Wit Grzesik

A revised model for predicting surface roughness in turning

Abstract The objective of this paper is to demonstrate how tribological interactions at the interface between the chip and the tool control the surface roughness generation in finish turning with a single-point tool. It was assumed that one of the major sources which cause visual differences between the theoretical and natural surface roughness is relatively strong adhesion at the rake-chip interface. By considering this exceptional tribological effect, expressed mathematically in the form of the condition for the transition from ploughing to micro cutting, the minimum undeformed chip thickness was estimated. Consequently, the theoretical surface roughness was calculated based on Brammertzs formula. From the obtained data it is evident that the proposed methodology results in a better evaluation of the natural surface roughness produced by turning.

A computational approach to evaluate temperature and heat partition in machining with multilayer coated tools

Abstract In this paper, analytical models for estimating the interface temperature and heat partition to the chip in continuous dry machining of steels with flat-faced tools treated with multilayer coatings are presented. The database for modeling includes changes in the thermal properties of both workpiece and substrate/coating materials and the Peclet and Fourier numbers occurring at actual interface temperatures. Process outputs involve the average tool–chip interface temperature, the tool–chip contact length, the friction energy and the heat balance between the moving chip and stationary tool. It was found that the heat partition coefficient varies significantly from 0.65 to 0.8 when using multilayer coated tools, and changes from 0.5 to 0.6 for uncoated carbide tools. This implies that the use of multilayer coated tools causes about 30% more heat generated due to friction to be transferred into the moving chip. In general, both power and linear models can be used to estimate the interface temperature.

The influence of thin hard coatings on frictional behaviour in the orthogonal cutting process

New knowledge about the tribological response deriving from the interaction of the substrate/coating-chip system, with special attention to the orthogonal cutting process when chatter-free end turning using natural contact tools, is developed. In order to evaluate the frictional behaviour of this process under modified contact conditions, experimental investigations including the contact temperature, the contact loads, friction and the frictional heat flux per unit area were carried out. In contrast to the most obvious approach, the coefficient of sliding friction versus the cutting speed, the contact temperature, the normal pressure and the interface control factor is considered. A number of different coating structures, starting from single up to three and four layer films, in combination with medium carbon and austenitic stainless steels, were tested. It is pointed out that the results obtained provide a modified approach to the frictional behaviour of the cutting process and its controllability. Among various responses, specific for such tribo-contact pairs, of particular interest is the self-adaptation resulting in controlled generation of friction energy and conduction of the frictional heat flux.

The mechanics of continuous chip formation in oblique cutting with single-edged tool — Part I. Theory

Abstract The paper presents a new point of view on oblique cutting mechanics when cutting with a single-edged tool. This is based on the main assumption that plastic flow of material during chip forming occurs in the set of planes normal to the cutting edge. As a consequence of the transformation of various orthocartesian systems some new generalized relationships for kinematics of oblique cutting operation and plastic deformation in the shear zone are derived.

FINITE DIFFERENCE METHOD-BASED SIMULATION OF TEMPERATURE FIELDS FOR APPLICATION TO ORTHOGONAL CUTTING WITH COATED TOOLS

ABSTRACT A finite difference method was proposed to model the effect of a variety of tool coatings on the magnitude and distribution of temperatures through the tool-chip contact region and the coating/substrate boundaries. For each workpiece-tool pair tested the intensity of uniformly distributed heat flux and relevant analytically obtained values of the heat partition coefficient were assumed to change with variations of cutting speed and the corresponding friction. In this case the simulation of an orthogonal machining of AISI 1045 steel was performed using special computing algorithm with elementary balances of induced energies (MBE). It is concluded that the temperature contours obtained reflect favorable the specific contact conditions existing at the tool-chip interface and substantial differences can arise from coating effects.

Thermophysical-Property-Based Selection of Tool Protective Coatings for Dry Machining of Steels

A method is proposed for determining the optimum cutting conditions in dry turning of carbon and stainless steels using multilayer coated tools for physical criteria such as the maximum temperature of a cutting tool or maximum heat flux. The modified thermal number describing the tool-chip behavior, is introduced. The correlation between the thermal number and the average tool-chip interface temperature and frictional heat flux is examined for both flat-faced and grooved rake configurations. Although not shown here directly, the thermal barrier effect observed for multilayer coatings with an intermediate Al 2 O 3 layer increases the heat partition to the chip at substantially reduced cutting temperatures. The experimental results indicate that thermal and tribological outputs from the interface are sensitive to changes in the thermophysical properties of the workpiece and coating materials, in particular the thermal conductivity, thermal diffusivity and the heat transmission coefficient. It was proven that the described approach makes it possible to select the appropriate cutting conditions based on process constraints such as the heat transfer intensity and temperature on the tool face.

Mechanics of Cutting and Chip Formation

This chapter presents the basic knowledge on mechanics of the machining process in which the workpiece material hardened to 45–70 HRC hardness is machined with mixed ceramic or cubic boron nitride tools. Specific cutting characteristics, including cutting forces and cutting energy, and chip formation mechanisms are discussed in terms of process conditions. Additionally, currently developments in finite-element modelling are overviewed and some representative results are provided.

Surface integrity of machined surfaces

This chapter presents the basic knowledge on surface integrity produced in traditional and non-traditional machining processes. An extended overview of fundamental characteristics of surface finishes and surface integrity including surface roughness/surface topography, specific metallurgical and microstructure alterations and process-induced residual stresses is carried out. Surface roughness was determined by many important 3D roughness parameters and representative scanned surface topographies were included. They allow recognizing the structural features, i.e., determined and random components of the machined surfaces. Moreover, some practical formulae for prediction of the theoretical surface roughness in typical cutting operations (turning and milling) and grinding operations are provided. On the other hand, possible surface alterations resulting from abusive machining operations are demonstrated. Finally, the state-of-the-art of machining technology is addressed to many finishing cutting, abrasive and non-traditional (EDM, ECM, LAM, USM, etc.) operations to show how the manufacturing processes can be effectively utilized and optimized in practice.

Machining of Hard Materials

This chapter presents basic knowledge on the special kind of the machining process in which a workpiece material hardened to 45–70 HRC hardness or more is machined with mixed ceramic or CBN tools. An extended comparison with finish grinding, as well with other abrasive finishing processes, is carried out. Specific cutting characteristics, including cutting forces, chip formation mechanisms and tool wear modes with relevant interface temperatures are discussed in terms of process conditions. Currently developing finite element (FE) and analytical modelling is overviewed. A complete characterization of surface integrity including geometrical features of hard-machined surfaces, along with specific microstructural alterations and process-induced residual stresses, is provided. Finally, the state of the art of hard cutting technology is addressed for many cutting operations to show how manufacturing chains can be effectively utilized and optimized in practice.

The mechanics of continuous chip formation in oblique cutting with single-edged tool—part II. Experimental verification of the theory

Abstract Experimental results obtained during oblique cutting of annealed steel Ck 45 (SAE-AISI 1045) with a single-edged tool are presented. Extensive measurements of forces, cutting ratio, chip flow angle etc. have been carried out under a wide range of cutting conditions. The measured data obtained from these cutting tests are used to test assumptions proposed in the first part of this work. In relation to previous works dealing with oblique cutting problems the present one extends to tools with angles of tool obliquity ranging from 30° to 70° and having a large negative rake angle.