J. Rech

Surface integrity in finish hard turning of case-hardened steels

Abstract Highly stressed steel components, e.g., gears and bearing parts, are appropriate applications for hard turning. Therefore, the process effects on significant engineering properties of work materials have to be carefully analyzed. Roughness, residual stresses, and white layers as parts of surface integrity, are functions of the machining parameters and of the cuttability of the cutting edge, i.e. of the tool wear. The aim of this work was to study the influence of feed rate, cutting speed, and tool wear on the effects induced by hard turning on case-hardened 27MnCr5 gear conebrakes and to point out the technical limitations in mass production.

OPTIMIZATION OF THE CUTTING EDGE GEOMETRY OF COATED CARBIDE TOOLS IN DRY TURNING OF STEELS USING A FINITE ELEMENT ANALYSIS

This paper investigates the effects of edge radius of a round-edge coated carbide tool on chip formation, cutting forces, and tool stresses in orthogonal cutting of an alloy steel 42CrMo4 (AISI 4142H). A comprehensive experimental study by end turning of thin-walled tubes is conducted, using advanced coated tools with well-defined cutting edge radii ranging from 5 to 68 microns. In parallel, 2-D finite element cutting simulations based on Lagrangian thermo-viscoplastic formulation are used to predict the cutting temperatures and tool-stress distributions within the tool coating and substrate. The results obtained from this study provide a fundamental understanding of the cutting mechanics for the coated carbide tool used, and can assist in the optimization of tool edge design for more complex geometries, such as chamfered edge. Specifically, the results obtained from the experiments and simulations of this study demonstrated that finite element analysis can significantly help in optimizing the design of coated cutting tools through the prediction of tool stresses and temperatures, especially within the coating layer.

Workpiece Surface Integrity

This chapter presents an analysis of workpiece surface integrity. The definition and material and mechanical aspects of surface integrity are discussed.

Effects of Lubrication Mode on Friction and Heat Partition Coefficients at the Tool–Work Material Interface in Machining

The quantification of friction coefficient along the tool–work material interface in machining remains an issue in tribology. This article aims at identifying the evolution of friction coefficient for a large range of sliding velocity during the machining of an AISI 4140 steel (290 HB) with a TiN-coated carbide tool. The influence of various lubricants (straight oil or emulsion) and lubrication modes (flow or mist) is investigated and compared to a dry sliding situation (dry machining). It has been shown that, in dry machining, the friction coefficient decreases with the sliding velocity until reaching a lower limit around 0.2. On the contrary, the presence of a straight oil significantly decreases friction coefficients to a value around 0.1. Emulsion enables a significant decrease of friction coefficient to around 0.2 for low sliding velocities, whereas its action is absent for higher sliding velocities. Oil mist exhibits an intermediate behavior. Finally, it has been shown that all kinds of lubrication lead to a large decrease of heat flux transmitted to cutting tools, even if heat flux is almost similar irrespective of the nature of oil and of its application mode. Straight oils decrease the heat flux transmitted to cutting tools due the decrease of friction coefficient, whereas they are not able to modify the heat partition coefficient at the interface. On the contrary, emulsions limit the transmission of heat flux to cutting tools due to their lower heat partition coefficient, even if their influence on friction is more limited.

Energetic analysis of cutting mechanisms in belt finishing of hard materials

Belt finishing has been tested successfully as a complementary process to hard turning. This technology improves the surface texture and generates compressive residual stress. However, the mechanisms and characteristics of this new process have not yet been fully explained. This article provides a comprehensive characterization of cutting mechanisms generated by belt finishing. First, an analytical analysis based on cutting forces is developed. Then, the macroscopic specific energy is dissociated into a cutting specific energy responsible of shearing and ploughing mechanisms and a sliding specific energy due to adhesion. It has been demonstrated that cutting is more predominant than sliding in belt finishing process. The omnipresence of cutting demonstrates the effectiveness and the profitability of belt finishing operation.

Characterization of friction coefficient and heat partition coefficient between an AISI4140 steel and a TiN-coated carbide – influence of (Ca, Mn, S) steel's inclusions

Abstract This work aims at characterizing the influence of Ca—Mn—S inclusions in an AISI4140 steel (280 HB) during friction against a TiN-coated carbide under extreme conditions. A specially designed open tribometer has been used to characterize the friction coefficient and heat partition coefficient at the contact under specific extreme contact conditions simulating those occurring at the tool—chip—workpiece interface in dry cutting. It has been shown that sliding velocity is the most influential parameter. A great decrease of the friction coefficient and heat partition coefficient is observed under high sliding velocities. Moreover, it has been revealed that inclusions lead to a great decrease of friction at low sliding speeds, irrespective of contact pressure. On the contrary, under high sliding velocities, there is no significant influence of inclusions. Additionally, it has been observed that inclusions are not able to modify the heat partition coefficient at the interface. Finally, this work provides quantitative data of the friction coefficient and heat partition coefficient versus sliding velocity and contact pressure in order to be implemented in any analytical and numerical model.

Analytical modeling of thrust force and torque in drilling

The prediction of thrust force and torque in drilling remains a key issue. There are three main ways to determine these forces based on experimental, numerical and finally analytical approaches. The major drawback with numerical and analytical methods concerns their reliability compared to phenomenological models. As a consequence, several studies use a resetting method in order to correct parameters of their analytical or numerical models so that they correspond to experimental results. The goal of this article is to introduce a new analytical model in drilling based on the discretization of the cutting edge. Local forces are estimated with a semiorthogonal analytical model based on a modified Merchant’s model. Parameters have been identified by a basic semiorthogonal cutting test for a large range of cutting speed and feed rates, by friction tests for a range of sliding velocities and by a variable shear angle model. The macroscopic feed force and torque are estimated by the sum of each local force along the cutting edge. Two drills applied in a large range of cutting conditions are investigated to validate this approach.

Determination of the maximum diameter of free fines to assess the internal stability of coarse granular materials

In granular soils, suffusion may occur only if three conditions are met: (i) the hydraulic gradient is high enough; (ii) the fine particles are not blocked by the constrictions of the larger ones; and (iii) the fine particles are free to move, i.e. they are not prevented to move by a steric condition. The aim of the paper is to determine the maximum diameter of the particles for the third condition, which are called the “free fines”. Three methods are examined: (i) the direct measurement of the bulk volume of binary mixtures of fine and coarse grains, (ii) the method proposed by Ghafghazi and Azhari in 2012 and (iii) a new method based on the calculation of the fine content for which the fines fill the voids between the coarse grains. Some methods, proved difficult to apply in practice, whereas the new method appeared consistent with the available experimental results. In particular, the method permitted to predict the results of flow tests carried out on binary mixtures of glass balls as well as gap-graded granular materials or materials with continuous grain size distributions.

Analysis of the internal stability of coarse granular materials according to various criteria

Flow tests under high hydraulic gradients were carried out to estimate the internal stability of various granular materials with grains ranging from .08 to 16 mm. Twenty grain size distributions were tested, either continuous or discontinuous (gap-graded). After the tests, the materials were separated into three layers and the grain size distribution of each layer was measured. The soils were classified as stable if the maximum difference between the passing percentages of the upper and lower layer did not exceed 5% and as unstable otherwise. The experimental data were compared with the results given by some internal stability criteria which are obased on very different approaches. The same method was applied to about 40 soils from the literature. It was found that the simple criterion of Kézdi, which consists in separating the soils into ‘coarse’ and ‘fine’ fractions at different values of the diameter and applying Terzaghi filter rule to the two fractions, led to global agreement with the experimental data, whereas the two other criteria gave unsatisfactory results. However, a few differences were noted between Kézdi classification and the results of the tests, showing the need for further research, especially in one case (over 65) where the soil was classified as stable and proved unstable.

Emissivity calibration for temperature measurement using infrared thermography in orthogonal cutting of 316L and 100Cr6 grinding

Material removal operations such as turning or grinding are prone to generate very high temperatures at the tool/chip and tool/workpiece interfaces. These phenomena are involved in studies concerning tools or workpieces, and their estimation is a key point for predicting damages. Temperature elevation is the main cause in workpieces worsening because it generates residual stresses and metallurgical modifications. It is also linked to the tools wear because of the thermal fatigue phenomena and the thermally activated diffusion process. In this paper, a first attempt to measure the temperature fields during 316L orthogonal cutting and 100Cr6 grinding is presented and can be divided in three parts. In the first part the physics of temperature measurement using infrared thermography are presented. Then, the calibration of the infrared camera is realized and allows to obtain of the emissivity curves of 316L and 100Cr6 steels. To do so, an experimental device has been set up to reproduce the luminance recording conditions encountered during the machining operations. The last step is the computation of all the experimental data to obtain the temperature fields from the recorded luminance and the 316L and 100Cr6 emissivity curve. At last, temperature level measured is compared to those presented in the bibliography.