The influence of abrasive waterjet cutting conditions on the surface quality of graphite/epoxy laminates

An experimental investigation was conducted to determine the influence of cutting parameters on the surface roughness and kerf taper of an abrasive waterjet machined graphite/epoxy laminate. Experimental design was used to systematically measure the influence of cutting parameters on the surface roughness and kerf taper of laminate specimens. Stylus prolifometry was used to measure the surface roughness and a visual inspection including scanning electron microscopy (SEM) was conducted. Profilometry measurements supplemented with microscopy analysis suggests that three regions of surface topography are evident on the machined surface of the laminate specimens. ANOVA techniques indicate that the influence of cutting parameters on the surface roughness changes as a function of cutting depth. Mathematical models were developed to predict the surface roughness and kerf taper in terms of the cutting parameters of a graphite/epoxy laminate to cutting depths of 16 mm.     

Material removal analysis in abrasive waterjet cutting of ceramic plates

Fine ceramics have been recognized increasingly in structural and functional applications on account of their merits of hardness, corrosion resistance, electromagnetic response and bio-compatibility. Due to the need for dimensional control or production optimization, post-sintering machining can be required. Cutting by conventional means is most often practiced, but the associated heavy tool wear is hard to overcome. A waterjet at transonic speed carrying abrasive particles provides an effective means for hard-material removal. Undesired material fracture at entrance and exit can be reduced significantly by sequential abrasive micro-machining.

The present paper discusses the kerf formation of a ceramic plate cut by an abrasive waterjet. The mechanism and the effectiveness of material removal are studied first. Different materials are found to possess different removal rates in machining and there also exists a critical combination of hydraulic pressure, abrasive flow rate and traverse speed, below which through-cut for a certain thickness cannot be achieved. The wall finish achieved is determined by the mesh size of the abrasives: sufficient hydraulic pressure with fine abrasives will produce a smooth surface comparable to that from grinding. The kerf is slightly tapered with wider entry due to decreased cutting energy with kerf depth. A high-power input per unit length produces a small taper but a wide slot.     

THE EROSIVE PROCESS IN ABRASIVE WATERJET CUTTING OF POLYMER MATRIX COMPOSITES

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Abrasive waterjet cutting of polycrystalline diamond: A preliminary investigation

Although abrasive waterjet (AWJ) machining has been employed in different setups (e.g. through cutting, milling, turning, cleaning) to generate surfaces in various workpiece materials (e.g. metallic alloys, ceramics, composites), up to now there is scarce information on the use of this technology in cutting super-hard materials such as diamond-based materials. The paper reports for the first time on a preliminary study of the capability of AWJ cutting of polycrystalline diamond (PCD) using abrasive media with different hardness, i.e. aluminium oxide (Al₂O₃), silicon carbide (SiC) and diamond. While keeping some operating parameters constant (pump pressure, stand-off distance and size of abrasives) the feed speed has been adjusted to enable full jet penetration for each type of abrasives. It was found that not only the material removal rates vary significantly with the employment of different types of abrasives but also the nozzle wear ratios, with further implications on the kerf quality (width, taper angle) of diamond cut surfaces. Furthermore in-depth studies of the cut surfaces helped to reveal the material removal mechanism when different types of abrasives are employed: Al₂O₃—low intensity erosion; SiC—medium erosion with undesired cracking; diamond—high intensity erosion. The experimental results showed that while Al₂O₃ and SiC abrasive media yield modest material removal rates (comparable with those obtained by electro discharge machining-EDM) the use of diamond abrasives can greatly increase (>200 times) the productivity of AWJ through cutting of PCD test pieces at acceptable roughness (Ra<1.6 μm) and integrity (i.e. crack-free) of the cut surfaces. Despite some limitations (e.g. cost of diamond abrasives, extensive nozzle wear rates) that can be overcome through further developments, it is believed that this preliminary research gives an indication of the capability of the AWJ to profile diamond-based structures for high-value engineering applications where conventional methods (e.g. EDM, laser) cannot be applied or are not productive enough.     

The effect of cutting head vibrations on the surfaces generated by waterjet cutting

After a first period in which the research has been focused on the optimisation of the process parameters, the attention is now focused on aspects that were usually neglected. However, they are very important in order to understand the physics of the waterjet/abrasive waterjet cutting process and to improve the cutting quality.Particularly, it has been demonstrated that, in the pure waterjet cutting (and in the abrasive waterjet cutting too), there are irregularities, called striations, along the generated surface. The striation formation depends mainly on the jet instability caused by vibrations during the cutting process. Vibration signals have been measured whilst varying the cutting conditions. A model has been studied which estimates the mean spacing and the frequency of the striations, as a function of the period and the amplitude of the jet vibration. This model has been completely validated through measurements of plasticine surfaces generated by waterjet cutting.     

金刚石线锯切割绝缘陶瓷的切缝精度研究

为了探寻一种切缝窄、切缝轮廓平直的金刚石线锯切割工艺,将电火花线切割机床改装为金刚石线锯切割机床,并进行了金刚石线锯恒力进给切割陶瓷的实验,探究了丝速和配重对切缝宽度和切缝轮廓平直度的影响;用光学显微镜测量切缝宽度,并观察切缝轮廓的平直度。实验结果表明:随着丝速的增大,切缝宽度变化不明显,随着配重的增大,切缝宽度明显减小;随着丝速的增大,切缝轮廓逐渐出现锯齿状,随着配重的增大,切缝轮廓发生弯曲甚至崩碎。     

超声辅助磨料水射流加工机制及去除模型研究

为了增强磨料水射流的加工效果,设计了超声辅助微细磨料水射流加工系统。通过喷嘴内变幅杆的超声振动,将声能与射流压力能叠加,增强磨料水射流的脉动行为,形成脉冲射流。通过数值计算的方法研究了流场轴线上的声压分布及磨粒在流场中的运动特性,探究了脉冲射流的加工机制及硬脆材料去除机制。通过切槽实验分析了系统压力、振幅及靶距对冲蚀深度的影响规律,实验结果证明,施加超声振动可有效降低系统压力,最佳靶距为8～10 mm。基于实验结果,利用MATLAB建立了硬脆材料的去除深度预测模型。     

Influence of kinematic operating parameters on kerf geometry in abrasive waterjet machining of silicon carbide ceramics

Silicon carbide (SiC) is extensively used for manufacturing of highly engineered parts due to its high hardness, low coefficient of friction, wear resistance and high decomposition temperature. However, generating 3D surfaces (e.g. pockets) in such structural ceramics by conventional machining is a difficult task. In this context, abrasive waterjet (AWJ) machining, with its capability to cut any material with low specific cutting forces, seems to be the “ideal” processing technique for such materials; nevertheless machining 3D shapes by AWJ milling is still in its infancy. 3D shapes can be generated by “enveloping” them with successions of jet footprints (kerf geometries) generated by varying the process operating parameters. To enable this, the present work investigates the influence of key kinematic operating parameters (i.e. α-jet impingement angle and v-jet feed rate) on the kerf geometry and its dimensional characteristics. Furthermore, the kerf generation mechanism under multi-pass jet erosion was analysed to get control over erosion depth in multi-pass machining. It was found that by varying α (90°–40°), the symmetric/asymmetric kerf geometry is intimately dependent on the variation of standoff distance (SOD), abrasive particle velocity distributions and their local impact angles accounted across the jet footprint. Variation in v influences the exposure time of material to jet and enhances the erosion capability of abrasives impacting at shallow angle that results in different erosion rates along the kerf profile; this combined effect leads to departure of kerf geometry from simple cosine profile approximation to more elliptical type with the decrease of feed speed. Further, at lower jet feed rates, the depth of erosion increased and the low energy abrasive particles along trailing edge of jet plume get enough time to erode the material that results in variation of slope of kerf walls and hence, overall geometry. Based on these observations, the multi-pass trials showed that the successive passes have to account for both the local impact angles of abrasive particles as well as the actual SOD (SOD+initial kerf depth). In this way, by understanding the influence of key kinematic operating parameters (α and v) on the kerf geometry and its dimensional characteristics, the paper establishes a good basis for developing strategies for controlled 3D AWJ machining of complex shapes.     

Modeling of chip–tool interface friction to predict cutting forces in machining of Al/SiCp composites

In Al/SiCp metal matrix composites, in addition to machine, tool and process-related parameters, a change in composition (size and volume fraction of reinforcement) has a influence on machining force components. In the analytical models in the literature, the effect of abrasive reinforcement particles, which affects the coefficient of friction and consequently the friction angle, has not been considered while predicting cutting forces in machining of MMCs. In this paper, chip–tool interface friction in machining of Al/SiCp composites has been considered to involve two-body abrasion and three-body rolling caused due to presence of reinforcements in composites. The model evaluates resulting coefficient of friction to predict the cutting forces during machining of Al/SiCp composites using theory of oblique cutting. Further, the model considers various frictional forces on the wiper geometry on the cutting edge that has been found to improve the integrity of machined surface on composites. The predicted cutting force values were found to agree well with the corresponding experimental values for finer reinforcements composites with the assumption that 40% of the reinforcement particles contribute to the abrasion at chip–tool interface. However, for the coarser reinforcement composites, assumption that the 60% of the particles contribute to the abrasion yields better results.     

A two-fluid model of abrasive waterjet

Waterjet flow added with abrasive solid particles, the so called abrasive waterjet, can enhance its performance for cutting various materials from soft food products to very hard titanium alloys. In this study, a theoretical analysis is conducted in order to develop a flow model for the abrasive waterjet. The main concern is whether the abrasive particles can be treated as a pseudo-fluid phase. A two-fluid model is developed based on the fundamental laws of conservation. A control volume method is used to discretize the equations, and a phase-coupled SIMPLE algorithm is adopted to solve the pressure–velocity coupling equations. The quasi two-dimensional flow field outside a conventional nozzle used in abrasive waterjet is analyzed and computed to validate the model. Good agreement is observed comparing the numerical results with the experimental measurements.     

Modeling of the influence of the abrasive waterjet cutting parameters on the depth of cut based on fuzzy rules

Currently, the abrasive waterjet cutting parameters for the milling operation have to be determined by a combination of prior experience and trial and error. It is shown that the selection of the abrasive waterjet cutting parameters for a required depth of cut in the given material can be effectively done by applying the principles of the fuzzy set theory. This approach will eliminate the need for extensive experimental work in order to select the magnitudes of the most influential abrasive waterjet parameters on the depth of cut. Fuzzy logic provides a methodology and imitation of a human's way of making decisions which is very useful in such applications where the mathematical model of the process does not exist, and one of such processes is indeed abrasive waterjet cutting. A number of case studies are performed to verify the validity of the proposed methodology for selecting the abrasive waterjet cutting parameters in order to achieve the predetermined depth of cut.     

Experimental study on abrasive waterjet polishing for hard–brittle materials

The rapid growth of hard–brittle materials necessitates the development of compatible machining techniques, especially for the precision machining. The abrasive waterjet (AWJ) machining is a powerful tool in processing hard–brittle materials. In the last decades, some of AWJ machining technologies, such as AWJ cutting, AWJ milling and AWJ drilling have gradually become mature and steady. However, a few investigations on precision surface machining for hard–brittle materials by AWJ had been carried out. In this research, the ductile erosion mechanism of hard–brittle materials by AWJ in small erosion angle has been analyzed. In theory, the ductile erosion can achieve micromaterial removal and the surface eroded is smooth and without any fracture. Based on the ductile erosion mechanism, the feasibility of polishing for hard–brittle materials by the AWJ has been investigated. A group of polishing experiments is performed. The polished surfaces of workpieces were observed with scanning electron microscope (SEM) and measured by atomic force microscopy (AFM). The results of these polishing experiments indicate that AWJ has a great potential to be used as a precision surface machining technology.     

Geometrical modelling of abrasive waterjet footprints: A study for 90° jet impact angle

Modelling of abrasive waterjet footprints is of critical importance when aiming to generate controlled freeform surfaces. The paper reports on a geometrical model of the jet footprint (kerf) in maskless controlled-depth milling applications. The model firstly needs to find the material specific erosion (etching) rate that is obtained from the jet footprint by taking the limiting conditions (high jet feed rates) of the model. Once this is found, the jet footprint can be predicted accurately for any jet feed speed. An example of model validation is presented for 90° jet impingement angle against a SiC ceramic as target material.     

Model-based path planning for laser cutting of curved trajectories

An analytical model of evaporative laser cutting over a curved beam trajectory is presented. A curved cutting path produces three main changes in the kerf profile compared with straight-line cutting: (1) an increase in the kerf width, (2) a shift of the kerf centerline position towards the center of rotation, and (3) asymmetry between the inner and outer kerf walls. Laser-cutting experiments were performed on polymethyl-methacrylate (PMMA) workpieces with energy density ranging from 100 J/mm² to 500 J/mm² in order to validate the model. Good agreement was obtained between model estimates and experimental results at low energy densities, while model estimates underestimated kerf eccentricity and asymmetry as energy density approached 500 J/mm². Curvature effects become dominant as the curvature ratio (ratio of radius of beam path curvature to beam radius) decreases below 50 for PMMA. This modeling approach can be used to plan dimensional accuracy, wall taper and material removal rate for laser processing in a number of applications, including laser blanking, sharp comer cutting, form cutting over a curved workpiece, and die board/pattern cutting.     

微磨料空气射流加工技术的发展

微磨料空气射流加工(MAJM)技术是对硬脆材料进行微细加工的一种非常有潜力的技术，特别是对复杂的三维微细结构的加工。微磨料空气射流加工技术是基于传统的喷砂技术发展起来的，通过由空气喷射磨料微粒形成高速气流冲击工件表面而去除工件材料。与其它的加工技术相比，微磨料空气射流加工具有环境友好、易于控制、无热影响区、切口质量好等优点。本文介绍了此技术的基本加工原理、特点以及加工过程中的影响因素，论述了微磨料空气射流加工的材料冲蚀机理和切口特征。重点分析了一些主要的加工参数，例如空气压力、磨料材料、尺寸、供给率、喷嘴的形状和尺寸、喷射距离以及工件材料，对切削性能和切口特征的影响。并提出了微磨料空气射流加工技术中有待进一步深入开展的研究工作。     

A new model for predicting the depth of cut in abrasive waterjet contouring of alumina ceramics

A new predictive model for the depth of cut in abrasive waterjet (AWJ) contouring of alumina ceramics is developed using a dimensional analysis technique. The model is then experimentally verified when cutting an 87% alumina ceramic within the practical range of process variables. It is found that the model can give adequate predictions of this cutting performance measure with about 1% average error.     

Processes and apparatus developments in industrial waterjet applications

This paper is based on an abrasive waterjet cutting process that helps solve problems in processing of modern hard-to-cut materials, enabling wider industrial application. A detailed explanation of the recent developments in the main components of abrasive waterjet systems are given. Factors such as water pressure, grain diameters of abrasive feed rate, and traverse speed influencing surface roughness and depth of cut are studied using experimental data. Taking account of industrial applications, advantages–disadvantages, and limitations of the process are assessed.     

Waterjet—An innovative tool for manufacturing

The use of multi-axis waterjet machines as a manufacturing tool is rapidly increasing. A wide range of materials can be machined ranging from carbon fibre composites on the latest aircraft fuselages to difficult to machine exotic alloys and state of the art metal matrix composite materials. This work presents an overview of the range of materials and gives examples of geometries that can now be formed using this technique. The surface finish and processing interaction during machining is also outlined. Cutting, drilling and to a limited extent, milling will be considered.     

A study of abrasive water jet machining process on glass/epoxy composite laminate

Surface roughness (R_a) and kerf taper ratio (T_R) characteristics of an abrasive water jet machined surfaces of glass/epoxy composite laminate were studied. Taguchi's design of experiments and analysis of variance were used to determine the effect of machining parameters on R_a and T_R. Hydraulic pressure and type of abrasive materials were considered as the most significant control factor in influencing R_a and T_R, respectively. Due to hardness of aluminium oxide type of abrasive materials, it performs better than garnet in terms of both machining characteristics. Increasing the hydraulic pressure and abrasive mass flow rate may result in a better machining performance for both criteria. Meanwhile, decreasing the standoff distance and traverse rate may improve both criteria of machining performance. Cutting orientation does not influence the machining performance in both cases. So, it was confirmed that increasing the kinetic energy of abrasive water jet machining (AWJM) process may produce a better quality of cuts.     

A strategy for efficient and quality cutting of materials with abrasive waterjets considering the variation in orifice and focusing nozzle diameter

In Abrasive Waterjet (AWJ) cutting, orifice and focusing nozzle diameter undergo continuous change in their dimensions due to erosive nature of high velocity abrasive waterjet. This particular phenomenon can affect the efficiency and quality of the process. To achieve maximum efficiency and desired quality with this process, the parameters need to be optimally selected from time to time considering the changes in the dimensions of orifice and focusing nozzle. In an effort to develop strategies for this purpose and to build the knowledge base for adaptive control of the process, the present work aims to study the influence of orifice and focusing nozzle diameter variation on the performance of abrasive waterjets in cutting 6063-T6 aluminum alloy. The performance was assessed in terms of different parameters such as depth of cut, material removal rate, cutting efficiency, kerf geometry and cut surface topography. In order to maintain the desired performance, it is essential to monitor the condition of nozzles and suitably adjust the process parameters with a view to control the process. Towards the latter, the present work attempts to suggest a strategy that can aid in replacing the nozzles at an appropriate time for maintaining the performance of process within certain limits so as to maintain the precision in machining with abrasive waterjets.