Single-point diamond flycutting and two-axis diamond turning produce high-quality metal mirrors, while the fast-tool servo system adds nonrotationally symmetric capabilities.
Michael R. McKinney, Scott Mitchell, and John Reid
Diamond turning is the precision machining of optics with diamond-tipped tools. Whether it is through a rotating optic and nonrotating diamond tool, or a rotating diamond tool and nonrotating optic, diamond turning provides an economic means of optically machining reflective materials such as copper, aluminum, and brass into mirrors. Such mirrors may have a surface finish of less than 5 nm root-mean-square (RMS) and a figure accuracy of a quarter-wave (l/4) peak-to-valley at 0.6328 µm. Diamond turning also enables the optical machining of complex geometries including aspheres, diffractives, and conic sections. Distinct types of diamond turning include flycutting and two-axis turning; in addition, a "fast-tool servo" system has been introduced that is capable of three-axis turning, including the optical machining of nonrotationally symmetric designs.
Single-point diamond flycutting uses a flywheel-mounted diamond tool. The flywheel spins above a nonrotating optic. The optic is mounted to a precision single-axis track (machine slide) that tracks the optic beneath the spinning flywheel. As the optic tracks beneath the flywheel, the diamond tool makes contact with and tracks across the surface of the optic, shaving off minute threads of material while creating a highly reflective, smooth, and optically flat surface (see Fig. 1).
Flycutting machines are engineered to maintain extremely rigid machining tolerances, with machine slides possessing sufficient inherent stiffness to withstand any lateral or vertical movement. The latter is also true for the air-bearing spindles that spin the flywheels: typically, most spindles have axial and radial runouts in the 0.1-µm range. To isolate the flycutting machines from external vibration, granite bases of substantial size and mass are used in both active and passive modes. Finally, precision motion control of the machine slides is provided through a feedback control loop. (Typically, this control loop is maintained by sophisticated electronics, in which axis movement and machine control are passed to the operator by means of computer numerical control.)
Single-point diamond flycutting is capable of producing surface finishes on oxygen-free high-conductivity copper of less than 5 nm RMS, surface finishes on 6061 aluminum of less than 6 nm RMS, and figure accuracies of l/4 peak-to-valley at 0.6328 µm on both materials.
Some of the many complex geometries that can be produced by diamond flycutting include industrial laser beam-delivery mirrors, industrial laser-cavity mirrors, steering and head mirrors for military and aerospace use, deformable mirrors, faceted lenses and mirrors for laser beam integration, pyramidal polygons for laser scanning, components for space and cryogenic applications, and optical housings. (While optical housings are often overlooked, excellent geometry control of precision optical housings can be achieved, and this control provides the mechanical and optical designer the luxury of bolt-together optical designs.)
Most of the materials that can be optically machined into mirrors via single-point flycutting are nonferrous metals. If weight reduction and stiffness is necessary in the optic, ferrous metals to which a diamond-machinable surface plating (for example, electroless nickel or pure aluminum) has been added can also be used.
The two-axis diamond-turning process is used to produce rotationally symmetric components for applications ranging from carbon dioxide lasers to infrared (IR) imaging systems (see Fig. 2). Products include parabolic beam-focusing optics, spherical mirrors for laser cavities, aspheric focusing lenses, telecentric lenses for micro-via drilling, reflective beam expanders, cylindrical optics, custom aerospace components, reflective telescopes and seeker/sensor optics, and components for space and cryogenic applications.
With two-axis turning, a nonrotating diamond tool is mounted on a machine slide that is arranged perpendicular to the optic to be machined. The optic is mounted on a spinning platform, or spindle. The spindle moves the optic laterally across the diamond tool, thus giving the diamond tool two axes of motion relative to the optic. The position of both the diamond tool and the spinning optic are controlled by mathematical equations that define the desired cross-sectional curve, or finished shape of the optic.
As with flycutting machines, to produce high-quality optics, steps must be taken to minimize friction and vibration generated by the two-axis machine itself and to isolate the machine from external disturbances. Furthermore, to achieve the necessary machining precision, the machine slide ways ("tracks") must be lapped extremely flat (0.5 µm over any 150-mm section is a popular specification) and must also be assembled to a very high level of perpendicularity. The resolution of the positioning feedback system must also be very good. Some machines use laser interferometers with better than 2-nm resolution, but because of their expense and extreme sensitivity the current trend is to use linear glass scales with better than 10-nm resolution. The spindle that rotates the part must also be very precise in its motion, so air-bearing spindles with very low levels of runout are needed. With these features in place, optics with good form accuracies can be produced.
Friction and vibration must also be controlled to allow good surface finish to be achieved and to reduce temperature fluctuations that can be detrimental to form accuracies. Air-bearing spindle and hydrostatic oil-bearing slideswhich do not exhibit any metal-to-metal contactare used to minimize the effects of friction and vibration. The diamond turning machine must also have features to reduce the effect of vibration from external sources such as heating, ventilation, and air-conditioning equipment; nearby roads or highways; construction equipment; or other production machinery. Large granite bases coupled with either pneumatic air bags or rubber isolation pads combat this effect.
The diamond tool must be of very high quality for good optics to be produced. The most popular tools are natural, single-crystal, gem-quality diamonds of about 0.25 carat. Some tool manufacturers also use synthetic diamonds. The two-axis diamond-turning process makes use of an arc on the tool dependent on the gradient of the part being machined; the diamond tool itself must have a very accurate radius, since the diamond tool is essentially replicating itself onto the optic. In other words, if the tool has form inaccuracies, then the optic will have form inaccuracies also. Approximately 0.05 µm is the current level of tool accuracy produced by diamond-tool manufacturers. A very accurate relationship between the axis of revolution of the air-bearing spindle and the tip of the diamond tool must also be established. A popular method is to diamond-turn a spherical test piece and test it with an interferometer. The established tool relationship is then adjusted until the test piece exhibits acceptable form accuracy.
A newly available technology in two-axis diamond turning is the fast-tool servo system. This system, introduced commercially in the United States by II-VI Inc., has in essence a three-axis turning capability that allows nonrotationally symmetric geometries to be produced by driving the diamond tool with a piezoelectric transducer at high frequencies (up to 1000 Hz). With the current technology, the maximum range of the nonrotationally symmetric portion is 30 µm. This option had been available at the research level for several years prior to its recent entry into the commercial marketplace. Potential products include laser-beam-shaping optics, laser-resonator mirrors, and toroidal mirrors (see Fig. 3).
With the capability to machine various metals into optical-quality mirrors at an economical price, as well as the capability to create complex geometries not achievable through other machining means, diamond turning is the method of choice for machining rotationally symmetric metal mirrors. Fast-tool servo capabilities further extend the reach of diamond turning, positioning it as the method of choice for the manufacture of nonrotationally symmetric optics made from machinable materials.
MICHAEL R. MCKINNEY is graphic communications coordinator and SCOTT MITCHELL and JOHN REID are project engineers at II-VI Inc., 375 Saxonburg Blvd., Saxonburg, PA 16056; e-mail: [email protected].