GlossaryKind of term: Manufacturing termUltra-precision machining
What is ultra-precision machining?
Ultra-precision machining reaches sub-micrometre form accuracy and nanometre roughness, mostly by diamond turning in a temperature-controlled room.
Its accuracy lies beyond what a standard CNC machining centre achieves; roughness is then expressed as an Ra of a few nanometres. The best-known process is diamond turning with a single-crystal diamond tool; diamond milling, fly cutting and ultra-precision grinding also belong to it. The boundary with ordinary precision machining moves with the state of the art and is not fixed in a standard.
Where does the accuracy of ultra-precision machining come from?
The accuracy comes from the whole system: spindles and slides on air or hydrostatic bearings, a stiff and well-damped machine, temperature controlled to fractions of a degree and a cutting edge that is close to perfectly sharp. Not every material can be cut with diamond. Aluminium, copper, brass, high-phosphorus electroless nickel, many plastics and infrared materials such as germanium and silicon can. With iron, nickel and titanium the diamond wears chemically fast, because at cutting temperature its carbon diffuses into the metal or reacts with it. A steel mould therefore often gets an electroless nickel layer that is then diamond turned.
How does ultra-precision machining work in practice?
The work comes from optics, mirrors, lens moulds and precision mechanics. Machine, controlled room and interferometer inspection usually put the hourly rate of such a machine well above that of an ordinary lathe. For many shops diamond turning is therefore a subcontracted step, as is the electroless nickel plating before it. Measurement is preferably done in an environment as well controlled as the machining, sometimes on the machine itself: form with an interferometer or a contact profilometer, roughness optically, for example by white light interferometry.
What is ultra-precision machining often confused with?
Ultra-precision is not high precision on a standard machining centre, where a few micrometres are already tight. And diamond turning is not polishing: polishing lowers roughness and can spoil the form, while diamond turning produces form and finish in the same cut, along a defined toolpath.
Frequently asked questions
Which materials can be diamond turned?
Among others aluminium, copper, brass, gold, high-phosphorus electroless nickel, plastics such as PMMA and polycarbonate, and infrared materials such as germanium and silicon. Steel, titanium and nickel alloys hardly at all, while glass and most ceramics are ground and polished.
Why can't steel be diamond turned?
Because at cutting temperature the carbon of the diamond diffuses into the iron, so the edge wears chemically fast. A steel part therefore often gets an electroless nickel layer that can be diamond turned.
What is the difference between micro-machining and ultra-precision machining?
Micro-machining is about small features, cut with tools under a millimetre across. Ultra-precision is about form accuracy and roughness, and applies to a large mirror as much as to a small lens.