Machining theory for orthogonal metal cutting

Machining theory for orthogonal metal cutting


Nowadays, most manufacturing industries, including aerospace industries, and particularly the machining sector is looking for producing parts with improved functional performance using environmentally friendly processes. The functional performance and in-service life of mechanical components are known to be significantly influenced by the machined surface integrity. Thus, it is worth studying the effects of tool/workpiece interaction and machining system parameters on the produced surface integrity.
To increase part performance in structural applications, new advanced material with highstrength and light-weight materials are, continuously, produced and precipitations treatable aluminum alloys such as AA6061-T6 and AA7075-T6 were ones of the most used lightweight alloys. However, the machining of such alloys is always accompanied by built-up edge formation and tangled chips which can affect the stability of the machining system and thereafter the surface quality of the machined parts. These shortcomings induce many technical issues in automatic control of the process during CNC machining. Moreover, to reduce the tool wear and improve the productivity, cutting fluid has traditionally been used.
On the other hand, the use of dry high cutting speed which increases metal removal rate (MRR), reduces the formation of built up edges (BUE) and burrs (Rao et Shin, 2001) have been looked as an alternative, but it affects the surface integrity of the machined parts (Pawade, Joshi et Brahmankar, 2008). Traditionally the machining of aluminum alloys was performed using positive rake angle. However, it was found that negative rake angle with coated KC910 grade inserts gave the best performance in machining aluminium based metal matrix composite (MMC) (Abdullah, 1996). Vernaza-Peña et al. (Vernaza-Peña, Mason et Ovaert, 2003) reported that as the rake angle decreases, the cutting temperature is mainly generated by shearing in the primary shear zone rather than the friction at the tool/chip interface, in orthogonal machining of 6061-T6 aluminum alloy, which allow reducing the tool wear. Moreover, ultra-precision machining of 6061-T6 alloy was carried out using high negative rake angle (-25°) and encouraging results in terms of surface quality were achieved as documented in (Abou-El-Hossein, Neethling et Olufayo, 2013).

Research objectives

Currently, fundamental understanding of the chip formation (chip morphology, cutting forces, cutting temperature, etc.) and its effect on surface integrity during the machining of ductile alloys and induction hardened steels is still lacking. To do so, the Oxley’s machining theory should be extended to both classes of materials (ductile and hardened) by mean of a selected constitutive equation. In this context, the present study will investigate experimentally and theoretically the chip formation and surface integrity characteristics during the machining of two aluminum alloys (AA6061-T6, AA7075-T651) commonly used for structural applications and an induction hardened steel (AISI 4340) which is typical for mechanical applications. Hence, the main objectives of the present study are:
i) Investigate experimentally the chip formation process and its effect on the surface integrity of AA6061-T6, AA7075-T651, and induction hardened AISI 4340 (58-60 HRC) under high speed dry orthogonal machining conditions.
ii) Develop a methodology to identify the proper material constants of a selected bconstitutive equation (Marusich’s constitutive equation) to simulate the high speed machining of the aluminums AA6061-T6, AA7075-T651, and the induction hardened AISI 4340 (58-60 HRC).

Generalities about the machining process

The earliest machining or cutting of material experiments has been carried out from the latter part of the eighteenth century (Boothroyd, 1988). The term machining deals with any process in which material is removed gradually from a workpiece. The narrower term cutting is intended to include operations in which a thin layer of material, the chip, is removed by a wedge-shaped tool (Grzesik, 2008). Generally, the machined surface is generated by a relative motion between the cutting tool and the workpiece (Figure 1.1). Since the present work deals with the turning process, we will only focus on the lathe machine and cutting tools used in turning. Two kinds of relative motion must be provided by a lathe machine-tool:
The primary motion (rotational or linear) is the main motion, known also as the cutting speed (Astakhov, 2006). The feed motion (feed rate/cutting feed) is a linear motion that is
additionally provided to the tool or workpiece.

 Surface integrity characteristics in machining

Field and Kahles (1964) were the first to introduce the concept of ’’surface integrity” by means of defining « the inherent or enhanced condition of surface produced in machining or other surface generation operation » (M’Saoubi et al., 2008). Their pioneer work led to the subsequent establishment of an American National Standard on surface integrity (ANSI B211.1, 1986). The machining affected surface layer was defined as the layer from the geometrical surface inward that shows changed physical and sometimes chemical properties, as compared with the material before machining (Youssef et El-Hofy, 2008a). These modifications include topographical features (surface finish), mechanical, and metallurgical alterations of the machined surface (Figure 1-11) and their relationship to functional performance (Griffiths, 2001).

 Machining of aeronautic aluminum alloys

The machining of aeronautical aluminum parts is characterized by the generation of complex shapes (Figure 1-18). The volume of metal to be removed during the machining of such parts is significantly high which increases the machining time. To be more efficient and competitive, most of machining workshops working in aeronautical industry have integrated the high speed machining HSM. The use of high cutting speed increases metal removal rate (Rao et Shin, 2001); however, it affects the chip formation mechanisms and surface integrity characteristics of the machined parts (Pawade, Joshi et Brahmankar, 2008).

 Surface integrity effects in hard machining

Hard machined parts are exposed to different alterations including mechanical, metallurgical, thermal and chemical, during the machining (Youssef et El-Hofy, 2008b). the mechanical alterations are represented by the residual stresses, plastic deformations, and microhardness gradients (softening or hardening). The metallurgical alterations include phase changes (white layer, dark layer).

 Comparison between hard machining and grinding

The hard machining process was developed in order to replace the grinding operations during the manufacturing of hard materials. Compared to grinding, hard machining enables relatively high material removal rates, great flexibility, the manufacture of complex shapes in a single setup, and hence, substantial cost reduction. Additionally, since hard turning is a single-point cutting tool process, it is possible to precisely modify the rake angle to control tool wear and surface integrity and to adapt hard turning to both roughing and finishing operations. This is not possible for grinding, due to multiple edges which are randomly scattered on the grinding wheel, and the effective rake angles vary over a large range (Dahlman, Gunnberg et Jacobson, 2004). Therefore, hard machining, particularly hard turning is increasingly accepted as a competitive process and as an effective alternative for grinding operations.


The current research work addressed a comprehensive investigation on materials behaviour in high speed machining of some high strength aluminum alloys (AA6061-T6, AA7075- T651) and induction hardened AISI 4340 steel used in structural aeronautic components. This investigation involved the characterization of the surface integrity of the machined surfaces and chip formation during machining. For the experimental investigation, the focus was on the establishment of relationships among the surface integrity characteristics (surface finish, residual stress, microhardness, and microstructure), technological parameters (cutting speed and feed rate), and machining data (cutting forces, cutting temperatures, chip thickness, etc.). The machining tests were carried out under orthogonal cutting conditions with varied cutting speeds and feed rates. The cutting conditions were selected to cover a wide range of cutting speed and ratio of the uncut chip thickness to the tool cutting edge radius.
Observing the experimental results, it was argued that the surface integrity characteristics can be related directly to the machining data such as cutting forces, shear and friction angles, and cutting temperature. The prediction of such quantities is of great importance and could help in understanding the mechanisms of surface alterations over a wide range of machining conditions.

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Table des matières

1.1 Introduction
1.2 Generalities about the machining process
1.2.1 Cutting tool angles
1.2.2 Chip formation in machining Cutting forces system in turning Cutting forces system in orthogonal machining
1.2.3 Surface integrity characteristics in machining Surface topography of the machined parts Mechanical alterations Metallurgical alterations
1.3 Machining of aeronautic aluminum alloys
1.3.1 Chip formation in machining aluminum alloys
1.3.2 Surface integrity in machining aluminum alloys Surface finish in machining aluminum alloys Residual stress in machining aluminum alloys
1.4 Machining of hardened steel (Hard machining)
1.4.1 Chip formation and cutting forces in hard machining
1.4.2 Surface integrity effects in hard machining Surface finish in hard machining Residual stress in hard machining Plastic deformation and work hardening in hard machining White layer formation in hard machining
1.4.3 Comparison between hard machining and grinding
1.5 Modeling of the machining process
1.5.1 Basics of Oxley’s machining theory (Hastings, Mathew et Oxley,
1980; Kopalinsky et Oxley, 1984; Oxley et Hastings, 1977)
1.5.2 Generalization of the Oxley’s machining theory
1.5.3 Material’s constitutive equations applied to the machining modeling
1.6 Concluding remarks
2.1 Introduction
2.2 Experimental Procedure
2.3 Results and Discussion
2.3.1 Surface Finish
2.3.2 Residual Stress
2.4 Conclusions
3.1 Introduction
3.2 Material and methods
3.3 Results
3.3.1 Induction hardening treatment
3.3.2 Influence of the tool retraction
3.3.3 Surface finish
3.3.4 Residual stress
3.3.5 Microstructure
3.3.6 Microhardness
3.4 Discussion
3.5 Conclusions
4.1 Introduction
4.2 Experimentation
4.2.1 Work materials
4.2.2 Experimental methods
4.2.3 Design of experiment (DOE)
4.3 Response surface modeling (RSM)
4.4 Identification of Marusich’s constitutive equation (MCE)
4.5 Validation of M1 and M2 models using dynamic tests
4.6 Finite element modeling
4.6.1 Numerical validation and sensitivity analysis
4.6.2 Prediction of machining forces
4.6.3 Prediction of chip morphology and tool/chip contact length
4.6.4 Prediction of cutting temperature distributions
4.7 Conclusions
5.1 Introduction
5.2 Machining theory for orthogonal metal cutting
5.2.1 Extension of the Oxley’s predictive theory
5.2.2 Assumptions and model geometry
5.2.3 Calculation of state variables in the primary shear zone AB
5.2.4 Calculation of state variables along the tool/chip interface
5.2.5 Some issues in calculating the strain rate constant 􀡯􀫙 and 􀢾
5.3 Model validation and discussion
5.3.1 Aluminum AA6061-T6
5.3.2 Aluminum AA7075-T651
5.3.3 Hardened steel AISI4340
5.4 Discussion
5.4.1 Cutting force prediction
5.4.2 Chip thickness and contact length
5.4.3 Effects on 􀡯􀫙 and 􀢾 values
5.5 Conclusions

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