Tech Briefs Magazine - May 2021 - PIT-20

imum weight it is able to machine.
Depending on the kinematics of the
machine, the optic might be spun
and/or translated and the motors that
affect these motions need to have
enough torque to generate the required
acceleration. In some cases, this means
that the machine has to be configured
specifically for heavy workpieces, which
might lead to longer cycle times and
thus higher cost.
In general, machine manufacturers
select motors strong enough to machine
the weights typical of workpieces in the
size class, so this should be less of an
issue. However, keep in mind that during
manufacturing the optic is typically bonded to a carrier for easy transfer and alignment between machines and measuring
equipment, which also adds weight.

Profilometer
Stylus

Aspheric Lens

Figure 2. Stylus of a tactile profilometer for asphere metrology.

Metrology Considerations
Diameter
Speaking about measurements, the
limitations of the metrology equipment
should not be overlooked. And, of
course, the metrology platform needs to
have enough travel to reach the full
diameter of the optic.

Figure 3. Surface quality inspection stations.

grinding and polishing machines. Many
machine manufacturers conveniently
label their models (e.g. CNC100,
CNC200 or CNC300), which tends to be
related to range of motion of the
machine. Unfortunately, this does not
mean that a " CNC200 " machine can be
used to machine a 200 mm diameter
large asphere.
For starters, during the manufacturing process a larger diameter blank is
used, which is then edged-down to the
final diameter in one of the last processing steps. But more importantly, the
size limitation of a machine is given by
a combination of the machine's kinematics and the shape of the optic. For
instance, let us assume that the optic is
placed facing up on a spindle in the
center of the machine, and a spinning
disk tool is moved radially, starting at
one edge and finishing at the other,
and the vertical position of the tool is

adjusted by the machine as required by
the shape of the optic. From there, it
follows that for a concave optic the tool
has to travel much less horizontally to
machine the same sized part than for a
convex optic.
The optical fabrication engineer might
be able to squeeze out an extra few millimeters of range by tweaking the process
parameters, but these most likely will negatively impact cost and/or quality and/or
lead time. In the example above, one
might reduce the diameter of the wheel,
but this will limit the cutting speed and
increase the process time and increase the
tool wear. As such, these labels do not
indicate a hard limit, but a transition from
economical to expensive to not feasible.
Weight
In addition to the dimensions of an
optic, an asphere grinding and polishing machine also has a limit on the max-

Sagittal Height
During manufacturing, an asphere is
typically measured using a tactile profilometer. With increased size of the
optic, it is also likely that the sagittal
height is increased (but this very much
depends on the actual design of the
asphere). Another limiting factor of a
tactile profilometer, in addition to the
travel range, is the height of the stylus
used. This limits how much it is able to
reach over the vertex of a convex
asphere to measure the profile of the
surface on the opposite side (Figure 2).
A concave optic has an analogous limitation for reaching into the optic to
measure the vertex. There are some
tricks that the optical fabrication engineer can apply to squeeze some more
mileage out of the metrology platform
that he/she has at their disposal, but
these will again affect cost and/or quality
and/or lead time.
Accuracy
In addition, having to use a larger stylus may negatively affect the accuracy of
the metrology due to the increased
weight, flex, and instability, and thus also
negatively affect cost and/or quality
and/or lead time.
Photonics & Imaging Technology, May 2021

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Tech Briefs Magazine - May 2021

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