森田 昇

We present a numerical and experimental study on the microdrilling of glass with UV laser pulses with the repetition rates from 1 kHz to 20 kHz. Pulse repetition frequency dependency on ablation threshold at the surface of the glass was discussed experimentally and numerically. It was confirmed that the pulse repetition rate influenced heat accumulation and temperature of illuminated area changed.

On the turning operations, the thrust force causes flexural deformation of the work-piece and adversely affects machining accuracy. Minimization of the thrust force is desirable to attain high dimensional precision in the turning operations. In this study, we proposed the equations of predicting the cutting force to decide the conditions of thrust-force-free turning theoretically. We named them modified equations. The modified equations was applicable to a conditions of low feed rate even if size effect occurred. The calculation accuracy and applicable conditions of the equations was confirmed. In order to further improve the calculation accuracy, the models of the projected area of cut was changed and the discretized modified equations were proposed.

We describe a technique to improve diamond cutting tools used in nanometer- and micrometer-scale machining and formed via focused-ion-beam (FIB) micromachining. Although FIB irradiation is an effective means of fabricating arbitrary miniature shapes in diamond cutting tools, FIB irradiation induces a non-diamond phase, as well as Ga ion implantation, in the irradiated area. This adversely affects the performance of the ultra-precision machining process, especially in terms of tool life and the quality of the machined surface. To eliminate the affected layer, we applied heat-treatment techniques and investigated the optimum thermal profiles. A temperature of 500℃ applied to the cutting tool provided optimal machining of nickel phosphorus. The tool life was significantly improved, and a tool life similar to that of a non-irradiated diamond tool was obtained. The quality of the machined surface was also improved markedly owing to superior tool wear and adhesion resistance.

Demands of laser scribing to glass increase in electronic device relevant market.Untill now, laser scribing is analyzed by an steady state. However, since a laser scribing phenomenon is a result of movement of being accompanied by propagation of a crack and displacement by thermal expansion of a minute domain, it needs the unsteady thermo elasticity analysis based on an equation of motion. The three-dimensional thermal field model and the two-dimensional stress field model were developed, and they were calculated using numerical computation. Under the condition table feed rate 200mm/s, the laser power 58.7W, it was found that the maximum temperature is 560℃, maximum stress is lOMPa.Distribution of the phase differences observed by the experiment and derived by analysis were similar. Result of analysis is reliable because the phase difference is generated by stress.

In this study, we propose the new equations which predict cutting forces under the conditions of a small feed (2.5μm) and negative approach angle. They are based on Nakayama's equations. According to our previous research, the experimental cutting forces were larger than the calculated forces with Nakayama's equations. The differences were caused by size effect. One of the causes of size effect was a cutting edge radius. Therefore, the new equations were formulated with taking into account the cutting edge radius. The cutting force calculated by the new equations were approximately equal to the experimental results. The accuracy of the new equations is validated under the conditions of thrust force free cutting.

The propagation of thermal stress in sheet glass during single pulsed laser irradiation was clarified using a one-dimensional model. A thermoelastic equation based on an equation of motion was applied. In the case of a 0.1 μs pulse width, a thermal stress cycle of propagation, reflection, propagation, interference, and separation was revealed. Upon the reflection of stress wave at the free ends, the polarity of the stress change. Alternating stresses of approximately 0.5 MHz appeared inside the sheet glass. Because a crack may propagate in the glass as a result of the stress, there is a possibility of high-cycle fatigue. It was found that the thermal stress generated by single pulsed laser irradiation exhibited wavelike properties.

In order to improve the laser micro-machinability of borosilicate glass, the glass surface was doped with metal (silver or copper) ions by electric field-assisted solid-state ion exchange. Doped ions drifted and diffused into the glass substrate under a DC electric field. The drift-diffusion behavior of metal ions in glass was numerically analyzed using standard explicit finite-difference method. The calculated penetration depths of both silver and copper showed good agreement with experimentally measured values. However, there was a difference between measured and calculated ionic fluxes, especially for the early-stage of ion exchange. This discrepancy was likely to be caused by an imperfect initial contact between metal foil and glass substrate. Therefore, the increase in electric current path with ion exchange time was necessary to be taken into consideration. The modified calculation enabled more accurate estimation of ionic penetration depths.

Thermal elastic behavior of glass plate during laser cleaving was investigated by one-dimensional model based on thermo-elastic theory. Unsteady state thermal conduction equation with moving heat source by laser beam was used. Thermo-elastic equation based on equation of motion with damping force in proportion to deformation velocity was applied. Numerical calculation was performed by finite difference method based on implicit method with regard to time evolution. Proportion damping factor was adopted for the calculation. In stationary heat source, compression stress on the heating region changes into tensile stress at a moment of 0.1 second to switch over from heating to cooling. In moving heat source, the highest tensile stress was provided in a domain with the proportion damping factor less than 0.1. The proportion damping factor changes to small value with increasing of the moving velocity.