UNCOMMON OPTICAL GRINDING and polishing materials such as boron carbide, sapphire dust, titanium oxide, diamond dust, together with uncommon facts about some common abrasives, are the subject of a systematic survey by John M. Holeman, 305 Thayer Drive, Richland, Wash. He writes:

Today, many other substances than glass are being optically worked. To name a few: metals, especially hard ones that do not tarnish; crystals, transparent salts and plastics, each of which seems to be best worked with some particular combination of abrasives and laps.

There is a trend toward harder and harder abrasives because they lessen time of working and thereby lower the cost of the finished product. The makers of diamond tools advertise:

"Diamond is the cheapest abrasive you can use." In many cases this turns out to be true. A dollar's worth of diamond will remove more glass than a dollar's of any other abrasive, and with expenditure of less effort.

The arbitrary Mohs scale of hardness used by mineralogists is very deceptive. Diamond on this scale is 10, sapphire 9, topaz 8, quartz 7, feldspar 6, apatite 5, fluorite 4 and so on. These indicate the relative hardness of the minerals but not their abrasive power and not the breadth of the steps in between the various degrees. One might be led to believe that each degree of hardness represented by a number meant a uniform increase in hardness and abrasive durability. Experience shows that this is far from true. Actually, the steps from fluorite 4 to feldspar 6 are very close but from topaz 8 to diamond 10 much greater. The hardness numbers do not reveal this disparity.

In the discussions that follow there will be no attempt at completeness nor will all the means of using any particular abrasive be indicated. The information offered represents the experience of only one amateur.

Sand: This, the cheapest abrasive to buy, is still used where nothing else is available or where cost is paramount. Plate glass is ground flat with sand. During the War when better abrasives were under priority some amateurs used sand. TNs stranded on Pacific islands used sand. Pure sand is quartz and has a hardness of 7. This is greater than that of glass, which varies from 5 1/2 for soft crowns to 6 for hard flints. Sand may be crushed, sieved, washed and levigated and has been used to produce excellent work on both metal and glass. In the form of a sand blast it is the quickest means of removing glass, and blown through suitable rubber stencils it can be used for chanelling glass tools and will perform glass removing operations that would be very difficult by any other means.

Crushed steel: This inexpensive byproduct of the steel mills (see "A.T.M.") is available only in coarse sizes, but is useful for bringing a large piece of glass to shape. It cuts Pyrex readily.

Emery: A form of corundum (not Carborundum) which came originally from natural sources, but in this country practically all of it that is used as an abrasive is manufactured, permitting better control of the purity and uniformity. Chemically, emery is the same as sapphire, both being aluminum oxide, and has a hardness of 9.

Emery is available in all grades from very coarse to extremely fine and in some countries is used in the coarse sizes almost to the exclusion of silicon carbide, Carbo, for example. It is generally believed that the pits and scratches left by emery are different from those left by Carbo and are easier to polish out. Consequently emery is almost universally used as a final abrasive. Similarly, because it seems to chip sharp edges less, it is often used for the fine grinding stages of roof prisms and other sharp-edged elements, this being a case where the fastest abrasive is not always the best.

Manufactured emery is a clean white powder easy to refine and levigate. In many plants used emery is saved, washed, and re-levigated to produce finer grades. The best description of this process, which is more or less applicable to refining all abrasives, is found in Deve's "Optical Workshop Principles."

Silicon Carbide: Chemically, Carborundum, usually shortened to "Carbo," and Crystolon, are silicon carbide and have a hardness greater than 9. In coarse grades, at least, these remove glass much faster than the same grade of emery-which, as we have seen, is only a fraction of a hardness degree less hard. Silicon carbide is today available in grades from very coarse to 600 grit. No. 1000 Carbo, used in wartime, has been discontinued. The price, like that of any abrasive, varies with the fineness and is less per pound than fine grades which are more difficult to produce. In any case, it is considered an extremely cheap, hard abrasive. Silicon carbide is used to cut quartz and metals as well as glass and, in the form of inexpensive resin- or rubber-bonded thin disks, is used to saw glass and other materials.

Boron Carbide: Sold as "Norbide," this has hardness 9 1/2 and is therefore much harder than silicon carbide; it is the hardest material known excepting the diamond. Commercially it is available in at least two grades, 100 and 400 grit, from the Norton Co., Worcester, Mass., and it represents the ultimate in loose abrasives at present. Though it is only a fraction of a hardness degree higher than silicon carbide and emery, it breaks down much less rapidly and can be used to rough out a 6" Pyrex mirror in half an hour when properly applied. This material is also useful to charge biscuit cutters and wire saws for cutting glass and harder substances.

In 1947 boron carbide cost 50 times as much as silicon carbide, but the price will probably decrease. In appearance it resembles silicon carbide, being black and yet crystalline, but when broken down and wet it forms a dirty black slurry that is much more difficult to clean up than other abrasives. Metal tools that have become embedded with it are difficult to clean and will continue to scratch for a long time due to the difficulty of breaking down the very hard grains. This is one worker's experience. Another says almost the opposite. Because boron carbide grains are blocky in shape they will not charge metal tools as readily as do the sharper abrasives; in fact, when it is actually desired to cause a metal tool to take and hold a charge of it, cast iron having plenty of voids should be selected. [The Bill of Rights of the United States Constitution guarantees the freedom of workers who may wish to choose sides in this little dispute, and an account of their findings will be welcome.-Ed.]

Sapphire Dust: This is a by-product of the artificial sapphire industry which during the war made millions of jewel bearings for instruments. Sapphire crystals are grown to large size, then sawed up to make bearings, all the sawings becoming waste. The Linde Air Products Company, East Chicago, Indiana, has several grades at surprisingly low prices. A half pound of No. A5175 "Polishing Powder" cost $5 in 1946. It has a 9 grain size comparable to very fine emery. Though chemically it is the same as emery and, like it, has a hardness of 9, it is a very different abrasive. It cuts and polishes glass, Pyrex, quartz, sapphire, and metals more readily and to a higher polish. Used on glass laps it will 9 put the sharpest possible edge on microtome and other knife blades. Used on hard laps it grinds and polishes (though not optically) stainless steel, Stellite, and many other difficult metals. Used on pitch or felt it polishes quartz and harder materials in a fraction of the usual time, though for best finish a final I treatment with rouge should be added. This snow white powder is clean to 9 handle and, of course, can be levigated like emery or diluted with talc to reduce its cutting speed.

Diamond: During the war greatly improved techniques for using diamond were developed. Previously, rotary diamond saws were made by rolling or pressing diamond chips into slots or nicks in the edge of a copper disk. Such saws cut well until they snagged a sharp edge and the diamonds were dislodged and lost. Further, the soft copper blade was so flexible that it was difficult to make an oblique cut and the saw wandered off on a crooked path. Most postwar saws are made of steel with the diamonds brazed on, so that it is practically impossible to remove them. Such saws last almost indefinitely. A 10" blade costs about $6. They will cut glass, quartz, rocks, firebrick, hardened steel or any hard substance. In a recent demonstration with portable equipment, an 8" thick concrete highway was thus sawed in two in 30 minutes at much less cost than the job could have been done with air hammers.

Another development of the diamond saw is the hole saw or biscuit cutter used for making large holes in glass and other hard materials. Such hole cutters are now available so cheaply that it hardly pays to make one. They cut faster, cooler and with less danger 9 of chipping than the usual cutter charged with loose abrasives. When they are properly operated the hole produced has a beautiful finish with only extremely fine chips on the exit side, and the removed plug looks as if it had been turned and fine ground.

A recent development is a diamond boring tool made by Felkin Tool Company, Torrance, Calif., which forces lubricant (usually water) under its cutting edge through a stationary water connection on its rotating shank. Such a saw cuts ten times as fast as the familiar diamond hole saw and I have seen an operator cut sixty 1" diameter holes in thick glass in less than two hours.

Besides saws and hole saws, diamond milling cutters and lathe tools are available. The former are metal cylinders with the surface covered with brazed-on diamond chips. They can be used either on a milling machine or a surface grinder. It is amazing to see such a cutter smoothing off the top of a line of glass prism pressings on power feed at the rate of several feet a minute. Diamond lathe tools can be used to turn glass disks round or to shape lenses. The deep curve of a Schmidt mirror, for example, can be quickly cut out with such a tool. In fact, if diamond cutters are used, it is possible to work glass very much like metal and on the same machine tools.

Probably the most advanced development of this kind is the lens generator which makes precision lenses from glass blanks without the use of laps or loose abrasive. The blank is clamped in a holder and a diamond cutter adjusted by a graduated wheel runs over the surface at the desired curvature. The resulting lens is of good quality and even has a fair polish. A simple adjustment of the dial allows a different radius to be cut.

Diamond drills for small holes consist of a single properly shaped diamond fastened to the end of a steel shaft. Such drills are mass produced in the size, about 0.05" diameter, that is used for drilling mounting holes in spectacle lenses, and can be made to order in other sizes. The selection of the stone, with the mounting and shaping of the point, are a job for the expert and may be considered reasonable at $35 for the size described. Small holes can be drilled in hard materials, using a vibrating hardened steel wire and diamond dust, by a process that is exactly the same as star drilling in concrete. The National Bureau of Standards, Washington, D.C., has recently developed some very superior chemical and electrical methods for drilling extremely small holes in diamonds which would probably be applicable to other hard materials also.

Diamond is too expensive to use loose like silicon carbide and emery. Instead, a tool or lap is made and charged with diamond powder or chips. Lapidaries, in smoothing and polishing gems, spread a small amount of diamond dust in oil vehicle on a smooth soft iron or copper lap; then, with the lap rotating at low speed, they press down on the surface with a small hardened roller, which forces the abrasive into the soft metal. The cutting action of such a charged lap is a combination of grinding and polishing. It is like grinding' in that relatively large particles of abrasive are used and consequently there is rapid removal of material, and it is like polishing, in that the abrasive particles are fixed in a comparatively soft lap and are not free to roll. The result is that a diamond lap cuts rapidly but leaves a semipolished surface covered with scratches caused by the larger particles. The spaces between the scratches are pretty well polished and this is very convenient in working glass, as the surface always has a good enough finish to give a reflection for test purposes.

Next month, more about diamond abrasives; how to make diamond laps; data on titanium oxide; and Barnesite, its composition, how it should be applied, where it may be obtained. Also garnet fines as a finishing "emery."

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