Innovations in Ultra-Precision Machine Tools:
Design and Applications
Dan E. Luttrell
Moore Nanotechnology Systems, LLC
Abstract – Ultra-precision machine tools represent a special class of machines that perform at an extreme
level of precision. Performance of these machines has steadily evolved with time and this has enabledcontinuous development of new capabilities. Not only can these machines directly generate surfaces
capable of performing a variety of optical functions, the overall shape of these surfaces is not limited torotational symmetry. Surfaces can be freeform and discontinuous. In addition, very large ultra-precision
machine tools have now been developed to generate large scale micro-structured surfaces for high-volume
manufacturing. Further development of these ultra-precision machines will continue and enabled byadvances in measurement systems and electronics. Future machine systems will address the need for
greater size, lower costs, and higher productivity.

Introduction and Historical Review
Machine Technology
Ultra-precision machine tools can be defined as thosewith the ability to generate surfaces suitable for
performing optical functions. This definition does notlimit the machines from being used to make surfaces andfeatures intended for mechanical functions. However,this definition does serve as a way to differentiate thisclass of machine from more conventional precision andhigh-precision machine tools. Conventional precision
machine tools are designed specifically for generating
features with mechanical functions and are designed andbuilt to operate at an at a lower level of repeatability andaccuracy (precision).
Surfaces and features that manipulate light require
overall form error on the order of 0.1 ʅm/100 mm (inoptical terms, about O/4 or better) with surface finish
less than 10 nm Ra. In fact, most ultra-precision machine
tools are now capable of generating surface finishes on
the order of 1 nm Ra in many materials. In the case of agrinding process, the surfaces can be on a very hardmaterial. Fig. 1 shows the measurement of an opticalmold made from tungsten carbide.

Figure 1. Grinding results on Tungsten Carbide: 30 mm dia,Form PV = 0.065 ʅm, Surface Finish Ra = 3.635 nm.

Ultra-precision machine tools can achieve this form
accuracy and surface finish at the same time on the sameworkpiece. This is one significant characteristic thatdifferentiates these machines from the moreconventional machine tools.

These ultra-precision machine tools have evolved overthe last 50 years and they are now commerciallyavailable from several manufacturers [1]. Some common
features of these machines over time have been the useof fluid bearings, high resolution measurement systems,
non-influencing actuators, and fast, high bandwidth(BW) control systems.
Typical today are machines with aerostatic spindles
having error motions < 20 nm. Linear slides use oilhydrostatic bearings constructed with an overall
geometry < 1ʅm/1m (10-6 or better). Linear
measurement encoders have effective resolution < 0.1
nm over 1 m of travel. Spindles and linear slides use
minimum influencing brushless DC motors. Control
systems are based on DSP and gate array hardware
tightly integrated with the measurement and actuatorsystems.
Initial Applications
These machines were originally developed to generate
optical surfaces with single point diamond cutting tools.
This provided a method that is more deterministic thanpolishing with the added benefit of reduced cycle time.
Early applications were hard disk substrates, photocopierdrums, VHS read/write heads, and faceted laser scanningmirrors.
One particular shape these machines could generate ina deterministic way was an aspheric optical surface. The
ability to do this opened up a large number ofapplications over the years starting with infrared optics
such as those used in night vision systems and laser
cutting systems. Many aspheric surfaces are produced
this way today. This has been an enabling technology formany consumer, industrial, and military devices.
From this beginning, many new technologies havebeen employed and the applications addressed by ultraPage 1 of 4

July 2010

precision machine tools have grown at an impressiverate. The following are some of the more widely
implemented applications along with the technologies
that have enabled them.

Slow Slide Servo
For non-rotationally symmetric applications that require
displacements beyond a few millimeters, a few of themore recent ultra-precision machine tools have been
designed and built so the main linear slides of themachine can be controlled with precision to track apredetermined path as a function of main spindle
rotation. One key technology advance that enables this is
the use of brushless linear motors. Early ultra-precision
machine tools used ball screws for the slide actuatorsand this severely limited the slide bandwidth androbustness. As with the FTS, improvement in control
systems has also been a key enabling technology.

Figure 2. Deterministic Machining of a Highly AsphericSurface.

Applications and Enabling Technologies
Fast Tool Servo
Initial development of the Fast Tool Servo (FTS) for
direct machining of non-rotationally symmetric surfaces
took place in the mid to late 1980s [2]. The firstcommercial application was for contact lenses and thisbegan in the early 1990s. Early applications of thisdevice were limited because control systems at the time
could not handle the speeds and complexity needed.
Improvements in computational speeds and the use of
more flexible open-architecture control systems haveenabled this device to address many more applications.

Figure 4. Slow Slide Servo Setup for Surface with Large
Departure from Rotational Symmetry.

The early FTS devices were based on piezoelectric
actuators which enabled high bandwidth operation but
were limited in the amount of displacement; typicallyless than 50 ʅm. As designs evolved, the displacement
was increased through the use of flexures withmechanical advantage [3,4]. This type of tool servo canachieve on the order of 500 ʅm displacement and theseare still used in manufacturing applications today.
Recent innovations in FTS design include long stroke
tool servos [5]. Different actuator technology has beendeveloped which enables displacements up to 1 mm at
100 Hz BW and 6 mm at 20 Hz BW. Some even use acounter-mass to minimize influence of the high speed
motion on the rest of the machine.

Figure 3. Examples of Complex Surfaces Enabled by Fast
Tool Servo Technology.

Figure 5. Examples of Optical Molds Made with Slow Slide
Servo.

Freeform Shapes
The development of an ultra-precision linear verticalaxis has enabled the machining of optical shapes that are
not limited to some approximation of rotational
symmetry. Surfaces can be of a complex shape, or
freeform, and generally limited only by the setup andtooling constraints.
Unlike the turning processes described above,generation of freeform shapes has the workpiece
relatively stationary and the cutting tool rotating on themain spindle. Motion of the machine’s axes iscoordinated so the cutting tool is moved across theworkpiece surface along a series of parallel scanning
Page 2 of 4

July 2010

lines with very close spacing. This raster type scanningallows the tool to follow very complex contours. Fig. 6
shows one particular type of freeform optical surface
that is machined using this process.

thermal environment on a large scale has been critical todevelopment of these large machines.
Many variations of products based on microstructured surfaces have been manufactured and the useof Fast Tool Servos has been an enabling technology.
Fig. 8 shows two examples of BEF pattern modified
using an FTS. Advancements continue to be made in thegeneration of the surfaces for large scale manufacturing.

Figure 6. Freeform Surfaces Generated by Raster Flycutting

Micro-Structured Surfaces
The manufacture of products with micro-structured
surfaces began in the early 1960s with development of
the polymer Fresnel lens used in overhead projectors [6].
From this small beginning, many products have beendeveloped based on micro-structured surfaces. Most
notable among them are the Brightness Enhancement
Film (BEF) used in LCD displays and the retroreflective material used in traffic and personal safety
products.
Ultra-precision machine tools are the foundation forgenerating the master tooling for production of theseproducts. Initially, the master tools were small and often
pieced together to scale up for manufacturing [7]. This
process is still used today for some products. Morerecently, much larger ultra-precision machine tools havebeen designed and built specifically for generation oflarge master tooling. This has enabled companies toincrease the volume and lower the cost of manufacturing.
Fig. 7 shows a 2 meter ultra-precision drum lathe used
for generating master tools for production of optical film
for displays.

Figure 8. Brightness Enhancement Film Modified with Fast
Tool Servo.

Micro-Milling
As with the machining of freeform surfaces, thedevelopment of a linear, ultra-precision vertical axis has
also enabled the process of micro-milling opticalsurfaces. One particular type of workpiece that can now
be made is a complex array of small lenses. Because
each lens is made individually, they are not limited tospherical shapes. In fact, they can not only be asphericalbut also non-rotationally symmetric.

Figure 9. Micro Lens Arrays Generated by Micro-Milling
Each Lens Individually.

Figure 7. Large Capacity Ultra-Precision Drum Lathe andFilm Manufacturing Line.

The use of hydrostatic bearings for linear slides andspindles was an important enabling technology for thedesign of these large machine tools. Perhaps more
important has been the use of glass linear encoders(instead of interferometers) and the sophisticated openarchitecture control systems that are now available. But
as these machines have grown in size, the importance ofthermal control has grown as well. Technology
developed over the last 20 years for control of theOne application of such a lens array is for the moldingof small camera modules for hand-held devices. A lensarray mold can be used to mold lenses directly onto awafer of CCD detectors which can then be diced into
individual cameras. This has enabled cost reduction tothe point where camera modules can be manufactured
for less than one US dollar each. This is an example
where the capability of an ultra-precision machine tool
has enabled a downstream manufacturing process tomove from discrete module assembly to a parallel
process and thus reduce cost.

Page 3 of 4

July 2010

of these areas at the same time, while challenging, willlead to new designs and enable new processes.

Figure 10. A Conventional Camera Module Compared with aWafer Camera Module.

Anticipated Future Developments
Automation
Most conventional precision machine tools offer
automation features that increase productivity. Thesefeatures have been well developed on conventional
machines, but these ideas have yet to be used to any
great degree on ultra-precision machine tools. This is
one obvious area for further development. Performance
has always been the key motivation behind developmentof ultra-precision machines and their applications, but
productivity is quickly becoming an important factor.
Electronics and Software
The basic principles of ultra-precision mechanicaldesign have been well developed over the last 100 years
or more. However, when combined with future highbandwidth control systems and new measurement
sensors, ultra-precision machine tools will continue to
add capabilities and address many new applications.
Along with this, work must continue in thedevelopment of software and firmware to take fulladvantage of the latest generation of electronic hardware.
Opportunities exist to more fully integrate sensors andsignals into the control system.
Design Challenges
To help understand future challenges in ultra-precision
machine design, it is useful to categorize machines basedon a few key attributes. For ultra-precision machines theattributes of size, speed, and price offer a way toconsider, or anticipate, future developments.

Figure 11. Normally Exclusive Attributes in Ultra-Precision
Machine Design.

Each of these attributes tends to have design
objectives that are in conflict with the others so they are
somewhat exclusive of each other. Development in two

For example, the large lathes designed for generating
micro-structured master tooling generally require long
cycle times to machine a single workpiece. It is adifficult task to design a large, ultra-precision machine
tool that can operate at higher speeds to reduce cycle
time without losing precision. However, this is a requestfrom those who work to optimize manufacturing.
Application Driven Challenges
There are many types of application challenges
presented to developers. One is related to the increased
complexity of workpieces. These will require more
complex tooling setups and will present a more complex
programming task. Software will become a moresignificant challenge.
Many applications require the use of harder materials.
Longevity of the workpiece in subsequent
manufacturing processes often dictates this. Working
with hard materials will require further development intooling and a better understanding of the material
removal process.
Workpieces with smaller features will continue to bepresented. This will require better measurement andsetup processes and the sensors and software to support
this. It will also put increased emphasis on accuracy andquality of the cutting tools.
Finally, many applications require longer cycle times
to complete. This generally increases the amount ofcutting done by a single tool and tool life becomes animportant issue. For example, on the large cylindricaldrums used for making display film, chip lengths caneasily exceed 20 km on a single pass of the tool acrossthe workpiece. Longer cycle time also increases theimportance of thermal stability of the machine, thetooling and fixtures, and the workpiece.
References[1] Marsilius, N.M., “A Short History of Diamond Turning,”
Proc. ASPE 24th Annual Conf., Oct. 2009.
[2] Falter, P.J., Dow, T.A., A Diamond Turning Apparatus
for Fabrication of Non-Rotationally Symmetric Surfaces,
Ultraprecision in Manufacturing Engineering, SpringerVerlag, 1988.
[3] Falter, K.J., Youden, D.H., “The Characterization andTesting of a Long Stroke Fast Tool Servo,” Proc. 8th Intl.
Precision Engineering Seminar, May, 1995.
[4] O’Neill, C.G. “Piezoelectric Positioner,” U.S. Patent
6,040,653, March 21, 2000.
[5] Weck, M., H. Oezmeral, et.al., “A New Hybrid Concept
for a Long Stroke Fast-Tool-Servo System,” Proc. ASPE
10th Annual Conf., Oct. 1995.
[6] Stewart, T.A., “3M Fights Back”, Fortune, Feb. 5, 1996.
[7] Scott, S., “Manufacturing Methods for Large Area Microstructured Applications by Recombination,” presented at
1st Aachen Precision Days Conference, April, 2008.

Page 4 of 4July 2010