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Multi-axis electrical discharging machining (EDM) is the most commonly used manufacturing method for shrouded blisks. The discharge points and discharge gap distance are time-varying in a multi-axis EDM process. It is difficult to obtain an optimal servo feedrate. In traditional multi-axis EDM servo control systems, servo sensitivity is usually chosen as a smaller constant to avoid arc discharges. Thus, the servo feedrate is lower than the material removal rate in a machining process. The open circuit rate is high and the machining efficiency is decreased. This paper proposes an adaptive servo sensitivity control algorithm for multi-axis EDM of shrouded blisks. Servo sensitivity is adjusted according to discharge status, so as to decrease the open circuit rate and to improve the machining efficiency. Experiment results show that the proposed adaptive servo sensitivity control algorithm can improve the machining efficiency of shrouded blisks by 28.02% without deteriorating surface integrity. [ABSTRACT FROM AUTHOR]

Observations of the discharge gap were conducted to investigate the discharge plasma motion in reciprocated traveling wire electrical discharge machining (RT-WEDM). It was found that a discharge plasma continuously slides on the surface of a workpiece. An oscillating moving heat source is proposed to better describe the characteristics of discharge plasma motions. Based on an oscillating moving heat source, a novel thermal-fluid coupling model by adopting a level-set method is proposed to investigate the formation of a crater and the effects of machining parameters on craters. Discharge plasma sliding, latent heat, and molten pool forces are comprehensively considered in the proposed model. Simulation and experimental results show that a sliding discharge plasma has a significant effect on a crater. At the end of a discharge, a considerable proportion of molten materials remains in the molten pool. A high sliding speed of discharge plasma can lead to a low aspect ratio in a crater and a thin recast layer. Due to discharge plasma sliding, an increase in pulse duration can significantly increase the length of a crater and decrease the aspect ratio, while an increase in peak current can significantly increase the depth and volume of the crater. Simulation and experimental results also show that morphologies of the simulated crater and experimental craters are consistent, verifying the feasibility and accuracy of the proposed simulation model in explaining the mechanism of crater formation in RT-WEDM. [ABSTRACT FROM AUTHOR]

This paper presents a wire breakage prevention method and a corresponding system for reciprocated traveling wire-EDM based on discharge location detection. The wire breakage risk is evaluated using the consecutive discharge concentration coefficient (CDCC) value and the number of arcs. CDCC characterizes the proximity of a sequence of consecutive discharge locations occurring in a short time, which takes both time and spatial domain into consideration. The system employs two fuzzy logic controllers to adjust machining parameters according to the degree of discharge concentration and the servo tracking performance detected. Verifications are conducted by cutting two workpieces with variable heights, including a stair-shaped and a hollow one. The machining tests show that the proposed system can significantly gain machining efficiency without wire breakage hazards. [ABSTRACT FROM AUTHOR]

Fast electrical discharge milling (fast ED-milling) has become a promising technology in the manufacturing industry for machining complex structures such as diffuser-shaped film cooling holes. However, the mechanism of the efficient removal of materials in this technology is not yet fully understood. To gain a further insight into this matter, an experimental investigation on the morphology of discharge craters including the absolute material removal volume per discharge, material residual volume per discharge, and directionality, is firstly carried out. The obtained results imply that a high-pressure inner flushing can significantly promote the expelling of molten material from a molten pool and is a fundamental reason why fast ED-milling can be of higher machining efficiency than regular ED-milling. To explore the mechanism behind, a novel thermal-fluid coupling model is developed to simulate the evolution process of the molten material under the effect of a flow field. The results of numerical simulation show that during a discharging, the molten material moves along the workpiece surface towards the outlet of a gap channel and solidifies at the side of a crater that is away from the electrode center. Another interesting finding is that an inappropriately high flushing pressure can result in a low machining efficiency because the severe heat convection will consume a large part of the heat generated by a discharge. This well explains the phenomenon that occurred during the experimental investigation. [ABSTRACT FROM AUTHOR]

The gap discharge state monitoring system for wire electrical discharge machining (WEDM) has an important impact on machining performance. Due to sampling noise and the slight differences between different types of discharge pulses in reciprocating traveling WEDM, it is easy to misjudge discharge pulse types based solely on the gap voltage and current in the conventional discharge state monitoring system. To provide more accurate feedback signals and achieve better machining performance, this study proposes to monitor gap discharge states using radio frequency signals. Radio frequency signals are analyzed in both the frequency and time domains. After determining the range of radio frequency signals through frequency domain analysis, radio frequency signals in a specific frequency range are amplified after filtering out noise and undesired signals. In addition, for sampling convenience, the radio frequency signal is processed using an envelope detection circuit for frequency down-conversion. In the time domain analysis, the available radio frequency signal is obtained and analyzed for different types of discharge pulses. Based on an ISO-pulse power supply, the characteristic analog radio frequency signal is used to classify the discharge pulses. The radio frequency signal monitoring system is then used to provide feedback signals for the servo feed rate control system. The experimental results indicate a substantial improvement in the classification accuracy. The material removal rate is improved by about 10% in roughing, and the surface quality is improved slightly. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In multi-axis electrical discharging machining (EDM), to maintain a stable discharging gap, a gap width controller regulates the feedrate of a tool electrode with respect to a workpiece. However, in the existing EDM machine, feedrates in G-codes are computed with an extended linear displacement which takes 1∘ as 1 mm and does not take the diameters of rotational axes into account. To adjust feedrates with actual distances between an electrode and a workpiece, kinematics from the tool frame to the workpiece frame is needed. In this paper, screw theory is utilized to derive the kinematics of a six-axis EDM machine consisting of three translational axes and three rotational axes. With the rigid transformation of each axis described by an exponential of a twist, the overall kinematics is obtained as a product of exponentials of twists. With the forward kinematics, the feedrate post-processing algorithm (FPPA) is used to plan feedrates in G-code blocks. Experimental results showed that the feedrate planning according to the forward kinematics can reduce the machining time by 23.5% in the EDM for a shrouded blisk. Discharge gap statistics showed that the use of the FPPA algorithm increases the normal discharge ratio. [ABSTRACT FROM AUTHOR]

Multi-axis electrical discharging machining (EDM) is the main manufacturing approach for shrouded blisks. To maintain a stable discharging, the feedrate of an electrode must be regulated appropriately with respect to a workpiece. The feedrate control in multi-axis EDM is a challenge due to two facts. The first is that the feedrate fluctuations occur at line segment junctions along the feeding trajectory which is expressed in G01 codes. The second is that, with involvement of rotary axes, the feedrate control must consider the nonlinear mapping between the workpiece frame and the machine frame. This paper proposes an extended unit arc length increment interpolation (UALII) for generalized NURBS curves. Given the point coordinates in the machine frame, the coordinates in the workpiece frame can be obtained by the forward kinematics. By using the UALII, the curve parameter is first obtained. To achieve synchronization among involved axes, the curve parameter is substituted into the generalized NURBS curve. Due to the nonlinear mapping, each interpolation of one basic length unit (BLU) can give the axial increments which are more than one BLU. Digital differential analyzer (DDA) is used for a finer interpolation for each axial increment. Machining tests show that the use of UALII for generalized NURBS curves can decrease the machining time by 42.19%, 29.65% and 26.42%, respectively, as compared with the cases of no feedrate planning, the weighted average movement ratio algorithm (WAMRA) and the feedrate post-processing algorithm (FPPA). The extended UALII algorithm can also reduce the electrode wear. • Data points in workpiece frame computed from G-codes by six-axis kinematics. • Unit arc length increment interpolation (UALII) used for NURBS curve. • Synchronization achieved by using the same parameter for two NURBS curves. • Machining time and electrode wear decreased by extended UALII. [ABSTRACT FROM AUTHOR]

Shrouded blisks play a vital role in aero and rocket engines. Multi-axis EDM is regarded as the most commonly used manufacturing method for shrouded blisks. For a linear jump path, the jump motion only needs to consider the starting and end point of the jump and there is no direction change during the jump. However, when dealing with multi-axis jump motions, the path of a jump motion is a curve which is represented by a number of line segments. For this case, the jump speed of an electrode will change abruptly in terms of direction and magnitude during the jump motion. In this paper, multi-axis corner rounding solution (MCRS) is proposed to smooth multi-axis jump paths. Two three-dimensional subspaces, a linear subspace and a rotary subspace, are built to describe six-axis jump paths for a six-axis EDM machine tool. A three-axis jump path smoothing solution can be applied for the subpath in each subspace. By using a bidirectional scanning method as well as an S-shaped profile, the smoothness of the multi-axis feedrate schedule is obtained. Simulation results show that the MCRS achieves a smoother scheduled feedrate profile as well as a higher efficiency. Experimental results show that the machining time can be decreased by 19.68% with the use of the MCRS. [ABSTRACT FROM AUTHOR]

As compared to traditional machining approaches, multi-axis electrical discharge machining (EDM) has its unique advantages in machining components which are made of difficult-to-cut materials and have complex structures, such as shrouded blisks. Manufacturing processes of such complex parts involve synchronized movements of both linear and rotary axes. Due to different dimensions of angular and linear velocities, there is a discrepancy between demanded and actual feedrates. This feedrate discrepancy, on the one hand, can cause feedrate fluctuations leading to a machining instability, and on the other hand, can cause inaccurate heights of electrode jumps. In order to ensure the machining stability, this paper proposes a feedrate planning method for multi-axis EDM of shrouded blisks. In this method, CAD models of both an electrode and an shrouded blisk are utilized to extract characteristic radii for feedrate planning in G-code generations. A feedrate post-processing algorithm for multi-axis EDM for shrouded blisks is proposed to reduce feedrate fluctuations and achieve a proper electrode jump height. Data analysis demonstrates the resultant feedrate can be kept in a proper range. In comparative multi-axis EDM for shrouded blisks, the machining time has been reduced by up to 21.43 % by using the proposed feedrate planning method. [ABSTRACT FROM AUTHOR]

Gap voltage can be used as an indicator on the direction of the electrode movement along a desired tool path in electrical discharging machining (EDM) processes. However, due to the noise induced by electrical discharges, the estimation of gap voltages is difficult due to the lack of an appropriate state space model. In this paper, gap voltage signals are considered to be generated as a summation of colored noise through a linear filter and measurement noise. Obtained by the Yule–Walker auto-covariance method, the transfer function of the linear filter can be converted into a state space model. The composite process noise and the composite measurement noise are defined to derive the composite noise covariance matrices. A Kalman filter can thus be designed based on the state space model and the noise covariance matrices. Experimental results showed that, as compared with the traditional 10-point moving average filter, the Kalman filter can decrease the average machining time as well as improve the discharging gap status. [ABSTRACT FROM AUTHOR]

Abstract: The model-based Taylor series expansion error compensation (TSEEC) method has been shown to be capable of eliminating contour errors in biaxial CNC machines. However, it has been developed for only linear and circular contours. In this paper, an extension of TSEEC, a generalized TSEEC or GTSEEC, is proposed which is capable of compensating for the contour errors of arbitrary two-dimensional contours. A new real-time algorithm is used to estimate the contour error so that GTSEEC can be applied even in cases where the function of the contour is unknown. Simulation results using linear, circular and parabolic contours are presented which show that, with perfect knowledge of the axial dynamics, GTSEEC can perfectly eliminate the contour errors for any contour, even when there is significant mismatch in the axes'' dynamics. Experiments on a small two-axis computer-controlled machine indicate the effectiveness and better performance of GTSEEC in reducing contour errors for free-form contour following tasks when compared with two other leading approaches, the zero phase error tracking control (ZPETC) and cross-coupled control (CCC). [Copyright &y& Elsevier]

In this paper, a new model-based Taylor series expansion error compensation (TSEEC) strategy is proposed to improve the contouring accuracy for computer numerically controlled (CNC) machines. In TSEEC, the contour error compensation problem is formulated as a Taylor series expansion problem, in which the value of the contour error is expanded around the reference points and the compensation components are calculated as the deviations from the reference points. Simulations show that, with perfect knowledge of the axial dynamics, zero contour errors can be achieved with TSEEC for both linear and circular contours. Due to modeling errors, external disturbances, and measurement noise, some modifications and experimentation need to be made to determine suitable parameters for implementation of the TSEEC scheme on a real machine. These measurements include a low-pass filter, a choice of a compensation target, and a compensation gain. Experimental results show the effectiveness of TSEEC in reducing contour errors and demonstrate the superiority of TSEEC over inverse feedforward compensation and cross-coupled control in improving the contouring accuracy. [ABSTRACT FROM AUTHOR]

Abstract: Contour errors usually result from unmatched dynamics, limited axial bandwidth and undesirable nonlinearities. For circular contours, friction, especially static friction, deteriorates the circular contouring accuracy at quadrant positions where there is reversal of velocities or motion from standstill. A two-stage tracking error-based static friction compensation scheme is proposed to compensate for static friction during the transition from the presliding regime to the sliding regime. The compensation consists of two phases, which are implemented sequentially in time. The first is a function of tracking error and the second is a function of tracking error and velocity. The compensation signals are then imposed at the input of the velocity control loop. Experimental results show that the two-stage tracking error-based static friction compensation scheme can effectively improve the circular contouring accuracy at quadrant positions without complicated modeling. [Copyright &y& Elsevier]

Abstract: In this paper, an integral sliding mode controller (ISMC), based on input–output models, is proposed as a refinement from a two-degree-of-freedom controller with independent objectives for tracking and regulation. Thus, the knowledge of pole placement can be utilized in ISMC. The robustness is improved by a disturbance estimation, which results in an equivalent control. To eliminate the problem of chattering, two measures adopted are an appropriate choice of the sliding surface and an integral control action. It was found that the choice of a slower natural frequency of the sliding surface dynamics than that of the open loop can ease the problem of chattering. The choice of k i , the integral control coefficient, is based on the two-degree-of-freedom controller. Root locus is used in assisting in choosing an appropriate value of k i to ensure the closed-loop stability. The proposed ISMC was implemented and experimentally tested in a mini-CNC machine. The contouring accuracy of the mini-CNC machine was greatly improved by the proposed ISMC. Furthermore, no chattering was observed, which is beneficial to machine actuators. [Copyright &y& Elsevier]

Abstract: Contour errors in continuous-path computer numerical control (CNC) systems usually result from unmatched axial dynamics and undesirable nonlinearities. The errors due to unmatched dynamics can be largely reduced by using a simple controller so as to keep the order of axial dynamics low and by tuning appropriately the proportional gains to match the dynamics. Experimental results show that the resulting matched axial dynamics can, on one hand, effectively eliminate the linear contour errors, and, on the other hand, largely reduce the circular contour errors due to unmatched axial dynamics. On the basis of the matched axial dynamics, circular contours will be made circular with accuracies greatly improved. However, because of limitation in the bandwidths of the axial dynamics, there will be radial contour errors. The radial contour error can be further reduced by a simple approach of feedforward compensation. Experiments conducted show the effectiveness of feedforward compensation in improving circular contouring accuracies. [Copyright &y& Elsevier]

Abstract: Some industrial and scientific processes require simultaneous and accurate control of temperature and relative humidity. In this paper, support vector regression (SVR) is used to build the 2-by-2 nonlinear dynamic model of a HVAC system. A nonlinear model predictive controller is then designed based on this model and an optimization algorithm is used to generate online the control signals within the control constraints. Experimental results show good control performance in terms of reference command tracking ability and steady-state errors. This performance is superior to that obtained using a neural fuzzy controller. [Copyright &y& Elsevier]

Deep narrow slots (DNSs) are typical difficult-to-machine structures. At present, EDM is the main machining method of DNSs. However, due to the difficulty in evacuating discharge products, machining of DNSs becomes difficult and time-consuming when the machining depth grows. Preliminary studies have shown that certain machining attitudes have a higher machining efficiency than the traditional downward-feeding machining attitude during a fixed attitude machining. To further accelerate the evacuation of discharge products and improve the machining efficiency of DNSs, a dynamically variable attitude EDM machining method for DNSs is proposed. The simulation of motions of discharge products in the gap flow field shows that the dynamically variable attitude EDM machining method can promote the evacuation of discharge products. Experiments are carried out to investigate the effect of different process parameters in the dynamically variable attitude method on the machining efficiency of machining DNS structures. The results show that the dynamically variable attitude method has higher machining efficiency than the fixed attitude machining method. The machining time under the dynamically variable attitude method is shortened by 13.02% compared with the horizontal fixed attitude machining and 19.04% shorter than the traditional downward-feeding machining. To find out the reasons for the improvement of the machining efficiency caused by the dynamically variable attitude method, gap states during the whole machining process are collected and analyzed. [ABSTRACT FROM AUTHOR]

Film cooling is an effective technology for improving the thrust-to-weight ratio of an aero-engine. The axial orientation of film cooling holes is one of the key factors determining the film cooling effect. However, the machining quality of cooling holes is still carried out by manual inspection, whose accuracy and consistency cannot be guaranteed. To address this problem, this paper proposes an axial orientation extraction algorithm based on laser point cloud processing. Firstly, the blade surface is scanned by a line laser sensor and a blade point cloud is obtained. Secondly, a single film hole can be segmented from the blade point cloud based on scanning line fitting residual. Thirdly, an improved Gaussian map is to complement the arc belt obtained by Gaussian map to obtain a complete circle along an error-sensitive direction. Finally, the axial orientation is estimated by random sample consensus with an initial value. The accuracy and consistency of the algorithm are verified by comparing the extraction result by the algorithm with those by the Geomagic software. The result shows that the axial accuracy extracted by the algorithm can reach 0.405°. [ABSTRACT FROM AUTHOR]

With variations in gap discharge state or workpiece height, the servo feedrate of wire electrical discharge machining (WEDM) often fluctuates, and the machining process easily becomes unstable, thereby resulting in a decrease in material removal rate or an increase in risk of unexpected wire breakage. With developments of ISO-pulse discharge pulse power supplies and reciprocating traveling WEDM, a precise discharge state monitoring system for the servo federate control system is required. In this paper, with each single discharge pulse being precisely classified and counted, more accurate feedback information can be provided to the servo feedrate control system. Then a new adaptive servo feedrate control system with a self-tuning regulator and a variable gain linear proportional controller has been proposed to improve the material removal rate and control performance. The scheme of an adaptive servo feedrate control system is analyzed. It is found that the new adaptive servo feedrate control system by using a variable gain linear proportional component has a better control performance. By adjusting servo parameters, the discharge ratios are regulated, and the servo feedrate control system can keep the gap discharge state stable. Experimental results show that the material removal rate can be improved significantly by up to 19% with the new adaptive servo feedrate control system, without sacrificing the surface quality. [ABSTRACT FROM AUTHOR]

As a typical non-traditional machining method, reciprocated traveling wire electrical discharge machining (RT-WEDM) has been widely used in mold and die manufacturing industry. However, at present RT-WEDM still suffers from the problem that a large portion of molten material remains in a molten pool after a discharge, due to a relatively long discharge duration. This phenomenon limits the improvement of material removal efficiency and resulting in a thicker recast layer. To facilitate the expelling of the molten material in RT-WEDM, a two-stage discharge current regulation method is proposed in this study, which includes oscillating-current regulation (OCR) during discharge duration and explosive-current regulation (ECR) right before the end of the discharge. A pulse power generator with discharge current regulation modules is developed, and the control principle for generating an oscillating current and an explosive current is elaborated. Regulation parameters are determined based on an analysis of discharge plasma behavior. Both single-pulse discharge experiments and consecutive-pulse machining experiments are carried out to verify the effect of the proposed two-stage regulation method. The results show that the material removal rate (MRR) has been increased by 16.5% and the average thickness of recast layer on the workpiece surface has been reduced by about 15%. [ABSTRACT FROM AUTHOR]

This paper proposes a novel non-circular parametric curve interpolation algorithm based on the unit arc length increment method (UALI). In this method, axial coordinates are taken as functions of arc length. The reference point moves by 1 basic length unit (BLU) along a curve in each interpolation period, and the corresponding projections on each coordinate are added into accumulators of each axis respectively. An axis will then move by 1 BLU when its accumulator exceeds 1 BLU. Theoretically, based on the UALI method, an interpolation algorithm is derived for parametric curves such as involutes, Archimedes spirals, cycloids, and parabolas. This interpolation algorithm is applicable for ruled surfaces of a workpiece with different parametric curves in the upper and lower planes. Tool compensation is considered for both non-tapered workpieces as well as tapered workpieces. Simulation results and experiment results demonstrate the advantages of this interpolation algorithm, and the accuracy of this interpolation algorithm is analyzed. [ABSTRACT FROM AUTHOR]

Multi-axis electrical discharging machining (EDM) is the most commonly used manufacturing method for shrouded blisks. However, debris is difficult to be evacuated from a discharge gap, severely affecting the machining efficiency. Spatial planetary movements of a tool electrode can generate a non-uniform flow field in a discharge gap, promoting the evacuation of debris. This paper proposes a spatial planetary movement method for EDM of inclined holes. Screw theory is used to derive the kinematics of a spatial planetary movement, and the coordinate increments of each axis in an interpolation period can be calculated. The trajectories of the spatial planetary movements are simulated. The reliability of the method is confirmed by the simulation. Then, a multi-axis EDM machine can be controlled to drive a tool electrode to realize the spatial planetary movement. Machining experiments are carried out to explore the influences of the spatial planetary movement parameters on the machining performance of multi-axis EDM. Experiment results show that the increase in the amplitude of spatial planetary movements can improve discharge rate and gap status. The machining depth and machining efficiency of multi-axis EDM can be increased ulteriorly. And the speed of spatial planetary movements has no significant effect on the performance of multi-axis EDM. [ABSTRACT FROM AUTHOR]

In computer numerical control (CNC) systems, parametric curves can be used instead of a large amount of linear blocks to describe tool paths for freeform surface or curve machining. However, existing parametric curve interpolation methods may cause large feed fluctuations or even a failure of the machining process near the sharp corners of a parametric curve. Therefore, a parametric curve interpolation method with an error correction and failure prevention scheme is required. In this paper, the augmented Taylor's expansion (ATE) method for computing B-spline curve parameters is proposed. A group of calibrators consisting of the knots and the arc lengths between adjacent knots are pre-computed before the interpolation starts. The parameter is computed based on Heun's method in a prediction–correction manner, and the accumulated errors caused by the cut-off errors of Taylor's expansion are eliminated by the calibrators at the knots. To cope with the extreme cases that usually occur near the sharp corners of a curve, a linear parametric interpolation between the previous parameter and its next calibrator is carried out when Heun's method fails to obtain a parameter in the domain. Simulation and experimental results show that, when the arc length increments are kept small enough near the sharp corners, the ATE method attains high accuracy and robust computation. The proposed method is also applicable to the NURBS curves. [ABSTRACT FROM AUTHOR]

Deep narrow slot (DNS) structures are commonly used in slots for inserts in tire molds, die-casting molds of heat-dissipating fins, grooves in flexible joints of gyros, and seal slots in turbine blades. Electrical discharge machining (EDM) of DNSs is a time-consuming process, because traditional EDM machines adopt a vertical downward feed direction to remove materials, resulting in debris deposition in the gap by means of gravity. In this paper, systematic experiments are carried out to investigate the effect of different feed directions on the machining efficiency of EDM DNS structures. Thirteen feed directions with an incremental angle of 15° from −90 to 90° have been tested without electrode jumps. Among them, the feed directions of 0° and 15° have the highest machining efficiency, which even exceeds the machining efficiency achieved with electrode jumps. A machining efficiency model with respect to feed direction is built based on experimental results. The gap discharge state is acquired by an FPGA circuit based on discharge state analyzer, associated with machining efficiency curves. For most feed directions, the discharge rate drops significantly after the machining depth around 4 mm, leading to an obvious turning point in the machining efficiency curve. To explore the reasons for machining efficiency changes during the machining of DNS, the movements of bubbles are observed by a high-speed camera for calculating their speeds and sizes. [ABSTRACT FROM AUTHOR]