IMPULSIVE CHIP BREAKING IN METAL MACHINING: A PROOF-OF-CONCEPT STUDY

An innovative non-conventional technique, called impulsive chip breaking, is developed in the present study to break difficult-to-break chips that are often generated in machining high toughness or soft gummy materials, such as pure aluminum, pure copper, aluminum alloys, copper alloys, low carbon steels, and stainless steels. These materials have a wide variety of engineering applications. In impulsive chip breaking, the machine tool spindle rotational speed periodically increases to a prescribed higher speed within a set short period of time and then resumes to its normal constant speed to continue machining operations. The experimental investigations covering a range of cutting conditions on a selected work material are preformed to confirm the feasibility of impulsive chip breaking and study its basic mechanism as well as the characteristic variations of machining performances, including the chip morphology, the cutting forces, the machining vibrations, and the surface roughness of the machined workpiece. It is demonstrated that as long as the impulsive rotational speed of the machine tool spindle is appropriately selected or optimized, both requirements of breaking chips and maintaining the machined surface quality can be simultaneously satisfied.     

Effect of very high cutting speeds on shearing, cutting forces and roughness in dry turning of austenitic stainless steels

Behavior of austenitic stainless steels has been studied at very high cutting speeds. Turning tests were carried out using the AISI 303 austenitic stainless steel. In particular, the influence of cutting speed on tool wear, surface quality, cutting forces and chip geometry has been investigated. These parameters have been compared when performing machining at traditional cutting speeds (lower than 350?m/min) versus high cutting speeds. The analysis of results shows that the material undergoes a significant change in its behavior when machining at cutting speeds above 450?m/min, that favors the machining operation. The main component of cutting forces reaches a minimum value at this cutting speed. The SEM micrographs of the machined surfaces show how at the traditional cutting speeds the machined surfaces contain cavities, metal debris and feed marks with smeared material particles. Surfaces machined at high cutting speeds show evidence of material side flow, which is more evident at cutting speeds above 600?m/min. Tool wear is located at the tool nose radius for lower cutting speeds, whereas it slides toward the secondary edge when cutting speed increases. An analysis of chips indicates also an important decrement in chip thickness for cutting speeds above 450?m/min. This study concludes that there is an unexplored range of cutting speeds very interesting for high-performance machining. In this range, the behavior of stainless steels is very favorable although tool wear rate is also significant. Nevertheless, nowadays the cost of tool inserts can be considered as secondary when comparing to other operation costs, for instance the machine hourly cost for high-end multitasking machines.     

Development of a programmable vibration cutting tool for diamond turning of hardened mold steels

There are two drawbacks in the performance of the resonant vibration cutting tools currently available. One is that the vibration frequency cannot be changed arbitrarily and the other is that the cutting force is low. It is impossible to turn mirror plates with many kinds of materials using these resonant vibration cutting tools because they cannot match the desirable turning speeds that correspond to the machined materials and the vibration frequency needs to be increased or decreased to obtain the desired surface roughness. Resonant vibration cutting tools do not allow the vibration frequency to be set arbitrarily. The small cutting force of these tools also causes some difficulties in diamond turning of metals other than soft metals such as aluminum and copper. In this research, we developed a vibration cutting tool that can generate two-dimensional vibration shapes without distortion by placing two piezoelectric actuators at right angles. This tool can machine the many kinds of mirror plates with different materials to the desired surface roughness. It is also possible to machine hard materials such as mold steels because the amplitudes of the vibrations do not decrease, even when large cutting forces are required. In this paper, we describe the design of the nonresonant vibration cutting tool, and show by the diamond turning of oxygen-free copper and aluminum that our nonresonant vibration cutting tool can machine desired shapes. Finally, it is verified by actual machining experiments that the proposed vibration cutting tool can machine hardened mold steels such as STAVAX with HRC 54.     

AN EXPERIMENTAL STUDY OF ORTHOGONAL MACHINING OF GLASS

An experimental study of machining glass with a geometrically defined cutting tool is presented. Orthogonal cutting conditions are employed to permit a focus on the fundamental modes of chip and surface formation. Analysis of the machined surfaces under an optical microscope identifies four regimes that are distinctly different with respect to either chip formation or surface formation. For a very small target uncut chip thickness, one on the order of the cutting edge radius, pure rubbing of the edge with no chip formation is observed. Edge rubbing imparts light scuffmarks on the machined surface giving it a frosted appearance. At a larger uncut chip thickness, ductile-mode chip formation occurs ahead of the cutting edge and a scuffed surface remains after the subsequent rubbing of the edge across the freshly machined surface. A further increase in uncut chip thickness maintains a ductile-mode of chip formation, but surface damage initiates in the form of surface cracks that grow down into the machined surface and ahead of the tool. The transition to this machining mode is highly dependent on rake angle. Increasing the uncut chip thickness further causes brittle spalling of chips leaving half-clamshell shaped divots on the surface. This experimental identification of the machining modes and their dependence on uncut chip thickness and rake angle supports the use of geometrically defined cutting tools to machine glass in a rough-semi-finish-finish machining strategy as is traditionally employed for machining metals.     

AN EXPERIMENTAL STUDY OF ORTHOGONAL MACHINING OF GLASS

Abstract

An experimental study of machining glass with a geometrically defined cutting tool is presented. Orthogonal cutting conditions are employed to permit a focus on the fundamental modes of chip and surface formation. Analysis of the machined surfaces under an optical microscope identifies four regimes that are distinctly different with respect to either chip formation or surface formation. For a very small target uncut chip thickness, one on the order of the cutting edge radius, pure rubbing of the edge with no chip formation is observed. Edge rubbing imparts light scuffmarks on the machined surface giving it a frosted appearance. At a larger uncut chip thickness, ductile-mode chip formation occurs ahead of the cutting edge and a scuffed surface remains after the subsequent rubbing of the edge across the freshly machined surface. A further increase in uncut chip thickness maintains a ductile-mode of chip formation, but surface damage initiates in the form of surface cracks that grow down into the machined surface and ahead of the tool. The transition to this machining mode is highly dependent on rake angle. Increasing the uncut chip thickness further causes brittle spalling of chips leaving half-clamshell shaped divots on the surface. This experimental identification of the machining modes and their dependence on uncut chip thickness and rake angle supports the use of geometrically defined cutting tools to machine glass in a rough-semi-finish-finish machining strategy as is traditionally employed for machining metals.     

Analysis of cutting force coefficients in high-speed ball end milling at varying rotational speeds

In high-speed ball end milling, cutting forces influence machinability, dimensional accuracy, tool failure, tool deflection, machine tool chatter, vibration, etc. Thus, an accurate prediction of cutting forces before actual machining is essential for a good insight into the process to produce good quality machined parts. In this article, an attempt has been made to determine specific cutting force coefficients in ball end milling based on a linear mechanistic model at a higher range of rotational speeds. The force coefficients have been determined based on average cutting force. Cutting force in one revolution of the cutter was recorded to avoid the cutter run-out condition (radial). Milling experiments have been conducted on aluminum alloy of grade Al2014-T6 at different spindle speeds and feeds. Thus, the dependence of specific cutting force coefficients on cutting speeds has been studied and analyzed. It is found that specific cutting force coefficients change with change in rotational speed while keeping other cutting parameters unchanged. Hence, simulated cutting forces at higher range of rotational speed might have considerable errors if specific cutting force coefficients evaluated at lower rotational speed are used. The specific cutting force coefficients obtained analytically have been validated through experiments.     

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.     

高体积分数SiCp/Al复合材料旋转超声铣磨加工的试验研究

由于大量高硬度增强相SiC颗粒的存在,高体积分数铝基碳化硅（SiCp/Al）复合材料的机械加工十分困难。旋转超声加工被认为是加工这种材料的有效方法。通过超声辅助划痕试验,分析高体积分数SiCp/Al复合材料旋转超声铣磨加工的材料去除机理。在超声振动的作用下,材料中铝基体发生塑性变形,其表面得到夯实;SiC增强相被锤击成细小的颗粒而发生脱落,形成较大的空洞。由于材料加工的缺陷大多产生于SiC颗粒的去除过程中,SiC颗粒的去除方式对加工表面的质量起着决定性的作用,选择合适的工艺参数可以有效提高加工表面质量。旋转超声加工工艺特征试验表明,超声振动可有效降低切削力;主轴转速对轴向切削力的影响最大,其次是进给速度,切削深度对轴向切削力的影响较小;另外主轴转速对表面质量的影响效果也最大,并随主轴转速的增大表面粗糙度增大。因此在加工过程中,可以适当加大切削深度,在保证加工质量的基础上,选择较大的进给速度,在保证刀具寿命的前提下,选择合适的主轴转速,以获得较优的加工表面质量和加工效率。     

Characteristics of chip generation by ultrasonic vibration cutting with extremely low cutting velocity

Recently, mirror-surface machining of brittle materials such as ferrite, glass, and optical plastics has become more important, as these materials are used in optical communications and precision devices. Non-ferrous metals such as aluminium and copper were readily turned with diamond tools, but as the need for both infra-red and reflective optics escalated, the need to machine brittle materials arose. In this paper, ultrasonic vibration cutting at 20 kHz at extremely low cutting velocity for the precision machining of brittle plastics used for optical lenses is suggested and tested. The mechanism of chip generation, and characteristics of surfaces in the ductile mode, machined by ultrasonic vibration cutting are investigated. As a result, when micro cutting by ultrasonic vibration, it was confirmed that the chips generated by ductile mode cutting are obtained at 1/40 of the critical cutting velocity of the ultrasonic vibration cutting system, which is an extremely low cutting velocity.     

高硅铝合金切削性能的试验研究

由于Al-Si合金比重小、强度高、导热性好和便于回收利用等优点,广泛应用于飞机、汽车和电子科技等行业产品中。因其切削加工过程中易发生刀屑粘结、刀具磨损严重和划伤已加工表面等问题,难以获得良好的表面质量和较高的加工精度。为研究高硅铝合金的切削性能,本文以切削力和切削温度为研究对象,采用正交试验法试验,分析切削速度、进给量、铣削宽度和背吃刀量对铣削力和温度的影响。通过本研究,期望改善生产中刀具磨损严重和加工表面质量差的状况。     

Problems and solutions in machining of titanium alloys

Titanium alloys are known as difficult-to-machine materials. The problems of machining titanium are many folds which depend on types of titanium alloys. This paper investigates the underlying mechanisms of basic challenges, such as variation of chip thickness, high heat stress, high pressure loads, springback, and residual stress based on the available literature. These are responsible for higher tool wear and worse machined surface integrity. In addition, many cutting tool materials are inapt for machining titanium alloys as those materials are chemically reactive to titanium alloys under machining conditions. To address these problems, latest techniques such as application of high pressure coolant, cryogenic cooling, tap testing, thermally enhanced machining, hybrid machining, and use of high conductive cutting tool and tool holder have also been discussed and correlated. It seems that all the solutions are not yet well accepted in the industrial domain; further advancement in those fields are required to reduce the machining cost of titanium alloys.     

Specific cutting force, tool wear and chip morphology characteristics during dry drilling of austempered ductile iron (ADI)

Dry machining is being recognized as ecological machining due to its less environmental impact and manufacturing cost. However, the choice of dry machining is mainly influenced by the workpiece material properties, machining operation and cutting conditions. The recent emergence of austempered ductile iron (ADI) can be considered a significant economic advantage to the increasing industrial demand for cost- and weight-efficient materials. However, due to its microstructure-induced inherent properties, ADI is considered hard-to-machine material. Thus, the dry drilling of ADI is investigated in this paper. The ADI material used in the present study is produced using an innovative process route for near net shape casting production. Drilling experiments are conducted on a DMU80P Deckel Maho five-axis machining centre using PVD-coated carbide tools under dry cutting environment. The dry drilling of ADI under different cutting conditions is evaluated in terms of specific cutting force and tool wear analysis. The influence of cutting conditions on chip morphology and surface roughness is also investigated. The experimental results revealed that the combination of the low feed rate and higher cutting speed leads to the higher mechanical and thermal loads on the tool's cutting edge, resulting in higher specific cutting force values. This behaviour is further supported by the chip morphology analysis, which revealed the formation of segmented chips at higher cutting speed with segment spacing increase with an increase in feed rate. Depending upon the cutting parameters, different modes of tool failures including crater wear, flank wear, chipping, breakage and built-up edge were observed. Surface roughness analysis revealed the influence of tool wear and chip morphology on the machined surface finish.     

Selection of the Machining Inclination Angle in High-Speed Ball End Milling

High-speed machining is a key issue for current die and mould manufacturing. Though this technology has great potential for high productivity, tool wear accelerated by high cutting speeds and hard materials is a barrier. In this work, we attempted to reduce tool wear by controlling the machining inclination angle between the tool and the workpiece. To do this, the range of cutting angles engaged in the cutting using a ball end cutter is obtained from the boundary lines describing machined sculptured surfaces represented by the cutting envelop condition and the geometric relationships of successive tool paths. Then, the chip cross-sectional area and chip length can be obtained from the calculated cutting edge and the rotational engagement angle. The simulation results showed that a machining inclination angle of 15° was good enough from the point view of machineability, and this value was verified by a cutting experiment using high-speed ball end milling.     

Statistical model to determine surface roughness when milling hastelloy C-22HS

The aim of this study is to develop the surface roughness prediction models, with the aid of statistical methods, for hastelloy C-22HS when machined by PVD and CVD coated carbide cutting tools under various cutting conditions. These prediction models were then compared with the results obtained experimentally. By using response surface method (RSM), first order models were developed with 95 % confidence level. The surface roughness models were developed in terms of cutting speed, feed rate and axial depth using RSM as a tool of design of experiment. In general, the results obtained from the mathematical models were in good agreement with those obtained from the machining experiments. It was found that the feed rate, cutting speed and axial depth played a major role in determining the surface roughness. On the other hand, the surface roughness increases with a reduction in cutting speed. PVD coated cutting tool performs better than CVD when machining hastelloy C-22HS. It was observed that most of the chips from the PVD cutting tool were in the form of discontinuous chip while CVD cutting tool produced continuous chips.     

Some Observations of the Chip Formation Process and the White Layer Formation in High Speed Milling of Hardened Steel

Abstract

With the advent of recent advances in machine tools design (main spindle, feed drives, etc.), high-speed milling has become a cost-effective manufacturing process to produce products with high surface quality, low variations in the machined surface characteristics, and excellent dimensional accuracy. In taking into account the most obvious advantages of high-speed machining over conventional machining, a key issue is to identify the effective range of cutting speed that corresponds to high-speed machining producing improved machining performance. The simple reason for this is the fact that machining performance improves when entering the high-speed region but, large increase in cutting speed is not cost-effective due to rapidly increasing tool-wear rates and high power consumption. In order to address this issue requiring a trade-off, an attempt has been made in this paper by formulating an approximate procedure which is based on the analysis of chip-formation mechanisms and a chip-shape analysis, together with the use of metallographic methods. This procedure includes fundamental understanding of the well-known phenomena of white layer formation during the high-speed machining of hardened steels. Essentially, the white layer generated on a machined surface represents a surface defect. Therefore, it is necessary to determine the factors influencing its generation and its prevalent characteristics. There is lack of knowledge in this area, which tends to present the influence of the white layer on the surface integrity and performance of the machined part as a function of machining conditions. This article provides a basis for the determination of the optimal range of cutting speeds and feed rates in high-speed milling of hardened steels ensuring minimized influence of the white layer on the workpiece quality and machined surface integrity.     

高速切削淬硬钢的研究进展

回顾了近10年来高速切削淬硬钢中切屑变形机理、刀具及磨损、切削力、切削温度以及表面质量的研究进展。指出了高速切削淬硬钢技术研究的不足,并讨论了其将来的研究趋势。     

Numerical analysis of chip formation and shear localisation processes in machining the Ti-6Al-4V titanium alloy

A finite element modelling was carried out to analyse the chip morphology and adiabatic shear banding localisation processes when high-speed machining refractory titanium alloys. A thermo-visco-plastic model for the machined material and a rigid with thermal behaviour for the cutting tool were assumed. The study tries to understand the effect of the material behaviour on the produced chip morphology. One of the main characteristics of titanium chips is a segmented shape for a wide range of cutting conditions. This kind of morphology was found only dependent on adiabatic shear banding without material damage effect in the shear zones (primary and secondary shear zones). The influence of the material characteristics (strain softening, thermal softening, etc.) and machining parameters on the cutting forces and chip morphology were analysed. Three flow-stress laws and different friction coefficients (low and high friction) at the tool-chip interface was particularly analysed to explain the different morphologies obtained for refractory titanium chips.     

Surface topography and roughness of high-speed milled AlMn1Cu

The aluminum alloy AlMn1Cu has been broadly applied for functional parts production because of its good properties. But few researches about the machining mechanism and the surface roughness were reported. The high-speed milling experiments are carried out in order to improve the machining quality and reveal the machining mechanism. The typical topography features of machined surface are observed by scan electron microscope(SEM). The results show that the milled surface topography is mainly characterized by the plastic shearing deformation surface and material piling zone. The material flows plastically along the end cutting edge of the flat-end milling tool and meanwhile is extruded by the end cutting edge, resulting in that materials partly adhere to the machined surface and form the material piling zone. As the depth of cut and the feed per tooth increase, the plastic flow of materials is strengthened and the machined surface becomes rougher. However, as the cutting speed increases, the plastic flow of materials is weakened and the milled surface becomes smoother. The cutting parameters (e.g. cutting speed, feed per tooth and depth of cut) influencing the surface roughness are analyzed. It can be concluded that the roughness of the machined surface formed by the end cutting edge is less than that by the cylindrical cutting edge when a cylindrical flat-end mill tool is used for milling. The proposed research provides the typical topography features of machined surface of the anti-rust aluminum alloy AlMn1Cu in high speed milling.     

Some Observations of the Chip Formation Process and the White Layer Formation in High Speed Milling of Hardened Steel

With the advent of recent advances in machine tools design (main spindle, feed drives, etc.), high-speed milling has become a cost-effective manufacturing process to produce products with high surface quality, low variations in the machined surface characteristics, and excellent dimensional accuracy. In taking into account the most obvious advantages of high-speed machining over conventional machining, a key issue is to identify the effective range of cutting speed that corresponds to high-speed machining producing improved machining performance. The simple reason for this is the fact that machining performance improves when entering the high-speed region but, large increase in cutting speed is not cost-effective due to rapidly increasing tool-wear rates and high power consumption. In order to address this issue requiring a trade-off, an attempt has been made in this paper by formulating an approximate procedure which is based on the analysis of chip-formation mechanisms and a chip-shape analysis, together with the use of metallographic methods. This procedure includes fundamental understanding of the well-known phenomena of white layer formation during the high-speed machining of hardened steels. Essentially, the white layer generated on a machined surface represents a surface defect. Therefore, it is necessary to determine the factors influencing its generation and its prevalent characteristics. There is lack of knowledge in this area, which tends to present the influence of the white layer on the surface integrity and performance of the machined part as a function of machining conditions. This article provides a basis for the determination of the optimal range of cutting speeds and feed rates in high-speed milling of hardened steels ensuring minimized influence of the white layer on the workpiece quality and machined surface integrity.     

CHIP MORPHOLOGY CHARACTERIZATION AND MODELING IN MACHINING HARDENED 52100 STEELS

Hard machining is attracting more and more attention as an alternative to grinding in finish machining some hardened steels. The saw-toothed chips formed in hard machining have their own unique characteristics. The saw-toothed chip morphology is of great interest since the understanding of the saw-toothed chip morphology and its evolution in machining helps unveil hard machining chip formation mechanisms as well as facilitate hard machining implementation into industry. In this study, the effect of tool wear and cutting conditions on the saw-toothed chip morphology was examined in machining 52100 hardened 52100 bearing steel. It was found that the chip dimensional values and segmentation frequency were affected by tool wear and cutting conditions while the chip segmentation angles were approximately constant under different tool wear and cutting conditions. The shear band spacing has also been predicted at the same order of magnitude as the measurement, and improved spacing modeling accuracy is expected if the cutting process information can be better predicted first.