Single-Point Diamond Turned Technique

1. 8/7/2012 OPTI 521 1 Single-Point Diamond Turned Technique Ping Zhou

2. 8/7/2012 OPTI 521 2 Outline Introduction Materials SPDT Machines Surface Figure Surface Finish Post Finish Advantages & Disadvantages of SPDT Technique

3. 8/7/2012 OPTI 521 3 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

4. 8/7/2012 OPTI 521 4 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.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.

5. 8/7/2012 OPTI 521 5 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

6. 8/7/2012 OPTI 521 6 Surface figure—radius of curvature & irregularity Radius of curvature 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 irregularityExample: 4 fringes of power, 1 fringe surface irregularity

7. 8/7/2012 OPTI 521 7 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

8. 8/7/2012 OPTI 521 8 Surface figure test Talysurf contacting profilometer Scan the part Measure the departure of the surface from theoretical shape Interferometer Overall performance Null test

9. 8/7/2012 OPTI 521 9 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

10. 8/7/2012 OPTI 521 10 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. 8/7/2012 OPTI 521 11 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

12. 8/7/2012 OPTI 521 12 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 productionNot efficient for mass production

13. 8/7/2012 OPTI 521 13 Example

14. 8/7/2012 OPTI 521 14 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