Workpiece Height Research Articles

Online estimation of workpiece height by least squares support vector machines in wire electrical discharge machining

When machining workpieces with complicated and intricate shapes by wire electrical discharge machining, the workpiece height usually varies along a machining path. To ensure a stable and efficient machining, the machining parameters must be appropriately tuned based on the estimated workpiece height. Though an offline workpiece height estimation model can be established by traditional support vector regression with a satisfactory accuracy, the offline model is unable to cover all possible machining conditions. In this paper, least squares support vector machine is proposed to build up online a workpiece height estimation model. Due to the use of equality constraints in the formulation of least squares support vector machine, all data points are treated as support vectors, thus sparsity is lost as compared with traditional support vector machine. For online applications, a low computational load as well as a limited memory storage are required for an online algorithm. To meet the requirements of online workpiece height estimation, two measures are thus adopted. One is the use of a projection method which measures the relevance of a new data point with existing basic vectors by calculating its residue. The residue is used as criteria for admission of the new data point as a new member of the basic vector set. The other is the restriction on the size of the basic vector set. Removal of an insignificant basic vector is determined by its contribution to the model, which is measured by its coefficient in the model (or support value). Experimental results show that, by online learning, the estimation model can achieve an estimation error less than 2 mm at smooth parts of a workpiece. Based on the estimated workpiece height, proper machining parameters can be set and a stable machining can be achieved.

Workpiece height estimation in wire electrical discharge machining by using support vector regression

The risk of wire breakage adversely affects the full potential of wire electrical discharge machining since the overall process efficiency is considerably deteriorated. Online estimation of workpiece height can avoid wire breakage effectively as the estimated height can provide a reference for setting machining parameters so as to improve the machining speed and stability. In this article, a discharge frequency monitoring system based on voltage waveforms of wire electrical discharge machining is developed, and support vector regression is used to build up a model for online estimation of workpiece heights in wire electrical discharge machining. Experimental results show that besides at stairs of a workpiece where the value of estimation errors can reach up to 6 mm due to the poor flushing condition in machining, the developed model can successfully estimate the workpiece height with an estimation error of less than 2 mm. Based on the estimated workpiece height, the discharge frequency can be controlled at an optimal value without the risk of wire breakage when cutting a workpiece with a variable height.

On-line Workpiece Height Estimation and its Application in Servo Feed Control of WEDM Process

On-line estimation of workpiece height is essential for wire breaking prevention in WEDM process. In addition, it can provide the reference of machining parameters setting for increasing machining speed and stability. A new on-line WEDM workpiece height estimation system for the iso-energy type wire EDM machine is presented. Based on the proposed specific discharge energy (SDE), a mathematical model relating workpiece height, feed rate and discharging frequency by a proportional constant is derived. This constant coined thickness identification coefficient (TIC) is found a function of workpiece height. Hence, a simplified approach is proposed. The value of TIC of a known workpiece height is taken to estimate the workpiece height during the WEDM process. The actual workpiece height is obtained by multiplying the estimated value by the correction factor of the associated workpiece height. Experimental results show that the system works satisfactorily. The estimation error of workpiece height is within 1mm while the response time is less than 1 second in general. In the practical application of machining a workpiece with dynamic variations in thickness, a model relating optimum servo voltage and workpiece height that leads to maximum machining speed is established in advance. The servo voltage is adjusted according to the estimated height. It is found that the process can be maintained very stable at maximum machining speed without wire breaking.

Applications of acoustic mapping in electrical discharge machining

The spatial distribution of discharges in electrical discharge machining (EDM) comprises valuable process information, which is not accurately obtained from electrical signals that are utilized extensively for process monitoring and control. This research hence explored the application of acoustic emission (AE) to map the discharges, in consideration of the acoustic time lag. In particular, the work refers to realistic process conditions, wherein AE from successive discharges cause repeated signal interference, which is detrimental to reliable time lag estimation. The applications of this capability for the respective identification of electrode length and workpiece height in fast-hole EDM and wire EDM are presented.

Influence of workpiece height on the induction heating process

A set of 2D steady state finite element numerical simulations of electromagnetic fields, eddy currents distribution and induction heating pattern was performed for different heights of a metal workpiece. Comparison between the calculation results show the height of the workpiece body with respect to the induction coil has a marked effect on the electromagnetic field distribution, eddy currents profile, heating structure and coil efficiency in the system. If the workpiece height is equal to or less than the coil height, then overheating at the workpiece ends is high and makes a nonuniform heating pattern along the workpiece wall.

Numerical evaluation of transient thermal loads on a WEDM wire electrode under spatially random multiple discharge conditions with and without clustering of sparks

Thermal load on wire electrode under randomly located multiple discharge condition is the most important consideration for predicting wire breakage in wire electrical discharge machining process. Sometimes the discharges form clusters as observed experimentally by different researchers and may occur because of inadequate evacuation of the debris generated during each discharge. Formation of clusters is more likely in thick work pieces. Clusters are spread randomly along the wire while sparks in each cluster too are random. Such clustering of sparks enhances the intensity of thermal load on the wire. In the present investigation, a one-dimensional explicit finite-difference thermal model is proposed for estimating the transient temperature distribution along the length of the wire under the conditions of randomly located spatial sparks with and without the formation of clusters. While each of the electric discharges is simulated as a volumetric heat source present within the wire over the discharge channel width, which in turn is calculated from the available literature, the successive sparks and cluster of sparks are located on the wire by means of a random function. The predicted values of maximum wire temperatures indicate the degree of wire rupture risk, which has been found to be different for short and long elapsed times. Accordingly, random pulse and clusters models are suggested for predicting thermal loads while machining thin or thick work pieces, respectively. The effects of work-piece height, power input, pulse frequency, duty factor, wire velocity, wire diameter, and the convective heat transfer coefficient have been reported. The one-dimensional thermal models may be used for setting rules of selection for an expert system for the safe operating conditions of wire electro-discharge machining.

Monitoring and control of the micro wire-EDM process

A new pulse discriminating and control system has been developed for process monitoring and control in micro wire-EDM. The pulse discriminating and control system identifies four major gap states classified as open circuit, normal spark, arc discharge and short circuit based on the characteristics of gap voltage waveform. The effect of pulse interval, machining feedrate and workpiece thickness on the variations of the proportion of normal spark, arc discharge and short circuit in the total sparks (defined as normal ratio, arc ratio and short ratio, respectively) were investigated. It is found that a long pulse interval results in an increase of the short ratio under a constant feedrate machining condition. A high machining feedrate or an increase of workpiece height results in an increase of the short ratio. To achieve the stability of the machining operation, a control strategy is proposed by regulating the pulse interval of each spark in real-time according to the identified gap states. Experimental results indicate that the developed pulse discriminating and control system can significantly reduce the arc discharge and short sparking frequency as well as achieve stable machining under the condition where the instability of machining operation is prone to occur.

A neural network approach for the on-line estimation of workpiece height in WEDM

In this paper, a feed-forward neural network is used to estimate the workpiece height and distinguish the machining condition in wire electrical discharge machining (WEDM). Some experiments have been carried out to verify the effectiveness of this approach. Based on the on-line estimated workpiece height, a rule-based strategy is proposed to maintain optimal and stable machining. According to the rule-based strategy, servo voltage and power settings can be adjusted correctly to suit the workpiece profile. Experimental results demonstrate that high machining efficiency and stable machining can be achieved by means of the rule-based control strategy.

On-line Estimation of Workpiece Height by Using Neural Networks and Hierarchical Adaptive Control of WEDM

Wire breakage and unstable machining drastically reduce the machining efficiency and accuracy in wire electrical discharge machining (WEDM). When a stair-shaped workpiece is machined, poor electrolyte flow around the steps leads to wire rupture or unstable machining. This paper presents a WEDM adaptive control system that maintains optimal machining and improves the stability of machining at the stair section where workpiece thickness changes. A three-layer back propagation neural network is used to estimate the thickness of a workpiece. The developed adaptive control system is executed in the hierarchical structure of three control loops, using fuzzy control strategy. In the first control loop, the total sparking frequency is controlled within a safe level for wire rupture suppression. In the second control loop, the proportion of abnormal sparks is maintained at a pre-determined level for process control purposes. Based on the estimated thickness of a workpiece, adaptive parameter optimisation is carried out to determine the optimal machining settings and to provide the reference targets for the other two control loops. Experimental results demonstrate that the workpiece height can be estimated by using a feed-forward neural network. The developed adaptive control system results in faster machining and better machining stability than does the commonly used gap voltage control system.

Effect of gas sparging on the rate of mass transfer during electropolishing of vertical plates

Rates of electropolishing of vertical copper plates in phosphoric acid using a gas sparged rectangular cell were measured under different conditions of superficial gas (N 2) velocity, workpiece height, physical properties of the solution and macroroughness of the surface. The limiting current was found to increase with increasing N 2 superficial velocity to the power of 0.24 while increasing workpiece height ( L) was found to decrease the limiting current ( I) according to the equation, I= aL −0.19. The data were correlated by the equation j=0.235(Re Fr) −0.253 L d e −0.19 Where j, Re and Fr are the mass transfer j factor, Reynolds number and Froude number, respectively. Macroroughness was found to increase the limiting current by a modest amount depending on the peak-to-valley height of the roughness elements. The economic importance of using gas sparging in mass transfer controlled metal finishing processes is pointed out.

WEDM-Adaptive Control with a Multiple Input Model for Indentification of Workpiece Height

The variation in power density during wire electrical discharge machining (WEDM) results in reduced productivity and wire rupture. This paper presents a WEDM adaptive control system that optimizes on-line the sparking frequency by estimating the workpiece height with a new multiple input model. This model describes the dynamic and stochastic relationships between average gap feedback voltage and spark frequency and machine table feed rate. Extensive experiments have been carried out to verify the new model. The proposed system correctly identifies the workpiece height and avoids the sudden rise in the estimated height found in existing system.

Monitoring and self-learning fuzzy control for wire rupture prevention in wire electrical discharge machining

Wire breaking is a serious problem in the application of wire electrical discharge machining (WEDM). A WEDM sparking frequency monitoring and control system based on the characteristics of the voltage waveform of WEDM is developed. A new self-learning fuzzy controller is proposed to control the sparking frequency at a safe level by regulating the pulse off-time in real time for avoiding wire rupture and maintaining a state of high metal removal rate. The developed control strategy is tested under the conditions of cutting a workpiece with continuous sharp angles, a change in workpiece height during machining process, and machining with a high feed-rate. Experimental results show that this monitoring and control system can control the sparking frequency at a predetermined level without the risk of wire rupture.

WEDM Identification and Adaptive Control for Variable-Height Components

Wire rupture in wire EDM is caused by high temperature due to high power density determined by discharge frequency and workpiece height. This paper reports a new WEDM adaptive control system that monitors and controls the spark frequency according to the on-line identified workpiece height. The workpiece height is estimated by using a non-linear discrete model relating cutting speed with spark frequency. This system controls the spark frequency at optimal levels to avoid wire rupture and to maintain the maximum productivity. The error of the workpiece height identification is 1 mm and the response to changes in workpiece height is 1 second.

Elementary theory of hot rolling

This article deals with the elementary theory of hot rolling. The well-known equation of state of strain-hardening creep theory is taken as a basis. The plane-section hypothesis and the assumption of the uniformity of strains and stresses across the workpiece height are used. Formulae were obtained to determine the contact pressure and the shear stress.

A stochastic explanation of some structural properties of a ground surface

A stochastic theory of grinding is advanced from which some important statistical properties of the resulting ground surface are derived in terms of the basic parameters and dimensions of the grinding wheel used. If a Gaussian abrasive height distribution is assumed it is shown that the workplace height distribution will be negatively skewed. Use of a more rigorous height distribution based on the physical structure of the abrasive predicts that the workpiece height distribution will initially be skewed but will tend to Gaussian with further grinding. The model explains the reduction in roughness of the ground surface when the wheel is sparked out and permits its numerical prediction. Good agreement is found between theory and experiment.

Bearings for tough jobs : Steel, v. 140, Apr. 8, 1957, p. 114 + 3 pages

The spatial distribution of discharges in electrical discharge machining (EDM) comprises valuable process information, which is not accurately obtained from electrical signals that are utilized extensively for process monitoring and control. This research hence explored the application of acoustic emission (AE) to map the discharges, in consideration of the acoustic time lag. In particular, the work refers to realistic process conditions, wherein AE from successive discharges cause repeated signal interference, which is detrimental to reliable time lag estimation. The applications of this capability for the respective identification of electrode length and workpiece height in fast-hole EDM and wire EDM are presented.