In case of small or micro mold (diameter: Φ0.3–Φ5 mm), the wheel shaft has a risk of contacting the workpiece edge; the grinding spindle is 45 degrees tilted in order to use a much small wheel and overcome the problem as shown in Figure 4.87. In order to increase the grinding speed, ultra high grinding air spindle is used even though the small wheel is used. By using a diamond wheel of Φ0.3 in minimum diameter, the mold of Φ0.3 mm can be ground ultraprecisely. Form accuracy of 0.05 μm P-V and surface roughness of 2 nm Ra are obtained [130].
Figure 4.87.
Microaspheric grinding machine tilted 45 degrees
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The microstructured Fresnel shape of the mold can be ground by the sharp-edged grinding wheel. A schematic illustration of the micro Fresnel grinding method is shown in Figure 4.88. For the wheel, a disk shape of diamond grinding wheel, whose edge is trued as a sharp knife-edge, is used. The wheel scans along the workpiece radial position vertically. The orientation of the wheel air spindle is vertical and the workpiece air spindle is horizontal. The feature of this grinding method is that the wheel rotates in parallel with the workpiece rotational direction at the grinding point. The axis-symmetric aspheric Fresnel shape of the mold can be generated by the sharp-edge grinding wheel. The sharpness of the wheel is important in order to generate the structured surface precisely. Form accuracy of 0.1 μm P-V and surface roughness of 2 nm Ra are obtained [131].
Figure 4.88.
Microstructured grinding of Fresnel shape