Naotake Mohri

Some Considerations to Machining Characteristics of Insulating Ceramics-Towards Practical Use in Industry-

Abstract Machining of insulating ceramics can be realized in EDM by using the assisting electrode method. After a tool electrode cuts through the assisting electrode, a carbon layer covers the ceramics surface during EDM. This carbon layer, formed from the decomposition of the hydrocarbon working oil, enhances the ceramics surfaces conductivity. In this paper, machining phenomena of insulating ceramics are considered towards practical use in industry. Several kinds of insulating ceramics and assisting electrode materials are investigated under considerations of various machining characteristics; and high speed machining in W-EDM is carried out under conditions of lower tension than usual.

Probability of precision micro-machining of insulating Si3N4 ceramics by EDM

Abstract Electrical discharge machining (EDM) is used as a precision machining method for the electrically conductive hard materials with a soft electrode material. But recently we succeeded to machine on insulating material by EDM. The technology is named as an assisting electrode method. The EDMed surface is covered with the electrical conductive layer during discharge. The layer holds the electrical conductivity during discharge. For micro-EDM, the wear of tool electrode becomes lager ratio than the normal machining. So the micro-machining is extremely difficult to get the precision sample. In this paper to obtain a fine and precise ceramics sample, some trials were carried out considering the EDM conditions, tool electrodes material and assisting electrode materials. Insulating Si 3 N 4 ceramics were used for workpiece. The machining properties were estimated by the removal rate and tool wear ratio. To confirm the change of micro-machining process, the discharge waveforms were observed. The micro-machining of the O0.05xa0mm hole could be machined with the commercial sinking electrical discharge machine.

Layer Generation Process on Work-piece in Electrical Discharge Machining

Abstract In recant years, surface modification of metals and machining of insulating ceramics by electrical discharge machining (EDM) have been successfully carried out. In surface modification by EDM with semi-sintered electrodes, worn substances in the gap region form the material source of the layer generated on the work-piece surface. In the machining of insulating ceramics by EDM, a crystallized carbon layer or carbide layer from the working oil covers the surface of the insulator. Increase in the thickness of the generated layer, however, tends to stop at a certain maximum value in both surface modification by EDM with semi-sintered electrodes and machining of insulating ceramics by EDM processes. In these machining operations, accretion and removal phenomena occur alternately. In this paper, the mechanisms of machining insulators and the accretion process are discussed considering the characteristics of the generated layers on the work-piece surface.

Micro-pin Electrodes Formation by Micro-Scanning EDM Process

In recent micro-EDM processes various kinds of thin tool manufacturing methods, including the wire electrical discharge grinding (WEDG) have been proposed to satisfy the growing need for very thin tools. This study describes a new thin tool micro-formation by EDM where a rod electrode is driven through the center of a controlled slit formed between two electrically isolated metal plates to provide a critical path for the rod electrode tool during the process. A micro-pin of the desired shape and diameter is obtained, under stable conditions, in a very short time interval.

System Identification of Wire Electrical Discharge Machining

Abstract In wire electrical discharge machining (wire-EDM), it is very important to restrain the vibration of the tool wire electrode for the improvement of machining accuracy. In this paper, investigation is carried out toward the dynamic wire vibration mechanism and a mathematical model is derived. This model is compared with experimental results. The measured displacement of a wire electrode in machining a thin plate is analyzed with impulsive force measured through impulse response by a single discharge. The force acting on the wire depends on the direction of the wire movement in vibration. A 3rd order system equation for the wire EDM system is derived considering machining removal and vibrational features of the system are discussed with the equation. As the discharge circuit suddenly changes according to gap condition, this vibration system is essentially nonlinear. Simulation of wire-EDM is performed with the modeled system.

Three-dimensional machining of insulating ceramics materials with electrical discharge machining

The insulating ceramics were processed with sinking and wire cut electrical discharge machining(EDM). The new technology was named as the assisting electrode method. In the machining, the electrical conductive material was adhered on the surface of insulating workpiece as the starting point of electrical discharge. As the processing operated in oil, the electrical conductive product composed of decomposition carbon element from working oil adhered on the workpiece during discharge. The discharges generated continuously with the formation of the electrical conductive layer. So, the insulating ceramics turn to the machinable material by EDM. We introduced the mechanism and the application of the machining of insulating ceramics such as Si3N4 and ZrO2.

Time-resolved observation of the change of electrode shape in single discharge: Rapid self-forming of a thin electrode

A very thin, needlelike electrode can be formed at the tip of thin tungsten electrode by a single discharge with rather high discharge current. We have demonstrated some practical applicability of such very thin electrode. Understanding the formation mechanism of such electrodes is an important issue to utilize the phenomenon to practical applications. We should see what happens on the electrode during the discharge. In this study, the change of electrode shape at tip part of a thin metal electrode induced by single discharge with a large current was observed dynamically by a stroboscopic imaging system with high time resolution. In the imaging system, the second harmonics of a Q-switched neodyum: yttrium-aluminum-garnet laser is used for lighting of charge-coupled device camera and it can take a photo with time resolution of about 10ns. Electrode materials were tungsten and copper of 100μm diameter and the discharge current was 30A with 400μs duration. Both the positive and negative polarities of electro...

Effect of Surface Oxidation on Micromachinability of Monocrystalline Silicon

Abstract Microcutting experiments are carried out under an atomic force microscope (AFM) using workpieces of silicon monocrystals that have been exposed to air for various lengths of time before cutting. The results are observed under the same AFM with decreased tip force. The results show that difficult-to-cut areas appear locally after 24 hours of exposure time and these areas extend with increasing exposure time until the whole surface is covered after 120 hours. It is also found that exposure of workpieces to air produces a SiO 2 surface layer in which residual compressive stress is generated and whose hardness and/or elastic constant are lower than those of bulk Si. The molecular dynamics simulations carried out based on the above results show that the deterioration of machinability of monocrystalline silicon is caused by the viscoelastic/plastic properties of SiO 2

Coarse motion of 'seal mechanism' with three degrees of freedom by using difference of frictional force

This paper deals with a small mobile device, a `seal mechanism, with three degrees of freedom (DOFs) for a scanning probe microscope. This device has two piezoelectric actuators, two controlled electromagnets A and B and electromagnet C, that generates a constant friction. The frictional forces at electromagnets A and B are alternated by an on-off control. The whole device wriggles with electromagnets C when the piezoelectric actuators extend and contract. The 3-DOF device can move in up to micrometre-steps in the x- and y-directions and up to submilliradian steps in the ?-direction. The movable range is infinite in principle. The seal mechanism performs as well as the inchworm mechanism. The seal mechanism needs a smaller number of controlled actuators and is driven by a smaller number of steps than the multiple-DOF positioning stages employing the inchworm mechanism.

Detection of acoustic emission wave by using optical fiber sensor during single pulse discharge

This paper describes some experimental results on a fundamental phenomenon of the single pulse discharge. The single pulse discharge phenomenon was measured by using the optical fiber sensor, piezoelectric sensor and current transducer. In order to investigate the generation factor and propagation characteristic of acoustic emission (AE) by single pulse discharge in oil, the voltage across the gap, the discharge current and the displacement velocity were measured. Detected the signals using the optical fiber sensor and piezoelectric sensor have been compared with each other. As the results, optical fiber sensor was more resist than piezoelectric sensor in an electromagnetic noise. It is likely that the optical fiber sensor was more useful than piezoelectric sensor for AE detection with electromagnetic field. In order to investigate the relationships between the single pulse discharge phenomena and the detected AE signals, numerical analysis by using finite element method (FEM) was adapted. The numerical analysis result using the FEM and the experiment result along with the single pulse discharge was compared. As the result, it was estimated that the generation factor of the AE wave was the 1μs duration force. The signal detected by the optical fiber sensor has a good agreement with the signal from the FEM one. It is found that optical fiber sensor can detect the displacement velocity with wide band. Moreover, it was considered that frequency response of the optical fiber sensor is approximately flat. The optical fiber sensor was effective in analysis of AE source.