tant consideration and is usually
specified for diamond-turned parts. But,
oddly enough, the tool wear characteristics of materials do not correlate with
the level of surface roughness that
can be routinely achieved with them.
Specifically, for diamond-turned CaF2
parts, a surface roughness in the 50-Å
range is typically achieved, while ZnS
and ZnSe can be routinely machined to
the 40-Å level. Silicon wears tools rapidly but cuts cleanly, and a 15-Å surface roughness value is possible with
this material. Germanium is again a
standout here, and surface roughnesses
as low as 10 Å can be achieved on it. Specifying surface roughness values significantly lower than those enumerated here is
likely to drive up production costs substantially.
For all materials, aspect ratio (diameter:thickness ratio) is another important consideration. Components with a high aspect
ratio are particularly difficult to produce and tend to have some
edge roll-off (an area where the actual shape departs from the
desired curve). Typically, the clear aperture (the diameter over
which specified tolerances and performance must be maintained)
for diamond-turned optics is 90 percent of the part diameter.
Specifying a larger clear aperture than this on a component with
a relatively high aspect ratio would drive production costs up. In
other words, if the shape of the part must be held near to the
edge, it’s better to make the part thicker.
One advantage of diamond turning
is that accommodating features such as
through holes or mounting flanges while
the part is being turned is fairly easy.
However, the production of center holes
in particular can drive up fabrication
costs because it eliminates the curve
vertex that is often used as the reference
point for subsequent measurements of
Because of their shapes, aspheric
surfaces typically cannot be measured
using exactly the same techniques em-
ployed with spherical surfaces. The simplest and least expensive
method for gauging aspheric surface shape is with the use of a
contact profilometer. These instruments operate by drawing a
small stylus over the surface and measuring its up-and-down
movement. Each individual measurement is performed along only
a single line, so measurements at multiple different azimuthal an-
gles may sometimes be required to build up a three-dimensional
picture of part shape.
Laser interferometers, which are used almost universally for
testing spherical optics to very high degrees of precision, also can
be adapted for measuring aspheres, usually with the addition of
correction optics (called null optics) that suitably modify the
wavefront. This is typically accomplished with a computer-gener-ated hologram (CGH) specifically designed to make the neces-
Figure 3. Single-point diamond turning can be used on a variety
of materials and can produce features such as through holes,
mounting flanges and even grating profiles.