Diamond turned optics are no longer just for prototypes. In fact, we coat quite a few medium to large volume substrates manufactured using single point diamond turning (SPDT). In this tech note, we’ll explore the advantages of SPDT, our coating capabilities in this area, and the unique benefits we offer to transition seamlessly from quick turn prototyping of SPDT optics to volume production.

What is SPDT?

Example of prism used in augmented reality gogglesSingle Point Diamond Turning (SPDT) is a process by which diamond cutting tools are used to machine high-precision optics from polymers and plastics, crystals, and metals. Diamond turned optics have low surface form errors and high surface finish quality, eliminating the need for additional polishing or processing prior to coating. Each optic is machined individually, making the process ideal for quickly generating unique or complex surface profiles for research or prototypes simply by feeding the optical design directly into a CNC machine.

Injection molded optics are very cost-effective in volume, but the cost of tooling design and fabrication adds both expense and design risk to development projects. For optical designers accustomed to waiting 12 weeks for a mold to be created, the 2-3 week leadtime of SPDT offers the ability to quickly prove out new parts and optimize system designs prior to committing to a certain geometry.

It also offers concrete design advantages. SPDT optics are much more accurate than molded parts in many cases, particularly for difficult, awkward, or large geometries and those involving focusing. Some specifications, such as those for virtual and augmented reality systems, simply can’t be met any other way. Diamond turning permits much more complex designs to be created and on a shorter time scale. The cost invested in their design can be recovered after just a few dozen parts, as opposed to 100+ pieces for molded optics.

As designers have moved to take advantage of the many benefits of SPDT, the number of vendors have increased, yields have improved, and the cost of diamond turned optics have come down. SPDT is now a routine production technique for medium to large volume fabrication as well, facilitated by its ability to be coated with no further optical processing. While SPDT optics are excellent substrates for multi-layer dielectrics or metals, the transition from fabrication to coating chamber isn’t necessarily “clean”.

Why AccuCoat?

The coating of diamond turned optics brings together many of the things that AccuCoat does best: specialized cleaning processes, expert tooling & fixtures, and the ability to coat complex shapes and a wide variety of materials. Given the increased cost of SPDT optics, it is crucial to work with a vendor who can dependably deliver the right coating at high yield. AccuCoat understands how to properly clean, handle, inspect, fixture, and coat these optics in order to ensure high quality and adhesion.

Cleaning SPDT Optics

precision ultrasonic cleaningThe injection molding process typically produces optics than can go direct from mold to coating chamber without cleaning. As a machining process, however, diamond turning is messier – often leaving behind residue, milled material, and even fingerprints due to handling. Parts must therefore be carefully cleaned prior to coating to ensure good adhesion and long-term reliability. AccuCoat has developed unique processes for pre-cleaning and preparing SPDT parts for coating. Through a series of studies and extensive experience, we’ve identified special detergents and optimized our cleaning processes to help remove the residues of the diamond turning process, as well as fingerprints left in handling.

Tooling & Fixtures

The flexible nature of the SPDT process allows it to create some very unusual geometries, which can be challenging both to mount and coat. Over time, we’ve built up a large inventory of adjustable, adaptable fixtures that are well suited to the wide variety of SPDT optic shapes, and have learned how different geometries need to be mounted to ensure precise, uniform coating. Diamond-turned optics were initially millimeters to a few inches in size, but can now be up to 8 inches across, all of which we can accommodate.

Coatings, Geometries & Materials

Single point diamond turning can be performed on plastic, polymer, or metal substrates to create very unique geometries and large optics. As use of SPDT processing does not limit coating compatibility, AccuCoat can apply all of our standard coatings to SPDT optics, from AR coatings to high reflectors, beamsplitters, polarizers, and specialized material coatings.

The high quality and rigorousness of the cleaning processes we have developed allows us to reliably coat the many materials optical designers use with SPDT, including all types of plastics and polymers. By optimizing the cleaning process to the substrate material, we are able to consistently meet the stringent environmental specification requirements of many MIL-SPEC standards.

Conclusion

Single point diamond turning now allows optical engineers to quickly create and cost-effectively refine optical designs while achieving highly accurate parts with sophisticated surface profiles. By pairing this capability with AccuCoat’s ability to reliably coat SPDT optics with any of our standard coatings in small or large volume, it is possible to create optical prototypes that accurately reflect the performance to be expected of the final product.  

Though SPDT circumvents the setup cost and leadtime of molded parts and significantly speeds the prototyping process, it still results in higher per-piece prices, making it essential to work with a coating vendor like AccuCoat who can deliver on every aspect of the process, from cleaning and fixturing to coating performance and reliability. Contact us to discuss how we can help take you from prototype to production with ease.

SPDT Optic Geometries

SPDT affords the optical designer tremendous flexibility in shape, surface structure, and size. AccuCoat is comfortable working with a wide range of SPDT optic geometries and form factors, including:

  • Aspheres
  • Toric lenses
  • Bi-conics
  • Cylinders
  • Diffractive lenses
  • Fresnel lenses
  • Polynomial freeform
  • Micro lens arrays
  • Gratings/rulings

Typical SPDT Materials

Many different polymers and plastics can be diamond turned directly. AccuCoat has coated a variety of SPDT substrate materials, including:

  • Acrylic / PMMA
  • Zeonex
  • OKP 1 & 4
  • Polyester
  • Ultem
  • Polycarbonate
  • Polypropylene
  • Polystyrene
  • Metals
  • Infrared (IR) materials