The conclusions reached were:
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ELID grinding is stable, efficient, and economical.
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Some problems in achieving ductile- or semiductile-mode grinding of micro-optical components occurred with low grinding speed, instability of ultrafine abrasive and small-size wheels, difficulty in achieving precise and efficient trueing and dressing of the wheels, and difficulty in obtaining precise and effective fixturing.
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Coarse grit size wheels (#325) do not show any difference in the final roughness when ELID is applied.
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Finer #4000 wheels, however, resulted in lower surface roughness when ELID was employed.
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ELID high-precision grinding of microspherical lens with cup wheels (ELID-CG grinding) achieved high spherical accuracy and low roughness around Ra 20 nm.
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ELID-CG grinding can be successfully utilized to fabricate microspherical lens with a more stable process, higher efficiency, and better surface quality than conventional grinding.
ELID Grinding of Large Optical Glass Substrates [5]
In this research, ELID grinding was employed to grind optical components 150–250 mm in diameter. ELID grinding using fine-mesh superabrasive wheels produced spectacularly low roughness of 4–6 nm Ra on brittle surfaces, including BK-7 glass, silicon, and fused silica. For some applications, ELID grinding eliminated polishing or lapping operations.
ELID Precision Internal Grinding [25] and [26]
Little has been reported on mirror-finish internal grinding due to the limitation in abrasive grit size applicable to nonmetallic-bond grinding wheels. A novel method to carry out ELID grinding of internal cylindrical surfaces on an ordinary grinding tool, named interval ELID and presented in Figure 7.28, was developed. The wheel was dressed at intervals (before each stroke), and the abrasive grains remained protruding. After a predressing operation, the insulating oxide layer was 30 μm thick and increased the external diameter of the grinding wheel.
Figure 7.28.
Schematic of interval ELID grinding [25]
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The characteristics of the electrical current utilized for interval ELID grinding are shown in Figure 7.29.
Figure 7.29.
Current fluctuation in interval ELID grinding [25]
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In internal grinding, abrasive wear occurs rapidly due to the smaller diameter of the wheel. These problems were overcome by interval ELID grinding technique. Two new techniques for internal grinding, namely ELID II and ELID III, are also described in the literature, Figure 7.30. For ELID II, a fixed cylindrical dressing electrode dresses the cast iron fiber–bonded cubic boron nitride (CIB-CBN) grinding wheel before each stroke. For ELID III, the metal-resin bonded grinding wheel is connected to the positive terminal of the power supply, while the workpiece itself is connected to the negative pole. Investigations reached the following conclusions:
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Due to the limitation on wheel diameter, wheel speed can be adjusted only within a small range. The effect of wheel speed on output parameters was not significant.
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A higher wheel speed, within the limited range, resulted in a finer surface roughness.
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The obtained values of grinding parameters after interval ELID grinding are similar to those obtained after conventional internal grinding.
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The surface quality of the ground workpieces was better for increased mesh size values of the wheel. Fine abrasive wheels can, therefore, be used to grind smooth surfaces, without risking the stability of the surface roughness.
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Internal mirror-finish was possible for pieces made of bearing steel and alumina.
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Pipe-shaped dressing electrodes are superior to other shape electrodes.
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It is possible to achieve a mirror finish with a coarse grinding wheel. The roughness obtained after ELID III grinding with #2000 grit size was almost the same as the roughness obtained after ELID grinding with a #400 CBN wheel.
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A combination of ELID II and ELID III can be employed for finishing operations, especially when small diameters are applicable.
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Rough and finish grinding can be performed on the same machine tool using ELID II and ELID III procedures.
Figure 7.30.
ELID II and ELID III internal grinding processes [26]
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ELID Grinding of Hard Steels [27]
Hardened bearing steels such as M50 were ground to produce an optical quality surface, finer than 10 nm in Ra, using a 76 μm CBN grain size and 500 μm depth of cut, as shown in Figure 7.31 and Figure 7.32. The final surface roughness was reduced by the burnishing action of the worn CBN grits. ELID grinding was employed to reduce surface roughness by maintaining the protrusion and sharpness of the CBN grits and to avoid the pullout of carbides in the secondary finishing zone phenomenon.
Figure 7.31.
Surface finish versus feed rate and depth of cut [27]
Figure options
Figure 7.32.
Surface finish versus feed rate for ELID and non-ELID grinding [27]
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Another grinding technique employed to minimize microcracking, surface burn, and phase transformation is low-stress grinding (LSG). However, LSG places special demands on machine tool stiffness, low and controllable vibration levels, low wheel speed, and frequent wheel dressing. LSG is characterized by low removal rates, low grinding ratios, and significantly increased production costs. Also, it was found that some localized surface damage and surface roughness in the range of Ra 100–200 nm were obtained.
Onchi and colleagues reported a roughness of Ra 30 nm achieved after grinding of SAE 52100 with a porous CBN wheel, yet with relatively low removal rates and very fine CBN grits [29].
After superfinishing hardened steel pieces with #500 grit size fused-alumina stones, Puthanangady and colleagues reported a surface finish parameter Ra of up to 60 nm [30].
In another instance, Stephenson and colleagues employed CBN grinding wheels having 30 μm grain size for roughing, 2 μm grain size for intermediate finishing, and 0.7 μm grain size for the final mirror finish [27].
A 100 mm–diameter D151 electroplated diamond wheel at 3000 rev/min was employed with a traverse rate of 5 mm per revolution and an in-feed of 1–4 μm per pass. The electrical power supply parameters were 60 V, peak current of 10 amps, on-time 6 μs, and off-time 2 μs with a square pulse wave.
Important findings are described here:
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A repeatable surface roughness less than Ra 10 nm was obtained with 75 μm CBN grit and 500 μm depth of cut.
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The lowest surface roughness of Ra 2.3 nm was obtained with a 200 μm depth of cut.
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Chip thickness was estimated at 1–10 nm.
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Carbide pullout of the CBN grits could be avoided by employing ELID dressing.
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Optical quality surfaces were considered to have been obtained by a combination of processes in the primary and secondary finishing zones of the cup wheel, with the final surface finish enhanced by the burnishing action of worn CBN grits.
ELID Mirror Grinding of Carbon Fiber-Reinforced Plastics [28]
Carbon fiber-reinforced plastics (CFRP) are used in the aerospace industry and for machine tool spindles, power-transmission shafts, and robotic arms. It was found that:
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Surface roughness of the CFRP improved substantially for grinding with diamond wheels of increasing mesh number up to #4000.
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For diamond wheels having mesh number greater than #4000, roughness did not noticeably improve.
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The upper limit of surface roughness for a #6000 wheel was Rmax 0.65 μm.
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Surface roughness obtained for CFRP was finer than for brittle materials ground under similar ELID conditions, which was explained by the elastic deformation of this material.
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Mirror-finish is strongly dependent on the grinding direction. Grinding at 90° with respect to fiber direction favors the best results.
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The sparkout effect on roughness was significant for rapid-feed grinding but small for creep-feed grinding.
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Mirror-finish is accompanied by a homogenization mechanism resulting from grinding heat and chip smearing.
ELID Grinding of Chemical Vapor–Deposited Silicon Nitride [4]
Chemical vapor–deposited silicon nitride (CVD-SiC) is the second-most ideal material for deflection mirrors used in short wavelength laser systems, surpassed only by the crystalline diamond. Conventional polishing techniques were unable to finish CVD-SiC mirrors. The study concluded that:
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ELID grinding can achieve extremely smooth surfaces.
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Surface roughness reduced with grain size.
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Fewer pits and whiskers were produced in ELID grinding during brittle fracture when the abrasive grains crush and plow the surface of the workpiece compared to other dressing techniques. The number of pits and whiskers reduced with decreasing grain size.
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The ratio of the ductile- versus brittle-fracture mechanisms was higher for ELID grinding than for conventional dressing. The ratio increased with decreasing grain size.
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Ductile-mode removal was realized with ELID, even on a conventional less-stiff and less-precise grinder, by optimum control of depth and composition of the insulating oxide layer.