Achieving Small Structures in Thin NiTi Sheets for Medical Applications with Water Jet and Micro Machining: A Comparison

NiTi shape memory alloys (SMA) are used for a variety of applications including medical implants and tools as well as actuators, making use of their unique properties. However, due to the hardness and strength, in combination with the high elasticity of the material, the machining of components can be challenging. The most common machining techniques used today are laser cutting and electrical discharge machining (EDM). In this study, we report on the machining of small structures into binary NiTi sheets, applying alternative processing methods being well-established for other metallic materials. Our results indicate that water jet machining and micro milling can be used to machine delicate structures, even in very thin NiTi sheets. Further work is required to optimize the cut quality and the machining speed in order to increase the cost-effectiveness and to make both methods more competitive.     

Chatter avoidance in the milling of thin floors with bull-nose end mills: Model and stability diagrams

The milling of thin parts is a high added value operation where the machinist has to face the chatter problem. The study of the stability of these operations is a complex task due to the changing modal parameters as the part loses mass during the machining and the complex shape of the tools that are used. The present work proposes a methodology for chatter avoidance in the milling of flexible thin floors with a bull-nose end mill. First, a stability model for the milling of compliant systems in the tool axis direction with bull-nose end mills is presented. The contribution is the averaging method used to be able to use a linear model to predict the stability of the operation. Then, the procedure for the calculation of stability diagrams for the milling of thin floors is presented. The method is based on the estimation of the modal parameters of the part and the corresponding stability lobes during the machining. As in thin floor milling the depth of cut is already defined by the floor thickness previous to milling, the use of stability diagrams that relate the tool position along the tool-path with the spindle speed is proposed. Hence, the sequence of spindle speeds that the tool must have during the milling can be selected. Finally, this methodology has been validated by means of experimental tests.     

Challenges in using waterjet machining of NiTi shape memory alloys: An analysis of controlled-depth milling

The nickel-titanium shape memory alloys (NiTi SMAs) have a very high potential for a wide variety of applications thanks to their unique mechanical properties: shape memory effect and pseudoelasticity. However, they have been proved to be more challenging to cut than other advanced engineering materials because of their high ductility, crystal-oriented and stress-oriented mechanical properties. In stark contrast to the extensive work on the metallurgical/microstructural properties of the SMA, there is limited research regarding non-conventional machining of this group of special alloys.Waterjet technology is well-known for cutting advanced difficult-to-cut materials owing to its benefits of reduced mechanical and thermal damages to workpiece surfaces. This paper reports for the first time the use of waterjet technology to mill the functional shape memory alloys and thus to open new avenues for the utilisation of these alloys for advanced engineering applications (e.g. aerospace, medical fields). However, when it comes to NiTi SMAs (characterised by low temperature phase martensite and parent phase austenite), the insignificant waterjet temperatures become critical to the material behaviour as their crystal structures are sensitive to the variations in both temperature and mechanical compression. This makes the processing (particularly waterjet controlled-depth milling) a real challenging task.By taking into consideration both of the waterjet temperatures at different material removal conditions (i.e. with and without abrasives in the focussing tube) and the transformation temperatures of NiTi, three different working zones (100% martensite; mix of austenite and martensite; 100% austenite) under waterjet process have been proposed. In addition, a combined phase and stress-strain diagram for shape memory effect in martensitic phase and pseudoelasticity in austenite phase of NiTi has been suggested. In this paper, Ni_49.8Ti_50.2 shape memory alloy was considered in which its transition temperature range is overlapped with the waterjet operating temperature; two approaches of waterjet processes (plain and abrasive waterjet milling) were proposed so as to investigate the mechanical and metallurgical effect provoked by the relationship between operating temperatures and transformation temperatures. It was found that abrasive waterjetting is more viable than plain waterjetting for controlled-depth milling of NiTi shape memory alloys.     

Effect of machining fluid on the process performance of electric discharge milling of insulating Al2O3 ceramic

Wire electric discharge machining (WEDM) and electrical discharge machining (EDM) promise to be effective and economical techniques for the production of tools and parts from conducting ceramic blanks. However, the manufacturing of insulating ceramic blanks with these processes is a long and costly process. This paper presents a new process of machining insulating ceramics using electrical discharge (ED) milling. ED milling uses a thin copper sheet fed to the tool electrode along the surface of the workpiece as the assisting electrode and uses a water-based emulsion as the machining fluid. This process is able to effectively machine a large surface area on insulating ceramics. Machining fluid is a primary factor that affects the material removal rate and surface quality of the ED milling. The effects of emulsion concentration, NaNO₃ concentration, polyvinyl alcohol concentration and flow velocity of the machining fluid on the process performance have been investigated.     

Tribological behavior of micro structured surfaces for micro forming tools

Mechanical micro machining processes, like milling and grinding are appropriate technologies for the flexible production of precise molds with complex shapes for metal forming processes. In most cases machining strategies are orientated towards form accuracy of the desired forming tool only. Thus, the generation of tribologically advantageous surfaces is often carried out in subsequent machining steps like honing. In micro scale the subsequent treatment of complex surfaces is very difficult. For that reason it is desirable to create the shape and a suitable surface texture with one tool in one step.This paper is focusing on the comparison of the tribological behavior of polished surfaces with structured surfaces machined by micro milling and micro grinding processes. Micro milling tools and grinding pins with ballend shape are used to create micro structured surfaces. The machining strategy (tool path and line pitch) was varied for both tool types in the same manner. The experiments were carried out on hardened cold working steel using tungsten carbide micro cutters with TiAlN coating and micro grinding pins with an abrasive diamond layer. White light interferometry was used to characterize the machined surfaces and determine the surface parameters. Moreover, a strip drawing test was set up to investigate the tribological behavior of the system consisting of the machined surfaces and thin sheet metals. The results of the strip drawing test suggest a relationship between micro structure and tribological behavior. Finally, the dependencies between machining technology, surface parameters and tribological behavior will be discussed.     

Microfabrication and Performance of Annealed NiTi Shape Memory Thin Films by Sputtering for Microdevice Applications

SINCE NiTi alloy is one of the promising candidatesfor sensors and actuators due to its excellentmechanical properties[1-9].Nickel-titanium shapememory thin film is expected to become one of thepromising materials for microdevice applications[1-14],which demonstrates advantages of low cost,fastresponse,and large output energy density.In thisresearch,we will focus on microfabrication andperformance of annealed nickel-titanium shapememory thin film with preferred structures for micro-device ap…     

Development and evaluation of an on-machine optical measurement device

Demand for fabricating micro-features such as fine holes, micro-cavity for injection moulding, and micro-pin using both conventional (turning, milling, etc.) and non-conventional edge detection method (EDM), wire cut EDM, etc.) processes is increasing significantly. To successfully achieve micro-machining, development of a miniature machine tool, process technology, and on-machine measurement is essential. However, in such tool-based micro-machining processes, proper tool shape monitoring, precision processing, and dimensional control require significant attention. Since these are tool-based machining processes, tool shape monitoring and control are also important technologies to be established.In this study, an on-machine measuring device was developed based on non-contact optical method to inspect dimensions of the fabricated tools (e.g. electrodes for EDM) as well as the wear of tools used for the respective processes. The developed inspection system uses a laser light source and a photo-electronic device. To minimize errors due to the change of tool measurement position and the Fresnel diffraction of laser light, an edge detection algorithm using a linear discrimination function is proposed in this study. Furthermore, an intensity measuring method was added for specimen with a smaller diameter. The experimental results show that the developed on-machine optical inspection system has the accuracy and stability to effectively monitor the fine dimensions of tools and their wear.     

整体式硬质合金微铣刀的几何结构优化与在位放电制备

基于ANSYS有限元分析软件,对微铣刀进行模态分析和应力变形分析,讨论微铣刀的刀头形状、悬伸量、刀杆直径、刀颈半锥角、刀头长径比等几何结构参数对其动力学性能的影响规律,对比D形、三角形、"一"字形等简单刀头截面形状结构及传统螺旋结构微铣刀的强度和刚度,进而获得微铣刀的几何结构优化参数。采用线电极电火花磨削的方式,在位放电制备出刀头直径约100μm、刃口锋利的D形微铣刀。     

Influences on specific cutting forces and their impact on the stability behaviour of milling processes

To withstand global competition, nowadays it is essential for companies to assure high productivity and high quality. To reach this aim permanent technical innovation and further developments are necessary. In machining industry and especially in the field of milling the development of possibilities to increase chip removal is the major goal. The optimisation of the cutting process is one way to achieve this aim. Here, the use of stability prediction models is essential to reduce the effort in time and costs. To implement a stability prediction tool with a high accuracy in representing reality, all relevant influencing parameters and their interactions within the cutting process have to be analysed. This article describes one possibility for the experimental identification of instable milling processes. Furthermore, the influences of spindle speed and temperature on specific cutting forces and the temperature influence on the stability behaviour in milling processes are shown.     

Effect of laser microcutting on thermo-mechanical properties of NiTiCu shape memory alloy

The machining of shape memory alloys (SMAs), such as NiTi based alloys, is a very interesting and relevant topic for several industrial applications in the biomedical, sensor and actuator fields. Laser technology is one of the most suitable methods for the manufacturing of products in the aforementioned fields, mainly when small and precise features have to be included. Due to the thermal nature of this process, study of its effect on the functional properties of these materials is needed. Except for binary NiTi, few results on the laser machining of NiTi based alloys are available in the literature. In this work, thin sheets of Ni40Ti50Cu10 (at.%) were processed by a fibre laser and the effect of process speed on the material properties was analysed. Scanning electronic microscopy was adopted for observation of the laser cut edges’ morphology. Chemical composition of the processed material was evaluated by energy dispersion spectroscopy and nanohardness measurements were used to estimate the heat affected zone. SMA functional properties were studied on both base and laser machined material. These characteristics are affected by laser machining for the presence of melted material; this effect can be minimised by increasing the laser process speed.     

CVD金刚石薄膜涂层整体式刀具的制备与应用

化学气相沉积(chemical vapor deposition,CVD)金刚石薄膜具有硬度高、摩擦系数低、耐磨性强以及表面化学性能稳定等优异的机械及摩擦学性能,这使其在硬质合金工模具领域具有广阔的应用前景.本文采用热丝化学气相沉积法(hot filament chemical vapor deposition,HFC...     

Analysis of the manufacturing chain of CVD diamond coated shaft type cutting tools

Hypereutectic aluminium silicon alloys, e.g. casted AlSi17Cu4Mg, are commonly used in the automotive and aeronautical industries. These alloys consist of hard, abrasive silicon particles in a soft aluminium matrix and thus place high mechanical loads on the tool during machining processes. Polycrystalline Diamond or CVD (chemical vapour deposition) diamond based cutting tools can be used for the high speed machining of these alloys due to their high hardness and wear resistance. Diamond thin film coatings of different film morphologies are commonly applied on cemented carbide tools using Hot Filament CVD. The distinguishing characteristic to other coatings is utmost hardness resulting in high resistance to abrasion, low tendency to adhesion and low friction coefficient. The manufacturing of CVD diamond coated shaft type cutting tools is challenging due to the complex design of the cutting edges and the demanding stress behaviour during tool application. The influencing parameters of substrate type, chemical and mechanical substrate pre-treatment as well as diamond film modification on the tool cutting performance are discussed. The manufacturing route of CVD diamond coated thread milling drills is analysed with the use of material and tribological tests. The complex thread manufacturing tools are then applied in the machining of AlSi17Cu4Mg, whereby the tool performance is characterised with respect to their wear behaviour, the process forces and temperatures as well as the workpiece quality.     

Thermal modelling of end milling

Determination of the temperatures during machining is one of the most important challenges for accurate milling simulations. Coupled with excessive shearing, plastic deformation and friction in a small region of cutting, the temperatures in milling may have very significant impact on parts and tools such as dimensional errors, residual stresses and tool wear. Temperature exhibits a non-linear complex-modelling problem in milling process. In this article, for the first time, a novel thermal modelling is introduced for fast and accurate prediction of temperatures in end milling processes. A theoretical modelling approach and experimental validations are presented for various cutting conditions.     

Cylindrical milling tools: Comparative real case study for process stability

Critical comparison is presented related to the stability behaviour of milling processes performed by conventional, variable helix and serrated milling tools. The paper presents a general milling model linked to any non-proportionally damped dynamic system. Extended multi frequency solution and semi-discretization are implemented and used to calculate the stability of stationary milling. Measurements performed in industrial environment validate the general numerical algorithm that is able to predict the stability conditions of milling processes carried out by cylindrical cutters of optional geometry. Both the calculations and the measurements confirm that, for roughing operations, the highest stability gain can be achieved by serrated cutters. It is also demonstrated that variable helix milling tools can achieve better stability behaviour only if their geometry is optimized for the given cutting operation.     

A geometric approach to calculating tool wear in screw rotor machining

Screw compressor rotors are machined today by grinding or milling, usually in two stages; roughing, when the work piece is machined to its approximate size, and finishing, when the rotors are machined to their final dimensions. The material, or cutting stock, which is removed in the finishing operation, is determined by the design of both the cutting tools. Since screw compressor rotors are of a helical shape, each point of the tool traverses a different contact length during the cutting process. Thus the rate of wear of the finishing tool, along its profile is not uniform. The envelope theory of gearing is used here to calculate the relative motion between each point of the tool and the rotor during the cutting process. The rate of tool wear at each point is then calculated on the assumption that it is proportional to this relative motion. The calculation results are compared with experimental values of rates of tool wear and good agreement is noticed. On this basis, it is shown how to produce a roughing tool that results in semi-finished stock of variable thickness that enables the finishing tool to wear at a uniform rate. A similar technique is applicable to many machining processes in which form tools are used.     

Modeling and simulation of milling processes including process damping effects

Especially in high speed milling of aluminum alloys in the aviation industry, chamfered milling tools have proven themselves. Due to the chamfer, an extended contact between the tool and the workpiece at the flank face is evoked, which leads to additional process damping forces opposed to tool vibrations. Hence, the cutting process shows improved stability characteristics. This article presents an approach for the identification and modeling of these process damping effects in transient milling simulations. For this purpose, a simulation- and experiment-based procedure for the identification of required simulation parameters depending on the tool chamfer geometry is introduced and evaluated. Finally, the identified parameters are used for transient simulations of milling processes with extended stability due to the tool chamfer. The suitability of the proposed identification method and simulation model for milling with process damping is finally proved by a comparison between simulations and experiments.     

A new perspective on the stability study of centerless grinding process

Regenerative chatter is one of the most complex dynamic processes in machine tools. It is characterized by the presence of self-excited vibrations during machining, limiting the achievable tolerances in the workpieces. In order to predict the set-up conditions that produce these vibrations, it is necessary to model the regenerative mechanism responsible of their appearance accurately, so that the system stability can be studied solving the characteristic equation of the chatter loop. Although the dynamic behavior of machining processes like milling, turning or drilling is governed by a time delayed differential equation with one time delay term, a very particular problem is presented in centerless grinding. In this process, in addition to the dynamic instabilities, geometric instabilities must be analyzed, which are another important factors limiting the workpiece tolerances and lead to three time delay terms in the modeling procedure. This fact complicates its study remarkably, and the resolution of the characteristic roots of the dynamic process of these kinds of machines has not been tackled in the specialized literature as extensively as in other machining processes, being this field a challenging research line. According to this, in this paper an original and efficient method is presented to solve the roots of the characteristic equation of the centerless grinding process, based on the application of the root locus method. The main features of the proposed procedure are its ability to obtain the solutions accurately and that it is capable of determining the origin of the instabilities, so it constitutes a powerful tool to predict machine response for different set-up conditions. These interesting properties are demonstrated through the simulation results presented in this paper.     

An investigation of factors controlling part growth and surface finish of form tools in the automatic screw machine test

In order to investigate factors controlling part growth and surface finish in the automatic screw machine test, a set of eight separate experiments was carried out. It was observed that flank wear is the major factor in controlling rough-form part growth, but does not account for all of it. Other possible factors were reviewed. The elastic deflection of the workpiece estimated from the increase of the taper of rough-formed part with machine time appears to account for most of the difference between the observed part growth and the estimated values from flank wear. BUE overhang and thermal expansion of the workpiece could also contribute to rough-form part growth. The contribution of BUE overhang could be either positive or negative depending on how overhang size changes with machine time. Thermal expansion of the workpiece would give rise to negative part growth. Interestingly, during the initial 30 min of machining, negative part growth for rough-form tool and rapid positive part growth for finish-form tool were observed. Thermal expansion for the rough-form tool and the change in the size of BUE overhang for both tools are believed to be at least partially responsible for these effects. A procedure to start measuring part growth after approximately 30 minutes of machining is thus recommended. For finish-formed surface finish in this investigation, uneven BUE is believed to be the dominant factor in determining surface roughness.     

基于有限元法的微径铣刀变形分析

文章基于有限元法建立了微径铣刀的欧拉悬臂梁结构动力学模型,求解了不同刀头直径,不同刀头长度和不同刀柄组合的微径铣刀的固有频率,可知随着悬伸长度的减小,固有频率表现出明显的尺度效应。通过计算微径铣刀的振型及变形得知,当悬伸长度相同时,在静态作用力下,随着刀头直径的减小,刀具变形增大;在动态作用力下,刀具变形对微径铣刀的铣削频率和固有频率的平方较敏感,铣削频率离微径铣刀的固有频率越近,刀具变形越大。微径铣刀的动力学变形分析有助于研究微细铣削力、合理选择微细铣削参数、提高铣削过程稳定性,从而获得良好的加工质量。     

Lightweight semi-actively damped high performance milling tool

Long cantilevered high performance milling tools tend to vibrate during machining operation due to process excitation. This impairs the quality of the workpiece surface and limits the achievable material removal rate. An optimisation of the dynamic properties of these tools enables an increased machining performance. This paper introduces a lightweight design of a shell end milling tool with an integrated semi-active damping system based on magnetorheological fluids. The investigations show that this approach allows an adjustment of the dynamic behaviour of the tool. In machining experiments a significant increase of the material removal rates and improved surface quality are achieved.