The specifications of polygon mirror wheels (Fig. 6.1) can be met by micromachining and are verified best by interferometry, both on the machine at the multiple clampings and unclampings on a dividing head and after final dechucking. Facet angles, facet flatness, absolute and relative pyramidal angular errors, the axial base surface and, for some applications, the front surface are all subject to interferometric quality control. The planar base, a granite surface plate, will permit the component to become part of an ultraprecise scanning system; the polygon’s base can usefully serve as an axial air bearing of a spindle with an integrated drive (Fig. 6.2).
An imbalance of the drive system may impair the finest relative pyramidal quality of the polygon scanner, as will skewed assembly of the polygon on the bearing axis. Dynamic deviations from accuracy areminimized in designs, making the polygon component part of the spindle, as shown in Fig. 6.2. A complete set of specifications for mass-produced polygons is provided in Table 6.1.
Designing the polygon wheel to be an integral part of an air-bearing spindle is the ultimate precision solution for a scanner. The unit goes through various production and assembly stages that involve interferometric testing. Adherence to the specifications in Table 6.1 requires a sequence of delicate operations. The following is a summary of the essential work steps involved in the production of an air-bearing polygon scanner:
1. Prepare and verify the axial surface of the polygon.
2. Diamond machine and verify facet flatness and angles.
3. Measure relative pyramidal error.
4. Attach polygon to spindle bearing (the polygon is the spindle face plate) and ascertain that no deformation of the component by the air-film pressure has occurred. The polygon body must withstand the mechanical force exerted onto its base by the air pressure needed for the air-bearing operation.
5. Ascertain that the drive of the spindle with an integrated motor is error free.
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