pitch is used in the finishing of precision optics for laser applications,
from NIF phosphate
glass laser slabs used in the National
Ignition Facility (NIF) at LLNL, to large aspheric focusing
lenses in OMEGA, to the refractive optics in UV steppers for photolithography.
The reasons are as follows:
- Pitch polishers
give the smoothest surface finishes on most glasses.
- Pitch polishers
may be scaled up to polish very large, meter class flats with
corners (rectangle, square).
- Three people
(one per shift) can operate several large planetary polishers,
each with several polishing stations, 24 hours per day to produce
relatively large quantities of large parts.
polishing pitch is the critical consumable for high precision optical
finishing in most shops. The recipes for the manufacture of a very
popular product, Swiss Gugolz® pitch, was sold by the originators
three years ago. Those companies in the USA that depended upon the
consistency of the Gugolz® products were thrown into disarray
when performance of new batches, made according to the "old
recipe", were judged to be unacceptable. They are still recovering.
LLE Optical Fabrication Shop is a typical example. Months of shop
time were lost to "bad" pitch on the 36" continuous
polishing machine (CP), since the only measure of acceptability
was to pour the pitch onto the CP, "cut" a pattern into
its surface, and polish parts. The pitch was judged unacceptable
because, after 4 weeks of trial and error on fused silica parts,
our optician could not produce flat parts. He could not find a parameter
space (temperature, location of weighted conditioning flat, type
and concentration of polishing abrasive, groove pattern in pitch)
that would yield surfaces flat to l/20
@ 633 nm over a 190 mm clear aperture. He removed the "bad"
pitch and tried another candidate.
is a very complex material. [See http://www.physics.uq.edu.au/pitchdrop/pitchdrop.shtml]
Under load, it exhibits an instantaneous elastic strain, a delayed
elastic strain, and creep or permanent deformation. Its ability
to transfer load to the polishing abrasive that contacts the glass
surface is one key to material removal and smoothing to sub-nm levels.
The ability of pitch to creep or flow under load to match the surface
being polished is the other key attribute for uniform removal over
the part surface, and for figure control.
is limited information in the literature regarding the composition
and properties of optical polishing pitch. Compositions are mostly
proprietary. It is generally understood that the material consists
of various amounts of the following: residues distilled from tar,
oil, or wood; rosin, a derivative of turpentine which comes from
sap of pine trees or stumps, to increase melting point and tackiness;
beeswax or linseed oil to lower melting point; asphalt; flake shellac;
paraffin wax; wood flour; or walnut shell flour. Properties of importance
are viscosity (stated to be in the range of 107 to 109
Pa·s), softening point, (55-70 °C), penetration hardness
(60-80 by Shore D), coefficient of friction (tackiness), and groove
Performing a Pitch Characterization Test
2001 we began a three year research project to establish a database
of physical properties for optical polishing pitches. With commercially
available instrumentation we began to characterize batches supplied
to us from our own shop and other shops across the country. We are
looking to understand why there are batch-to-batch variations, and
just which properties are best able to predict consistent performance.
Pitch Database Pages
2004 we conducted a series of characterization tests on a synthetic
polishing pitch called Acculap (Sutton Technologies, Star NC). Because
it is a totally synthetic product developed without the organic
volatiles present in natural pitches, it is thought that this form
of pitch might exhibit smaller batch-to-batch variations.
days to drop from funnel to dish
Varshneya, J. E. DeGroote, L. L. Gregg, and S. D. Jacobs, "Characterizing
Optical Polishing Pitch," Optifab 2003 (SPIE, Bellingham, WA,2003),
Vol. TD02, pp. 87-89.
E. DeGroote, S. D. Jacobs, L. L. Gregg, A. E. Marino, J. C. Hayes,
and R. Varshneya "A Data Base for the Physical Properties of
Optical Polishing Pitch," in Optical Fabrication and Testing
Digest (Optical Society of America, Washington, DC, 2002), pp.
E. DeGroote, S. D. Jacobs, and J. M. Schoen, "Experiments on
Magnetorheological Finishing of Optical Polymers," in Optical
Fabrication and Testing Digest (Optical Society of America,
Washington, DC, 2002), pp. 6-9.
E. DeGroote, S. D. Jacobs, L. L. Gregg, A. E. Marino, and J. C.
Hayes, "Quantitative Characterization of Optical Polishing
Pitch," in Optical Manufacturing and Testing IV, edited
by H. P. Stahl (SPIE, Bellingham, WA, 2001), Vol. 4451, pp. 209-221.