Papers by Author: Thet Thet Mon

Abstract: This research centers on experimental laser-micromachining of silicon using solid state pulsed Nd:YAG laser. The laser is equipped with 3-axis controllers with resolution of 1 µm. The main objective is to find out the possibility of using this laser machining system to fabricate a micro feature on silicon. Simple straight lines were generated on silicon surface. The two process parameters – the laser traverse speed and pulse energy- were considered in the experimental design. Two forms of experiments were carried out: (1) processing with air assist gas and (2) processing without the assist gas using the same experimental design. Standard full factorial design of 3^k was used to design the experiments. Repeatability of the machine and nonlinearity were taken into account in the design by adding center points. Sequence of experiments was also randomized. Statistical analysis of experimental results could not show any significant factor. However, the surface plot did provide general information on desirable regions for the response line width, which was consistent with the published results. Micrograph study of the featured lines revealed that the laser processing without the assist gas produced preferable results.

Abstract: Machining of titanium and its alloys is still the subject of research and researchers’ interest despite some improvement in its machinability from several machining methods. This research presents performance of nitrogen gas in machining titanium. Machining of titanium is carried out on conventional turning center with triangular insert and holder according to ISO designation. Compressed nitrogen gas contained a cylindrical tank is supplied to the cutting zone via speciallydesigned valve that controls pressure and volume of nitrogen. The gas outlet pipe of diameter 2 mm is directed to just-above the tool rake face. During machining, the gas is supplied with high pressure so that the cutting zone receives an effective cooling as well as the chip will easily break. The effectiveness of this new cooling strategy is demonstrated by tool condition after machining, and also by comparing with performance of conventional coolant. The result is found to be excellent in terms of relative amount of tool wear. The cutting insert has surprisingly been almost intact when using nitrogen gas as coolant whereas tool wear at failure state has occurred with conventional coolant for the same machining parameters.

Abstract: This research presents performance of nitrogen gas as a coolant in machining titanium. Compressed nitrogen gas stored in a cylindrical tank is supplied to the cutting zone via the stainless steel tube of 2x8x25mm (inside diameter x outside diameter x length) connected to the flexible hose and specially-designed valve with pressure controller. Machining experiments are carried out on conventional turning center. The cutting tool used is triangular insert of ISO-TPGN160308 with the holder (ISO-CTGPR3232K). The cutting insert grade is KC5010 (TiAlN3 coated carbide) as recommended by Kennametal for machining titanium. During machining, the tube is manually directed to be just-above the tool rake face and the nitrogen gas is supplied with high pressure so that the cutting zone receives an effective cooling as well as the chip brakes easily. The effectiveness of this new cooling strategy is demonstrated by the cutting edge condition and surface finish after machining at various speeds, and also by comparing with performance of conventional coolant. The result is found to be excellent in terms of relative amount of tool wear and surface finish. The cutting insert has surprisingly remained almost intact when using nitrogen gas coolant whereas severe tool wear occurred with conventional coolant even at low cutting speed. This cryogenic strategy also improved machined surface quality greatly.

Abstract: Orthogonal metal cutting process involves large plastic deformation accompanied by excessive heat generation. This work addresses the thermal-mechanical responses of the workpiece material at the tool-workpiece contact. In this respect, the orthogonal cutting process of Ti-6Al-4V using CVD diamond tool is simulated using finite element method. The cutting condition consists of cutting speed, V=180 m/min, feed rate, t=0.125 mm/rev and width of cut of 1.25 mm. Eulerian formulation with coupled thermal-mechanical analysis is employed in the model. The Johnson- Cook constitutive equation is employed for Ti-6Al-4V workpiece material to accurately simulate the formation of shear bands. The stick-slip friction condition is modeled at the tool-chip interface. The sliding coefficient of friction of 0.8 and the limiting shear stress of 700 MPa for stick-slip condition are determined experimentally. Results show that high temperature and temperature gradient concentrate in the primary shear zone and the contact area between the tool rake face and the chip. A primary shear band is predicted in the workpiece ahead of the tool-workpiece contact face while the secondary shear band is formed in the chip. This highly-deformed shear band is revealed in the microstructure of etched chips. The predicted high strain rate results in build-up edge at tool cutting edge-chip contact. Low cutting condition of V=150 m/min, t=0.125 mm/rev promotes stagnant zone formation that helps preserve the cutting edge of the tool. The maximum predicted temperature at the cutting edge is in excess of 700 °C. Such high temperature level facilitates diffusion of carbon elements into the chips and conversely, elements of titanium into the CVD diamond tool.