Mechanics of high speed cutting with curvilinear edge tools

https://doi.org/10.1016/j.ijmachtools.2007.08.015Get rights and content

Abstract

High speed cutting is advantageous due to the reduced forces and power, increased energy savings, and overall improved productivity for discrete-part metal manufacturing. However, tool edge geometry and combined cutting conditions highly affects the performance of high speed cutting. In this study, mechanics of cutting with curvilinear (round and oval-like) edge preparation tools in the presence of dead metal zone has been presented to investigate the effects of edge geometry and cutting conditions on the friction and resultant tool temperatures. An analytical slip-line field model is utilized to study the cutting mechanics and friction at the tool-chip and tool–workpiece interfaces in the presence of the dead metal zone in machining with negative rake curvilinear PCBN tools. Inserts with six different edge designs, including a chamfered edge, are tested with a set of orthogonal cutting experiments on AISI 4340 steel. Friction conditions in each different edge design are identified by utilizing the forces and chip geometries measured. Finite-element simulations are conducted using the friction conditions identified and process predictions are compared with experiments. Analyses of temperature, strain, and stress fields are utilized in understanding the mechanics of machining with curvilinear tools.

Introduction

The design of cutting edge geometry and its influence on machining performance have been a research topic in metal cutting for a long time. Emerging machining techniques such as hard turning, hard milling, and micromechanical machining where the uncut chip thickness and the tool edge dimension are in the same order of magnitude require cutting edges which can withstand high mechanical and thermal stresses, hence wear, for a prolonged machining time. It is known that sharp tools are not suitable for such machining operations, therefore, tool manufacturers introduced different types of tool edge preparations such as chamfered, double chamfered, chamfer+hone, honed, and waterfall hone (oval-like) edge designs. Chamfered tools are usually used in roughing and interrupted turning. The stable trapped material (dead metal zone—DMZ or cap) in front of the chamfered cutting edge increases the strength of the tool tip; however, it also increases cutting forces. Honed tools are employed in finish turning operations since the application of hone to the tool tip increases the impact resistance. Waterfall hone edge geometry combines the appropriate characteristics of chamfered and honed tools such as increased tool tip strength and increased rake angle. Its oval-like geometry eases the flow of work material in front of the tool.

The proper selection of edge preparation (edge radius, chamfer angle, and height) can be possible once the behavior of material flow around the cutting edge is well understood. The effect of edge preparation on the mechanics of cutting has been investigated by many researchers by using various methods such as analytical [1], [2], [3], [4], [5], [6], [7], computational [8], [9], [10], [11], [12], and experimental [13], [14], [15], [16] methods. The initial motivation of studying the effects of edge preparation was to understand the ploughing phenomena [1], [13], [17]. A tertiary shear zone at the tool–workpiece interface was believed to be responsible for additional cutting forces. In an early study, Mayer and Stauffer [18] compared the performance of the honed and chamfer tool inserts with sharp tools during non-interrupted machining of AISI 1045 steel. They found that the increasing hone radius and chamfer width and angle results in increased forces and decreased tool life.

In modeling the mechanics of cutting in the stagnant metal zone by using analytical techniques, two major approaches have been proposed. The first approach is based on the existence of a stagnation point on the tool round edge where the material flow is diverted upwards and downwards [5], [7]. The second approach considers a stable build up of material in front of the tool edge like a DMZ which diverts material flow [4]. Waldorf [19] compared these two approaches for AL 6061-T6 aluminum and AISI 4340 steel, and concluded that the model with stable build up describes experimental results better. Stable trapped work material zone formation was observed by Kountanya et al. [20] for honed tools. As for stagnation point based approaches, Manjunathaiah et al. [5] utilized equivalent chamfer geometry for the honed tool by identifying the stagnation point on the tool. Fang [7] presented a detailed slip-line field analysis for rounded-edge tools based on stagnation point assumption. Later Fang and Wu [21] compared honed and chamfered edges during machining of aluminum alloys. All the work mentioned above, especially the ones based on stagnation point assumption, considered cutting tools with a positive rake angle. Recently, Ranganath et al. [22] investigated the effects of edge radius for machining of cast iron. Honed cutting tools with various edge radii and rake angles were tested. Proposed mechanistic model based on stagnation point assumption successfully captured the effect of hone radius on cutting forces during cutting with positive rake angle tools, however high prediction errors were obtained at zero and negative rake angle cutting conditions. In this study, DMZ assumption is adopted due to the fact that negative rake tools are used.

Complex material flow around honed tools, especially in finish machining conditions, can be modeled and simulated with finite element modeling (FEM) techniques. In FEM models, workpiece material properties and the edge geometry of the cutting tool can be defined and process variables such as forces, temperature distributions, stresses, etc. can be obtained. Kim et al. [8] studied the effects of honed edge preparation on the forces and temperatures in orthogonal cutting by using finite element analysis and showed that tool edge radius influences field variables such as temperature distributions and strain rate. They observed increasing cutting forces and temperatures, decreasing maximum effective strain rate with increasing edge radius. In another finite-element simulation based study, Yen et al. [11] observed the effect of various tool edge geometries on the field variables. They also observed increasing average rake face temperature in the tool, increasing effective strain distribution in the chip and workpiece with increasing edge radius. Recently Chen et al. [12] investigated the performance of honed and chamfered PCBN cutting tools for hard turning of AISI 52100 steel. They concluded that the optimum selection of edge preparation depends on machining parameters.

It must also be noted that the correct definition of friction conditions are crucial in order to obtain meaningful results from the finite-element models. Sartkulvanich et al. [23] performed a sensitivity analysis and showed the effect of friction and flow stress models on the outputs of 2D cutting finite element simulations. Özel [24] investigated the tool–chip interfacial frictional models by using FEM and concluded that when frictional properties and workpiece material behavior are properly modeled, FEM models can offer accurate and viable predictions.

The goal of this work is to understand the mechanics of high speed machining and complex material flow around the curvilinear (rounded) cutting edge tools. Orthogonal cutting tests and slip-line modeling is performed to identify friction factors and DMZ angles. A cutting speed range of 125–175 m/min is selected for the cutting tests that is considered transition to the high speed machining range for AISI 4340 steel [32]. Recommended cutting speeds by the cutting tool suppliers are much more conservative. These two cutting speeds represent two values toward higher end of the recommended cutting speeds. Furthermore, selecting very high cutting speeds (anything above 200 m/min) is avoided in order to be able to use plasticity based slip-line field analysis and Johnson–Cook material model for FEA. The performances of honed and waterfall hone type of edges is compared in terms of cutting forces, temperature, and stress distributions. The basic geometrical comparison of round hone (rε) and waterfall hone (rε/2: rε) is given in Fig. 1. In the waterfall hone edge design, the ratio of the side of the edge to its top is usually taken as 1:2. Current edge preparation technology can provide about 0.005 mm (0.0002″) repeatability for the curvilinear edges (hone and waterfall hone) for CBN tools [33]. That is possible because the high resistance to wear presented by the CBN. On this superhard material the edge erosion is slow enough that edge preparation process be controlled much closer than if it were ceramic or carbide cutting tool material [33].

Section snippets

Slip line modeling for machining with curvilinear edge tools

Slip line filed analysis based on plasticity theory has been used to model orthogonal metal cutting as reviewed in Childs et al. [25] and Fang et al. [26]. Abebe and Appl [27] proposed a slip-line field model for machining with large negative rake angle tools by considering stagnant metal zone in front of the cutting tool. Waldorf et al. [4] proposed a slip-line model to study ploughing and tool wear mechanisms in round edge cutting tools. Fang et al. [26] presented a universal slip-line field

Experimental setup and results

Orthogonal turning of thin webs (2.5–2.8 mm) were performed on annealed AISI 4340 steel using CBN cutting tool inserts (TNG-423) with five different hone and waterfall hone and a chamfered edge design (20° chamfer angle and 0.1 mm chamfer height) in a rigid CNC turning center as illustrated in Fig. 3. The tool holder provided a negative 7° rake angle; hence a negative 27° angle is formed at the chamfer face. The images of the round and waterfall (oval-like) edge preparation of the CBN insert

Identification of slip-line and DMZ angles and friction factors

By using the friction factor identification procedure, rake face friction factor m3, DMZ angle α, DMZ friction factor m1, and slip-line angle θ can be determined. Fig. 7 shows the relationship between the ratios of uncut chip thickness to edge radius to the rake face friction factors for different cutting speeds. According to these results, as the ratio of uncut chip thickness to edge radius increases, friction factor on the rake face decreases. As cutting speed increases, rake face friction

Finite-element analysis

In order to compare field variables such as temperature distributions, strains, and maximum effective stresses in the tool, finite-element simulations were performed by using commercial software DEFORM-2D®. Johnson-Cook [29] work material constitutive constants for AISI-4340 steel A=1504 MPa, B=569 MPa, n=0.22, C=0.003, and m=1.17 was adapted from Gray et al. [30] and used in simulations under rigid–plastic material deformation conditions. As mentioned before, friction definition is crucial in

Conclusions

In this paper, the tool–chip friction characteristics of curvilinear edge tools are investigated by utilizing orthogonal cutting tests, slip-line field analysis, and finite-element simulations. Orthogonal cutting tests were used to identify slip-line angles which yielded tool–chip friction characteristics of curvilinear edge cutting tools. Finite-element simulations, which make use of the friction factor findings of the slip-line field analysis, are used to study temperature, strain, and stress

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    Present address: Industrial Engineering, Bilkent University, Turkey.

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