at the substrate. This can smallerbe seen grits in Figure is good 5. toThe control; uniformity the grits of the have grit a largerdistribution area to diminishes populate at with the substrate.rising grit Thissizes. can The be seenbiggest in Figure examined5. The grit uniformity size, 8–16 ofµm, the is grit too distributionlarge for the diminishesgiven substrate with diameter rising grit of sizes.36 µm The to achieve biggest µ µ examineda feasible grit grit size, concentration. 8–16 m, is tooIn largeaddition, for the larger given grits substrate require diameter a much of 36largerm togrit achieve concentration a feasible gritincreasing concentration. the costs In addition,of the manufacturing larger grits require proces as much tremendously. larger grit concentrationThe minimal increasinggrit concentration the costs ofrequired the manufacturing to manufacture process an MPGT tremendously. with a grit The size minimal of 8–16 µm grit is concentration 23 g/L. In comparison, required towe manufacture found that anfor MPGT a grit withsize aof grit 1–2 size µm, of a 8–16concentrationµm is 23 g/L. of 0.15 In comparison,g/L is sufficient we foundto manufacture that for a gritan MPGT size of 1–2withµ m,a adiameter concentration of 50 µm of 0.15[27]. g/LExperimental is sufficient results to manufacture revealed the angrit MPGT size of with 6–12a µm diameter (Figureof 5e) 50 toµ bem the [27 ]. Experimentallargest grit size results to deliver revealed MPGTs the gritwith size a sufficient of 6–12 µgritm (Figureconcentration.5e) to be the largest grit size to deliver MPGTs with a sufficient grit concentration. 3.2. A Case Study in Micro Grinding 3.2. A Case Study in Micro Grinding The grit size not only is the most crucial factor for the manufacturing of the MPGTs, but also for theirThe application. grit size not To onlydemonstrate is the most this, crucial MPGTs factor coated for thewith manufacturing the grit sizes 1–2 of the µm MPGTs, and 3–6 but µm also were for theirapplied application. with different To demonstrate process parameters this, MPGTs in a ca coatedse study with (see the Table grit 3). sizes The 1–2 workpieceµm and material 3–6 µm werewas applied16 MnCr5, with hardened different to process 665 ± parameters15 HV30. After in a clam caseping, study the (see workpiece Table3). Thewas workpiece face ground material to achieve was 16maximum MnCr5, hardenedevenness. toA 665water-soluble± 15 HV30. metal After working clamping, fluid the was workpiece applied at was a flow face groundrate of 100 to achievemL/h. maximumFigure 6 shows evenness. the process A water-soluble kinematics metalfor the working case study. fluid was applied at a flow rate of 100 mL/h. Figure6 shows the process kinematics for the case study. Table 3. Parameters used in the case study. Table 3. Parameters used in the case study. Grit Size Feed Rate Rotational Speed Depth of Cut Groove Length Case Case Grit Size(µm) (µ m) Feed(mm/min) Rate (mm/min) Rotational(rpm) Speed (rpm) Depth(µm) of Cut (µm) Groove(mm) Length (mm) 1 1–2 0.1 30,000 5 1 1 1–2 0.1 30,000 5 1 22 3–63–6 0.1 0.1 30,000 30,000 5 5 1 1 33 1–21–2 0.2 0.2 50,000 50,000 5 5 1 1 44 3–63–6 0.2 0.2 50,000 50,000 5 5 1 1

FigureFigure 6.6. Micro grinding process kinematics.

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TheThe experimentsexperiments werewere performedperformed on the precision milling milling machine machine presented presented in in Figure Figure 2.2. GroovesGrooves withwith a length of of 1 1 mm mm at at a adepth depth of ofcut cut of of5 µm 5 µ werem were machined machined for each for each case casepresented. presented. All Allexperiments experiments and and measurements measurements were were done done thrice thrice.. Figure Figure 7 7shows shows the the MPGTs MPGTs after after machining machining and and the entry point in the workpiece of the respective grooves. the entry point in the workpiece of the respective grooves.

FigureFigure 7. 7.MPGTs MPGTs afterafter machiningmachining (leftleft column), entry area area of of grooves grooves ( (middlemiddle column column) )and and process process conditionsconditions ( right(right columncolumn),), forfor casecase 11 ((aa),), casecase 22 ((b), case 3 ( c) and case 4 4 ( (dd).).

TheThe casescases 1,1, 2,2, andand 44 werewere performedperformed successfully with all all nine nine tools tools (including (including statistical statistical repetitions)repetitions) intactintact afterafter machiningmachining the grooves (Figur (Figuree 77a,b,d).a,b,d). In contrast, only one one out out of of the the three three tools of case 3 managed to machine the complete groove length successfully. Figure7c shows one of

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tools of case 3 managed to machine the complete groove length successfully. Figure 7c shows one of thethe MPGTsMPGTs that failed to to complete complete the the entire entire groov groovee length. length. That That is, the is, theMPGTs MPGTs with with grit gritsizes sizes of 1–2 of 1–2µmµ performm perform poorly poorly at higher at higher feed feed rates rates and and higher higher rotational rotational speeds. speeds. Higher Higher feed feed rates rates result result into into higherhigher chip thicknesses and and hence hence higher higher loads loads of of the the grains grains and and the the abrasive abrasive body. body. In that In thatsense, sense, the thenegative negative influence influence of higher of higher feed feed rates rates on on the the pe performancerformance of of the the tools tools is is expectable. expectable. In In contrast, contrast, higherhigher rotationalrotational speeds speeds result result into into higher higher cutting cu speedstting speeds and hence and lower hence chip lower thicknesses, chip thicknesses, commonly improvingcommonly machiningimproving conditionsmachining in conditions grinding. in However, grinding. with However, the given with kinematics the given the kinematics cutting speed the dropscutting to speed zero towardsdrops to zero thecenter towards of the the center tools. of The the grits tools. at The the grits center at the of the center tool of tip the hence tool tip only hence rub andonly ploughrub and and plough generate and generate heat. At heat. higher At higher feed rates, feed rates, the grits the grits towards towards the centerthe center of the of the tool tool tip becometip become more more loaded, loaded, obviously obviously resulting resulting in bond in overload. bond overload. Another Another factor is thefactor thermal is the expansion thermal coefficient.expansion coefficient. This coefficient This iscoeffici muchent larger is much for the larger nickel for bondthe nickel than forbond cemented than for carbidecemented basic carbide body, causingbasic body, stresses causing between stresses the between substrate the and substrate the bond. an Obviously,d the bond. all Obviously, that is compensated all that is compensated by larger grit sizes,by larger as case grit 4sizes, performed as case well, 4 performed which can well, also wh beich seen can in also the be resulting seen in bottomthe resulting surface bottom roughness surface of theroughness grooves of (Figure the grooves8). (Figure 8).

Figure 8. Bottom surface roughness of fi ﬁrstrst 100 µmµm groove length.

TheThe bottombottom surfacesurface roughnessroughness of the grooves was measured at the respective entrance entrance are are for for thethe fourfour casescases (Figure 88).). The measurement was was do donene according according to to DIN DIN EN EN ISO ISO 4287 4287 [29], [ 29 i.e.,], i.e., a ameasurement measurement length length of of 400 400 µmµm in in direction direction of of the the maximum maximum roughness roughness profile profile was was maintained; maintained; exceptexcept forfor casecase 3,3, where a lower measurement length had had to to be be used used as as the the groove groove length length was was shorter. shorter. ThreeThree mainmain conclusions conclusions can can be be drawn: drawn: higher higher rotational rotational speeds speeds result result in a more in astable more processstable (smallerprocess standard(smaller standard deviations) deviations) and a lower and surface a lower roughness, surface roughness, and higher and grit higher sizes resultgrit sizes in a result lower in roughness. a lower roughness.The more stable behavior at higher rotational speeds may be caused by the spindle characteristics (characteristicThe more excitation stable behavior in the axial at direction)higher rotational rather thanspeeds by toolmay specifications, be caused by and the should spindle not becharacteristics overrated. (characteristic A lower roughness excitation at higher in the rotationalaxial direction) speeds rather is the than result by tool of smallerspecifications, uncut chipand thicknesses,should not be a well-knownoverrated. A interdependencylower roughness fromat higher macro rotational grinding speeds processes. is the Aresult rather of unusualsmaller uncut result ischip the thicknesses, positive influence a well-known of higher interdependency grit sizes. In macro from grinding,macro grinding those resultprocesses. in a higherA rather roughness unusual dueresult to ais risingthe positive uncut chipinfluence thicknesses. of higher Here, grit it sizes. is vice In versa. macro This grinding, could be those traced result back in to a different higher roughness due to a rising uncut chip thicknesses. Here, it is vice versa. This could be traced back to material removal mechanisms. Possibly, the smaller grit sizes and chip space could result in a more different material removal mechanisms. Possibly, the smaller grit sizes and chip space could result in ploughing-dominated material removal regime rather than cutting-dominated, thus diminishing the a more ploughing-dominated material removal regime rather than cutting-dominated, thus surface finish. Also, glazing of the tools could intensify the ploughing domain. Smaller grit sizes entail diminishing the surface finish. Also, glazing of the tools could intensify the ploughing domain. smaller chip spaces and hence are more prone to glazing. In addition, larger grits provide higher grain Smaller grit sizes entail smaller chip spaces and hence are more prone to glazing. In addition, larger retention forces. Wear flats are generated on the grits resulting into smaller chip thicknesses and hence grits provide higher grain retention forces. Wear flats are generated on the grits resulting into smaller a lower surface roughness. The smaller grits break out before wear flats are generated and hence larger chip thicknesses and hence a lower surface roughness. The smaller grits break out before wear flats chip thicknesses result. are generated and hence larger chip thicknesses result.

Micromachines 2017, 8, 261 11 of 18 Micromachines 2017, 8, 261 11 of 18

3.3. Manufacturing of Ultra SmallSmall Diameter Micro Milling Tools To demonstratedemonstrate thethe possibilitiespossibilities andand limitationslimitations of manufacturingmanufacturing USM-mills, tools with an effective milling cutter diameter of 10 µµmm andand 5050 µµmm werewere manufacturedmanufactured (Figure(Figure9 9).). TheThe detaileddetailed view reveals some limitations when reducing the tooltool diameter. While the tool with diameter 50 µµmm provides sharpsharp cutting cutting edges edges and and the the faces faces appear appear quite quite smooth, smooth, the cutting the cutting edges of edges the small of the diameter small tooldiameter appear tool blunt appear and blunt the grinding and the groovesgrinding can grooves clearly can be clearly seen. Thisbe seen. shows This that shows the tool that diameter the tool cannotdiameter be cannot reduced be byreduced only scaling by only the scaling tool geometry. the tool geometry. The tool’s The micro tool’s geometry micro geometry has to be has adapted to be toadapted the tool to diameter.the tool diameter. Also, the Also, tool substratethe tool substrat (namelye (namely the grain the size grain of the size cemented of the cemented carbide) carbide) and the manufacturingand the manufacturing parameters parameters have to behave adjusted to be adjusted in order in to order master to the master occurring the occurring size effects. size effects.

Figure 9. 9. MicroMicro End End mills mills with with different different diameters diameters (a1 and (a1 a2and) 10a2 µm) 10 (b1µ andm ( b1b2)and 50 µmb2 )at 50 differentµm at differentmagnifications. magniﬁcations.

3.4. A Case Study in Micro milling—Influnecemilling—Influnece of Cemented Carbide SpecificationsSpecifications In macro machining, the specificationspecification of the material a tool is made of has a high influenceinfluence on the performance of of the the tool. tool. In In micro micro machining, machining, this this influence influence can can be be expected expected to tobe beeven even higher, higher, as asthe the tool’s tool’s cutting cutting edge edge has hasto be to very be very sharp sharp in micro in micro machining. machining. This is This due is to due the to fact the that fact very that small very smalluncut uncutchip thicknesses chip thicknesses necessitate necessitate very verysmall small cutting cutting edgeedge rounding rounding to avoid to avoid ploughing ploughing [30,31]. [30, 31As]. Asthere there areare very very few few investigations investigations on on the the influenc influencee of of the the cemented cemented carbide carbide specifications specifications on the performance of micro tools,tools, aa smallsmall casecase studystudy willwill bebe presentedpresented toto ascertainascertain this.this. Three different specificationsspecifications ofof cemented carbide were investigated,investigated, firstlyfirstly differing in the grain size of of the the tungsten tungsten carbide carbide content. content. Average Average grain grain sizes sizes of 0.3 of µm 0.3 (ultrafine-grained),µm (ultrafine-grained), 0.6 µm 0.6(finest-µm (finest-grained),grained), and 3 µm and (coarse-grained) 3 µm (coarse-grained) were considered. were considered. The specificatio The specificationsns and properties, and as properties, well as a asSEM well (scanning as a SEM electron (scanning microscopy)-image electron microscopy)-image of the cross of thesections cross sectionscan be found can be in found Figure in Figure10. With 10. Withrising rising grain grain size, size,the hardness the hardness as well as well as the as the transverse transverse rupture rupture stre strengthngth (TRS) (TRS) decline. decline. In In addition, addition, the amount of the binderbinder phasephase (cobalt)(cobalt) isis rising.rising. Milling tools were manufactured according to the approach described in Section 2.3 using the three types of cemented carbide. The single edge US USM-millsM-mills with a flat flat face had an effective milling ◦ cutter diameter of 50 µµm,m, aa minorminor cuttingcutting edgeedge angleangle ofof χχ’’rr == 1212°,, and were optimizedoptimized for a spindlespindle run-out of 3 µµmm as well asas aa feedfeed perper toothtooth ffz << 3 3 µm.µm.

Micromachines 2017, 8, 261 12 of 18 Micromachines 2017, 8, 261 12 of 18

FigureFigure 10. SpecificationsSpeciﬁcations of cemented of cemented carbide carbide for the fordifferent the different grain sizes grain of (top sizes) 0.3 ofµm, (top (middle) 0.3 )µ 0.6m, (µmmiddle and) ( 0.6bottomµm and) 3 µm (bottom ) 3 µm

The influence of the grain size of the cemented carbide on the quality of the manufactured tools The influence of the grain size of the cemented carbide on the quality of the manufactured tools can be seen Fin Figure 11. The tools made of ultrafine-grained (0.3 µm) cemented carbide exhibit a can be seen Fin Figure 11. The tools made of ultrafine-grained (0.3 µm) cemented carbide exhibit a very very homogenous and smooth cutting edge geometry. The main cutting edge does not show any homogenous and smooth cutting edge geometry. The main cutting edge does not show any breakouts breakouts and the minor cutting edge only a few. The intended ideal geometry as depicted in Figure and the minor cutting edge only a few. The intended ideal geometry as depicted in Figure3 was closely 3 was closely matched. In addition, virtually no grooves or pile-ups resulting froulm tool grinding matched. In addition, virtually no grooves or pile-ups resulting froulm tool grinding resulted. resulted. The finest-grained cemented carbide (0.6 µm) also results in a very homogenous shape and surface The finest-grained cemented carbide (0.6 µm) also results in a very homogenous shape and of the tool. However, in contrast to the ultrafine-grained cemented carbide, the tool exhibits grinding surface of the tool. However, in contrast to the ultrafine-grained cemented carbide, the tool exhibits grooves resulting from its manufacturing. The main cutting edge shows some bulgings, being pile-ups grinding grooves resulting from its manufacturing. The main cutting edge shows some bulgings, from tool grinding. The minor cutting edge has some breakouts. being pile-ups from tool grinding. The minor cutting edge has some breakouts. The coarse-grained cemented carbide exhibits a rough surface finish, grinding grooves can be The coarse-grained cemented carbide exhibits a rough surface finish, grinding grooves can be seen very clearly. The main and the minor cutting edge show breakouts. seen very clearly. The main and the minor cutting edge show breakouts. The tools were employed in slot milling. The investigated material was commercially pure titanium grade 2 (maximum amount of iron Fe < 0.2% and oxygen O < 0.18%); specifications can be

Micromachines 2017, 8, 261 13 of 18

found in Figure 12. Titanium was chosen due to its chemical resistance [32] and good bio- compatibility [33], making it a widespread material in the aerospace and medical sectors. An example Micromachines 2017, 8, 261 13 of 18 are pace makers [34], needing micro structures that could be manufactured by micro milling.

FigureFigure 11. 11.Inﬂuence Influence of cementedof cemented carbide carbide speciﬁcations specifications on on tool tool quality quality and and wear wear in dependencein dependence of theof the graingrain size size (from (from left left to right:to right: 0.3 0.3µm, µm, 0.6 0.6µm µm and and 3 µ m)3 µm) before before (top ()top and) and after after machining machining (bottom (bottom). ).

The tools were employed in slot milling. The investigated material was commercially pure titanium grade 2 (maximum amount of iron Fe < 0.2% and oxygen O < 0.18%); specifications can be found in Figure 12. Titanium was chosen due to its chemical resistance [32] and good bio-compatibility [33], making it a widespread material in the aerospace and medical sectors. An example are pace makers [34], needing micro structures that could be manufactured by micro milling. Grooves were milled at a cutting speed of vc = 7.54 m/min at a feed per tooth of f z = 1.2 µm and a depth of cut of 5 µm. The length of the grooves was 50 mm for the WC grain sizes 0.3 µm and

0.6 µm and 25 mm for the grain size of 3 µm. The tools were recorded via SEM after their application (Figure 11). Micromachines 2017, 8, 261 14 of 18 Micromachines 2017, 8, 261 14 of 18

FigureFigure 12.12.Etched Etched cross-section cross-section (left (left) and) and properties properties of commerciallyof commercially pure pure (CP) (CP) titanium titanium grade grade 2 (right 2 ) after(right [35) after]. [35].

TheGrooves wear were of the milled ultrafine-grained at a cutting (0.3speedµm) of cementedvc = 7.54 m/min carbide at toola feed was per the tooth lowest of f inz = this 1.2 research.µm and Therea depth is slightof cut cutting of 5 µm. edge The rounding, length of but the the grooves tool is was still capable50 mm for of cutting.the WC Somegrain adhesionssizes 0.3 µm of titaniumand 0.6 couldµm and be detected,25 mm for but the there grain was size no of build-up 3 µm. The edge tools formation were recorded at the rake via face.SEM after their application (FigureThe 11). finest-grained (0.6 µm) showed wear of the minor flank face. Especially the edges between the majorThe and wear minor of flankthe ultrafine-grained face were severely (0.3 rounded. µm) cemented There wascarbide a considerable tool was the build-up lowest in edge this formationresearch. onThere the rakeis slight face. cutting edge rounding, but the tool is still capable of cutting. Some adhesions of titaniumThe coarse-grainedcould be detected, (3 µ m)but cementedthere was carbideno build-up tools edge were formation evaluated at after the a rake feed face. travel of 25 mm as no toolThe was finest-grained able to reach (0.6 50 mmµm) without showed failure. wear of On th thee minor contrary, flank no face. build-up Especially edge the formation edges between could be detectedthe major after and 25 minor mm offlank feed face travel were and severely the wear rounded. of the flank There and was rake a faces considerable was very build-up low. However, edge breakoutsformation resultingon the rake from face. abrasion can be seen at the main cutting edge. It can be concluded that those causeThe the coarse-grained abrupt failure of(3 thoseµm) cemented tools due tocarbide the rise tools of thewere cutting evaluated forces. after a feed travel of 25 mm as noTo tool further was evaluateable to reach the performance 50 mm without resulting failure. from On thethe cementedcontrary, carbideno build-up specifications, edge formation forces werecould recorded be detected during after milling25 mm of feed the grooves travel and with the a wear dynamometer of the flank (Figure and rake 13). faces The was feed very travel low. of theHowever, two finer breakouts grain sizesresulting (0.3 fromµm andabrasion 0.6 µ canm) be were seen extended; at the main while cutting that edge. of the It can finest-grain be concluded size (0.6thatµ thosem) was cause extended the abrupt to 1600 failure mm of until those tool tools failure, due the to the feed rise travel of the of thecutting ultrafine-grain forces. size (0.3 µm) was extendedTo further up evaluate to 16,000 the mmperformance without resulting failure. Atfrom that the point, cemented the experimentcarbide specifications, was interrupted. forces Somewere recorded conclusions during can easilymilling be of drawn the grooves by this with investigation. a dynamometer First (Figure of all, the 13). finer The thefeed grain travel size, of the the longertwo finer the grain achievable sizes (0.3 feed µm travel. and 0.6 Secondly, µm) were the extended; finer the while grain that size of the the smaller finest-grain the process size (0.6 forces. µm) Especially,was extended the to feed 1600 force mm was until considerably tool failure, smaller the feed for travel the ultrafine-grain of the ultrafine-grain size. This size could (0.3 µm) be traced was backextended to the up tool to manufacturing.16,000 mm without As discussed,failure. At thethat intended point, the ideal experiment geometry was of the interrupted. tool was closelySome conclusions can easily be drawn by this investigation. First of all, the finer the grain size, the longer matched using the ultrafine-grained cemented carbide. As described in Section 2.3, the facettes on the the achievable feed travel. Secondly, the finer the grain size the smaller the process forces. Especially, flank face are adjusted to a certain feed per tooth. When this feed per tooth is exceeded, the flank face the feed force was considerably smaller for the ultrafine-grain size. This could be traced back to the touches the workpiece, what can result in severe wear or abrupt tool failure. The data implies that tool manufacturing. As discussed, the intended ideal geometry of the tool was closely matched using this occurred for the finest-grained and coarse-grained cemented carbide tools. While the effective the ultrafine-grained cemented carbide. As described in Section 2.3, the facettes on the flank face are feed force was very low for the ultrafine-grained tools, those were considerably higher for the other adjusted to a certain feed per tooth. When this feed per tooth is exceeded, the flank face touches the two grain sizes. This can be traced back to the flank face contacting and pushing against the workpiece. workpiece, what can result in severe wear or abrupt tool failure. The data implies that this occurred The surface roughness shows no significant difference between the three grain sizes micro tools at for the finest-grained and coarse-grained cemented carbide tools. While the effective feed force was the beginning of each slot (Figure 14). The arithmetic mean roughness Ra differs only in the range of very low for the ultrafine-grained tools, those were considerably higher for the other two grain sizes. 10%, the range in which also the standard deviation varies. The same is true for the average roughness. This can be traced back to the flank face contacting and pushing against the workpiece. Despite those small deviations between the grain sizes, it can be stated that both roughness values are lowest for the coarse grain size 3 µm. This can be explained by the wear behavior of the USM-mill. ◦ The unworn tool has a minor cutting edge angle of χ’r = 12 which reduces the contact of the cutting edge with the slot bottom but on the other side increases the kinematic roughness. When this minor cutting edge angle is reduced by the wear of the minor cutting edge, the kinematic roughness of the

Micromachines 2017, 8, 261 15 of 18 slot bottom is reduced, resulting in lower roughness values. As the coarse-grained USM-mill wears rapidly,Micromachines a lower 2017, bottom 8, 261 surface results. 15 of 18

FigureFigure 13. 13.Forces Forces inin dependencedependence ofof thethe cemented carbide specifications speciﬁcations (top (top left: left: 0.3µm, 0.3µm, top top right: right: 0.6 0.6 Micromachinesmmmm and and 2017 3 3 mmmm, 8, (bottom 261(bottom left)left) whenwhen micromicro millingmilling (process speciﬁcationsspecifications bottom right). 16 of 18

The surface roughness shows no significant difference between the three grain sizes micro tools at the beginning of each slot (Figure 14). The arithmetic mean roughness Ra differs only in the range of 10%, the range in which also the standard deviation varies. The same is true for the average roughness. Despite those small deviations between the grain sizes, it can be stated that both roughness values are lowest for the coarse grain size 3 µm. This can be explained by the wear behavior of the USM-mill. The unworn tool has a minor cutting edge angle of χ’r = 12° which reduces the contact of the cutting edge with the slot bottom but on the other side increases the kinematic roughness. When this minor cutting edge angle is reduced by the wear of the minor cutting edge, the kinematic roughness of the slot bottom is reduced, resulting in lower roughness values. As the coarse- grained USM-mill wears rapidly, a lower bottom surface results. Concluding this case study, the grain size was revealed to be a crucial influencing factor concerning the micro milling process and tool performance. While there was no influence on the resulting bottom surface roughness, a significantly higher tool life and smaller process forces were the result of small grain sizes of the cemented carbide. FigureFigure 14. 14.Bottom Bottom surfacesurface roughnessroughness inin dependencedependence of the cemented carbide carbide grain grain size size (from (from left left to to right:right: 0.3 0.3µ µm,m, 0.60.6µ µmm andand 33µ µm).m).

5. Conclusions In this paper, state of the art manufacturing of ultra-small micro tools and their application were presented. The investigations covered micro milling with micro end mills and micro grinding with micro pencil grinding tools. Concerning micro end mills, it was shown that the specifications of the cemented carbide the blank is made of highly influences the quality of the tool as well as its applicability. Sharper, more homogeneous cutting edges without breakouts can be achieved with smaller grain sizes of the cemented carbide. The application of the tools revealed smaller forces and significantly higher tool life with smaller grain sizes. Considering micro pencil grinding tools, the decisive properties are those of the abrasive layer covering the cemented carbide blank. The case study revealed that larger grit sizes are preferable due to a lower achievable bottom surface roughness of the machined groove as well as higher achievable feed rate and a more stable process behavior. This rather surprising result needs to be further researched. That is, the process parameters as well as the range of grit sizes should be increased and an in depth-analysis of tool wear and tool glazing is necessary to ascertain the material removal mechanisms. The studies on electroless plating of the tools revealed that the increase of grit size is limited. The uniformity of the grit distribution diminishes with rising grit size and at the same time larger grit sizes necessitate longer plating times. The plating time, however, is limited by the lifetime of the plating solution. For both processes, further studies on cooling and lubrication have to be conducted. It is well known from macro machining that this highly influences the process. Process results in micro machining, such as in [36], show the positive influence of metal working fluids, especially in combination with rough micro tool surfaces where the lubricant’s adhesion tends to be better [37].

Acknowledgments: This research was funded by the German Research Foundation (DFG) within the Collaborative Research Center 926 “Microscale Morphology of Component Surfaces”

Author Contributions: Benjamin Kirsch supervised the present study and helped to discuss and analyze the results. Martin Bohley has conceived, designed and performed the micro end milling experiments. Peter Arrabiyeh has conceived, designed and performed the micro grinding and electroless plating experiments. Jan C. Aurich initiated the study. All authors were highly involved in writing the paper.

Conflicts of Interest: The authors declare no conflict of interest.

Micromachines 2017, 8, 261 16 of 18

Concluding this case study, the grain size was revealed to be a crucial influencing factor concerning the micro milling process and tool performance. While there was no influence on the resulting bottom surface roughness, a significantly higher tool life and smaller process forces were the result of small grain sizes of the cemented carbide.

4. Conclusions In this paper, state of the art manufacturing of ultra-small micro tools and their application were presented. The investigations covered micro milling with micro end mills and micro grinding with micro pencil grinding tools. Concerning micro end mills, it was shown that the specifications of the cemented carbide the blank is made of highly influences the quality of the tool as well as its applicability. Sharper, more homogeneous cutting edges without breakouts can be achieved with smaller grain sizes of the cemented carbide. The application of the tools revealed smaller forces and significantly higher tool life with smaller grain sizes. Considering micro pencil grinding tools, the decisive properties are those of the abrasive layer covering the cemented carbide blank. The case study revealed that larger grit sizes are preferable due to a lower achievable bottom surface roughness of the machined groove as well as higher achievable feed rate and a more stable process behavior. This rather surprising result needs to be further researched. That is, the process parameters as well as the range of grit sizes should be increased and an in depth-analysis of tool wear and tool glazing is necessary to ascertain the material removal mechanisms. The studies on electroless plating of the tools revealed that the increase of grit size is limited. The uniformity of the grit distribution diminishes with rising grit size and at the same time larger grit sizes necessitate longer plating times. The plating time, however, is limited by the lifetime of the plating solution. For both processes, further studies on cooling and lubrication have to be conducted. It is well known from macro machining that this highly influences the process. Process results in micro machining, such as in [36], show the positive influence of metal working fluids, especially in combination with rough micro tool surfaces where the lubricant’s adhesion tends to be better [37].

Acknowledgments: This research was funded by the German Research Foundation (DFG) within the Collaborative Research Center 926 “Microscale Morphology of Component Surfaces”. Author Contributions: Benjamin Kirsch supervised the present study and helped to discuss and analyze the results. Martin Bohley has conceived, designed and performed the micro end milling experiments. Peter Arrabiyeh has conceived, designed and performed the micro grinding and electroless plating experiments. Jan C. Aurich initiated the study. All authors were highly involved in writing the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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