Electrical Discharge Machining Drilling Research Articles

Research on the key techniques of composite processing of EDM and vibration ultrasonic drilling

PurposeUtilizing electrical discharge machining (EDM) to process micro-holes in superalloys may lead to the formation of remelting layers and micro-cracks on the machined surface. This work proposes a method of composite processing of EDM and ultrasonic vibration drilling for machining precision micro-holes in complex positions of superalloys.Design/methodology/approachA six-axis computer numerical control (CNC) machine tool was developed, whose software control system adopted a real-time control architecture that integrates electrical discharge and ultrasonic vibration drilling. Among them, the CNC system software was developed based on Windows + RTX architecture, which could process the real-time processing state received by the hardware terminal and adjust the processing state. Based on the SoC (System on Chip) technology, an architecture for a pulse generator was developed. The circuit of the pulse generator was designed and implemented. Additionally, a composite mechanical system was engineered for both drilling and EDM. Two sets of control boards were designed for the hardware terminal. One set was the EDM discharge control board, which detected the discharge state and provided the pulse waveform for turning on the transistor. The other was a relay control card based on STM32, which could meet the switch between EDM and ultrasonic vibration, and used the Modbus protocol to communicate with the machining control software. FindingsThe mechanical structure of the designed composite machine tool can effectively avoid interference between the EDM spindle and the drilling spindle. The removal rate of the remelting layer on 1.5 mm single crystal superalloys after composite processing can reach over 90%. The average processing time per millimeter was 55 s, and the measured inner surface roughness of the hole was less than 1.6 µm, which realized the micro-hole machining without remelting layer, heat affected zone and micro-cracks in the single crystal superalloy.Originality/valueThe test results proved that the key techniques developed in this paper were suite for micro-hole machining of special materials.

Investigating mechanisms of debris removal in ultrasonic vibration-assisted EDM drilling

In traditional electrical discharge machining (EDM) of micro-holes, the debris removal becomes increasingly challenging as hole depth increases, significantly affecting the machining of difficult-to-machine materials. To address this issue,we first developed a micro-hole gap flow field model for a novel longitudinal–torsional ultrasonic vibration (LTV) electrode EDM drilling method. We analyzed the effects of three types of electrodes — rotation electrodes, longitudinal ultrasonic vibration (LUV) electrodes, and LTV electrodes — on the flow velocity in the micro-hole interstitial flow field. Next, we simulated the motion of fluid and debris in the interstitial flow field of micro-holes, theoretically verifying the advantage of electrode ultrasonic vibration in enhancing debris removal efficiency. Additionally, we observed the electrode tip morphology, the elemental composition on the electrode surface, and the morphology of the micro-hole inlet/outlet and inner wall to further elucidate the debris removal mechanism and confirm the superior drilling performance of the ultrasonic vibration electrodes. At an optimal ultrasonic amplitude (AL=4μm), the LUV electrode demonstrated improved consistency in the micro-hole inlet/outlet and achieved a micro-hole taper of 0.014°. The material removal rate (MRR) using the LTV electrode reached 5.14 × 105μm3/s, nearly 1.5 times higher rotation electrode. This method successfully produced a positively tapered hole suitable for injector nozzles.

A study on the EDM drilling of reaction-bonded silicon carbide using different electrode materials

Today, there is a growing demand for efficient hole manufacturing technology in many industries such as aeronautics, automotive and nuclear, among others. Thus, the present study deals with the machining of through holes on SiSiC advanced ceramic by using Electrical Discharge Machining (EDM) drilling technology. Since recommendations related to the electrode characteristics and settings parameters are found to be scant for the industrial use of EDM drilling of SiSiC ceramics, this research work comes to cover this gap as it presents a complete study focused on the influence on different electrodes under rough and finish machining conditions. In particular, the influence of four electrodes materials (copper, copper-tungsten, graphite and copper infiltrated graphite) and three different electrode diameters ranging from 2 to 4 mm are investigated. In addition, the rotational speed of the electrode is also analysed. From the experimental results, both electrode material and machining regime, seem to be the most relevant factors of all. In the case of 2 mm diameter electrode, material removal rate (MRR) with Cu electrode was, approximately, 4.5 times higher than that obtained with a C electrode. In fact, it was found that copper electrode rotating at 20 rpm combined with high values of discharge energy (I = 2 A; ti = 70 µs) is the most economical option in terms of production cost and production time, as it gives a high MRR of 0.4754 mm3/min and a minimum electrode wear (EW) value of 7.52%. Moreover, slightly higher values of MRR were achieved for CuC electrode compared to those obtained with C electrode, indicating that the addition of Cu in the electrode contributes to a greater removal of material. However, a value of Ra of 0.37 µm could be obtained by setting low current intensity values (I = 0.5 A; ti = 45 µs) combined with C electrodes and with no rotation.

Microstructure Effects of the Electrical Discharge Machining Drill on Aerospace Super Alloys

This paper presents an analysis of the parent material damage and electrode wear of a handheld electrical discharge machining (EDM) drill. The motivation of this study is to investigate the material effects of EDM drilling on aerospace alloys at a microstructure level. The specific effects that are being investigated are heat affected zones (HAZ), grain structure changes, material composition changes, and fracturing. A secondary goal of this study is to determine if there is potential for electrode wear prediction models for supply chain management. The study consisted of making cuts into both Inconel and titanium test specimens while varying cut depths and electrode diameters. The results of the study found that the electrode wear while cutting Inconel showed some amount of predictable behavior. On the contrary, the titanium specimens did not indicate a predictable behavior and did not appear to follow an intuitive trend. Interestingly, when drilling into titanium, the active electrode increased in length, indicating that the titanium is being deposited onto the electrode. The EDM drill caused notable damage to the parent material, including a substantial recast layer of up to approximately 150 μm in thickness, incorporation of electrode material into the parent material through the recast layer, and formation of microcracks that can cross the recast layer/parent material interface. These noted characteristics can have an impact on material performance and longevity due to the nonuniform nature of the damage and the introduction of microcracks crossing from the recast layer into the parent material.

Bubble flushing effect in micro EDM drilling and its relation with debris

In micro electrical discharge machining (EDM) drilling with a liquid dielectric, the bubble flushing effect is significant to the dielectric exchange and debris evacuation. Clarifying its mechanism is critical to guild the machining and avoid the deterioration of machining environment, which is a common problem in deep hole drilling with micro EDM. However, the investigation of bubble flushing is always disturbed by debris, which affects bubble production through disturbing the discharge process, because the debris is always conductive as a byproduct of metal materials. In this paper, the bubble flushing effect is systematically investigated from the in situ observation to statistical analysis to clarify the driven force for bubble movement or escape in the narrow gap and assess the evolution of bubble flushing effect with increasing hole depth. Moreover, the influence of debris on the bubble flushing effect is quantitatively analyzed through the contrast experiments in single crystal SiC and stainless steel SUS304 (SS304). It is confirmed in this paper that the debris of SiC has nearly no effect on the discharge due to its special material property. The visible and quantitative investigations reveal the nature bubble flushing effect in micro EDM drilling. The contrast analysis of bubble flushing effect under two cases (with/without influence of debris) clarifies how bubble and debris affect one another and their influences on the drilling process. This study provides new insights into the mechanism of micro EDM drilling and benefits to optimize the actual machining processes for the industry and manufacturer.

Experimental study of EDM milling of 3D-shaped diffuser for film cooling holes on turbine blades

Cylindrical and diffuser-shaped film cooling holes on aero-engine turbine blades are usually machined by electrical discharge machining (EDM) by means of drilling and die-sinking approaches in a sequence. However, this sequential machining on different types of EDM machines is prone to deteriorate the machining accuracy since the inevitable secondary clamping error. Besides, since the nature of electrode wear, the shape of the tip of tool electrode is constantly changing during machining. To ensure the geometry accuracy of the diffuser, the worn tool electrode has to be replaced or dressed online frequently, resulting in a low machining efficiency. To address these problems, this paper proposes a machining method for diffuser shaped film cooling holes by a compound approach in combination of high speed EDM drilling and milling implemented on an EDM drilling machine with tubular electrodes and high-pressure inner flush. To obtain the optimal machining parameters, experimental investigation is carried out based on the experimental design method. Machining performance in terms of longitudinal tool electrode wear ratio, machining time, and machining stability under different machining parameters, such as pulse duration, pulse interval, peak current, capacitance, spindle speed, and flushing pressure, were taken into account. And the range analysis method is then adopted to obtain an optimal set of machining parameters. By applying the obtained optimal machining parameters, the proper compensation parameter and tool path overlapping ratio of the EDM milling process can be obtained. In the milling of an inversed pyramid, the material removal rate can reach as high as 14.02 mm3/min, the relative geometrical error can be controlled within 3.58%, and the maximum recast layer thickness was 6.8μm. Machining experiments of diffuser shaped film cooling holes verify the capability and effectiveness of the proposed method.

Observation of molten material and recast layer distribution of fast EDM drilling on inclined surface

The recast layer on film cooling holes of turbine blades processed by fast EDM (electrical discharge machining) drilling is the core issue that affects the quality of blades. Since the film cooling hole machining method is to drill holes on the inclined surface of the workpiece, deviation and reciprocating vibration of electrode often occur during the stage of contact. This article observes the instability phenomena and investigates the influence of the contact instability on the recast layer during the contact stage with fast EDM drilling on inclined surface. Through the self-built molten material observation platform and the single pulse discharge detector, observation experiments were carried out to observe the instability of the contact phase. According to the observation experiment, the contact instability phenomena was observed and captured, while the movement and distribution of the molten material were dynamically tracked. Then the distribution of the recast layer was observed under the influence of contact instability. Finally, combined with the results of the dynamic observation of the molten material and the statistics of the discharge state during the contact instability, the thermo-mechanical coupling analysis of the recast layer distribution by the fast EDM drilling on the inclined surface is carried out.

Study on the evolution process of recast layer for fast EDM drilling based on observation experiment and a novel thermal-fluid coupling model

The existence of a recast layer of film cooling holes on turbine blades imposes a crucial problem that affects the quality of fast electrical discharge machining (EDM) drilling. This paper focuses on an evolution process from molten material generation to recast layer formation. Single pulse discharge observation experiments with a high speed camera and a self-built observation platform were carried out to observe the formation and movement of the molten material. A novel thermal-fluid coupling model is proposed to combine fast EDM drilling methods with special materials and geometric properties of film cooling hole machining. A heat transfer module, a turbulent flow computational fluid dynamics module, a phase change module and a level set method are combined in this model. The complete process of generation, flow and solidification of molten materials is investigated by simulation with a single pulse discharging. The simulation results agree well with the experimental observation results. A thermal-mechanical coupling analysis of a molten material evolution process is carried out based on the temperature, velocity and pressure fields from the thermal-fluid model. Simulation results show that the molten material is generated at 1 μs after the starting of a discharge, and solidifies within 20 μs after the ending of the discharge. Under the influence of a high-speed flushing fluid, molten materials will not splash into a gap channel along the radial directions. Most of the molten material enters the gap channel along the workpiece surface with a velocity of over 25 m s−1. The moving direction of the molten material is consistent with that of a flushing fluid. The thickness of a recast layer gradually increases from bottom to top in the fast EDM drilling.

Elucidation of the mechanism of the deteriorating interelectrode environment in micro EDM drilling

In drilling with micro electrical discharge machining (EDM), gradual deterioration of the interelectrode environment with increasing drilling depth results in bad process control, low machining efficiency and poor shape accuracy. The reasons are attributed to three key factors: tool wear, bubble and debris. Tool wear changes the initial tool shape and results in poor machining accuracy. Bubble behavior affects dielectric exchange and debris evacuation by virtue of the bubble flushing effect. Debris plays the decisive role on the dielectric contamination. This paper reveals the characteristics of three factors at the micro level and elucidates how they affect the drilling process. First, the influence of tool wear on the material removal process is investigated by presenting the evolution of micro hole shape based on the in situ observation of micro hole profile. Second, the bubble flushing effect is quantitatively evaluated depending on the standard statistical analysis of bubble behavior. Third, the complex debris agglomeration is observed and explained by the colloidal chemistry theory, then the dielectric contamination is quantitatively evaluated based on weighing the debris accumulation amount in the gap. Results show that tool wear leads to an enlarged discharge area, especially the lateral discharge area, and results in uneven material removal. The nature of bubble flushing effect is a self-vibration process, which is weakened seriously with increasing feed depth due to severely decrease of vibration frequency. The existence of debris agglomeration indicates that debris particles have complex behavior but not just freely distributed in the dielectric. Comprehensive analysis indicates that deterioration of the interelectrode environment is inevitable in micro EDM drilling, attributed to the weakened bubble flushing effect and increasing difficulty of debris evacuation and control of discharge area. Visible and quantitative investigations provide specific evidences of how three factors affect the discharge process compared to the vague description from empirical consensus in previous researches. The insights elucidate complex interelectrode phenomena, enhance the understanding of mechanism of micro EDM and provide the theoretical guide for the manufacturers.

Path optimization for multi-axis EDM drilling of combustor liner cooling holes using SCGA algorithm

Multi-axis electrical discharge machining (EDM) drilling technology is widely used in the manufacture of film cooling and jet cooling holes of combustor liners. However, there can be more than 3000 film cooling holes with variant orientations in a typical combustor liner. Drilling of thousands of holes without process planning will result in a low machining efficiency. Traditional path optimization methods are carried out in a workpiece coordinate system, which is usually not optimal when applied to multi-axis machining processes. Aiming at minimizing the total non-productive time in a combustor liner multi-axis EDM drilling process, a mathematic model, which takes tool traveling time, tool switching time, and compensation moving time into consideration, is firstly derived. To solve the optimization problem efficiently, a spectral clustering/genetic algorithm (SCGA) is applied as the model has a large number of 0–1 variables. To improve the search performance, pairwise inter-reshuffle (PIR) based initial population and modified mutation are utilized. The performance of SCGA is evaluated by solving a 3616-hole optimization problem and the results are compared with those by other heuristic methods. Comparison results show that the SCGA significantly outperforms other comparison algorithms in terms of minimum non-productive time and convergence speed.

Completion detection and efficiency improvement for breakout stage of fast EDM drilling

Electrical discharge machining (EDM) is a non-conventional machining process that removes material by consecutive electrical sparks. Fast EDM drilling is one of such processes, which employs rotary tubular electrodes and inner flushing to enhance the machining efficiency and stability. It is widely used to produce small holes on various kinds of products. The breakout event occurs when the electrode just penetrates the workpiece during the drilling process, which marks the start of the breakout stage of the process. At this moment, the outlet of the hole is smaller than the desired due to severe radial tool wear, and the hole is not completed. As a result, further feeding is needed before the electrode fully penetrates the workpiece. To avoid back-strikes, the hole completion detection is crucial. However, the uncertainty of longitudinal and radial tool wear makes it difficult. This paper proposes a novel method to detect the hole completion by analyzing the discharge signals in the breakout stage. Besides, for the machining efficiency to become much lower in the breakout stage, the control strategy is also proposed to improve it by adjusting the reference voltage and gain factor of the servo controller. The optimal parameters were found by a full-factorial experiment. Experiments were conducted to validate the proposed methods by drilling holes at various inclinations. The results show that the proposed hole completion detection method could successfully sense the hole completion without misjudgments. And the electrode was fed at most 1.81 mm out of the hole outlet when the detection was made. Besides, the proposed control strategy improved the machining efficiency by up to 56.2% in the breakout stage in the experiments.

Experimental investigations of material removal rate on Mg/SiCp-flyash hybrid metal matrix composites by electrical discharge machining

The widening growth of the mechanical industries largely associated with enormous requirement of composite materials due to their light weight and enhanced mechanical properties. Electrical Discharge Machining (EDM) is an imperative modernistic machining technology employed for cutting difficult-to-machine and new age-band of materials. In this research work, Mg/SiCP-Fly-ash hybrid metal matrix composites (MMCs) was fabricated by powder metallurgical route and machined by EDM machining. The pulse on time, pulse off time and the applied current were the parameters used to study the material removal rate in EDM process. EDM drilling operations were conducted on an ELEKTRA PLUS spark erosion electrical discharge machine. 10 mm diameter brass tool was chosen to drill the MMC specimens. A mathematical model was computed to show a relationship of EDM machining process variables such as applied current, pulse on time and pulse off time on the MRR. The ANOVA (analysis of variance) method was performed to analyze the various input data on the output responses.

Minimum-variance self-tuning regulator in EDM drilling processes for ultra-high-aspect-ratio small holes

High speed small hole electrical discharge machining (EDM) drilling processes can easily become unstable, due to changing flushing conditions, inappropriate preset electrical parameters, and machining debris residues. Although there are already successful attempts in controlling EDM processes, cost-effective adaptive control of high speed small hole EDM drilling has not been reported. Process stability can contribute to the improvement of machining efficiency and surface finish. To maintain a higher process stability, a minimum-variance self-tuning regulator (MVSTR) is designed based on an auto-regressive moving average exogenous (ARMAX) model. To determine specifications of the system, the time and frequency responses are analyzed, and the validation is executed for the offline system model. A series of high aspect ratio holes drilling experiments with different electrode diameters of 0.3 mm, 1.0 mm, and 3.0 mm show that, with the use of the MVSTR control scheme, a higher machining efficiency and a lower tool wear can be achieved as compared with the original controller. Microscopic pictures of drilled holes and electrodes show better surface quality.

Super Dielectric Based EDM Process for Drilling of Inconel 718

ABSTRACT A noncontact machining process such as electrical discharge machining (EDM) is generally employed for machining electrically conductive materials irrespective of its hardness. However, the process becomes unstable and slow when deep features are to be machined. Furthermore, the standard dielectric-based EDM process is unable to meet the requirements of today’s fast moving manufacturing industry. Therefore, in this study the authors present the development of a novel super dielectric which is used in the EDM drilling process, and experimentally demonstrate how the machining efficiency in the drilling of a deep hole is improved significantly. In addition the effect of super dielectric fluid on the plasma current is presented and the experimental observations have been validated through the plasma physics theory. The efficiency of the super dielectric-based EDM process is experimentally validated with the successful drilling holes of aspect ratio more than 100 in Inconel 718 within 6 minutes.

Influence of Electrical Discharge Machining on Thermal Barrier Coating in a Two-Step Drilling of Nickel-Based Superalloy

The influence of electrical discharge machining (EDM) on thermal barrier coating (TBC) in a two-step drilling of nickel-based superalloy has been studied. The TBC was coated on the superalloy using electron beam physical vapor deposition (EB-PVD). Simulation result shows that the temperature at TBC/metal interface increases with pulse-on time and peak current. The temperature will exceed the melting point of the coating material when using relatively large pulse energy. Experimental result reveals that large pulse-on time or high peak current caused defect such as cracking at coating sidewall and delamination at the TBC/metal interface. For EDM drilling holes with inclined angle, the leading edge had worse condition of debris evacuation, and consequently caused secondary discharge which resulted in melt and spalling of the coating material. In addition, relatively larger pulse-on time and greater peak current might result in greater thermal stress gradient at the interface and subsequently lead to cracking or spalling at the coating layer. Therefore, process conditions of short pulse-on time, longer pulse-off time, and small peak current are recommended for EDM drilling of the EB-PVD coated nickel-based superalloy.

The effect of magnesium content on drilling of Al-Mg-Ti alloy by hole electrical discharge machining process

This study presents an experimental investigation of small hole electrical discharge machining of Al-Mg-Ti alloys. A series of drilling operations were carried out for exploring the effect of magnesium content. Holes of 2 mm diameter and 15 mm depth were drilled using tubular single-hole rotary brass electrodes. The rates of material removal and electrode wear, surface roughness, overcut, average recast layer thickness, taper height and angle were studied for Al-Mg-Ti alloys contain 2%, 4%, 6%, 8%, 10%, 12%, and 14% Mg. The results show that the material removal rate is increasing with increasing Mg content while the rate of electrode wear is almost unchanged. Due to decreasing the melting temperature of the Al-Mg-Ti alloy with increasing Mg content, more metal melts and vaporizes during electrical discharge machining drilling. Therefore, more overcut and taper, thicker white layer, and rougher surfaces were measured for higher Mg content.

Study on electrode vibration in the touch-down stage of fast electrical discharge machining drilling

Fast EDM (electrical discharge machining) drilling is broadly used in the manufacturing industry. When drilling inclined holes, the machining efficiency in the touch-down stage is much lower than that of drilling normal holes for a general-purpose fast EDM drilling machine. To uncover the reasons, this paper conducted observations of inclined hole drilling processes by using a high-speed video camera. It was found that the vibration of the tubular tool electrode was the major factor that led to the unstable state, resulting in low machining efficiency. And the vibration was driven by the discharge reaction force and the hydrodynamic force. The vibration enlarged the gap width and caused open circuit pulses, which misled the servo control system to feed the electrode downward. The overfeeding eventually caused short circuits and the subsequent retraction of the electrode. The iterative feeding and retraction of the electrode resulted in low machining efficiency. Then a fluid-structure interaction (FSI) model was built to analyze the vibration behavior. The impacts of the inner flow speed and the free length of electrode were studied with the model. It was found that the vibration is inevitable on general-purpose fast EDM drilling machines. Finally, to reduce the vibration impact, a new gap servo control strategy for the touch-down stage was proposed. The strategy adjusts the reference gap voltage by fuzzy rules. Experimental results showed that with the new gap servo control strategy, the performance was improved in the touch-down stage, with the machining efficiency improved by 38.75% and the area of discharge affected zone around the hole inlet reduced by 13.44%.

Self-tuning breakthrough detection for EDM drilling micro holes

In electrical discharge machining (EDM), accurate and timely detection of the breakthrough of the tool electrode is rather intricate when drilling though-holes due to considerable longitudinal tool wear. This paper proposes a novel method for detecting the completion of the drilling process using the cumulative sum (CUSUM) control chart. In this method, variations in the amount of normal discharge pulses per unit time are monitored for detecting the completion of the drilling process. The actual process conditions are sensed at the start of the drilling process an the control limit for detecting the hole completion is accordingly self-tuned. The robustness of the detection method to different process conditions is supported by a design of experiments study. Experimental validation has been carried out by drilling through micro holes at relative angles between the tool and workpiece of 30°, 45°, and 60°. The proposed method has been shown to guarantee the hole completion, while in the same time avoiding machining a surface situated 1 mm below the outlet of the drilled hole. This is critical for EDM drilling applications in the aerospace and automotive sectors.

Method of Machining Diffusors in Inconel 718 Turbine Blades for Film Cooling using EDM Drilling and Shaping

Electrical Discharge Machining (EDM) is a well-known process, which is mainly used to machine difficult-to-cut, conductive materials. Die-sinking EDM, wire EDM and EDM drilling are widely used in turbine blade machining. Optimization of EDM machining strategies is permanently aimed to achieve the highest precision in minimal machining time, and therefore, to reach better quality and higher efficiency. Reduction of the process chain, which is proposed in this work by performing more operations using a single machine tool, allows the reduction workpiece reclaiming and leads to positioning error minimization. The new strategy in EDM milling and shaping described in this work allows to machine complex geometries by using a standard cylindrical shape rotary electrode, which with some limitations can be used instead of the complex die-sinking electrode. This new strategy is used for complex-shaped diffusors machining in Inconel 718 turbine blades. The comparison between state-of-art layer-by-layer milling strategy and the new method is shown. Correlation of the electrode wear pattern after EDM drilling and EDM milling with the geometry of the machined cavities is applied. The presented new method is more than 15% faster in comparison with the layer-by-layer machining strategy, whereas has higher geometrical accuracy at the same time.

Investigation of Advanced Control Schemes for High-speed Small-Hole EDM Drilling

High-speed small-hole electrical discharge machining (EDM) drilling is extensively applied to machine cooling film holes of turbine blades and diesel nozzle spray holes due to its non-contact characteristics and dedicated features, e.g. high-speed rotation and high-frequency pulse power generator. However, with increasing drilling depth, the process stability could be easily deteriorated by frequent debris due to restricted flushing conditions. Consequently, the machining efficiency and surface finish of high-aspect-ratio holes cannot be guaranteed. To address this problem, this paper proposes a sliding mode and self-tuning control scheme to regulate the discharging process. An integral function is presented as the switch mode to avoid chattering, and a direct algorithm that reparameterizes the model in terms of controller parameters is considered to reduce computation. The presented comparative drilling experiments with high aspect ratio (nearly 100) show the benefits of the adaptive controller.