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Keywords: Rotary tool; Hard turning; Chip morphology; Tool wear
1. Introduction Turning instead of grinding hardened steel is an economical method to generate a high quality machined surface. During the past few years, there has been significant industrial interest in using dry machining rather than grinding of hardened steel and other difficult-to-machine materials. As an example, dry hard turning of automotive differential side gears is a successful industrial application of this technology. This technology reduces both the machining time and the specific cutting energy, and eliminates the health and environmental hazards associated with coolant usage in conventional machining operations. Although a large volume of literature exists on hard turning [1–10], the control of the tool wear and its effect on the machined surface’s physical properties represent a major technical challenge. Understanding the chip formation mechanism is essential to achieve a better insight of the machining process fundamentals. Saw toothed chip formation is observed during hard turning
Abstract This paper presents a performance assessment of rotary tool during machining hardened steel. The investigation includes an analysis of chip morphology and modes of tool wear. The effect of tool geometry and type of cutting tool material on the tool self-propelled motion are also investigated. Several tool materials were tested for wear resistance including carbide, coated carbide, and ceramics. The self-propelled coated carbide tools showed superior wear resistance. This was demonstrated by evenly distributed flank wear with no evidence of crater wear. The characteristics of temperature generated during machining with the rotary tool are studied. It was shown that reduced tool temperature eliminates the diffusion wear and dominates the abrasion wear. Also, increasing the tool rotational speed shifted the maximum temperature at the chip–tool interface towards the cutting edge. 2002 Elsevier Science Ltd. All rights reserved.
∗
Corresponding author. Fax: +1-506-453-5025. E百度文库mail address: kishawy@unb.ca (H.A. Kishawy).
and was the subject of interest by several researchers [1–9]. Several models for the chip removal mechanism were presented. Some researchers explained the mechanism of saw toothed chip formation by the adiabatic shear theory. However, other researchers attributed the nature of its morphology to crack propagation. Recent studies using quick stop mechanism confirmed that the saw toothed chip is caused by cyclic crack propagation. The integrity of the surface produced by hard turning is another important subject. Controlling the machining induced residual stresses is an important aspect for wide spread industrial application of this technology [3,7]. Hard turning required tool materials that exhibit high wear resistance and an ability to endure the specific cutting forces and high temperature generated. In addition, high indentation hardness of at least three times the workpiece hardness is essential [10]. Since tool wear and plastic deformation of the cutting edge affect the quality and integrity of the machined surface, ceramics and PCBN tools are commonly used for hard turning. Although in earlier studies [11,12] rotary tools made of different materials have shown superior wear resistance and prolonged tool life, their performance during hard turning was only investigated using CBN tipped
H.A. Kishawy ∗, J. Wilcox
Department of Mechanical Engineering, University of New Brunswick, Fredericton, New Brunswick, Canada, E3B 5A3 Received 15 February 2002; received in revised form 7 October 2002; accepted 14 October 2002
rotary tools [13]. In addition, there were no attempts in the open literature to model the temperature characteristics in machining with rotary tools. In this paper an attempt to evaluate the cutting performance of rotary tools, made of different materials, during hard turning is presented. In addition a temperature model is presented to describe the heat transfer characteristics and self-cooling feature of this tool. 2. Experimental procedure A comprehensive testing procedure was carried out to evaluate the rotary tool performance during hard turning. Dry hard turning tests were performed using a 10 hp CNC lathe. Bars of heat-treated AISI 4340 steel (54-56 HRC) having a 75 or 100 mm diameter, and a 200 mm length, were used. The tests were conducted using carbide and TiN coated carbide inserts. Cutting speeds of 100, 130 and 270 m/min, with a feed rate of 0.2 mm/rev, and depths of cut of 0.1 and 0.2 mm were used. Circular inserts having a diameter of 25.4 mm were used. The tool wear was measured at four locations, approximately equidistant, along the perimeter of the insert using a tool maker’s microscope. These values were then averaged to obtain the value of tool wear. Chips were collected for different cutting conditions. These chips were then mounted in epoxy, ground, polished, and etched using a 1.5% Nital solution. The cross-sections of these chips were examined and photographed using an optical microscope. Optical and scanning electron microscopes (SEM) were used to analyze the collected chips and to analyze the modes of tool failure. The tool spinning speed was measured using an optical tachometer. 3. Essential features of rotary tools In a pioneer work, Shaw et al. [14] presented a study of a lathe-type cutting tool in the form of a disk that rotates around its center. The continuous spinning of the tool around its center allows for the use of the entire circumference of the insert. As a result of tool spinning, a fresh portion of the cutting edge is provided and therefore a better distribution of tool flank wear over the entire cutting edge is expected. The spinning action of the tool provides a way for carrying the fluid to the tool point at a high cutting speed as in the case of a journal bearing. In addition, this tool offers a self-cooling feature by which the heat is continuously carried away from the cutting zone. Rotary tools are found in two forms: driven or selfpropelled. The tool spinning action in the driven tool is supplied by an independent external source. In the selfpropelled tool, the spinning action is achieved by the interaction between the tool and the chip. The driven
0890-6955/03/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0890-6955(02)00239-0
434
H.A. Kishawy, J. Wilcox / International Journal of Machine Tools & Manufacture 43 (2003) 433–439
International Journal of Machine Tools & Manufacture 43 (2003) 433–439
Tool wear and chip formation during hard turning with selfpropelled rotary tools
1. Introduction Turning instead of grinding hardened steel is an economical method to generate a high quality machined surface. During the past few years, there has been significant industrial interest in using dry machining rather than grinding of hardened steel and other difficult-to-machine materials. As an example, dry hard turning of automotive differential side gears is a successful industrial application of this technology. This technology reduces both the machining time and the specific cutting energy, and eliminates the health and environmental hazards associated with coolant usage in conventional machining operations. Although a large volume of literature exists on hard turning [1–10], the control of the tool wear and its effect on the machined surface’s physical properties represent a major technical challenge. Understanding the chip formation mechanism is essential to achieve a better insight of the machining process fundamentals. Saw toothed chip formation is observed during hard turning
Abstract This paper presents a performance assessment of rotary tool during machining hardened steel. The investigation includes an analysis of chip morphology and modes of tool wear. The effect of tool geometry and type of cutting tool material on the tool self-propelled motion are also investigated. Several tool materials were tested for wear resistance including carbide, coated carbide, and ceramics. The self-propelled coated carbide tools showed superior wear resistance. This was demonstrated by evenly distributed flank wear with no evidence of crater wear. The characteristics of temperature generated during machining with the rotary tool are studied. It was shown that reduced tool temperature eliminates the diffusion wear and dominates the abrasion wear. Also, increasing the tool rotational speed shifted the maximum temperature at the chip–tool interface towards the cutting edge. 2002 Elsevier Science Ltd. All rights reserved.
∗
Corresponding author. Fax: +1-506-453-5025. E百度文库mail address: kishawy@unb.ca (H.A. Kishawy).
and was the subject of interest by several researchers [1–9]. Several models for the chip removal mechanism were presented. Some researchers explained the mechanism of saw toothed chip formation by the adiabatic shear theory. However, other researchers attributed the nature of its morphology to crack propagation. Recent studies using quick stop mechanism confirmed that the saw toothed chip is caused by cyclic crack propagation. The integrity of the surface produced by hard turning is another important subject. Controlling the machining induced residual stresses is an important aspect for wide spread industrial application of this technology [3,7]. Hard turning required tool materials that exhibit high wear resistance and an ability to endure the specific cutting forces and high temperature generated. In addition, high indentation hardness of at least three times the workpiece hardness is essential [10]. Since tool wear and plastic deformation of the cutting edge affect the quality and integrity of the machined surface, ceramics and PCBN tools are commonly used for hard turning. Although in earlier studies [11,12] rotary tools made of different materials have shown superior wear resistance and prolonged tool life, their performance during hard turning was only investigated using CBN tipped
H.A. Kishawy ∗, J. Wilcox
Department of Mechanical Engineering, University of New Brunswick, Fredericton, New Brunswick, Canada, E3B 5A3 Received 15 February 2002; received in revised form 7 October 2002; accepted 14 October 2002
rotary tools [13]. In addition, there were no attempts in the open literature to model the temperature characteristics in machining with rotary tools. In this paper an attempt to evaluate the cutting performance of rotary tools, made of different materials, during hard turning is presented. In addition a temperature model is presented to describe the heat transfer characteristics and self-cooling feature of this tool. 2. Experimental procedure A comprehensive testing procedure was carried out to evaluate the rotary tool performance during hard turning. Dry hard turning tests were performed using a 10 hp CNC lathe. Bars of heat-treated AISI 4340 steel (54-56 HRC) having a 75 or 100 mm diameter, and a 200 mm length, were used. The tests were conducted using carbide and TiN coated carbide inserts. Cutting speeds of 100, 130 and 270 m/min, with a feed rate of 0.2 mm/rev, and depths of cut of 0.1 and 0.2 mm were used. Circular inserts having a diameter of 25.4 mm were used. The tool wear was measured at four locations, approximately equidistant, along the perimeter of the insert using a tool maker’s microscope. These values were then averaged to obtain the value of tool wear. Chips were collected for different cutting conditions. These chips were then mounted in epoxy, ground, polished, and etched using a 1.5% Nital solution. The cross-sections of these chips were examined and photographed using an optical microscope. Optical and scanning electron microscopes (SEM) were used to analyze the collected chips and to analyze the modes of tool failure. The tool spinning speed was measured using an optical tachometer. 3. Essential features of rotary tools In a pioneer work, Shaw et al. [14] presented a study of a lathe-type cutting tool in the form of a disk that rotates around its center. The continuous spinning of the tool around its center allows for the use of the entire circumference of the insert. As a result of tool spinning, a fresh portion of the cutting edge is provided and therefore a better distribution of tool flank wear over the entire cutting edge is expected. The spinning action of the tool provides a way for carrying the fluid to the tool point at a high cutting speed as in the case of a journal bearing. In addition, this tool offers a self-cooling feature by which the heat is continuously carried away from the cutting zone. Rotary tools are found in two forms: driven or selfpropelled. The tool spinning action in the driven tool is supplied by an independent external source. In the selfpropelled tool, the spinning action is achieved by the interaction between the tool and the chip. The driven
0890-6955/03/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0890-6955(02)00239-0
434
H.A. Kishawy, J. Wilcox / International Journal of Machine Tools & Manufacture 43 (2003) 433–439
International Journal of Machine Tools & Manufacture 43 (2003) 433–439
Tool wear and chip formation during hard turning with selfpropelled rotary tools