Investigation on micro milling of cemented carbide with ball nose and corner radius diamond coated end mills

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Introduction
The micro milling of brittle hard materials in a ductile cutting mode o↵ers a potentially valuable real-world industrial application.It enables intricate 3D features production with excellent finishing and tight tolerances, applicable to the manufacture of advanced tooling components such as powder compaction tools, micro injection moulds and dies.Still, the cutting phenomena at small scale di↵er significantly from those observed at the macroscopic level.Size e↵ects tend to occur once the scale of material removal, for instance the uncut chip thickness, t, or even the grain size, are comparable to the dimension of the cutting edge radius.When these length scales become comparable, several key aspects play a pivotal role in the machining behaviour.Due to the larger ratio of area to volume, tool-chip surface forces (adhesion and friction) become more significant, resulting in di↵erent cutting load scenarios thus a↵ecting tool wear, surface finish and chip formation.The critical uncut chip thickness, t c , is considered a key parameter in the attainment of brittle-fracture-free cutting [1][2][3][4][5][6][7][8].This is especially relevant in cemented carbide workpieces given the minimized occurrence of surface defects (i.e.chipping and cracks) while allowing for micro milling of complex geometries that otherwise would require relatively more time-consuming manufacturing processes, such as powder metallurgy, electrical discharge machining or even manual operations.Even though they can be noticed in both macro and micro milling [9], the size e↵ects become more challenging to control in the latter case [10], promoting more significant defects (i.e.surface ploughing, excessive springback elastic recovery and thermal e↵ect) when removing material at the scale of micron.According to Balazs et al. [11], size e↵ects do not correlate linearly with tool size reduction.Even though there may be a linear relationship between t c and the tool cutting edge radius, the workpiece grain size is also highly relevant, as stated by Li et al. [12].For t in the same order of magnitude of grain size, chip formation might occur within a few or only a single grain of the material.Because of this, the workpiece material may not exhibit a homogenous response, which is reflected on the cutting forces and the surface quality [1,9].In contrast with conventional milling processes, where material can be considered to be homogeneous and isotropic, in the micro milling process the workpiece material must be seen as heterogeneous and, in some cases, anisotropic.As the tool moves from one metallurgical phase to another, it is expected that the cutting process will behave di↵erently.These changes manifest themselves as load variations in the process that may lead to higher levels of vibration and pronounced tool wear/ breakage.Vogler et al. [13] support such dependency by using a mechanistic model for the micro milling process that explicitly accounts for the di↵erent phases while machining heterogeneous materials, capable of capturing higher cutting load frequencies than those that can be explained by the cutting process kinematics.Wojciechowski et al. [14] present a numerical-analytical model considering the chip thickness variation within current tool rotation (full immersion micro endmilling process of AISI 1045 steel), enabling the determination of a threshold relation for the transition from burnishing-dominant to chip formation-dominant regime (t c /f z =0.66).Komatsu et al. [15]s t u d i e d the influence of grain size on cutting force and specific cutting energy concluding that smaller grain sizes result in a larger cutting force and specific cutting energy.This conclusion was also reached by Lauro et al. [16].In micro milling the magnitude of the cutting forces is smaller compared with the conventional milling, due to the size reduction.However, the ratio of cutting force, F c ,t o the passive force, F p , di↵ers significantly, whereby the passive force has more significant e↵ect on the chip removal process in micro milling.Due to the higher influence of passive force, the ploughing e↵ect increases and becomes more dominant than shearing responsible for chip formation.During micro machining of hard and brittle material like cemented carbide and ceramics the critical uncut chip thickness, t c , can be estimated using the empirical formulas proposed by [17][18][19], in order to avoid unwanted scale e↵ects and non-cutting behaviours, such as pulling of hard particles and tearing of soft phase, commonly observed in micro milling [20,21].
As suggested by Astakhov [22], much of our knowledge and theoretical work on conventional machining cannot be directly applied to the macro machining of hard materials.In the field of micro machining, the larger knowledge gap evidences the need for further studies on such materials.Despite the limited data, Chen et al. [18]h a v es t u d i e dt h e micro milling of soft-brittle crystals, showing a predominance of ploughing e↵ects when compared to shearing e↵ect for feeds per tooth smaller than the cutting edge radius, r β .T h i sm a yr e s u l ti n poor surface quality due to the higher machined surface roughness.The authors also state the existence of higher specific cutting forces for smaller values of feed per tooth due to ploughing e↵ect, mainly for micro end mills with a blunt cutting edge radius.Therefore, it is crucial to find out a good compromise between feed per tooth and tool sti↵ness, thus providing a shearing action of the cutting edge over than ploughing, ensuring a good surface quality.By performing orthogonal cutting experiments on WC-11.8Co and WC-17.5Co,Ottersbach et al. [23] report di↵erences in chip formation mechanisms due to Cobalt content, having identified a maximum chip thickness of 18 µm, with a decreasing tendency for lower Co content.The minimum uncut chip thickness for both materials was found in the range of 4 to 6 µmm and correlates well with the r β /t c of 0.3, which is characteristic of di↵erent other brittle materials such hardened alloys and steels.Wu et al. [24] machined WC-15Co with a Polycrystalline Diamond (PCD) micro end mill and they revealed that the t c , of this material is 1.49 µm for the tested cutting conditions.Brittle cracks and micro pits emerged (the latter due to the pull out of broken WC grains) during brittle cutting mode.These phenomena are the main causes of poor surface quality and high surface roughness.Chen et al. [18] performed experiments of micro milling in a potassium dihydrogen phosphate crystal, which is a brittle material, reporting the existence of micro cracks and micro pits in the brittle cutting mode.These last authors state that for a ratio f z /r β larger than 1, cracks and brittle areas start occurring in the machined surfaces, with total brittle cutting mode from f z /r β =3.On the other hand, 0.7¡f z /r β ¡ 1 correspond to ductile cutting conditions.Bian et al. [19] demonstrated the defects on the machined surface of ZrO2 ceramic that were originated in a brittle cutting mode.The defects were mainly micro pits and micro cracks, which increasingly occurred with tool wear evolution.Huo et al. [20] reported a partial ductile cutting mode when machining single-crystal silicon using a CBN micro end mill and a diamond coated WC micro end mill.The partial cutting mode was considered to exist only when few surface defects such as irregular streaks and micro pits were left on the machined surface.When micro pits dominated the entire surface, the authors considered that brittle cutting mode was the main mechanism.The partial ductile cutting mode is related for other authors as a transition zone between the ductile cutting mode and the brittle cutting mode.Another example of such is the work conducted by Choong et al. [21] that addressed this transition zone and explaining it by the occurrence of partial brittle fracture.All mentioned authors relate cutting mode with feed per tooth.The ductile cutting mode is verified for lower values of feed per tooth and depth of cut.Cutting tools definition presents as a significant impact on the control of the cutting regime.Wu et al. [24] have attained ductile cutting direct micro milling of cemented carbide (WC-15Co) using a PCD micro end mill with a large tool tip radius (prepared by wire electrical discharge machining).Using identical tool material, Yuan et al. [? ] investigate the influence tool structural design on the surface morphology and roughness of cemented carbide workpiece.The authors claim that micro ball end mills with single-edged and spherical flank faces can avoid the phenomenon of chip blocking and depositing.[25] studied the cutting mechanisms of ultrasmall feed rates using diamond-coated carbide end mills on tungsten carbide workpieces.The authors report the negative impact of feed rates higher than the cross sectional thickness of the coating film and the importance of keeping feed rates bellow such values for maximizing tool durability.Focusing on the surface quality of cemented carbide, Okada et al. [26] have compared direct cutting using diamond-coated carbide end mill with two di↵erent common processing routes of cemented carbide: mechanical polishing and electric discharge machining (EDM).Despite having found that identical mirror-like finish to mechanical polishing can be obtained via direct cutting, finished surface resulting from direct cutting were comparatively rougher than those resulting from mechanical polishing (and roughner than EDM).The authors have identified two types of surfaces: (i) flat portions of the tungsten carbide grains generated by the ductile mode cutting and (ii) uneven portions composed of fine grains generated by the brittle mode cutting.Moreover, a surface morphology with uniform distribution of tungsten particles is noticed especially in the case of large feed rate and depth of cut conditions.In this paper the influence of HF-CVD diamond-coated cemented carbide end mills geometry is inspected on the micro milling of cemented carbide.The influence of cobalt content and the operational parameters of the process are additionally studied.The micro milled surface formation, chip geometry and damage mechanisms are analysed via scanning electron microscopy in order to infer on the mechanics of micro milling, particularly on the occurrence of ductile chip formation.

Experimental setup 2.1 Cutting tools, workpiece and process parameters
The cutting tools were composed of a WC-5Co substrate with a WC average grain size of 1.5 µm and 1780 HV10 hardness, fully manufactured by Palbit S.A. Their coating was achieved through Hot-Filament Chemical Vapor Deposition (HF-CVD) method, using both nano-and microcrystalline diamond layers, hereinafter referred to respectively as NCD and MCD, as depicted in figure 1, where the respective approximate thickness of each layer is illustrated.The coating design consists of a nucleation layer of NCD, followed by a transition layer with increasing crystalline size, culminating in the MCD layer located in the middle section of the coating.Above the MCD layer, the grain size gradient is reversed, with crystalline size diminishing progressively until the top NCD layer, completing a total of 16 µm of coating average thickness.Two distinct tool geometries were employed in this study: (i) a micro bull nose end mill with an tip radius of 0.1 mm, herein referred to as "bull nose" and a (ii) two-flute micro ball end mill, herein referred to as "ball nose", with a 0.5 mm tool radius (refer to figures 2b and 2d), both with a 1 mm diameter.Their geometry is described in table 1.A small length-to-diameter ratio was designed in order to minimize tool deflection in the micro milling process.The distinct corner radius of each tool result in a cutting edge length of 0.5 mm for ball nose end mill and 1.2 mm for bull nose end mill.The rake angle and flank angle were 0 .The inclination angle of the bottom edge was 0 .The bottom and side first relief angle was 6 , which was designed to reduce the contact area between the tool bottom surface and machined surface.The cutting edges radius was measured posteriorly to cutting tools production (8 µm).
The diamond-coated micro end mills were tested in the machining of distinct cemented carbide workpieces composed by WC-15 wt.% Co and WC-10 wt.% Co, both with 3 µm WC grain size, hardness of 1160 ± 40 HV10 and 1385 ± 40 HV10, respectively.The grades used in the experiments have 0.7% of γ-phase composed by metallic carbides, namely TiC, TaC and NbC, which are thermal stabilisers and grain growth inhibitors (this phase arises in the cemented carbide sintering stage).
A CNC milling machine (Makino IQ500) with air cooling was used for conducting the milling   experiments.The cutting parameters, f z , v c and n are defined as constant.Only the a p and the a e took di↵erent values depending on the test, as listed in table 2.T h ed i ↵ e r e n tm i l l i n gd e p t h and feed per tooth levels were selected in order to confirm and identify the distinct chip removal conditions of cemented carbide during micro milling.The spindle rotating speed was fixed at 45000 rpm, corresponding to a cutting speed of 141 m/min at maximum end mill diameter.Additionally important to note is the surface preparation process prior to the micro milling experimental campaign (refer to figure 3).The sequence of machining operations and the appropriateness of the previous machined surface is decisive in attaining high dimensional accuracy and, thus, functionality of the micro milled parts.All samples were submitted to a first micro roughing stage in order to ensure flatness of the surface and parallelism reference.A second preparation operation (micro finishing) is conducted to improve surface quality and ensure consistency of the experimental campaign.Lubricant oil was used in the preparation stages in order to achieve an optimal surface baseline for all remaining experiments.A cemented carbide workpiece (cube) with 20x20x15 mm has been employed for the whole (surface preparation and experimental) campaign clamped to the CNC machine through a modular EROWA ITS system.The experimental campaign was performed under dry cutting conditions allowing for chip collection.
The cutting parameters of each surface preparation stage are described in figure 3a and 3b. Figure 3c shows the location for roughness measurements of each test (3 regions of the micro milling path  were analysed) as well as the methodology for roughness measurements (figure 3d) and an example of one analysed location of a micro milled tested region (figure 3e).During the test campaign, the cutting tests were always repeated once to ensure repeatability of the controlled variables.

Setup and measurement equipment
The observation of the machined surface is crucial to identify characteristic defects that can be associated with the brittle cutting mode.The chips formed during cutting process also indicate the cutting mode, so their observation is also essential.The morphology and the roughness of the machined surfaces and edge radius were measured with the Bruker Alicona Infinite-FocusSL, non-contact 3D measurement system, based on focus-variation.Moreover, Scanning Electron Microscopy (SEM) was also employed to observe defects occurrence in the machined surface and chips formed during cutting tests.The resulting chip morphology (i.e.size and shape) can provide valuable information about the cutting mode and surface quality according to the operational conditions (by varying cutting parameters and milling tool definition) enabling an identification of the most appropriate cutting scenario.With regards to the machining operation, a copy (micro) milling strategy has been employed (refer to figure 4a) with movement in both x and y directions, including up-and down-milling.The engagement condition of each cutting tool is described in figure 4b) and 4c).Chip thickness definition is thereby illustrated, according to each mill radius, maximum thickness (h max ), corresponding to 0.41fz for the ball nose and 0.91fz for the bull nose end mills.Despite the much smaller h max promoted by the usage of the ball mill, it o↵ers improved radial contact with the workpiece, leading to increased stability during cutting, particularly in the current scenario of shallow axial depths.Alternatively, despite its smaller radial contact with the workpiece, the bull nose may constitute an adequate cutting solution given its larger cutting diameter, combining productivity with cutting stability, which is highly required given the workpiece brittleness.
3 Results and analysis

Chips formation and their size
Chip formation consistently occurred regardless of cutting conditions and WC-Co grade, for both bull nose and ball nose end mills.The chip free surface (opposite to the side that contacts with the tool rake surface) shows striation marks due to the intense compressive and shear stresses, whereas its back surface (the side of the chip that contacts with the tool rake face) is smoother, as shown in figure 5.This smooth chip back surface aspect is a good indicator of the presence of the ductile cutting mode, promoted by tool-chip sliding with low friction conditions and also, as concluded by Bian et al. [19] and Antwi et al. [27].On the cutting test with the bull nose end mill there seems to be a difference in the chip formation behaviour, especially in conditions of lower average chip thickness.Still, brittle cutting mode does not seem to completely dominate the chip formation mechanism, otherwise only long material particles could be seen and no chips could be perfectly detached, as referred by Liu et al. [28].As shown in figure 5 and further supported by figures 6, 7 and 8 the critical uncut chip thickness, t c was achieved for a portion of the full chip length in both micro mills.This means that due to the variable thickness of the obtained chips, a portion of its length is under the t c ductile threshold, whereas another portion is above it, explaining the very significant di↵erences in size of chips and fragments, illustrated in figure 5.
Figure 6 and 7 show a representative impact of WC-15Co and WC-10Co collected chips for each tested condition of micro-milling, using the same cutting tool (ball nose).For both tested material grades and operative conditions (a e and a p ) small chip fragments can be observed, more evidently on the carbide grade with the lower amount of Co binder.The fragments can also be resultant from chips that get into the tool path (interference, as dry cutting process zone), during the micro-milling process.From figures 6 and 7,i ti s additionally possible to observe that the needleshaped chips formed with the two-flute ball nose end mill have a flat rather than curled appearance for both WC-Co grades.
From the analysis and direct comparison of figures 6 and 8 it is possible to confirm the considerable impact of the cutting edge configuration presented in figure 4. The ball nose micro end mill edges (with a bigger radius) allow a cutting process with larger plastic deformation and longer chips, the same does not seem valid for the smaller radius tool (bull nose), despite its larger average uncut chip thickness.
Figure 9 shows the relation between chip average size in function of operative conditions for both WC-15Co and WC-10Co materials.Despite some scatter (potentially resultant from chip fragmentation degree), it is verified that chip size increases with axial depth of cut, which reinforces the idea inferred by figures 6, 7 and 8.Even though a representative sample of chips was selected (> 100 samples), average chip size will certainly be influenced by the fragmentation phenomenon.Also interesting to note is that the linear fit to the experimental data tend to a non-zero chip size for very small axial depths of cut, evidencing the existence of a minimum chip size L ch which, depending on the operational conditions, may amount to 8 to 18 µm, approximately.
While the ductile-to-fragile transition is readily discernible in various materials, it remains less clearly defined within this particular composite material.The geometry condition of the cutting edge isn't direct represented in critical depth model of equation presented by Bifano et al. [29] nor detailed on micro milling evaluations of Neo et al. [30] and Cheng et al. [31].
While the bull nose and ball nose chips generated during micro-milling exhibit distinct visual di↵erences at a macroscopic level (refer to figure 5), a closer examination at high magnification (refer to figure 10) reveals a remarkable equivalence between the two.Despite their macroapparent variation in shape, both chip types retain the elements of the removed material, namely tungsten carbide (WC) and cobalt (Co).This can be confirmed by the EDS analysis conducted for each type of chip (refer to figure 10).Such observation emphasizes the underlying consistency in the material removal process and highlights the presence of those elements within the chips, suggesting identical (very large deformation) fundamental principles governing both micro end mills on cemented carbides workpieces.

Machined surface characterization
The arithmetical mean height (Sa) results, obtained through 3D micro topography are shown in figure 11, enabling surface quality assessment and control.Those results show that surface quality is not sensitive to the axial depth of cut, regardless of the tested operative conditions or %Co of the workpiece.This observation is consistent with the idea that the minimum chip thickness was reached within stable chip formation and, thus, resulting in the possible lowest roughness values regardless of remaining process variables.Alternatively, the relatively improved surface quality of the ball nose end mill over the bull nose is evidenced, supporting the significant  influence of the tool edge definition, in particular, the extent of radial contact with the workpiece.The more uniform engagement of the ball nose end mill seems to result in enhanced cutting stability, avoiding tool chatter which could lead to surface imperfections.In sum, the geometrical influence of the tool seems to surpass any variation to the cutting phenomenon imposed by the distinct tested operational conditions.Figure 12 depicts the resultant machined surfaces and defects occurrence in the micromilling experiments.In particular, figure12ashows the transition between original (non-milled) and micro-milled surface, using the two-flute ball nose micro-milling cutter.A larger micro-milled area can be seen in figure 12b, where surface defects (such as micro-pit occurrence and cracking can be noticed).Furthermore, it's worth noting that tool engagement can lead to noticeable degradation of the workpiece edges.For that reason, it is crucial to better control the micro-milling strategies near the workpiece edges.An alternative to the proposed machining strategy could be a peripheral finishing stage after the surface end machining.In figure 12c that edge degradation is closely demonstrated and suggesting brittle fracture of those workpiece edges, given that WC grains do not seem to undergo any morphological changes apart from cracks occurrence, which may propagate, in the workpiece edge region, to the generated micro-milled surface, compromising the functionality of added-value components.Even though figure 12c particularly shows WC-15Co defects, these identically occurred for both %Co workpiece materials and tested tools.In some of the observed micro-pits, detailed in figure 12d and 12e, an EDS analysis revealed the existence of Ti, Ta and Nb (characteristic elements of the WC-Co γ-phase, which is rich in TiC/TaC and NbC).Unlike cobalt this γ-phase does not undergo plastic deformation (due to its very high hardness) being consequently pulled o↵ the machined surface.Micro-pit occurrence was verified in almost all experiments, in both WC-15Co and WC-10Co materials.
Depending on the operational cutting conditions, traces/marks on the milled surfaces were observed, such as the illustrated on figure 12b, which are resultant from (i) the cutting test or (ii) from surface preparation.Due to the limited material removal during the micro-milling tests (especially the ones for smaller a p ), there might be instances where the residual marks from preceding tools, along with surface imperfections such as micro-pits, cannot be entirely removed.Figure 12f confirms that below a p ≤6µ the generated ploughing layer does not completely cover the machined surface, indicating a reduction in the ductile deformation on the material, with increasingly large areas of surface defects and exposed WC carbide.
Figure 13 illustrates the deformation phenomenon that occurs in both WC-15Co and WC-10Co workpiece materials by analysing the a↵ected layer under the machined surface (possible due to its exposure promoted by workpiece edge deterioration.The a↵ected layer shows evidence of ploughing, as a consequence of the micro machining size e↵ects, as referred by the literature.Figures 13b and 13c demonstrate the presence of some cracked WC grains below the thin ploughing layer.
In both tested WC-Co grades the identification of ploughing layer limits and its thickness calculation revealed to be a challenging task.Although, this layer appears to have a thickness in the nanometre range, the SEM analysis seems to show that the ploughing layer is a combination of Co with small fragments of WC grains.This morphology of ploughing layer appears to be valid for both material grades and tested tools.Since both material grades have a grain size of 3µm, this may indicate that machining leads to fragmentation of WC grains during chip removal, or a rearrangement of the ploughing layer and substrate.

Conclusions
The developed work contributes to the comprehension of the intricate nature of micro fabrication.The downsizing of products presents new challenges and calls for novel machining approaches.This transition benefits from the reassessment of tool technology, material behaviour, as well as mechanical/tribological cutting conditions especially in what concerns ductile chip formation in brittle material workpieces.The ductile chip formation process remained consistent irrespective of cutting conditions and WC-Co grade, despite the inherent brittleness of the material.Even though the more consistent chip morphology obtained with the ball nose micro end mills (needle-shape), that phenomenon was observed regardless of tool geometry.Despite the predominance of ductile cutting mode, responsible for smooth surfaces brittle cutting seems to coexist (presence of small workpiece material fragments) resulting in small surface defects.The average chip size is smaller for the WC-10Co material than for the WC-15Co material, for both tested micro-cutters.Such is related with the lower Co content of the former material grade and, thus, its relatively higher brittleness (lower toughness).
The results of these investigations can be summarized as follows: 1.A linear tendency in chip size with depth of cut, corroborates the ductile chip formation mechanism and suggests a minimum chip size, given its non-zero prediction for near-zero depth of cut, data tend to a non-zero chip size for very small axial depths of cut, evidencing the existence of a minimum chip size Lch which, depending on the operational conditions, may amount to 8 to 18 µm, approximately.Surface quality remains independent of operative conditions or workpiece %Co.This consistency aligns with minimum chip thickness occurring during stable chip formation, resulting in optimal (minimal) roughness.Moreover, the ball nose's uniform engagement seems to enhance stability, preventing tool chatter and surface defects.2. Considerable impact of the cutting edge configuration confirming impact on the ducting cutting regime, ball nose micro end mill edges (with a bigger radius) allow a cutting process with larger plastic deformation and longer chips, even in conditions of smaller average chip thickness resulting from the edge configuration.With lower cutting conditions than ap=50µm ae=10µmf z = 4 µm/z using bull nose end mill the deformation regime are very unstable but brittle cutting mode does not seem to completely dominate the chip formation mechanism, otherwise only long material particles could be seen and no chips could be perfectly detached.3. Tool geometry prevails over variations from diverse operational conditions in shaping the cutting phenomenon.SEM images reveal a ploughing layer for all ap values, for both WC-Co grades with the two-flute micro ball end mill with sub grain thickness, which is an evidence of the ductile cutting regime.Also, seems to exist a minimum depth of cut (6 microns), below which it is not possible to have a fully processed material with a continous ploughing layer a↵ecting the surface quality.Deformation phenomenon that occurs in both WC-15Co and WC-10Co workpiece materials were detect, controlled layer shows evidence of ploughing, as a consequence of the micro machining size e↵ects, as referred by the literature with a size lower than 1µm, this phenomenon can have an impact on certain tribological demands on surfaces.4. Surface quality is not sensitive to the axial depth of cut, regardless of the tested operative conditions or %Co of the workpiece, relatively improved surface quality of the ball nose end mill over the bull nose is evidenced, supporting the significant influence of the tool edge definition. 5. Micro-pits surface defects related to cemented carbide γ-phase high hardness were confirmed in both carbide grades.
6. Optimization strategies and relevant e↵ects/defects on machined surfaces in mechanical micro-machining are being presented in various works, in particular reporting cutting parameters and tool characteristics and those impact of machined surface quality, during the cutting test were identified tool engagement can lead to noticeable degradation of the workpiece edge, micro-milling strategies near the workpiece edges adopting peripheral finishing stage after the surface end machining should have a positive impact on edge degradation.

Declarations
Funding

Fig. 1
Fig. 1 Diamond coating definition of the HF-CVD micro tools used in the study showing crystallite size distinction by layer on the tungsten carbide substrate.

Ta b l e 1
Geometrical parameters of each end mill.

Fig. 2
Fig. 2 Micro end mills used in the present study: (a) rake surface of bull nose end mill; (b) rake surface of ball nose end mill; (c) flank surface of bull nose end mill; (d) flank surface of ball nose end mill.

Fig. 3
Fig. 3 Stages of surface preparation and experimental campaign: (a) micro roughing first surface preparation stage; (b) micro finishing second surface preparation stage; (c) experimental campaign and surface measurement location at each micro milled slot; (d) surface roughness results; (e) surface roughness analysis.

Fig. 4
Fig.4Scheme of the employed surface copy milling strategy, including up-and down (micro) milling (a); micro milling maximum chip thickness (hmax)i l l u s t r a t i o nf o rb u l l( b )a n db a l l( c )n o s ee n dm i l l s ,u n d e rt h es pe c i fi e do pe r a t i o n a lc u t t i n g conditions.

Fig. 5
Fig. 5 Characterization of the WC-Co chips formed with: a) two-flute micro bull nose b) and micro ball end mill using ap =4µma n dae =5µm.

Fig. 9
Fig.9Influence of axial depth of cut, ap,andradialdepth of cut, ae,on the chip size evolution using a ball nose end mill on %15 and %10 cobalt content tungsten carbide.

Fig. 10
Fig. 10 EDS analysis and morphology of selected chips from ball nose (a) and bull nose (d) end mills, using an ap of 10 µm, fz of 4 µma n dae of 10 µmo nt h e% 1 5 C ot u n g s t e nc a r b i d ew o r k p i e c e ,e v i d e n c i n gt h ee x i s t e n c eo fb o t hW Ca n dC o regardless of chip morphology.

Fig. 11
Fig. 11 Surface quality (Sa) obtained from the performed experimental campaign.

Fig. 12
Fig. 12 Micro milled surface observation under scanning electron microscopy:(a) original (non-milled) and milled surfaces, evidencing the intense deformation of the WC grains; (b) micro pit and edge deterioration defects; (c) surface crack defects at workpiece edge surface; (d) micro pit defect submitted to (e) EDS analysis; ()f) effect of depth of cut on surface processing appearance.

Fig. 10 EDS
Fig. 10 EDS analysis and morphology of selected chips from ball nose Ta b l e 2 Tested levels for each cutting parameter and for each cutting tool geometry.