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 <i>Industrie 4.0</i>, 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.
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.
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.
The segmentation of the primary mirror is the only promising solution for building the next generation of ground telescopes. However, manufacturing segmented mirrors presents its own challenges. The edge mis-figure impacts directly on the telescope’s scientific output. The ‘Edge effect’ significantly dominates the polishing precision. Therefore, the edge control is regarded as one of the most difficult technical issues in the segment production that needs to be addressed urgently. This paper reports an active edge control technique for the mirror segments fabrication using the <i>Precession's</i> polishing technique. The strategy in this technique requires that the large spot be selected on the bulk area for fast polishing, and the small spot is used for edge figuring. This can be performed by tool lift and optimizing the dell time to compensate for non-uniform material removal at the edge zone. This requires accurate and stable edge tool influence functions. To obtain the full tool influence function at the edge, we have demonstrated in previous work a novel hybrid-measurement method which uses both simultaneous phase interferometry and profilometry. In this paper, the edge effect under ‘Bonnet tool’ polishing is investigated. The pressure distribution is analyzed by means of finite element analysis (FEA). According to the ‘Preston’ equation, the shape of the edge tool influence functions is predicted. With this help, the multiple process parameters at the edge zone are optimized. This is demonstrated on a 200mm crosscorners hexagonal part with a result of PV less than 200nm for entire surface.
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.
The next generation ground-based giant telescope, the European Extremely Large Telescope (E-ELT), under
development by the European Southern Observation (ESO) <sup>1</sup>, 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 <i>Precessions</i> sub-aperture bonnet polishing plays an important role providing capability for polishing the surface
and correcting the form to meet this target <sup>2,3</sup>.
BoX<sup>TM</sup> grinding has been adopted. This technology has the advantage of fast generating of aspheric surface with very
low subsurface damage (SSD) <sup>4</sup>. 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 BoX<sup>TM</sup> 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 mm<sup>3</sup>/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.
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.