Ultra-precision machining of grayscale pixelated micro images on metal surface

In this paper, a novel machining technique, diamond micro lithography (DML), is developed to provide a possible solution to generate grayscale micro images on metal surfaces. This technique is capable of converting a grayscale image into a pixelated array of customized micro features. The micro feature cell is designed to be a micro inverted pyramid structure as the constituent pixel, which can be engraved by a V-shape sharp diamond tool using a multi-axis ultra-precision machine. Based on an assumption that the perceived luminance by the observer is directly related with the size and geometry of the respective pyramid structure machined, the cell size of each micro feature is designed to have a value linearly related with the grayness value of the corresponding pixel. The distribution of feature-based pixels is determined by superimposed projection algorithm, a customized image processing technique developed in this study to convert the grayscale image to CNC machining codes. A series of machining tests are conducted to evaluate the effect of cell aperture size and cell aspect ratio on the grayness of pixels as well as to prove the assumption made on the relationship between cell geometry and pixel grayness. To explore the performance of DML, the size of machined grayscale image on metal surface is continuously scaled down from 2 mm to 100 μm radius, which is still able to be recognized with the aid of a microscope, demonstrating the capability of DML in transferring pixelated image on metal surface with high fidelity and intelligibility in a wide range of image size. The machined metal workpiece has the potential to be used as mold to replicate the vivid grayscale images onto polymer products for anti-counterfeiting purpose.