Initial polishing of the aluminium

Polishing aluminium, being a ductile material, is extremely troublesome and required research. The author was trying to develop a technique for an extremely challenging task of putting a reasonable polish on a ductile material. It was necessary because the radius of curvature of the mirror required an accuracy of 31900 mm +0 -200 mm and the surface quality required an accuracy before coating of 10 microns +0-10 microns. Albert Franks of the N.P.L. [91] stated that there could be considerable print through problems when attempting to polish the over coated surface layer and that any defects in the substrate would not be totally removed in the subsequent polishing of the coating. A literature search to confirm this was undertaken without success. Nevertheless in a one off project such as this, Franks statement was of some considerable concern. Therefore it was necessary to polish the surface at least to a moderate quality and, moreover, it was essential for optical testing for determining the figure before coating.

The larger of the two grinding (1.4 m dia) laps was populated with 200, 75 mm x 75 mm square pitch facets. Each facet was made from No 73 Gugolz pitch manufactured by Loh [70] was 10 mm thick and spaced 10 mm apart. Gulgolz pitch is supplied in five varieties. No 55 (soft), 66, 73, 82, and 91 (hard), with the relative hardness equating to its melting point. No 55 has the lowest melting point and No 91 the highest. With the knowledge gained through manufacturing smaller optics in similar ambient temperatures. No 73 was considered the most appropriate for this application.

The pitch and lap were heated (35°C) and pressed on the mirror. At elevated temperatures the pitch will readily flow and conform to the mirror profile. Over 200 Kgs were added to the lap to reduce the time required for pressing. Full contact over the area of the pitch was achieved after five cycles of heating and pressing.

Next, the excess pitch was trimmed and 65 mm x 65 mm squares of Texmet 1000 polishing cloth stuck to each pitch facet. Texmet is a proprietary polishing cloth manufactured by Buehler Park Crammer Ltd [52]. It is white woven cloth with a paper texture and has a self-adhesive backing. The lap was cold pressed over night to establish good contact with the mirror. Experiments are detailed in chapter 4 concerning the choice of appropriate polishing cloths.

Mass of lap including the polishing arms Polishing pressure

Area of polishing facets

130 Kgs 15.38 gms/cm' 8450 cm^

Initial polishing began using 600 grade SiC and within 5 minutes 12 Texmet facets had detached from the lap. The detached facets came from the outer one third of the lap. This section of the lap passes over the periphery of the mirror with the normal polishing action. The Texmet facets had clearly became trapped between the lap and the mirror resulting in pitch facets being broken. Particles of pitch adhered to the mirror surface causing other cloth facets to tear off. A dangerous situation was reached within half a dozen strokes. The lap was repaired and retried with the same result. It was thought that the single facets were the problem, so the lap was faced with a full sheet of Texmet over the pitch facet. Figure 5.6. shows the 1.4 meter lap with individual Texmet facets before commencing polishing and figure 5.7. shows the lap with a full sheet of Texmet after polishing Polishing resumed and the surface began to brighten, but after one hour the cloth tore again. Inspection of the Texmet showed that the cloth became detached from the adhesive when the cloth moistened.

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Figure 5.6; 1.4 m lap with cloth facets

Buehler Park Crammer Ltd [52] were consulted and stated that they were using a new adhesive that was water soluble, and duly supplied replacement polishing cloth with their original adhesive. This cloth worked for considerably longer, but again tore at

the identical position as previously. To improve the productivity the polishing compound was changed to Calcite alumina with 6-micron particle size. Calcite alumina was considered to be the most efficient polishing medium to work the aluminium, after consultation with the abrasive supplier [69]. It will rapidly polish the aluminium but leaves an orange peel surface, which was considered acceptable for this stage in production. After a number iterations of polishing and replacing the cloth, the surface had receive a sufficient polish for an optical test. Polished aluminium does not have a good optical surface. The surface was grey with a grainy texture (orange peal) but the reflectivity was sufficient to enable the overall form of the mirror to be tested.

The form requirement was that the surface be within 10 microns of true (10% of the predicted nickel coating thickness), before the mirror is coated. The author considered 10% a sufficient margin to polish and figure the nickel coating, from experience gained on previous work. The main reason for requiring the surface accuracy of the aluminium to be within 10 microns is commercial. The closer the base curve is ground to true, the less material has to be ablated by polishing to complete the optic. This saves time and money.