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Mechanisms of surface response to overlapped abrasive grits of controlled shapes and positions: An analysis of ductile and brittle materials
CIRP Annals - Manufacturing Technology 63 (2014) 321–324
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Mechanisms of surface response to overlapped abrasive grits of controlled shapes and positions: An analysis of ductile and brittle materials Paul Butler-Smith, Dragos Axinte (1)*, Mark Daine, Ming Chu Kong Machining and Condition Monitoring Group, Faculty of Engineering, University of Nottingham, UK
A R T I C L E I N F O
A B S T R A C T
Keywords: Grinding Abrasion Material removal
Abrasive surfaces can nowadays be produced incorporating precision cutting features of controlled shape, size, protrusion and location. This study investigates the influence of defined abrasive shapes (square/ triangular/conic pyramidal frusta) of designed groups of overlapped abrasives on the successive removal of ductile (Cu) and brittle (Al2O3) materials. Scanning electron microscopy studies combined with detailed micro-topographical evaluations of scratches/adjacent material have revealed the progressive actions of material removal/displacement of copper and the fracture and brittle/plastic transitions of sapphire for the different abrasive shapes, aiding a fundamental understanding of the influence of defined microgeometries on the grinding process and resulting material surface topography. ß 2014 CIRP.
1. Introduction The need for improvements to the performances of abrasive tools to enable higher surface quality and precision of ground/polished parts for value-added industries have been documented and researched both from experimental and simulation point of view [1,2]. For this, in order to avoid constitutive (grits distribution and/or orientation) randomness of the abrasive tools in the last years, efforts have been reported in developing engineered solutions: placing diamond crystals in particular positions on a tool shank [3]; generating macro-features of agglomerated (composite) abrasives [4]; producing (via EDM [5], laser ablation [6]) micro-cutting edges in solid/composite solid abrasives of regular shapes/geometries. While the first two approaches resolve the issue on uniform distribution of abrasives on the active surface of the tools, the last solution solves also the problem of having abrasives of similar micro-geometry/shape/protrusion; the implementation of these solutions reported on the improvement of workpiece surface roughness and reduced tool clogging hence, grinding forces [7]. In this context, there is now an open ground to design truly bespoke abrasive tools of particular applications. Nevertheless, to fully exploit this new technology there is a need for scientific understanding on the relationship between the shapes and spatial arrangement of the grits and outcomes of the abrasive process. On this respect, studies of the influence of single random [8,9] and controlled [10] micro-geometries of the abrasives on the influence the morphology and geometry of the scratch marks on ductile/brittle materials were reported. Although this might be the first step into this research field, of real relevance for these engineered grinding/ polishing tools [6,7] is to understand how material removal occurs under the successive/overlapped passes of the controlled-shaped
* Corresponding author. http://dx.doi.org/10.1016/j.cirp.2014.03.024 0007-8506/ß 2014 CIRP.
abrasive grits. Indeed, some studies on surface texture obtained after multi-grit cutting passes have been reported [11,12] but as the geometry of the employed abrasives is not controlled, it is challenging to draw robust qualitative/quantitative conclusions about the interdependence between material removal mechanism of one abrasive grit the follow-up ones coming on an overlapped trajectory. To address this knowledge gap, the paper presents a study on how workpiece surfaces, of different mechanical properties, respond to successive passing of overlapped abrasive grits of controlled geometry (circular/square/triangular base frusta); this was enabled by accurately generating, using pulsed laser ablation, primitive abrasive shapes of accurate dimensions and well defined spacing. Micro-topography and scanning electron microscopy (SEM) analyses of copper and sapphire workpiece surfaces led to the understanding of the fundamental differences in removal mechanism of ductile and brittle materials that are subjected to successive cutting actions of abrasive grits with overlapped cutting trajectories. Thus, this research aims to put a scientific methodological basis on how to decide which shapes and what positions should be selected for the controlled-geometry grits to enable improved grinding efficiencies of these engineered abrasive tools for particular workpiece materials. 2. Concept and investigatory approach With the possibility to generate controlled-shapes abrasives, up to now there has not been a study of the mechanism of how the workpiece material responds to successive grit passes; this would enable a conscious design of the grinding/polishing tools Fig. 1a [7,13], with grits of regular arrayed shapes. To address this research query, abrasive grits of three shapes (circular, square, triangular base frustums) have been generated at a fixed overlap (B = 40%) and tested against two workpiece materials: copper–ductile; sapphire–brittle. The selection of the
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grit shapes replicates some possible shapes of abrasives with various Number of Cutting Edges (NoCE) that act along the cutting direction: circular (NoCE = 0); square (NoCE = 2); triangular (NoCE = 3); these generate different degrees of cutting/rubbing actions depending on the workpiece mechanical properties. While the dimensions of the shapes were selected to replicate the 140 mesh size (FEPA), the layout and degree of overlapping were chosen to emulate a typical grit density found on diamond electroplated tools; this provides an undisturbed flat surface to the leading and trailing grits to machine a combination of part of the track of the leading grit and part of non machined material.
angle of the array of grits could be considered to be zero. The setup of the abrasive arrays to zero depth of cut (ap = 0) was carried out by use of an acoustic emission sensor (Physical Acoustics, WD type), from where tests with ap = 1, 2 and 3 mm were carried out on two materials of different machining behaviour: ductile – Cu (UTS 220 MPa, Yield 70 MPa, Mohs Hardness 3); brittle – single crystal Al2O3 (UTS 1900 MPa, Yield 400 MPa, Mohs Hardness 9); such a selection of workpiece materials was driven by the need to understand how the progressive rubbing/fracturing mechanism occurs on the overlapped grit scratch marks resulting in a particular complex of properties (e.g. texture, morphology) of the workpiece surfaces. With the overlapped grit scratch marks generated on the workpiece surfaces, micro-profilelograms (Talysurf CLI 1000), scanning electron microscopy (Philips XL30) enabled the analysis of plastic deformations/shearing/fracturing phenomena occurring on the scratch marks and the extent of material removed/displaced/ fractured to be quantified along each of the produced scratches. These analyses also allowed comparisons to be made between the scratches produced by single [10] and overlapped controlled geometry grits and thus, an in-depth understanding of workpiece material response to the different shapes of the abrasive grits and the progressive actions of the grits of defined shapes/cutting edges.
3. Research methodology
4. Results and discussion
The address the research objectives, arrays of circular (top diameter of 125 mm – Fig. 2a)/square (top side of 125 mm – Fig. 2b)/triangular (top side of 125 mm – Fig. 2c) at overlapping of 40% between the frusta have been generated on monocrystalline chemical vapour deposited diamond (CVD-MCC100) using a Nd:YAG Q-switched pulsed laser (wavelength 1064 nm; focal point 40 mm; irradiance 11.14 106 W cm 2; frequency 50 kHz; beam speed 400 mm s 1); all the geometries have constant heights of 40 mm and generatrix angle of ca. 608 from the horizontal plane.
4.1. Surface topography
Fig. 1. Examples of engineered grinding tools with (a) ordered shaped abrasives [13] and schematic of grits overlapping study (b).
An initial thought was that knowing the profile of a single grit scratch mark, the micro-topography of the overlapped arrays of controlled-shaped abrasives could be known. This was not the case due to secondary effects: pile-up and fracture of the ductile and brittle workpiece materials respectively (example for square grit Figs. 4a and 5a). In such instances there was a need to first understand these mechanisms from the scanned single grit scratches and then compare them to those produced by the multi grits of the same shape (Figs. 4b and 5b). This understanding was aided by measuring the volumetric removal within the scratch (M_S)/pileup (M_P) of material in each case and comparing the results with the calculated volume of the scratch (C–S) produced by the theoretical cross section of the respective grits under the same conditions (see references on Figs. 4c, d and 5c, d). This
Fig. 2. SEM images of circular (a), square (b), triangular (c) overlapped frusta grits arrays generated by pulsed laser ablation in solid monocrystalline CVD diamond (CVD-MCC100) structures.
These arrays have been mounted on a grinding wheel hub (diameter 140 mm, cutting speed 35 m s 1) and overlapped scratch marks have been produced on the selected workpieces using a Makino A55 machining centre while applying Hocut3380 emulsion cutting fluid (Fig. 3); note that having the abrasive arrays ablated on a flat diamond log allowed their precise relative setup in relation to the workpiece surface and in all the trials, the clearance
Fig. 3. Setup overlapped grit grinding tests (a) and examples their marks for square base frustum on Cu (b) and sapphire workpieces (c).
Fig. 4. Profilometric scans of scratches in Cu produced by single (a) /overlapped (b) grits and their corresponding volumetric evaluations along the scratch length for single (c) and overlapped (d) grits trials.
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moreover, due to the wall fracture, the scratches were typically wider than the extremities of the grit’s sectional contact profile. Similarly the traces produced by the three sets of overlapped grits were less pronounced as a result of extensive scratch wall and floor fracture (evident in SEM images discussed later), i.e. they were not a defined replication of the sectional profile of the group of overlapped grits that produced them (Fig. 5b). From the comparisons of the measured volume removed by the single and overlapped grits with the theoretical volumetric removal in each case (Fig. 5c and d), it was interesting to observe that the single square grit for example produced a proportionally greater volumetric removal, suggesting that the multi grits produced less collateral fracture in the adjacent material. These phenomena may be as a result from the machining of pre-fractured material opposed to a greater extent of fracture initiation by the leading grit. Thus, from observations made in relation to ductile and brittle materials it was possible to understand the implications of grit overlapping on the produced workpiece micro-topography, quantify the associated secondary effects (pile-up/fracturing); this opens the avenue to develop future models for predicting surface texture when utilizing tools of controlled-shaped grits. However, to understand the implications for overlapping of grits, surface integrity analysis is needed. Fig. 5. Profilometric scans of scratches in Al2O3 produced by single (a) /overlapped (b) grits and their corresponding volumetric evaluations along the scratch length for single (c) and overlapped (d) grits trials.
comparison between the trace topographies produced by the single and multi grits provided information on the progressive nature of the material removal. From an initial analysis of the ductile material (Cu) for the specific grit shapes it has been observed that the single grit produces a trace resembling the cross sectional profile of the grit in combination with displaced material on the shoulders of the trace [10]. However, in detailed examinations carried out for the current work, it was observed for all the single grit shapes, in specific regions of the scratch once sufficient material was removed, the sum of the sectional areas of adjacent material pile-up was found in some instances to be greater than the sectional area of the scratch concavity. Performing this analysis (Fig. 4c), it was identified that at the beginning of the grit engagement into the workpiece, little material is displaced/piled up but as the scratch progresses, there is strong evidence that the material is piled up in an accumulative manner, culminating in more displaced material towards the end of the scratch (Fig. 4c). It can furthermore be noted that in the ductile material, the overall envelope of the overlapped grits can be observed on the overlapped scratch profiles with associated material pile-up on the adjacent sides of the scratch (Fig. 4d); this leads to the conclusion that material piled-up by the leading 1st grit, having ‘‘no support’’ on the sides, is removed by the trailing 2nd and 3rd grits in the group. In addition, the piled-up material produced by the group of overlapped grits in relation to the volume of material removed is proportionally less than with the single grit case (Fig. 4c); this is likely because on the scratches generated by the trailing grits there is a likely a side flow of the material that having no support form the bulk material results in its removal. These observations prove that the material removal process in ductile materials, e.g. Cu, is significantly influenced by its ‘‘historic’’ topography, i.e. influenced by the actions of previous abrasive grits. Also single grit grinding trials are comprehensive for understanding how the final surface roughness is generated after grinding. Such mechanisms suggest that that as the population of grits increases, the amount of pile up in relation to material removed would therefore progressively reduce. In contrast to the response of a ductile material, as sapphire is predominantly removed under a shearing/fractural mechanism, the scratches produced by the three single grit geometries [10] were less pronounced and with no pile-up effects (Fig. 5a);
4.2. Morphology of scratched surfaces with overlapped grits The spatial arrangement of the grits offer the opportunity to study the morphology of the surfaces generated by the action of a single and multi cutting edge actions and thus, to understand the mechanism of material removal is influenced by the initial surface properties; this is of particular importance when low chip thickness and cutting edges with negative rake angles, such as those in grinding, are utilized. SEM images of surfaces of the ground scratches produced in Cu and Al2O3 have provided further information on the mechanisms by which the respective material are removed/ploughed when subjected to the progressive action of the multi-grits with differing geometries. The ductility of Cu is indicated by the significant plastic deformations on the bottom of the scratch marks and of the material pileup on the shoulders constituting material displacement. While the nature of the material displacement varied relating to the grit geometry (see Section 4.1), the surface morphology of the scratches in Cu were similar (example Fig. 6a), the scratch floor and walls showing good definition, as produced by the extremities of the collective profile of the multi grits. As a result of the grits’ layouts (Fig. 2), while the effects from the cutting edges of the leading grit were not clearly defined (i.e. those producing the initial scratch) trace marks from the cutting edges/inner extremities of the trailing grits were evident. Such ‘edge marks’ of the grits evident along the major part of the grind trace (Fig. 6a for triangular grits) can be ascribed to microdisplacement or pile up of this ductile material along the grind length of the trace. As there is little evidence of ‘edge marks’ left by the leading grit, theses results suggest that the removal of the initial grind track left by the leading grit was effectively achieved by the trailing grits.
Fig. 6. Examples of morphology of scratches obtained by overlapped triangular grits on Cu (a) and Al2O3 (b) testpieces.
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From the SEM analysis of the scratches produced by the groups of shaped multi grits in sapphire, with its low fracture toughness, by contrast with the above, predominantly exhibits crystallographic fracture (Fig. 6b) by the intermittent micro cleaving of its lattice structure on a micro scale. In addition, the SEM micrographs reveal extensive amounts of collateral material removal through surface tear-outs, produced as a result of crack propagations, which are evident on the floor, walls and shoulders of the grind traces, thereby reducing the overall profile definition as a result of these topographical characteristics. From in-depth analysis of Al2O3 testpieces, it can be observed (Fig. 7a–c) that differences exist in the surface morphologies of the traces produced by the tested multi/overlapped grit shapes: the circular grit (Fig. 7a) resulted in a predominantly fractured surface; the overlapped triangular grits (Fig. 7b) producing a combination of plastic deformations and surface fracture; the square grit (Fig. 7c) resulting in a surface exhibiting predominantly plastic deformation. These morphological differences are likely the result of the effective number of cutting edges (NoCE) and grit micro-geometries: the square grits (NoCE = 2) in the tested orientation present orthogonally orientated edges in the direction of the travel, providing a shearing (cutting) action in the machining direction: the triangular grits (NoCE = 3) orientated with their apex and two flanks producing a combination angular shearing and sliding; the circular grits (NoCE = 0) producing a rubbing action.
volumetric removal was more efficient with the group of grits but not as efficient as with brittle materials which exhibited greater volumetric removal due to collateral fracturing of the walls/floor of the scratch. For the brittle material, the secondary fracture was proved to reduce as the result of the follow up action of the cutting edges of the trailing grits when compared with the case of the leading/initial grit. This phenomenon is expected to be as a result from the machining of pre-fractured material initiated by the leading grit. This raise the point of careful consideration of results of single scratch tests while putting a more scientific base for the determination of surface texture obtained with such highly engineered grinding/polishing tools. Morphological examinations of the ductile material revealed that the grit shapes evaluated had little influence on the produced wall/floor finishes of the scratch. However the grit shape defining the number of cutting edges in the overlapped group had a distinct influence on the surface morphologies of the produced scratches in brittle materials: conical frusta (NoCE = 0) producing pronounced plastic deformation; triangular frusta (NoCE = 3) producing a combination of fracture and shear; square frusta producing a predominantly fractured morphology. These were consistent with the findings for single grits. The study aids with the understanding of the influence of abrasive shape on the behaviour of individual and groups of defined overlapped girts, being the fundamental collective element of an abrasive surface, providing a unique understanding of the role of abrasive micro geometries which are utilized in the design of engineered abrasive surfaces. Thus, the paper presents for the first time a set of unique results that sets a scientific route to specify the grit geometry and density so grinding/polishing tools which can be customized for particular workpiece materials and applications. Acknowledgement The authors would like to acknowledge Mr. Cem Akgun, BEng at University of Nottingham for contributing to part of experimental trials. References
Fig. 7. SEM micrographs (low and high magnification – upper and lower rows respectively) of the surface characteristics of the middle region of scratches produced by (a) circular, (b) triangular and (c) square overlapped grits in sapphire.
These detailed analyses of the cutting behaviour of the engineered shape and position controlled overlapping grits in the materials selected for the test not only show the significance of the number of cutting edges and therefore the grit micro geometries on the material removal process, but in the progressive action of identically shaped grits in the removal of ductile and brittle materials. 5. Conclusion In order to understand the response ductile/brittle materials to grinding tools with orderly and controlled-shapes abrasives, an indepth topographical and morphological study of surfaces generated under overlapped cutting edges of different geometries (circular/square/triangular frusta) is presented in this study. The key findings can be summarized as follows: The 2D topography of the overlapped scratches showed that the sectional profiles of the scratches provided a good replication of the envelope geometry of the collection of grains that produced them. The pile-up secondary effect phenomenon was accumulative as the scratch progressed and decreased on the follow-up pass by the cutting edges of the trailing grits when compared with the case of a pass with a leading/initial grit. As a result, the
[1] Brinksmeier E, Aurich JC, Govekar E, Heinzel C, Klocke F (2006) Advances in Modeling and Simulation of Grinding Processes. Annals of CIRP 55(2):666–696. [2] Aurich JC, Herzenstiel P, Sudermann H (2008) High-Performance Dry Grinding Using a Grinding Wheel with a Defined Grain Pattern. Annals of CIRP 57(2):666–696. [3] Tsai M, Chen S, Liao Y, Sung J (2009) Novel Diamond Conditioner Dressing Characteristics of CMP Polishing Pad. International Journal of Machine Tools and Manufacture 49(9):722–729. [4] Silva EJ, Oliveira JFG, Bottene AC (2013) Advances in Part Texturing by Grinding. 22nd International Congress of Mechanical Engineering, Sao Paulo, Brazil, 8532–8538. [5] Luo SY, Yu TH, Liu C (2009) Grinding Characteristics of Micro-Abrasive Pellet Tools Fabricated by a LIGA-Like Process. International Journal of Machine Tools and Manufacture 49(3–4):212–219. [6] Butler-Smith PW, Axinte DA, Daine M (2009) Preferentially Oriented Diamond Micro-Arrays: A Laser Patterning Technique and Preliminary Evaluation of Their Cutting Forces and Wear Characteristics. International Journal of Machine Tools and Manufacture 49:1175–1184. [7] Butler-Smith PW, Axinte DA, Daine M (2011) Ordered Diamond Micro-Arrays for Ultra-Precision Grinding – An Evaluation in Ti–6Al–4V. International Journal of Machine Tools and Manufacture 51:54–66. [8] Kannappan S, Malkin S (1972) Effects of Grain Size and Operating Parameters on the Mechanics of Grinding. Journal of Engineering for Industry – Transaction of the ASME 94:833–842. [9] Opoza T, Chen X (2012) Experimental Investigation of Material Removal Mechanism in Single Grit Grinding. International Journal of Machine Tools and Manufacture 63:32–40. [10] Axinte D, Butler-Smith P, Akgun C, Kolluru K (2013) On the Influence of Single Grit Micro-Geometry on Grinding Behavior of Ductile and Brittle Materials. International Journal of Machine Tools and Manufacture 74:12–18. [11] Matsuo T, Toyoura S, Oshima E, Ohbuchi Y (1989) Effect of Grain Shape on Cutting Force in Superabrasive Single-Grit Tests. Annals of CIRP 38(1):323–326. [12] Fugimoto M, Ichida Y, Sato R, Morimoto Y (2006) Characterisation of Wheel Surface Topography in cBN Grinding. Japanese Society of Mechanical Engineers C 49(1):106–113. [13] Butler-Smith PW, Axinte DA, Daine M (2012) Solid Diamond Micro-Grinding Tools: From Innovative Design and Fabrication to Preliminary Performance Evaluation in Ti–6Al–4V. International Journal of Machine Tools and Manufacture 59:55–64.
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Mechanisms of surface response to overlapped abrasive grits of controlled shapes and positions: An analysis of ductile and brittle materials Paul Butler-Smith, Dragos Axinte (1)*, Mark Daine, Ming Chu Kong Machining and Condition Monitoring Group, Faculty of Engineering, University of Nottingham, UK
A R T I C L E I N F O
A B S T R A C T
Keywords: Grinding Abrasion Material removal
Abrasive surfaces can nowadays be produced incorporating precision cutting features of controlled shape, size, protrusion and location. This study investigates the influence of defined abrasive shapes (square/ triangular/conic pyramidal frusta) of designed groups of overlapped abrasives on the successive removal of ductile (Cu) and brittle (Al2O3) materials. Scanning electron microscopy studies combined with detailed micro-topographical evaluations of scratches/adjacent material have revealed the progressive actions of material removal/displacement of copper and the fracture and brittle/plastic transitions of sapphire for the different abrasive shapes, aiding a fundamental understanding of the influence of defined microgeometries on the grinding process and resulting material surface topography. ß 2014 CIRP.
1. Introduction The need for improvements to the performances of abrasive tools to enable higher surface quality and precision of ground/polished parts for value-added industries have been documented and researched both from experimental and simulation point of view [1,2]. For this, in order to avoid constitutive (grits distribution and/or orientation) randomness of the abrasive tools in the last years, efforts have been reported in developing engineered solutions: placing diamond crystals in particular positions on a tool shank [3]; generating macro-features of agglomerated (composite) abrasives [4]; producing (via EDM [5], laser ablation [6]) micro-cutting edges in solid/composite solid abrasives of regular shapes/geometries. While the first two approaches resolve the issue on uniform distribution of abrasives on the active surface of the tools, the last solution solves also the problem of having abrasives of similar micro-geometry/shape/protrusion; the implementation of these solutions reported on the improvement of workpiece surface roughness and reduced tool clogging hence, grinding forces [7]. In this context, there is now an open ground to design truly bespoke abrasive tools of particular applications. Nevertheless, to fully exploit this new technology there is a need for scientific understanding on the relationship between the shapes and spatial arrangement of the grits and outcomes of the abrasive process. On this respect, studies of the influence of single random [8,9] and controlled [10] micro-geometries of the abrasives on the influence the morphology and geometry of the scratch marks on ductile/brittle materials were reported. Although this might be the first step into this research field, of real relevance for these engineered grinding/ polishing tools [6,7] is to understand how material removal occurs under the successive/overlapped passes of the controlled-shaped
* Corresponding author. http://dx.doi.org/10.1016/j.cirp.2014.03.024 0007-8506/ß 2014 CIRP.
abrasive grits. Indeed, some studies on surface texture obtained after multi-grit cutting passes have been reported [11,12] but as the geometry of the employed abrasives is not controlled, it is challenging to draw robust qualitative/quantitative conclusions about the interdependence between material removal mechanism of one abrasive grit the follow-up ones coming on an overlapped trajectory. To address this knowledge gap, the paper presents a study on how workpiece surfaces, of different mechanical properties, respond to successive passing of overlapped abrasive grits of controlled geometry (circular/square/triangular base frusta); this was enabled by accurately generating, using pulsed laser ablation, primitive abrasive shapes of accurate dimensions and well defined spacing. Micro-topography and scanning electron microscopy (SEM) analyses of copper and sapphire workpiece surfaces led to the understanding of the fundamental differences in removal mechanism of ductile and brittle materials that are subjected to successive cutting actions of abrasive grits with overlapped cutting trajectories. Thus, this research aims to put a scientific methodological basis on how to decide which shapes and what positions should be selected for the controlled-geometry grits to enable improved grinding efficiencies of these engineered abrasive tools for particular workpiece materials. 2. Concept and investigatory approach With the possibility to generate controlled-shapes abrasives, up to now there has not been a study of the mechanism of how the workpiece material responds to successive grit passes; this would enable a conscious design of the grinding/polishing tools Fig. 1a [7,13], with grits of regular arrayed shapes. To address this research query, abrasive grits of three shapes (circular, square, triangular base frustums) have been generated at a fixed overlap (B = 40%) and tested against two workpiece materials: copper–ductile; sapphire–brittle. The selection of the
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P. Butler-Smith et al. / CIRP Annals - Manufacturing Technology 63 (2014) 321–324
grit shapes replicates some possible shapes of abrasives with various Number of Cutting Edges (NoCE) that act along the cutting direction: circular (NoCE = 0); square (NoCE = 2); triangular (NoCE = 3); these generate different degrees of cutting/rubbing actions depending on the workpiece mechanical properties. While the dimensions of the shapes were selected to replicate the 140 mesh size (FEPA), the layout and degree of overlapping were chosen to emulate a typical grit density found on diamond electroplated tools; this provides an undisturbed flat surface to the leading and trailing grits to machine a combination of part of the track of the leading grit and part of non machined material.
angle of the array of grits could be considered to be zero. The setup of the abrasive arrays to zero depth of cut (ap = 0) was carried out by use of an acoustic emission sensor (Physical Acoustics, WD type), from where tests with ap = 1, 2 and 3 mm were carried out on two materials of different machining behaviour: ductile – Cu (UTS 220 MPa, Yield 70 MPa, Mohs Hardness 3); brittle – single crystal Al2O3 (UTS 1900 MPa, Yield 400 MPa, Mohs Hardness 9); such a selection of workpiece materials was driven by the need to understand how the progressive rubbing/fracturing mechanism occurs on the overlapped grit scratch marks resulting in a particular complex of properties (e.g. texture, morphology) of the workpiece surfaces. With the overlapped grit scratch marks generated on the workpiece surfaces, micro-profilelograms (Talysurf CLI 1000), scanning electron microscopy (Philips XL30) enabled the analysis of plastic deformations/shearing/fracturing phenomena occurring on the scratch marks and the extent of material removed/displaced/ fractured to be quantified along each of the produced scratches. These analyses also allowed comparisons to be made between the scratches produced by single [10] and overlapped controlled geometry grits and thus, an in-depth understanding of workpiece material response to the different shapes of the abrasive grits and the progressive actions of the grits of defined shapes/cutting edges.
3. Research methodology
4. Results and discussion
The address the research objectives, arrays of circular (top diameter of 125 mm – Fig. 2a)/square (top side of 125 mm – Fig. 2b)/triangular (top side of 125 mm – Fig. 2c) at overlapping of 40% between the frusta have been generated on monocrystalline chemical vapour deposited diamond (CVD-MCC100) using a Nd:YAG Q-switched pulsed laser (wavelength 1064 nm; focal point 40 mm; irradiance 11.14 106 W cm 2; frequency 50 kHz; beam speed 400 mm s 1); all the geometries have constant heights of 40 mm and generatrix angle of ca. 608 from the horizontal plane.
4.1. Surface topography
Fig. 1. Examples of engineered grinding tools with (a) ordered shaped abrasives [13] and schematic of grits overlapping study (b).
An initial thought was that knowing the profile of a single grit scratch mark, the micro-topography of the overlapped arrays of controlled-shaped abrasives could be known. This was not the case due to secondary effects: pile-up and fracture of the ductile and brittle workpiece materials respectively (example for square grit Figs. 4a and 5a). In such instances there was a need to first understand these mechanisms from the scanned single grit scratches and then compare them to those produced by the multi grits of the same shape (Figs. 4b and 5b). This understanding was aided by measuring the volumetric removal within the scratch (M_S)/pileup (M_P) of material in each case and comparing the results with the calculated volume of the scratch (C–S) produced by the theoretical cross section of the respective grits under the same conditions (see references on Figs. 4c, d and 5c, d). This
Fig. 2. SEM images of circular (a), square (b), triangular (c) overlapped frusta grits arrays generated by pulsed laser ablation in solid monocrystalline CVD diamond (CVD-MCC100) structures.
These arrays have been mounted on a grinding wheel hub (diameter 140 mm, cutting speed 35 m s 1) and overlapped scratch marks have been produced on the selected workpieces using a Makino A55 machining centre while applying Hocut3380 emulsion cutting fluid (Fig. 3); note that having the abrasive arrays ablated on a flat diamond log allowed their precise relative setup in relation to the workpiece surface and in all the trials, the clearance
Fig. 3. Setup overlapped grit grinding tests (a) and examples their marks for square base frustum on Cu (b) and sapphire workpieces (c).
Fig. 4. Profilometric scans of scratches in Cu produced by single (a) /overlapped (b) grits and their corresponding volumetric evaluations along the scratch length for single (c) and overlapped (d) grits trials.
P. Butler-Smith et al. / CIRP Annals - Manufacturing Technology 63 (2014) 321–324
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moreover, due to the wall fracture, the scratches were typically wider than the extremities of the grit’s sectional contact profile. Similarly the traces produced by the three sets of overlapped grits were less pronounced as a result of extensive scratch wall and floor fracture (evident in SEM images discussed later), i.e. they were not a defined replication of the sectional profile of the group of overlapped grits that produced them (Fig. 5b). From the comparisons of the measured volume removed by the single and overlapped grits with the theoretical volumetric removal in each case (Fig. 5c and d), it was interesting to observe that the single square grit for example produced a proportionally greater volumetric removal, suggesting that the multi grits produced less collateral fracture in the adjacent material. These phenomena may be as a result from the machining of pre-fractured material opposed to a greater extent of fracture initiation by the leading grit. Thus, from observations made in relation to ductile and brittle materials it was possible to understand the implications of grit overlapping on the produced workpiece micro-topography, quantify the associated secondary effects (pile-up/fracturing); this opens the avenue to develop future models for predicting surface texture when utilizing tools of controlled-shaped grits. However, to understand the implications for overlapping of grits, surface integrity analysis is needed. Fig. 5. Profilometric scans of scratches in Al2O3 produced by single (a) /overlapped (b) grits and their corresponding volumetric evaluations along the scratch length for single (c) and overlapped (d) grits trials.
comparison between the trace topographies produced by the single and multi grits provided information on the progressive nature of the material removal. From an initial analysis of the ductile material (Cu) for the specific grit shapes it has been observed that the single grit produces a trace resembling the cross sectional profile of the grit in combination with displaced material on the shoulders of the trace [10]. However, in detailed examinations carried out for the current work, it was observed for all the single grit shapes, in specific regions of the scratch once sufficient material was removed, the sum of the sectional areas of adjacent material pile-up was found in some instances to be greater than the sectional area of the scratch concavity. Performing this analysis (Fig. 4c), it was identified that at the beginning of the grit engagement into the workpiece, little material is displaced/piled up but as the scratch progresses, there is strong evidence that the material is piled up in an accumulative manner, culminating in more displaced material towards the end of the scratch (Fig. 4c). It can furthermore be noted that in the ductile material, the overall envelope of the overlapped grits can be observed on the overlapped scratch profiles with associated material pile-up on the adjacent sides of the scratch (Fig. 4d); this leads to the conclusion that material piled-up by the leading 1st grit, having ‘‘no support’’ on the sides, is removed by the trailing 2nd and 3rd grits in the group. In addition, the piled-up material produced by the group of overlapped grits in relation to the volume of material removed is proportionally less than with the single grit case (Fig. 4c); this is likely because on the scratches generated by the trailing grits there is a likely a side flow of the material that having no support form the bulk material results in its removal. These observations prove that the material removal process in ductile materials, e.g. Cu, is significantly influenced by its ‘‘historic’’ topography, i.e. influenced by the actions of previous abrasive grits. Also single grit grinding trials are comprehensive for understanding how the final surface roughness is generated after grinding. Such mechanisms suggest that that as the population of grits increases, the amount of pile up in relation to material removed would therefore progressively reduce. In contrast to the response of a ductile material, as sapphire is predominantly removed under a shearing/fractural mechanism, the scratches produced by the three single grit geometries [10] were less pronounced and with no pile-up effects (Fig. 5a);
4.2. Morphology of scratched surfaces with overlapped grits The spatial arrangement of the grits offer the opportunity to study the morphology of the surfaces generated by the action of a single and multi cutting edge actions and thus, to understand the mechanism of material removal is influenced by the initial surface properties; this is of particular importance when low chip thickness and cutting edges with negative rake angles, such as those in grinding, are utilized. SEM images of surfaces of the ground scratches produced in Cu and Al2O3 have provided further information on the mechanisms by which the respective material are removed/ploughed when subjected to the progressive action of the multi-grits with differing geometries. The ductility of Cu is indicated by the significant plastic deformations on the bottom of the scratch marks and of the material pileup on the shoulders constituting material displacement. While the nature of the material displacement varied relating to the grit geometry (see Section 4.1), the surface morphology of the scratches in Cu were similar (example Fig. 6a), the scratch floor and walls showing good definition, as produced by the extremities of the collective profile of the multi grits. As a result of the grits’ layouts (Fig. 2), while the effects from the cutting edges of the leading grit were not clearly defined (i.e. those producing the initial scratch) trace marks from the cutting edges/inner extremities of the trailing grits were evident. Such ‘edge marks’ of the grits evident along the major part of the grind trace (Fig. 6a for triangular grits) can be ascribed to microdisplacement or pile up of this ductile material along the grind length of the trace. As there is little evidence of ‘edge marks’ left by the leading grit, theses results suggest that the removal of the initial grind track left by the leading grit was effectively achieved by the trailing grits.
Fig. 6. Examples of morphology of scratches obtained by overlapped triangular grits on Cu (a) and Al2O3 (b) testpieces.
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From the SEM analysis of the scratches produced by the groups of shaped multi grits in sapphire, with its low fracture toughness, by contrast with the above, predominantly exhibits crystallographic fracture (Fig. 6b) by the intermittent micro cleaving of its lattice structure on a micro scale. In addition, the SEM micrographs reveal extensive amounts of collateral material removal through surface tear-outs, produced as a result of crack propagations, which are evident on the floor, walls and shoulders of the grind traces, thereby reducing the overall profile definition as a result of these topographical characteristics. From in-depth analysis of Al2O3 testpieces, it can be observed (Fig. 7a–c) that differences exist in the surface morphologies of the traces produced by the tested multi/overlapped grit shapes: the circular grit (Fig. 7a) resulted in a predominantly fractured surface; the overlapped triangular grits (Fig. 7b) producing a combination of plastic deformations and surface fracture; the square grit (Fig. 7c) resulting in a surface exhibiting predominantly plastic deformation. These morphological differences are likely the result of the effective number of cutting edges (NoCE) and grit micro-geometries: the square grits (NoCE = 2) in the tested orientation present orthogonally orientated edges in the direction of the travel, providing a shearing (cutting) action in the machining direction: the triangular grits (NoCE = 3) orientated with their apex and two flanks producing a combination angular shearing and sliding; the circular grits (NoCE = 0) producing a rubbing action.
volumetric removal was more efficient with the group of grits but not as efficient as with brittle materials which exhibited greater volumetric removal due to collateral fracturing of the walls/floor of the scratch. For the brittle material, the secondary fracture was proved to reduce as the result of the follow up action of the cutting edges of the trailing grits when compared with the case of the leading/initial grit. This phenomenon is expected to be as a result from the machining of pre-fractured material initiated by the leading grit. This raise the point of careful consideration of results of single scratch tests while putting a more scientific base for the determination of surface texture obtained with such highly engineered grinding/polishing tools. Morphological examinations of the ductile material revealed that the grit shapes evaluated had little influence on the produced wall/floor finishes of the scratch. However the grit shape defining the number of cutting edges in the overlapped group had a distinct influence on the surface morphologies of the produced scratches in brittle materials: conical frusta (NoCE = 0) producing pronounced plastic deformation; triangular frusta (NoCE = 3) producing a combination of fracture and shear; square frusta producing a predominantly fractured morphology. These were consistent with the findings for single grits. The study aids with the understanding of the influence of abrasive shape on the behaviour of individual and groups of defined overlapped girts, being the fundamental collective element of an abrasive surface, providing a unique understanding of the role of abrasive micro geometries which are utilized in the design of engineered abrasive surfaces. Thus, the paper presents for the first time a set of unique results that sets a scientific route to specify the grit geometry and density so grinding/polishing tools which can be customized for particular workpiece materials and applications. Acknowledgement The authors would like to acknowledge Mr. Cem Akgun, BEng at University of Nottingham for contributing to part of experimental trials. References
Fig. 7. SEM micrographs (low and high magnification – upper and lower rows respectively) of the surface characteristics of the middle region of scratches produced by (a) circular, (b) triangular and (c) square overlapped grits in sapphire.
These detailed analyses of the cutting behaviour of the engineered shape and position controlled overlapping grits in the materials selected for the test not only show the significance of the number of cutting edges and therefore the grit micro geometries on the material removal process, but in the progressive action of identically shaped grits in the removal of ductile and brittle materials. 5. Conclusion In order to understand the response ductile/brittle materials to grinding tools with orderly and controlled-shapes abrasives, an indepth topographical and morphological study of surfaces generated under overlapped cutting edges of different geometries (circular/square/triangular frusta) is presented in this study. The key findings can be summarized as follows: The 2D topography of the overlapped scratches showed that the sectional profiles of the scratches provided a good replication of the envelope geometry of the collection of grains that produced them. The pile-up secondary effect phenomenon was accumulative as the scratch progressed and decreased on the follow-up pass by the cutting edges of the trailing grits when compared with the case of a pass with a leading/initial grit. As a result, the
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