4
Milling tools for precision optics, spectacle lenses and technical components
Precision optics covers all kinds of optical components for imaging systems such as objective lenses, microscope
components, telescope mirrors, laser systems, nuclear engineering, data processing and many other applications.
Spectacle lens manufacturing is an area within precision optics.
Technical components covers all other components, some of which require extreme accuracy with techniques similar
to those used in precision optical machining.
The steps in machining with diamond tools are:
Rough milling - fine milling - cutting off
for spherical, aspherical and flat surfaces.
Fine milling with pellet tools
Pellets are metal or resin bond diamond tools, largely cylindrical in shape, in the diameter range 4...10...(15)mm; before
application, they have to be bonded or brazed into position. Their surface geometry has to be adjusted to the desired
shape, within the specified tolerances, by grinding them over before using them for machining.
The introduction of pellet tools improved the geometric accuracy and thus the life of fine milling tools so much that loose
abrasive for lapping (not to be confused with polishing agents) was replaced, and considerable progress was made in
rationalisation and automation of processes.
Definitions of optical terms
Used to describe all lenses which have a spherical surface at least on one side, defined by a radius of
curvature.
Used to describe surfaces which are neither spherical nor flat, e.g. a parabolic surface, or special non-
spherical shapes for spectacle lenses.
Used to describe a surface having two curves superposed on one another with a certain angle between
them, i.e. the "base curve". This is comparable with the surface of a car tyre - the base curve corresponds
to the outer diameter of the tyre, while the cylindrical curve corresponds to the curve across the tread of
the tyre.
This is the unit of strength or focal power of a lens. The diopter is the international standard unit of power.
One diopter (1D) is equivalent to the reciprocal of the focal length (B) in metres:
1D = 1 m, 10D = 0.1 m =100 mm.
The formula for calculation of the radius of curvatureis:
Radius of curvature R = (n
d
- 1) : B
where n
d
is the refractive index of the type of glass used.
Example: n
d
= 1.523 for crown glass BK7 (Schott), desired power D = 3.
B = 1 : 3 = 0.333 mm.
Thus radius of curvature R = (1.523 - 1) : 0.333 = 1.5705 m.
Diamond tools
Spherical
Aspherical
Toric
Diopter
Precision optics
Spectacle lenses
Technical glass
components
Fine milling with pellet tools
diamond tools
