DEVELOPMENT AND APPLICATION OF A PREDICTIVE MACHINING THEORY

ABSTRACT

A predictive machining theory that 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 can be used to predict cutting forces, temperatures, etc., in terms of cutting conditions and work material properties. Suggestions are made regarding the possible application of the theory to selecting optimum cutting conditions. Finally, some consideration is given to the future directions machining research might take.     

Influence of the Temperature in the Tool–Workpiece Contact Zone on the Deformational Dynamics in Turning

Simulation of the thermal processes in the turning of metals is considered. The proposed model assesses the thermal energy in the tool–workpiece contact zone, taking account of the power of the irreversible transformations. The influence of irreversible transformations on the thermal energy in the tool–workpiece contact zone is described by a model with a hereditary aspect. That may be attributed to the dissipation of thermal energy in the workpiece. Feedback in the model takes account of the influence of the temperature in the cutting zone on the reactive forces in shaping motion of the tool. The results show that such feedback plays a significant role in stabilization of the cutting process.     

MODELING TOOL STRESSES AND TEMPERATURE EVALUATION IN TURNING USING FINITE ELEMENT METHOD

ABSTRACT

The proposed approach to model stresses in cutting tools leading to evaluation of tool temperature distribution, and to eventually model the combined thermo-elastic stress state, is primarily intended for the development of efficient cutting tools rather than process control. The highlights of the approach are the semianalytical modeling of the tool-chip and tool-work contract stresses and friction, incorporation of the role of the secondary shear effects (albeit empirical) and the use of finite element method (FEM)-based estimation of the elastic (2-D) stress field. The contact stress information is used subsequently to model the temperature distribution in the tool. This approach was successfully evaluated in the case of single point turning tools using results from the experimentally measured temperature field in the tool based on the Binder Phase Transformation (BPT) technique. The temperature distribution in the tool for both dry and wet conditions as predicted by the FEM approach agreed quite well, in general, with experimentally obtained isotherms. Deviations in the observed results in the vicinity of the flank region appears to be related to the simplifications used in modeling the contact stresses therein.     

On the contribution of primary deformation zone-generated chip temperature to heat partition in machining

In machining, the percentage of heat flux that enters the cutting tool can have a critical impact on tool wear especially in dry cutting or high speed machining. In previous work, heat partition was evaluated by iteratively reducing the secondary deformation zone heat flux to the tool until the finite element simulated temperatures matched the experimental measured rake face temperatures. This follow-on work quantifies the contribution of primary zone heat flux to heat partition in machining. In this study, an analytical model was used to evaluate the rise in chip temperature due to primary deformation zone heat source. The heat partition and thermal modelling on the rake face was then conducted with an appropriate initial rake face temperature. Thus primary zone heat loads and shear-force-derived secondary zone heat flux were applied in finite element transient heat transfer analysis to evaluate heat flux into the cutting tool. External dry turning of AISI/SAE 4140 with tungsten carbide-based multilayer TiCN/Al₂O₃-coated tools was conducted for a wide range of cutting speeds between 314 and 879 m/min. Results further support the dominance of secondary zone heat flux on heat partition. The contribution of primary zone heat generation to the cutting tool heat flux in machining was less than 9.5 %. These findings suggest that, to address the thermal problem in machining, research and development should also focus on reducing friction on the rake face (e.g. coating innovations) and reducing contact areas (e.g. rake face design) in addition to the modification of shear angle and hence primary zone heat intensity.     

Thermal modeling of drilling process in titanium alloy (Ti-6Al-4V)

Abstract

In drilling in titanium alloys, heat trapped in a hole adversely affects tool life, hole surface quality and integrity. Therefore, modeling temperature distribution in drilling is vital for effective heat dissipation and improving quality of drilled surfaces. The existing numerical and finite element models consider only frictional heat, whereas the effect of shear heat generation and tertiary heat generation is neglected. In the present work, a comprehensive thermal model of the drilling process is developed by considering all heat generated in shear, friction and tertiary zones. The drill cutting edges are divided into a series of independent elementary cutting tools (ECT). The calculated heat flux loads are applied on an individual ECT in the finite element model to determine the temperature distribution and the maximum temperature around the cutting edge. The temperature in the drill was also measured experimentally with the help of an Infrared (IR) camera. The results of numerical simulations lie within the error of ～8.75% when compared to the prior studies, and ～5.41% when compared to our experimental work. The thermal model gives the temperature distribution, and the maximum temperature observed at the corner of cutting edge was 604.2°C at a cutting speed of 35?m/min.     

THE ADAPTABILITY OF A TOOL WEAR MONITORING SYSTEM UNDER CHANGING CUTTING CONDITIONS

The process of metal cutting is a complex phenomenon that has been researched for many years but the aim of practical cutting tool condition monitoring has yet to be achieved. Previous work by the current authors using two neural networks (to classify acquired data) moderated by an Expert System (based on Taylor's tool life equation) has shown that it is possible to accurately monitor tool wear with a single machine/tool/material/cutting condition combination and to identify any inconsistencies between the predictions of the neural networks and engineering practice. This paper investigates the effects that minor inconsistencies in cutting conditions might have on such a system by determining the ‘zone of influence’ of this working system by systematically varying the cutting conditions whilst keeping all other variables fixed. The investigation has found that the zone of influence is small but usable, and an approach to the utilisation of the system in a machine shop is suggested.     

THE INTERACTION BETWEEN TOOL MATERIAL,ENVIRONMENT, AND PROCESS CONDITIONS IN THE MACHINING OF ALUMINIUM ALLOYS

ABSTRACT

Friction between the rake face of a cutting tool and the freshly formed chip surface plays a vital role in influencing both the ease of cutting and the quality of the resultant machined surface. The existence of clean surfaces together with the high local hydrostatic stresses favour the formation of strong adhesion between the cutting tool or insert and the machined component. These adhesive bonds can lead to poor surface integrity although their extent can be limited by the provision of a suitable machining lubricant.

In an effort to identify the essential lubricating aspects of fluid activity, as opposed to any role as a coolant, experiments involving the orthogonal machining of precipitation hardened aluminium alloys, principally 2014, have been carried out in controlled low pressure environments in which, for a given feed (that is the depth of cut), speed and temperature have been varied while using a variety of lubricating species in vapour form in combination with high speed steel cutting inserts. The results indicate that there can be unexpectedly subtle, but significant, interactions between the metallurgy of the workpiece, the surface of the tool and the surrounding environment. These are not wholly consistent with conventional theories of vapour phase lubrication in which transport of the lubricant has been assumed to control the effectiveness of the lubricating agent. The implications of these observations for the complex tribological system constituted by the combination of workpiece, tool surface and local environment are discussed.     

FINITE ELEMENT ANALYSIS OF CHIP FORMATION IN GROOVED TOOL METAL CUTTING

Abstract

This paper presents the simulation of chip formation in grooved tool cutting using DYNA3D, 3D FEM software for dynamic nonlinear analysis that was used to simulate the orthogonal cutting problem. First, a flat-face cutting tool was employed in the simulation to verify the validity of the FEM model. Next, the same simulation techniques were used to study the effects of different groove geometries on the chip formation process in grooved tool cutting. In the first set of grooved tool simulations, the depth of the groove was constant while the width was decreased. In the second set, the width was constant and the depth was increased. By analyzing the chip flow, chip curl, chip thickness, stress and strain in the chip, the effects of different groove widths and depths on the chip formation process were then discussed.     

A new reverse engineering method to combine FEM and CFD simulation three-dimensional insight into the chipping zone during the drilling of Inconel 718 with internal cooling

ABSTRACT

The use of cooling lubricants in metal machining increases both the tool life and the quality of workpieces and improves the overall sustainability of production systems. In addition to fulfilling these main functions, the focus of machining processes is also related to the reduction of environmental pollution. This can for example be achieved by an optimized arrangement of the cutting tool cooling channels. Therefore, the active cutting edges of the tool should be effectively supplied with a sufficient amount of cooling lubricant. An analysis of the tribological stress is rather difficult because the complex contact zone is inaccessible. Hence, optical investigations are often limited to only observing the chip formation or analyzing the process without considering the influence of the chips.

This article presents an innovative method, which enables a deeper three-dimensional insight into the chip formation zone during drilling with internal cooling channels, considering the cooling lubricant distribution and chip formation. The chip formation simulation based on the finite element method and the computational fluid dynamics flow simulation are combined. In this way, the differences between the different geometric models that do not allow any joint generation of numerical information due to missing interfaces are overcome.     

Analysis of tool temperature fluctuation in interrupted cutting

A unidimensional model for temperature distribution in the tool during intermittent cutting is presented. The tool-chip interface heating is approximated by a periodic rectangular heat flux. The effects of cutting time ratio, frequency of temperature fluctuation and thermal diffusivity of the tool material on internal temperature distribution and on thermal stresses developed in the tool have been discussed. With increasing cutting frequency, the temperature gradient in the cutting zone increases, but with higher thermal diffusivity of the tool material, it diminishes. The magnitude of thermal stresses increases with increase in amplitude of temperature fluctuation     

Serrated chip formation mechanism analysis for machining of titanium alloy Ti-6Al-4V based on thermal property

Based on the software ABAQUS/Explicit, a finite element (FE) model for orthogonal cutting was established. The FE model was validated by comparing the cutting forces and serrated degree of chips obtained by orthogonal cutting experiments under the cutting speeds 40, 80, 120, and 160 m/min. Based on the developed FE model, the influence of thermal conductivity on the degree of chip segmentation and the adiabatic shear localization were investigated. Furthermore, the plot contours on undeformed shape of cutting simulation was used to investigate the temperature distribution, and the high temperature zone was identified, which can help enhance the understanding of the serrated chip formation. Finally, cracks located in the adjacent segments of chips were observed. The results show that with the increase in thermal conductivity, the degree of adiabatic shear decreases. It can be concluded that the poor thermal conduction performance should be primarily responsible for the formation of serrated chips during machining Ti-6Al-4V alloy. Due to the high temperature at contact surface between cutting tool and workpiece, the increasing of cutting speed facilitates the formation of serrated chips during machining.     

A Note on Interpreting Tool Temperature Measurements from Thermography

Thermography (thermal imaging) is a well-established experimental method for studying cutting tool temperature distributions. In one form, cutting edge temperatures within the chip / tool contact area are deduced from thermal images of tool faces normal to the cutting edge but offset from the contact region. In general practice, the offset is made as small as possible (<< 1 mm) and it is assumed that the observed temperature is the same as that within the contact. In this short communication an approximate analytical model is developed for the influence of the offset on the observed temperature. The predictions from the model are compared with previously unpublished existing results on the machining of Ti alloys (Ti6Al4V and Ti5Al4V) and on steel (AISI 4140). It is shown that ignoring the offset may introduce underestimates of cutting edge temperature of ≈ 30% or more. This is large compared to the usually considered uncertainties of ± 5% from camera and tool emissivity calibration. There is a need for a dedicated study of this effect.     

ANALYSIS OF INITIAL CONTACT AND TOOL FRACTURE IN THE MILLING PROCESS

Abstract

This paper presents an analysis of the entry process and tool fracture in the milling operation. Equations representing different types of initial contacts are derived. Experimental and theoretical results reveal that the direction of the cutting force and the initial location on the rake face determine the entry fracture in milling cutters.     

Hybrid analytical–numerical solution for the shear angle in orthogonal metal cutting — Part II: experimental verification

The hybrid analytical–finite element model described in Part I is applied to predict the shear angle for a range of cutting velocity, uncut chip thickness, and two tool orthogonal rake angles. Experimental results and an empirical equation are also presented for the influence of the cutting conditions and cutting tool geometry on the chip–tool contact length. It is shown that there is a linear dependence between the chip–tool contact length/uncut chip thickness ratio and chip thickness/uncut chip thickness ratio over the range of cutting conditions assumed. The increase of the shear angle with the tool orthogonal rake is mostly due to the reduction of the specific shear energy in the primary shear zone and the specific friction energy in the secondary shear zone accompanied by a reduction of the chip–tool contact zone. The uncut chip thickness and cutting velocity influence the shear angle through their effect on the interface temperature and hence on the material flow stress in the secondary shear zone. The change in both parameters does not change significantly the specific shear energy in the primary shear zone. The model results are compared with the experimental results for a work material 0.18% C steel. The agreement between the predicted and experimental results is seen to be exceptionally good.     

ESTIMATION OF TOOL WEAR OF CARBIDE TOOL IN ORTHOGONAL CUTTING USING FEM SIMULATION

In metal cutting, tool wear on the tool-chip and tool-workpiece interfaces (i.e. flank wear and crater wear) is strongly influenced by the cutting temperature, contact stresses, and relative sliding velocity at the interface. These process variables depend on tool and workpiece materials, tool geometry and coatings, cutting conditions, and use of coolant for the given application. Based on the predicted temperatures and stresses on the tool face from the finite element analysis (FEA) simulation, tool wear may be estimated with acceptable accuracy by incorporating an empirical wear model.

The overall objective of this study is to develop a methodology to predict the tool wear evolution and tool life in orthogonal cutting using FEM simulations. To approach this goal, the methodology is proposed with three different parts. In the first part, a tool wear model for the specified tool-workpiece pair is developed via a calibration set of tool wear cutting tests in conjunction with cutting simulations. In the second part, modifications are made to the commercial FEM code used to allow for tool wear calculation and tool geometry updating. The last part includes the validation of the developed methodology. This paper is mainly focused on the modifications made to the commercial FEM code in order to make reasonable tool wear estimates (the second part).     

THE OXLEY MODELING APPROACH,ITS APPLICATIONS AND FUTURE DIRECTIONS

Abstract

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.     

Study on micro-interacting mechanism modeling in grinding process and ground surface roughness prediction

In the research on grinding process modeling, the stochastic nature of grain sizes and locations need to be considered. A new numerical model was developed which will describe the micro-interacting situations between grains and workpiece material in grinding contact zone. The model was established based on a series of reasonable assumptions, the critical conditions of starting points of plowing and cutting stages, and the redefined grinding contact zone. It indicated that there are four types of grain existing in grinding contact zone: uncontact, sliding, plowing, and cutting grains. The number of grains per unit wheel volume (N _v ) and the undeformed chip thickness (h _cu,max), which are key parameters in grinding process modeling, were firstly obtained. The numbers and distributions of different grain types along grinding contact zone were then obtained and analyzed. Calculation results showed that only a small fraction of grains participate in cutting interactions and the changing laws of each grain types along grinding contact length are very different from each other, which gives a deeper insight into grinding process and can be a good foundation for more precise grinding force prediction and thermal analysis. Another important application of this model is for ground surface roughness prediction and a new method on this purpose was developed. At last, two comparisons were made between calculation results and existing experimental data for validating the work on paper. Comparison results showed that the roughness of ground surface can be well predicted and gave the method theoretically to reduce ground surface roughness.     

Analysis of precision deburring using a laser —An experimental study and FEM simulation

The purpose of this study is to develop an effective methods for automated deburring of precision components. A high power laser is proposed as a deburring tool for complex part edges and burrs. For the laser experiments, rectangular-shaped carbon steel and stainless steel machined specimens with burr along one side were prepared. A 1500 Watts CO₂ laser was used to remove burrs on the workpieces. The prediction of the heat affected zone (HAZ) and cutting profile of laser-deburred parts using finite element method is presented and compared with the experimental results. This study shows that the finite element method (FEM) analysis can effectively predict the thermal affected zone of the material and that the technique can be applied to precision components.