Advances in Ultra Precision Manufacturing
O. Riemer
Laboratory for Precision Machining (LFM), Bremen, Germany

Abstract: In the past many qualified technologies have been established for the manufacture of optical, precision andmicro parts as well as precision structured surfaces. In this field mechanical machining processes, like diamondmachining, play a dominant role. Novel mechanical manufacturing processes, further development of ultra precision
machine tools and the precision machining of challenging materials yield a great potential as enabling technologies.
The growing markets of ultra precision technologies will fuel many industrial sectors.
Ultra precision manufacturing, machines, tools, processes

1. Introduction
Mechanical ultra precision machining processes arealmost universal and have a long technical tradition.
This is due to the fact that a huge class of engineering
materials like metals, semiconductors, ceramics, opticalglasses, plastics can be processed and a large variety ofsurfaces with optical, mechanical or other properties canbe generated. Therefore, ultra precision manufacturing
has become a powerful tool for controlling surface
properties and sub-surface integrity of parts with often
optical but also other functionalities.
Over the last decades ultra precision machining has
provided deterministic methods for the generation of
freeform surfaces and complex micro-structured
surfaces with unique accuracy and cost effectiveness.
The amazing diversity of shapes that can be createdtoday in optical or near-optical quality by multi-axis
machining has stimulated the design of complex optics.
Application of ultra precision manufacturing ranges
from automotive to medical, illumination, astronomy,
optics and metrology.
According to Taniguchi [TANI1983] "precision" isa relative idea changing its meaning with the enduring
pursuit for higher accuracy. Today, ultra precision
machining is associated with relative positional errors<10-6. There is, however, an equally important effort
towards higher complexity in machine tool building
providing more degrees of freedom for controlling theposition and orientation of the tool with respect to thework piece, which is a prerequisite for non-rotationallysymmetric complex shapes. Diamond machined opticalcomponents are needed for projection systems, displays,
laser scanners, sensors, scientific instruments, medical
and defence equipment, laser beam guiding,illumination systems and many more. These products
exhibit a multitude of different surfaces ranging fromrotational symmetric aspheres to free form andstructured surfaces with arrays, Fresnel or prismaticelements.
A classification of processes towards micro
machining has been proposed by Masuzawa [MASA2]
and the approach is valid for most existing machining
methods. Two guidelines direct towards micro
machining reducing the unit removal and improving
equipment precision. Masuzawa proposes different
types of micro machining processes distinguished by theremoval process. These processes are removal bymechanical force, melting and vaporization, ablation,
chemical or electrochemical dissolution, plastic
deformation, solidification, lamination, and recomposition.
Among the conventional machining processes basedon material removal from a work piece, the mostpopular ones are those in which the useless part of thework piece is removed by mechanical force throughplastic or brittle breakage. Referring to Masuzawa, in
micro metal cutting, the first requirement for micro
machining, small unit removal, is satisfied when a highstress that causes shearing of material is applied to avery small area or volume of the work piece. This meansthat a highly concentrated force must be applied to an
appropriate position of the work piece. Therefore,
assuming that the desired unit removal is around
100 nm, a tool that has its edge sharpened to a radius
smaller than 1 m is necessary.
2. Ultra precision machining
A major prerequisite for ultra precision machining isa remarkable precision of the tools, machines andcontrols down the nanometer range. This technology has
originated as diamond machining from the 1950s to
1970s and was originally designed for machining of
metal optics at macroscopic dimensions with so far
unreached tolerances. During the following decades themachine tools, the mono crystalline diamond cutting
tools, the work piece materials and the machining
processes advanced to even higher precision andflexibility.
Today, several kinds of fast tool machining andmulti axis machining operations can be applied for
diamond machining of ultra precision components aswell as micro optical elements. These parts can either bemachined directly as single or individual component or
as mold insert for mass production by injection moldingof plastics or hot pressing of glass.
Machining of such surfaces requires at least three
numerically controlled machine axes. The respective
machining processes can be classified into line contact
processes, e.g. non-circular turning and asymmetriccontour grinding, turning with fast tool servos, flycutting and contour boring, and point contact processes,
like raster fly-cutting and raster grinding, asymmetriccontour milling and ball-end milling. The technology offigure evaluation of free forms and micro-structured
surfaces as well as non-destructive measurement of the
micro topography of structured surfaces still needfurther development.
Multi-axis machining is characterized by theabsolute positional errors normal to the surface whichhave to be extremely small, usually in the order of a few
nanometers. This is imperative to all point contact
methods where the tool travels a long distance in space
before returning to the same coordinate. This imposes ahigh degree of sophisticated machine design, error
motions of the individual axes should be separated
instead of being superimposed, backlash is prohibitive,reference scales must be invariant to changing ambientconditions. Thus, the arrangement of slides and rotary
axes must carefully be considered.
Furthermore, machining times in ultra precision
machining can be extremely long, e.g. up to several days
translating into a requirement for outstanding thermal
stability of both environment and machine components
like motors and spindles. These conditions also have an
impact on the machine’s numerical control and feedback
loops. There are two key elements which enable theevolution of mechanical machining towards higher
precision:• accuracy of the machine tools• accuracy of the tools
The development of ultra precision machining overthe last decades has been summarized by Evans[EVAN1989], Chapman [CHAP2004] and Marsilius[MARS2009] and some important landmarks regarding
machine design, feedback systems, control andaccessories are given in the following. The
developments make today’s ultra-precision machining

systems more productive, more precise, and lower inprice:
x Thermal and mechanical stability as well as good
damping properties through machine bases madefrom epoxy granite or natural granite.
x Linear axes equipped with hydrostatic oil bearings
for improved damping and wear free smooth motions
at highest geometrical accuracy.
x High-resolution linear scales with resolution of 1 nm
and below replacing laser interferometers fornanometric axis position and improved geometricalaccuracy.
x High feed rates and excellent dynamic stiffness
obtained by linear motors.
x Environmental control by air-conditioning ofmachine housings and customized systems forenhanced vibration isolation.
x High load capacity and stiff aerostatic spindles
guaranteeing high-speed applications.
x Grinding and polishing of diamond tools with welldefined geometry and sub-micron cutting edge
waviness.
x High speed PC based computer numerical controls
allowing data exchange and large part programs.
x Advanced drive and feedback devices to improve
work piece accuracy.
x On machine work piece measurement and error
compensation systems to access residual work pieceerrors.
x Multi-axis machines, fast tool servo and slow slide
servo turning for freeform machining.
x Dedicated software for free form machining and insitu metrology.
A common limitation of single point diamondturning and grinding machines, is their inherent ability
to generate only rotationally symmetrical surfaces.
Although these geometries cover the majority ofrequirements, there is an increasing demand for optics toincorporate more random, freeform geometries. Thesesurfaces might require raster fly-cutting, or raster
grinding, depending on the material.
Today it has become possible to generate free form
shapes and structured surfaces by ultra precision
machining techniques, even in combination. Increasing
complexity is associated with a loss of symmetry of thesurface and hence with an increase of the number ofdegrees of freedom needed for moving a tool past asurface, i.e. an increase of the number of controllable
machine axes. This development of machining systems
for ultra precision machining towards higher accuracy
and increase in complexity is illustrated in figure 1.

rotation of the tool. Point contact methods arecharacterized by long machining times but bear the
potential of machining almost arbitrary shapes. Line
contact fly-cutting changes into point contact fly-cutting
when the radius of the fly-cutter is smaller than the local
radius of curvature in the fly-cut plane. Figure 2 shows atypical set-up for raster milling of a free form mirror.

Figure 1: Development of ultraprecision machining
systems towards higher accuracy and complexity.
Raster machining processes require the part to be
mounted in a static condition, while its relationship withthe cutting tool or wheel might move simultaneously in
up to five axes. Many advanced design features are built
in to such a machine, an example of this is the integral
axis configuration, to improve system stiffness, reduce
thermal effects, and reduce geometrical errors.
Automatic systems have been developed for establishingtool/wheel radius, height, and position on the machine
relative to spindle center-line. These devices usuallyemploy kinematic mounting techniques to ensure fastand precise location on the machine and tactile or
optical probe technology combined with automatic
setting software.
3. Application of ultra precision machining processes
In line contact machining processes continuous
cutting is maintained along a certain line in space. The
most obvious way of achieving a well-defined departurefrom a rotationally symmetric asphere is to modify aturning process by moving the tool parallel to the workpiece spindle axis C as a function of angular position.
This movement can be performed either with themachine’s Z-axis slide (so called Slow Slide Servo SSS) or with a superimposed Z'-axis, a so-called fast
tool servo (FTS).
Diamond milling or fly-cutting is a well-known
method for generating toroidal surfaces on a 2-axis
lathe. If the midpoint of the fly-cutter can be moved
freely in space, arrays of toroidal surfaces can begenerated.
Point contact is established, if the interaction
between the cutting edge and the work piece is restricted
to a small surface element and is periodicallyinterrupted, usually at a high frequency, due to the

Figure 2: Raster millingof a free form mirror.
Contour boring with a half-arc diamond tool is a line
contact process that can be used on a multi-axis machinefor generating arrays of concave spheres. If a lateralfeed is superimposed to the motion of a half-arc
diamond tool spinning about its axis, a transition to ballend milling takes place. Ball-end milling allows themachining of surfaces with much smaller radii ofcurvature than is attainable by raster fly-cutting. In order
to avoid burnishing of material at the centre of therotating tool, pseudo ball-end milling diamond tools
have been proposed by Takeuchi which exhibit a small
gap between the axis of rotation and the beginning of
the cutting edge [TAKE1993].
Structured and functional optical surfaces are key
components in many optical systems. And ultra
precision machining opens up many opportunities forthese applications. A new machining process, diamondmicro chiseling (DMC), has been developed which iscapable of generating micro retro reflectors and othermicro optic structure geometries not machinable so far[FLUC2008]. The potential of DMC for allowing moredegrees of freedom in optical design for micro structures
becomes obvious as shown in figure 3, a large scalecorner cube array manufactured by DMC.
Future goals for DMC are to improve process
stability, machining time and to increase the complexityof machinable structures by development of dedicated
CAM strategies, improved diamond tools and in-situ
quality control. Structure arrays will be machined onto
flat, spherical and also aspherical work pieces, followed

by molding experiments to assess if the DMC machined
molding tools fulfill the requirements in terms ofaccuracy and tool life, which are necessary for theirapplication in mass production of micro structured
optical parts.

surface roughness. Furthermore, structure accuracy, burr
formation and tool wear have to be taken into account.

Figure 4: Nanometerstroke fast tool servo (nFTS)
turning for the manufacture of diffractive structures.

Figure 3: Large scale corner cube array
manufactured by Diamond Micro Chiseling (DMC).
The final aim is to apply DMC for the manufactureof advanced molding tools for replication of cube corner
retro reflectors or hybrid optics, which containmicrostructures for integration of additional optical
functions.
Furthermore, refractive components are more andmore substituted by diffractive optical elements (DOE)
due to their more complex functionality. Despite theextensive amount of research on FTS no existing system
offered a suitable combination of stroke, operating
frequency and positioning accuracy for fabricating highresolution diffractive optics. Thus, Brinksmeier et al.
[BRIN2010b] have recently presented a 350 nm stroke
FTS operating at frequencies up to 10 kHz which can beused for the generation of such holograms in metal
surfaces for security applications. This so called nFTS
system allows for machining DOEs with submicron
precision of the structural dimensions. Such DOEs canbe used as optical keys for security applications, having
the advantage of fast fabrication within a single process
step and each DOE having an individual structure.
The nFTS process has successfully been applied tomachine diffraction gratings for wavelengths in therange of 600 nm (see figure 4). Despite of the opticaldesign and machining accuracy the work piece material
is of essential importance. The material has to besuitable for diamond machining to achieve a sufficient

Figure 5 summarizes the application of all ultra
precision machining processes discussed above withrespect to the basic geometrical dimension and theattainable geometrical complexity.

Figure 5: Complexity of ultra precision machining
processes.
4. Metrology of ultra precision machined surfaces
Figure evaluation of freeform surfaces andassessment of micro topography are decisive for all
types of ultra precision machining processes. The
difficulties of figure evaluation of rotationallysymmetric aspherics are even exceeded by those
imposed by free forms. Most measuring devices,mechanical and optical, do not accept surface slopes
beyond a certain limit. Moreover, there are restrictions
in size or in departure from certain standard geometries.

Any assessment of surface geometry consists of predefinition of the part coordinate frame with respect tothe coordinate frame of the measuring device, retrievalof surface data with sufficient accuracy and resolution,and finally data interpretation. The most universal
technique for evaluating the geometry of free form
surfaces - although comparatively slow - is collecting
surface data with an ultra precise 3D coordinate
measuring machine. The urgent need for moreconvenient, faster and inexpensive methods for testingof aspheres and free forms has been recognized for along time. Computer generated holograms which arecostly and inflexibile are nowadays complemented byinterferometric stitching techniques.
For the qualitative imaging of the micro topographyof precision machined structured surfaces scanningelectron microscopy is commonly employed, althoughprobe size sometimes poses a problem. However, nondestructive quantitative measurements are possible
today only within narrow limits. The biggest problems
are associated with large slopes and aspect ratios.
White-light interferometry, like other optical methods,
cannot cope with surface slopes larger than approx. 25°.
Stylus instruments accept a somewhat larger slope, butdeliver an image convoluted with stylus geometry which
is disadvantageous for the assessment of sharp corners.
The non-destructive assessment of the topography along
steep slopes and vertical walls remains essentiallyunsolved.
5. Conclusion
Ultra precision machining is generally regarded as a
key technology of the 21st Century. Everyday products
such as televisions, video players and cameras, contactlenses, binoculars, security systems, compact discplayers, personal computers, and many more, rely on
advanced manufacturing techniques to produce highperformance optics and precision components in a costeffective way.
For many products this has resulted in a need to
produce parts with improved size and geometrictolerances, often with the aid of in-process measurement
and error compensation techniques. To achieve highprecision motion, in the presence of non-linear
disturbances, advanced CNC systems require the use of
feed forward techniques to achieve zero tracking errors
and robust synchronization of motion. The continued
increases in processor speed and measuring resolution
have advanced the technology to a stage where current
machines tools, tools, control algorithms andarchitectures are capable of very high performance interms of ultra precision machining. The need to produce
precision parts in a mass scale introduces new
production requirements for the machine and equipment

manufacturers in terms of availability, cost-effectivenessand reliability.
Particularly ultra precision machining processes liketurning, milling and drilling using mono crystalline
diamond tools and deterministic grinding processes
show greatest potential aiming at higher complexity inpart geometries combined with higher productivity
driven by market requirements. Advances in associatedcomputer assisted technologies like CAD/CAM forcomplex optics, in-situ metrology and data feedback
lead to deterministic ultra precision machining.
Promising and still challenging is the increasing
demand for precision components with structured or
micro structured surface for optical or mechanicalapplications. New processes are under development formachining even features with sub-micron size, i.e.
diffractive optical elements. At the same time,measuring structures is yet not solved satisfactory for all
required characteristics.
Furthermore, the demand for ultra precision
machined novel materials, e.g. molds for replication
processes, leads to the need to apply precision grinding
with all associated challenges. Optical components
require surfaces without sub-surface damage, which canbe achieved in grinding explained by the phenomenonof ductile-to-brittle transition. Nevertheless, in ultraprecision grinding the situation is different togeometrically defined precision cutting processes. Whileraster fly-cutting is a well-established process, ductile
regime raster grinding is still under development. Since
the critical uncut chip thickness of most brittle materials
is in the submicron range, it is evident that ductile mode
grinding can only be achieved with ultra-precision
machines and grinding wheels exhibiting an excellent
roundness. Additionally, dressing of the grinding wheels
is of highest importance. Here electrolytic in-processdressing (ELID) is a well-established technique toenable ductile mode material removal [OHMO1995].
Additionally, new grinding tool concepts with coarse
grains and binderless fine grained diamond tools areunder preparation [BRIN2010a]. Finally, precision
grinding has significant influence on machine design toensure that the stiffness of the machine is suitable tohandle the increased grinding forces, and that theguarding and coolant containment measures are up to
the increased demands [CHAP2004].
Also promising and extensively and long-time
investigated is the search for solutions for the precision
machining of steel. The catastrophic wear of diamond
tools when machining ferrous metals cannot be
explained alone by the differences in the mechanicalproperties of the materials like hardness and fracture
toughness. The presence of a chemical component indiamond tool wear as has been pointed out by Paul andEvans [PAUL1996]. The reduction of diamond toolwear was achieved by ultrasonic vibration cutting ofsteels, in which the contact between the cutting edge andthe work piece material is interrupted periodically at ahigh frequency [MORI1995].
Todays ultra precision machining systems are built
on fundamental precision engineering principles coupled
to leading edge technologies in controls, drives, andfeedback devices. Important technological advances in a
number of disciplines – first of all process technologies,
but also including measuring and testing, quality
assurance as well as the production environment - arenecessary for further advances including thedevelopment of ultra precision processes, machines, andcontrol systems which can achieve nanometer tolerancesand sub nanometer surface finishes as well as new
analytical techniques that can observe, measure andprovide 3-dimensional images of features at thenanometer level. Finally, it can be concluded that novel
mechanicalmanufacturing
processes,
furtherdevelopment of ultra precision machine tools and theprecision machining of novel materials offer a great
potential as enabling technologies in ultra precision
manufacturing; ultra precision cutting processes willproceed in a bright future.
The growing markets of precision, optics and microtechnologies and systems yield a high potential forengineering tasks in ultra precision manufacturing. In
the future, machine developments will continue to be
driven by market requirements and progress in the ultra
precision machining area will fuel many industrial
sectors.
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