Jooho Hwang

An Ultra-precision Lathe for Large-area Micro-structured Roll Molds

We report an ultra-precision lathe designed to machine micron-scale features on a large-area roll mold. The lathe can machine rolls up to 600 mm in diameter and 2,500 mm in length. All axes use hydrostatic oil bearings to exploit the high-precision, stiffness, and damping characteristics. The headstock spindle and rotary tooling table are driven by frameless direct drive motors, while coreless linear motors are used for the two linear axes. Finite element method modeling reveals that the effects of structural deformation on the machining accuracy are less than . The results of thermal testing show that the maximum temperature rise at the spindle outer surface is approximately . Finally, performance evaluations of the error motion, micro-positioning capability, and fine-pitch machining demonstrate that the lathe is capable of producing optical-quality surfaces with micron-scale patterns with feature sizes as small as on a large-area roll mold.

Estimation of Axial Displacement in High-speed Spindle Due to Rotational Speed

This paper presents an estimation procedure for axial displacement in spindle equipped with angular contact ball bearings due to rotational speed. High-speed spindle-bearing system experiences axial displacement due to thermal expansion and rotational speed-dependent characteristics of angular contact ball bearings. This paper deals with the axial displacement caused by the rotational speed-dependent effects such as centrifugal force and gyroscopic moments. To this end, a bearing dynamic model is established that includes all the static and dynamic properties of angular contact ball bearing. An analytical formula to calculate the axial displacement based on contact angles between ball and races is derived to discuss the physics regarding the axial displacement in spindle. The proposed dynamic model is compared with a reference and a commercial program. Numerical examples are presented to show the effects of centrifugal force and gyroscopic moment on the axial displacement. The proposed model is also validated with an experimental result.

Fabrication processes and multi-functional applications of carbon nanotube nanocomposites

The microstructures and properties of carbon nanotube (CNT)/metal nanocomposites, which are fabricated by the molecular level mixing process, have been investigated for multifunctional applications as structural and functional materials. The molecular level mixing process provides a homogeneous dispersion of carbon nanotubes in metal matrices by inducing the ionic bonding between functionalized carbon nanotubes and metal ions. For the structural applications, CNT/Cu nanocomposites showed an outstanding enhancement in mechanical properties such as strength, modulus, and wear resistance. For the functional applications, CNT/Co nanocomposites showed excellent field emission behavior for possible applications on next generation field emission displays. SiC/W nanocomposites, fabricated by using CNT/W nanocomposite powders, showed good ablation properties for the applications as rocket propulsion materials.

Estimation of Rotational Motion Accuracy for Rotary Units

The error motion of a machine tool spindle directly affects the surface errors of machined parts. Those are usually due to the imperfectness of bearings, stiffness of spindle, assembly errors, external force or unbalance of rotors. The error motions of the spindle have been needed to be decreased to desired goal of spindles performance. The level of error motion is needed to be estimated during the design and assembly process of the spindle. In this paper, the estimation method for the five degree of freedom (5 D.O.F) error motions for rotary units such as a spindle and rotary table are suggested. To estimate the error motions of the rotary unit, waviness of bearings and external force model were used as input data. The estimation model considers geometric relationship and force equilibrium of the five degree of the freedom motions.

Analysis and Measurement of the Dynamic Motions of a Large-Scale Rotating Roll Workpiece

This paper describes the analysis and measurement of the dynamic motions of a large-scale rotating roll workpiece, which is used for a large-scale lathe, under the gravity condition. A dynamic model which includes the gravity effect was first established. In order to investigate the behavior which comes from stiffness differences of the journal bearing at the headstock and the tailstock, a spring-mass-damper system was also established. In addition to the simulations, the behavior of the rotating roll workpiece was investigated with respect to the different rotating speed. In the experiment, a mirror finished hollow type roll workpiece with a diameter of 320 mm, a length of 1800 mm, a weight of about 340 kg was fabricated by a large-scale lathe. Five capacitive type displacement sensors were mounted on the sensor holders which were arranged along the axis of rotation of the spindle. The behavior measurement was carried out at 5 sections of the roll workpiece along the axial direction. The rotating speed was set to be 5, 50, 100, 150, 200, 250 and 300 rpm. The dynamic model for the analysis with simulations and the measurement result of the behavior of a rotating roll workpiece according to the different rotating speed are presented in this paper.

Study on the Frictional Torque in the Angular Contact Ball Bearing for Machine Tool Spindle by Empirical Formula

Ball and roller bearings are commonly used machine elements for supporting rotating motion about shafts in simple devices including bicycles, in-line skates, and electric motors, as well as in complex machines. Heat is generated by the friction in the bearings, which causes the temperature inside the bearing to increase. If the heat is not appropriately removed from the bearing, elevated temperatures may give rise to premature failure. It is, therefore, important to be able to calculate the temperature in the bearings due to friction.Here, we describe a method to estimate the frictional torque in bearings using an empirical formula developed using a method based on bearing analysis tool and the measured frictional torque in a spindle system. Thermal analysis of the spindle system including the bearings was achieved using the finite element method (FEM), and the bearing temperature was compared with measured data to verify the empirical formula.

Experimental Investigation for Rotational Error Motion Simulation of Inherently Compensated Aerostatic Journal Bearing

It is an important thing for a designer to simulate and predict the performance of a spindle and a rotary table. In addition to the general performance such as static stiffness, the error motion performance information is beneficial to the designer in many cases. However for an aerostatic bearing the fluid film physical status should be calculated in order to simulate those performances and the calculation time is another obstacle for a simple performance simulation. In this paper the investigation on experiment and simulation is performed in order to find a more effective simulation method for the rotational error motion.

An Optical Surfacing Technique of the Best-fitted Spherical Surface of the Large Optics Mirror with Ultra Precision Polishing Machine

This paper describes a novel method to surface large optics mirror with an extremely high hardness, which could replace the high cost of the repetitive off-line measurement steps and the large ultra- precision grinding machine with ultra-positioning control of 10 nm resolution. A lot of diamond pellet to be attached on the convex aluminum base consists of a grinding tool for the concave large mirror, and the tool was pressured down on the large mirror blank. The tool motion at an interval on the spiral path was controlled with each feed rate as the dwell time in the conventional computer-controlled polishing. The shape to be surfaced was measured directly by a touch probe on the machine without any separation of the mirror blank. Total 40 iterative steps of the surfacing and measurement could demonstrate the form error of RMS 7.8 µm, surface roughness of Ra 0.2 µm for the mirror blank with diameter of 1 m and spherical radius of curvature of 5400 mm.