|Author:||Wong, Hau Chung|
|Title:||A study of ultra-precision diamond turning of optical polymers|
|Advisors:||Lee, W. B. (ISE)|
Chan, C. Y. (ISE)
|Subject:||Hong Kong Polytechnic University -- Dissertations|
|Department:||Department of Industrial and Systems Engineering|
|Pages:||xi, 178 pages : color illustrations|
|Abstract:||Ultra-precision machining of optical grade polymers is difficult. Due to the low melting temperature point of most polymers and the high temperature generated in the heated affected zone in machining, the adhesion of the chip debris melted on the cutting tool causes adhesive tool wear. This results in a poor surface finish of the machined part. In ultra-precision machining, using a diamond tool, the situation is even more critical in the machining of precision optics as the required surface finish and form accuracy is in the order of nanometers and micron respectively. Usually, lenses made of polymers are injection molded and hot embossed are produced in large quantities. However, for small quantity and prototyping of precision optical components such as microlens array used in laser optics, direct machining is preferred. Each lenslet in the microlen array is of the order of a micron in diameter and, over a small area, thousands of lenslet need to be machined without tool changes. This imposes extreme demand on the integrity of the diamond tool. In this research, a novel class of cutting fluid consisting of stable nanometric sized emulsified water and oil molecules has been developed by the author to diamond turn optical grade polymers. Nanofluid is generated by breaking the emulsified oil and water into stable small molecules with ultrasound energy. The separation time of oil and water can last for a few days of storage in a closed environment and the cutting fluid is made more stable. Such nano droplet cutting fluid (NDCF), due to its small sizes, is able to penetrate into the interface between the chip and the tool. In machining, the specific cutting pressure is high, of the order of 100MPa. Conventional cutting fluid would be shut off and the fluid only being able to serve as a coolant and not as a lubricant. To evaluate the effectiveness of the nanodroplet fluid, comparison were made in diamond turning PMMA and PC in dry cutting, and cutting with water, conventional cutting fluid and with the nanodroplet approach respectively, as the cutting fluid in a series of taper cutting experiments. Aluminium alloy A0661 was also machined to determine if there is any difference in the effectiveness of the fluid in machining metals and polymers. The diamond tool wear was measured by the change in tool radius after cutting. A cutting tool evaluation monitoring method has been developed to monitor the cutting tool condition without dismantling the diamond tool from the machine. Instead of measuring the tool directly, the groove radius at various depth of cut was measured with a precision optical profiler. The contours of the groove profiles were taken which showed clearly when the groove radius would deviate from the measured radius of the tool before cutting. Software was written to calculate the tool radius after cutting. The tool wear indicates the extent to which adhesive tool wear has occurred. The tool wear as indicated by the change in the diameter of the cut groove is found to be less in the order of NDCF, oil, water and dry cutting. Surface finish was also found to be the highest in cutting with NDCF.|
The effectiveness of the cutting fluid to reduce friction is measured by the change in cutting force. The cutting forces in three directions are monitored by a force sensor attached to the tool. If the coolant is able to penetrate into the interface, this should reduce friction and be reflected in a decrease of the cutting force. The fluctuation of the cutting force in machining is analyzed by Fast Fourier Transform (FFT). Any interruption in the cutting process will result in the tool vibration and hence poor surface finish. The extent of cutting tool vibration, as reflected by the fluctuations of cutting forces, varies with the cutting fluid being used. Higher fluctuation is found in dry cutting, followed by machining with water and oil. Less cutting force vibration is seen in machining with oil and the best result is found in cutting with NDCF. The reduction in cutting force, as well the degree of force vibration damping, reflects the effectiveness of the cutting fluid lubricant in penetrating the cutting tool and workpiece interface. A PC microlens array was also successfully machined with a nanometric surface finish and form error in the order of microns. Overall, NDCF makes a significant enhancement to cutting tool life and produces a better surface finish in the diamond turning of optical polymers. As this research is mainly an experimental study in the evaluation of a new class of cutting fluid for machining optical polymers, further theoretical investigation is needed to explore the cutting mechanism of this new class of nanodroplet cutting fluid. Such fluid is able to penetrate into the interface between the chip and diamond tool interface to reduce tool wear and hence the cutting force. The performance of the nanodroplet cutting fluid is evaluated and comparisons are made with different lubricants in a series of taper cutting experiments. This new class of nanodroplet cutting fluid made of oil and water has been used in the diamond turning of optical polycarbonates. The performance of the cutting fluid on the tool wear and cutting force are evaluated through a three dimensional surface topography map and Fast Fourier Transform analysis. The use of the nanodroplet cutting fluid is found to reduce the cutting force vibration and tool wear which results in a better surface quality for microlens applications.
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