Neha Khatri

Experimental Investigation on Uncontrollable Parameters for Surface Finish during Diamond Turning

Single point diamond turning is one of the ultraprecision methods to generate high quality surfaces with highest possible profile accuracies. However, diamond turning process also introduces some unique errors which affect the surface finish. Some of these errors are controllable, while some are noncontrollable. Controllable parameters need proper selection, whereas uncontrollable parameters require adequate understanding of the process behavior. In this study only the effect of uncontrollable parameters on surface finish is explored. Tool overhang results tool–workpiece vibration whereas dynamic unbalance of vacuum responsible for workpiece wobble and both affect the surface finish considerably. Chipping of tool edge, improper extraction of chips, and slipping of workpiece on vacuum chuck are the main reasons of random scratch marks on surface and are contributing factors for degradation of surface finish.

Optimization of Process Parameters to Achieve Nano Level Surface Quality on Polycarbonate

grade plastics are increasingly being used as magnifiers in Ophthalmic Optical Instrumentation applications. In its effort to develop indigenously aspheric technology-based Ophthalmic Optical aids, the Aspheric Group at CSIO has studied the machining and surface characteristics of optical grade plastics. Generally, PMMA and polycarbonate considered as suitable candidates for aspheric Visual aids. In the study presented, optical grade polycarbonate is explored for its single-point diamond turning (SPDT) features and its profile characteristics. This study focuses on the optimization of SPDT machining parameters viz: tool feed rate, depth of cut, spindle speed for a given tool nose radius. In this study, the machining sensitivity in terms of surface roughness and profile error (Pt) is investigated. It is found that machining parameters play a major role in surface quality optimization in terms of roughness and profile. Based on optimized machining parameters, good quality aspheric lens is developed. Keywordspoint diamond turning (SPDT), Surface roughness, Profile error, machining parameters and tool path

An experimental study on the effect of magneto-rheological finishing on diamond turned surfaces

Magneto-rheological materials are a class of smart materials whose rheological properties can be rapidly varied by applying a magnetic field. Magneto-rheological finishing utilizes magneto-rheological fluid, which consists of magnetic particles, non-magnetic abrasives and some additives in water or other carrier to polish the materials. Single-point diamond turning is able to remove hundreds of microns of material and generate surface with micron accuracies. Residual turning marks are the most important factor limiting the performance in diamond turning process. Magneto-rheological finishing has inherent ability to improve micro-roughness, remove subsurface damage and reduce residual stresses induced during diamond turning process. Combining single-point diamond turning and magneto-rheological finishing creates a deterministic process for manufacturing highly finished surfaces. In this article, an attempt has been made to improve the finish of diamond turned surface with magneto-rheological finishing and to investigate the effects of parameters like current, spacing, wheel speed, feed rate and magnetic field on the final surface finish. Based on the parametric study, an optimum combination of process parameters is identified using analysis of variance. Various image processing techniques have been used for the comparison of the surface analysis of diamond turned surfaces and the magneto-rheological finished surfaces.

An experimental investigation on the influence of machining parameters on surface finish in diamond turning of silicon optics

Silicon is widely used in IR optics, X-Ray optics and electronics applications. These applications require Silicon of optical quality surface as well as good form accuracy. To get the desired finish and dimensional accuracy, diamond turning is preferable. Taylor-Hobson Nanoform-250 diamond turning equipment is used to machine flat Silicon mirror. Negative rake diamond tool is used with a tool nose radius of 1.5 mm. A series of SPDT machining operations are performed in the sequential combinations of tool feed rate, Spindle Speed and depth of cut. In order to find out the effect of machining parameters on the Surface Roughness during turning, Response Surface Methodology (RSM) is used and a prediction model is developed related to average Surface Roughness (Ra) using experimental data. The surface quality is analyzed in terms of arithmetic roughness (Ra) and Power Spectral Density for uniform evaluation. In addition, a good agreement between the predicted and measured Surface Roughness is observed.

Molecular dynamics simulation of the elliptical vibration assisted machining (EVAM) of pure iron

Abstract It is well known that diamond wears out rapidly (within several metres of cutting length) when machining low carbon ferrous alloys and pure iron. The past few years have seen a growing interest in the field of elliptical vibration-assisted machining (EVAM) due to it being successful in the micromachining of difficult-to-cut materials including steel. During EVAM, a cutting tool is prescribed an oscillatory motion perpendicular to the direction of cutting, thereby causing the tool to be relieved intermittently from chemical and physical contact with the workpiece. This phenomenon serves as a guideline to develop the simulation test bed for studying EVAM in this work to compare it with conventional cutting. The pilot implementation of the EVAM came as a quasi-3-dimensional (Q3D) elliptical cutting model of body-centred cubic (BCC) iron with a diamond cutting tool using molecular dynamics (MD) simulation. The developed MD model supplemented by the advanced visualization techniques was used to probe the material removal behaviour, the development of the peak stress in the workpiece and the way the cutting force evolves during the cutting process. One of the key observations was that the cutting chips of BCC iron during conventional cutting underwent crystal twinning and became polycrystalline, while EVAM resulted in cutting chips becoming highly disordered, leading to better viscous flow compared to conventional cutting.

A hybrid fabrication approach and profile error compensation for silicon aspheric optics

Aspheric optics is widely used for many optical applications due to their advantages, that is, light weight, cost-effectiveness and efficiency. There are many fabrication challenges which affect the quality of aspheric optics used for infrared-based applications. Diamond turning is one of the most suitable techniques for fabrication of infrared aspheric lens with high profile accuracies, due to its deterministic approach. However, for optics with large sag value, multiple machining cycles are required to make the best fit surface. Repeated machining cycles result in generation of inherent stresses leading to subsurface deformation and poor quality. In this study, hybrid approach of grinding and machining is proposed for fabrication of silicon infrared optics in large volume. The proposed approach results in reduced fabrication time and subsurface deformation with improved surface quality and tool life. The profile accuracy after compensation of profile error (Pt) is 0.21 µm and surface roughness (Ra) 10.5 nm is achieved.

Investigation of Surface Roughness of Single Point Diamond Turned Germanium Substrate by Coherence Correlation Interferometry and Image Processing

Germanium is a widely used material in the infrared range. Single crystal germanium is used as semiconductor and optical material due to its salient features like high refractive index and proper working in cryogenic conditions. Thus, germanium is an important substrate for infrared lens having many applications in thermal imaging cameras, optical telescopes and miniaturization of infrared optical elements. These applications require optical elements of excellent surface quality and high dimensional accuracy. In addition to fulfil the demands, ultraprecision machine is used to fabricate the optical components. In this work, single crystal germanium (111) mirror is fabricated by using single point diamond tool with, negative rake angle. A large number of experiments are performed to achieve the surface finish of nanometric range. The best and worst combinations of process parameters are found on the basis of surface roughness with the help of coherence correlation interferometry(CCI) measurement and image processing using Canny, Prewitt, Roberts and Sobel edge filters and histogram. These results can be used for fabrication of diffractive optical elements and aspheric lenses of germanium.