Diamond turning
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Diamond turning is a process of mechanical machining of precision elements using Computer Numerical Control (CNC) lathes equipped with natural or synthetic diamond-tipped cutting elements. This process is also known as Single-Point Diamond Turning (SPDT). The process of diamond turning is widely used to manufacture high-quality aspheric optical elements from glass, crystals, metals, acrylic and other materials. Optical elements produced by the means of diamond turning are used in optical assemblies in telescopes, TV projectors, missile guidance systems, scientific research instruments, and numerous other systems and devices.
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[edit] Process
Diamond turning is a multi-stage process. Initial stages of machining are carried out using a series of CNC lathes of increasing accuracy. A diamond-tipped lathe tool is used in the final stages of the manufacturing process to achieve sub-nanometer level surface finishes and sub-micrometer form accuracies. The surface finish quality is measured as the peak-to-valley distance of the grooves left by the lathe. The form accuracy is measured as a mean deviation from the ideal target form. Quality of surface finish and form accuracy is monitored throughout the manufacturing process using such equipment as contact and laser profilometers, laser interferometers, optical and electron microscopes.
[edit] The machine tool
For best possible quality natural diamonds are used as single-point cutting elements during the final stages of the machining process. A CNC SPDT lathe rests atop a high-quality granite base with micrometer surface finish quality. The granite base is placed on air suspension on a solid foundation, keeping its working surface strictly horizontal. The machine tool components are placed on top of the granite base and can be moved with high degree of accuracy using a high-pressure air cushion or hydraulic suspension. The machined element is attached to an air chuck using negative air pressure and is usually centered manually using a micrometer. The chuck itself is separated from the electric motor that spins it by another air suspension.
The cutting tool is moved with nanometer precision by a combination of electric motors and piezoelectric actuators. The motion of the tool is controlled by a list of coordinates generated by a computer directly from a CAD model. The final surface is achieved with a series of cutting passes of decreasing depth.
[edit] Quality control
Despite all the automation involved in the diamond turning process, the human operator still plays the main role in achieving the final result. Quality control is a major part of the diamond turning process and is required after each stage of machining and, sometimes after each pass of the cutting tool. Even a minute error during any of the cutting stages would result in a defective part if it were not immediately detected. The extremely high requirements for quality of diamond-turned optics leave virtually no room for error.
The SPDT manufacturing process produces a relatively high percentage of defective parts, which must be discarded. As a result, the manufacturing costs are high compared to conventional polishing methods. Even with the relatively high volume of optical components manufactured using the SPDT process, this process cannot be classified as mass production, especially when compared with production of polished optics. Each diamond-turned optical element is manufactured on an individual basis with extensive manual labor.
