SinglePoint Diamond Turned Technique

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Single-Point Diamond Turned Technique

• Ping Zhou

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Outline

• Introduction
• Materials
• SPDT Machines
• Surface Figure
• Surface Finish
• Post Finish
• Advantages Disadvantages of SPDT Technique

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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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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

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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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Surface figureradius 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 (gt 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)

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Surface figureaspheric 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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Surface figure test

• Talysurf contacting profilometer
• Scan the part
• Measure the departure of the surface from
  theoretical shape
• Interferometer
• Overall performance
• Null test

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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
• Dont use scratch/dig specification

Surface roughness 40 Å rms
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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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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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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

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Example
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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