What is single point diamond turning?

How diamond turning works

Diamond turning is turning, as on any lathe: the part spins and a tool with one cutting edge moves across it and removes a thin chip. Three things make it different from ordinary turning.

• The machine. An ultra-precision lathe has a spindle running on air bearings, slides on oil bearings, and sits on a vibration-isolated base. It positions the tool in steps of nanometers and runs in a temperature-controlled room.
• The tool. The cutting edge is a single crystal diamond, polished so sharp and smooth that its edge is nearly perfect even under a microscope.
• The cut. The final pass removes only a few micrometers, with a very slow feed. The edge of the tool is copied onto the part, turn after turn.

The result is a surface with a roughness of a few nanometers and a shape accurate to a fraction of a micrometer. It reflects or transmits light straight from the machine.

Why the tool is a single crystal diamond

An optical surface can only be as good as the edge that cut it. A single crystal diamond is one continuous crystal, with no grains and no binder, so its edge can be polished sharper and smoother than any other cutting material. It is also the hardest material known and carries heat away quickly, so the edge keeps its shape over a long cut.

Tools made from diamond grains, such as PCD, are tougher, but their edge is limited by the size of the grains and cannot cut an optical finish. Diamond turning tools are made from natural diamond or from grown single crystal, MCD. For how these differ, see our guide to natural diamond, MCD and PCD.

Which materials can be diamond turned

What cannot be diamond turned:

• Steel and other ferrous metals. At cutting temperatures the carbon in the diamond reacts with iron, and the edge wears out after a very short cut. This is why steel mold inserts are coated with electroless nickel first: the nickel layer is diamond turned, not the steel.
• Glass. Optical glass is too brittle to turn in normal practice. Glass lenses are ground and polished, or pressed in molds.
• Very abrasive materials, which wear the edge too quickly to hold an optical shape.

Brittle crystals such as germanium and silicon can be turned, but only with very small cuts and a tool with a negative rake angle. Under those conditions the material comes off as a fine chip, as a metal does, and does not crack.

What diamond turning is used for

• Infrared optics. Germanium, silicon and zinc selenide lenses for thermal cameras and sensors.
• Metal mirrors. Flat, spherical and off-axis mirrors in aluminum and copper, for lasers, scanners and telescopes.
• Mold inserts. The nickel-plated inserts that shape plastic lenses for phone cameras, headlamps and head-up displays.
• Aspheres, freeform and diffractive surfaces. Shapes that are slow and costly to polish but only a tool path for a lathe.
• Fresnel lenses and fine grooves, cut with a sharp V-shaped tool.
• Contact lenses and intraocular lenses, turned from plastic blanks on small diamond turning lathes.

Turning, fly cutting and tool servos

• Turning. The part spins, the tool moves. This is the standard for round, symmetric optics.
• Fly cutting. The diamond tool spins on a large arm or head and the part moves past it. It is used for flat mirrors and long, straight grooves.
• Slow and fast tool servo. The tool moves in and out during each turn of the spindle, so the lathe can cut surfaces that are not round and symmetric: freeform optics and lens arrays.

What the tool edge has to do

Because the edge is copied onto the part, a diamond turning tool is specified in more detail than an ordinary lathe tool.

• Nose radius. The radius of the tip, from a few hundredths of a millimeter to several millimeters. A larger radius gives a smoother surface at the same feed; a smaller one reaches steep and small features.
• Rake angle. Usually 0° for metals and plastics, and negative, often around −25°, for germanium, silicon and other brittle crystals.
• Clearance angle. The relief under the edge. It has to be large enough for the tool to clear the steepest part of the surface.
• Waviness. How far the round edge strays from a perfect circle. That error is printed into the surface, so optical tools are made to a controlled waviness, from about a micrometer down to tens of nanometers.
• Included angle, for V tools that cut grooves and Fresnel patterns.

Diamond turning or grinding and polishing?

Traditional optics are ground and then polished, which works on glass and gives the smoothest surfaces of all. Diamond turning is faster for small and medium series, makes aspheres and freeform shapes as easily as spheres, and works on metals, plastics and crystals that are hard to polish.

Its weak point is the turning marks: a very fine, regular groove pattern from the tool feed. For infrared optics it does not matter. For visible light with the highest demands, parts are sometimes given a light polish after turning.

When the tool is worn

A worn edge shows first in the part: the surface turns hazy, fine lines appear, or the measured shape drifts. A single crystal diamond tool is not thrown away at that point. The edge is relapped to its original radius and waviness, and the tool can be resharpened several times.

Tools for diamond turning

Bulunmaz makes radius tools, V tools, fly-cutting tools and negative-rake tools in natural diamond and MCD, to the radius and angles on your drawing. See diamond turning tools for optics and lens molds, or send us your material and drawing.

We do not diamond-turn parts for customers.