We present two methods for generating novel hyper-crossing tool paths for use with CNC and robotic sub-aperture polishing techniques. One generation method utilizes an optimization based on a Voronoi diagram. The second method is seeded by a unicursal pseudo-random pattern to generate a hyper-crossing tool path which features many self-crossings over the entire part surface. Each instance of generation for a given surface for either algorithm will result in a new hyper-crossing pattern. Tool paths can be generated for any surface area including those with interior holes. We also present results of an experiment using a hyper-crossing tool path to remove diamond turning marks.
Aluminum (pure or alloy) mirrors attract increasing interest, having Young’s Modulus and density similar to glasses. Advantage of high diffusivity offsets disadvantage of high thermal expansion coefficient and means that the mirror reaches thermal equilibrium rapidly. High ductility supports extreme light-weighting and complex machining, including fluid-cooling channels in high-energy applications, and integral interface components. Aluminum mirrors are also tolerant to vibrations and shock loads. The material is amenable to single point diamond turning (SPDT) and does not require optical coating. However, SPDT tends to produce mid-spatial frequency artefacts, which are difficult to remove, especially for aspheres and free-forms. These introduce diffraction effects and compromise stray light performance. In our previous research, we have demonstrated the potential of industrial robots to automate manual interventions with CNC polishing machines, and to provide surface-processing capabilities in their own right. We have also presented research concerning the mismatch between rigid and semi-rigid tools (including non-Newtonian tools), and aspheric surfaces. In this paper, we report on polishing of spherical and aspheric aluminum mirrors using an industrial robot. This includes tool-design, tool-path generation, texture control and removal of the mid-spatial frequency artefacts. We have investigated removal-rates and textures achieved, using different specialized slurries, polishing pads and special tool-paths. An effective process has been established, achieving Sa of 5nm on a 400mm square witness sample and a 490mm elliptical off-axis parabolic mirror.
In the context of Industrie 4.0, we have previously described the roles of robots in optical processing, and their complementarity with classical CNC machines, providing both processing and automation functions. After having demonstrated robotic moving of parts between a CNC polisher and metrology station, and auto-fringe-acquisition, we have moved on to automate the wash-down operation. This is part of a wider strategy we describe in this paper, leading towards automating the decision-making operations required before and throughout an optical manufacturing cycle.
A new surface metrology instrument, the ‘Swinging Part Profilometer’ (SPP), has been developed for in-situ measurement of optics undergoing robot-processing in the ground (non-specular) state. In this paper, we present the hardware-design of the SPP, together with software for hardware-control, data-acquisition and surface-reconstruction. First results on a sample part are presented, compared with interferometric metrology, and error-contributions considered. Notably, during each individual scan of a measurement-cycle, the probe remains fixed. This lends itself to automated probe-deployment by the same robot as performs surface-processing, as probe stability is required on only the time-scale for a single scan.
After the formal acceptance of our fabrication of E-ELT segments, we aim to further accelerate the mass production by introducing an intermediate grolishing procedure using industrial robots, reducing the total process time by this much faster and parallel link. In this paper, we have presented research outputs on tool design, tool path generation, study of mismatch between rigid, semi-rigid tool and aspheric surface. It is indicated that the generation of mid-spatial frequency is proportional to the grit size and misfit between work piece and tool surfaces. Using a Non-Newtonian material tool with a spindle speed of 30 rpm has successfully reduce the mid-spatial error. The optimization of process parameters involve the study the combination effects of the above factors. These optimized parameters will result in a lookup table for reference of given input surface quality. Future work may include the higher spindle speed for grolishing with non- Newtonian tool looking for potential applications regarding to form correction, higher removal rate and edge control.
Although computer controlled polishing (CCP) of aspheres and freeforms is one of the best understood state-of-the-art fab processes today, there are yet some unsolved issues: e.g. compared to bonnet polishing, fluid jet polishing is taking less iteration steps reaching the same form accuracy and ion beam figuring eventually is reaching much higher shape accuracies. This paper is a first move into solving this matter by introducing a novel footprint recording approach for CCP. To that aim, a new method for measuring the impact of a single tool mass acceleration value onto footprint shape is presented, the second derivative footprint recording (SECondo) method. First experimental evidence of the SECondo effect is presented, demonstrating that for bonnet polishing, acceleration of tool mass significantly alters the pressure distribution within the footprint and consequently affects its cross section.
This paper builds on previous reported work describing the marriage of robots and CNC polishing machines, both for the pre-processing of parts, and to automate operations hitherto manually conducted on the CNC platforms. This paper reviews strategies for metrology, then takes the work a stage forward by reporting the use of a robot to automate the exchange of a part between CNC machine and metrology station, the probing of the part, and the capture of interferometer data. This constitutes an important step towards realization of an automated manufacturing cell.
Following formal acceptance by ESO of three 1.4m hexagonal off-axis prototype mirror segments, one circular segment, and certification of our optical test facility, we turn our attention to the challenge of segment mass-production. In this paper, we focus on the role of industrial robots, highlighting complementarity with Zeeko CNC polishing machines, and presenting results using robots to provide intermediate processing between CNC grinding and polishing. We also describe the marriage of robots and Zeeko machines to automate currently manual operations; steps towards our ultimate vision of fully autonomous manufacturing cells, with impact throughout the optical manufacturing community and beyond.
An IRB6620 industrial robot from ABB Co. Ltd. (Zurich, Switzerland) is used as a processing platform for optical processing, and computer-controlled optical surfacing is applied as a key technology. The function of each coordinate system of the robot in processing is reviewed, as well as the relationship of each coordinate system and coordinate transformation. An algorithm governing coordinate transformations is provided. In order to assess the functionality of the robot as a polishing instrument, experiments have been designed so that the removal rate and surface form error correction of the robot facility have been compared with those from established computer numerical control polishing. The importance for the application of industrial robot in optical processing is also presented.
We report on the first-ever demonstration of grinding and polishing full-size, off-axis aspheric, mirror segments as
prototypes for an extremely large telescope, processed entirely in the final hexagonal shape. We first describe the overall
strategy for controlling form and mid spatial frequencies, at levels in the vicinity of <10nm RMS surface. This relies first
on direct CNC grinding of the base-form of these 1.4m segments, using the Cranfield BoX™ machine. The segments are
then mounted on a custom designed (Optic Glyndwr Optoelectronic Engineering Group) three segment hydraulic
support, and CNC polished on a Zeeko IRP 1600 machine using a variety of custom tooling. We overview the fullaperture
and sub-aperture metrology techniques used to close the process-loop and certify quality, all of which operate
with the segment in-situ on the IRP1600. We then focus on the pristine edge-definition achieved by the combination of
tool-lift and smoothing operations; results never previously demonstrated on full-size pre-cut hexagonal segments.
Finally, the paper discusses the feasibility of scaling the process to deliver 931 segments in seven years, as required for
the E-ELT project.
The next generation ground-based giant telescope, the European Extremely Large Telescope (E-ELT), under
development by the European Southern Observation (ESO) 1, will have nearly 1000 hexagonal segments of 1.45m across
the flats. Fast processing of these segments with high form and edge specifications has proven to be a challenge. The
Zeeko Precessions sub-aperture bonnet polishing plays an important role providing capability for polishing the surface
and correcting the form to meet this target 2,3.
BoXTM grinding has been adopted. This technology has the advantage of fast generating of aspheric surface with very
low subsurface damage (SSD) 4. This will avoid the need of removing thick layer of stock at polishing stage to remove
SSD. However the result grinding signatures has proven to be problematic for direct polishing with Zeeko’s standard
bonnet technology. A novel ‘grolishing’ process which stands between ‘grinding’ and ‘polishing’ has been developed to
deal with mid-spatial features left by BoXTM grinding. This tool is designed base on Zeeko’s R80 bonnet which will fits
directly into the company’s IRP series machines. The process parameters have been optimised to have signatures less
than 10 nm PV. The edge profile is 1μm upstand within 40 mm edge zone.
The ‘grolished’ surface can be directly pre-polished together with all the form corrections. To meet the fabrication time
target, R160 bonnet is used with 50 mm polishing spot, this will provide removal rate of 9.8 mm3/minute, which can be
employed at pre-polishing stage and some form correction. Process parameters have been developed to leave slow
upstand at edge zone without any form of sharp edge downturn. The following form correction stage, which employs
smaller polishing spot of about 20 mm diameter, will continue to remove form errors of spatial frequency between 0.02 –
0.05 1/mm. Furthermore, the upstand edge will be, to a large part, removed at this stage. It is demonstrated that the form
specs can be achieved after this process. The following smoothing process will improve surface textures and remove
edge errors. Local edge rectification is normally necessary to bring the edge at same level. A final smoothing process
will bring the bulk area and edge zone to meet all the specifications.
As the development of modern optical technology, especially space optical science, more high precision mirrors with
large apertures are needed. But it is difficult to manufacture high precision large aperture optical components. The
method of optical polishing using an ultra-precise bonnet is based upon the technology of computer controlled optical
surfacing. A bonnet filled with air is applied as a precise polishing tool which is flexible and able to adapt itself well to
the shape of the part, which is superior to other polishing methods. A material removing model of bonnet precessed
polishing is established according to kinematic principle based on the Preston equation. The model is modified in terms
of Hertz contact theory using the physical characteristics of polishing bonnet tools. A satisfactory result was obtained for
one of the surfaces of a wedge mirror with a diameter of 570mm. The resulted PV and RMS parameters are 1/8 λ and
1/75 λ respectively.
This paper addresses two challenges in establishing a new process chain for polishing hexagonal segments for
extremely large telescopes:- i) control of edge and corner profiles in small-tool polishing of hexagons, and ii)
achieving the required smoothness of the bulk aspheric form. We briefly describe the performance of a CNC-grinding
process used to create the off-axis asphere, which established the input-quality for subsequent processing. We then
summarize processes for smoothing ground mid-spatials and pre- and corrective polishing using Zeeko CNC
machines. The impact of two cases is considered; i) all processing stages are performed after the segment is cut
hexagonal, and ii) final rectification of a hexagon after cutting from an aspherised roundel, as an alternative to ionfiguring.
We then report on experimental results on witness samples demonstrating edges and corners close to the EELT
segment specification, and results on a full-aperture spherical segment showing excellent surface smoothness.
We describe progress on a novel process-chain being used to produce eight 1.4m hexagonal segments as prototypes for
the European Extremely Large Telescope - a Master Spherical Segment as a reference, and seven aspheric segments. A
new pilot plant integrates a bespoke full-aperture test-tower designed and built by OpTIC Glyndwr, with a Zeeko 1.6m
polishing machine. The process chain starts with aspherising hexagonal segments on the Cranfield BoX™ grinder,
followed by smoothing, corrective-polishing and edge-rectification using the Zeeko CNC platform. The paper describes
the technology and progress, and anticipates how the process-chain is expected to evolve through the seven segments to
increase both process-speed and surface-quality.
Rigid tools can confer advantages at certain stages of manufacturing off-axis mirror segments, but the misfit due to surface asphericity and asymmetry poses constraints on their application. Types of misfit are classified and, using least squares, the best-fit tool forms with different distances from the pole of the parent asphere are calculated. The outer mirror segment for the European extremely large telescope is taken as a case-study, assuming a rigid tool size of 150 mm. A simple independent approximation validates the calculation. A close parallel is wavefront misfit in subaperture interferometry, which is also considered.
In this paper we address two interrelated issues important to primary mirror segments for extremely large telescopes - edge-control, and the detailed topography over the segment surface. Both affect the intensity and distribution of stray
light and infrared emissivity. CNC polishing processes typically deploy spiral or raster tool-paths, tending to leave
repetitive features. We compare and contrast two novel families of pseudo-random tool-paths for Precessions CNC
polishing. We then show how CNC control of the three-dimensional tool-path can optimize edge-profiles. Finally, we
demonstrate fluid-jet polishing used to clean up residual edge defects.
A new ultra precision large optics grinding machine, BoX® has been developed at Cranfield University. BoX® is
located at the UK's Ultra Precision Surfaces laboratory at the OpTIC Technium. This machine offers a rapid
and economic solution for grinding large off-axis aspherical and free-form optical components.
This paper presents an analysis of subsurface damage assessments of optical ground materials produced using
diamond resin bonded grinding wheels. The specific materials used, Zerodur® and ULE® are currently under
study for making extremely large telescope (ELT) segmented mirrors such as in the E-ELT project.
The grinding experiments have been conducted on the BoX® grinding machine using wheels with grits sizes of
76 μm, 46 μm and 25 μm. Grinding process data was collected using a Kistler dynamometer platform. The
highest material removal rate (187.5 mm3/s) used ensures that a 1 metre diameter optic can be ground in less
than 10 hours. The surface roughness and surface profile were measured using a Form Talysurf. The subsurface
damage was revealed using a sub aperture polishing process in combination with an etching technique.
These results are compared with the targeted form accuracy of 1 μm p-v over a 1 metre part, surface roughness
of 50-150 nm RMS and subsurface damage in the range of 2-5 μm. This process stage was validated on a 400
mm ULE® blank and a 1 metre hexagonal Zerodur® part.
The 'Zeeko Classic' polishing process is implemented in a series of CNC machine-tools. The standard tooling utilizes
inflated membranes ('bonnet') covered with standard polishing cloths, and flooded by a supply of re-circulating
polishing slurry. The usual input quality is a part off a precision CNC grinding machine, and the process both polishes
and corrects form. In this paper we demonstrate how dynamic range can be substantially extended using three distinct
Zeeko Grolishing processes that are hybrids between loose-abrasive polishing and bound-abrasive grinding. The output
quality and volumetric removal rates of these processes are compared and contrasted. Finally, we note how these hybrid
processes can extend the capabilities of the machine from polishing and form control, to smoothing parts with inferior
input-quality, removing larger volumes of material during form control, and addressing harder materials.
This paper reports on the commissioning of the first of Zeeko's "IRP1200" 1.2m capacity 7-axis automated CNC polishing machines. These combo machines now support five different removal regimes, which are described. The machines differ substantially from Zeeko's more familiar 200mm machines on which we have focused before, in terms of overall architecture and detailed design. Large and small optics place different demands on part-fixturing, tooling, machine speeds and accelerations, metrology, slurry-handling, part-loading and access etc. These have had a profound effect on the development-path from 200 to 1.2m machines. Moreover, an advance in the kinematic design has extended the allowable range of surface slope-angles from typically 30° up to a hemisphere. The paper presents results from the pass-off trials, the first fluid-jet experiment, and the development of tooling to address a requirement to smooth a part with a local defect.
The requirements of space and defence optical systems and ground-based astronomy (especially extremely large telescopes) are providing optical fabricators with new challenges. These challenges particularly concern process speed, determinism and automation, and tighter tolerances on surface form and texture. Moreover, there is a growing demand for complex off-axis and 'freeform' surfaces and for larger components of the ~1m scale.
With this in view, we first report on form-correction on a smaller analogue of the IRP1200: an IRP400 in service in industry. We then report on the design, commissioning and preliminary process-development results from the first of the scaled-up 1.2m capacity CNC polishing machine from Zeeko, Ltd. This machine delivers the 'Classic' bonnet-based process, together with two new processes: fluid-jet polishing and the hybrid soft-grinding/polishing process called 'Zeeko-Grolish.' We indicate how this trio of processes running on the same machine platform with unified software can provide an unprecedented dynamic range in both volumetric removal rate and removal spot-size. This leads into a discussion of how these processes may be brought to bear on optimal control of texture and form. Preliminary performance of the 1.2m machine is illustrated with results on both axially-symmetric and more complex removal regimes. The paper concludes with an overview of the relevance of the technology to efficient production of instrumentation-optics, space optics and segmented telescope mirrors.