Thet Thet Mon

Ductile streaks in precision grinding of hard and brittle materials

Ductile streaks produced during diamond grinding of hard and brittle materials have aided the subsequent process of polishing. Two novel techniques were used to study the formation of ductile mode streaks during diamond grinding (primary process) of germanium, silicon, and glass. In the first technique, aspheric surfaces were generated on Ge and Si at conventional speeds (5000 rpm). In the second technique, diamond grinding of plano surfaces on glass and Si surfaces using high speed (100,000 rpm) was carried out. Form accuracy, surface finish and ductile mode grinding streaks are discussed in this paper. It was found that resinoid diamond wheels gave more ductile streaks than metal-bonded wheels but better form accuracy was obtained with the latter. Ductile streaks were obtained more easily with pyrex rather than with BK 7 glass thus necessitating very little time for polishing. Ductile streaks appeared in abundance on germanium rather than silicon. Both the novel grinding techniques were used on CNC machining centres.

Design of electrostatic comb actuators based on finite element method

Electrostatic comb actuators are commonly used to provide displacement-invariant force in micro electro-mechanical system (MEMS). Major application can be found in resonator, inertial sensor, accelerometer, and gyroscope. The size of the comb may be a few microns to millimeters. Principally, the electrostatic force is produced in the comb structure due to potential difference between the electrodes, which is used to actuate the system attached to it [1]. The higher forces are very often desirable for high sensitivity and performance. However to meet this demand, micro-scaled structures are very often fabricated on trial-error basis because of lack of well-established fabrication method. In this situation, designing on a computer prior to the actual fabrication would be very helpful. Moreover, in a virtual device, parameters can be changed much more quickly than trial-and-error fabrication reducing the time to market and also the cost to develop a commercial device considerably [2].

Precision micro-machining of silicon and glass

This research work is concentrated on machining of flat surfaces on silicon and glass for applications like IC chips and small optical lenses. Aspheric surfaces were also generated on silicon and germanium. Grinding was done to give a good surface finish and at the same time focused on getting the maximum amount of ductile streaks. After grinding the surfaces were lapped and polished. The polishing curve is a saturation curve where rapid improvement in surface finish is obtained followed by a gradual decrease in the Ra, with the curve flattening out parallel to the horizontal time axis. It was found that the initial roughness after grinding was not the sole criterion for rapid polishing but rather the amount of ductile streaks. The amount of ductile streaks is not only dependant on the feed and the depth of cut but also on the grit size of the diamonds as well as its concentration, and not least of all on the type of bond. Brightness of silicon surfaces and lightness of glass workpieces and their relationship with roughness have been determined.

Performance of Cryogenic Machining with Nitrogen Gas in Machining of Titanium

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.

Experimental Micromachining of Silicon with Nd-YAG Laser

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 3k 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.

Design of an Electrostatic Comb Actuator Based on Finite Element Method

Electrostatic comb actuators were designed using finite element modeling and analysis, so‐called finite element method (FEM). Design objective was to generate maximum actuating force within the constraints. 2D and 3D FE models of the comb structures were developed in general‐purpose FE code. The element geometries were 4‐node plate element for 2D model and 8‐node brick element for 3D models. Electrostatic field strength and voltage analysis of the FE models were performed to compute generated voltage and electrostatic force in the structure. Subsequently done was the structural analysis to examine structural response to the electrostatic force. The initial finite element model was verified with the published experimental result. Based on the amount of force generated and lateral deflection of the comb fingers, the best possible design of choice was determined. The finite element computations show that the comb structure having high aspect ratio with smaller gaps can provide higher actuation force.

Virtual Design of Multi-axis Positioning for Robotic Application

With the development of information technology virtual design is now considered as one of the most important phases in the process of the overall design in engineering. In this paper, multi-axis positioning was virtually designed using commercial finite element code with the intention of application in robot structure. In order to be a realistic design, actual robot model fanuc M-6iB was chosen for reference and virtual robot having five axes similar to the reference robot was designed. The 3D geometry of multi-axes integrated into the robot was created in solid works CAD environment. Solid work model was then imported into finite element (FE) environment for physical analysis. By using joint method and manipulating boundary conditions available in commercial FE package, motion of each manipulator was defined. The element types used for manipulators and joints were brick and beam elements respectively. Materials were specified according to the published report. Currently, linear material model was assumed. Mechanical event simulation was employed to analyze dynamic behavior of the manipulators. Physical response such as stress, strain due to dynamic effect were predicted. Finite element predictions provide consistent rotational displacements with the reference robot. Stress, strain and deformation were also found to be reasonable.

Design Analysis of Silicon Cantilever for Label-less Sensing using Finite Element Method

Silicon cantilevers are principal sensing components in measuring physical parameters, chemical and biochemical sensing, and monitoring DNA changes. In this paper, silicon cantilever that can sense micro/nano objects without the aid of any label for sensing has been designed and analyzed. In order to do so, deformation of cantilever itself will act as sensing unit. Finite element method was employed for design implementation. Design target is to achieve high sensitivity in bending without the use of fluorescence or radioactive label. Design parameters under consideration were length and thickness of the cantilever. Finite element model was developed as a thin cantilever of uniform rectangular cross-section discretized with 8-node quadrilateral brick elements. The micro-scaled mechanical properties of silicon were taken from published reports. For the purpose of having objects to be sensed, differential surface stress and additional weight in sub-micron scale were created as disturbance to the cantilever. Linear static stress analyses were performed to extract the amount of bending and the stress induced by different loadings. Finite element modeling and design analysis were implemented in general-purpose FE code. The results show that the feasible size of silicon cantilever that can sense any object at micron or sub-micron scale is 1000 times 100 times 0.4 mum provided that it does not impose the fabrication problem.

Thermal-Mechanical Responses of Ti-6Al-4V during Orthogonal Cutting Process

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.

Performance of nitrogen gas as a coolant in machining of titanium

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.