Another man who had worked with both materials comments as follows:

"Glass-like all insulators of electricity-is no good mechanically; but it is homogeneous and it is seldom that a noticeable variation in hardness can be detected in any particular sample. Hence, when one piece is rubbed against another with an abrasive between, it is to be expected that all parts of the same piece will be similarly affected by the same rubbing speed and pressure. Not so with the metals. The metal is composed of crystals and hence is of varying hardness. If the steel is tempered, the lack of uniformity of hardness is even more noticeable.

"The chances are that the metal mirror maker has two strikes on him before he even starts to work. The chap who uses glass can depend on what he is getting, and be reasonably sure that his next piece will be pretty much the same as the piece he is now using. However, metal is much easier to handle than glass.

"Steel," continues the same man, who wishes to remain anonymous, "cannot be ground disk on disk, because of the tendency to gall. Little pieces pick up and roll into large balls, making deep, torn furrows in the work. Hardened steel is much better in this respect. Hence, I grind and fine-grind the mild steels on a lead lap. The fringes soon become visible and I can almost finish the job on lead. Then a little polishing on pitch, with rouge, gives a gorgeous finish."

A man who has worked with metals for flats is Sydney J. Needs, The Fairfax, 43rd and Locust Streets, Philadelphia, Pa. He is not an amateur telescope maker but an engineer who was aided by "A.T.M." in learning to make metal flats for use in an extended non-optical but high precision mechanical research on boundary films of lubricant in machine bearings, an account of which he published in The Transactions of the American Society of Mechanica1 Engineers, May 1940, pages 331-345.

As he did not there include data on his method of working metal flats, he was invited to describe his technique for these columns. Much of his description will seem strange to the worker in glass but, since amateur opticians often are called on to do precision work on metal surfaces, the technique he developed after much laborious experimentation should go into the permanent record and not be lost. Your scribe has a neat little flat of hardened tool steel presented by Needs. It tests flatter than 1/20 wavelength-a millionth of an inch. Needs writes as follows, in describing his working technique: "A metal surface produced by the lathe or grinder may be fine ground on a lead lap in preparation for the final polish. The lead lap has the advantages of being easily prepared; it wears away very slowly; and the degree of surface perfection produced may be varied within wide limits. In fact, the lead lap is capable of producing finishes quite satisfactory for most purposes without the necessity of further polishing.

FOR work up to about 6" diameter, an 8" lap is ample. A lap of this diameter should be about 2" thick, but this thickness ratio is arbitrary and unnecessarily high for laps of larger diameter. The casting is prepared by melting lead of the purest and cleanest grade obtainable and pouring into an oversized wooden mold. To produce a good surface on the work it is necessary that the lap be free of foreign particles. For this reason, scrap lead should not be used. When machining the lead casting, the horizontal table of a small boring mill will be found preferable to the chuck or face plate of a lathe. The outside diameter and the upper face of the casting are finished in one setting. Half inch lifting holes about 1" deep may be drilled radially from opposite sides midway between the two faces. These holes are useful for holding the lap on the machine table, while the second face is machined parallel with the first, and are necessary for handling the lap when in use.

"Both sides of the lap may be used, one for coarse grades of Carborundum or emery, the better side being used with only the finest grade of abrasive.

"After machining, the faces of the lap should be scraped to fit a plane surface plate. Lead will be found very difficult to scrape, and a poor surface will result unless a few simple precautions are observed. The scraper used is the ordinary flat double-edged hand scraper forged from a discarded file and hardened. Time will be well spent stoning the cutting edges of the scraper until they are smooth and sharp. Lead is so soft that the scraper will remain sharp indefinitely. Place a piece of carpet or several opened newspapers on the bench to form a support for the lead lap. Moisten a clean cloth with lard oil and wipe the upper face of the lap. A thin film of lard oil will remain on the lead and, thus lubricated, the scraper will cut smoothly and without tearing the metal Scrape the surface until all machine tool marks have been removed. Now clean both lap and surface plate thoroughly, moisten both with lard oil, and wipe dry with a clean cloth. A film of lard oil will remain on each and when the lap is gently rubbed on a surface plate, the trace of lard oil present will be sufficient to lubricate the points in contact. Upon removal of the lap these high points will be found brightly polished; and sufficient lard oil will still be present to lubricate the scraper removing them.

"After a test on the surface plate, remove all high spots, moving the scraper in one direction. After the next test, remove the high spots by moving the scraper at right angles to the previous direction, thus preventing the formation of furrows. Before each test clean the lap and reference plane thoroughly and apply a fresh film of lard oil; then re. move as- much of it as possible with a clean dry cloth. Should scratches be found on the lead after testing, more care in cleaning is probably indicated. When the contact spots are quite close together and well distributed over the surface of the lead, the lap will be found to float quite freely with no perceptible friction when first placed on the reference plane. This is due to a film of air separating the surfaces and an appreciable time is required for the air to escape. When this occurs, the lap is fairly flat but the scraping may be carried on as long as patience will permit.

ONE important detail concerning the use of the scraper must be added. Lead is so soft that merely reducing the pressure on the scraper at the end of a forward stroke is not sufficient to raise the scraping edge and stop the cutting. The scraper must be lifted from the work while still moving forward at the end of each stroke. If this is not done, each stroke will end below the surface of the surrounding metal and a burr will be raised where the scraper is stopped. The forward motion of the scraper removing the high spot has exposed metal, free of lard oil. If the scraper is moved back or a second forward stroke made over this unlubricated surface, scratching and tearing of the metal will result. The chip removed by the forward stroke will be found on the edge of the scraper and must be removed before the next stroke is started. After a few trials, the trick of raising the scraper and removing the chip with a finger will become automatic and require no further attention.

"After the scraping, the lead lap is ready to be charged and used. No grooves are necessary since the lap is used dry.

"For charging, a flat cast-iron plate is convenient, having approximately the same diameter as the lap but it need not be as thick. After it is machined all over, one side of the cast-iron plate is finished plane and smooth, either by scraping to fit the reference plane or by the time-honored process of grinding three plates against each other in pairs. Lifting holes similar to those in the lead lap are drilled in the side of the cast-iron plate. To charge the lead lap, No. 600 Carborundum is sprinkled on its scraped surface and spread as evenly as possible with absorbent cotton. The cast-iron plate is gently placed on the lap and moved in several directions with short straight strokes. Ten or 15 strokes should suffice to spread the abrasive evenly and drive some of it into the surface of the lead. After removing the plate, all excess carbo is removed with a cloth and the lap is ready to use (with the work on top). At first the lap will cut rapidly but, after 40 or 50 strokes, recharging is necessary. Before recharging, however, much can be gained by rubbing the work over the cast-iron plate, since this also was charged when charging the lead lap. Being harder, the charged cast-iron plate will cut more rapidly than the lead lap, but the scratches in the work will be much deeper. By using the cast-iron plate and the lead lap on small work, the time required for removing the tool marks will be so short that there seems to be no advantage in using any of the coarser grades of abrasive. The lead lap produces a surface that is quite bright and, almost from the start, the surface contour may be observed by interference fringes under a master glass.

"In addition to charging the lap, the cast-iron plate makes it possible to keep the work flat as the lapping proceeds. It has been found that, if the lead lap is flat, the work will be slightly convex, particularly if of small diameter. This is probably due to elastic deflections of the lap caused by the pressure applied to the work moving over it. To get the work flat, it is necessary that the lap be slightly convex. This is accomplished during charging by rubbing the cast-iron plate on the lead lap with the proper stroke. The plate will tend to become concave and the lap convex in the same manner as spherical mirrors ground by the telescope makers. Should the work become concave the lap is too convex, and this is corrected during charging by reversing the process and rubbing the lap on the cast-iron plate. Thus surface control may be maintained during the process of the work.

"As soon as all tool marks have been removed from the work it is no longer necessary to use the cast-iron plate as a lap, and the finish is made on the lead.

"It is important that the lap be dusted free of all loose abrasive after each charging. The lap will cut faster when loose abrasive is present, but the edge of the work will be badly turned. When the abrasive is sprinkled on the lap, it will fall in small mounds. If these mounds are not broken up the cast-iron plate will be placed on several little hills and the focal pressures over such small areas will be sufficient to distort the lead permanently, causing humps to rise around the depressions. Hence, the necessity of spreading the abrasive as evenly as possible before using the cast-iron plate.

"As stated above, the lead lap cuts comparatively rapidly when freshly charged and it follows that the scratches in the work will then be deepest. When all tool marks and visible flaws have been removed from the work and the test glass shows the surface to be of the required flatness, the polish may be considerably improved by continued working on the lead lap with no further recharging.

"From time to time, the lap should be washed with soap and water, washed again with clean water to remove as much of the soap as possible, and thoroughly dried with a clean cloth. Continued effort will reduce the depth of the scratches, with a corresponding improvement in polish. Unless removed mechanically or chemically, traces of soap will remain on the lap and these seem to assist by slightly lubricating the surfaces.

"The limit seems to be reached when the surface of the work begins to lose its brightness and take on a cloudy film. This is probably due to all the abrasive being forced into the surface of the lap, thus permitting the work to come into actual contact with lead. By stopping just before this point, the best results with the lead lap will have been achieved. Viewed through the low-power objective of the microscope at this time, the surface of the work is seen to be a mass of tiny scratches. The scratches, however, are very fine indeed and may be entirely removed in a few minutes by rouge on a pitch lap. If the pitch lap is flat at the beginning of the final operation, the polishing action on the work will be uniform and the surface contour unchanged by the pitch polisher."

With regard to metals for optical surfaces, Needs largely concurs with the anonymous writer first quoted, stating in the Transactions A.S.M.E. article cited above that: "There appears to be no metal which, when polished and examined even under low powers of the microscope, will present a surface of uniform surface even remotely approaching that of well-polished glass." If this is so decidedly the case, why then publish anything at all on the working technique for metals? Because of legitimate special uses for metal, and perhaps a little because of human nature. Amateurs will, and do, make metal mirrors.

Needs has told how he avoids turned edge without recourse to the exquisite skill developed by long-experienced makers of flats-simply the "surround" principle ("A.T.M.," page 53). "Turn up a ring (Figure 1) of the same material as used for the flat. Let the outside diameter be about 1" larger than the flat. Turn the outside of the flat a neat sliding fit in the ring. The fit must be good. If ever so little tight the flat and ring will both be sprung. If too loose, fine Carborundum will get between flat and ring and cause trouble when polishing. Heat gently and cement the flat in the ring with beeswax. Finish up, and the turned down edge will be on the ring. To separate, heat, push through hole A, and the ring will drop off, leaving the edge undisturbed."

SIXTEEN-INCH CLUB: In July, 1941, we told how Clyde W. Tombaugh, Lowell Observatory, Flagstaff, Arizona, sponsored a "club" of amateurs who, if as many as 20 members could be rounded up, would get a greatly reduced price on 16" Pyrex disks; and in November, 1941 we told of the success of that attempt. We now bring the story up to date.

Some months ago Tombaugh gave the names of 19 of the Sixteen-Inch Club "members," thus: Dr. C. O. Lampland, Flagstaff, Ariz.; Dr. James G. Baker, Harvard College Observatory, Cambridge, Mass.; Dr. H. Sidney Newcomer, New York; Messrs. John Kshir, 1702 Green St., Phila., Pa.; Harold Simmonds, 517 20th St., Sacramento, Calif.; Arthur Brear, 50 Willow St., Lawrence, Mass; Lyman H. Allen, 100 Franklin St., Boston, Mass.; Irvin H. Schroader, Angwin, Calif.; Byron S. Warner, 610 S. Ree Pl., Burbank, Calif.; Prof. Newton, Angwin, Calif., W. L. Whitson, Washington Missionary College, Wash., D. C.; Harry A. Shaw, 2401 Mar Vista Ave., Altadena, Calif.; James J. Connors, 2318 S. 61 St., Cicero, Ill.; A. M. Mackintosh, 70 Arnold Rd., Jamaica, B.W.I.; Franklyn Creese, 269 Davis St., San Leandro, Calif.; Arthur Hugenberger, 2322 Kenworth Rd., Columbus, Ohio, Otto Griener, 55 Plane St., Newark, N. J.; Clyde Tombaugh, Flagstaff, Ariz. These ordered a total of 21 disks.

In August, Tombaugh wrote that the total had risen to 33 members and 37 disks, and of the former he knew of the following: Messrs. C. L. Taylor; B. L. Souther, 24 Harrison Ave., New Canaan, Conn.; H. A. Lower 1032 Pennsylvania St., San Diego, Calif.; J. R. Smith; Russell S. Booker; C. H. Gamble, Marvin J. Vaun; J. W. Fecker, but did not know the addresses of some. "It would nice if you could publish the list of buyers," he writes, "to keep interest alive." This club certainly exceeded my expectations. But I don't think I can do a thing more with the Sixteen-Inch Club until the war is over, as I am organizing civilian defense and really am busy. After the war I hope to get the members organized into doing some serious observing."

Admittedly, the lists as given are far from complete or definite. Data are now difficult to round up, because of the war, and personnel is in a relative state of flux. The club has partly passed out of original organizing control (it really never was a definite organization but a convenience to guarantee Corning the cost of a master mold). As word of availability of this ideal size for a bigger-than-average telescope has got around people have gone direct to Corning Glass Works, at Corning, N. Y. for the disks.

Suppliers and Organizations

The Society for Amateur Scientists
5600 Post Road, #114-341
East Greenwich, RI 02818
Phone: 1-877-527-0382 voice/fax

Internet: http://www.sas.org/

At Surplus Shed, you'll find optical components such as lenses, prisms, mirrors, beamsplitters, achromats, optical flats, lens and mirror blanks, and unique optical pieces. In addition, there are borescopes, boresights, microscopes, telescopes, aerial cameras, filters, electronic test equipment, and other optical and electronic stuff. All available at a fraction of the original cost.

SURPLUS SHED
407 U.S. Route 222
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Phone/fax : 610-926-9226
Phone/fax toll free: 877-7SURPLUS (877-778-7758)
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