HARD TURNING PLUS GRINDING-A COMBINATION TO OBTAIN GOOD SURFACE INTEGRITY IN AISI O1 TOOL STEEL MACHINED PARTS

The establishment of adequate machining guidelines requires the study of several factors (residual stresses, roughness, hardness, microstructural changes, etc.) that define the surface integrity generated in the part by a machining operation. This work studies the surface integrity generated in AISI O1 tool steel by four hard turning (conventional, laser assisted, MQL and conventional with worn tool) and two grinding (production and finishing) processes, as well as by a combined machining process (conventional hard turning + finishing grinding). Hard turning generates tensile stresses and strong structural changes in the machined surface while grinding causes compressive stresses and negligible structural changes. Below the surface, grinding generates slightly tensile or nearly null stresses whereas turning generates strong compressive stresses. The results obtained show that an optimum machining process would imply the combination of hard turning plus a slight final grinding.     

EFFECTS OF CUTTING SPEED WHEN TURNING AGE HARDENED INCONEL 718 WITH PCBN TOOLS OF BINDERLESS AND LOW-CBN GRADES

Application of polycrystalline cubic boron nitride (PCBN) tools as an alternative for ceramic and cemented carbide tools in machining superalloys has been frequently identified as a solution for enhancing process efficiency but only a limited number of studies has been done in this area. The current study explores the effect of the cutting speed, which was varied in a wide range (2–14 m/s), on machinability of age hardened Inconel 718 with PCBN tools. Performance of binderless PCBN grade and grade with low-cBN content was evaluated in terms of tool life, tool wear, cutting forces and surface quality. Chip formation and process dynamics were analyzed as well. It was found that low-cBN grade provided 70–90% better surface finish and tool life than the binderless at moderate speeds (5–8 m/s). Performance of both grades at low and high speed ranges was non-satisfactory due to notching and flaking respectively. At low cutting speed adhesive wear plays a major role while as the speed increases a chemical wear becomes dominant.     

EFFECT OF CUTTING SPEED AND SURFACE CHEMISTRY OF CUTTING TOOLS ON THE FORMATION OF BUL OR BUE AND SURFACE QUALITY OF THE GENERATED SURFACE IN DRY TURNING OF AA6005 ALUMINIUM ALLOY

In the present investigation, AA6005 (ISO: AlSiMg) alloy was machined in turning operation with different cutting tools, such as uncoated cemented carbide insert, PVD TiN coated, CVD diamond coated and PCD insert, under dry environment. Effect of cutting speed was studied for each of the cutting tools with regard to the formation of built-up layer (BUL) or built-up edge (BUE). The rake surface of the tools was characterized by optical microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopic microanalysis. Particular emphasis was given on wear mechanism of PVD TiN coated insert, conventionally used in machining ferrous alloys, during dry turning of AA6005 alloy. It has been observed that increase of cutting speed from 200 m/min to as high as 1000 m/min could not substantially reduce formation of BUL over tool rake surface during dry machining of AA6005 alloy with uncoated or PVD TiN coated cemented carbide inserts. The potential of diamond-based tools in dry machining of aluminium alloy was also studied. Finally, the effect of cutting speed on surface finish of the workpiece machined with different cutting tools was studied during dry turning of AA6005 alloy.     

纳米切削工艺中刀具原子尺度磨损机理的分子动力学分析

为研究刀具磨损的微观机理,以单晶铝为例,采用分子动力学方法对纳米切削中刀具失效的原子级物理本质进行研究。模拟结果显示,随着切削深度的增加,能够形成化学键的配对原子数也急剧增加,导致刀具的扩散磨损加剧;晶体的各向异性对刀具扩散磨损影响很小,说明扩散磨损主要是一种化学过程;扩散磨损生成的积屑瘤代替刀具进行切削,使得超精密加工的表面质量恶化,切削区域温度上升,进一步加剧扩散磨损过程。     

A NEW CONCEPT FOR DECOUPLING THE CUTTING FORCES DUE TO TOOL FLANK WEAR AND CHIP FORMATION IN HARD TURNING

Determining the temperature field in metal cutting when the tool flank is progressively worn requires the knowledge of the forces due to tool flank wear and that due to chip formation. In the past, these forces have been computed from data experimentally measured with a dynamometer, under the assumption that the chip formation configuration remained unaltered when the tool flank is progressively worn. This approach has been used in the literature even though there has been evidence that it is not correct. The error introduced by this doubtful assumption in computing the maximum surface temperature in the work-piece can be significant.

Of late there has been considerable interest in employing hard turning as the final finishing process in place of grinding and superfinishing. Consequently, the ability to accurately predict the maximum surface temperature and its distribution in the workpiece is now most desirable, for avoiding thermal damage to the machined surface. This paper discusses a new method based on the thickness of the microstructural change in chips to decouple the tool-flank forces for predicting the maximum surface temperature and its distribution in the workpiece.     

EFFECT OF CRYSTALLOGRAPHIC ORIENTATION ON CUTTING FORCES AND SURFACE FINISH IN DUCTILE CUTTING OF KDP CRYSTALS

In order to investigate the influence of material anisotropy in ductile cutting of Potassium Dihydrogen Phosphate (KDP) crystals, experiments of face cutting of (001) plane of KDP crystals are carried out by using an ultra-precision lathe with a single point diamond tool. The cutting forces, surface finish, and surface roughness in all crystallographic orientations of the machined surface are measured, and a power spectrum analysis method is used to reveal the cutting force patterns. The experimental results show that the cutting forces and surface roughness vary greatly with different crystallographic orientations of KDP crystal, and that amplitude variation of cutting forces and surface finish is closely related with the cutting parameter of the maximum undeformed chip thickness. With the maximum undeformed chip thickness below 30 nm, the amplitude variation of cutting force and surface finish is minimized, and a super-smooth surface with consistent surface finish in all the crystallographic orientations can be achieved. The surface roughness is 2.698 nm (Ra) measured by Atomic Force Microscope (AFM). These findings provide criteria for achieving a large-scale KDP crystal with consistent super-smooth surface using ductile cutting technology.     

INVESTIGATION OF THE TOOL WEAR AND SURFACE FINISH IN LOW-SPEED MILLING OF STAINLESS STEEL UNDER FLOOD AND MIST LUBRICATION

This paper examines the performance of AlN/TiN coated carbide tool during milling of STAVAX® (modified AISI 420 stainless steel) at a low speed of 50 m/min under conventional flood and mist lubrication. Abrasion, chipping, fracture resulting in the formation of crater and catastrophic failure are the wear mechanisms encountered during machining under flood lubrication. The flank wear, and the likeliness of the cutting tool to fracture, chip and fail prematurely increased with an increase in the hardness of the workpiece and a reduction in the helix angle of the tool. Small quantity of mineral oil sprayed in mist form was effective in reducing the flank wear and severity of abrasion wear, and preventing the formation of crater and the occurrence of catastrophic failure. In milling 35 and 55 HRC-STAVAX® using a feed rate of 0.4 mm/tooth and a depth of cut of 0.2 mm under mist lubrication, the cutting edge of the 25° and 40° helix angle tools only suffered small-scale edge chipping and abrasive wear throughout the entire duration of testing. The influence of the ductility of the workpiece on the surface finish and the effectiveness of mist lubricant in improving the surface finish are also discussed.     

OPTIMAL MQL AND CUTTING CONDITIONS DETERMINATION FOR DESIRED SURFACE ROUGHNESS IN TURNING OF BRASS USING GENETIC ALGORITHMS

The evolving concept of minimum quantity of lubrication (MQL) in machining is considered as one of the solutions to reduce the amount of lubricant to address the environmental, economical and ecological issues. This paper investigates the influence of cutting speed, feed rate and different amount of MQL on machining performance during turning of brass using K10 cemented carbide tool. The experiments have been planned as per Taguchi's orthogonal array and the second order surface roughness model in terms of machining parameters was developed using response surface methodology (RSM). The parametric analysis has been carried out to analyze the interaction effects of process parameters on surface roughness. The optimization is then carried out with genetic algorithms (GA) using surface roughness model for the selection of optimal MQL and cutting conditions. The GA program gives the minimum values of surface roughness and the corresponding optimal machining parameters.     

AN INVESTIGATION INTO DIMENSIONAL ACCURACY AND SURFACE FINISH ACHIEVABLE IN DRY TURNING

This paper presents experimental and analytical results of an investigation into dimensional accuracy and surface finish achievable in dry turning. The Taguchi method and Pareto ANOVA analysis are used to determine the effects of the major controllable machining parameters, viz. cutting speed, feed rate and depth of cut, on three key quality characteristics, viz. diameter error, surface roughness and circularity, and subsequently to find their optimum combination. The work and tool materials selected are alloy steel AISI 4340 and enriched cobalt-coated carbide, respectively. The results indicate that while the surface roughness can be optimized through proper selection of feed rate, optimization of diameter error and circularity is difficult due to complex interactions between the input parameters.     

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.     

CUTTING PERFORMANCE AND WEAR MECHANISM OF Si3N4-BASED NANOCOMPOSITE CERAMIC CUTTING TOOL IN MACHINING OF CAST IRON

A type of Si₃N₄-based nanocomposites ceramic cutting tool material was prepared by the addition of nano-scale Si₃N_4W whisker and nano-scale TiN particle. Cutting performance of the Si₃N₄/Si₃N_4W/TiN nanocomposite ceramic tool in machining of cast iron was investigated in comparison with a commercial sialon ceramic tool, and the tool wear mechanism was studied. The two types of cutting tools have similar cutting performance at relatively low cutting parameters, while Si₃N₄/Si₃N_4W/TiN nanocomposite tool exhibits a better wear resistance than sialon tool at the relatively high cutting parameters. The wear of sialon ceramic cutting tool is dominated by the plastic deformation, abrasive action, microcracking, pullout of grains and chemical action at the higher cutting parameters. The higher mechanical properties, and the refined matrix grains, intragranular TiN grains and dislocation in the microstructure improve the resistances to plastic deformation, microcracking, and pullout of grains for Si₃N₄/Si₃N_4W/TiN nanocomposite ceramic cutting tool. The wear of Si₃N₄/Si₃N_4W/TiN nanocomposite ceramic cutting tool is dominated by the abrasive and chemical actions at the higher cutting parameters.     

IN-PROCESS TOOL CONDITION MONITORING USING ACOUSTIC EMISSION SENSOR IN MICROENDMILLING

Recently researchers and manufacturers have shown keen interest in fabricating micro-components through tool based mechanical micromachining processes namely micromilling, microdrilling, microturning, etc. In this scenario, microendmilling is used in the manufacture of micro-molds, micro-dies, micro-channel, micro-gear, etc. The major issue in microendmilling process is the unpredictable life of the micro-tool and its premature failure during operations. Therefore in this work, an attempt has been made to monitor the tool condition (in-process) using acoustic emission (AE) sensor in microendmilling of different materials such as aluminum, copper and steel alloys. From this study, it is observed that there is a strong relationship between the tool wear (flank wear) and acoustic emission (AE_RMS) signals, surface roughness (R_a) as well as chip morphology. In order to understand the mechanism of tool wear, SEM and EDAX analyses were carried out on the microendmill after machining. Scanning Electron Microscope (SEM) and energy dispersive X-ray spectroscopy (EDAX) analyses indicated occurrence of the tool wear mechanism such as adhesion and plastic deformation in all three materials. Coating delamination is also observed while machining steel alloy. This work provides significant and new knowledge on the usage of AE sensor in monitoring the tool condition and understanding the tool wear mechanism in microendmilling of different materials.     

基于遗传算法的超精密切削表面粗糙度预测模型参数辨识及切削用量优化

建立易于分析各切削用量对粗糙度影响关系的表面粗糙度预测模型和最优的切削用量组合,是超精密切削加工技术的不断发展的需要。针对最小二乘法和传统优化方法的不足,提出了将遗传算法用于超精密切削表面粗糙度预测模型的参数辨识,并用于求解最优切削用量,给出了金刚石刀具超精密切削铝合金的表面粗糙度预测数学模型和切削用量优化结果,进行了遗传算法和常规优化算法的比较,结果表明遗传算法较最小二乘法和传统的优化方法更适合于粗糙度预测模型的参数辨识及保证切削用量的最优。     

SURFACE INTEGRITY CHARACTERIZATION AND PREDICTION IN MACHINING OF HARDENED AND DIFFICULT-TO-MACHINE ALLOYS: A STATE-OF-ART RESEARCH REVIEW AND ANALYSIS

This paper presents a review of the state-of-art research on surface integrity characterization, especially the characteristics of residual stresses produced in machining of hardened steels, titanium and nickel-based superalloys using the geometrically defined tools. The interrelationships among residual stresses, microstructures, and tool-wear have been discussed. Current research on residual stress modeling and simulation using finite element method has been critically assessed. Also, the rationale for developing multi-scale simulation models for predicting residual stresses in machining has been presented. At the end, possible future work has been proposed.     

TOOL CONDITION MONITORING USING ACOUSTIC EMISSION,SURFACE ROUGHNESS AND GROWING CELL STRUCTURES NEURAL NETWORK

The monitoring of tool wear is a most difficult task in the case of various metal-cutting processes. Artificial Neural Networks (ANN) has been used to estimate or classify certain wear parameters, using continuous acquisition of signals from multi-sensor systems. Most of the research has been concentrated on the use of supervised neural network types like multi-layer perceptron (MLP), using back-propagation algorithm and Radial Basis Function (RBF) network. In this article, a new constructive learning algorithm proposed by Fritzke, namely Growing Cell Structures (GCS) has been used for tool wear estimation in face milling operations, thereby monitoring the condition of the tool. GCS generates compact network architecture in less training time and performs well on new untrained data. The performance of this network has been compared with that of another constructive learning algorithm-based neural network, namely the Resource Allocation Network (RAN). For the sake of establishing the effectiveness of GCS, results obtained have been compared with those obtained using Multi Layer Perceptron (MLP), which is a standard and widely used neural network.     

EFFECT OF COOLANT PRESSURE,NOZZLE DIAMETER,IMPINGEMENT ANGLE AND SPOT DISTANCE IN HIGH PRESSURE COOLING WITH NEAT OIL IN TURNING Ti-6AL-4V

Though titanium alloys are being increasingly sought in a wide variety of engineering and biomedical applications, their manufacturability, especially machining and grinding imposes lot of constraints. Titanium alloys are readily machinable provided the cutting velocity is in the range of 30–60 m/min. To achieve higher productivity, if the cutting velocity is enhanced to 60–120 m/min and beyond, rapid tool wear takes place diminishing the available tool life. Tool wear in machining of titanium alloys is mainly due to high cutting zone temperature localised in the vicinity of the cutting edge and enhanced chemical reactivity of titanium with the tool material. Rapid tool wear encountered in machining of titanium alloys is a challenge that needs to be overcome. High pressure cooling in machining is a very promising technology for enhancing tool life and productivity via appropriate cooling and lubrication. The present investigation is an attempt to study the effects of jet application parameters, i.e., coolant pressure, angle of impingement of the jet, spot distance and nozzle diameter on tool wear and chip morphology and to compare the effectiveness while turning Ti-6Al-4V bars under high pressure cooling with neat oil. Results indicated that at a cutting speed of 85 m/min and feed of 0.2 mm/rev, high pressure cooling provided a tool life of 24 min vis-à-vis 12 min under cryogenic cooling.