wood lathe in a home workshop is remarkably similar to Livermore's
Large Optics Diamond Turning Machine. Both spin a workpiece while
a cutting tool cuts the revolving surface. But their end products
bear little resemblance. Built to form large, irregularly shaped
mirrors for experimental lasers, the LODTM (pronounced "load 'em")
leaves behind a gleaming reflective surface that often needs no
further finishing. It is the most accurate large machine tool in
Diamond turning is routinely
used today to manufacture contact lenses and parts for videocassette
recorders. Defense contractors also use diamond turning to make
lenses for heat-seeking missiles and other weapons. All of these
products are transmissive optics, meaning that light passes through
them. They are also relatively small with a regular, curved shape.
Says engineer Jeff Klingmann, leader of the Precision Systems and
Manufacturing Group, "That type of diamond turning is a whole different
animal from the large, reflective optics we do. Reflective optics—mirrors—are
often ground and polished. But that doesn't work for mirrors with
aspheric shapes. When the Department of Defense needed large, aspherical
metal mirrors back in the early 1980s, Livermore built LODTM. Producing
aspherical shapes is no problem. We just program the shape in, and
the diamond tool goes to work."
LODTM can handle a workpiece
with a diameter of up to 1.65 meters, a height up to 0.5 meters,
and a weight of as much as 1,360 kilograms. A diamond the size and
quality of a half-carat engagement ring is secured to a steel shank
and carried on the end of a vertically moving tool bar. The workpiece
rotates about 50 times a minute on the horizontal face plate while
the diamond tool cuts gossamer threads of aluminum, copper, silicon,
gold, or nickel with unprecedented precision. The LODTM can produce
parts with tolerances to 28 nanometers (about a millionth of an
inch), accuracy more than 1,000 times greater than that of a conventional
example of the aspherical mirrors that the Large Optics Diamond
Turning Machine first produced.
of an Ultraprecision Machine
the 1970s, researchers were considering the development of powerful
experimental lasers as an element of missile defense. These ideas
became part of the Strategic Defense Initiative, or Star Wars, a
program born in the 1980s during the Reagan administration. The
laser system's optics had to be extremely large, exotically shaped,
and fabricated with a precision corresponding to a small fraction
of the wavelength of light. In meeting those requirements, LODTM
achieved levels of accuracy that defied measurement by existing
methods. Even today, the machine's accuracy is such that it cannot
be corroborated by the National Institute of Standards and Technology.
Livermore's Precision Engineering
Program designed the machine as the culmination of research in machine
tool accuracy. They had determined in the late 1970s that by pushing
the limits of precision, they could develop a diamond-turning machine
for machining large, oddly shaped optics to exacting tolerances.
LODTM incorporated the results of an exhaustive analysis and elimination
of factors that cause machine errors, from the heat of a human body
to the vibration from a heavy truck passing by.
For example, LODTM has several
ways to handle the temperature fluctuations that are typically the
largest single cause of diamond-turning machining error. Air temperature
in the LODTM enclosure is maintained at precisely 20 degrees Celsius. After the
tool is set up, machining does not begin for at least 12 hours to
allow the effect of the machinist's body heat to dissipate. All
personnel remain outside the LODTM enclosure while a part is being
cut. What little heat the diamond cutting tool generates is carried
away by cutting oil, also maintained at 20 degrees Celsius.
Engineer Jim Hamilton, who
translates client needs into specific instructions for LODTMÕs machinists,
says, "We were concerned that construction for the National Ignition
Facility over the last several years might cause us problems. Our
building is only about 100 meters away from the NIF construction
site. But the earth moving and other heavy work didn't affect the
The heart of the machine's
accuracy is a metrology (measurement) frame isolated from the environment
by temperature-controlled water flowing through expanded stainless-steel
panels. The frame is made of super invar, a steel–nickel–cobalt
alloy with one of the lowest coefficients of thermal expansion of
any metal. The frame "floats" on LODTM, moving independently from
the main machine to give an unstressed, undeformed reference. The
part being machined is thus made relative to this frame, not the
main machine components. Seven interferometers on the metrology
frame continuously measure the location of the tool relative to
the part. The machine controller uses this information in real time
to dictate all machining. This continuous measurement from an unchanging
platform eliminates errors from machine geometry and temperature
changes so they do not appear in the part.
National Aeronautical and Space Administration engineer Holly
Cagle examines SPARCLE's primary mirrors on the Large Optics
Diamond Turning Machine (LODTM) spindle. (b) A close up of a
mirror and the diamond tool on LODTM.
LODTM continues to produce
one-of-a-kind, prototype optical devices for possible future space-based
defense systems. The ultimate client is the U.S. Air Force, with
Livermore's technical requirements coming from TRW Inc. These conical
mirrors are made of silicon for a simple, light, uncooled laser
Previously, Livermore used
LODTM to produce three secondary mirrors for the Keck telescopes
on Mauna Kea on the Big Island of Hawaii. The Keck telescopes, the
largest and most powerful in the world, gather infrared light rather
than visible light. For infrared astronomy, diamond turning was
the only viable process because the mirrors had to be accurate right
to the edge of the reflective surface. Processes such as grinding
and polishing round off or taper the edge of the critical surface.
Livermore used two precision
machining tools, the Diamond Turning Machine #3 and LODTM, to produce
the primary mirrors for SPARCLE, an experiment on National Aeronautical
and Space Administration's (NASA) Space Shuttle. SPARCLE will demonstrate
the ability to measure wind speeds using a space-based lidar system.
Diamond Turning Machine #3 first semifinished an aluminum blank
that was then coated with electroless nickel. LODTM did final "figuring"
in the nickel layer. After leaving Livermore, the mirrors were polished
and gold coated for final use.
The next big project for
LODTM may be for NASA scientists who are planning a new space-based
telescope. LODTM has the capability to machine some or the mirrors
for this next-generation version of the Hubble telescope.
A staff of seven operates
and maintains LODTM, about half the number required when the machine
first came on line. Over the years, many original, custom-made parts
have been replaced by commercial ones. The result is a more efficient
and reliable machine that is easier to operate and maintain.
But LODTM is nevertheless
a unique machine, and it must machine parts to extremely tight tolerances.
Says Steve Bretz, head machinist on LODTM, "We spend about 80 percent
of our time keeping the machine running properly. Before I came
to Livermore, I was a machinist in a regular machine shop. Working
on LODTM is entirely different. Here we have to work very closely
with engineers and experts in computers, electronics, and control
systems to eliminate deviations and maintain the required tolerances."
They must be doing something
right. Eighteen years after LODTM's first operations, measuring
devices are still not sophisticated enough to confirm the machine's
Keck telescopes, Large Optics Diamond Turning Machine (LODTM), National
Aeronautical and Space Administration (NASA), precision engineering,
Strategic Defense Initiative.
information contact Jeff Klingmann (925) 423-8328 (firstname.lastname@example.org).