Statistical analysis of the effects of machining parameters and workpiece hardness on the surface finish of machined medium carbon steel

The objective of this study is to ascertain the effect of machining parameters and workpiece hardness on surface roughness of machined components and to develop a better understanding of the effect of process parameters on the machined surface. Such an understanding can provide insight into the problems of controlling the finish of machined surfaces when the process parameters are adjusted to obtain a certain surface finish. The collected data were analyzed using parametric analyses of variance (ANOVA) with surface finish as the dependent variable and hardness of the workpiece material, cutting tool position from the surface of the clamping device (chuck), depth of cut, cutting velocity, and cutting feed as independent variables. The results showed that surface roughness is significantly affected by the workpiece hardness, cutting feed, cutting speed, depth of cut, cutting tool position from the chuck, and their interactions with each other. The results suggest that feed rate and cutting speed can be adjusted to produce a certain surface finish when the position of the cutting tool from the surface of a clamping device or the hardness of the workpiece is changed.     

Performance evaluation of pure CBN tools for machining of steel

Ultra-precision machining is one of the most important machining technologies for the manufacture of precision dies and molds. Typically, single point diamond cutting tools are used to machine molds which are coated with electroless nickel (NiP) for such applications. The high cost of diamond cutters and electroless nickel plating, coupled with problems of pre-mature failure of the coating in service and long lead time are negative factors in this approach. Hence, there is a strong need for the direct ultra-precision machining of mold steel and to develop relevant technologies to address the problem of tool wear. In the machining of alloy steel, cubic boron nitride (CBN) has long been used as an ideal cutting tool material but recently binderless CBN or pure CBN (PCBN) with superior mechanical properties has been developed by Sumitomo Electric Industries in Japan. The objective of this paper is to explore the feasibility of using PCBN tools for direct ultra-precision machining of Stavax, a type of alloy steel from ASSAB. The performance characteristics in terms of surface roughness and tool wear of PCBN (Sumitomo IZ900) and conventional CBN (Sumitomo BN600) under different machining conditions were studied and their results were compared. Based on the experimental results, PCBN has been found to perform better in terms of wear resistance compared to conventional CBN tool. It is also able to achieve near mirror finish of less than 30 nm R_a, and hence it appears to be a promising tool for direct cutting of die and mold materials.     

Modelling of temperature and forces when orthogonally machining hardened steel

This paper initially considers heat generation in single-point metal cutting and the direct/indirect techniques employed to measure cutting temperatures. The development of analytical models of the cutting process is briefly reviewed, including more recent work involving finite element (FE) methods. Details are given of the different FE packages and formulation methods used by different researchers. Following on from this, an FE model is presented using FORGE 2® to simulate cutting forces and temperature distributions when orthogonal turning a hardened hot work die steel, AISI H13 (52HRC), with polycrystalline cubic boron nitride (PCBN) tooling. Experimental data from infrared chip surface temperature measurements and cutting force output are used to validate the model. Good correlation was obtained between experimental and modelled results for temperature; however, the FE analysis underestimated feed force results due to a lack of adequate workpiece property data and simplistic tool/chip friction assumptions.     

Cutting performance of PVD-coated carbide and CBN tools in hardmilling

In this study, cutting performance of CBN tools and PVD-coated carbide tools in end-milling of hardened steel was investigated. In high-speed dry hardmilling, two types of CBN tools were applied: the CBN-rich type and an ordinary one. In the case of relatively low-speed milling, on the other hand, a few coated carbide tools were selected where four kinds of coating films, TiN, TiCN, TiAlN and multi-layered TiAlN/AlCrN, were deposited on the K10 and P30 grade carbide. The cutting performance was mainly evaluated by tool wear, cutting temperature, cutting force and surface roughness. In dry cutting of hardened carbon steel with the ordinary CBN tool, the cutting tool temperature rose rapidly with increase in cutting speed; and tool temperature reached approximately 850 °C at the cutting speed of 600 m/min. In the case of the CBN-rich tool, the cutting temperature decreased by 50 °C or more because of its high thermal conductivity. It is remarkable that tool wear or damage on a cutting tool was not observed even when the cutting length was 156 m in both CBN tools. In the case of coated carbide tools, the temperatures of TiN-, TiCN- and TiAlN-coated carbide tools rose as cutting proceeded because of the progress of tool wear, but that of TiAlN/AlCrN-coated carbide tool hardly rose due to little tool wear. When the base material was K10 grade carbide, tool temperature was lower than that of P30 with any coating. The tool flank wear depends considerably on hardness and oxidizing temperature of the coating film.     

Comparison of tool life between ceramic and cubic boron nitride (CBN) cutting tools when machining hardened steels

This paper describes a comparison of tool life between ceramics and cubic boron nitride (CBN) cutting tools when machining hardened bearing steels using the Taguchi method. An orthogonal design, signal-to-noise ratio (S/N) and analysis of variance (ANOVA) were employed to determine the effective cutting parameters on the tool life. First order linear and exponential models were carried out to find out the correlation between cutting time and independent variables. Second order regression model was also extended from the first order model when considering the effect of cutting speed (V), feed rate (f), hardness of cutting tool (TH) and two-way of interactions amongst V, f, TH variables. The results indicated that the V was found to be a dominant factor on the tool life, followed by the TH, lastly the f. The CBN cutting tool showed the best performance than that of ceramic based cutting tool. In addition, optimal testing parameter for cutting times was determined. The confirmation of experiment was conducted to verify the optimal testing parameter. Furthermore, the second order regression model and exponential model supported the first order model regarding the prediction capability. Improvements of the S/N ratio from initial testing parameters to optimal cutting parameters or prediction capability depended on the S/N ratio and ANOVA results. Moreover, the ANOVA indicated that the cutting speed was a higher significant but other parameters were also significant effects on the tool lives at 90% confidence level. The percentage contributions of the cutting speed, tool’s hardness, and feed rate were about 41.63, 32.68, and 25.22 on the tool life, respectively.     

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.     

Tool-life and wear mechanisms of CBN tools in machining of Inconel 718

The demand for increasing productivity when machining heat resistant alloys has resulted in the use of new tool materials such as cubic boron nitride (CBN) or ceramics. However, CBN tools are mostly used by the automotive industry in hard turning, and the wear of those tools is not sufficiently known in aerospace materials. In addition, the grade of these tools is not optimized for superalloys due to these being a small part of the market, although expanding (at 20% a year). So this investigation has been conducted to show which grade is optimal and what the wear mechanisms are during finishing operations of Inconel 718. It is shown that a low CBN content with a ceramic binder and small grains gives the best results. The wear mechanisms on the rake and flank faces were investigated. Through SEM observations and chemical analysis of the tested inserts, it is shown that the dominant wear mechanisms are adhesion and diffusion due to chemical affinity between elements from workpiece and insert.     

Statistical analysis of surface roughness and cutting forces using response surface methodology in hard turning of AISI 52100 bearing steel with CBN tool

The present work concerns an experimental study of hard turning with CBN tool of AISI 52100 bearing steel, hardened at 64 HRC. The main objectives are firstly focused on delimiting the hard turning domain and investigating tool wear and forces behaviour evolution versus variations of workpiece hardness and cutting speed. Secondly, the relationship between cutting parameters (cutting speed, feed rate and depth of cut) and machining output variables (surface roughness, cutting forces) through the response surface methodology (RSM) are analysed and modeled. The combined effects of the cutting parameters on machining output variables are investigated while employing the analysis of variance (ANOVA). The quadratic model of RSM associated with response optimization technique and composite desirability was used to find optimum values of machining parameters with respect to objectives (surface roughness and cutting force values). Results show how much surface roughness is mainly influenced by feed rate and cutting speed. Also, it is underlined that the thrust force is the highest of cutting force components, and it is highly sensitive to workpiece hardness, negative rake angle and tool wear evolution. Finally, the depth of cut exhibits maximum influence on cutting forces as compared to the feed rate and cutting speed.     

The performance of CBN tools in the machining of titanium alloys

Advancements in the aerospace, nuclear and other industries require the enhanced in-service performance of engineering components. These requirements have resulted in the large scale development and use of heat-resistant and high-strength materials such as titanium alloys, which pose considerable machining problems. In this study on machining of titanium alloy using CBN tools, the machining performance was evaluated in terms of cutting force, specific cutting pressure, cutting temperature, chip strain and surface finish.     

Residual stress and surface roughness when facing age hardened Inconel 718 with CBN and ceramic cutting tools

The demand for increasing productivity when machining heat resistant super alloys has resulted in the use of advanced cutting tools such as ceramics and cubic boron nitride (CBN). However, the effects of these tools on the surface integrity, especially the residual stresses created, in the high speed facing operation of Inconel 718 has not been dealt with. In this paper, the residual stresses and the surface roughness when facing age hardened Inconel 718 using CBN and mixed ceramic cutting tools at their respective optimum performance based on productivity has been investigated. The residual stress and surface finish generated during facing with CBN cutting tools have been investigated as a function of speed, depth of cut, coolant, tool geometry and nature of the tool coating. In addition, mixed ceramic cutting tools have been investigated for comparison. The results show that mixed ceramic cutting tools induce tensile residual stresses with a much higher magnitude than CBN cutting tools. The residual stresses and the surface roughness generated by CBN cutting tools are more sensitive to cutting speeds than depth of cut. The use of coolant results in either compressive residual stresses or lowers the magnitude of the tensile residual stresses, whereas dry cutting always resulted in tensile residual stresses. From this investigation, it is suggested that round CBN cutting tools should be used at slow cutting speeds (150 m/min) and small depths of cut (0.05 mm) and with the use of coolant to achieve compressive or minimal tensile residual stresses and good surface finish.     

Analysis of orthogonal finish machining using tungsten carbide and diamond tools of different heat transfer coefficients

An orthogonal cutting model for finish machining, using diamond and tungsten carbide tools which have different coeffficients of thermal conductivity, was simulated and analyzed. It was assumed that the tool had a minute amount of tool flank wear. The distribution of strain rate and stress within the machined workpiece and the determination of the cutting force were obtained after simulation. The generation and distribution of temperature and stress within the chip through cutting of the workpiece were also acquired. In addition, the temperature of the tool, the workpiece and the chip during finish machining by the two different tools, that show the effects of the different friction coefficients of the diamond tool and the tungsten carbide tool on cutting, were compared. Finally, the cutting forces predicted by the model for orthogonal finish machining were compared with those obtained by experiment, and it appears that the present orthogonal finish machining model is reasonable.     

Prediction of flank wear by using back propagation neural network modeling when cutting hardened H-13 steel with chamfered and honed CBN tools

Productivity and quality in the finish turning of hardened steels can be improved by utilizing predicted performance of the cutting tools. This paper combines predictive machining approach with neural network modeling of tool flank wear in order to estimate performance of chamfered and honed Cubic Boron Nitride (CBN) tools for a variety of cutting conditions. Experimental work has been performed in orthogonal cutting of hardened H-13 type tool steel using CBN tools. At the selected cutting conditions the forces have been measured using a piezoelectric dynamometer and data acquisition system. Simultaneously flank wear at the cutting edge has been monitored by using a tool makers microscope. The experimental force and wear data were utilized to train the developed simulation environment based on back propagation neural network modeling. A trained neural network system was used in predicting flank wear for various different cutting conditions. The developed prediction system was found to be capable of accurate tool wear classification for the range it had been trained.     

Predicting the effect of vibration on ultraprecision machining surface finish as described by surface finish lobes

Surface finish error resulting from unwanted relative tool/workpiece harmonic motion in the in-feed direction is studied for the ultraprecision face turning operation. Specifically consideration is given to the manifestation of this motion in the feed direction of the workpiece for a broad range of relative tool/workpiece motion disturbance frequencies. The concept of surface finish lobes is presented and is used to describe the feed direction surface finish error spatial frequency expected on a workpiece surface, for a given disturbance frequency. The surface finish lobes make it possible to know, for a broadband of disturbance frequencies, the resulting error on the workpiece surface in the feed direction. The surface finish lobe methodology is validated with experimental findings. Finally, potential applications of the surface finish lobes are discussed, including their use in shifting waviness errors to a very long wavelength, thus reducing the impact of unwanted relative tool/workpiece harmonic motion on ultraprecision machining quality.     

Performance of nano/micro CBN particle coated tools in superfinish hard machining

A two-stage composite coating method has been developed for coating of nano/micro cubic boron nitride (CBN) particles on cutting tools. Since nano/micro CBN particle coated tools are more cost-effective than solid polycrystalline CBN (PCBN) tools, a comprehensive study on the coated tools is required. This paper studies the performance of these tools in superfinish hard machining. Specimens were machined by a solid PCBN tool and CBN particle coated tools with two different CBN particle size distributions: less than 0.5 and 2 μm. The specimen machined by a tool with small CBN particle coating (less than 0.5 μm) showed more compressive residual stresses and less thermal damage below the machined surface than other specimens. Furthermore, the specimen machined by a tool with small CBN particle showed less residual stress scatter than other specimens. The rolling contact fatigue life was predicted by using a rolling contact fatigue life model. The rolling contact fatigue life predictions indicate that the predicted life of the specimen machined by a tool with small CBN particle coating is longer than that of other specimens. The results thus indicate that a tool with small CBN particle coating provides better performance than other tools in superfinish hard machining.     

Contour finish turning operations with coated grooved tools: Optimization of machining performance

This paper presents a new methodology for optimization of machining performance in contour finish turning operations. Two machining performance measures, chip breakability and surface roughness, are considered as optimization criteria due to their importance in finishing operations. Chip breakability covers two major factors: chip shape and size. Comprehensive case studies are presented to demonstrate the determination and application of optimal cutting conditions through experimental validation.     

Estimating the effect of cutting parameters on surface finish and power consumption during high speed machining of AISI 1045 steel using Taguchi design and ANOVA

The present paper outlines an experimental study to investigate the effects of cutting parameters on finish and power consumption by employing Taguchi techniques. The high speed machining of AISI 1045 using coated carbide tools was investigated. A combined technique using orthogonal array and analysis of variance was employed to investigate the contribution and effects of cutting speed, feed rate and depth of cut on three surface roughness parameters and power consumption. The results showed a significant effect of cutting speed on the surface roughness and power consumption, while the other parameters did not substantially affect the responses. Thereafter, optimal cutting parameters were obtained.     

Optimum cutting tool geometry when interrupted cutting carburized steel by CBN tool

Interrupted turning, a quite severe application for the CBN tool, was attempted. In the turning of a two-groove bar, gradual wear controlled the tool life. However tool failure determined the life in the turning of a spline shaft. The type of failure in the latter case changed depending on the way in which the tool collided with the tool tip—catastrophic damage could be avoided with substantial improvement of the tool life and surface roughness of the workpiece. For further improvements of the life, a new tool was developed having a land on the flank face. Turning with this new tool resulted in a prolonged life, as much as 26 times that of the conventional tool previously developed for continuous turning.     

Surface finish on hardened bearing steel parts produced by superhard and abrasive tools

New technological process consisting of hard turning (HT) followed by abrasive machining, in place of the widely used method in industry, i.e., hard turning versus grinding, has lately been launched in the automotive industry. This is because, many transmissions parts, such as synchronizing gears, crankshafts and camshafts require superior surface finish along with appropriate fatigue performance. This paper provides a comprehensive characterization of part surface finish produced in dry turning of a hardened AISI 52100 bearing steel using mixed ceramic (MC) and PCBN tools, and also its modification after special abrasive finishing operations including superfinishing (SF) and belt grinding (BG). In this investigation, some important 2D and 3D surface roughness parameters, as well as profile and surface characteristics, such as the amplitude distribution functions, bearing area curves, surface topographies and contour maps obtained for the four surface types selected, were determined and analyzed. Experimental data gained during measurements indicate that each of the finishing abrasive processes provides a specific set of surface topologies. The transformation of bearing properties of surfaces, generated through two optional PCBN HT-BG and MC HT-SF machining sequences, are highlighted. As a result, the modifications of surface profiles achieved by means of special abrasive machining operations can distinctly improve the bearing properties of previously hard turned surfaces, and exemplarily, they shorten the running-in period.     

Ultra-precision surface finish of the hardened stainless mold steel using vibration-assisted ball polishing process

A vibration-assisted spherical polishing system driven by a piezoelectric actuator has been newly developed on a machining center to improve the burnished surface roughness of hardened STAVAX plastic mold stainless steel and to reduce the volumetric wear of the polishing ball. The optimal plane surface ball burnishing and vibration-assisted spherical polishing parameters of the specimens have been determined after conducting the Taguchi's L₉ and L₁₈ matrix experiments, respectively. The surface roughness R_a=0.10 μm, on average, of the burnished specimens can be improved to R_a=0.036 μm (R_max=0.380 μm) using the optimal plane surface vibration-assisted spherical polishing process. The improvement of volumetric wear of the polishing ball was about 72% using the vibration-assisted polishing process compared with the non-vibrated polishing process. A simplified kinetic model of the vibration-assisted spherical polishing system for the burnished surface profile was also derived in this study. Applying the optimal plane surface ball burnishing and vibrated spherical polishing parameters sequentially to a fine-milled freeform surface carrier of an F-theta scan lens, the surface roughness of R_a=0.045 μm (R_y=0.65 μm), on average, within the measuring range of 149 μm×112 μm on the freeform surface, was obtainable.     

Modeling of velocity-dependent chip flow angle and experimental analysis when machining 304L austenitic stainless steel with groove coated-carbide tools

This paper presents an analysis of experimental cutting forces and the study of the chip flow angle when machining 304L austenitic steel with a groove coated tool under dry condition. Experiments were conducted on a wide range of cutting conditions with a particular attention to ensure a great confidence in the obtained results. A detailed analysis of experimental cutting forces and the identification of empirical cutting force equations similar to that usually used for flat tools are proposed. The main focus of this work is on the study of chip flow angle deduced here from experimental cutting forces, considering that the chip flow direction is collinear to the friction force. From a comparison between experiments and two often used approaches, it appears that the experimental chip flow angle estimation, based on neglecting the complex tool geometry and adopting a zero rake angle, is bounded by the two considered modelings that renders useful for the proposed study. From experiments it is also observed an increase of the chip flow angle as the cutting velocity is increased. A velocity-dependent modeling with two distinct strategies of identification is then proposed in order to capture the cutting velocity effect on the chip flow angle.