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

2. Outline Introduction Materials SPDT Machines Surface Figure
Surface Finish
Post Finish
Advantages & Disadvantages of SPDT Technique
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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
Computer controlled cutting tool, monitored by interferometer
Vacuum chuck
Spindle with air bearing
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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.
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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
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OPTI 521

6. 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
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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
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OPTI 521

8. Surface figure test Talysurf contacting profilometer Interferometer
Scan the part
Measure the departure of the surface from theoretical shape
Interferometer
Overall performance
Null test
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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
Surface roughness: ~40 Å rms
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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
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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
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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 production
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13. Example
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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
11/13/2018
OPTI 521