A comparative study of multilayer and functionally graded coated tools in high-speed machining

Multilayer-coated tool systems have been effective in controlling mechanical and thermal loads, especially in high-speed cutting regime. In this study, cutting performance of tungsten carbide tools with restricted contact length and multilayer chemical vapour deposition deposited coatings, TiCN/Al₂O₃/TiN (in series) and TiCN/Al₂O₃–TiN (functionally graded), was investigated in dry turning. Cutting tests were conducted on low carbon alloy steel AISI/SAE 4140 over a wide range of cutting speeds between 200 and 879?m/min. Results including cutting forces, chip compression ratio, shear angle, contact area inclusive of sticking and sliding phenomena and tool flank wear are presented. In particular, prediction of heat partition into the cutting tool inserts was carried out using a combination of experimental tests and the finite element method. The results show that coating layouts and cutting tool edge geometry can significantly affect heat distribution into the cutting tool. The paper clearly shows the role and potential benefits of applying different top coats on the rake and flank faces with regards contact phenomenon, impact on thermal shielding and tool wear. An appropriate coating layout selection is crucial in controlling tool wear, especially in high-speed machining.     

A comparative study of the tool–chip contact length in turning of two engineering alloys for a wide range of cutting speeds

Tool chip contact length is an important parameter in machining, as it provides an indication of the size of area of interaction between the hot chip and the tool surface and hence the interface heat transfer zone. Heat transfer and thermally activated wear modes usually dominate tool wear in the high speed machining of steels and machining of titanium alloys at most cutting speeds. In this study, existing models for the prediction of tool–chip contact length are reviewed and examined for their suitability in high speed machining of two widely used engineering alloys. Orthogonal turning tests for AISI 1045 steel and Ti6Al4V titanium alloy are conducted for a range of cutting speeds from conventional to high speeds. New contact length models are presented for both materials covering a wide range of cutting speeds. More significantly, these contact length models are appropriate for high speed machining where thermal loads significantly influence process performance. Additionally, the work discusses how the machinability of engineering materials influences the ability to predict contact length.     

Analytical prediction of cutting tool wear

An analytical method is presented which enables the crater and flank wear of tungsten carbide tools to be predicted for a wide variety of tool shapes and cutting conditions in practical turning operations based only on orthogonal cutting data from machining and two wear characteristic constants. A wear characteristic equation is first derived theoretically and verified experimentally. An energy method is developed to predict chip formation and cutting forces in turning with a single-point tool from the orthogonal cutting data. Using these predicted results, stress and temperature on the wear faces can be calculated. Computer simulation of the development of wear is then carried out by using the characteristic equation and the predicted stresses and temperatures upon the wear faces. The predicted wear progress and tool life are in good agreement with experimental results.     

Tool Wear Prediction and Surface Improvement in Vibration Cutting

Prolongation of tool life in metal cutting is an effective factor to produce lower cutting forces and better machined surfaces. In this study, the influence of ultrasonic vibration is analyzed using experimental and numerical methods. Accordingly, turning tests are carried out on an AISI 4140 steel bar in two types of machining: conventional and ultrasonic-assisted turning. After verification of the developed model, tool wear results are discussed with respect to analysis of heat and stress distributed on tool faces. Finally, it was revealed that periodic movement of the cutting tool in vibratory turning resulted in reduced contact time, resulting in lower heat conduction from the deformed chip to tool rake face. As a result, lower wear has been propagated on tool faces compared to a tool worn in conventional turning. In addition, the effect of cutting parameters on surface roughness is investigated by measurement and 3D analysis of surface topography.     

Influence of the tool corner radius on the tool wear and process forces during hard turning

Hard turning has become an alternative machining process for grinding processes of hardened steels. One challenge during hard turning is the increasing wear during the operation time of the tool and the hereby influenced workpiece surface and subsurface properties. This causes unfavorable changes of the microstructure and residual stress state or rather damages of the subsurface. Important factors are the contact conditions between the tool and the workpiece. The width of flank wear land influences the size of the passive force significantly. This has a direct impact on the subsurface properties of the workpiece. One solution is to modify the contact conditions and thereby the specific mechanical and thermal loads that are applied to the tool as well as to the workpiece. This article presents an experimental approach of modified corner radius geometry of cutting tools for hard turning processes. Hereby, the size and direction of the contact length of the cutting edge are adjusted as well as the load impact during machining. The aim is to reduce the tool wear performance. The results show the potential of the load-specific tool design concerning the tool wear and the workpiece subsurface properties. Furthermore, a new approach for predicting the process forces during hard turning is presented.     

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.     

平面铣削刀具前刀面瞬态切削温度的研究

刀具切削温度对刀具寿命、刀具磨损等有重要影响。因此在实际加工之前预测出刀具温度,对合理选择切削参数、优化数控程序等均具有重要意义。平面铣削等断续切削过程的热条件不同于车削等连续切削过程。用数学物理方法建立了平面铣削过程刀具的一维传热学模型,用解析的方法预测平面铣削过程中刀具前刀面的温度分布,考虑了刀具切出时空气强化对流散热对刀具前刀面温度的影响。结果表明,刀具切入时间和切出时间对刀具温度有较大影响。用文献中断续车削刀具温度实验数据对铣削刀具前刀面温度的传热学预测模型进行了验证,结果表明二者趋势一致,但平面铣削预测的刀具温度略低于断续车削的刀具温度。     

The monitoring of flank wear on the CBN tool in the hard turning process

In precision hard turning, tool flank wear is one of the major factors contributing to the geometric error and thermal damage in a machined workpiece. Tool wear not only directly reduces the part geometry accuracy but also increases the cutting forces drastically. The change in cutting forces causes instability in the tool motion, and in turn, more inaccuracy. There are demands for reliably monitoring the progress of tool wear during a machining process to provide information for both correction of geometric errors and to guarantee the surface integrity of the workpiece. A new method for tool wear monitoring in precision hard turning is presented in this paper. The flank wear of a CBN tool is monitored by feature parameters extracted from the measured passive force, by the use of a force dynamometer. The feature parameters include the passive force level, the frequency energy and the accumulated cutting time. An ANN model was used to integrate these feature parameters in order to obtain more reliable and robust flank wear monitoring. Finally, the results from validation tests indicate that the developed monitoring system is robust and consistent for tool wear monitoring in precision hard turning.     

An integrated approach to evaluating the tribo-contact for coated cutting inserts

The orthogonal machining process when end turning medium carbon and austenitic stainless steels with cemented WC-Co tools coated with single-layer (TiC), two-layer (TiC/TiN), and three-layer (TiC/Al₂O₃/TiN) hard thin films was investigated. Extensive experimental investigations including the thermal, mechanical and tribological responses of the tribo-contact between the coating–substrate system and the chip, under different cutting conditions, were carried out. The study sheds light on the cutting forces, the interface temperatures and the tribo-contact conditions, including the friction energy dissipated at the tool–chip interface, the frictional heat flux conducting into either the chip or the insert, the mean coefficient of sliding friction and the contact loads exerted on the tool rake face. Finally, it was demonstrated how the intrinsic coating properties control the heat flux flowing into both components of such a closed tribo-system and the mechanical stresses on the contact area.     

Finish Machining of Nickel-Base Inconel 718 Alloy with Coated Carbide Tool under Conventional and High-Pressure Coolant Supplies

Single-point turning of Inconel 718 alloy with commercially available Physical Vapour Deposition (PVD)-coated carbide tools under conventional and high-pressure coolant supplies up to 20.3 MPa was carried out. Tool life, surface roughness (Ra), tool wear, and component forces were recorded and analyzed. The test results show that acceptable surface finish and improved tool life can be achieved when machining Inconel 718 with high coolant pressures. The highest improvement in tool life (349%) was achieved when machining with 11 MPa coolant supply pressure at higher speed conditions of 60 m · min^?1. Machining with coolant pressures in excess of 11 MPa at cutting speeds up to 40 m · min^?1 lowered tool life more than when machining under conventional coolant flow at a feed rate of 0.1 mm · rev^?1. This suggests that there is a critical coolant pressure under which the cutting tools performed better under high-pressure coolant supplies.

Cutting forces increased with increasing cutting speed due probably to reactive forces introduced by the high-pressure coolant jet. Tool wear/wear rate increased gradually with prolonged machining with high coolant pressures due to improved coolant access to the cutting interface, hence lowering cutting temperature. Nose wear was the dominant tool failure mode when machining with coated carbide tools due probably to a reduction in the chip-tool and tool-workpiece contact length/area.     

Effect of tool edge wear on the cutting forces and vibrations in high-speed finish machining of Inconel 718: an experimental study and wavelet transform analysis

High-speed machining has been receiving growing attention and wide applications in modern manufacture. Extensive research has been conducted in the past on tool flank wear and crater wear in high-speed machining (such as milling, turning, and drilling). However, little study was performed on the tool edge wear??the wear of a tool cutting edge before it is fully worn away??that can result in early tool failure and deteriorated machined surface quality. The present study aims to fill this important research gap by investigating the effect of tool edge wear on the cutting forces and vibrations in 3D high-speed finish turning of nickel-based superalloy Inconel 718. A carefully designed set of turning experiments were performed with tool inserts that have different tool edge radii ranging from 2 to 62???m. The experimental results reveal that the tool edge profile dynamically changes across each point on the tool cutting edge in 3D high-speed turning. Tool edge wear increases as the tool edge radius increases. As tool edge wear dynamically develops during the cutting process, all the three components of the cutting forces (i.e., the cutting force, the feed force, and the passive force) increase. The cutting vibrations that accompany with dynamic tool edge wear were analyzed using both the traditional fast Fourier transform (FFT) technique and the modern discrete wavelet transform technique. The results show that, compared to the FFT, the discrete wavelet transform is more effective and advantageous in revealing the variation of the cutting vibrations across a wide range of frequency bands. The discrete wavelet transform also reveals that the vibration amplitude increases as the tool edge wear increases. The average energy of wavelet coefficients calculated from the cutting vibration signals can be employed to evaluate tool edge wear in turning with tool inserts that have different tool edge radii.     

Observations of the tool–chip boundary conditions in turning of aluminum alloys

Interface boundary conditions are critical to the determination of the tool temperatures and stresses which in turn are needed to design better tools and select better cutting conditions. Friction at the interface has been studied for 60 years and yet the accurate modeling of friction has presented a formidable challenge, especially in operations such as turning where the interface is inaccessible due to the continuous contact between chip and tool. A historical perspective of friction in machining is provided to better evaluate the purpose of this article. The contradictions arising in the assumptions regarding friction are analyzed. This is substantiated by experimental observations regarding seizure and sliding and their domains of validity at the interface. This paper concentrates on turning operations in the machining of aluminum workpieces using carbide cutting tools, at a range of speeds common to conventional practice.     

A metallurgical evaluation of tool wear and chip formation when machining pearlitic grey cast irons with dissimilar graphite morphologies

The need for superior in-service strength has meant that an increasing number of engineering components are now being made from pearlitic cast irons containing spheroidal graphite, rather than the more traditional cast irons containing flake graphite. Such changes of workpiece material have resulted in a rapid decline in tool life in many machining operations, particularly turning and facing.An investigation into the factors involved during chip formation which result in the observed patterns of tool wear is described in the work presented here. A series of turning tests were made on pearlitic grey cast irons containing flake (GA iron) and spheroidal (SG iron) graphite morphologies with cemented carbide (coated and uncoated) and ceramic tool materials. Built-up edge persisted to higher cutting speeds when cutting SG iron than GA iron, its periodic detachment causing attrition or fracture of the cutting edge. Smooth wear processes, probably caused by dissolution-diffusion and small strain discrete plastic deformation, were predominant on the rake and flank faces of the coated and ceramic tools when cutting both cast irons at high speed. Smooth wear was less rapid when cutting GA iron than SG iron because tool temperatures were reduced and “protective” nonmetallic layers, deposited from the chip-workpiece, interrupted dissolution-diffusion. When cutting SG iron, rapid wear of the uncoated cemented carbides was caused by attrition, while the relatively slower smooth wear, when cutting GA iron, was caused by dissolution-diffusion.     

2D FEM estimate of tool wear in turning operation

Finite element method (FEM) is a powerful tool to predict cutting process variables, which are difficult to obtain with experimental methods. In this paper, modelling techniques on continuous chip formation by using the commercial FEM code ABAQUS are discussed. A combination of three chip formation analysis steps including initial chip formation, chip growth and steady-state chip formation, is used to simulate the continuous chip formation process. Steady chip shape, cutting force, and heat flux at tool/chip and tool/work interface are obtained. Further, after introducing a heat transfer analysis, temperature distribution in the cutting insert at steady state is obtained. In this way, cutting process variables e.g. contact pressure (normal stress) at tool/chip and tool/work interface, relative sliding velocity and cutting temperature distribution at steady state are predicted. Many researches show that tool wear rate is dependent on these cutting process variables and their relationship is described by some wear rate models. Through implementing a Python-based tool wear estimate program, which launches chip formation analysis, reads predicted cutting process variables, calculates tool wear based on wear rate model and then updates tool geometry, tool wear progress in turning operation is estimated. In addition, the predicted crater wear and flank wear are verified with experimental results.     

Cooling performance of micro-texture at the tool flank face under high pressure jet coolant assistance

Micro-texture at the tool face is a state-of-the-art technique to improve cutting performance. In this paper, five types of micro-texture were fabricated at the flank face to improve the cooling performance under the condition of high pressure jet coolant assistance. By using micro-textures consisted of pin fins, plate fins and pits fabricated 0.3 mm away from the cutting edge, heat transfer from the tool face to coolant was enhanced. The conditions of tool wear, adhesion and chip formation were compared between the micro-textured and non-patterned tools in the longitudinal turning of the nickel-based superalloy Inconel 718. As a result, micro-textured tools always exhibited the reduced flank and crater wear compared with the non-patterned tool, and the rate of tool wear was influenced by the array and height of fin. The energy dispersive spectroscopy analysis of worn flank faces and the electromotive forces obtained from the tool-work thermocouple supported better cooling performances of micro-textured tools. In addition, coolant deposition at flank face evidenced that heat transfer could be promoted by micro-texture near the border of the contact area between the flank wear land and machined surface. Finally, the changes of flow patterns with pit depth are analyzed for pit type tools by computational fluid dynamics. This investigation clearly showed the function of micro-textures for increasing the turbulent kinetic energy and cooling the textured tool face.     

切削速度对精密干式车削淬硬工具钢切削力的影响

通过使用PCBN刀具精密干式车削淬硬Cr12MoV工具钢(62±1 HRC)的试验,分析了切削速度对三向切削力的影响,得出了最优切削速度。试验表明:随切削速度提高,三向切削力先急剧增大,后急剧减小,再又缓慢增大。若从最小车削合力与提高加工效率两个角度来优化切削速度,则226 n/min是最优切削速度。试验结果也对精密干式切削淬硬工具钢具有实际指导意义与参考价值。     

Preparation of tungsten disulfide (WS2) soft-coated nano-textured self-lubricating tool and its cutting performance

This paper proposes a new effective dry cutting tool named tungsten disulfide (WS₂) soft-coated nano-textured self-lubricating tool which is fabricated by two steps. First, nano-texture is made on the tool–chip interface of rake face of uncoated YS8 (WC + TiC + Co) cemented carbide cutting inserts by femtosecond laser micromachining technology. Second, WS₂ soft coating is deposited on the nano-textured tool by medium-frequency magnetron sputtering, multi-arc ion plating and ion beam assisted deposition technique. Dry turning tests on 45# quenched and tempered steel were carried out with three kinds of cutting tools: conventional YS8 tool, nano-textured tool (CFT), and WS₂ soft-coated nano-textured self-lubricating tool (CFTWS). Results show that the cutting forces, cutting temperature, the friction coefficient at the tool–chip interface, and the antiadhesive effect of the nano-textured tools were significantly reduced compared with those of the conventional one. The CFTWS tool had the best cutting performance among all the tools tested under the same test conditions. Through cutting force and cutting temperature theoretical analysis and experimental results, four mechanisms responsible were found. The first one is explained as the formation of the WS₂ lubricating film with low shear strength at the tool–chip interface, which was released from the surface nano textures and smeared on the rake face, and served as lubricating additive during dry cutting processes to reduce the cutting forces and cutting temperature. The second one is explained by the reduced contact length at the tool–chip interface of the nano-textured tools; the smaller direct contact area between the chip and tool rake face leads to less friction force, which can also contribute to the decrease of cutting forces and cutting temperature. The third one can be explained that because of the excellent lubricity of the WS₂ lubricating film, the antiadhesive effect can be significantly improved which can reduce adhesive wear of the cutting tool and prolong the tool life. The fourth one can be explained that the advantage of CFTWS tool in cutting forces and cutting temperature is obvious in relatively high-speed and high-temperature conditions may be because of ultra-low friction coefficient, high temperature resistance, and the high oxidation resistance of WS₂ soft coating which is not sensitive to high cutting temperature and high cutting speed can significantly improve the severe dry cutting environment.     

Performance of ceramic tools in high-speed cutting iron-based superalloys

A series of turning tests were conducted to investigate the cutting performance of ceramic tools in high-speed turning iron-based superalloys GH2132 (A286). Three kinds of ceramic tools, KY1540, CC650, and CC670 were used and their materials are Sialon, Al₂O₃–Ti(C,N), and Al₂O₃–SiC_w, respectively. The cutting forces, cutting temperatures, tool wear morphologies, and tool failure mechanisms are discussed. The experimental results show that with the increase in cutting speed, the resultant cutting forces with KY1540 and CC670 tools show a tendency to increase first and then decrease while those for CC650 increase gradually. The cutting temperature increases monotonically with the increase in cutting speed. The optimum cutting speeds for KY1540 and CC650 when turning GH2132 are less than 100 m/min, while those for CC670 are between 100 and 200 m/min. Flank wear is the main reason that leads to tool failure of KY1540 and CC670 while notch wear is the main factor that leads to tool failure of CC650. Tool failure mechanisms of ceramic tools when machining GH2132 include adhesion, chipping, abrasion, and notching. Better surface roughness can be got using CC670 ceramic tools.     

Parametric tool wear estimation solution of HSC appropriate machining

New strategies are used in manufacturing enterprises due to global competition. High-speed cutting offers a very appropriate opportunity to reduce run times since the high cutting speeds and feed rates involved permit the reduction of production times and minimise rework. There are, however, difficulties in judging tool wear [1]. This paper analyses the formation mechanism of tool wear and presents a complete solution to calculate wear using a ball end cutter for high-speed cutting. Chip geometry generated to calculate tool wear is affected by machining conditions such as turning speed, cutting depth and geometries of tool and work piece which in turn determine the key parameters of chip sections such as length of cut and mean chip thickness. An improved algorithm and a knowledge-based decision model developed to calculate effective tool contact are also discussed to help reduce calculation time and improve calculation efficiency. The calculation results include output form and a 3D wear model showing wear data distributed on the tool contour.     

Investigation of tool wear in High Speed Machining by using a ballistic set-up

Investigation of tool wear in High Speed Machining (HSM) by using a ballistic set-up is proposed. An originality of this work consists in analyzing the previous models in sight of the real temperatures fields measured during the cutting process. Results show the high potentiality of the presented device associated with wear modeling. The chip formation and the temperature distribution are observed in real time under perfect orthogonal cutting conditions at very high cutting speeds with an intensified CCD camera. Observations highlight the importance of the evolution of the tool-chip contact during the crater wear process, especially when low feeds are selected. For low feeds, the crater profile seems control the rolling-up of the chip which also participates to accentuate the wear. Experimental results such as temperature distribution and contact length are input into diffusion wear models under different assumptions for predicting the evolution of crater profiles. Taking into account the mechanical action of the chip on the tool rake face, the prediction of tool wear is reached with a great accuracy.