Fragmented chip formation mechanism in high-speed cutting from the perspective of stress wave effect

In this study, fragmented chip formation mechanism in high speed cutting (HSC) process is explored by using continuum-based discrete element method (DEM) based on the stress wave theory. The continuum-based DEM model established with Godunov frame is suitable for capturing multiple cracks propagation under high-speed impact. The chip forming mechanism based on stress wave theory is quantitatively analysed by DEM simulations against experimental data, which shows that the unloading wave reflected by the free surface of chips under high-speed conditions has a fundamental influence on chip formation.     

Implementation of a process and structure model for turning operations

The consideration of the dynamic interaction between the machine tool structure and the cutting process is a prerequisite for the simulative prediction and optimization of machining tasks. However, existing cutting force models are either dedicated to already examined manufacturing operations or require extensive measurements for the determination of cutting coefficients. In this context this paper outlines a modular, analytical cutting force model applicable to common turning processes. It takes into account the dynamic material behavior, nonlinear friction ratios on the rake face as well as heat transfer phenomena in the deformation zones. On the part of the machine tool structure a parametric model based on the Finite Element Method (FEM) is implemented. Both models are coupled for the simulation of process and structure interactions, whereas the influence of the control system is considered as well. The simulation results were verified experimentally on a turning center.     

Modeling of the influence of the abrasive waterjet cutting parameters on the depth of cut based on fuzzy rules

Currently, the abrasive waterjet cutting parameters for the milling operation have to be determined by a combination of prior experience and trial and error. It is shown that the selection of the abrasive waterjet cutting parameters for a required depth of cut in the given material can be effectively done by applying the principles of the fuzzy set theory. This approach will eliminate the need for extensive experimental work in order to select the magnitudes of the most influential abrasive waterjet parameters on the depth of cut. Fuzzy logic provides a methodology and imitation of a human's way of making decisions which is very useful in such applications where the mathematical model of the process does not exist, and one of such processes is indeed abrasive waterjet cutting. A number of case studies are performed to verify the validity of the proposed methodology for selecting the abrasive waterjet cutting parameters in order to achieve the predetermined depth of cut.     

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.     

A cutting force model for finishing processes using helical end mills with significant runout

Analytical cutting force models play an important role in a wide array of simulation approaches of milling processes. The accuracy of the simulated processes directly depends on the predictive power of the applied cutting force model, which may vary under specific circumstances. End milling processes with small radial cutting depths, e.g. finishing processes, are particularly problematic. In this case, the tool runout, which is usually neglected in established cutting force models, can become quite significant. Within this article, well-known cutting force models are implemented for runout-prone finishing processes and modified by integrating additional parameters. A method is presented for how these additional runout parameters can be efficiently determined alongside commonly used cutting coefficients. For this purpose, a large number of milling experiments have been performed where the cutting forces were directly measured using a stationary dynamometer. The measured cutting forces were compared with the simulated cutting forces to verify and assess the modified model. By using the presented model and calibration method, cutting forces can be accurately predicted even for small radial cutting depths and significant tool runout.     

FE-simulation of machining processes with a new material model

In the field of materials mechanics the influence of the state of stress on the plastic deformation behavior of metals is known since decades. However, the state-of-stress influences are usually not considered in structural or processing simulations. Nevertheless, its application in the numerical investigation of manufacturing processes seems very promising since, for example, machining processes are characterized by complex states of stress. Consequently, its incorporation in the computation of the workmaterial's flow stress may increase the physical conformity and accuracy of cutting FE-analysis.This paper presents the creation and experimental validation of a 3D-FEM model of the longitudinal turning process with an extended modified Bai–Wierzbicki material model (extended MBW model). This newly developed material model evaluates the influence of state of stress as well as damage on the strain hardening behavior. In addition, it takes temperature and strain rate effects into consideration, whose influences are both typically higher in cutting processes than in structural–mechanical problems.For the validation of the proposed material model, longitudinal turning experiments were conducted on AISI 1045 steel. Four different cutting tools and process conditions were investigated, which cover a broad range from finishing to roughing. A high speed camera was used to film the chip formation and chip flow in order to compare it to the simulation results. The three cutting forces components were also collected. Measured chip temperatures were taken from the literature. The validation showed that the implementation of the selected material model results in a close agreement between experimentally obtained and predicted chip geometries, cutting forces and chip temperatures.     

Fluid structure interaction (FSI) modelling of deep hole twist drilling with internal cutting fluid supply

This paper presents a novel approach for coupling of thermal FE and CFD simulations to predict the temperature distribution in the cutting process. The developed FSI model considers experimentally validated workpiece temperature to simulate the heat convection interactions in drilling operations. This innovative method allows not only for the common analysis of the flow behaviour, but additionally for the detailed investigation of the temperature distribution within the cutting fluid. The simulation provides indications for an insufficient fluid supply of the cutting edge and the results can contribute significantly to the further optimisation of thermally high stressed cutting tools and processes.     

Holistic modelling of process machine interactions in parallel milling

Parallel milling allows to increase the manufacturing capacity in industry due to a higher cutting productivity. The latter is often negatively influenced by dynamical instabilities of the parallel milling process entailing a reduced profitability. In this paper a holistic model of interactions between a double spindle machine and a parallel milling process is presented that enables an identification of the dynamic compliance of parallel milling machines and a simulation of their stability behaviour. With that an investigation of stability optimization means is possible. A significant improvement of the process stability and, thus, the effective cutting productivity was observed in the parallel milling processes, optimized with help of the holistic model.     

Influence of the cutting edge rounding on the chip formation process: Part 1. Investigation of material flow, process forces, and cutting temperature

In order to meet the ever-increasing demand for high quality and low cost products, machining processes with geometrically defined cutting edges such as high speed cutting, or hard turning are being used. Due to the fact that cutting is accomplished through a physical interaction between the cutting edge and the workpiece, the characteristics of the cutting edge itself play a key role in influencing the machining process, which determines the product quality and the tool life. As a result, cutting edge design has attracted the focus of many researchers, and crucial improvements have been achieved. Rounded cutting edges have been found to improve the tool life, and the product quality. However, to better understand the impact of prepared cutting edges on the aspects of the machining processes, and to produce tailored cutting edges for specific load profiles, further investigation on the influence of the cutting edge design on the machining processes needs to be carried out. In this study, the effect of symmetrically and asymmetrically rounded cutting edge on the material in the vicinity of the cutting edge has been investigated using finite element simulation techniques. The results obtained from this investigation show that process forces and material flow under the flank face are mainly influenced by the micro-geometry S_??. However, the magnitude and the location of maximum nodal temperature are influenced by S_?? as well as S_??.     

Interaction of manufacturing process and machine tool

Analysing the machine tool and the machining process individually is necessary in order to tackle the challenges that both have to offer. Nevertheless, to fully understand the manufacturing system, e.g. vibrations, deflections or thermal deformations, the interactions between the manufacturing process and the machine tool also have to be analysed. In cutting, grinding and forming there are important effects that can only be explained through these interaction phenomena. This paper presents the current state of research in process-machine interactions for a wide variety of manufacturing processes. It is based on the findings of the CIRP research group “Process Machine Interaction (PMI)” and on the international publications in this field. Cutting with defined and undefined cutting edges as well as sheet and bulk metal forming are the key processes. The emphasis is on understanding, modelling and simulating all modes of interaction. Additional needs of research in process-machine interaction are identified for future projects.     

The prediction of cutting force in ball-end milling

Due to the development of CNC machining centers and automatic programming software, the ball-end milling have become the most widely used machining process for sculptured surfaces. In this study, the ball-end milling process has been analysed, and its cutting force model has been developed to predict the instantaneous cutting force on given machining conditions. The development of the model is based on the analysis of cutting geometry of the ball-end mill with plane rake faces. A cutting edge of the ball-end mill was considered as a series of infinitesimal elements, and the geometry of a cutting edge element was analysed to calculate the necessary parameters for its oblique cutting process assuming that each cutting edge was straight. The oblique cutting process in the small cutting edge element has been analysed as an orthogonal cutting process in the plane containing the cutting velocity and chip flow vectors. And with the orthogonal cutting data obtained from end turning tests on thin-walled tubes over wide range of cutting and tooling conditions, the cutting forces of ball-end milling could be predicted using the model. The predicted cutting forces have shown a fairly good agreement with test results in various machining modes.     

Analysis of cutting force during milling with regards to the dependency on the penetration angle

In recent years the modeling of dynamic processes in milling has become increasingly important especially in the determination of process stability. In the future the importance of these models will increase in order to establish a virtual machine tool technology. In literature several mathematical models are used for the prediction of cutting forces assuming simplified dynamic (chatter free) conditions. These models are based on different parameters which are identified in various ways. The aim of this paper is to improve the identification process for the coefficients of a basic state of the art cutting model by using a numerical optimization method. Based on this optimized model, which reflects the measured cutting force chart more precisely, dependencies of the cutting force parameters on the penetration angle are proven.     

Influences on specific cutting forces and their impact on the stability behaviour of milling processes

To withstand global competition, nowadays it is essential for companies to assure high productivity and high quality. To reach this aim permanent technical innovation and further developments are necessary. In machining industry and especially in the field of milling the development of possibilities to increase chip removal is the major goal. The optimisation of the cutting process is one way to achieve this aim. Here, the use of stability prediction models is essential to reduce the effort in time and costs. To implement a stability prediction tool with a high accuracy in representing reality, all relevant influencing parameters and their interactions within the cutting process have to be analysed. This article describes one possibility for the experimental identification of instable milling processes. Furthermore, the influences of spindle speed and temperature on specific cutting forces and the temperature influence on the stability behaviour in milling processes are shown.     

Identification of the specific cutting force for geometrically defined cutting edges and varying cutting conditions

Cutting force modeling is a major discipline in the research of cutting processes. The exact prediction of cutting forces is crucial for process characterization and optimization. Semi-empirical and mechanistic force models have been established, but the identification of the specific cutting force for a pair of tool and workpiece material is still challenging. Existing approaches are depending on geometrical idealizations and on an extensive calibration process, which make practical and industrial application difficult. For nonstandard tools and five axis kinematics there does not exist a reasonable solution for the identification problem.In this paper a co-operative force model for the identification of the specific cutting forces and prediction of integral forces is presented. The model is coupled bidirectionally with a multi-dexel based material removal model that provides geometrical contact zone information. The nonlinear specific forces are modeled as polynomials of uncut chip thickness. The presented force model is not subjected to principal restrictions on tool shape or kinematics, the specific force and phase shift are identified with help of least square minimization. The benefit of this technique is that no special calibration experiments are needed anymore, which qualifies the method to determine the specific forces simultaneously during the machining process. In this paper, experiments with different cutting conditions are analyzed and systematically rated. Finally, the method is validated by experiments using different cutting conditions.     

Analysis of heat effects in laser cutting of steels

Efficient control of laser cutting processes is closely related to knowledge of heat effects in the cutting front and its surroundings. Similar to other machining processes using high power densities, in laser cutting processes it is very important to monitor the heating phenomena in the workpiece material due to heat input. In laser cutting processes with oxygen as an auxiliary gas, cutting energy is a combination of laser beam energy and the energy of the exothermic reactions occurring in the cutting front. The presence of oxygen in the process increases cutting efficiency, but also causes additional physical processes in the cutting front that render a more detailed analysis of the cutting phenomena difficult. The aim of this article is to analyze the emission of infrared rays from the cutting front with a photodiode, statistically analyze the temperature signals, and optimize the laser cutting process based on a critical cutting speed. The measured infrared radiation temperature signal was, on the basis of calibration, converted into a temperature that was related to the formation of macro- and microstructures and to the change in microhardness in the surface layer of the cut. On the basis of experimental results, it was proved that heat effects in the cutting front decisively influenced the quality of cut. Finally, factor analysis was used to establish statistical relations among variables of the laser system, variables of the cutting process, and geometrical characteristics of the cut.     

Analytic model of process forces for orthogonal turn-milling

The manufacturing process of turn-milling offers high productivity, high geometrical flexibility, safe chip breaking and machining of twist-free surfaces. However, the achievable accuracies and surface qualities do not allow the substitution of finish grinding processes. Thereby the dynamic profile of process forces is one of the main disturbance values. This paper presents a new process model of orthogonal eccentric turn-milling without axial feed. Thus the focus lies on the analysis of the process forces and their dynamic behavior caused by the changing chip cross section area on the face cutting edge. The developed model considers single and multiple cutting edge tools and has a geometrical-empirical character. This model describes the process kinematics and the error inducing process forces in orthogonal turn-milling. Their dynamic profile can be analyzed and minimized using multiple cutting edge tools and customized pre-machining of the workpiece surface topography with the goal of creating a robust process to substitute the finish grinding process in high precision applications. Such applications include, for example, the main and pin bearing seats of crank shafts used in the automotive and truck industry or even for large ship engines. The key feature is to minimize the form deviation of the cylindricity, which results from straightness and roundness of the workpiece surface to below 10 µm for crank shafts used in the truck industry. The model is adjusted and verified by experimental tests.     

Increasing productivity in sculpture surface machining via off-line piecewise variable feedrate scheduling based on the force system model

Sculpture surface machining is a critical process commonly used in various industries such as the automobile, aerospace, die/mold industries. Since there is a lack of scientific tools in practical process planning stages, feedrates for CNC machining are selected based on the trial errors and previous experiences. In the selections of the process parameters, production-planning engineers are conservative in order to avoid undesirable results such as chipping, cutter breakage or over-cut due to excessive cutter deflection. Currently, commonly used CAD/CAM programs use only the geometric and volumetric analysis, but not the physics of the processes, and rely on experience based cutting tool database and users’ inputs for selection of the process parameters such as feed and speed. Usually, the feeds and cutting speeds are set individual constant values all along the roughing, semi-finishing, and finishing processes. Being too conservative and setting feedrate constant all along the tool path in machining of sculpture surfaces can be quite costly for the manufacturers. However, a force model based on the physics of the cutting process will be greatly beneficial for varying the feedrate piecewise along the tool path.The model presented here is the first stage in order to integrate the physics of the ball-end milling process into the selection of the feeds during the sculpture surface machining. Therefore, in this paper, an enhanced mathematical model is presented for the prediction of cutting force system in ball end milling of sculpture surfaces. This physical force model is used for selecting varying and ‘appropriate’ feed values along the tool path in order to decrease the cycle time in sculpture surface machining. The model is tested under various machining conditions, and some of the results are also presented in the paper.     

Study of polycrystalline Al2O3 machining cracks using discrete element method

Discrete element method (DEM) was introduced to simulate crack initiation and propagation of polycrystalline alumina during the brittle model machining process. A bonded particle model (BPM) was employed in the DEM simulations procedure to generate a particle assembly system similar to the micro-structure of the polycrystalline alumina. Particle and parallel bond properties, which were calibrated through a series of numerical tests, were subsequently used in the simulations of polycrystalline alumina cutting process and scratching tests. It is found that the cracks initiated right under or in front of the machining tool. There were many micro-cracks remained on the machined surface, some of them propagated downwards to form macro-cracks or forwards to lead material removal. Both DEM simulations and acoustic emission measure experiments have found that the fracture became acute when the normal and the tangential force changed suddenly, causing the crack number to increase. In 3D DEM scratching simulation, the surface cracks length and subsurface cracks depth linearly increased with the scratching depth, the value agreed well with the experimental results, and the surface-damage width decreased gradually with the depth to the surface, looking like half of a coin.     

A Model for Surface Roughness in Ultraprecision Hard Turning

Numerical modelling procedures to predict surface roughness in turning processes have been in use for more than forty years. However, the procedures available to date do not correlate well with hard turning. A novel numerical model is presented which incorporates process disturbances such as tool cutting edge defects and machine vibration in hard turning and thus their effect on the achievable surface roughness. It includes a material partition equation to address the behaviour of chip removal and deformations during the cutting process; it also allows additional information to be derived about the mechanism of generation involved at a given point on the surface. Experimental results show good correlation of calculated with measured roughness parameters even at low feed rates.     

Chemical Aspects of Machining Processes

Machining processes used to create surfaces are influenced by the mechanical, thermal, and chemical loading in the contact zone. In addition, the tribo-physical and tribo-chemical interactions between the cutting tool, workpiece, metalworking fluid and surrounding medium have an influence on the properties of the resulting surface. In order to design efficient machining processes and control the chemical state of the surface produced, a basic understanding of the chemical mechanisms in the contact zone is needed. The chemical effects of metalworking fluids on the processes of machining and grinding are discussed, including the chemical interactions which occur between the various participating surfaces. The impact of the resulting chemical state of the surface produced is addressed.