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Tool making

Ultra-precision tools as the key element

Aixtooling GmbH uses ultra-precision grinding with point contact kinematics as a fine machining technology, permitting the production, thanks to its great geometrical flexibility of aspherical, toroidal, or free-form contours depending on the axis configuration.

Ultra-precision machining
luftgelagerte Werkzeugspindeln der Toschiba ULG-100 D (SH3)<br>Source: Toshiba Machine Co.Ltd.
Werkzeugspindeln
Schleifbearbeitung

Principle

Ultra-precision grinding is the most important production technology that Aixtooling GmbH is intensively developing, and it is used to fabricate a variety of high-precision molding tools.

To produce the high surface qualities required for optical components, a ductile machining process is essential. In such a process, the abrasive grain penetrates into the workpiece surface during chip formation, leading initially to an elastic deformation of the material at the point of entry. Chip formation begins when the phase of elastic deformation yields to plastic deformation of the material. This process occurs only below a critical chip thickness. Small diamond grain sizes in high concentrations, low feed velocity, and high cutting speeds are required by this chip formation mechanism.

Depending on the workpiece geometry, different intervention conditions may apply to the grinding disk. These may also have effects on the results of the grinding process, in the form of shape and dimensional inaccuracy, as well as surface quality and border zone influences.

The grinding process is subject to several factors that can lead to the surface contour obtained deviating from the ideal contour. These are:

  • the grinding tool geometry
  • the relative axis position
  • the flexibility of the grinding coating
  • wear of the grinding coating
  • the flexibility of the spindle bearings
  • the flexibility of the grinding disk collet
  • the thermal expansion of the components involved in the process


In all, this means that it is essential to record the shape produced using a suitable measurement system (2D, 3D), to prepare the data and information obtained, and to feed it back into the machine controller for corrective machining. In this manner, it is possible to iteratively approach an optimum shape, the challenge being to meet all other requirements, for example regarding roughness or errors with medium frequency.

Machine technology

To satisfy the requirements of replicative optics, form precision of up to 100 nm PV (< 15 nm RMS) must be achieved during tool making. Extremely precise machine tools are needed, which are available from only a few manufacturers in the world. These machines are characterized by the highest possible axis precision (e.g. < 200 nm evenness over the travel length of 300 mm) and the highest possible roundness precision (< 50 nm) in the aerostatic spindle systems used. Today, driving the dynamic axes using linear motors also permits high-precision simultaneous movement in multiple axes, necessary for the production of complex rotationally asymmetrical geometries.

Aixtooling GmbH works with UP system technology from different manufacturers ? focusing on machines from Toshiba Machine Co. or Moore Nanotechnologies.

Tool systems

Besides the machining systems, tool systems also take on enormous significance in ultra-precision grinding. To achieve surface roughnesses of < 3 nm Ra right after the grinding process, the surface border zones must be machined fully ductile, that is, the individual grain chip thicknesses must be reduced to a minimum as described above.

To attain that goal, the grinding tools used must first be intensively prepared for use, that is, intensive, high-precision orienting and balancing processes must be used in order to avoid any pulsed load on the grinding disk, while still obtaining a cutting-friendly disk topography. Due to the extreme hardness of the workpiece materials, there must be wear mechanisms in the grinding tool. To make grinding possible at all, diamond grinding tools are used almost exclusively.

Machining kinematics

Aixtooling is fabricating tools today in a variety of forms. Tools can vary greatly, both in size and in shape. For this reason, different grinding kinematics are developed and qualified for optical requirements. Here is a list of some examples of kinematic variants:

  • Point contact grinding with X-Z kinematics
  • Point contact grinding with Y-Z kinematics
  • 4-axis insert grinding with line contact and X-Y-Z-C kinematics
  • Profile grinding with X-Z or Y-Z kinematics
  • 4-axis slow tool servo grinding with X-Y-Z-C kinematics
  • and many more

Finish machining - polishing processes optimize surface topography
Formprüfinterferometerische Messung

Final finish machining for the finest surfaces

The basic goal of the UP-CNC grinding process is to achieve form and dimension precision specifications as well as to obtain the finest possible surface quality. Despite this, the associated grain penetrations still mean that surfaces are characterized by microscopically small, but directed, grooves. In optical applications, these microgrooves can lead to negative phenomena, for example, scattering or refractive effects, and must therefore be removed in many cases.

In the polishing process, the ground tools are therefore subject to finish machining, in which the topography left by the grinding process is smoothed down to the component surface. A polishing agent carrier holds an abrasive agent, and is brought into contact with the workpiece with pressure and relative velocity. The abrasion/wear hypothesis determines how microchip formation produces the surface quality desired.

Since no mechanical polishing concepts are available for the processing of such complex geometrical surfaces, or they are impractical from a business standpoint, manual polishing is used in many applications. Here, a high degree of experience is necessary to avoid endangering the precision already achieved during the polishing process.

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© 2009 Aixtooling GmbH
Steinbachstraße 17, 52074 Aachen, Deutschland
Phone +49 241 8904-499
Fax +49 241 8904-6499
E-Mail: info@aixtooling.de
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