Study the effects of process parameters (speed, feed, depth of cut etc.) associated with SPDT for manufacturing of micro lens let arrays. Also find out the ranges of these process parameters and values of optimal process parameters.
Submitted by: Marmeek Kishor Kumar Kosambia (09010332)
Submitted to : Dr. S.N. Joshi
Date of Submission: 17/01/13
A microlens array is composed of a series of micro lens distributed in a regular pattern and has been usedin a wide range of photonic products. A micro lens is a small lens, generally with a diameter less than a millimetre (mm) and often as small as 10 micrometers (µm). The small sizes of the lenses means that a simple design can give good optical quality but sometimes unwanted effects arise due to optical diffraction at the small features. A typical micro lens may be a single element with one plane surface and one spherical convex surface to refract the light. Because micro lenses are so small, the substrate that supports them is usually thicker than the lens and this has to be taken into account in the design. More sophisticated lenses may use aspherical surfaces and others may use several layers of optical material to achieve their design performance.Since surface roughness affects the performance of the lens, one needs to generate finely machined surface with minimum roughness
Single Point Diamond Turning machining is a technique which removes materials from a few microns to sub-micron level to achieve ductile mode machining on hard-to-machine materials such as electro less nickel plating, silicon, quartz, glass and ceramics with no subsurface defects. Such a machining process is able to achieve mirror surface finish of less than 10 nm and form error of less than 1um easily. If properly applied to a specific range of diamond turnable materials, the process is far superior to grinding and polishing where shape control is more difficult and processing time is longer.
The selecting and optimization of machining parameters is one of the main factors that could influence the machining accuracy. The main machining parameters are tool feed rates, spindle speed and depth of cut. The tool feed rate is normally expressed in terms of either distance travelled by the tool per unit time (mm/min) or distance travelled per unit rotation (mm/revolution). It is most common to see the distance per revolution as it is directly related to the anticipated theoretical surface finish. For a given tool feed rate, larger the tool nose radius, lower the roughness and the better the optical surface finish. The surface quality depends to great extents on the material characteristics like: grain size, micro structure or crystal boundary, crystal uniformity and annealing procedures adopted. . Casual selection of combination of machining parameters may affect the surface quality, so it is required to optimize the machining parameters before final SPDT process. Machining of aspheric surface is more complicated than spherical and flat surfaces because of complicated tool path and uneven material removal. To achieve required profile tool path should be optimized.
Surface roughness with respect to variable Feed rate
Feed rate is most important parameter and variation of this havea great impact on the surface finish. Theoretical surface finishdepends on feed rate and tool radius. Depth of cut 2 μm andRPM 1000 kept constant and tool feed rate is varied from 0.5μm/rev to 5.0 μm/rev. the results of the experiments are asfollows.
From the above experiments, it is observed that the surface finish is going down as we reduce feed. The surface roughness is 54.8 nm at feed 0.5 μm where the depth of cut is 2 μm and 1000 RPM was maintained. It is investigated that although look of the surface was good but it is not of optical quality at these parameters. We have varied the depth of cut at feed rate of 0.5.
Surface roughness with respect to...