Spark Assisted Chemical Engraving Research Articles

Intelligent characterization of spark-assisted chemical engraving (SACE) process using time series classification

Intelligent characterization of spark-assisted chemical engraving (SACE) process using time series classification

On the use of the current signal in spark assisted chemical engraving for micromachining process control

Spark-Assisted Chemical Engraving (SACE) is a hybrid manufacturing process used to micromachine glass and other hard-to-machine materials. A tool-electrode, integrated in an electrochemical cell, is used to generate sparks and locally heat the workpiece surface, which enables material removal through thermally-promoted etching by an alkaline electrolytic solution.A challenge in SACE machining is the precise in-situ monitoring of material removal for process control; (electro)mechanical solutions exist, however they are complex to realize, hence limiting the use of the SACE process for precision glass machining in industry and academia. The present study introduces novel, quantitative and practical methods of obtaining process information by measuring the current flowing through the electrodes for both workpiece surface referencing by probing and SACE machining feedback and control strategies.During workpiece surface probing operations, a spike in the average current signal was observed at a repeatable tool height with respect to the workpiece, which can be used to accurately and repeatably measure the machining gap in a probing operation. Furthermore, three distinct current modes were identified and related to a different gas film state during machining of microchannels, paving the way for active control of gas film stability through monitoring of the current signal.

SACE scanning process assisted with ultrasonic vibration on quartz glass

ABSTRACT Spark-assisted chemical engraving (SACE) is suitable to machine hard and brittle insulating materials such as quartz glass; however, the processing efficiency is quite low for achieving good machining quality. In this research, a scanning process of ultrasonic vibration-assisted SACE (UV-SACE) is proposed for high-efficiency processing on quartz glass without surface microcracks. For revealing the microscope mechanism under different vibration amplitudes, a high-speed camera is used to observe the motion of micro bubbles, morphology of gas films, and instantaneous discharges. Then through the processing experiments with two-factor coupling effect of feed rate and vibration amplitude, it is found that the cutting depth is significantly affected by feed rate due to the effect of ultrasonic vibration. The optimal input conditions are obtained by using the NSGA-II optimization method. Furthermore, the removal mechanism of UV-SACE scanning process on quartz is analyzed and applied: Lower feed rate (scanning speed) as the fully discharging case can improve the material removal rate (MRR), while higher feed rate as the insufficient discharging case can improve the processing accuracy and surface quality. Machining experiments verified the proposed mechanism and the optimized process.

Effect of Machining Limiting Factors on Drilling Progress during Spark Assisted Chemical Engraving (SACE): General Trends

Spark Assisted Chemical Engraving (SACE) is a micro-machining technology for non-conductive materials, mainly glass, based on thermal assisted etching. Generally, during SACE, drilling proceeds at a fast rate reaching 100 µm/s for the first 100 µm and then it slows down for depths higher than 300 µm. While several techniques have been proposed to establish faster drilling, they mainly rely on tuning the machining parameters to enhance the machining performance. However, with this approach machining parameters need to be constantly tuned to achieve certain machining performance depending on the size of the tool and the features needed. Therefore, this necessitates further work to enhance understanding regarding the SACE machining process fundamentals in order to enhance machining speed and quality. Since SACE is a thermal assisted etching process, both local heating and flushing of electrolyte in the machining zone are required. However, to the authors’ knowledge there is not any study that attempts to analyze the effect of each of these machining limiting factors on the machining performance. This work attempts to clarify the effect of each flushing and heating on the drilling progress for hole depths higher than 100 microns. It therefore provides a deeper understanding of the fundamentals of the SACE machining process.

Glass precision micro-cutting using spark assisted chemical engraving

Manufacturing industry faces new challenges with the emergence of the need for the production of small batches of personalized parts. Such production methods demand for a capability to integrate multiple machining operations in one manufacturing process to reduce setup and calibration time and tooling costs. This requirement is especially challenging for difficult-to-machine materials such as glass, since there exist only a limited number of glass machining technologies and further these technologies often require specialized tooling. Glass cutting is among the crucial machining operations, which is frequently required for glass products.The presented study discusses free-form micro-cutting by Spark Assisted Chemical Engraving (SACE), determining cut parameters, in terms of tool feed-rate F and depth-of-cut p in function of machining voltage. A simple model is discussed allowing to predict the maximal product F⋅p which can be used to cut glass by SACE. The presented data and model allow to reduce the time-consuming trial and error process in determining appropriate cutting parameters. An interesting finding is that lowest cutting times can be achieved with tools of 100-μm diameter. Cut surface roughness of initial cuts can be reduced by deploying subsequently incremental finishing (polishing) passes performed at lower machining voltage, lower tool feed rates and higher angular tool rotation. It is demonstrated that very smooth cut surfaces (Rz ~ 1 μm) can be achieved.

The Capabilities of Spark-Assisted Chemical Engraving: A Review

Brittle non-conductive materials, like glass and ceramics, are becoming ever more significant with the rising demand for fabricating micro-devices with special micro-features. Spark-Assisted Chemical Engraving (SACE), a novel micromachining technology, has offered good machining capabilities for glass and ceramic materials in basic machining operations like drilling, milling, cutting, die sinking, and others. This paper presents a review about SACE technology. It highlights the process fundamentals of operation and the key machining parameters that control it which are mainly related to the electrolyte, tool-electrode, and machining voltage. It provides information about the gas film that forms around the tool during the process and the parameters that enhance its stability, which play a key role in enhancing the machining outcome. This work also presents the capabilities and limitations of SACE through comparing it with other existing micro-drilling and micromachining technologies. Information was collected regarding micro-channel machining capabilities for SACE and other techniques that fall under four major glass micromachining categories—mainly thermal, chemical, mechanical, and hybrid. Based on this, a figure that presents the capabilities of such technologies from the perspective of the machining speed (lateral) and resulting micro-channel geometry (aspect ratio) was plotted. For both drilling and micro-channel machining, SACE showed to be a promising technique compared to others as it requires relatively cheap set-up, results in high aspect ratio structures (above 10), and takes a relatively short machining time. This technique shows its suitability for rapid prototyping of glass micro-parts and devices. The paper also addresses the topic of surface functionalization, specifically the surface texturing done during SACE and other glass micromachining technologies. Through tuning machining parameters, like the electrolyte viscosity, tool–substrate gap, tool travel speed, and machining voltage, SACE shows a promising and unique potential in controlling the surface properties and surface texture while machining.

Numerical and experimental investigations to analyze the micro-hole drilling process in spark-assisted chemical engraving (SACE)

With the increased utilization of glass in micro-electromechanical systems (MEMS) applications, need is generated to produce micro-features on glass with excellent surface quality, i.e., better hole circularity and less thermal cracks. Spark-assisted chemical engraving (SACE) has proven to be an emerging and reliable technology for micro-machining of glass that includes local joule heating of the work material for its removal. However, some problems need to be covered in the micro-hole drilling process such as low machining depth, low aspect ratio, high thermal cracks, and an increase in machining time with the increase in depth. Moreover, few finite element modeling (FEM) studies have been reported to analyze the material removal aspects in SACE. This present study developed the FEM-based thermal model to analyze the material removal rate (MRR) and investigated the different machining conditions to improve the micro-hole drilling process in SACE. Improvement in depth and aspect ratio can be achieved by using a different tool’s shape which variably enhances the flow of electrolytes and thereby increases the formation rate of gas film. Pointed and cylindrical tools were used for micro-drilling operations. MRR, hole entrance diameter, machining depth, aspect ratio, thermal cracks, and machining time were observed under different machining conditions. FEM study revealed that MRR improves with the increase in both the applied voltage and electrolyte concentration. The MRR was predicted by utilizing the plots of temperature distributions and was found in accordance with the experimental results of MRR. The experimental study concluded that the pointed tool produced micro-holes with a higher machining depth and aspect ratio at an improved machining rate, while the cylindrical tool produced micro-holes with a smaller number of thermal cracks and better circularity.

Energy action model of spark assisted chemical engraving (SACE) for improving surface quality of micro cavities in ZrO2 ceramics

ZrO2 ceramic has a wide range of applications, such as in the thermal barrier coating (TBC) of turbine blades, micro actuators, and gas sensors. However, this material is challenging to machine due to its high hardness and brittleness. Although spark assisted chemical engraving (SACE) can be used to machine ZrO2 ceramics, the traditional SACE models for glass are difficult to apply to ZrO2 ceramics since the melting point of ZrO2 is much higher than that of glass. A theoretical basis for applying the SACE process to ZrO2 ceramics is still lacking, which makes applying the SACE process to machine micro cavities with high surface quality in ZrO2 ceramics very challenging. This paper proposes an energy action model based on the processing voltage, pulse width and machining gap by analyzing the energy transfer process of SACE. The energy model expresses difference in the spark energy between physical removal and chemical removal. Through machining experiments, the contact effect of the SACE process on ZrO2 ceramics was found, and the reasons why the SACE process is sensitive to the machining gap were clarified. Furthermore, physical and chemical removal process models with or without the discharge constraint onto the end of the tool electrode were established. Using the above theoretical models, a circular ring microcavity without micro cracks on the surface was achieved in a ZrO2 ceramic workpiece by using the SACE process with the regulating energy effect trending to the chemical removal. Additionally, considering the contact effect and the process models applied in the SACE scanning process of ZrO2 ceramics, a tool electrode with a spring structure was employed to solve the bending problem of the micro tool electrode. As a result, a square micro-cavity with high surface quality was machined successfully.

Micro-SACE scanning process with different tool-surface roughness

ABSTRACT As a non-traditional machining process, spark assisted chemical engraving (SACE) can machine hard, brittle, and non-conducting materials. The spark discharges on tool-electrode sidewall result in energy loss and overcut edges. A tool electrode covered with an insulating film on its sidewall is usually used to constrain the discharges to the tool-electrode end. However, not only the preparation of the insulating film is cautious and fussy, but also it is easy to split away under the high temperature and pressure action of SACE. In this research, a novel method constraining the micro-SACE discharges to the tool-electrode end was proposed by changing surface roughness of the tool electrode. The special micro-tool electrode with rough end and smooth sidewall was fabricated on-machine. The constraint effect was experimented by observing the distribution of SACE bubbles and discharges around the tool-electrode end, and the optimal parameters were obtained by evaluating the bubble aspect ratio. Furthermore, machining experiments of the micro-SACE scanning process by constraining discharges to tool-electrode end were verified on ZrO2 ceramics as a kind of difficult-to-machine material. Micro-circular grooves without micro cracks were successfully machined by the use of a Φ300 μm tool electrode.

Suppression of cathode tool wear by a forward-bias voltage in pulsed spark-assisted chemical engraving

Pulsed spark-assisted chemical engraving (SACE) is a promising technology for micromachining of insulating materials, with no burrs, no cracks and no residues left after processing. Cathode tool wear, however, is one of the problems that must be overcome to enable large-scale application of SACE. This paper presents a comprehensive study of the phenomena and mechanisms involved in cathode tool wear in pulsed SACE. By analyzing the electrolytic current and pulsed voltage waveform during the process, it is demonstrated that cathode tool wear is determined by the reverse current and is particularly affected by the pulse-off time and applied voltage. A longer pulse-off time results in more significant tool wear. The reverse current is mainly caused by the discharge of electrical double layer capacity when a pulsed voltage is applied. By applying a forward-bias voltage during the pulse-off time, cathode tool wear can be completely suppressed. The research results of this paper provide a reference for prevention of tool wear in precision micromachining with the pulsed SACE method.

Scanning process of spark assisted chemical engraving (SACE) on ZrO2 ceramics by constraining discharges to tool electrode end

Zirconium dioxide (ZrO2) is a ceramic material used in a wide range of applications. However, the mechanical properties of ZrO2 ceramics, such as brittleness, high hardness, and high temperature resilience, make it challenging to machine. SACE (spark assisted chemical engraving) is an advanced technology for machining ZrO2 ceramics that has also been used successfully in glass machining. However, the sidewall of the SACE tends to generate spark discharges, which not only waste energy but also affect machining accuracy. In a set of exploratory SACE experiments, it was found that a large percentage of discharges occur at the end of the tool electrode under certain conditions. It is believed that the sharp square edge of the tool-electrode end is conducive to the end discharges by large. Therefore, a cylindrical notched tool electrode with an increased number of sharp edges to improve discharge constraint is proposed in the SACE process. SACE scanning experiments were conducted on ZrO2 ceramics using the proposed tool electrode and found that it significantly improved machining efficiency and accuracy.

Investigating the surface layer of micro-features machined by Spark Assisted Chemical Engraving

Abstract This work investigates the potential of modifying the surface of micro-features machined on glass by Spark Assisted Chemical Engraving (SACE). It is shown that micro-particles added to the electrolyte could be bonded to the glass surface on some locations. It is believed that these particles were attached to a glass surface layer that is reformed during machining. The thickness of this layer could not be quantified. Future work will investigate the nature of the particle-glass bond and the thickness of the surface layer.

Hybrid method combining SACE micro-machining and laser processing to fabricate glass micro-features with special surface properties

Abstract In this work, a relatively simple and low-cost hybrid method that combines spark assisted chemical engraving (SACE) and high intensity laser induced reverse transfer (HILIRT) to fabricate glass micro-features with specialized surface properties is presented. The capability of SACE to machine glass coated with Titanium nanofibers is demonstrated. The results will be applied to fabricate Lab-on-chips devices for different applications.

Electrochemical Manufacturing to Support Industry 4.0 – Spark Assisted Chemical Engraving (SACE)

Manufacturing industry is facing new challenges as there is a growing demand for mass-personalized products at low cost. A new kind of processes need to be developed. This new revolution was recognized recently by industry and in Germany the key word industry 4.0 [1] was introduced to characterize this “fourth industrial revolution” for the entire manufacturing value chain. A major hurdle to overcome for successful production of mass personalized products are setup and tooling costs. According to recent studies (made by the Universities of Michigan and Cincinnati for the World Economic Forum) hybrid technologies, including electrochemical technologies, are promising to address these manufacturing challenges [2]. Among the established electrochemical technologies can be cited electrochemical (discharge) machining, electro-deposition/-forming and electropolishing e.g. as post-process for metal additive manufactured (AM) parts. None is yet used for mass personalisation, where the custom products are no longer an assembly of individual parts (i.e. modular design), but they are fully personalized, i.e. the shapes of the parts change too. Suitable manufacturing processes for personalized batch-size-1 production must be highly flexible and have little overhead, particularly for the tooling. Hybrid technologies are very promising as these processes require little to no specialized tooling and can handle virtually any shape, including inner surfaces. At the same time, glass, existing for millions of years in its natural form, has fascinated and attracted much interest from both the academic and industrial world. The application of glass science to the improvement of industrial tools occurred only in the past century, with a few exceptions. Glass has been employed in many forms to fabricate glazing and containers for centuries while it is now entering new applications that are appearing in micro and even nanotechnology like fibers, displays and Micro-Electro-Mechanical-System (MEMS) devices [3]. Many qualities make glass attractive since it is transparent, chemically inert, environmentally friendly and its mechanical strength and thermal properties. In fact, no other materials being mass-produced have shown such qualities over so many centuries. Nowadays glass offers recycling opportunities and allows for tailoring new and dedicated applications. Moreover, glass is radio frequency (RF) transparent, making it an excellent material for sensor and energy transmission devices. Another advantage of using glass in microfluidic MEMS devices [4] is its relatively high heat resistance, which makes these devices suitable for high temperature microfluidic systems [5] and sterilization by autoclaving. However, glass is a hard to machine material, due to its hardness and brittleness. Machining high-aspect ratio structures is still challenging due to long machining times, high machining costs and poor surface quality [6]. Hybrid methods like Spark Assisted Chemical Engraving (SACE) [7] perform well to machine high aspect ratio and smooth surface structures on glass. These assets of SACE technology combined with its relative high machining speeds compared to chemical methods and low-cost compared to femto-laser technologies make SACE perfectly suitable for rapid prototyping of micro-scale glass devices. In this thermochemical process, a voltage is applied between tool- and counter-electrode dipped in an alkaline solution (typical NaOH or KOH). At high voltages (around 30 V), the bubbles evolving around the tool electrode coalesce into a gas film and discharges occur from the tool to the electrolyte through it. Glass machining becomes possible due to thermally promoted etching (breaking of the Si-O-Si bond) [7]. In the present communication, it is shown how electrochemical processes can be used to design new high precision manufacturing processes for industry 4.0. In particular hybrid machining by SACE technology is discussed for hard-to-machine materials like glass. Some other examples are highlighted as well in the field of post-processing technologies for metal AM parts and fabrication of high-precision complex metal structures based on 3D printed high resolution polymer models. [1] Deloitte, “Industry 4.0. Challenges and solutions for the digital transformation and use of exponential technologies”, Deloitte, pp. 1–30, 2015. [2] J. Ni, J. Lee “Emerging and Disruptive Technologies for the Future of Manufacturing” Case study no.7 World Economic Forum Global Agenda Council on the Future of Manufacturing [3] E. Le Bourhis, “Glass, Mechanics and Technology.”, Wiley-VCH, 2014. [4] G. M. Whitesides, “The origins and the future of microfluidics.”, Nature, vol. 442, no. 7101, pp. 368–373, 2006. [5] D. Sinton, “Energy: the microfluidic frontier.”, Lab Chip, vol. 14, no. 17, 2014. [6] L. Hof, J. D. Abou Ziki, “Micro-hole drilling on glass substrates – a review”, Micromachines, vol. 8, no.53, 2017. [7] R. Wüthrich and J. D. Abou Ziki, “Micromachining Using Electrochemical Discharge Phenomenon.”, Elsevier, 2015. Figure 1

Rapid prototyping of packaged glass devices: Eliminating a process step in the manufacturing workflow from micromachining to die singularizing

Direct glass-to-glass bonding after micro-patterning in glass is often challenging, especially in the frame of rapid prototyping, as several special cleaning or other post-processing steps are needed before bonding is possible. In this study, we demonstrate that glass-to-glass bonding is possible directly after Spark Assisted Chemical Engraving (SACE) micromachining without any special post-treatments. This approach enables flexible prototyping of glass devices at relatively low cost, which is illustrated by fabrication of functional microfluidic devices. The machined and bonded glass device is evaluated both qualitatively and quantitatively on performance and shows good results.

Experimental investigation of magnetohydrodynamic effect in electrochemical discharge machining

Electrochemical discharge machining (ECDM) which is also known as spark assisted chemical engraving (SACE) is a machining process to machine non-conducting and brittle materials. Micro-ECDM presents several desirable properties featuring high flexibility, smoothly machined surfaces, and high drilling speed comparing with other micromachining technologies. The unstable gas film around the tool electrode in which the electrical discharges take place is unpredictable as the machining depth increases, resulting in inaccurate geometry and inconsistent machining. The material removal rate is significantly decreased due to insufficient electrolyte flow around tooltip in the hydrodynamic regime. These are the serious drawbacks of the process that need to be improved. This paper presents an analytical analysis of the magnetohydrodynamic (MHD) effect in the ECDM process. The mechanism of the magnetohydrodynamic effect in electrochemical discharge machining was investigated. The high-speed camera was used to record the formation of a gas film on the tool electrode with and without magnetohydrodynamics effect. The experimental results showed that the MHD effect induced by the magnetic field improved electrolyte circulation and higher machining efficiency was achieved. Furthermore, it was observed that the thickness of the gas film was decreased. The radius of the machined hole is reduced from 528 µm to 430 µm while the machining time was decreased from 50 to 16 s. The counter resistant feeding method and magnetic field can be applied simultaneously by the newly designed set-up. This hybrid method made significant enhancement in accuracy and throughput of the drilling process, as well as improving the roundness of the machined hole.

Characteristics of the Arcing Plasma Formation Effect in Spark-Assisted Chemical Engraving of Glass, Based on Machine Vision.

Spark-assisted chemical engraving (SACE) is a non-traditional machining technology that is used to machine electrically non-conducting materials including glass, ceramics, and quartz. The processing accuracy, machining efficiency, and reproducibility are the key factors in the SACE process. In the present study, a machine vision method is applied to monitor and estimate the status of a SACE-drilled hole in quartz glass. During the machining of quartz glass, the spring-fed tool electrode was pre-pressured on the quartz glass surface to feed the electrode that was in contact with the machining surface of the quartz glass. In situ image acquisition and analysis of the SACE drilling processes were used to analyze the captured image of the state of the spark discharge at the tip and sidewall of the electrode. The results indicated an association between the accumulative size of the SACE-induced spark area and deepness of the hole. The results indicated that the evaluated depths of the SACE-machined holes were a proportional function of the accumulative spark size with a high degree of correlation. The study proposes an innovative computer vision-based method to estimate the deepness and status of SACE-drilled holes in real time.

Spark assisted chemical engraving (SACE) mechanism on ZrO2 ceramics by analyzing processed products

Traditional machining processes are usually unworkable for ZrO2 ceramics due to the mechanical properties of fragility, high hardness and high-temperature resistance. The process of spark assisted chemical engraving (SACE) has been explored for machining ZrO2 ceramics, but up to now the processing mechanism has not been clearly understood yet. To understand the microscopic mechanism, the phase transition paths of ZrO2 ceramics under various physical and chemical conditions were analyzed. A backstepping approach based on structures and components of SACE processed products was proposed to verify the physical and chemical mechanisms. According to the crystal phase composition of the processed debris resulting from Raman spectrum analysis, it was found that the debris exhibited crystalline and amorphous forms which provided the evidence of physical crushing and chemical reactions. Furthermore, the chemical removal mechanism was proven by the nearly saturated concentration of Zr ion in the processed solution according to the ICP-OES assay of SACE electrolyte. The quantitative proportion of physical and chemical effect can be successfully determined by distinguishing and analyzing two kinds of debris with different crystal phase compositions which yield distinctly identifiable appearances under an optical microscope.

Industry 4.0 – Towards fabrication of mass-personalized parts on glass by Spark Assisted Chemical Engraving (SACE)

With the fourth industrial revolution manufacturing industry faces new challenges. Small batches of personalized parts must be produced in an economically viable manner. In this study, we show the feasibility of an approach using Spark Assisted Chemical Engraving to achieve personalized parts of hard-to-machine materials like glass with a low-cost setup. Key is the use of low-cost rapid prototyping and in-situ fabrication of the needed tooling, eliminating high indirect costs and long lead times. This approach can be used for on-demand manufacturing of personalized high precision applications of glass such as smartphone covers, advanced medical devices or fiber optic telecommunications.

Fabrication of precise microchannels using a side-insulated tool in a spark assisted chemical engraving process

ABSTRACT Microfluidics is an emerging field giving rise to a number of scientific developments with wide applications in proteomics, genomics, DNA analysis, drug delivery systems and lab-on-a-chip. Among the various microfluidic structures, microchannels form an integral part of Lab-on-a-chip devices for clinical diagnosis. The geometrical accuracy and surface finish quality of the microchannels affect the performance of microfluidic systems, thereby demanding a precise microfabrication technique for realizing microchannels. Spark assisted chemical engraving (SACE) is an effective hybrid micromachining technique used for micro-structuring non-conducting substrates such as glass and ceramics. In the present work, we report on a room temperature coated thick insulating film for improving the quality of the microchannels realized using a SACE process. The insulator film coated around a tool electrode confines the spark discharges only to the bottom surface of the tool. Moreover, the proposed insulator coating methodology does not require any cost intensive equipment, carrier gases, vacuum units and a clean room environment. Experimental results demonstrate that for same process parameters a micro-channel fabricated using an insulated tool has its overcut reduced by 57.8% and machining depth improved by 19.53% with a smooth surface and regular edges as compared to the channels realized using an uninsulated tool.