Tzu-Yao Tai

Relationship between electrode size and surface cracking in the EDM machining process

This paper presents a study of the EDM machining of H13 and D2 tool steels using electrodes of different diameters. Scanning electron microscopy is employed to analyze the machined surface, and the concept of a Crack Critical Line (CCL) is introduced to explore the influence of electrode size, EDM parameters and material thermal conductivity on surface cracking. The current results reveal that the surface crack distribution is influenced by the machining parameters, the electrode diameter and the material conductivity. It is noted that cracks tend not to appear when the machining is performed with a decreased pulse current and an increased pulse-on duration. Furthermore, it is observed that changing the electrode diameter causes a parallel shift of the CCL location within the crack distribution map. The intercept of the line depends on the electrode size. When small diameter electrodes are employed in the machining process, the location of the CCL shifts upwards. This causes the no-crack zone to enlarge, and therefore permits a wider choice of machining parameters to be adopted.

Application of EDM Hole-Drilling Method to the Measurement of Residual Stress in Tool and Carbon Steels

This study adopts the application of the electrodischarge machining (EDM) hole-drilling method to the measurement of residual stress in AISI D2 cold work tool steel, AISI H13 hot work tool steel, and AISI 1045 medium carbon steel. A calibration procedure based on the thermal conductivity of the material is conducted to compensate for the additional compressive stress induced in the workpiece by the EDM hole-drilling operation. Since the formation of this white layer influences the magnitude of the induced stress, the scanning electron microscopy, transmission electron microscopy, and nanoindentation techniques are used to examine the microstructure and hardness of the white layer resolidified on the EDMed surface. The experimental results reveal that combination of the hole-drilling strain-gage method (ASTM standard E837) with an EDM drilling process provides the effective means of determining the residual stress in materials with high hardness and good wear resistance.

Effect of Material Physical Properties on Residual Stress Measurement by EDM Hole-Drilling Method

When measuring the residual stress within a component using the electrical discharge machining (EDM) strain-gage method, a metallurgical transformation layer is formed on the wall of the measurement hole. This transformation layer induces an additional residual stress and therefore introduces a measurement error. In this study, it is shown that given an appropriate set of machining conditions, this measurement error can be compensated directly using a calibration stress factor σ cal computed in accordance with the properties of the workpiece material. It is shown that for EDM machining conditions of 120 V/12 A/6 μs/30 μs (discharge voltage/pulse current/pulse-on durationlpulse-off duration), the hole-drilling induced stress reduces with an increasing thermal conductivity (k) in accordance with the relation σ cal =325.5k ―0.65 MPa and increases linearly with an increasing carbon equivalent (CE) in accordance with σ cal =7.6×(CE) +22.4 MPa. Therefore, a given knowledge of the thermal conductivity coefficient or carbon equivalent of the workpiece material, an accurate value of the true residual stress within a component can be obtained simply by subtracting the computed value of the calibration stress from the stress value obtained in accordance with the EDM hole-drilling strain-gage method prescribed in ASTM E837.

Development of a 3D touch trigger probe using micro spherical stylus machining by micro-EDM for micro-CMM

This study develops a low-cost three-dimensional touch trigger probe for micro-CMM using micro spherical stylus machining by micro-EDM. The tip ball of the stylus, with a diameter smaller than 100 μm, is made by a micro electro discharge machine with the method of wire electro discharge grinding (WEDG). The current study uses tungsten carbide (WC) as the material for the stylus. Experimental results show that, the tip ball roundness could amount to 2 μm. Since the tip ball is not easily observable by naked eyes, we designed a micro imaging system to observe the probing in real time. This system is created with modified commercial webcam and microscope. The general design of the touch trigger probe is combined the stylus and suspension structure. When the stylus touches workpiece, it can be determined the time of probe trigger by measuring the motion of suspension structure. The stylus machined by micro-EDM was very fragile and the stylus handle of the tip ball was very thin. Therefore, the stylus handle bends easily that would result in the large measurement error or stylus break when the stylus touched workpiece. In this study we designed a measuring probe using a small chuck and an electrical circuit to measure the probe trigger. When the stylus touches the workpiece, the parallel circuit will switch on immediately and pick up the voltage variation of the resistor. Because the transistor always turns ON, it can avoid the problems of excessive energy and electrical noise.