WHICH OPTICAL SHOP methods are best-professional or amateur?

Neither. In each the needs are different. The professional makes many of a given thing, the amateur usually one. Such considerations as this call for adaptability of point of view. At least two professionals have related to your scribe with chuckles how they had employed amateurs and soon heard them saying, "The methods in this shop are wrong-'A.TM.' says . . ." Thanks for sticking up for "A.T.M." but the mind should always be kept free and limber. Amateurs do single jobs.

One of the functions of this department is to dig stuff out of professional confines and offer it to amateurs. Few professionals these days find that this harms them. Most are more farsighted and are not like one of the old school hear whom Porter worked in Washington during World War I. To a request of Porter's he replied, yes, he'd be willing to show some of his pet methods but "German spies, you know, are everywhere these days. Might overhear."

In the following two-part article, Patrick A. Driscoll, formerly of Rochester, now of Poplar Hill Road, Lima, N. Y., describes one method of making telescope mirrors, using a hand-lever spindle similar in principle to those shown in "A.T.M.," page 163, at C, and in "A.T.M.A.," page 151. Driscoll is an experimental lens maker in the Eastman Kodak Company Hawk-Eye Works and is the amateur who, in an article on making objective lenses, published in this department March last, described himself as "an amateur professional amateur"-he started as an amateur. His article:

THE barrel-head merry-go-round of grinding and polishing by hand, coupled with the amateur's almost universal use of the equal diameter grinder and mirror, leaves much to be desired. Grinding and polishing by hand often is laborious to the point of monotony; but, given the proper simple machinery and a fair amount of help, the basement optician can evolve perfect optical surface and have fun in doing it, reducing grinding and polishing to a minimum. The following notes describe standard optical practice and can be relied upon to give good results.

Building the grinding and polishing machine shown in Figure 1 will call for an assortment of angle iron and so on, and a raid on the local junkyard. We shall also need a 1/4-horsepower motor or, as a last desperate substitute resort, the family washing machine. The grinding machine will give excellent results both in working performance and the surfaces produced. It will handle a mirror up to 8" in diameter, using V-belting on the spindle drive. The stroke will be from side to side, not back and forth, and will be manually applied with the lever shown. Spindle speed for grinding will be about 200 r.p.m., and for polishing about 36 r.p.m.

Long experimentation proves that, when grinding by hand on a machine, a concave surface should be ground face down, grinder on bottom, mirror on top. The grinder should be 5/4 the diameter of the mirror. A convex surface should be ground face up, with the grinder on top, and the grinder should be 4/5 the diameter of the other disk.

In polishing, the mirror should be on top, and the polisher should be 6/5 the diameter of the mirror, but with a convex surface the polisher should be on top and 5/6 the diameter of the other disk. For makers of reflectors or refractors these rules may be considered practical. Discussion of their philosophy would require too much space to prove what is accepted as correct by expert technicians because it works.

We shall assume that we are grinding a 6" concave mirror and, in order to simplify division of diameters, all measurements will be given in millimeters, a smaller unit than inches. Since one inch is 25.4 mm, a 6" blank is 152 mm in diameter. Therefore, by the rule just cited, a glass grinder of 190 mm diameter is needed. For the mirror we purchase a rounded 24 x 152 mm.

The emeries will be numbers 180, 500, and 1200. On the machine these three sizes will give excellent results, and an elaborate sequence of grain sizes is unnecessary. If desired, 90 emery may be used to hog out the curve, but grinding to gage should be done in 180. Emeries 90, 180, and 500 may be had from The American Abrasive Co., Westfield, Massachusetts, and 1200 from the Bausch and Lomb Optical Co., Rochester, New York. (Carborundum also is good for roughing but as a finishing agent in precision work it leaves something to be desired. The amateur's troubles with scratches is traceable more often to Carbo than to carelessness, since its granules are not so consistent in size as emeries.) Since it will be used in a pit (Figure 1) this permits recovery and re-use during the rough grinding stage (180) but the 500 and 1200 are too likely to become contaminated, and should not be recovered.

The mirror blank requires a driving button (Figure 1) which will be attached to the glass with pitch and remain there throughout the grinding and polishing operations. To fasten it to the blank we melt the pitch slowly, heat the blank and button, dip the button into the pitch and quickly place it on the blank, centering it before it becomes cold. In the same manner we pitch the tool on the spindle pipe flange shown in Figure 1, centering it by rotating the spindle and shifting it until the tool runs concentric.

Since the grinder and the mirror blank have flat surfaces it will be necessary to start the curve by hand grinding for about 15 minutes. We shall use a sweeping round-and-round stroke, keeping the center of the blank over the edge of the grinder. When the edge of the grinder shows a ground ring extending in about 1" smaller than the mirror diameter we are ready to machine grind.

We replace the mirror on the grinder, engage the drive pin in the hole in the drive button and, with the mirror held offside on the grinder-say 2" off center-start the spindle.

Emery is kept in a jar or can and fed on the grinder with a 1" stiff bristle brush. The emery should be well submerged in water, as the cutting effect of emery is at its best in a thin solution. A 1-quart jar half full of 180 emery is recommended, also that water be added until the emery is saturated and the jar full.

As the curve starts to form on the 'grinder and blank, both disks will rotate more smoothly, and so we commence swinging the stroke arm of the machine from side to side. From this point on, the stroke will determine the ground figure we wish to evolve.

Figure 2 shows stroke positions. These are calculated on a basis of the grinder diameter, as divided into four equal points. If the stroke overhang brings the center of the mirror to the respective indicated points on the grinder, as we move outward, we obtain the short, medium, and long stroke. With the mirror on top a stroke that is excessively long will shorten the radius. A short stroke will have a reverse effect. A long stroke will grind the center more than the edge, and a short stroke will grind the edge more than the center.

The drive arm should be swept slowly right and left. Tests for curvature should be frequent, and the stroke shortened or lengthened as these tests indicate.

A rule that must be remembered is: never use so long a stroke that the edge of the mirror crosses the center of the grinder, and never a stroke so short that it holds the mirror inside the diameter of the grinder.

On a short stroke the mirror must overhang the edge of the grinder, if only l/4", in order that the grinder will be used all over its surface at each stroke.

Emery is applied by a stroke of the brush at each stroke of the drive arm. It will be ejected by the grinder into the pit and in this rough-grinding stage it can be reclaimed and used again so long as the ejected portion still contains unused particles.

The grinder and mirror will rotate in the same direction. It is fallacious that the disks must rotate in opposite directions, as many even advanced workers insist. Direction of rotation has nothing to do with the production of a truly spherical surface, which is accomplished by the stroke and rotation of the spindle causing the grinder and work never exactly to duplicate the conditions of any one stroke.

As a check on the curvature I recommend the glass gage ("A.T.M.", page 344). Even for the worker who owns a spherometer the glass gage can be of value. The spherometer can, of course, be used to check the gage and mirror for sagitta, but the gage will also show the overall variation of the mirror along its arc when used as a check on radius and sphericity. The glass should be as thin as can be found and the gage must be very carefully executed. A slight variation in the scribing apparatus will have a bad effect on the arc that carries the glass cutter. Pivot bearing and scribing bar should be very rigid and the cutter must be held very tightly to assure a perfectly even cut on the glass. Make a gage about 14" long, if for a 6" mirror, and cut out the portion having the most even curve. Grind the edges lightly together in 500 emery. A minute's work will bring them to a tight fit

The mirror, having been rough ground in 180 emery, must be brought to a tight fit on the gage before smoothing in 500 is undertaken.

We do not intend to grind by wets, as this method leaves the amount of glass removed in each size of emery to guesswork. By grinding exactly to curvature in each emery, and miking the glass each time, we shall be very sure of eliminating the pits from each previous size. This should lay to rest once and for all, the great disappointment of finding No. 180 pits holding over on the surface after it is polished.

The micrometer shown in Figure 3 with pointed tips over the anvils, will give accurate measurements of glass removal. The mirror must be marked on its edge with a permanent notch scratched in and filled with black paint, and this shall be the miking point.

After grinding with 180, the edge of the mirror is beveled. Hold a scrap of glass against the edge of the disk and, with the spindle running apply a little 500 emery until the bevel is about 2 mm wide. This will eliminate future scratches due to edge particles breaking free. Retain this bevel throughout smooth (500) and fine (1200) grinding and polishing. Keep the edge of the tool beveled also.

DRISCOLL'S article will be concluded next month. Observant readers will have discovered one important difference between the hand-lever spindles in "A.T.M." and "A.T.M.A." and Driscoll's. In the former the pressure on the work is regulated by the hand. In the latter the weight on the head of the free-floating pin regulates it uniformly

The philosophy of dragging and dragged disks, which Driscoll found too lengthy to discuss here, is discussed at length in Dévé's "Optical Workshop Principles."

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