Ultraprecision Machining

Owing to ultraprecision machining – a special area of the manufacturing technology -, it is possible to fabricate workpieces with highest precision from nonferrous metals, plastics or semiconductor materials such as silicon and germanium single crystals.
By "highest precision",

• geometrical accuracy in the submicrometre range
• surface roughness in the nanometre range

are to be understood.

Furthermore, these plane, cylindrical, spherical and nonspherical surfaces must often be of good optical quality, i.e. well reflecting, and the geometry must be reproducible in equal quality.

To meet these heavy requirements, very great demands must be placed on the machine tool, the tool and the environment as regards accuracy and temperature stability.

Rotationally symmetrical workpieces are fabricated on a CNC lathe of Rank Pneumo make.

Figure: Rank Pneuma lathe and some spherical/nonspherical workpieces

A 4 t granite block cushioned on three vertically adjustable air springs forms the frame of the machine which is placed on a foundation separated by a gap from the environment. It thus is ensured that even minimum shocks from the environment are kept away from the machine and are not transmitted from the machine to the environment.

On the frame two air-cushioned slides are arranged vertical to each other. The Z-slide carries the main spindle which is also air-cushioned and the X-slide carries the tool holder.

A laser interferometer detects the motion of the slides and transmits their position to the control unit. The minimum distance the laser interferometer can resolve is 0,01 µm.

The high precision of the machine can be profited from only if during machining the workpiece is kept free from stress. This is achieved with a vacuum chuck aspirating the workpiece and fixing it on the spindle.

According to the geometry of the workpiece, the accuracies reached are smaller than 0,2 µm. The mean roughness is usually better than 0,01 µm.

Only monocrystalline diamonds whose circular cutting edge can be fabricated with a geometrical accuracy of 0,05 µm and better are used.

For the fabrication of predominantly prismatic workpieces a CNC vertical milling machine with Fanuc control of SIP make is used.

Figure: SIP milling machine and typical workpieces of silicon

With a positioning accuracy smaller than 1,5 µm over the whole range of travel of 700 mm x 500 mm x 625 mm, this machine is regarded as the most precise milling machine available worldwide. The metrological basis are steel index gratings with a resolution of 0,1 µm.

Semiconductor materials such as silicon and germanium single crystals require high cutting speeds for the grinding process. To achieve these, as an alternative, a pneumatic or electrically driven high-speed grinding-wheel spindle is adapted to the main spindle of the CNC vertical milling machine so that the milling machine is turned into a coordinate grinding machine. The maximum speed which can be achieved is about 180000 rpm..