Ultraprecision machining is often defined generally terms because of the removal of fabric from a substrate utilizing a machine that operates at a resolution of 10 nm (0.4 μin.) or less. The machining process may take the shape of single-point diamond turning or free-form machining.
Although single-point diamond turning has been around for quite fifty years, the foremost significant technological advances first appeared within the 1980s. Today, diamond machining may be a well-established and affordable process for the fabrication of highly accurate optical components also as mechanical components requiring microinch dimensional tolerances.
Free-form machining has also proved to be an efficient manufacturing process. A free-form optical surface is one that’s not symmetric about any axis. Multiaxis machining and metrology systems are required to manufacture these optical components in various materials. Such a machine provides a reliable and proven deterministic method for producing a good sort of electro/optical components including thermal imaging and night-vision systems, reflective mirrors for CO2 and YAG laser applications, crystalline materials for UV and microlithography, electroless nickel lens mold inserts, plastic lenses, telecommunication components, and alignment devices.
When combined with an ultraprecision vibration-free machine, a compact rigid tool holder, and a stable well-balanced fixture, a single-crystal natural diamond cutter will remove material from the substrates cleanly and efficiently. due to the acute level of sharpness on the diamond tool’s leading-edge, very small forces are generated during the machining process. the top result’s a surface that exhibits optical qualities in both surface finish and form accuracy. additionally, single-crystal diamond cutting tools possess several other unique physical qualities that make them ideal for ultraprecision machining applications.
Minimal tool wear
When machining components to optical quality requirements, the slightest amount of tool wear can adversely affect the standard of the machined surface. due to their extreme hardness and resistance to wear, diamond tools maintain their high cutting-edge quality throughout the machining cycle better than others. This leads to highly reflective, low-scatter optical surfaces with figure accuracies on the order of two to 4 μin., peak-to-valley, as measured with a laser interferometer operating at 632.8 nm.
Unlike carbide or CBN (cubic boron nitride) tools, which have random grain structures, single-crystal diamond tools have a really clear and well-defined grain structure. When mounting the tool in its shank, tool manufacturers orient the tool so on make optimal use of the toughest point of the diamond. this is often formed at the intersection of the highest rake, which is generated within the 110-plane of the diamond, and therefore the nose radius, which is generated within the 100-plane of the diamond. This orientation provides for the longest possible tool life, while also maximizing the tool’s resistance to wear. due to the single-crystal grain structure, diamond tools are often sharpened to the extent of atomic spacing, approximately 3 to five Å.
Proper orientation of the diamond in its shank, combined with precise grinding, lapping, and polishing techniques, enables the cutting-edge radius to be accurately controlled. this suggests that the diamond tool’s form is often controlled to within microinch tolerances throughout the cutting arc (typically 100°) of the tool. Controlling edge waviness is extremely important when the last word in part-figure accuracy is required since it reduces the likelihood of errors being transmitted from the tool onto the workpiece.
Because diamond has a particularly low coefficient of friction, chips created during the machining process have a natural tendency to slip across the highest rake of the tool and far away from the workpiece. This eliminates the buildup of chips at the tool’s leading-edge, thus minimizing the scratching of the workpiece. Also, due to the particularly sharp leading edge and low coefficient of friction, little or no heat is generated during the method. the tiny amount of warmth that’s generated during the machining process is quickly drawn far away from the workpiece due to the high thermal conductivity of the diamond tool. and since diamond has a particularly low coefficient of expansion, the tool remains very stable during the machining process. this suggests that the tool geometry remains constant, which is critical when attempting to realize optimum quality optical surfaces.
Materials for machining
In most instances, many of the nonferrous metals lend themselves nicely to diamond machining. additionally, several polymers and crystals also are suitable. There are several materials that can’t be machined using single-point diamond machining, the foremost obvious being ferrous metals and glass. the lack of machine ferrous metals with diamond cutting tools results from a reaction within the presence of oxygen between the diamond and carbon. This reaction causes graphitization, which rapidly destroys the leading edge of the diamond tool.
The inability to diamond machine glass results from an identical condition. The reaction between the silicon content of the glass and therefore the carbon content of the diamond creates carbide, which also causes graphitization.
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