Restricted Contact Tools Research Articles

Machining performance evaluation and tool wear analysis of dry cutting austenitic stainless steel with variable-length restricted contact tools

Austenitic stainless steel during machining still faces several challenges in terms of poor machinability and severe tool wear. In this work, different rake surface restricted contact tools with an inconstant tool-chip contact length are designed and fabricated. An in-depth investigation is then performed to comprehend the influence of the variable-length restricted contact tools on the machining performance and tool wear during dry cutting of AISI 316L austenitic stainless steel. The results indicate that considerable improvements in tribological properties and tool life are achieved with the developed tools. The cutting force, cutting temperature, chip curl radius, and chip thickness decrease compared to traditional restricted contact tools. A dramatic reduction in flank wear of up to 44.18% is obtained in variable-length restricted contact tools compared with traditional restricted contact tools. Variable-length restricted contact structures also reduce the adhesion rate and abrasion damage of tool wear processes. The insights in this study exhibit an efficient method of dry machining stainless steel and may provide a basis for designing cutting tools for green machining of difficult-to-cut materials.

Analytical model of cutting temperature field in workpiece including uncut chip layer

To understand the nature of chip formation in cutting, the study of temperature field in the uncut chip layer of the workpiece is extremely important. In this study, the temperature field for the uncut chip layer is modeled using the heat source projection at different depths of the layer along the cutting speed direction to obtain the different heating times of points at corresponding depths. It is based on the model of the cutting temperature field in the machined surface layer of the workpiece, as proposed in the previous paper of the authors. The temperature distribution in both uncut chip and machined surface layers are thus achieved, and the temperature penetration depths in the machined surface layer are compared with experiment results. The results show that the computed isotherms on the boundary of the uncut chip and machined surface layers are continuous and smooth, indicating that the temperature field model for the uncut chip layer is correct. The temperature field for the nonplanar primary shear plane heat source is also modeled by introducing the curve integral. This is considerably significant in the conduct of further research on the temperature field in micromachining considering size effects and in machining using restricted contact tools, such as groove-type chip breaker tool, double-rake-angled tool, rounded-edge tool, and tool with built-up edge.

Chip inward curl and its influence on dimensional accuracy in groove cutting with a novel restricted contact tool

Tool geometry has a significant influence on cutting force, cutting temperature, and surface quality. Restricted contact tools show good advantages in reducing cutting force, decreasing temperature, and improving surface quality. In this article, a new pattern of chip curl, “chip inward curl,” and its influence on dimensional accuracy were investigated using a novel restricted contact tool when cutting grooves. This novel restricted contact tool has symmetrical tool/chip contact length which varies along the main cutting edge and has a smaller tool land length in the middle of the rake face. In order to analyze the chip inward curl and its influence on dimensional accuracy with this novel restricted contact tool, a theoretical method to predict the chip back-flow angle using a slip-line model taking into consideration of cutting tool edge was proposed. Dimensional accuracy of the grooves achieved by this novel restricted contact tool in groove cutting tests was investigated and compared with those cut by a conventional flat-faced tool. Also, three-dimensional finite element models were developed to analyze the chip back-flow angle and chip inward curl. Results indicate that this new type of restricted contact tool has significant advantages in the control of chip curl and obtains better dimensional accuracy in groove cutting.

Prediction of sticking and sliding lengths on the rake faces of tools using cutting forces

The algebraic relationships between friction force F and normal force N on the rake face of a tool, and between average stress distributions qF and qN, are derived for an assumed power law stress distribution of the normal contact pressure p between chip and rake face, i.e. for p=po(x/L)n, as used by Zorev, where x is measured towards the tip of the tool from an origin at the end of the contact region of length L, and po is the maximum pressure at the cutting edge. The derived expressions suggest a novel method of determining quantitatively the lengths s of stuck regions on the rake face, and unstuck lengths (L–s), just from the cutting forces without the use of special devices such as split tooling. Calculations for the variations of (s/L) and μapparent=qF/qN with uncut chip thickness t automatically give the variation in values of po and n. The theory is tested using experimental cutting force data in the literature from a wide range of materials in different thermomechanical states and the predictions are compared with independent data. It is demonstrated that the usually-illustrated version of the Zorev pressure distribution where the contact pressure rises ‘exponentially’ to the cutting edge (i.e. where n>1) applies only when (s/L) is less than about 0.5. When the sticking length s is a larger proportion of L, n<1 giving the experimentally-known different type of pressure distribution that levels out towards the cutting edge.Theory and experiments show that qF plots non-linearly against qN for all combinations of uncut chip thickness t and rake face contact length L. The plot emanates from the origin with an initial slope of μCoulomb. As soon as the sticking length s begins to increase, the slope diminishes and when (s/L)=1 at complete sticking, the local slope of the qF vs qN is zero. Increasing (s/L) corresponds to a reduction in (L/t) that may be achieved using restricted contact tools, but even in full-face cutting where L=Lff there is some sticking near the cutting edge at the largest (Lff/t).Plots of friction force F vs normal force N along the rake face are also predicted to be non-linear and emanate from the origin with slope μCoulomb. While some experimental results display this shape, most F vs N experimental data for full-face cutting follow linear “F=Fo+ϖN” relations (having high correlation coefficients). It is shown that such linear plots with intercepts are tangents to the more general non-linear relations, and are caused by the relatively small range of qF and qN encountered in full-face cutting which is caused by the interplay between rates of increase of cutting forces as t increases, and rates of change of Lff/t with increasing t. How (s/L), po and n may be determined from such plots without knowledge of μCoulomb is explained and calculations from experiments are made.The loads expected to be measured by split tools having a Zorev contact pressure distribution are also predicted and compare favourably with experiment.

Tensile testing of Al6061-T6 microspecimens with ultrafine grained structure derived from machining-based SPD process

Abstract

Study on the cutting force of cylindrical turning with novel restricted contact tools

Analysis of cutting force is important in research and development of metal cutting process and in designing cutting tools. This paper reports the theoretical computational formulae for cutting force for cylindrical turning using novel restricted contact tools that have inconstant tool/chip restricted contact length. These formulae are based on the minimum energy principle. The results of extensive cutting tests show that the derived theoretical computational formulae can predict the cutting force, especially the main cutting force, with reasonable accuracy. The novel RC tools-type II and type III can reduce cutting force as well as the conventional RC tools-type I. In addition, they can also effectively control the direction of chip curling, and type III RC tools may have a longer tool life due to the high strength of its cutting edge and the possibility of liquid coolant approaching the cutting zone. Finite element models have been developed to study the cutting force, and the results indicate that the main cutting force can be accurately predicted, while there are some inaccuracies for the feed force and thrust force because of the simplifications adopted during modeling.

A Quantitative Sensitivity Analysis of Cutting Performances in Orthogonal Machining with Restricted Contact and Flat-Faced Tools

This paper presents a new quantitative sensitivity analysis of cutting performances in orthogonal machining with restricted contact and flat-faced tools, based on a recently developed slip-line model. Cutting performances are comprehensively measured by five machining parameters, i.e., the cutting forces, the chip back-flow angle, the chip up-curl radius, the chip thickness, and the tool-chip contact length. It is demonstrated that the percentage of contribution of tool-chip friction to the variation of cutting performances depends on different types of machining operations. No general conclusion about the effect of tool-chip friction should be made before specifying a particular type of machining operation and cutting conditions.

On prediction of tool life and tool deformation conditions in machining with restricted contact tools

This paper first reviews the recently developed semi-empirical method for predicting tool life in machining with restricted contact (RC) tools. The method uses Oxley’s machining theory to predict cutting forces, tool-chip contact length and cutting temperatures for the corresponding plane face tool i.e., tool having the same cutting edge geometry but no RC. These predicted parameters and a set of empirical relations are then used to calculate the cutting temperatures and tool life under RC conditions. In this paper additional experimental results are used to verify the above method and validate the predictions. An attempt is then made to use the above method to predict tool deformation under RC conditions. Finally, a comparison between experimental and predicted results and the modifications implemented to improve the predictive method are given.

A new methodology for determining the stress state of the plastic region in machining with restricted contact tools

In rigid-plastic slip-line theory, once the geometry of the slip-line field is established, the stress state of the plastic region (including the primary and secondary deformation zones) in restricted contact machining is governed by the hydrostatic pressure P A (at a point on the intersection line of the shear plane and the work surface to be machined) and the frictional shear stress τ on the tool rake face. Based on the recently established universal slip-line model and a detailed study of six representative machining cases, a new methodology for determining the stress state of the plastic region, i.e. maximum value principle, is presented in this paper. According to this principle, the stress state of the plastic region can be determined by giving both P A and τ their theoretical maximum permissible values. The theoretical maximum permissible values of P A and τ can be found by satisfying four mechanical and geometrical constraint conditions under which the universal slip-line model applies. A comprehensive assessment factor is introduced in this paper. It is shown that the three machining parameters investigated in this present study, i.e. cutting force ratio, chip thickness ratio, and chip back-flow angle can be simultaneously considered to form a comprehensive criterion to compare predicted and experimental results. The applicable range of the maximum value principle is also discussed.

THE OXLEY MODELING APPROACH, ITS APPLICATIONS AND FUTURE DIRECTIONS

The Oxley machining theory which allows for the high strain-rate/high temperature flow stress and thermal properties of the work material is described. It is shown how the theory that was originally developed for the orthogonal process and later extended to oblique machining, can be used to predict cutting forces, temperatures and subsequently built-up edge formation conditions, tool life and cutting edge deformation conditions. It is also shown how the theory can be applied to obtain predictions in machining with restricted contact tools and in intermittent cutting processes, and to obtain work material properties using machining test results. Finally, some consideration is given to the future directions of machining research at UNSW. The Oxley Model can be used for predicting the performance parameters for different machining processes by taking into account the fundamentals of the chip formation process.

A universal slip-line model with non-unique solutions for machining with curled chip formation and a restricted contact tool

A universal slip-line model and the corresponding hodograph for two-dimensional machining which can account for chip curl and chip back-flow when machining with a restricted contact tool are presented in this paper. Six major slip-line models previously developed for machining are briefly reviewed. It is shown that all the six models are special cases of the universal slip-line model presented in this paper. Dewhurst and Collins's matrix technique for numerically solving slip-line problems is employed in the mathematical modeling of the universal slip-line field. A key equation is given to determine the shape of the initial slip-line. A non-unique solution for machining processes when using restricted contact tools is obtained. The influence of four major input parameters, i.e. (a) hydrostatic pressure ( P A ) at a point on the intersection line of the shear plane and the work surface to be machined; (b) ratio of the frictional shear stress on the tool rake face to the material shear yield stress ( τ/ k); (c) ratio of the undeformed chip thickness to the length of the tool land ( t 1/ h); and (d) tool primary rake angle ( γ 1), upon five major output parameters, i.e. (a) four slip-line field angles ( θ, η 1, η 2, ψ ); (b) non-dimensionalized cutting forces ( F c / kt 1 w and F t / kt 1 w); (c) chip thickness ( t 2); (d) chip up-curl radius ( R u ); and (e) chip back-flow angle ( η b ), is theoretically established. The issue of the “built-up-edge” produced under certain conditions in machining processes is also studied. It is hoped that the research work of this paper will help in the understanding of the nature and the basic characteristics of machining processes.

PREDICTION OF TOOL LIFE IN MACHINING WITH RESTRICTED CONTACT TOOLS

A semi-empirical method is described for predicting tool life in orthogonal machining with restricted contact tools. The method uses a well established machining theory to predict cutting forces, tool-chip contact length and cutting temperatures for the corresponding plane face tool i.e. tool having the same cutting edge geometry but no restricted contact. These predicted parameters and a set of empirical relations are then used to calculate the cutting temperatures and tool life for the restricted contact tool. A comparison has been made between predicted and experimental results obtained from the literature and from tests carried out by the authors.

PREDICTION OF CUTTING FORCES IN MACHINING WITH RESTRICTED CONTACT TOOLS

A semi-empirical method is described for predicting cutting forces in orthogonal machining with restricted contact tools. The method uses a well-established machining theory to predict cutting forces and tool-chip contact length for the corresponding plane face tool, i.e., a tool having the same cutting-edge geometry but no restricted contact. These predicted parameters and a set of empirical relations are then used to calculate the cutting forces for the restricted contact tool. A comparison between predicted and experimental results for two plain carbon steel work materials and a range of tool geometries and cutting conditions shows good agreement.

The Tool Restricted Contact Effect as a Major Influencing Factor in Chip Breaking: An Experimental Analysis

Chip breaking performances of six commercially available chip forming tool inserts have been assessed. The tool restricted contact effect determining the chip streaming (i.e. chip backflow) has been found to be a major influencing factor in chip breaking. No commercial chip former fully utilises the restricted contact effect. Chip up and side curling mechanisms have been investigated. Deforming the chip laterally across the chip-section contributes to the chip breaking. Commercial chip formers have been found to lie in an intermediary level between flat-faced conventional tools and restricted contact tools in terms of cutting power consumed.

What Can We Do About Chip Formation Mechanics?

Summary Simple shear zone models for orthogonal cutting describe satisfactorily the mechanics of chip formation in some cases while, in many practical cases, measured chip thicknesses are not as theoretically expected. Also, theoretical models suffer from the fact that process geometry is not directly related to the independent variables (tool rake angle and work material properties) but through a parameter describing the frictional conditions on the rake face, as the friction angle or the contact length factor Finally, the mathematical validity of simple models is often object of doubt, since application of the principle of minimum energy and assumption of a constant friction angle cannot be expected to hold under usual conditions. This paper contains a discussion of some steps which can be made to extend the range of validity of simple shear zone models together with their explicitness. It is suggested that stress distributions on the tool rake face and chip curl are interrelated and should be taken into account. A model for chip formation mechanics which incorporates stress distributions on the rake face is presented. The mechanism of chip straightening when using restricted contact tools is discussed

Cutting tools with restricted contact

The paper is concerned with the practical use of restricted contact tools (RC tools). A quantitative description of the mode of action of RC tools is given, based on a simple model for cutting recently produced. Characteristic curves for RC tools are established through cutting force and cutting temperature measurements. Results from measurements with different work materials and cutting data are presented. Examples of the use of RC tools are given, such as using RC tools instead of adding EP additives to cutting fluids.

Lubrication in Cutting—Critical Review and Experiments with Restricted Contact Tools

The paper is concerned with the lubricating action of cutting fluids. In a critical reivew of the different theories for lubrication in cutting, three main approaches are discussed: 1. the assumption that lubrication in cutting is due to the presence of a soft film between the tool and the chip; 2. the theory that the lubricating effect of a cutting fluid consists in a physio-chemical reduction of the work material shear strength; 3. the theory describing cutting fluid action in term of a reduction of the length of contact between the tool and the chip. An investigation on lubrication in cutting carried out with tools with restricted contact is described. Presented at the 35th Annual Meeting in Anaheim, California, May 5–8, 1980.

Controlled Contact Cutting Tools

A substantial reduction in power consumption, an increase in tool life, more effective utilization of cutting fluids, and improved surface finish on the machined workpiece have been achieved by suitably controlling the length of tool-chip contact. Reasons for these findings are discussed in terms of basic variables in chip formation mechanics. Artificially restricted contact tools open new avenues for metal cutting research. Machining data obtained with such tools provide further evidence of the invariant behavior of the dynamic shear stress of metals under high-speed cutting conditions, and unfold interesting information on the intricate nature of tool-chip contact.