Cutting a mirror
Most people think of lenses and mirrors as ground and polished. Diamond turning takes a different route. A tool with a single, extremely sharp diamond point cuts the final surface directly on a lathe, leaving a smooth, reflective finish with very small errors in shape. Surfaces made this way can be so smooth that they need little or no polishing, especially for metal mirrors and infrared optics.
The word “ultra-precision” matters. This is not a standard lathe with a diamond tool fitted. It is a purpose-built machine designed to move and cut with accuracy measured in tiny fractions of the wavelength of light. Specific accuracies vary by machine and are best taken from maker specifications.
The tool
The cutting tool is a single diamond, natural or synthetic, shaped and lapped into a wedge with an edge that is extremely sharp and smooth. Single crystal diamond is used because the edge can be polished to a very fine, nearly flaw-free finish, which is difficult with polycrystalline material. The crystal is oriented carefully, because diamond wears and splits differently in different directions, a consequence of the cleavage planes in the crystal.
Tools come in several shapes:
- Round-nose tools, with a small rounded tip, used for curved surfaces including spheres and aspheres.
- Flat or straight-edge tools, for facing flat surfaces and some grooving.
- Specialized shapes, for diffractive patterns, grooves and microstructures.
Any imperfection in the edge appears as a mark on the part, so tool quality is closely checked under high-powered microscopes.
The machine
Several design features separate these machines from ordinary lathes.
- Air or hydrostatic bearings. The spindle and slides float on a thin film of air or oil, which removes metal-to-metal friction and gives very smooth, repeatable motion.
- Very stiff, vibration-damped structure. Bases are often granite or similar materials, and the machines sit on isolation mounts.
- Fine position feedback. Linear scales or laser interferometers track the position of the axes to extremely small distances.
- Temperature control. Metals expand with temperature. Rooms and coolant are held at stable temperatures, since a small drift changes the dimensions.
- Clean coolant or mist. A fine, filtered coolant stream removes chips and manages heat without upsetting the part.
Some machines add a fast tool servo or slow slide servo, which moves the tool in and out as the part rotates. This allows non-rotationally-symmetric (freeform) surfaces to be produced.
Materials that can be diamond turned
Diamond turning works best on materials that are soft or ductile enough to be cut without causing damaging cracks, and that do not react with carbon.
- Aluminum alloys, especially specially processed grades, commonly used for mirrors.
- Copper and brass, including as molds and mirrors for high-power laser systems.
- Electroless nickel plating, deposited on a substrate to give a hard, fine-grained surface that cuts well. This is a common way to get mirror finishes on aluminum or other bodies.
- Infrared crystals, such as germanium, silicon, zinc sulfide and zinc selenide, for infrared optics. These are brittle, but under suitable conditions they can be cut in a ductile mode, with very small depths of cut.
- Some plastics, such as acrylic and other polymers used for lenses and molds.
Materials that are difficult
Steel and other ferrous metals cannot be diamond turned in the normal way because the diamond wears away rapidly through reactions with iron, the same core limit described for ferrous metal machining. Titanium and some other metals also present problems. Ordinary glasses are generally ground and polished instead, though some special glasses and brittle materials can be machined by ductile-regime grinding. Hard ceramics are normally ground with diamond wheels.
Why ductile mode matters for brittle materials
Normally, cutting a brittle material makes it crack. At very small depths of cut, however, the stress state can be such that material deforms plastically rather than fracturing, which allows a smooth surface without cracks. This is the principle that allows germanium or silicon to be turned. It requires extremely small feeds, high stiffness, and a good tool, and the process window is narrow.
What limits the result
- Tool wear. Even diamond wears slowly, and edge wear alters the surface. Long cuts on abrasive materials such as silicon-containing aluminum can be a problem.
- Machine errors. Slide straightness, spindle error motion and thermal drift show up directly in the part.
- Material quality. Inclusions, pores and grain boundaries in the workpiece show up as defects or step marks, which is why mirror-grade metals are carefully processed.
- Tool marks. A turned surface carries fine, regular grooves from the tool feed. Smaller feeds and a larger tool radius reduce this, at the cost of time.
- Measurement. Measuring such surfaces requires interferometers and profilometers, and the measurement setup often limits what can be verified.
Typical applications
Diamond-turned parts include infrared lenses for thermal imaging, mirrors for telescopes and laser systems, molds for lens production, laser scanner polygons, and precision components for sensing. Where the finished part is a mold, it is then used to replicate many optical parts in plastic or glass. In the broader field, diamond optics and diamond windows are different topics, where diamond is the workpiece rather than the tool.
Is it worth it
The machines and tools are expensive, and the process requires skilled operators and controlled environments. It tends to be justified when the shape is complicated (aspheric or freeform), when the part is metal or infrared material that is difficult to polish, or when it replaces several finishing steps. For simple spherical lenses in common glass, traditional methods usually remain more economical.