Historical Articles

April, 1954 issue of Plating

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Metal Reflector Finishing

E.B. Heyer, Heyer-Schultz, Inc., Cedar Grove, N. J.

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FIRST surface mirrors are used in many optical instruments and generally are made of aluminized glass. In some precision applications, reflectors of metal are preferred because of their optical stability at higher temperatures and their superior resistance to thermal shock. The following story deals with the miscellaneous finishing operations that are performed by the author’s company on metal reflectors for motion picture projection arc service.

Curvature correction of reflector blank on special lathe. Irregularities in a partially processed blank (inset) may be seen in the bottom and upper right sections.	Grinding of the reflector using a shaped rotating head.

Metal reflectors are made of a special soft-rolled, mill brass (0.025 mm grain size). The brass stock varies in thickness from 1/8 inch to 5/32 inch, depending on the design considerations of the final product. The first operation on the brass sheet is one where a circular blank is stamped to near optical curvature. The back of this reflector blank then is subjected to a grinding operation performed with a specially designed abrasive belt grinding wheel. The special design consists of an abrasive belt fastened to a soft rag wheel with disc cement. (Distix, supplied by Abrasive Machine & Supply Co., 261 South Street Newark N. J.) Use of a soft backing, such as is provided by this method, results in a high finish, which is a necessary design feature to meet the high temperature requirements in motion picture use.

Before and after curvature correction. Optical curvature corrected in top photo, uncorrected in bottom photo.	Buffing of the reflector prior to nickel plating.

The next operation is a slight spinning on a lathe which is performed to bring the near optical curvature a little closer to the desired correction. After this spinning step, the blank is ready for curvature cutting, an operation which transforms the stamped near curvature to a true optical value. This is done with a special mechanism which generates 100 per cent curvatures on all pieces. At this point in the manufacturing, an optical test is made to check the precision of the reflector’s curvature.

Grinding, the next operation, is carried out with a 400 grit aluminum oxide disc that is cemented to the face of a formed block. This grinding head is spun on the end of a motor shaft and brought up to the reflector face which is, in turn, spinning in a lathe. The formed grinding block rides in and out from center to periphery as the work spins. Again an optical test is made to-insure that no distortions have been introduced by faulty grinding.

A reflector in position for the rhodium plating operation.	A general view of the plating area showing a rack of completed reflectors.

Here a ”heavy” polishing operation is performed to remove the marks of the previous grinding step. The reflector is held in a jig supported by a rest. By means of leverage on this rest, a heavy even stroke of the polishing wheel over the face of the reflector is obtained. The coating compound (Matchless Metal Polish No. 731) used in this operation gives an exceptionally clean cut. Oddly enough, the material is a stainless steel cutting compound that is not recommended for brass. However, it has been found to give results superior to those obtained with other materials.

The work then is processed through the first step of the plating cycle, which consists of a single salt, room temperature, nickel bath from which a deposit of 0.0005 inch is plated. The gray or dull deposit thus obtained is colored using a lime compound. (Matchless Metal Polish No. 11-D.) After cleaning in a conventional cathodic type solution, the work is rinsed, then plated in a standard sulfuric acid rhodium electrolyte. Lead anodes are used in this bath. A plating time of approximately 90 seconds produces a bright rhodium deposit of 2 mg/sq in (0.00001 inch thick). Following the plating operation, a final optical test is made and each reflector is numbered serially to complete a work card data file, a practice which has been followed over the past 20 years by the author’s company.

Optical testing unit. The plated reflector, top, mirrors the light from an incandescent lamp through a variable diaphragm system to a viewing screen at the bottom. Optical imperfections in the reflector are easily detected by the operator in the form of shadow patterns

Old reflectors that have given years of good service are returned for reconditioning and are reprocessed as detailed below. For those that are in good condition, except for surface deposits of oxides and light scratches, the only preparation required is a cleanup buffing with the stainless steel compound. After the cleanup step, they are electrocleaned and replated with nickel and rhodium, as described previously. All processing information is recorded on an individual work card. In this manner a complete case history is kept, a study of which shows the effects of variations in manufacturing operations that have occurred over the years. Through such records, it has been possible to produce consistently superior quality metal reflectors.

Another metal reflector that has developed as-a result of the demands for more light by outdoor theaters, as well as by theaters showing 3D and wide-screen presentations, is the aluminized reflector. This reflector required the development of a special coating technique that would furnish an aluminized first surface mirror that would stand up under the high temperatures encountered in service. Tests of such aluminized reflectors have shown 15 per cent more screen light than rhodium plated units. The same careful procedures used to obtain optical precision in the rhodium units are carried through in the making of the aluminized product.

Photos showing a variety of reflector shapes which are rhodium plated or aluminized.

Units prepared in the manner described have many applications. Each of these may require variations in size and shape. Typical of these applications, in addition to that of motion picture projection, are (1) flood lighting for large outdoor areas such as airports and: factory storage sections; (2) search lights, both military and nonmilitary and (3) special inspection devices. Of special interest is the last named application. A small flat reflector is used to inspect the inside of hollow propeller blades. This inspection is performed by mounting the reflector on an extension arm which is inserted into the blade. Finishing operations that may have to be performed on the inside of such blades are guided by use of this reflector. Glass mirrors were found wanting in this use because of their poor resistance to mechanical shock to which they were subjected by accidental contact during the finishing procedure.