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Performance of single Si3N4 and mixed Si3N4+PCBN wiper cutting tools applied to high speed face milling of cast iron
International Journal of Machine Tools & Manufacture 45 (2005) 335–344 www.elsevier.com/locate/ijmactool
Performance of single Si3N4 and mixed Si3N4CPCBN wiper cutting tools applied to high speed face milling of cast iron Antoˆnio Maria de Souza Jr.a, Wisley Falco Salesb, Sandro Cardoso Santosc,*, Alisson Rocha Machadod a
b
Fiat-GM Powertrain Ltd, Betim, MG, Brazil Mechatronic Engineering, Polytechnic Institute—IPUC, Pontifical Catholic University of Minas Gerais—PUC Minas, IPUC, Belo Horizonte, MG, Brazil c Mechanical Engineering Department, Federal Technologycal Education Centre, CEFET-MG, Av. Amazonas, 7675, Nova Gameleira, Belo Horizonte 30410-000, MG, Brazil d Faculty of Mechanical Engineering, Federal University of Uberlaˆndia, Uberlaˆndia, MG, Brazil Received 22 October 2003; accepted 3 August 2004 Available online 19 October 2004
Abstract In this work two face milling cutter systems were used in high speed cutting of gray cast iron under cutting condition encountered in the shop floor. The first system, called ‘A’, has 24 Si3N4 ceramic inserts all with square wiper edges. The second system, called ‘B’, is a mixed tool material system, having 24 wiper inserts, 20 of them are Si3N4 intercalated by four PCBN inserts. Cutting speed (vc), depth of cut (doc) and feed rate per tooth (fz) were kept constant. Surface roughness (Ra and Rt) and waviness (Wt), tool life (based on flank wear, VBBmax) and burr formation (length of the burr, h) were the parameters considered to compare the two systems. System ‘B’ presented better performance according to all parameters, although only end of life criterion based on Rt parameter has been reached. q 2004 Elsevier Ltd. All rights reserved. Keywords: Si3N4; PCBN; Tool wear; Wear mechanisms; Surface finishing; Burr formation
1. Introduction Advances in milling processes technologies have been accelerating in recent years. Designers and users of cutting tools have worked to optimize shape and tool geometry, tool life, processes parameters related surface finish, productivity and finally, the most important factor, the machining cost per piece. Until recent years it was common to use single tool material or tool geometry in a tool holder. This paradigm in milling processes has been overcome, with the use of the mixed technology, that is, two or more tool materials or tools of more than one geometry mounted in a tool holder [1]. * Corresponding author. Tel.: C55 31 3319 5208; fax: C55 31 3319 5212. E-mail addresses: [email protected] (A.M. de Souza Jr.), [email protected] (W.F. Sales), [email protected] (S.C. Santos), [email protected] (A.R. Machado). 0890-6955/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2004.08.006
PCBN tools are used in machining operations, where tight dimensional tolerances are required due to their high elasticity modulus and hardness and low expansion volumetric coefficient. This material is recommended to machine practically all work piece materials [2], except low carbon steel [3]. This is because low carbon steel is ductile enough to generate a large chip/tool contact area, promoting strong chemical interactions and consequently activating diffusion wear mechanisms. In the last years, PCBN tools were used on shop floors in high speed machining of gray cast iron, white cast iron and hardened steels with low machining costs per piece. In these applications they tends to be economically advantageous [1,4,5]. Si3N4 ceramic tools are largely used to machine gray cast iron and hardened steels. When cutting gray cast iron, high speed machining is frequently used with low costs per pieces.
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The conventional milling processes use all inserts with the same geometry, mounted on the cutter. Normal octagonal inserts are frequently used for roughing cuts. For finishing cuts wiper systems offer much better surface finishing. When machining blocks and engine heads good surface finishing is required and wiper systems are usually used [5]. Milling cutters with two or more tool materials or tool geometries is a recent technology. This system uses octagonal inserts and intercalated by wiper inserts. Although, they show satisfactory results with respect to surface roughness the main reason for their choice is related to low machining costs, presented when compared to a system having only wiper tools [5]. With the need of better efficiency and performance of automotive engines and in order to be competitive car manufacturers have introduced in their engines a new metallic cylinder head junction. This provides a greater rigidity in the junction between the cylinder head and the block, with practically no deformation during lifetime and keeping the integrity of the junction. This reduces premature wear, which is usually observed in conventional head junctions, due to high load, and pressure variations that occur during the thermal cycles. The use of metallic head junctions demands better surface finishing (roughness parameters Ra and Rt, and waviness Wt) and reduction or elimination of burr formation to allow a good assembling of the cylinder head onto the cylinder block. Burrs are defined as ‘undesirable projections of material beyond the edge of the workpiece due to plastic deformation during machining’ [6]. The burrs generated in milling processes are extremely undesirable because they present accident risks to the operators in the assembly lines. Furthermore, they can hinder contacts between surfaces, compromising or damaging the desirable precision in assembled parts and most critical of all is that burrs can become detached and contaminate the lubrication and cooling lines, in the case of internal combustion engines. Burr removal is an expensive time consuming operation. Thus, the knowledge of the phenomenon of burr formation is of great importance because the milling process can be controlled, particularly the cutting parameters such as cutting speed (vc), feed rate (f ), depth of cut (doc), tool geometry, and approach angle, cr, so that burr formation can be minimized and, in some cases eliminated. With respect to wear, both flank and crater affect the burr formation process. The dimensions of the burr tend to increase with increasing wear levels. This is more critical after a certain amount of wear has been reached [7]. In the final stage of the process of burr formation a breakout can occur, particularly in brittle workpiece materials such as gray cast iron that can lead to scrap the part. In the production line both burr and breakout must be avoided because serious problem will happen lately at the serial assembly line. Occurrence of either will usually lead to the loss of the part. More critical is when this
phenomenon happens on the last machining operation after a reasonable amount of money had already been aggregated into the process. The objective of this work is to compare the performance of two milling cutter systems and to study the formation of burrs at the exit edge of the workpiece considering also and the surface quality and their relations with tool wear. Both systems have 24 wiper inserts. System ‘A’ is composed by Si3N4 ceramic tools while the system ‘B’ has 20 Si3N4 tools intercalated by four PCBN tools. The tests were all performed in the production line, using the same serial machinery for engine production. The cutting speed (vc), the depth of cut (doc) and feed rate per tooth (fz), were kept constant.
2. Experimental work 2.1. Machining process The top surfaces of engine blocks of car engines as shown in Fig. 1 were face milled. This surface is characterized by a complex system of tool entrance and exit to and from the workpiece due to the inherent geometry of the block. The engine block material was GH 190 UNI gray cast iron, with the following chemical composition: 3.2–3.5%C; 2.0–2.5%Si; 0.2%Cr; 0.15%S; 0.10%P and an average hardness 200 HV. The experiments were carried out on a transfer line machine during the engine block production of a car manufacturing industry. The milling machine with a Siemens 840D electronic control unit was driven by a tri-phasic asynchronous engine and has a GR ISO130 mono-mandrill. A constant spindle speed of 1270 rpm (vcZ1000 m/min) was always used. All the tests were performed during a normal production regime. Feed per tooth (fz) and depth of cut (doc) were 0.06 mm/tooth and 0.3 mm, respectively. Fig. 1 also shows schematically the milling cutter position with its rotation and feed directions, and
Fig. 1. Upper view of the engine block and indication of some machining operation details.
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of system ‘A’ all the 24 wiper inserts have individual axial adjustment. The maximum admissible axial deviation was 30 mm. The milling cutter of system ‘B’ had admissible maximum axial deviation of 40 mm among the Si3N4 inserts, and 20 mm among the PCBN inserts. The PCBN inserts were positioned 50 mm above the highest Si3N4 insert. 2.3. Measurement of tool wear, surface texture and burr size
Fig. 2. Positions of PCBN inserts on mill that characterize system B.
the region, where the maximum burr length was always observed. 2.2. Cutting tools Two different cutting tool assemblies, named system ‘A’ and system ‘B’ were evaluated in this investigation. Both systems used milling cutters of 250 mm diameter. System ‘A’ had 24 Si3N4 wiper inserts. These inserts had the following ISO designation: SNEN 1504ADTR with T 05!15 chanfer edge. The tool system ‘B’, had 24 inserts, 20 Si3N4 OPHN 0504 ZZN-A27 geometry and four PCBN wiper inserts OPHX 0504ZZR-A27 geometry. The PCBN inserts were positioned intercalating each five Si3N4 inserts, as shown in Fig. 2. The clamping system used a support pin for the tools and a mechanism with axial adjustments for the wiper tools. The adjustment of the tools was performed on an Ingersoll table, which a flat tip stylus. For milling cutter
A Wild-Heerbrugg optical microscope model 117.775 and an Omis Mini Optical Measurement Inspection System were used for tool wear measurements. Analysis of system ‘B’ tools was restricted to PCBN wiper tools because it greater responsibility to surface finish of machined parts. The worn surfaces of the tools were analysed within a Phillips scanning electronic microscope with objective to identify predominant shapes and wear mechanisms. Surface roughness and waviness measurements were performed by a Taylor Hobson MK3 profilometer. Cut-off length of 0.8 mm was adopted. The length of the burr was measured with a diamond stylus gauge (20 mm resolution). Five measurements were taken and the mean value considered. Lee et al. [8] was also utilized a similar procedure. 2.4. End of tool life criterion End of tool life criterion based on tool flank wear (VBBmaxZ0.7 mm) was adopted, and in order to comply with requirements from the design of the engine block to avoid workpiece rejection other criteria were also considered based on surface roughness parameters (maximum roughness average, RaZ1.6 mm; maximum roughness height, RtZ10 mm) and burr dimensions (maximum burr length, hZ1.8 mm). In each test the roughness parameters Ra and Rt, maximum flank wear VBBmax and burr length (h) were
Fig. 3. General view of Si3N4 cutting tool shows flank and chatter wear.
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Fig. 4. Detail shows some wear mechanisms such as micro-cracks, plastic deformation and abrasion.
measured after machining the first five workpieces. Next, up to the 100th engine block machined the intervals of measurements of theses parameters was enhanced to 10 workpieces. From the 100th to the 800th engine block machined the intervals of measurements was fixed in 100 workpieces and after the 800th engine block the interval was increased to 200 workpieces machined until one of the tool life rejection criteria was reached.
3. Results and discussion Results obtained are presented and discussed in this section, which is divided into two parts. First part presents a sequence of scanning electronic microscope photographs of worn cutting edges of wiper tools that will contribute for the discussion of the results of tool wear behavior, surface texture and burr size presented in the second part.
3.1. Scanning electronic microscope images of worn surfaces Worn surfaces of Si3N4 tools are shown in Figs. 3–6. Fig. 3 shows the general aspects of the worn surface, that allow identify flank wear as the main wear form. Smooth aspect of the topography of worn region can be also observed. Occurrence of this appearance may be attributed to adhesion of workpiece material, diffusive wear [9] or abrasion due the action of harder particles over softer materials [10]. Amplifications of worn region are shown in Figs. 4–6 that permit identify the presence of cracks in different regions of worn surface. Propagation of cracks causes chipping of some Si3N4 cutting tools that occurs during the tests. Smooth topography and flow of material over the worn surface indicates occurrence of adhesion. High chemical affinity of tool and workpiece material components also justifies the hypothesis of adhesion.
Fig. 5. Detail shows micro-cracks and abrasive wear.
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Fig. 6. Detail of worn surface of Si3N4 cutting tools shows micro-cracks.
Figs. 5 and 6 show the flow of workpiece material over the tool flank surface. Figs. 4 and 6 show details of cracks present at the worn area of the tool. Smooth surfaces near the cracks and surfaces that present abrasion marks can be observed. The gray cast iron workpiece material has in its composition silicon carbide, SiC, with high hardness (2500 HV). According to Refs. [11,12], the presence of hard particles in the tribological system can promote wear and the relationship between workpiece/abrasive hardness materials is an important effect on the wear regime. For hardness ratio less than 0.6 the so-called soft abrasion will occur while for ratio more than 0.6 the so-called hard abrasion regime will take place. Large system damage is encountered when hard abrasion is dominated. The tribological system evaluated presents Si3N4 tool material
with average hardness of 1600 HV and the hardness ratio is around 0.64, which situates on the hard abrasion regime. Fig. 7F shows cracks that propagate in several directions. The aspects of worn region of Si3N4 justify the tendency of catastrophic failure of the tools due chipping. Other wear mechanisms were reported by Silva et al. [13] and Vleugels and Van Der Biest [14]. The later showed the incompatibility of the use the of Si3N4 tools at high cutting speed and dry machining of steels because of the high interface temperatures and the chemical compatibility between the materials involved in the tribological system which encourage chemical wear which is dominant under those evaluated cutting conditions. Aspects of the worn surfaces of PCBN cutting tools are shown in Figs. 7–9, those are characterized by presence of
Fig. 7. General view of PCBN cutting tools shows flank wear.
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Fig. 8. Detail shows some wear mechanisms such as micro-cracks, plastic deformation and adhesion of the work piece material.
cracks running to the cutting edge. The presence of abrasion marks is also observed in these figures. This tribological system involves silicon carbides and a PCBN (4000 HV) and the hardness ratio workpiece/abrasive material is around 0.6. Therefore, the wear regime is a transition between soft and hard abrasion, but it is clear the presence of the abrasive wear. Aspects the worn surface suggest that thermal and mechanical cyclic load was the main factor to influence the wear of the cutting tools and fracture toughness resistance should be the main property required when machining with PCBN cutting tools. It was observed a tendency of the workpiece material to adhere into the cracks, hypothesis confirmed by EDX analysis. The results indicated the presence of Fe and Si, elements of the workpiece material. These portions contribute to cracks growth and propagation.
Gastel et al. [15] has studied the performance of PCBN tools on grey cast iron and compact graphite iron turning. They showed that oxidation of the tools and interdiffusion of constituting chemical elements between tool and workpiece were the dominant wear mechanisms. 3.2. Tool wear, surface texture and burr size Behavior of maximum flank wear of cutting tool during engine block production is shown in Fig. 10. System ‘A’ was able to produce 1600 parts, while system ‘B’ could produce 2577 blocks. Tools that compose system ‘A’ showed relatively low wear until 1400 parts were produced and after which a rapid growth was observed. System ‘B’ tools tended to present a uniform increase of flank wear during all the tool life.
Fig. 9. Detail shows evidences wear mechanisms such as abrasion and plastic deformation.
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Fig. 10. Evaluation of flank wear (VBBmax) as a function of number of blocks machined.
The type of wear observed on cutting tools can explain the behavior of the maximum flank wear curves. Si3N4 cutting tools presented a tendency to chip or break as a consequence of formation and propagation of cracks, as shown in Figs. 4–6. The occurrence of chipping implies in an abrupt change on tool geometry, which can be reflected in rapid increase in flank wear, as shown in Fig. 10. On the other hand, PCBN cutting tools presented a gradual increase of flank wear, which is compatible with wear aspects observed in these tools. Formation and growth of cracks were gradually observed, fact that implies in gradual increase of flank wear. Surface average roughnesses obtained by the two systems are shown in Fig. 11. Both systems presented the same behavior throughout the tool life. The exception of this is the last engine block machined with system ‘B’, when the Ra parameter was much higher than the end of tool life criterion RaZ1.6 mm. The PCBN and Si3N4 cutting tools presented different wear patterns, as shown by the different surface profiles of the machined surfaces. To complement the study of surface
texture two other parameters like maximum peak to valley distance (Rt) and waviness (Wt) were also measured as a function of the number of engine blocks machined. Results for Rt are presented in Fig. 12. System ‘A’ tends to produce higher values of Rt parameter during all the tool lives. Engine blocks milled by system ‘B’ tend to present relative uniform values of Rt, while system ‘A’ sowed crescent values. According to the results shown in Fig. 12, both systems exceeded the end of tool life criterion, RtZ10 mm. Results of waviness parameter Wt are shown in Fig. 13. As observed for Rt parameter, system ‘A’ presented higher values of Wt than system ‘B’ practically throughout the tool life. The wear of the PCBN tools was smaller than the wear of the Si3N4 tools. This fact is implied by the smaller cutting effort when machining with the former cutting tools, and consequently smaller deformation and vibration, which results in better surface finish. Burr height as a function of the number of blocks machined is shown in Fig. 14. The ceramic cutting tools tend to produce higher burrs and this difference increases
Fig. 11. Roughness (Ra) as a function of number of blocks machined.
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Fig. 12. Maximum peak to valley distance (Rt) as a function of number of the blocks machined.
when the number of parts machined increases. Again the higher wear of the ceramics compared to the PCBN tools is responsible for these results. According to Olvera and Barrow [16], the thickness and/or the length can be used to characterize the burr but the former is it’s the main dimension. The thicker burrs are the more difficult to be broken during a deburring operation. However, the measurement of its thickness is more difficult and laborious because metallographic preparation is usually required. On the other hand, the length of the burr can be easily measured, allowing a large number of measurements in a short period of time, and it can also be used for its characterization [17]. In the present work it was thus decided to measure the length of the burr instead of its thickness since the entire test were carried out in the production line. Work by Olvera and Barrow [16] in face milling AISI 1040 steel bars at a cutting speed of 142 m/min, investigated the influence of the depth of cut (doc) and the nose radii on the burr length. They founded that the burr length increases
when doc and nose radius increases until a value, called critical value, from which it starts to decrease. These transition points encountered were docZ0.5, 0.8 and 0.9 mm for the tool nose radii of 0.4, 0.8 and 1.2 mm, respectively. Although in the present investigation the work material is cast iron and the cutting speed (1000 m/min) is far higher than that used by Olvera and Barrow [16], which might imply here the development of higher cutting temperatures [9] the transition point might also be present. In this work the tool nose radius was 0.4 mm and the depth of cut was 0.3 mm and they did not vary, and therefore the transition point (or critical value) could not be determined. However, the influence of the wear or the burr length is clear. The burr length increases when the wear increases. This is in agreement with the mathematical model presented by Hashimura et al. [18] that indicates an increase in the burr sizes with increasing in the cutting section area. During all the tests no breakout phenomenon was observed. This means that the amount of wear was not enough to increase the plastic deformation zones to an
Fig. 13. Waviness parameter (Wt) as a function of the number of blocks machined.
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Fig. 14. Burr length (h) as a function of the number of motor blocks machined.
extension necessary to cause rupture of the negative shear plane [18–21] that would culminate in the breakout. 4. Conclusions The conclusions of this study can be summarized as follows: – Milling cutting system ‘B’ with four PCBN wiper cutting tools presented better performance compared with system ‘A’ regardless all parameters considered; – Among all the end of tool life criteria adopted, only the roughness parameter Rt was reached, which makes this the most critical index for product quality; – Si3N4 and PCBN cutting tools presented different dominant wear patterns and the other parameters studied can be related to them; – Concerning to tool materials, the dominant wear mechanisms for Si3N4 were micro-cracks, plastic deformation and abrasion while for PCBN were microcracks, adhesion and abrasion; – Both systems presented satisfactory performance, generating surfaces with topography parameters considerably below the limit established for the process.
Acknowledgements The authors would like to thank Fiat-GM Powertrain Ltd and FAPEMIG (Fundac¸a˜o de Amparo a` Pesquisa do Estado de Minas Gerais), for technical and financial support.
References [1] D.C. Correˆa, W.F. Sales, S.C. Santos, E.S. Palma, P.S. Neto, J.R. Ferreira, The use of PCBN tools for machining bimetallic bearings, Industrial Diamond Review Issue 1 (2003).
[2] B. Mills, Recent developments in cutting tool materials, Journal of Materials Processing Technology 56 (1996) 16–23. [3] A.M. Abra˜o, The machining of annealed and hardened steels using advanced ceramic cutting tools, PhD Thesis, University of Birmingham, UK, 1995. [4] M.W. Cook, P.K. Bossom, Trends and recent developments in the material manufacturing and cutting tool application of polycrystalline diamond and polycrystalline cubic boron nitride, International Journal of Refractory Metals and Hard Materials (2000) 147–152. [5] A.M. Souza Jr., W.F. Sales, High speed machining of motor blocks with PCBN and mixed ceramic inserts, Industrial Diamond Review Issue 4 (2002). [6] ASTME, Tool wear in the cutting of thin-gauge steel sheets, American Society of Tool and Manufacturing Engineers—ASTME Research Report no. 22, 1959. [7] G.L. Chern, D.A. Dornfeld, Burr/breakout model development and experimental verification, Journal of Engineering Materials and Technology 118 (1996) 201–206. [8] M. Lee, J.G. Horne, D. Tabor, The mechanism of notch formation at the depth of cut line of ceramic tools machining nickel base superalloys, Proceedings of the International Conference on Wear Materials, ASME, Darborn, MI, USA, 16–18 April 1979, pp. 460–469. [9] E.M. Trent, Metal Cutting, third ed., Butterworths–Heinemann, London, 1991, 273 pp. ISBN 0-7506-1068-9. [10] C. Yen-Qian, D. Xiang-G, W. Fu-Xing, C. Qi-Gong, Z. Zhang-Xiao, On wear mechanism of SIALON and metal in dry sliding, Wear 137 (1990) 175–186. [11] I.M. Hutchings, Tribology: Friction and Wear of Engineering Materials, CRC Press, Boca Raton, USA, 1992, p. 273. [12] K.H. Zum Gahr, Microstruture and Wear of Materials, Elsevier, Amsterdam, 1987. [13] R.F. Silva, F.J. Oliveira, F.P. Castro, J.M. Vieira, Modeling of chemical wear in ferrous alloys/silicon nitride contacts during high speed cutting, Acta Materials 46 (7) (1998) 2501–2507. [14] J. Vleugels, O. Van Der Biest, Chemical wear mechanisms of innovative ceramic cutting tools in the machining of steel, Wear 225– 229 (1999) 285–294. [15] M. Gastel, C. Konetschny, U. Reuter, C. Fasel, H. Schulz, R. Riedel, H.M. Ortner, Investigation of the wear mechanism of cubic boron nitride tools used for machining of compacted graphite iron and grey cast iron, International Journal of Refractory Metals and Hard Materials 18 (2000) 287–296.
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[16] O. Olvera, G. Barrow, Influence of exit angle and tool nose geometry on burr formation in face milling operations, Proceedings of the Institution of Mechanical Engineers 212 (part B) (1998) 59–72. [17] L.K. Gillespie, P.T. Blotter, The formation and properties of machining burrs, Transactions of the ASME (1976) 66–74. [18] M. Hashimura, Y.P. Chang, D. Dornfeld, Analysis of burr formation mechanism in orthogonal cutting, Journal of Manufacturing Science and Engineering 121 (1999) 1–7.
[19] A.J. Pekelharing, The exit failure uninterrupted cutting, Annals of the CIRP 27 (1) (1978) 5–10. [20] S.L. Ko, D.A. Dornfeld, Analysis of fracture in burr formation at the exit stage of metal cutting, Journal of Materials Processing Technology 58 (1996) 189–200. [21] S.L. Ko, D.A. Dornfeld, Burr formation and fracture in oblique cutting, Journal of Materials Processing Technology 63 (1996) 24–36.
Performance of single Si3N4 and mixed Si3N4CPCBN wiper cutting tools applied to high speed face milling of cast iron Antoˆnio Maria de Souza Jr.a, Wisley Falco Salesb, Sandro Cardoso Santosc,*, Alisson Rocha Machadod a
b
Fiat-GM Powertrain Ltd, Betim, MG, Brazil Mechatronic Engineering, Polytechnic Institute—IPUC, Pontifical Catholic University of Minas Gerais—PUC Minas, IPUC, Belo Horizonte, MG, Brazil c Mechanical Engineering Department, Federal Technologycal Education Centre, CEFET-MG, Av. Amazonas, 7675, Nova Gameleira, Belo Horizonte 30410-000, MG, Brazil d Faculty of Mechanical Engineering, Federal University of Uberlaˆndia, Uberlaˆndia, MG, Brazil Received 22 October 2003; accepted 3 August 2004 Available online 19 October 2004
Abstract In this work two face milling cutter systems were used in high speed cutting of gray cast iron under cutting condition encountered in the shop floor. The first system, called ‘A’, has 24 Si3N4 ceramic inserts all with square wiper edges. The second system, called ‘B’, is a mixed tool material system, having 24 wiper inserts, 20 of them are Si3N4 intercalated by four PCBN inserts. Cutting speed (vc), depth of cut (doc) and feed rate per tooth (fz) were kept constant. Surface roughness (Ra and Rt) and waviness (Wt), tool life (based on flank wear, VBBmax) and burr formation (length of the burr, h) were the parameters considered to compare the two systems. System ‘B’ presented better performance according to all parameters, although only end of life criterion based on Rt parameter has been reached. q 2004 Elsevier Ltd. All rights reserved. Keywords: Si3N4; PCBN; Tool wear; Wear mechanisms; Surface finishing; Burr formation
1. Introduction Advances in milling processes technologies have been accelerating in recent years. Designers and users of cutting tools have worked to optimize shape and tool geometry, tool life, processes parameters related surface finish, productivity and finally, the most important factor, the machining cost per piece. Until recent years it was common to use single tool material or tool geometry in a tool holder. This paradigm in milling processes has been overcome, with the use of the mixed technology, that is, two or more tool materials or tools of more than one geometry mounted in a tool holder [1]. * Corresponding author. Tel.: C55 31 3319 5208; fax: C55 31 3319 5212. E-mail addresses: [email protected] (A.M. de Souza Jr.), [email protected] (W.F. Sales), [email protected] (S.C. Santos), [email protected] (A.R. Machado). 0890-6955/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2004.08.006
PCBN tools are used in machining operations, where tight dimensional tolerances are required due to their high elasticity modulus and hardness and low expansion volumetric coefficient. This material is recommended to machine practically all work piece materials [2], except low carbon steel [3]. This is because low carbon steel is ductile enough to generate a large chip/tool contact area, promoting strong chemical interactions and consequently activating diffusion wear mechanisms. In the last years, PCBN tools were used on shop floors in high speed machining of gray cast iron, white cast iron and hardened steels with low machining costs per piece. In these applications they tends to be economically advantageous [1,4,5]. Si3N4 ceramic tools are largely used to machine gray cast iron and hardened steels. When cutting gray cast iron, high speed machining is frequently used with low costs per pieces.
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The conventional milling processes use all inserts with the same geometry, mounted on the cutter. Normal octagonal inserts are frequently used for roughing cuts. For finishing cuts wiper systems offer much better surface finishing. When machining blocks and engine heads good surface finishing is required and wiper systems are usually used [5]. Milling cutters with two or more tool materials or tool geometries is a recent technology. This system uses octagonal inserts and intercalated by wiper inserts. Although, they show satisfactory results with respect to surface roughness the main reason for their choice is related to low machining costs, presented when compared to a system having only wiper tools [5]. With the need of better efficiency and performance of automotive engines and in order to be competitive car manufacturers have introduced in their engines a new metallic cylinder head junction. This provides a greater rigidity in the junction between the cylinder head and the block, with practically no deformation during lifetime and keeping the integrity of the junction. This reduces premature wear, which is usually observed in conventional head junctions, due to high load, and pressure variations that occur during the thermal cycles. The use of metallic head junctions demands better surface finishing (roughness parameters Ra and Rt, and waviness Wt) and reduction or elimination of burr formation to allow a good assembling of the cylinder head onto the cylinder block. Burrs are defined as ‘undesirable projections of material beyond the edge of the workpiece due to plastic deformation during machining’ [6]. The burrs generated in milling processes are extremely undesirable because they present accident risks to the operators in the assembly lines. Furthermore, they can hinder contacts between surfaces, compromising or damaging the desirable precision in assembled parts and most critical of all is that burrs can become detached and contaminate the lubrication and cooling lines, in the case of internal combustion engines. Burr removal is an expensive time consuming operation. Thus, the knowledge of the phenomenon of burr formation is of great importance because the milling process can be controlled, particularly the cutting parameters such as cutting speed (vc), feed rate (f ), depth of cut (doc), tool geometry, and approach angle, cr, so that burr formation can be minimized and, in some cases eliminated. With respect to wear, both flank and crater affect the burr formation process. The dimensions of the burr tend to increase with increasing wear levels. This is more critical after a certain amount of wear has been reached [7]. In the final stage of the process of burr formation a breakout can occur, particularly in brittle workpiece materials such as gray cast iron that can lead to scrap the part. In the production line both burr and breakout must be avoided because serious problem will happen lately at the serial assembly line. Occurrence of either will usually lead to the loss of the part. More critical is when this
phenomenon happens on the last machining operation after a reasonable amount of money had already been aggregated into the process. The objective of this work is to compare the performance of two milling cutter systems and to study the formation of burrs at the exit edge of the workpiece considering also and the surface quality and their relations with tool wear. Both systems have 24 wiper inserts. System ‘A’ is composed by Si3N4 ceramic tools while the system ‘B’ has 20 Si3N4 tools intercalated by four PCBN tools. The tests were all performed in the production line, using the same serial machinery for engine production. The cutting speed (vc), the depth of cut (doc) and feed rate per tooth (fz), were kept constant.
2. Experimental work 2.1. Machining process The top surfaces of engine blocks of car engines as shown in Fig. 1 were face milled. This surface is characterized by a complex system of tool entrance and exit to and from the workpiece due to the inherent geometry of the block. The engine block material was GH 190 UNI gray cast iron, with the following chemical composition: 3.2–3.5%C; 2.0–2.5%Si; 0.2%Cr; 0.15%S; 0.10%P and an average hardness 200 HV. The experiments were carried out on a transfer line machine during the engine block production of a car manufacturing industry. The milling machine with a Siemens 840D electronic control unit was driven by a tri-phasic asynchronous engine and has a GR ISO130 mono-mandrill. A constant spindle speed of 1270 rpm (vcZ1000 m/min) was always used. All the tests were performed during a normal production regime. Feed per tooth (fz) and depth of cut (doc) were 0.06 mm/tooth and 0.3 mm, respectively. Fig. 1 also shows schematically the milling cutter position with its rotation and feed directions, and
Fig. 1. Upper view of the engine block and indication of some machining operation details.
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of system ‘A’ all the 24 wiper inserts have individual axial adjustment. The maximum admissible axial deviation was 30 mm. The milling cutter of system ‘B’ had admissible maximum axial deviation of 40 mm among the Si3N4 inserts, and 20 mm among the PCBN inserts. The PCBN inserts were positioned 50 mm above the highest Si3N4 insert. 2.3. Measurement of tool wear, surface texture and burr size
Fig. 2. Positions of PCBN inserts on mill that characterize system B.
the region, where the maximum burr length was always observed. 2.2. Cutting tools Two different cutting tool assemblies, named system ‘A’ and system ‘B’ were evaluated in this investigation. Both systems used milling cutters of 250 mm diameter. System ‘A’ had 24 Si3N4 wiper inserts. These inserts had the following ISO designation: SNEN 1504ADTR with T 05!15 chanfer edge. The tool system ‘B’, had 24 inserts, 20 Si3N4 OPHN 0504 ZZN-A27 geometry and four PCBN wiper inserts OPHX 0504ZZR-A27 geometry. The PCBN inserts were positioned intercalating each five Si3N4 inserts, as shown in Fig. 2. The clamping system used a support pin for the tools and a mechanism with axial adjustments for the wiper tools. The adjustment of the tools was performed on an Ingersoll table, which a flat tip stylus. For milling cutter
A Wild-Heerbrugg optical microscope model 117.775 and an Omis Mini Optical Measurement Inspection System were used for tool wear measurements. Analysis of system ‘B’ tools was restricted to PCBN wiper tools because it greater responsibility to surface finish of machined parts. The worn surfaces of the tools were analysed within a Phillips scanning electronic microscope with objective to identify predominant shapes and wear mechanisms. Surface roughness and waviness measurements were performed by a Taylor Hobson MK3 profilometer. Cut-off length of 0.8 mm was adopted. The length of the burr was measured with a diamond stylus gauge (20 mm resolution). Five measurements were taken and the mean value considered. Lee et al. [8] was also utilized a similar procedure. 2.4. End of tool life criterion End of tool life criterion based on tool flank wear (VBBmaxZ0.7 mm) was adopted, and in order to comply with requirements from the design of the engine block to avoid workpiece rejection other criteria were also considered based on surface roughness parameters (maximum roughness average, RaZ1.6 mm; maximum roughness height, RtZ10 mm) and burr dimensions (maximum burr length, hZ1.8 mm). In each test the roughness parameters Ra and Rt, maximum flank wear VBBmax and burr length (h) were
Fig. 3. General view of Si3N4 cutting tool shows flank and chatter wear.
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Fig. 4. Detail shows some wear mechanisms such as micro-cracks, plastic deformation and abrasion.
measured after machining the first five workpieces. Next, up to the 100th engine block machined the intervals of measurements of theses parameters was enhanced to 10 workpieces. From the 100th to the 800th engine block machined the intervals of measurements was fixed in 100 workpieces and after the 800th engine block the interval was increased to 200 workpieces machined until one of the tool life rejection criteria was reached.
3. Results and discussion Results obtained are presented and discussed in this section, which is divided into two parts. First part presents a sequence of scanning electronic microscope photographs of worn cutting edges of wiper tools that will contribute for the discussion of the results of tool wear behavior, surface texture and burr size presented in the second part.
3.1. Scanning electronic microscope images of worn surfaces Worn surfaces of Si3N4 tools are shown in Figs. 3–6. Fig. 3 shows the general aspects of the worn surface, that allow identify flank wear as the main wear form. Smooth aspect of the topography of worn region can be also observed. Occurrence of this appearance may be attributed to adhesion of workpiece material, diffusive wear [9] or abrasion due the action of harder particles over softer materials [10]. Amplifications of worn region are shown in Figs. 4–6 that permit identify the presence of cracks in different regions of worn surface. Propagation of cracks causes chipping of some Si3N4 cutting tools that occurs during the tests. Smooth topography and flow of material over the worn surface indicates occurrence of adhesion. High chemical affinity of tool and workpiece material components also justifies the hypothesis of adhesion.
Fig. 5. Detail shows micro-cracks and abrasive wear.
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Fig. 6. Detail of worn surface of Si3N4 cutting tools shows micro-cracks.
Figs. 5 and 6 show the flow of workpiece material over the tool flank surface. Figs. 4 and 6 show details of cracks present at the worn area of the tool. Smooth surfaces near the cracks and surfaces that present abrasion marks can be observed. The gray cast iron workpiece material has in its composition silicon carbide, SiC, with high hardness (2500 HV). According to Refs. [11,12], the presence of hard particles in the tribological system can promote wear and the relationship between workpiece/abrasive hardness materials is an important effect on the wear regime. For hardness ratio less than 0.6 the so-called soft abrasion will occur while for ratio more than 0.6 the so-called hard abrasion regime will take place. Large system damage is encountered when hard abrasion is dominated. The tribological system evaluated presents Si3N4 tool material
with average hardness of 1600 HV and the hardness ratio is around 0.64, which situates on the hard abrasion regime. Fig. 7F shows cracks that propagate in several directions. The aspects of worn region of Si3N4 justify the tendency of catastrophic failure of the tools due chipping. Other wear mechanisms were reported by Silva et al. [13] and Vleugels and Van Der Biest [14]. The later showed the incompatibility of the use the of Si3N4 tools at high cutting speed and dry machining of steels because of the high interface temperatures and the chemical compatibility between the materials involved in the tribological system which encourage chemical wear which is dominant under those evaluated cutting conditions. Aspects of the worn surfaces of PCBN cutting tools are shown in Figs. 7–9, those are characterized by presence of
Fig. 7. General view of PCBN cutting tools shows flank wear.
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Fig. 8. Detail shows some wear mechanisms such as micro-cracks, plastic deformation and adhesion of the work piece material.
cracks running to the cutting edge. The presence of abrasion marks is also observed in these figures. This tribological system involves silicon carbides and a PCBN (4000 HV) and the hardness ratio workpiece/abrasive material is around 0.6. Therefore, the wear regime is a transition between soft and hard abrasion, but it is clear the presence of the abrasive wear. Aspects the worn surface suggest that thermal and mechanical cyclic load was the main factor to influence the wear of the cutting tools and fracture toughness resistance should be the main property required when machining with PCBN cutting tools. It was observed a tendency of the workpiece material to adhere into the cracks, hypothesis confirmed by EDX analysis. The results indicated the presence of Fe and Si, elements of the workpiece material. These portions contribute to cracks growth and propagation.
Gastel et al. [15] has studied the performance of PCBN tools on grey cast iron and compact graphite iron turning. They showed that oxidation of the tools and interdiffusion of constituting chemical elements between tool and workpiece were the dominant wear mechanisms. 3.2. Tool wear, surface texture and burr size Behavior of maximum flank wear of cutting tool during engine block production is shown in Fig. 10. System ‘A’ was able to produce 1600 parts, while system ‘B’ could produce 2577 blocks. Tools that compose system ‘A’ showed relatively low wear until 1400 parts were produced and after which a rapid growth was observed. System ‘B’ tools tended to present a uniform increase of flank wear during all the tool life.
Fig. 9. Detail shows evidences wear mechanisms such as abrasion and plastic deformation.
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Fig. 10. Evaluation of flank wear (VBBmax) as a function of number of blocks machined.
The type of wear observed on cutting tools can explain the behavior of the maximum flank wear curves. Si3N4 cutting tools presented a tendency to chip or break as a consequence of formation and propagation of cracks, as shown in Figs. 4–6. The occurrence of chipping implies in an abrupt change on tool geometry, which can be reflected in rapid increase in flank wear, as shown in Fig. 10. On the other hand, PCBN cutting tools presented a gradual increase of flank wear, which is compatible with wear aspects observed in these tools. Formation and growth of cracks were gradually observed, fact that implies in gradual increase of flank wear. Surface average roughnesses obtained by the two systems are shown in Fig. 11. Both systems presented the same behavior throughout the tool life. The exception of this is the last engine block machined with system ‘B’, when the Ra parameter was much higher than the end of tool life criterion RaZ1.6 mm. The PCBN and Si3N4 cutting tools presented different wear patterns, as shown by the different surface profiles of the machined surfaces. To complement the study of surface
texture two other parameters like maximum peak to valley distance (Rt) and waviness (Wt) were also measured as a function of the number of engine blocks machined. Results for Rt are presented in Fig. 12. System ‘A’ tends to produce higher values of Rt parameter during all the tool lives. Engine blocks milled by system ‘B’ tend to present relative uniform values of Rt, while system ‘A’ sowed crescent values. According to the results shown in Fig. 12, both systems exceeded the end of tool life criterion, RtZ10 mm. Results of waviness parameter Wt are shown in Fig. 13. As observed for Rt parameter, system ‘A’ presented higher values of Wt than system ‘B’ practically throughout the tool life. The wear of the PCBN tools was smaller than the wear of the Si3N4 tools. This fact is implied by the smaller cutting effort when machining with the former cutting tools, and consequently smaller deformation and vibration, which results in better surface finish. Burr height as a function of the number of blocks machined is shown in Fig. 14. The ceramic cutting tools tend to produce higher burrs and this difference increases
Fig. 11. Roughness (Ra) as a function of number of blocks machined.
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Fig. 12. Maximum peak to valley distance (Rt) as a function of number of the blocks machined.
when the number of parts machined increases. Again the higher wear of the ceramics compared to the PCBN tools is responsible for these results. According to Olvera and Barrow [16], the thickness and/or the length can be used to characterize the burr but the former is it’s the main dimension. The thicker burrs are the more difficult to be broken during a deburring operation. However, the measurement of its thickness is more difficult and laborious because metallographic preparation is usually required. On the other hand, the length of the burr can be easily measured, allowing a large number of measurements in a short period of time, and it can also be used for its characterization [17]. In the present work it was thus decided to measure the length of the burr instead of its thickness since the entire test were carried out in the production line. Work by Olvera and Barrow [16] in face milling AISI 1040 steel bars at a cutting speed of 142 m/min, investigated the influence of the depth of cut (doc) and the nose radii on the burr length. They founded that the burr length increases
when doc and nose radius increases until a value, called critical value, from which it starts to decrease. These transition points encountered were docZ0.5, 0.8 and 0.9 mm for the tool nose radii of 0.4, 0.8 and 1.2 mm, respectively. Although in the present investigation the work material is cast iron and the cutting speed (1000 m/min) is far higher than that used by Olvera and Barrow [16], which might imply here the development of higher cutting temperatures [9] the transition point might also be present. In this work the tool nose radius was 0.4 mm and the depth of cut was 0.3 mm and they did not vary, and therefore the transition point (or critical value) could not be determined. However, the influence of the wear or the burr length is clear. The burr length increases when the wear increases. This is in agreement with the mathematical model presented by Hashimura et al. [18] that indicates an increase in the burr sizes with increasing in the cutting section area. During all the tests no breakout phenomenon was observed. This means that the amount of wear was not enough to increase the plastic deformation zones to an
Fig. 13. Waviness parameter (Wt) as a function of the number of blocks machined.
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Fig. 14. Burr length (h) as a function of the number of motor blocks machined.
extension necessary to cause rupture of the negative shear plane [18–21] that would culminate in the breakout. 4. Conclusions The conclusions of this study can be summarized as follows: – Milling cutting system ‘B’ with four PCBN wiper cutting tools presented better performance compared with system ‘A’ regardless all parameters considered; – Among all the end of tool life criteria adopted, only the roughness parameter Rt was reached, which makes this the most critical index for product quality; – Si3N4 and PCBN cutting tools presented different dominant wear patterns and the other parameters studied can be related to them; – Concerning to tool materials, the dominant wear mechanisms for Si3N4 were micro-cracks, plastic deformation and abrasion while for PCBN were microcracks, adhesion and abrasion; – Both systems presented satisfactory performance, generating surfaces with topography parameters considerably below the limit established for the process.
Acknowledgements The authors would like to thank Fiat-GM Powertrain Ltd and FAPEMIG (Fundac¸a˜o de Amparo a` Pesquisa do Estado de Minas Gerais), for technical and financial support.
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