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Characterisation of the wear behaviour of polycrystalline diamond (PCD) tools when machining wood-based composites
Journal of Materials Processing Technology 162–163 (2005) 665–672
Characterisation of the wear behaviour of polycrystalline diamond (PCD) tools when machining wood-based composites P. Philbin ∗ , S. Gordon Department of Manufacturing and Operations Engineering, University of Limerick, Limerick, Ireland
Abstract Wood-based composite materials are used extensively for interior and exterior construction applications. These materials are typically pressed into sheet form and then machined to the required size and profile using cutters such as circular saws, band saws and routers. These cutters are tipped with hard cutting tool segments that perform the cutting and prolong the life of the tool. Polycrystalline diamond (PCD) segments are increasingly used in woodworking due to extended tool life resulting from their superior properties over traditional tool materials. Estimating the degree and nature of tool segment wear for the machining of wood-based composite materials is difficult due to the long tool life involved. Under controlled machining conditions abrasive wear dominates, where the tool wears evenly with the amount of material cut. The tool sliding distance is a useful determinant of abrasive wear. This paper outlines a method for calculation of the tool sliding distance for the circular sawing of sheet materials based on a dimensional analysis. Using this information, a tool wear test was designed to test the performance of PCD tooling when sawing these products. Abrasive and chipping wear modes were found for sawing different materials. The underlying wear mechanisms appeared to be dislodgement of PCD grains and micro-fracture of the PCD respectively. © 2005 Published by Elsevier B.V. Keywords: Machining; Wood-based composites; PCD
1. Introduction A composite material consists of two or more base materials, combining the favourable characteristics of each [1]. Wood-based composite materials are increasingly used in manufacturing applications. There is a broad range of these materials, ranging from engineered wood boards to inorganic-bonded wood composites [2,3]. Tool wear, dimensional accuracy and the machined surface finish are important considerations in the machining of these materials [4]. Cutting tools used in traditional wood machining such as saws, drills and routers are now also used in the machining of wood-based composites. The main cutting tool materials used in woodworking are stellites, cemented carbides, and polycrystalline diamond (PCD). While each cutting tool material has found its own niche of applications, the extreme ∗
Corresponding author. E-mail address: [email protected] (P. Philbin).
0924-0136/$ – see front matter © 2005 Published by Elsevier B.V. doi:10.1016/j.jmatprotec.2005.02.085
hardness of diamond, its high thermal conductivity and low coefficient of friction make it an ideal tool material for the machining of wood and wood-based composites [5]. These properties are the reason for the superior performance of PCD and diamond-coated carbides over cemented carbide tools, such as increased wear resistance, improved ability to machine to closer tolerances and reduced acoustic emissions [6]. 1.1. PCD as a cutting tool material PCD tools are increasingly used in high volume machining of wood-based composites due to their superior tool life and associated cost savings resulting from the reduced downtime necessary for tool changing [7–13]. PCD is a synthesised, extremely tough inter grown mass of randomly oriented diamond crystals bonded to a tungsten carbide substrate. It is manufactured by sintering together micron sized diamond grains at high pressure and temperature in the presence of a solvent/catalyst metal, usually cobalt or cobalt/nickel al-
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loy [5]. During the sintering process, the voids between PCD grains are filled with cobalt binder. Unlike cemented tungsten carbide however, individual diamond grains actually bond to one another [14]. The result is a tough, hard product that will retain its shape and strength if some of the metal matrix is removed. In the case of cemented carbide, when the binder phase is removed the tungsten carbide grains break away from the parent material and from each other. This difference is readily observed by looking at fracture surfaces of PCD and of cemented tungsten carbide. Fracture surfaces of PCD show brittle fracture of the diamond crystals, whereas in the case of cemented carbide, ductile fracture of the binder phase and inter-granular fracture of the carbide phase are predominant [7]. This explains why the wear of cemented tungsten carbide woodworking tools occurs by erosion of the matrix and subsequent loss of carbide grains [15,16]. In the case of PCD, if the matrix is eroded grains are still held together by the bonding between them. 1.1.1. Wear modes of PCD in woodworking Micro-chipping, abrasive wear (edge rounding) and gross tool fracture are the main PCD tool wear modes in woodworking. Gross tool fracture results in a sudden catastrophic failure of the cutting edge, usually in the early stages of cutting. Foreign materials in the workpiece such as hard inclusions in chipboard are believed to cause tool fracture [11]. PCD is more vulnerable to fracture than tougher tungsten carbide tools [7]. Once a tool is fractured it is immediately removed from service because the resulting workpiece surface finish becomes poor. All other wear modes are gradual and do not lead to instant failure of the cutting edge. Micro-chipping of PCD can occur when machining hard materials at aggressive feed rates, e.g. 0.4 mm/tooth and above. Chipping occurs where the fracture toughness of the PCD is exceeded in a local area at the cutting edge. Research has shown that chipping wear occurs when machining inhomogeneous wood-based materials [17]. Fine grained PCD is less susceptible to edge chipping in the machining of wood-based composites [18]. The degree to which chipping will determine the life of the tool depends primarily on the quality of the finish required on the workpiece [15]. Abrasive wear occurs when the cutting edge recedes uniformly. It is thought that this type of wear occurs on the PCD edge when machining homogeneous wood-based materials [17]. The wear land will increase steadily with the amount of material machined, until a point is reached where the tool cannot cut effectively or produce the desired surface finish. The tool is then replaced and removed from service for disposal or regrinding where possible. In recent investigations into PCD tool wear modes in woodworking it was proposed that the PCD wear mechanism occurs by the initiation of micro-cracks [17]. These cracks were assumed to have occurred due to external impact loadings contributing to cleavage fracture within PCD
grains, thus weakening the tool edge and leading to wear. Inter-granular wear, grain cleavage, peeling and spalling of grains have been proposed as possible wear mechanisms of the PCD [17,19].
2. Sliding distance in panel sawing In circular sawing wood-based panels are fed, either by hand or mechanically, into a circular blade that cuts at speeds of 3000–5000 m/min. The teeth of a circular saw-blade cut consecutively along a circular arc at its circumference. A number of teeth are engaged during cutting. The linear distance machined is the total length of all cuts taken by a blade when the teeth are judged to have reached the end of tool life. It is easily measured by multiplying the number of cuts taken by the length of each cut. Linear distance machined is not a true measure of the length of work material passed through by the saw-blade tooth, as the saw-blade has a curved path. Sliding distance (L) is the true distance passed by the tool to cut a given amount of material. This length depends on the diameter of the cutter (D), the number of teeth on the cutter (n), the depth of cut (d) and the feed per tooth (Ft ). Knowledge of the sliding distance travelled by a cutting tool is important to enable comparisons with other machining processes on the basis of distance cut. In this section, sliding distance (L) is calculated from the linear distance machined, starting by defining the sliding distance for an individual cut. 2.1. Tool path for a single cut The length of cut for one revolution divided by the number of teeth on the blade gives the feed per tooth (Ft ). A number of teeth are engaged in cutting at any time depending on the blade projection (P) and the workpiece thickness (T). For this reason the tool passes through an arc and the tool path (l) is much longer than the value of Ft . The tool path for a single cut (l) in metres is obtained by multiplying the blade radius (r) by the included angle of cut surface to blade centre (θ) in rad: l = rθ
(1)
2.2. Included angle of cut surface to blade centre, θ In order to calculate the tool path for a single cut (l) the included angle of cut surface to blade centre (θ) must first be determined in terms of the blade radius (r), blade projection (P) and the workpiece thickness (T). In standardcutting (Fig. 1) the reference angles A and B are used to define θ. In groove-cutting (Fig. 2) the reference angles C and D are used. The angle θ is measured in radians for this analysis.
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The total number of cuts that can be taken per tooth (C) is the entire workpiece volume, i.e. the board length (B) by the board thickness (T) by the board width (W) divided by both the volume of material sawn in one cut (V) and by the number of teeth on the cutter (n): BTW BW C= = (5) [(k + st )TFt ]n (k + st )Ft n The total sliding distance per tooth (L) in panel sawing is given by the tool path for a single cut (l) multiplied by the total number of cuts taken per tooth (C): L = Cl, BW r − (P + T ) L= r cos−1 (k + st )Ft n r r−P −cos−1 r
Fig. 1. Included angle in standard-cutting.
For standard-cutting θ is defined as follows: r − (P + T ) r−P , cos(B) = , r r −1 r − (P + T ) −1 r − P θ = A − B = cos − cos r r
cos(A) =
(2) In the case of groove-cutting, the included angle θ is greater. Because the workpiece advances into the blade, each tooth starts to cut a distance of half the feed per tooth (Ft /2) before the vertical position. The depth of cut (d) is less than the workpiece thickness. In this case the formula for θ is Ft /2 r−d sin(C) = , cos(D) = , r r Ft r−d θ = sin−1 + cos−1 2r r
(3)
2.3. Sliding distance calculations The volume of material taken for one cut across the workpiece (V) equals the total sawn-off thickness, i.e. the sawblade kerf thickness (k) and sawn-off material (st ) multiplied by the workpiece thickness (T) and the feed per tooth (Ft ): V = (k + st )TFt
Fig. 2. Included angle in groove-cutting.
(4)
(6)
For groove-cutting the following equation applies: BW Ft r−d L= r sin−1 + cos−1 (k + st )Ft n 2r r (7)
3. Equipment Due to its superior properties over traditional cutting tool materials, cutting distances in excess of one million metres of wood-based composite have been reached with PCD without any significant tool wear [20–23]. In order to investigate the wear modes of PCD when sawing wood-based composites it was necessary that a test rig be developed to carry out a time compression test using the most abrasive wood-based composites available. A ‘single edge’ blade and CNC in feed was developed and fitted to a panel saw. The rig automated the cutting process where the tool wear was concentrated on a single tooth instead of being spread over 72 teeth in a standard PCD saw-blade. Design of the test rig used in this work has been reported previously [24]. The test materials selected were fibre cement board (FCB) and high pressure laminated flooring (HPL) (Fig. 3). FCB consists of wood chips and Portland cement. The cement contains silica which is abrasive to machine. HPL consists of a high-density fibreboard with a ceramic overlay and backing layer which are also abrasive to machine. Machining these abrasive materials caused rapid wear so that it was possible to investigate the initial wear of the PCD cutting tools. The saw teeth used in the sawing tests were prepared with PCD by the industrial partner in the research. A 10 m grain size, general purpose PCD grade was used in all of the machining tests. The PCD segments were wire EDM cut to size and then brazed on to a saw tooth. Initial tools were mechanically ground using an EWAG diamond-grinding machine. A slight chamfer was also ground on the PCD corners where
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the mineral content of the test materials were then calculated using the equations below, where w represents the initial wet mass of samples, d the mass of dried samples, and a the mass of ashed samples: w−d × 100, d a mineral (%) = × 100 w
moisture (%) =
(8)
4.2. Exploratory sawing tests Exploratory sawing tests were carried out using the ‘panel saw based’ test rig and workpiece materials described in Section 3. The test rig was semi-automatic. Each program cycle took a single cut off the end of a board and returned the saw table for the next cut. The duration of a program cycle depended on the feed rate used. Because of the low feed rates used with the single-tooth blade, testing time was several hours in some cases. A single board was sawn for FCB tests and three stacked boards were sawn for HPL tests. The width of cut chosen was 3 mm, half the width of the PCD cutting edge. This allowed comparison between the worn and unworn side of the cutting edge when a test was complete. A constant cutting speed of 2826 m/min (3000 rev/min) was used across all tests. Feed rates were varied across the tests, between 0.05 and 0.4 mm/tooth. The feed rate values were selected to match the feed per tooth values commonly used for PCD sawing. Larger sliding distances resulted from using lower feed rates (Fig. 4). Tests were terminated when sufficient wear was generated on the PCD cutting tools. The plan of exploratory tests with feed rates and associated distance cut is shown in Table 1.
Fig. 3. Wood-based composites: FCB (left) and HPL (right).
the relief faces meet. This was done to prevent chipping of the PCD at these edges. The tools used had an effective rake angle of 6◦ and an effective relief angle of 18◦ resulting in a cutting tool included angle of 66◦ .
4. Experimental procedure 4.1. Analysis of materials All wood-based materials contain a certain amount of moisture, organic and inorganic material. The inorganic component is generally abrasive and more difficult to machine than the organic component. An indication of the machinability of wood-based composite materials was found by determining the abrasive content of these materials. This weight percentage was calculated by a dry ashing procedure. Samples of FCB and HPL having a mass within the range 3–5 g were first cut to size and weighed. The samples were then dried out in an oven at 102 ◦ C for 24 h. Samples were then removed from the oven and cooled in a desiccator for 1 h so that moisture from the atmosphere would not enter the samples when cooling. Dried and cooled samples were then weighed to determine their dry mass. The samples were then reduced to ash by placing them in a muffle furnace at 800 ◦ C for 24 h. Subsequent to ashing the samples were weighed to determine the mass of the ash. The moisture content and
4.3. Microscopy of tools When testing was complete the cutters were first examined using a Meiji Stereo Optical Microscope at 15× and 45× magnification to identify tool wear modes. The wear was initially classified either as abrasive or chipping wear. As described previously, abrasive wear was defined as uniform and increasing linearly with the distance cut. The severity of the abrasive wear was determined by measuring the maxi-
Table 1 Exploratory testing sequence Tool
Workpiece
Feed rate (mm/tooth)
Feed rate type
Cuts taken (no.)
Linear distance machined (m)
Sliding distance (m)
1 2 3 4 5 6 7 8
FCB HPL FCB HPL FCB FCB HPL FCB
0.2 0.2 0.4 0.4 0.1 0.05 0.4 0.1
High High Aggressive Aggressive Standard Slow Fast Standard
440 373 440 373 440 440 166 147
101 89 101 89 101 101 34 34
15483 19462 7741 9731 30966 61932 9731 30966
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Table 2 Tool wear results Tool no.
Workpiece and feed
Wear mode
VB maximum (mm)
VB average (mm)
Chips on flank face
Chips on rake face
Chip size (mm)
1 2 3 4
FCB 0.2 HPL 0.2 FCB 0.4 HPL 0.4
0.112 0.164 0.125 0.086
0.057 0.080 0.050
2 1 – 3
– 4 – –
0.112 0.16 – 0.044
5 6
FCB 0.1 FCB 0.05
Chipping Chipping Abrasive Chipping and abrasive Abrasive Abrasive
0.109 0.15
0.109 0.15
– –
2 –
– –
Fig. 4. Sliding distance to machine a single board changes with feed per tooth used.
mum and average wear land (VB) values on the relief face. Chipping wear was thought to be stochastic however, so it may not be repeatable at a given feed rate. Therefore it was important to measure the concentration of the chipping wear, i.e. the number of chips on cutting edge and use this as a measure of repeatability. The severity of chipping wear, that is the depth of the chips on the relief face was also measured. Results are presented in Table 2. Later, a scanning electron microscope (SEM) was used to investigate the possible underlying tool wear mechanisms on a micron and sub-micron scale. Suitable tools for SEM were selected on the basis of optical microscope observations.
Fig. 5. Cross-sectional micrograph of HPL (45×) showing fibreboard and surface layers.
The FCB had a fairly uniform consistency throughout its cross-section (Fig. 6). Wood fibres were embedded in a matrix of Portland cement. Fibres varied in size up to approximately 20 mm long and 2 mm wide. The fibres were orientated in both directions in the plane of the board. 5.2. Machining results Tool wear results for sawing FCB and HPL materials are outlined and discussed below. While micro-cracks have been reported previously for PCD machining tests on wood-based
5. Results and discussion 5.1. Analysis of materials The analysis of work materials by ashing procedure revealed that the FCB consisted of 70% mineral content, 7.5% moisture, and 22.5% organic material. The HPL consisted of 1% mineral, 6.7% moisture, and 92.3% organic material. Cross-sectional micrographs were taken of the workpiece materials in order to further develop an understanding of their composition. These observations revealed that high-density fibreboard formed the vast majority of the cross-section of the 8 mm thick HPL material (Fig. 5). The top abrasive layer was approximately 80 m thick. Beneath this a decorative paper layer was approximately 55 m thick. A melamine-backing layer was approximately 0.22 mm thick.
Fig. 6. Cross-sectional micrograph of FCB (10×) showing wood fibres in cement matrix.
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Fig. 7. Adhesion at relief face after cutting FCB (10×).
Fig. 8. Worn tool used in FCB cutting (45×).
composites [17,19], no evidence of micro-cracks was noted in this study. 5.2.1. Sawing of FCB Sawing the FCB material produced dust and wood fibres as large as the width of cut. These were produced by the tool shearing off a fibre that was aligned in the plane of the board. No degradation in surface finish or edge chipping of the workpiece was observed. When sawing FCB a significant amount of adhering material was found a distance of about 0.4 mm down both the rake and clearance face of the tool (Fig. 7). It is thought that the cement material adhered to the clearance face of the tools under the conditions of cutting pressure and temperature. During the tests a substantial amount of white material in a powder form was found evenly distributed in the vicinity of the tool. This dust was thought to be the cement binder component of the FCB. Abrasive wear was the dominant PCD wear mode when sawing the relatively homogeneous FCB material; especially at the lower feed rates where sliding distance was greater. Testing indicated that a feed rate of 0.1 mm/tooth was best when sawing FCB to increase the sliding distance and generate the maximum amount of abrasive wear. The average wear land value was 0.1 mm. Small voids, around 10 m in size were noted in the wear surface when viewed under optical microscopy (Fig. 8) and SEM (Fig. 9). The voids seem to indicate locations where diamond grains were dislodged from the cutting edge. A close-up view revealed the damaged surface (Fig. 10). It is possible that diamond grains became weakened after abrasion of the cobalt binder around them, and then became dislodged from the tool edge when subjected to the intermittent cutting force in the panel sawing operation. 5.2.2. Sawing of HPL Sawing the HPL material produced dust and chips consisting of the laminate layer. No degradation in surface finish or edge chipping of the workpiece was observed. No ad-
Fig. 9. SEM of FCB tool.
Fig. 10. Close SEM of void.
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Fig. 11. Cutting HPL produced no adhesion (20×). Fig. 13. SEM view of edge chipping.
hering material was observed on tools used to cut the HPL material. Abrasive wear did not occur to as great an extent when sawing HPL, but chips were noted at the cutting edge. Repeated impacts with the cutting edge at the higher feed rates of 0.2–0.4 mm/tooth were thought to generate a sufficient load at the cutting edge that produced chipping wear. Chips of up to 160 m in size were observed (Figs. 11 and 12). The average chip size was 100 m and the average number of chips on an edge was 2. Closer examination of chips in the SEM (Fig. 13) revealed that the chipped surface is much more clearly defined. The surface resembles a fracture surface of PCD, with diamond grains clearly evident (Fig. 14). Chipping is likely to be caused by intermittent cutting of the abrasive laminate (Al2 O3 ) in the top layer. It seems likely that the chips were produced in one instance of fracture, when the strength of the cutting edge was exceeded.
Fig. 14. Close-up view of SEM of the chipped surface.
6. Conclusions
Fig. 12. Edge chipping of PCD used to cut HPL at 45× magnification.
1. The superior properties of polycrystalline diamond (PCD) have led to its increasing use as a tool material for both the machining of abrasive composite materials and highvolume sawing of wood-based composites. 2. Tool sliding distance depends on the linear distance cut, dimensions of the workpiece and the tool/workpiece orientation. Useful wear comparisons between tools used in different applications can be made with reference to the tool sliding distance of each. 3. A test rig for the panel sawing of wood-based composites was successfully implemented. The single edge blade allowed for some time compression of wear testing (sawing) on the work materials described. 4. The exact nature of PCD wear in the machining of woodbased composites has not yet been established. However, testing has revealed that abrasion of the cutting edge occurred when sawing relatively homogeneous FCB mate-
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rial. Chipping of the cutting edge occurred when sawing the inhomogeneous HPL material with a hard surface layer. 5. It is proposed that that abrasive wear occurred where the binder was removed followed by dislodging of the diamond grains, and that chipping wear occurred where tool impact with large workpiece particles gave rise to an instance of fracture.
Acknowledgements The authors wish to acknowledge the support of the Market Support Centre, Element Six, Shannon Co., Clare, Ireland, in this work.
References [1] A.K. Kaw, Mechanics of Composite Materials, CRC Press, Boca Raton, 1997. [2] S. Smulski (Ed.), Engineered Wood Products: A Guide for Specifiers, Designers and Users, PFS Research Foundation, 1997. [3] T. Dietz, K. Bohnemann, Calcium silicate hydrate in fiber cement sheets and autoclaved aerated concrete, in: Inorganic Bonded Wood and Fiber Composite Materials, 2000. [4] S. Gordon, M.T. Hillery, A review of the cutting of composite materials, Proc. Inst. Mech. Engrs., Part L: J. Mater. Des. Appl. 217 (1) (2003) 35–45. [5] J. Wilks, E. Wilks, Properties and Applications of Diamond, Butterworths/Heinemann, Oxford, 1991. [6] I.E. Clark, Polycrystalline diamond tooling in the woodworking industry, Finer Points 1 (1995) 6–20. [7] L. Prakash, Comparison of polycrystalline and cemented carbide tools in woodworking, Superabrasives (1991). [8] M. Jennings, PCD plays its part at Galtee, Ind. Diamond Rev. 4 (2000) 256–258.
[9] D. Hayes, Low noise PCD tooling from Japan, Ind. Diamond Rev. 1 (1999) 62–64. [10] H.J. Gittel, Cutting tool materials for high performance machining, Ind. Diamond Rev. 1 (2001) 17–21. [11] J. Gladu, Developments in applications of PCD in woodworking, Superabrasives (1991). [12] Q. Bai, Y. Yao, S. Chen, Research and development of polycrystalline diamond woodworking tools, Int. J. Refract. Met. Hard Mater. 20 (5–6) (2002) 395–400. [13] M.W. Cook, P.K. Bossom, Trends and recent developments in the material manufacture and cutting tool application of polycrystalline diamond and polycrystalline cubic boron nitride, Int. J. Refract. Met. Hard Mater. 18 (2–3) (2000) 147–152. [14] D.J. Anthony, Concepts of polycrystalline diamond tooling for the woodworking industry, in: Proceedings of the First International Symposium on Tooling for the Wood Industry, Raleigh, NC, 1990. [15] K. Steinmetz, PCD polycrystalline diamond — tailor made for woodworking applications, Superabrasives (1991). [16] J.Y. Sheikh-Ahmad, J.A. Bailey, The wear characteristics of some cemented tungsten carbides in machining particleboard, Wear 225–229 (1) (1999) 256–266. [17] S. Miklaszewski, M. Zurek, P. Beer, A. Sokolowska, Micromechanism of polycrystalline cemented diamond tool wear during milling of wood-based materials, Diamond Relat. Mater. 9 (3–6) (2000) 1125–1128. [18] Syndite Product Range. Element six, 2004. [19] Q.S. Bai, Y.X. Yao, P. Bex, G. Zhang, Study on wear mechanisms and grain effects of PCD tool in machining laminated flooring, Int. J. Refract. Met. Hard Mater. 22 (2–3) (2004) 111–115. [20] P.K. Sen, M.W. Cook, R.D. Achilles, Various diamond cutting tool materials for the machining of HPL wood flooring, in: Ultrahard Materials Technical Conference, Windsor, Ontario, Canada, 1998. [21] K. Steinmetz, P.J. Heath, SYNDITE CTC002 — a PCD material for woodworking, Ind. Diamond Rev. (1989). [22] M. Jennings, Medite uses SYNDITE, Ind. Diamond Rev. (1997). [23] J.L. Collins, M.W. Cook, T. Ninnis, New developments in ultrahard machining of wood, Ind. Diamond Rev. 1 (2001). [24] P. Philbin, S. Gordon, Machining of wood-based composites using a polycrystalline diamond (PCD) saw-blade, in: Proceedings of the 20th International Manufacturing Conference, Cork Institute of Technology, Cork, Ireland, CIT Press, 2003.
Characterisation of the wear behaviour of polycrystalline diamond (PCD) tools when machining wood-based composites P. Philbin ∗ , S. Gordon Department of Manufacturing and Operations Engineering, University of Limerick, Limerick, Ireland
Abstract Wood-based composite materials are used extensively for interior and exterior construction applications. These materials are typically pressed into sheet form and then machined to the required size and profile using cutters such as circular saws, band saws and routers. These cutters are tipped with hard cutting tool segments that perform the cutting and prolong the life of the tool. Polycrystalline diamond (PCD) segments are increasingly used in woodworking due to extended tool life resulting from their superior properties over traditional tool materials. Estimating the degree and nature of tool segment wear for the machining of wood-based composite materials is difficult due to the long tool life involved. Under controlled machining conditions abrasive wear dominates, where the tool wears evenly with the amount of material cut. The tool sliding distance is a useful determinant of abrasive wear. This paper outlines a method for calculation of the tool sliding distance for the circular sawing of sheet materials based on a dimensional analysis. Using this information, a tool wear test was designed to test the performance of PCD tooling when sawing these products. Abrasive and chipping wear modes were found for sawing different materials. The underlying wear mechanisms appeared to be dislodgement of PCD grains and micro-fracture of the PCD respectively. © 2005 Published by Elsevier B.V. Keywords: Machining; Wood-based composites; PCD
1. Introduction A composite material consists of two or more base materials, combining the favourable characteristics of each [1]. Wood-based composite materials are increasingly used in manufacturing applications. There is a broad range of these materials, ranging from engineered wood boards to inorganic-bonded wood composites [2,3]. Tool wear, dimensional accuracy and the machined surface finish are important considerations in the machining of these materials [4]. Cutting tools used in traditional wood machining such as saws, drills and routers are now also used in the machining of wood-based composites. The main cutting tool materials used in woodworking are stellites, cemented carbides, and polycrystalline diamond (PCD). While each cutting tool material has found its own niche of applications, the extreme ∗
Corresponding author. E-mail address: [email protected] (P. Philbin).
0924-0136/$ – see front matter © 2005 Published by Elsevier B.V. doi:10.1016/j.jmatprotec.2005.02.085
hardness of diamond, its high thermal conductivity and low coefficient of friction make it an ideal tool material for the machining of wood and wood-based composites [5]. These properties are the reason for the superior performance of PCD and diamond-coated carbides over cemented carbide tools, such as increased wear resistance, improved ability to machine to closer tolerances and reduced acoustic emissions [6]. 1.1. PCD as a cutting tool material PCD tools are increasingly used in high volume machining of wood-based composites due to their superior tool life and associated cost savings resulting from the reduced downtime necessary for tool changing [7–13]. PCD is a synthesised, extremely tough inter grown mass of randomly oriented diamond crystals bonded to a tungsten carbide substrate. It is manufactured by sintering together micron sized diamond grains at high pressure and temperature in the presence of a solvent/catalyst metal, usually cobalt or cobalt/nickel al-
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loy [5]. During the sintering process, the voids between PCD grains are filled with cobalt binder. Unlike cemented tungsten carbide however, individual diamond grains actually bond to one another [14]. The result is a tough, hard product that will retain its shape and strength if some of the metal matrix is removed. In the case of cemented carbide, when the binder phase is removed the tungsten carbide grains break away from the parent material and from each other. This difference is readily observed by looking at fracture surfaces of PCD and of cemented tungsten carbide. Fracture surfaces of PCD show brittle fracture of the diamond crystals, whereas in the case of cemented carbide, ductile fracture of the binder phase and inter-granular fracture of the carbide phase are predominant [7]. This explains why the wear of cemented tungsten carbide woodworking tools occurs by erosion of the matrix and subsequent loss of carbide grains [15,16]. In the case of PCD, if the matrix is eroded grains are still held together by the bonding between them. 1.1.1. Wear modes of PCD in woodworking Micro-chipping, abrasive wear (edge rounding) and gross tool fracture are the main PCD tool wear modes in woodworking. Gross tool fracture results in a sudden catastrophic failure of the cutting edge, usually in the early stages of cutting. Foreign materials in the workpiece such as hard inclusions in chipboard are believed to cause tool fracture [11]. PCD is more vulnerable to fracture than tougher tungsten carbide tools [7]. Once a tool is fractured it is immediately removed from service because the resulting workpiece surface finish becomes poor. All other wear modes are gradual and do not lead to instant failure of the cutting edge. Micro-chipping of PCD can occur when machining hard materials at aggressive feed rates, e.g. 0.4 mm/tooth and above. Chipping occurs where the fracture toughness of the PCD is exceeded in a local area at the cutting edge. Research has shown that chipping wear occurs when machining inhomogeneous wood-based materials [17]. Fine grained PCD is less susceptible to edge chipping in the machining of wood-based composites [18]. The degree to which chipping will determine the life of the tool depends primarily on the quality of the finish required on the workpiece [15]. Abrasive wear occurs when the cutting edge recedes uniformly. It is thought that this type of wear occurs on the PCD edge when machining homogeneous wood-based materials [17]. The wear land will increase steadily with the amount of material machined, until a point is reached where the tool cannot cut effectively or produce the desired surface finish. The tool is then replaced and removed from service for disposal or regrinding where possible. In recent investigations into PCD tool wear modes in woodworking it was proposed that the PCD wear mechanism occurs by the initiation of micro-cracks [17]. These cracks were assumed to have occurred due to external impact loadings contributing to cleavage fracture within PCD
grains, thus weakening the tool edge and leading to wear. Inter-granular wear, grain cleavage, peeling and spalling of grains have been proposed as possible wear mechanisms of the PCD [17,19].
2. Sliding distance in panel sawing In circular sawing wood-based panels are fed, either by hand or mechanically, into a circular blade that cuts at speeds of 3000–5000 m/min. The teeth of a circular saw-blade cut consecutively along a circular arc at its circumference. A number of teeth are engaged during cutting. The linear distance machined is the total length of all cuts taken by a blade when the teeth are judged to have reached the end of tool life. It is easily measured by multiplying the number of cuts taken by the length of each cut. Linear distance machined is not a true measure of the length of work material passed through by the saw-blade tooth, as the saw-blade has a curved path. Sliding distance (L) is the true distance passed by the tool to cut a given amount of material. This length depends on the diameter of the cutter (D), the number of teeth on the cutter (n), the depth of cut (d) and the feed per tooth (Ft ). Knowledge of the sliding distance travelled by a cutting tool is important to enable comparisons with other machining processes on the basis of distance cut. In this section, sliding distance (L) is calculated from the linear distance machined, starting by defining the sliding distance for an individual cut. 2.1. Tool path for a single cut The length of cut for one revolution divided by the number of teeth on the blade gives the feed per tooth (Ft ). A number of teeth are engaged in cutting at any time depending on the blade projection (P) and the workpiece thickness (T). For this reason the tool passes through an arc and the tool path (l) is much longer than the value of Ft . The tool path for a single cut (l) in metres is obtained by multiplying the blade radius (r) by the included angle of cut surface to blade centre (θ) in rad: l = rθ
(1)
2.2. Included angle of cut surface to blade centre, θ In order to calculate the tool path for a single cut (l) the included angle of cut surface to blade centre (θ) must first be determined in terms of the blade radius (r), blade projection (P) and the workpiece thickness (T). In standardcutting (Fig. 1) the reference angles A and B are used to define θ. In groove-cutting (Fig. 2) the reference angles C and D are used. The angle θ is measured in radians for this analysis.
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The total number of cuts that can be taken per tooth (C) is the entire workpiece volume, i.e. the board length (B) by the board thickness (T) by the board width (W) divided by both the volume of material sawn in one cut (V) and by the number of teeth on the cutter (n): BTW BW C= = (5) [(k + st )TFt ]n (k + st )Ft n The total sliding distance per tooth (L) in panel sawing is given by the tool path for a single cut (l) multiplied by the total number of cuts taken per tooth (C): L = Cl, BW r − (P + T ) L= r cos−1 (k + st )Ft n r r−P −cos−1 r
Fig. 1. Included angle in standard-cutting.
For standard-cutting θ is defined as follows: r − (P + T ) r−P , cos(B) = , r r −1 r − (P + T ) −1 r − P θ = A − B = cos − cos r r
cos(A) =
(2) In the case of groove-cutting, the included angle θ is greater. Because the workpiece advances into the blade, each tooth starts to cut a distance of half the feed per tooth (Ft /2) before the vertical position. The depth of cut (d) is less than the workpiece thickness. In this case the formula for θ is Ft /2 r−d sin(C) = , cos(D) = , r r Ft r−d θ = sin−1 + cos−1 2r r
(3)
2.3. Sliding distance calculations The volume of material taken for one cut across the workpiece (V) equals the total sawn-off thickness, i.e. the sawblade kerf thickness (k) and sawn-off material (st ) multiplied by the workpiece thickness (T) and the feed per tooth (Ft ): V = (k + st )TFt
Fig. 2. Included angle in groove-cutting.
(4)
(6)
For groove-cutting the following equation applies: BW Ft r−d L= r sin−1 + cos−1 (k + st )Ft n 2r r (7)
3. Equipment Due to its superior properties over traditional cutting tool materials, cutting distances in excess of one million metres of wood-based composite have been reached with PCD without any significant tool wear [20–23]. In order to investigate the wear modes of PCD when sawing wood-based composites it was necessary that a test rig be developed to carry out a time compression test using the most abrasive wood-based composites available. A ‘single edge’ blade and CNC in feed was developed and fitted to a panel saw. The rig automated the cutting process where the tool wear was concentrated on a single tooth instead of being spread over 72 teeth in a standard PCD saw-blade. Design of the test rig used in this work has been reported previously [24]. The test materials selected were fibre cement board (FCB) and high pressure laminated flooring (HPL) (Fig. 3). FCB consists of wood chips and Portland cement. The cement contains silica which is abrasive to machine. HPL consists of a high-density fibreboard with a ceramic overlay and backing layer which are also abrasive to machine. Machining these abrasive materials caused rapid wear so that it was possible to investigate the initial wear of the PCD cutting tools. The saw teeth used in the sawing tests were prepared with PCD by the industrial partner in the research. A 10 m grain size, general purpose PCD grade was used in all of the machining tests. The PCD segments were wire EDM cut to size and then brazed on to a saw tooth. Initial tools were mechanically ground using an EWAG diamond-grinding machine. A slight chamfer was also ground on the PCD corners where
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the mineral content of the test materials were then calculated using the equations below, where w represents the initial wet mass of samples, d the mass of dried samples, and a the mass of ashed samples: w−d × 100, d a mineral (%) = × 100 w
moisture (%) =
(8)
4.2. Exploratory sawing tests Exploratory sawing tests were carried out using the ‘panel saw based’ test rig and workpiece materials described in Section 3. The test rig was semi-automatic. Each program cycle took a single cut off the end of a board and returned the saw table for the next cut. The duration of a program cycle depended on the feed rate used. Because of the low feed rates used with the single-tooth blade, testing time was several hours in some cases. A single board was sawn for FCB tests and three stacked boards were sawn for HPL tests. The width of cut chosen was 3 mm, half the width of the PCD cutting edge. This allowed comparison between the worn and unworn side of the cutting edge when a test was complete. A constant cutting speed of 2826 m/min (3000 rev/min) was used across all tests. Feed rates were varied across the tests, between 0.05 and 0.4 mm/tooth. The feed rate values were selected to match the feed per tooth values commonly used for PCD sawing. Larger sliding distances resulted from using lower feed rates (Fig. 4). Tests were terminated when sufficient wear was generated on the PCD cutting tools. The plan of exploratory tests with feed rates and associated distance cut is shown in Table 1.
Fig. 3. Wood-based composites: FCB (left) and HPL (right).
the relief faces meet. This was done to prevent chipping of the PCD at these edges. The tools used had an effective rake angle of 6◦ and an effective relief angle of 18◦ resulting in a cutting tool included angle of 66◦ .
4. Experimental procedure 4.1. Analysis of materials All wood-based materials contain a certain amount of moisture, organic and inorganic material. The inorganic component is generally abrasive and more difficult to machine than the organic component. An indication of the machinability of wood-based composite materials was found by determining the abrasive content of these materials. This weight percentage was calculated by a dry ashing procedure. Samples of FCB and HPL having a mass within the range 3–5 g were first cut to size and weighed. The samples were then dried out in an oven at 102 ◦ C for 24 h. Samples were then removed from the oven and cooled in a desiccator for 1 h so that moisture from the atmosphere would not enter the samples when cooling. Dried and cooled samples were then weighed to determine their dry mass. The samples were then reduced to ash by placing them in a muffle furnace at 800 ◦ C for 24 h. Subsequent to ashing the samples were weighed to determine the mass of the ash. The moisture content and
4.3. Microscopy of tools When testing was complete the cutters were first examined using a Meiji Stereo Optical Microscope at 15× and 45× magnification to identify tool wear modes. The wear was initially classified either as abrasive or chipping wear. As described previously, abrasive wear was defined as uniform and increasing linearly with the distance cut. The severity of the abrasive wear was determined by measuring the maxi-
Table 1 Exploratory testing sequence Tool
Workpiece
Feed rate (mm/tooth)
Feed rate type
Cuts taken (no.)
Linear distance machined (m)
Sliding distance (m)
1 2 3 4 5 6 7 8
FCB HPL FCB HPL FCB FCB HPL FCB
0.2 0.2 0.4 0.4 0.1 0.05 0.4 0.1
High High Aggressive Aggressive Standard Slow Fast Standard
440 373 440 373 440 440 166 147
101 89 101 89 101 101 34 34
15483 19462 7741 9731 30966 61932 9731 30966
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Table 2 Tool wear results Tool no.
Workpiece and feed
Wear mode
VB maximum (mm)
VB average (mm)
Chips on flank face
Chips on rake face
Chip size (mm)
1 2 3 4
FCB 0.2 HPL 0.2 FCB 0.4 HPL 0.4
0.112 0.164 0.125 0.086
0.057 0.080 0.050
2 1 – 3
– 4 – –
0.112 0.16 – 0.044
5 6
FCB 0.1 FCB 0.05
Chipping Chipping Abrasive Chipping and abrasive Abrasive Abrasive
0.109 0.15
0.109 0.15
– –
2 –
– –
Fig. 4. Sliding distance to machine a single board changes with feed per tooth used.
mum and average wear land (VB) values on the relief face. Chipping wear was thought to be stochastic however, so it may not be repeatable at a given feed rate. Therefore it was important to measure the concentration of the chipping wear, i.e. the number of chips on cutting edge and use this as a measure of repeatability. The severity of chipping wear, that is the depth of the chips on the relief face was also measured. Results are presented in Table 2. Later, a scanning electron microscope (SEM) was used to investigate the possible underlying tool wear mechanisms on a micron and sub-micron scale. Suitable tools for SEM were selected on the basis of optical microscope observations.
Fig. 5. Cross-sectional micrograph of HPL (45×) showing fibreboard and surface layers.
The FCB had a fairly uniform consistency throughout its cross-section (Fig. 6). Wood fibres were embedded in a matrix of Portland cement. Fibres varied in size up to approximately 20 mm long and 2 mm wide. The fibres were orientated in both directions in the plane of the board. 5.2. Machining results Tool wear results for sawing FCB and HPL materials are outlined and discussed below. While micro-cracks have been reported previously for PCD machining tests on wood-based
5. Results and discussion 5.1. Analysis of materials The analysis of work materials by ashing procedure revealed that the FCB consisted of 70% mineral content, 7.5% moisture, and 22.5% organic material. The HPL consisted of 1% mineral, 6.7% moisture, and 92.3% organic material. Cross-sectional micrographs were taken of the workpiece materials in order to further develop an understanding of their composition. These observations revealed that high-density fibreboard formed the vast majority of the cross-section of the 8 mm thick HPL material (Fig. 5). The top abrasive layer was approximately 80 m thick. Beneath this a decorative paper layer was approximately 55 m thick. A melamine-backing layer was approximately 0.22 mm thick.
Fig. 6. Cross-sectional micrograph of FCB (10×) showing wood fibres in cement matrix.
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Fig. 7. Adhesion at relief face after cutting FCB (10×).
Fig. 8. Worn tool used in FCB cutting (45×).
composites [17,19], no evidence of micro-cracks was noted in this study. 5.2.1. Sawing of FCB Sawing the FCB material produced dust and wood fibres as large as the width of cut. These were produced by the tool shearing off a fibre that was aligned in the plane of the board. No degradation in surface finish or edge chipping of the workpiece was observed. When sawing FCB a significant amount of adhering material was found a distance of about 0.4 mm down both the rake and clearance face of the tool (Fig. 7). It is thought that the cement material adhered to the clearance face of the tools under the conditions of cutting pressure and temperature. During the tests a substantial amount of white material in a powder form was found evenly distributed in the vicinity of the tool. This dust was thought to be the cement binder component of the FCB. Abrasive wear was the dominant PCD wear mode when sawing the relatively homogeneous FCB material; especially at the lower feed rates where sliding distance was greater. Testing indicated that a feed rate of 0.1 mm/tooth was best when sawing FCB to increase the sliding distance and generate the maximum amount of abrasive wear. The average wear land value was 0.1 mm. Small voids, around 10 m in size were noted in the wear surface when viewed under optical microscopy (Fig. 8) and SEM (Fig. 9). The voids seem to indicate locations where diamond grains were dislodged from the cutting edge. A close-up view revealed the damaged surface (Fig. 10). It is possible that diamond grains became weakened after abrasion of the cobalt binder around them, and then became dislodged from the tool edge when subjected to the intermittent cutting force in the panel sawing operation. 5.2.2. Sawing of HPL Sawing the HPL material produced dust and chips consisting of the laminate layer. No degradation in surface finish or edge chipping of the workpiece was observed. No ad-
Fig. 9. SEM of FCB tool.
Fig. 10. Close SEM of void.
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Fig. 11. Cutting HPL produced no adhesion (20×). Fig. 13. SEM view of edge chipping.
hering material was observed on tools used to cut the HPL material. Abrasive wear did not occur to as great an extent when sawing HPL, but chips were noted at the cutting edge. Repeated impacts with the cutting edge at the higher feed rates of 0.2–0.4 mm/tooth were thought to generate a sufficient load at the cutting edge that produced chipping wear. Chips of up to 160 m in size were observed (Figs. 11 and 12). The average chip size was 100 m and the average number of chips on an edge was 2. Closer examination of chips in the SEM (Fig. 13) revealed that the chipped surface is much more clearly defined. The surface resembles a fracture surface of PCD, with diamond grains clearly evident (Fig. 14). Chipping is likely to be caused by intermittent cutting of the abrasive laminate (Al2 O3 ) in the top layer. It seems likely that the chips were produced in one instance of fracture, when the strength of the cutting edge was exceeded.
Fig. 14. Close-up view of SEM of the chipped surface.
6. Conclusions
Fig. 12. Edge chipping of PCD used to cut HPL at 45× magnification.
1. The superior properties of polycrystalline diamond (PCD) have led to its increasing use as a tool material for both the machining of abrasive composite materials and highvolume sawing of wood-based composites. 2. Tool sliding distance depends on the linear distance cut, dimensions of the workpiece and the tool/workpiece orientation. Useful wear comparisons between tools used in different applications can be made with reference to the tool sliding distance of each. 3. A test rig for the panel sawing of wood-based composites was successfully implemented. The single edge blade allowed for some time compression of wear testing (sawing) on the work materials described. 4. The exact nature of PCD wear in the machining of woodbased composites has not yet been established. However, testing has revealed that abrasion of the cutting edge occurred when sawing relatively homogeneous FCB mate-
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rial. Chipping of the cutting edge occurred when sawing the inhomogeneous HPL material with a hard surface layer. 5. It is proposed that that abrasive wear occurred where the binder was removed followed by dislodging of the diamond grains, and that chipping wear occurred where tool impact with large workpiece particles gave rise to an instance of fracture.
Acknowledgements The authors wish to acknowledge the support of the Market Support Centre, Element Six, Shannon Co., Clare, Ireland, in this work.
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