Intermittent metal cutting at small cutting depths—1. Dead zone phenomena and surface finish

Chip formation in intermittent metal cutting at small cutting depths was investigated by single edge experiments. Single cutting strokes were performed in a modified Charpy pendulum tester which offers force measurement, accurate selection of cutting speed and feed in the ranges typical of many intermittent high speed steel (HSS) tool operations. The pendulum is also provided with an excellent quick-stop mechanism.

The cutting performance of HSS tools in three widely used steel grades (including one plain carbon, one quenched and tempered and one austenitic stainless steel) was studied. A number of double rake micro geometries, with primary rake angles ranging from +20° (parrot bill) to −60°, all with a prepared 0.1 mm wear land were tested. The performance of the different edge geometries was investigated with respect to class of dead zone developed on the cutting edge, and its relation to chip curl and finish of the cut surface. The results are visualized in a dead zone map. The influence of cutting length, cutting speed, cutting depth and TiN-coating was treated specifically.

Among the most important observations were:

• the micro geometry of the edge influences the dead zone formation mechanism and hence the class of dead zone,

• the surface finish is strongly dead zone class dependent,

• the chip curl is determined by edge micro geometry and dead zone class.

The relationships between the varied parameters, generated dead zones and resulting cutting forces are presented in part 2 of this paper.     

Prediction of cutting forces in machining of metal matrix composites

This paper presents a mechanics model for predicting the forces of cutting aluminum-based SiC/Al₂O₃ particle reinforced MMCs. The force generation mechanism was considered to be due to three factors: (a) the chip formation force, (b) the ploughing force, and (c) the particle fracture force. The chip formation force was obtained by using Merchant's analysis but those due to matrix ploughing deformation and particle fracture were formulated, respectively, with the aid of the slip line field theory of plasticity and the Griffith theory of fracture. A comparison of the model predictions with the authors’ experimental results and those published in the literature showed that the theoretical model developed has captured the major material removal/deformation mechanisms in MMCs and describes very well the experimental measurements.     

The prediction of cutting forces in the ball-end milling process—II. Cut geometry analysis and model verification

This paper presents a model for the prediction of cutting forces in the ball-end milling process. The steps used in developing the force model are based on the mechanistic principles of metal cutting. The cutting forces are calculated on the basis of the engaged cut geometry, the underformed chip thickness distribution along the cutting edges, and the empirical relationships that relate the cutting forces to the undeformed chip geometry. A simplified cutter runout model, which characterizes the effect of cutter axis offset and tilt on the undeformed chip geometry, has been formulated. A model building procedure based on experimentally measured average forces and the associated runout data is developed to identify the numerical values of the empirical model parameters for the particular workpiece/cutter combination.     

Cutting forces modeling considering the effect of tool thermal property—application to CBN hard turning

Force modeling in metal cutting is important for a multitude of purposes, including thermal analysis, tool life estimation, chatter prediction, and tool condition monitoring. Numerous approaches have been proposed to model metal cutting forces with various degrees of success. In addition to the effect of workpiece materials, cutting parameters, and process configurations, cutting tool thermal properties can also contribute to the level of cutting forces. For example, a difference has been observed for cutting forces between the use of high and low CBN content tools under identical cutting conditions. Unfortunately, among documented approaches, the effect of tool thermal property on cutting forces has not been addressed systemically and analytically. To model the effect of tool thermal property on cutting forces, this study modifies Oxley’s predictive machining theory by analytically modeling the thermal behaviors of the primary and the secondary heat sources. Furthermore, to generalize the modeling approach, a modified Johnson–Cook equation is applied in the modified Oxley’s approach to represent the workpiece material property as a function of strain, strain rate, and temperature. The model prediction is compared to the published experimental process data of hard turning AISI H13 steel (52 HRc) using either low CBN content or high CBN content tools. The proposed model and finite element method (FEM) both predict lower thrust and tangential cutting forces and higher tool–chip interface temperature when the lower CBN content tool is used, but the model predicts a temperature higher than that of the FEM.     

Sensor signals for tool-wear monitoring in metal cutting operations—a review of methods

The state of a cutting tool is an important factor in any metal cutting process as additional costs in terms of scrapped components, machine tool breakage and unscheduled downtime result from worn tool usage. Several methods to develop monitoring devices for observing the wear levels on the cutting tool on-line while engaged in cutting have been attempted. This paper presents a review of some of the methods that have been employed in tool condition monitoring. Particular attention is paid to the manner in which sensor signals from the cutting process have been harnessed and used in the development of tool condition monitoring systems (TCMSs).     

An experimental method for studies of intermittent cutting at small cutting depths

Most devices for metal cutting experiments are designed to simulate continuous cutting at relatively large cutting depths. However, there is also a need for techniques to study the more complex situations prevailing in other important cutting operations like milling, sawing, hobbing, shaper cutting and grinding. These operations are characterized as being intermittent and having a relatively small and varying cutting depth per edge. In order to supply an experimental set-up for basic studies of chip formation and cutting forces under these conditions a new method for single stroke, single edge metal cutting has been developed.

The experiments are performed in a modified Charpy pendulum which offers force measurement and accurate selection of cutting speed and feed in the ranges typical of many intermittent cutting operations. The equipment is also provided with an excellent quick-stop mechanism to aid in chip formation studies.

The test method is described in detail and examples of metallographical and scanning electron microscopical studies of quick-stopped samples as well as registrations of specific thrust and cutting forces are presented.     

Cutting and ploughing forces for small clearance angles of hexa-octahedron shaped diamond grains

Investigations on the cutting behaviour of hexa-octahedron diamonds outlined an enormous influence of the grains’ clearance angle on the material removal process. Small negative clearance angles lead to increased specific cutting forces, decreased cutting force ratios and micro-structural changes. This is caused by additional ploughing of the material. This paper presents a kinematic-phenomenological model predicting the specific forces that are caused by the ploughed material. Therefore, the theoretical value of the specific ploughed volume is introduced as characteristic parameter. Results are subsequently compared for different grain cutting situations to experimental data allowing a validation of the proposed model.     

Cutting forces compatibility based on a plasticity model.: Application to the oblique cutting of the AA2024 alloy

In this work, a compatibility model for cutting force components is proposed. This model has been built by considering the plasticity hypothesis and cutting–shear force bond conditions. Its formulation gives rise to the Cutting Forces Compatibility Equation. This equation suggests that the cutting force components are related and, therefore, that they cannot be considered independent. An experimental study of AA2024 alloy has been achieved in order to determine the consistency of the model. This study was carried out by acquiring the orthogonal cutting force components through a piezoelectric dynamometer placed in the revolver of a computer numerically controlled lathe. The results obtained are in good agreement with the proposed model within the pre-established limits.     

Cutting forces and TEM analysis of the generated surface during machining metal matrix composites

Cutting of metal matrix composites (MMCs) has been considerably difficult due to the extremely abrasive nature of the reinforcements that causes rapid tool wear and high machining cost. An investigation was carried out to clearly understand the role played by the ductile matrix on the machining performance based on the estimation of line defects generated as a result of cutting. The microstructural studies were conducted using transmission electron microscopy (TEM) on the machined surface to reveal the deformation pattern of the work hardening matrix and its correlation with the forces generated during turning MMCs. Cracking and debonding of the reinforcement particles are the significant damage modes that directly affect the tool performance. It was found that the particle size and volume fraction affect the extent of deformation in the generated surface. Also the machining forces are correlated to the plastic deformation characteristics of the matrix material. This investigation provided valuable information on the deformation behaviour of particulate reinforced composites that can improve the performance and accuracy of machining MMCs.     

Process simulation using finite element method — prediction of cutting forces, tool stresses and temperatures in high-speed flat end milling

End milling of die/mold steels is a highly demanding operation because of the temperatures and stresses generated on the cutting tool due to high workpiece hardness. Modeling and simulation of cutting processes have the potential for improving cutting tool designs and selecting optimum conditions, especially in advanced applications such as high-speed milling. The main objective of this study was to develop a methodology for simulating the cutting process in flat end milling operation and predicting chip flow, cutting forces, tool stresses and temperatures using finite element analysis (FEA). As an application, machining of P-20 mold steel at 30 HRC hardness using uncoated carbide tooling was investigated. Using the commercially available software DEFORM-2D™, previously developed flow stress data of the workpiece material and friction at the chip–tool contact at high deformation rates and temperatures were used. A modular representation of undeformed chip geometry was used by utilizing plane strain and axisymmetric workpiece deformation models in order to predict chip formation at the primary and secondary cutting edges of the flat end milling insert. Dry machining experiments for slot milling were conducted using single insert flat end mills with a straight cutting edge (i.e. null helix angle). Comparisons of predicted cutting forces with the measured forces showed reasonable agreement and indicate that the tool stresses and temperatures are also predicted with acceptable accuracy. The highest tool temperatures were predicted at the primary cutting edge of the flat end mill insert regardless of cutting conditions. These temperatures increase wear development at the primary cutting edge. However, the highest tool stresses were predicted at the secondary (around corner radius) cutting edge.     

Fundamental studies of driven and self-propelled rotary tool cutting processes—II. Experimental investigation

An extensive experimental investigation has been carried out to verify the developed mechanics of cutting analyses for the fundamental driven and self-propelled rotary tool cutting processes. This involved testing the dynamic or perfect equivalence between the rotary tool and equivalent classical processes over a wide range of inclination angles, cut thickness and rake angles using statistical processing techniques. The collinearity conditions at the shear plane and rake face have also been tested as part of the model verification. It has been shown that all the force components, deformation and basic cutting parameter trends and quantities required for perfect equivalence have been satisfied as were the necessary collinearity conditions. The verified models provide a deeper understanding of the cutting mechanics and characteristics of these ingenious material removal operations and form the basis for the development of predictive cutting models for the fundamental and complex practical rotary tool operations.     

Influence of microstructure on surface integrity in turning—part II: the influence of a microstructure of the workpiece material on cutting forces

Analysis of cutting forces in fine turning is most frequently related to workpiece-material hardness and strength under different machining conditions. Material hardness and strength as well as material machinability, however, can be related to the material microstructure. Consequently, an additional influence exerted by the microstructure was included in the present study to find the relationship between the size of the soft phase described by intercept lengths and the magnitude of the cutting force measured during the fine-turning process. The magnitude of cutting force is usually treated in terms of its static and dynamic components. In our case, both the components are of importance. Because of different types of aluminium–silicon alloys, significant differences occur in the magnitude of the static and dynamic components of the cutting force mainly because of different types of microstructures. The latter can be described by the fraction and quantity of individual microstructure phases. The results obtained are represented by an average magnitude of static and dynamic components of the cutting force in relation to the intercept lengths of the soft phase.     

Fabrication of metal matrix composites by metal injection molding—A review

Metal injection molding (MIM) is a near net-shape manufacturing technology that is capable of mass production of complex parts cost-effectively. The unique features of the process make it an attractive route for the fabrication of metal matrix composite materials. In this paper, the status of the research and development in fabricating metal matrix composites by MIM is reviewed, with a major focus on material systems, fabrication methods, resulting material properties and microstructures. Also, limitations and needs of the technique in composite fabrication are presented in the literature. The full potential of MIM process for fabricating metal matrix composites is yet to be explored.     

Friction and cutting forces in cryogenic machining of Ti–6Al–4V

Conventional cutting fluid serves both as a coolant and lubricant. In cryogenic machining, liquid nitrogen (LN2) is recognized as an effective coolant due to its low temperature; however, its lubrication properties are not well known. The focus of this study was to investigate how the friction between the chip and the tool is affected by focused jetting LN2 to the cutting point in machining Ti–6Al–4V. Results of cutting force measurements indicated that the cold strengthening of titanium material increased the cutting force in cryogenic machining, but lower friction reduced the feed force. A mathematical model was developed to convert the measured 3D forces in oblique cutting into the normal and frictional force components on the tool rake face, and then to calculate the effective friction coefficient. It was found that the friction coefficient on the tool–chip interface was considerably reduced in cryogenic machining. Increased shear angle and decreased thickness of the secondary deformation zone, findings from a chip microstructure study, offer further evidence that friction is reduced.     

Basic geometric analysis of 3-D chip forms in metal cutting.: Part 2: implications

The geometric analysis of 3-D chip forms developed in Part 1 is extended and several new implications are identified: (i) the geometric properties at every point on the tool–chip separation line are fully determined once those at any one point are known, (ii) all possible 3-D chip forms are confined to a relatively restricted parameter space defining the chip velocity direction and the orientation of the axis of the helical chip, (iii) 3-D helical chips are only approximately conical, and (iv) the radii of up-curl and side-curl can be determined from a set of simple measurements of the chip-in-hand. Unlike past analyses, the new analysis paves the way to the study of chip forms from empirical data obtained from practical 3-D chips.     

Rare earth—platinum group mixed metal oxide systems

The phase development in rare earth—platinum group oxide (RE---PG---O) systems has been investigated by reacting selected compositions in sealed evacuated silica vials and characterizing the products using X-ray diffraction. PG(II), PG(III), PG(IV) and PG(V) species, where PG Ru, Ir, Rh, Pd and Pt, have been stabilized in these systems in various structure types. For PG(II) these include the Nd₂CuO₄-type structure and other fluorite-related derivatives; for PG(III), orthorhombically distorted perovskite; for PG(IV), the pyrochlore structure; for PG(V), - and γ-A₃BO₇ types, which are also ordered fluorite derivative structures. The rare earth has a secondary effect on RE---PG---O phase development, with the half-filled 4f shell of the Gd representing a transition point for structural stability in the RE series. Systems containing La and Ce exhibit anomalous behaviour with respect to other RE---PG---O systems. In the course of the present study 28 new phases including a new polymorphic type, designated γ-RE₃PGO₇, have been synthesized.     

Wear,cutting forces and chip characteristics when dry turning ASTM Grade 2 austempered ductile iron with PcBN cutting tools under finishing conditions

Experimental studies of wear, cutting forces and chip characteristics when dry turning ASTM Grade 2 austempered ductile iron (ADI) with polycrystalline cubic boron nitride (PcBN) cutting tools under finishing conditions were carried out. A depth of cut of 0.2 mm, a feed of 0.05 mm/rev and cutting speeds ranging from 50 to 800 m/min were used. Flank wear and crater wear were the main wear modes within this range of cutting speeds. Abrasion wear and thermally activated wear were the main wear mechanisms. At cutting speeds greater than 150 m/min, shear localization within the primary and secondary shear zones of chips appeared to be the key-phenomenon that controlled the wear rate, the static cutting forces as well as the dynamic cutting forces. Cutting speeds between 150 and 500 m/min were found to be optimum for the production of workpieces with acceptable cutting tool life, flank wear rate and lower dynamic cutting forces.     

Drilling of hybrid metal matrix composites—Workpiece surface integrity

The main concern in the present study is the surface roughness variations on the drilled surface and extension of surface and sub-surface deformation due to drilling. The influence of different tools and cutting conditions on Al2219/15%SiCp and Al2219/15%SiCp-3%Graphite (hybrid) composites is investigated experimentally. The composites are fabricated by liquid metallurgy method. The drilling tests are conducted with carbide and coated carbide tools. The surface roughness decreases with the increase in cutting speed and increases with the increase in feed rate. The surface is analyzed using scanning electron microscope (SEM). Microhardness profiles indicate that the subsurface deformation extends up to a maximum of 120 μm below the machined surface for Al2219/15SiCp-3Gr composite when compared to 150 μm in Al2219/15SiCp composite.     

A new experimental methodology to analyse the friction behaviour at the tool-chip interface in metal cutting

This paper investigates a new test to analyse the friction behaviour of the tool-chip interface under conditions that usually appear in metal cutting. The developed test is basically an orthogonal cutting process, that was modified to a high speed forming and friction process by using an extreme negative rake angle and a very high feed. The negative rake angle suppresses chip formation and results in plastic metal flow on the tool rake face. Through the modified kinematics and in combination with a feed velocity that is five to ten times higher than in conventional metal cutting, the shear and normal stresses are only acting in a simple inclined plane, allowing to calculate the mean friction coefficient analytically. In addition, the test setup allows to obtain the coefficient of friction for different temperatures, forces and sliding velocities. Experiments showed, that the coefficient of friction is strongly dependent on the sliding velocity for the example workpiece/tool material combination of C45E+N (AISI 1045) and uncoated cemented carbide.