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© WZL/Fraunhofer IPT
Ultraprecision Machining in Optics Manufacturing
Dipl.-Ing. Martin Weinzierl
Fraunhofer IPT, Aachen, Germany
4M Workshop / CAS Conference, Sinaia
October 15th 2008
Seite 1© WZL/Fraunhofer IPT
Contents
Introduction
Diamond turning of spherical and aspherical optics
Diamond milling and fly-cutting
Manufacturing of high precision freeforms with fast tool servos
Summary
Source: Schneider GmbH
Seite 2© WZL/Fraunhofer IPT
Contents
Introduction
Fundamentals
Diamond turning of spherical and aspherical optics
Diamond milling and fly-cutting
Manufacturing of high precision freeforms with fast tool servos
Summary
Source: Schneider GmbH
Seite 3© WZL/Fraunhofer IPT
Applications - High Quality Optics in Daily Use
Automotive – Head-up displaySource: Siemens VDO
Medical technology –Intraocular lensSource: HumanOptics AG
LED FrontlightSource: Audi
Display technology for home entertainment Source: Philips
Color displays for mobile communicationSource: Casio
Seite 4© WZL/Fraunhofer IPT
Process Chains for the Manufacture of Micro Structured Optics
PrototypingMastering
Galvanics
Recombination
Replication
Precision optics
Design
Process-iterations
Design based on reflective, refractive or diffractive principles
Prototyping or mastering of stamps or large area microstructures
Recombination of stamps to large area micro structured surfaces
Galvanic copying of microstructures into nickel
Replication into plastics
Design-iterations
Seite 5© WZL/Fraunhofer IPT
Mastering and Mould Making
- Laser lithography
- Electron- and Ionbeamlithography
- Nano-Imprint-Lithography
- Screening
- Precision milling and turning
- Micro milling
- Grinding
- Ultraprecision machining
- Polishing
- (ECM, EDM)
- Step & Repeat
- Silicon moulding
- UV-curing
- Galvanic Nickel
- Electroless Nickel
Mechanical processes
Lithographic processes
GalvanicsRecombination
Seite 6© WZL/Fraunhofer IPT
Ultraprecision Machining
Free form surface
Lens array
200 μm Processes
Turning (FTS), Milling, Fly-Cutting
CAD/CAM process chains for reduced geometrical complexity
Increasing machine-tool accuracy
Tools
Poly- and single crystalline diamond
Materials
Non-iron metals, Nickel-coatings, polymers
Precision
< 0.1 μm PV (depending on size and geometry)
Feature sizes down to 0.5 μm (V-grooves)
Surface quality
Optical, down to 2 nm Ra surface roughness
Micro pyramids
Seite 7© WZL/Fraunhofer IPT
Ultraprecision Machining with Single Crystalline Diamonds
ρs : Cutting edge radius
Poly crystalline(fine grain)
w ~ 0,78 μm
0,5 μm
2,5 μm
5 μm ρs ≈ 2 μm
Cu
ttin
g e
dg
e
Single crystal
Wav
ines
s0,5 μm
2,5 μm
w < 0,05 μm
Highest surface qualities with Ra < 5 nm
5 μm ρs ≈ 50 nm
w ~ 2,06 μm
0,5 μm
2,5 μm
ρs » 4 μm 5 μm
Poly crystalline(coarse grain)
Poly crystalline(fine grain)
Seite 8© WZL/Fraunhofer IPT
Base Materials for Diamond Milling and PlaningMetals
AluminiumBrassNickel silverCopperNickel coated steel samples
PlasticsPMMAPolycarbonate PCNylonAcetate
Brittle materialsGermaniumSilicon
Cu
8,96
129 pc
17
49-87 HV
1,69
Cu62Ni18
Zn20
8,72
120-135
16-17
75-190
29
ALZnMgCu0,5
2,76
72
23
170
18-22
Density [g/cm3]
Youngs modulus [GPa]
Thermal expansion [10-6 K-1]
Hardness [HB]
Elect. conductivity [μOhmcm]
Alum
iniu
m C
erta
l
Elec
trol
ess ni
ckel
Nickel
silv
er
OHFC
Bras
s MS
63
Cu63
Zn37
8,45
95-110
19-20
65-136
6,2-6,6
NI
8,9
199,5
13,3
100-190
6,9
Common materials for mould making
Seite 9© WZL/Fraunhofer IPT
Ultraprecision Machining ProcessesFly-cutting
Diamond milling
Diamond turning
Fast-Tool turning
Seite 10© WZL/Fraunhofer IPT
Contents
Introduction
Diamond turning of spherical and aspherical optics
Diamond milling and fly-cutting
Manufacturing of high precision freeforms with fast tool servos
Summary
Source: Schneider GmbH
Seite 11© WZL/Fraunhofer IPT
Historical Development
1960
1970
1980
1990
USA– Continuous development in energy systems,
computers, military, metrology, sensors– Development of the laser
Requirements– Components with optical surfaces– Complex geometries and highest
form accuracy
Development of special machines with ultraprecision
First Applications– Laser mirrors– Hard drives– IR-Optics
Development of series machines with correlating process technology
Further applications– Aerostatic bearings– Scanner optics– Video heads– Copy machines
Systems and processes – for the micro structuring of surfaces– machining of smallest components
Seite 12© WZL/Fraunhofer IPT
Precitech Ultra Precision Tuning Lathe
US-American machine tool builder since 1986Diamond tuning lathes since 1960 (Rank Pneumo)More than 1000 UP-Machines installed
Portfolio:
Diamond turning
Fast-, Slow-Tool
Fly-Cutting, Grooving
Milling
Focus:
Multi axes machines
Diamond tuning
Fly-Cutting, Grooving Source: Precitec
Seite 13© WZL/Fraunhofer IPT
Geometry:
Radius rε = 1 mm
Angle of rake γ = 0°
Clearance α =12°conic
Opening angle ε = 100°
Typical products:
– Turning of contact lenses
– Ruling or lenticularlenses
Single Crystalline Diamond Tools - Standards
Diamond: natural/synthetic
Radii: defined,5 μm - 10 mm
Negative and positive rake angles
Controlled waviness Standard optional < 0,25 μm < 0,05 μm
Source: Contour Fine Tooling
Seite 14© WZL/Fraunhofer IPT
Diamond Turning – Process Characteristics
Kinematics:
Transverse tool movement
Rotation of the work piece
Longitudinal turning and facing
Continuous cut
Rotational symmetric shapes
Advantages:
High geometrical variety
Direct machining of optical surfaces (Ra < 10 nm)
Realization of highest form accuracies (P-V < 250 nm)
Machining of metals, crystalline materials and polymers
Longitudinal turning Facing
Seite 15© WZL/Fraunhofer IPT
Off-Axis Diamond Turning
Process characteristics:Work piece is mounted at an offset of the spindle centreDiscontinuous cutSymmetric mounting of work piece (at least two) due to rotor balance
Advantages:Machining of non-symmetric work piece surfacesNo centre artefacts due to tool misalignment Turning of non-symmetric work piece surfaces without using fast- or slow-tools servos
Disadvantages:Discontinuous cutHigh centripetal forces may cause deviations in the work piece surface Difficult alignment of the work pieces on the spindle
Seite 16© WZL/Fraunhofer IPT
Challenges in Diamond Turning: Machining of Continuous Diffractive Structures
vf
Blazed diffractive structure
– Step hight down to 0.5 μm
– Contrast:Sharp corner geometrySmooth optical surface
15 μm
Diffractive structures:
LED technology
High performance optics
Structure: 0.5 – 5 μm
Blazed geometry
Aspheric base geometry
Seite 17© WZL/Fraunhofer IPT
Contents
Introduction
Diamond turning of spherical and aspherical optics
Diamond milling and fly-cutting
Manufacturing of high precision freeforms with fast tool servos
Summary
Source: Schneider GmbH
Seite 18© WZL/Fraunhofer IPT
Max. working area 1000 x 1000 x 200 mm³
Rotary table (C-axis)
Hydrostatic bearings for the main feed axes (not realised in vertical direction)
Two portal slides for either mass compensation or usage of two tools
Linear direct drives and hydrostatic lead screws
Equipped with standard NC controller
Ultraprecision Machine for Large Area Micro Structuring
Source: Fraunhofer IPT
Seite 19© WZL/Fraunhofer IPT
Basic Machine Design
Vertical slides(z1- and z2-axis)
Rotary table(c-axis)
Table(y-axis)
Hydrostatic leadscrew
Portal slides(x1- and x2-axis)
Portal bridges(cast iron)
Bridge support(granite)
Base(granite)
Seite 20© WZL/Fraunhofer IPT
Diamond Milling – Process Characteristics
Diamond ball-end milling tools
– Available tool radii:0.2 mm - 1.5 mm
Vertical alignment of the tool
Tilted alignment of the tool
Source: ALMT, Fraunhofer IPT
Seite 21© WZL/Fraunhofer IPT
Diamond Milling – Applications
Source: ALMT, Kaleido, Fraunhofer IPT
Channel Structures
Spherical cavities
Aspherical lens arrays
Seite 22© WZL/Fraunhofer IPT
Fly-Cutting – Process Characteristics
Rotating diamond toolAir bearing spindleLinear feed movement of the spindle over the work piece surfaceRotor revolutions between 2000- 3000 rpmInfeed up to 2 mmFeed rates between 5 – 100 mm/min Main process parameters:
– Spindle rotor revolution– Feed rate– Infeed
Applications:– Linear micro structuring – Planing of surfaces
Accuracy:– 0.1 μm / 100 mm– Ra < 20 nm
Rotational axis of the spindle rotor
aP = InfeedΔx= Structure pitch
Δx
aP
Feed direction
Single crystaldiamond tool
Micro structure 1 mm
Seite 23© WZL/Fraunhofer IPT
Single Crystal Diamond Tools for Surface Structuring
200 μm
Facet tool
20 μm
V-shape tool
50 μm
Radius blade
Single crystal diamond
Posalux tool shaft
a
b
g
a : Clearance angle: Wedge angleg
Tool geometry
Blade radius
Cutting edge radius
< 50 nm
Waviness < 250 nm
Seite 24© WZL/Fraunhofer IPT
Fly-Cutting – Applications
2 mm 50 mm
100 mm
10 mm
100 µm10 mm
0,5 mm 0,5 mm
Master of an illumination panel
Retroreflector element
Hot embossing tool
Master structure of a large surface reflector
Seite 25© WZL/Fraunhofer IPT
Planing – Process Characteristics
Rotating tool
Linear movement of the spindle across the work piece surface
Rotating speeds between 1000 - 3000 rpm
Infeed up to 1 mm
Feed rates between 5 – 100 mm/min
Process parameter:– Spindle revolution– Feed rate– Infeed
Applications:– Planing of optical
surfaces– Machining of
reference surfaces
Infeed
Feed direction
Schematic process description
z
xy
Feed direction
Diamond tool Rotational direction of the spindle
Raw work piece surface
Optical surface
In machine quality controlof the surface (Fizeauinterferometer)
Seite 26© WZL/Fraunhofer IPT
Planing - ApplicationsMould master with optical reference surfaces
Injection moulding toolwith optical mould inserts
Detail: Mould insert with a plane optical surface
Referencesurfaces
Functional surface
Seite 27© WZL/Fraunhofer IPT
Challenges in Diamond Milling: Large Area Micro Structuring
Workpiece:Structured area: 640 x 640 mm2
Structure: 4-sided pyramids
Process:Tool: single crystalline diamondSpindle rotation: 3000 rpmTooth engagement frequency: 50 HzFeed: 40 mm/minMachining time: 15 daysDistance of cut: 546 mInvestigation on tool wearActive compensation geometrical offsets 10 mm
Seite 28© WZL/Fraunhofer IPT
Tool Wear and Characteristic Damages During Fly-Cutting
Cutting edge break out:
Excessive load on the cutting edge :
– Mechanical reasons: e.g. planing at high infeeds
– Chemical reasons: Material (iron-contents!)
Effect: crucial to structure quality
Crater wear:
Long term non-stop machining in combination with high infeeds
Effect: drawbacks in surface quality
Cutting edge Break out:
Crater wear:
Seite 29© WZL/Fraunhofer IPT
Tool Referencing and Tool Wear Investigation
Quasi-continuous investigation of tool wear
Optical inspection of the cutting edge
High resolution CCD-camera (pixel size approx. 4 μm)
Hardware resolution of 0.4 μm by use of 10-fold telecentric objective
Interpolated resolution < 0.1 μm by measurement software
Automated tool exchange
In-process offset compensation via NC-control
Point 1Name
0.00000X
0.000018Y
0.000000Z
Tool tip
50 μm
Seite 30© WZL/Fraunhofer IPT
Contents
Introduction
Diamond turning of spherical and aspherical optics
Diamond milling and fly-cutting
Manufacturing of high precision freeforms with fast tool servos
Summary
Source: Schneider GmbH
Seite 31© WZL/Fraunhofer IPT
Fast Tool Servo
Design– Aerostatic– CFR-Light-weight– Voice-Coil– Invar housing
Specification– Stroke: 10 mm– Max. Force: 500 N– Resolution: 2 nm
(after interpolation)– Stiffnes/pad: 90 N/μm– Max. Force: 100 N
each direction– Accelaration:
amax=500 m/s2
– Control loop: 16 kHz, 32 kHz
Fast Tool ServoSource: FHG IPT
Seite 32© WZL/Fraunhofer IPT
Fast-Tool-Servo Machining – Composition of Free Form Surfaces
rotationally-symmetric optical element
non-rotationally-symmetricoptical element
Splitting of function-based surface encoding
z* = f (r, ϕ) = + f (r, ϕ)nrsf (r)rs
Seite 33© WZL/Fraunhofer IPT
Limitations of Fast-Tool-Servo MachiningNon-rotationally symmetric structure
micro lens array
Main limitation:
Tool geometry– Clearance angle limits
the infeed angle
Examples:
Linear groove– Clearance angle: 20°– Length: 20 μm– Depth: 5 μm– Usable length: 5 μm
Lens array– Clearance angle: 20°– Radius: 2,5 mm– Max. depth: 130 μm
angle of structurevc
R =
2,5
mm
D = 1,6 mm
20 μm
FTS-dynamic
vc
tool
5 μm
workpiece
Usable groove length
angle of structure
Seite 34© WZL/Fraunhofer IPT
Fast-Tool-Servo Machining – Applications
Examples:
Phase modulation mirror to reduce the transversal coherency of lasers
Facet mirror for beam shaping and beam guiding
Optical free form surfaces
– E.g. mould inserts for glass moulding
Materials:
Copper (OFHC)
Aluminium (6061, Certal)
Source: OEC AG, Fraunhofer IPT
00 μmMould insert Glass lens Application
Phase modulation mirror Facet mirror
Seite 35© WZL/Fraunhofer IPT
Challenges in Production
Design– Calculation of optical surface– Analysis of machinability
Data conformity– Interface compatibility– Fitting and referencing
FTS control system design– Online set point calculation (conformity)– Controller of system
Metrology– Characterisation of surface without symmetry– Data analysis
Shrinkage compensation (replication)– No symmetry for simplification– Volumetric shrinkage approach
Seite 36© WZL/Fraunhofer IPT
Contents
Introduction
Diamond turning of spherical and aspherical optics
Diamond milling and fly-cutting
Manufacturing of high precision freeforms with fast tool servos
Summary
Source : Schneider GmbH
Seite 37© WZL/Fraunhofer IPT
Summary
Optical mould and die making with diamond machining processes offers:
– Structure sizes of only a few microns with submicron resolution
– Optical surface quality – Great variety of machinable materials
Freeform Optics are the latest development in optics manufacturing and of high interest for beam shaping:
– Advertisement, illumination, compact optic design– Enabling element for LED illumination
Replication of optics requires an iterative optimization:– Data conformity– Machine & control optimization– Deterministic characterisation– New means for compensation
Source: Schneider GmbH
Seite 38© WZL/Fraunhofer IPT
Thank you for your attention !
For more information visit:
www.opticsmanufacturing.net