The Effect Of Friction On Surface Finishing During Orthogonal Cutting Machine

ABSTRACT

The generation of residual stresses in orthogonal machining is analysed by using an Arbitrary Lagrangian Eulerian (ALE) finite element approach. It is shown that a substantial level of tensile residual stresses can be obtained in the vicinity of the machined surface without any contribution of thermal effects. This motivates the development of a parametric study to analyse the effects of the thermomechanical coupling parameters on residual stresses. The roles of thermal expansion, of thermal softening and of the Taylor–Quinney coefficient (controlling the heat generated by plastic flow) are considered separately. The influence of friction is also analysed by assuming dry cutting conditions and a Coulomb friction law. The friction coefficient has a complex effect by controlling heat generation (frictional heating) along the tool rake and clearance faces and the propensity for the chip to stick to the tool. Geometrical effects such as the tool rake angle and the tool edge radius are also discussed.

CHAPTER ONE

1.0                                             INTRODUCTION

The friction along the tool-chip interface has a significant effect on almost all cutting parameters such as cutting forces, cutting temperature, tool life, workpiece surface integrity. Therefore, understanding the effects of different friction model and predicting them is of a crucial importance. Early analyses

of metal cutting were based on simple models, such as the shear-angle approach proposed by Oxley or Merchant [1], and lack of relevant models describing the frictional phenomena around the interfaces: tool-chip contact, tool-worpiece contact . Recently a new tribometer has been designed in order to reach this projective, and this system has been applied to indentify a new friction model in the context of dry cutting of AISI316L stainless steel [2]. In order to obtain a friction model usable in numerical modeling of cutting, a new identification method has been developed [3]. This model introduces a new concept that the most relevant parameter influencing the adhesive friction coefficient is the local sliding velocity of the worpiece against the cutting tool. The finite element studied by Komvopoulos and Erpenbeck [4] focuses its attention on chip formation in orthogonal metal cutting and on the effect of such factors as plastic flow properties of the workpiece material, friction at the tool-worpiece interface, and wear of the tool, on the cutting pro

cess, and the simulation of chip separation was achieved by using the distance tolerance ctiterion.

Moufki et al [5] proposed a temperature-dependent friction modeling of metal cutting. Their model depended on a shear angle solution, an estimation of the interface temperature, and an estimation of pressure distribution on the rake face. They have compared their predicted value of mean coffecients of friction with those deduced from cutting forces.

Although a great work that have been done in modeling metal cutting process of tool- chip friction, as indicated by the references cited above, there are still fundamental issues that remain open and deserve detailed analysis. The issue of the effect of tool-chip friction in orthogonal cutting AISI4340 is studied on thermo-mechanical influence, and to show the frictional behavior along the tool-chip interface. The finite element code ABAQUS 6.8 was used to simulate the orthogonal cutting according to a new friction coefficient calculation formulation.

1.1                                           BACKGROUND OF THE STUDY

Interest in the development of ultrafine machining technology has been growing over the past two decades due to the trend toward higher accuracy and smaller-sized components from current  and  emerging  technologies.  Micro- and  nano-technology  industries  need a deeper understanding of the fundamental mechanics governing nanometric machining processes where the removed layer of material is less than 100 nm.  Brittle materials such as semiconductors can undergo a ductile–brittle transition, which results in fracture within the substrate. Thus, it is essential to establish proper guidelines to achieve and sustain the desirable ductile mode of deformation during nanometric machining [1, 2]. However, it is unclear how applicable is the knowledge from conventional macroscopic machining, a now mature field, to nanometric machining. For instance, due to the small scales involved in nanomachining, the process is far more sensitive to interface effects such as surface adhesion and imperfections (tool and workpiece heterogeneity).

Apart from atomic force microscopy [3–5], experimental studies of nanometric machining are virtually non-existent due to the small scale of the specimens, the even smaller machining depths, and the small time scales during which the main mechanisms of the process take place. However, with the advent of massive computing and parallel molecular dynamics (MD) software [6], we are acquiring the ability to perform virtual experiments on such intricate processes. For sufficiently large system sizes and durations, and with appropriate interatomic potentials and initial and boundary conditions, MD simulations can provide detailed insight into the stress and temperature fields in the cutting region, the nucleation and motion of dislocations, the shear zone and chip formation process, and the resulting forces on the cutting tool all of which influence surface finish quality and tool wear.

Recently, the growing interest in understanding nanometric machining for nanotechnolog- ical applications has motivated several studies, almost all based on classical MD calculations of nanometric cutting. Several factors have been studied including the interatomic potential (Tersoff, Morse, embedded atom method (EAM) [7]), the system size (from a few thousand to a few million atoms [8]), and tool geometry (triangular, atomically sharp tip, AFM cantilever beam geometry, or rounded tip [9–11]). In addition, many studies have tried to determine the ideal machining parameters by varying the cutting conditions such as cutting speed (10–1000 m s−1) [12], cutting direction [13], substrate crystal orientation [13], cutting depth [14], and rake angle [1, 15]. The material systems have also been varied between metallic crys- talline materials such as copper, aluminum, gold [1, 4, 15, 16] and semiconductor crystalline materials such as silicon [2, 17, 18], which usually undergo the aforementioned undesirable ductile to brittle transition. Other relevant research efforts comprise the effect of third-body interactions on chip formation and tool wear [19] and studies which mimic AFM experiments by simulating scratching processes [3–5, 10, 20].

Despite this wealth of previous studies, it is our belief that the full potential of MD simulations has not been fully harvested. In particular, to the best of our knowledge, no previous work has investigated the evolution of friction at the tool–chip interface during orthogonal cutting, although this is an essential contributor to tool life and surface finish quality. Temperature fields have also been overlooked, although the choice of appropriate interatomic potentials and system sizes should allow the computation of temperature variations within the chip. The objective of this paper is to determine, through systematic parametric studies (tool piece geometry, lattice orientation, machining velocity and machined thickness), the underlying thermomechanical reasons dictating the level of resistance encountered by the tool and the degree of friction at the tool–chip interface during orthogonal nanometric cutting. In addition, whenever possible, we will make comparisons between conventional macro/micro- ultrafine machining and nanometric machining. With the noteworthy exception of the review work of Robinson and Jackson [21], limited attempts have been made to highlight differences and similarities with conventional machining. This paper intends to partially fill this gap.

1.2                                                 OBJECTIVE OF THE PROJECT

At the end of this work student involved will be able to learn and understand:

1. The orthogonal method of cutting iron
2. Understand the effect of friction on the surface finishing of metal during orthogonal metal cutting method

• The usefulness of friction of machining

1.3                                                    PURPOSE OF THE STUDY

The main purpose of the proposed research work is to highlight the effect of friction insert in the case of orthogonal cutting operation.

1.4                                                     SCOPE OF THE PROJECT

In this paper a new frictional model of cutting process [1] developed to gain better insight into the mechanics of frictional chatter is presented. The model takes into account the forces acting on the tool face as well as on the tool flank. Nonlinear dynamic behavior is presented using bifurcation diagrams for nominal uncut chip thickness (feed rate) as the bifurcation parameters. The influence of the depth of cut for different tool stiffnesses have been investigated. Finally, the influence of the tool flank forces on the system dynamics is studied.

1.5                                               SIGNIFICANCE OF THE STUDY

Friction plays a very important role in machining titanium and nickel alloys. It is the source for the high amount of heat generation, and as a result, the excessive tool  wear during machining these materials. The worn tool is known to create lower surface qualities with tensile surface residual stresses and machine-induced hardening at the surface, as well as high surface roughness. It is essential to create a method to determine how and to what extent the friction is built up on the tool. This study facilitates a determination methodology to estimate the effect of friction coefficients between the tool and the chip on the rake face, as well as the tool and the work piece on the flank face of the tool.