Single-Point Diamond Turned Technique Ping Zhou 11/13/2018 OPTI 521

Outline Introduction Materials SPDT Machines Surface Figure Surface Finish Post Finish Advantages & Disadvantages of SPDT Technique 11/13/2018 OPTI 521

Single-point diamond turning Use a single-crystal diamond cutting tool to accurately turn the surfaces to finished tolerances SPDT technique (1960s) Linear axes position feedback resolution: 8.6nm Rotary axis encoder angular resolution: 0.25 arc sec Mostly in IR application due to the surface finish Computer controlled cutting tool, monitored by interferometer Vacuum chuck Spindle with air bearing 11/13/2018 OPTI 521

Materials Materials that are not diamond-machinable Silicon based glass and ceramics, steel, beryllium, titanium, molybdenum... Diamond-machinable materials Metals Alluminum 6061, copper, brass, gold, nickel, silver, tin, zinc… Polymers (molecular length and elasticity) Acrylic, acetal, nylon, polycarbonate, polystyrene… Crystals ZnS, ZnSe, Ge, CaF2, Si… Ceramic: too brittle Al 6061: a preferred material for diamond machining because it is inexpensive, machines well, and has good reflectivity even uncoated. Al does not wear diamond tools rapidly Polymer: Molecular chain length, and elasticity characteristics Silicon can be diamond-turned but wear the cutting tools rapidly. Generally, single crystal materials machine to better surface textures than those machined from polycrystalline blanks. 11/13/2018 OPTI 521

Two basic types of SPDT machines Lathe type: Work piece rotates & diamond tool translates Axisymmetric surface Off-axis optics Flycutter type: Diamond tool rotates & work piece translates Flats Multi-faceted prisms 11/13/2018 OPTI 521

Surface figure—radius of curvature & irregularity Typical tolerance for radius of curvature: ±0.05% Relatively short radii: limits of spherometer accuracy: about 0.005% Long radii (> 2 m) : lower accuracy, error in the order of 0.1 – 1.0% Irregulairty The height departure from the ideal surface figure ¼ power specification (Rule of thumb) Example: 4 fringes of power, 1 fringe surface irregularity 11/13/2018 OPTI 521

Surface figure—aspheric figure Rotationally symmetric surface or off-axis section of the surface that has rotational symmetry General asphere with spherical, even and odd polynomial terms Toroid Concave or convex cylinders and axicons (almost impossible with conventional process) Fresnel lens, or grating type surface Surface defined by splines or differential equation Surface slope 2 arc sec or 10 microinches/ inch 11/13/2018 OPTI 521

Surface figure test Talysurf contacting profilometer Interferometer Scan the part Measure the departure of the surface from theoretical shape Interferometer Overall performance Null test 11/13/2018 OPTI 521

Surface finish A periodic residual grooves from the tool Groove height depends on the feed rate & the radius of the cutting tool Groove height: ~10nm Grooves scatter and diffract light Limits the application of SPDT technique in infrared Don’t use scratch/dig specification Surface roughness: ~40 Å rms 11/13/2018 OPTI 521

Post finish SPDT usually provide adequate surface figure accuracy Post finish can smooth out the residual grooves and minimize the scatter for the application of shorter wavelength Avoid post finish: it may change the surface figure 11/13/2018 OPTI 521

Advantages of SPDT Easy to program Setup can be changed quickly for short runs of optics High degree of precisions in mechanics Permits variations on surface shape, aspheric, toroidal, diffractive optics… Fast: directly turn the surface to its finished tolerances Cost effective 11/13/2018 OPTI 521

Limitation of SPDT Produce only one optic at a time The size of the work piece is limited by the size of the SPDT machine Mostly applied to infrared optics Not efficient for mass production 11/13/2018 OPTI 521

Example 11/13/2018 OPTI 521

Reference 1. The Handbook of Optical Engineering 2. Robert A. Clark, Design and Specification of Diamond Turned Optics 3. Hillary G. Sillitto, Analysis, tolerancing and diagnosis of diamond maching errors 4. Mark Craig Gerchman, Specifications and manufacturing considerations of diamond machined optical components 5. E.R. Freniere and J. Zimmerman, Specifications for diamond-turned surfaces 6. Paul R. Yoder, Opto-Mechanical System Design 11/13/2018 OPTI 521