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Advances in Precision Grinding
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Fig 428: (a) The Rank Taylor Hobson aspheric generating machine illustrating a chuck holding lens to be
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generated and a diamond tool holder (b) The Bell and Howell aspheric generator, showing the work spindle and the high-speed diamond burr [50]
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The Moore Special Tool Co also came up with a new method of generating aspherics, with three-axis CNC X-Z- The generator has an air bearing work spindle with a vacuum chuck, a three-axis adjustable tool post for a single-point diamond turning, a rotary table to ensure that the tool tip is always normal to the surface to be cut, a two-axis laser interferometer, and a computer numerical control
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Fig 429: A Moore s aspheric generator showing the path trace for convex and concave aspheric surfaces [55]
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The compression moulding process used in the 1960s and 1970s to make Fresnel lenses was a highly specialized process that had been developed to make precision optical microstructured products [51] Lewis [55] reported that Du Pont had spent 12 years developing the Ultra-precision Positioner and Shaper (UPPS) in collaboration with the Union Carbide Y-12 (nuclear weapons) plant in Oak Ridge, Tennessee, which used both conventional tools and diamond knives to produce military application components The process of developing a machine tool with Dupont began in 1962 and involved many of the components now associated with modern diamond machining equipments In nuclear weapons, small deviations from the ideal spherical form in the explosive and fissile core lenses cause instability, explaining the early interests at Y-12 and the development of the UPPS machine Herbert [56] reported that single-crystal diamond tools were used to produce a better than 50 nm finish on computer discs using machines with hydrostatic spindles at Mullard in Great Britain During the 1980s, it became possible to machine ultra-fine optical surfaces with a complex geometry by incorporating elaborate numerical control along with ultra-precision optical interferometric displacement transducers [57] A comparison is made between the production of an aspheric surface by high-quality conventional optical manufacturing processes and one generated and polished on a CNC jig grinding machine (the Moore Jig grinder) by Nicholas and Boon [58] The lens system described was one of the essential components used for laser plasma compression experiments A diamond grinding pin rotating approximately at 30000 RPM was used A workpiece was mounted on an angular face plate and driven by an external drive at 200 RPM The tool contacts the workpiece at the bottom edge After some 20 passes, about 1 hr of polishing a sufficiently polished surface was achieved Traditional methods of generating aspheric surfaces on glass have been found to be time consuming [60] A novel technique, which was developed, by Van Ligten and Venkatesh brought about heavy material removal without affecting the surface finish and the profile [59] This technique was extended to germanium and silicon using both metal-bonded and resinoid-bonded wheels
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Variable angle
Fig 430: The grinding of a lens using a grinding pin
Advances in Precision Grinding
To remove material quickly, and end up with the desired surface, the contact area between the grinding tool and the workpiece should be as large as possible As only spheres and toroids permit the condition of a full-area contact, partial-area contact, or line contact will be the best alternative The use of a machine with a rotating tool suggests that the contact surface must be symmetrically rotational In general, the shape of the workpiece is not predictable; hence, the condition of having a large contact area is put in jeopardy Thus, the method was chosen based on a long line contact between the tool and the workpiece during the first step of rough grinding During the subsequent steps of polishing, the use of a flexible tool allows conformity between the workpiece and the tool, approaching the original condition of the contact area Two cup-shaped identical sized diamond-grinding wheels with metallic (D20/30 MICL50M1/4) and resinoid (SD240-R1OO B69-6 mm) bonding were used The profile of the grinding edge is circular in this case, but not restricted to this shape, thus, forming a toroid The important feature is that the grinding surface shape is axially symmetrical It is now possible to programme the path of this tool on the CNC machine such that it is in line (or arc) contact with the workpiece as it cuts the desired shape on the glass To illustrate this, the grinding of a paraboloid is shown The cup tool can be thought of as consisting of a collection of circles whose planes are perpendicular to the axis of rotation of the tool When a plane, as shown in Figure 431, intersects the paraboloid the common line is an ellipse To cut a concave paraboloid, the tool must fit inside the paraboloid Hence, the tool must have a diameter smaller than the shortest radius found on the ellipse of intersection of any plane intersecting the paraboloid In the case of a paraboloid, the shortest radius of curvature on the eclipse of intersection is found when the plane contains the axis of symmetry of the paraboloid Any circle at the outer side of the tool can be contained in one of the planes intersecting the paraboloid The angle that this plane makes with the axis of the paraboloid can be adjusted such that I the arc of the circle and that of the ellipse (Figure 431) at d differs in the sag height by no more than a preset tolerance This condition sets a certain common arc length over which the difference in sag does not exceed a certain value, say 05 m B d Subsequently, the tool axis can be programmed to Plane take a slightly different position relative to the axis of rotation of the workpiece, as well as relative to Zone the apex P, of the paraboloid The sequence is then Paraboloid P repeated to form a neighbouring zone of the one indicated in Figure 431 The principle of the process is somewhat similar Fig 431: The basic principle of zone-based to the one used in producing spherical lenses Instead grinding [60] of making the whole lens surface with a spherical
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