INDUSTRIAL AND ENGINEERING CHEMISTRY
October, 1927
1127
Abrasives and Grinding By Lowell H. Milligan
I
T HAS been said that a Ford manufactured by former
methods would cost as much as a Rolls-Royce docs now, were it not for grinding. The satisfactory operation of motor carx, busses, tractors, trucks, and airplanes depends in a large measure upon the accuracy attainable in the fitting of their moving parts. Grinding methods of today produce a high degree of precision with rapidity and a t relatively ION cost. Grinding Wheels
of an automobile ball or roller hearines. and these could not be made without
Just as chemistry, physics, and allied sciences have modi-
the proper shape so that, after firing, it will fit accurately to constituent in the structure in the grinding wheel to the intexganular constituentsof
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modlfication of these factors in order to produce more e5cient and more uniform grinding action constitutes the contribution of chemistry and physics to the abrasive indu8try. The minding oneration itself is essentiallv mechanical. and a1tho;gh de;elipmenta from that source &e also extremely Important, they will not be considered here in detail.
arc chipped away, but the defects in modern alloy steels must be ground out as shown in Figure 2. Such materials have made it possible t o reduce the unit weight of an automobile or airolane Der horsenower develoned bv the engine. and
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I&\ DOS1'KI.IL AND ENQIIVEEI~IKGCHEMISI'KY
this, after all, is probably the rriaiii reason n-hy these inctliods of transportation have developed so rapidly. Stellitc, an alloy used for making inacliiniiig-tools to be operated a t high speeds, is anothcr inaterid that iriust be ground.
T'ol. 19, s o . 10
silicon carbide and known by trsde names such as Carborundum and Crystolon. PRODUCTION OF ALUMIKUMOXIDE TYPE-Crystalline aluminum oxide abrasive iu produced from iinpure alumiriuin oxide minerals. The raw material is usually dried, powdered, and mixed with the proper amount of powdered carbon so that in the electric furnace most of the impurities will bc reduced to metals. The furnaces consist of a cylindrical steel shell set on a carbon bottom. The electrodes are susponded from above and the heat is produced by electric arcs playiiig between them and the surface of the melt. The outside of the shell is cooled by 8 continual stream of water wliicl~ runs over i t to solidify a thin layer of alumina against tlie shell, and this then acts as a container for the rest of the melt. The mixture is fed a t intervals by hand and the electrodes iire slowly raised as the charge melts. The reduced metnls separate out as a layer below the liquid aluniina. Xearly a week is required after the electrodes are withdrawn for ti 10-ton "pig" of alumina to cool to room temperature. Figure 3 shows pigs of abrasivc, and furnaces in operation.
Figure 2-RouBh-GrindinQ the Surface Defects Off Alloy Steel Billeta
Other Uses for Abrasives Bbmyives are of service not alone as grindingwheels. Loose abrasives are used, suspended in grease or water, for grinding valves, for lapping piston pins and rings, crankshaft pins and bearings, and for surfacing plate glass. Glued on the surface of polishing wheels, abrasives serve to smooth and polish radiators, fenders, bumpers, and many other m&l parts. Fine abrasives cemented on paper or cloth are used for various finishing operations, conspicuous among which is the smoothing and surfacing of Duco and other lacquers 0:) aotoinohile hodies. Figure 4-Silicon
Fikure 3-Elecfric
Furnaces and Pic* of Cryslalllne Alumlna Abrasive
Types of Abrasives Satlira1 minerals were the first abrasives used hy man,quartz (often in the forni of sandstone), garnet, emery, and corundum. But. the grinding action of each of these was variable; t,heir composition mid structure could not be controlled. It. remained for the chemist to develop artificial abrasives which c(~uldb? made to specifications and maintained uniform in properties. Today there are two classof artificial abrasivesi both products of the intense heat of tlie electric furnace: the one consisting essentially of c r p talline aluminum oxide and renresented bv such trade names as Aluiidum and Aloxite; the other composed of crystalline
Carhide Furnace In Opemtion
For the production of high-purity aluminous abrasive, the furnacra are operabed in a dniilar WRY except that the raw niaterial is almost pure aluminum oxide, made by chemical methods from bauxite or other aluminous minerals, and no carbon needs to be added to the mixture. PRODUCTION OR SILICON CARBIDETYPE-Silicon carbide abrasive is produced in a resistance-type electric furnace, from a mixture of coke and sand, t.owther with sawdust and salt to give a porous mixture and ib facilitate reaction A furnace in operation is shown in Fimre 4. The Barnes are caused by -carbon monoxide gas that is liberated by cliernical reaction within the charge and escapes to the surface, where it burns. The silicon carbide is formed in the solid crystalline state in the furnace, and it is only necessary to cool the furnace off, open it up, arid break up the lumps of abrasive. COMPARATIVE I'HYSICAL fRoPEnTIEs--The physical propert.ies of alumina and silicon carbide abrasives are different, making them useful in different fields. The aluminous abrasives are intriiisically tougher, which makes them more suitable for mindina steel and other tourh materials of high tensile stre&li. Silicon carbide is somewhat harder-iext to the diamond in hardness-but it is more brittle, and of moyt use for grinding weaker metals and hard, brittle, or rock-like materials. Modification of Crystal Size But here main. in the matter of aluminous abrasives. the ~, chemist and physicist huve come to the fore, and by modi-
Figure SA-Dense Artificial Alumina of Medium-Size Crystal Structvre; ThIn Section, Transmitted Liehf. x 30
iying the crystal size or the porosity of the abrasive have been able to produce a more brittle product for special t y p u of grinding. A war-time abrasive, developed for surfacing optical glass, while not directly connected with the automobile industry, illustrates how crystal size may affect grinding action. Prior to the war emery was brought from Turkey to use for surfacing optical glass. Rut hostilities cut off importations and the optica.1 instrument industry was in dire distress because the armed forces of our country demanded ever-increasing numbers of linoculars. range finders, periscopes, and other optical instruments. The abrasives then available were too tough and sharp for surfacing glass; they scratched the glass without wearing down sufficiently. The problem was solved by the development of an aluminous abrasive of small crystal size, produced by pouring white-hot liqrrid alumina, molten at a temperature of over 2200” C., directly 011 a cast-iron plate, so as to chill the alumina rapidly in a thin layer. Figure 5 shows the magnified structure of artificial alnmina abrasive of medium crystal size. Production of Porous Abrasive
A porous crystalline alumina has been developed by having present in the raw materials for abrasive manufacture
Fisure 5B---Same as SA, hut In Plane P o l a r i d LiSht
some chemical substance which exerts an appreciable vapor pressure during the solidification of the fusion in tho electric furnace. I n practice, soda is commotily used and when the proper proportion of soda is present the product is as shown in Figure 6. Wlreels made with t.his abrasive find extensive use in t.ool and cutter grinding. I n all cases, of course, the lump abrasive has to be crnshed and carefully graded into definite grain sizes before being used or made into wheels. Bonding
Various bonds are employed to hold the abrasive grains together in the grinding wheel, and t.he resulting wheels have different properties and are for different purposes. “Vitrified” bonds are made of mixtures of clays and feldspars, and the wliecls are fired in ceramic kilns t.o mature the bond to a glass. “Silicate” bonds are essentially sodinm silicate, zinc oxide, and fillers. Shellac, hard ruhber, and synthetic resins, such as Bakelite, constitute the “organic” bonds commonly employed. The wheel which operates best is usually one that wean just enough t,o keep itself sharp, hut not so much ES to be wasted away too rapidly. To attain this result for different grinding operations requires that the abrasive grains he
I.VDUSI’RlAL A V D ENGIINEERING CHE.VISTRY
1130
bonded together with various degrees of strength 111 different dinarily this is accomplished by using different bond relative to abrasive grain in the manufacture of the wheels, and the products are said
heels ramie industries, except that the specific wbeels are mu& more rigid than for most The problems in hard rubber and synthet
ing ts
Vol. 19, No 10
from dust .I11 tliese probleiiis hale beerr solved and the resulting papers are used in quantities. In the preparation of abrasive papers and Is it is necessary that the abrasive grain be thoroughly wetted But abrasive grain readily &orbs oily e its ability to be wetted by glues. Often the open air for a week or two will contaminate the surface of the grain to an extent which is measurable by a test devised for the purpose. Chemical or phyical treatments which give a clean surface have beeii dcveloppd, and storage in especially designed and prepared kegs
cturers %ore coucerned unassociated with the industry can h a d complexity of the problems. Grinding wheels are being run at higher in order to obtain increased rates of p bonds must be madc stronger, tougher, and more uniform in order to maintain a sufficient factor of safety a t the high speeds. Much progress has been made along these line-,
ess of the wheel SfruCtnrP, for instance, are fed into the work very fast, and yet thclr must hold For this purpose a solidly shaped abr other purposes su one which contain
feet for similar v‘ capable of develop The uses of the The fact that they erties is an added
Perhaps the thinnest cutting-off wheels made have a hardrubber bond. These wbeels were developed for cutting-off tungsten wire and rod used for electric contact points in timers and coil boxes. They are only 0.015 inch thick, and are made very thin to avoid wastage of valuable tungsten metal. They are operated iu gangs with the sides supported by diamonds. Polishing Wheels and Abrasive Paper
For polishing wheels and abrasive paper the grain is Cemented onto a backing material. Until the chemists developed high-grade adhesives, emery was as good for these uses as were artificial abrasives. But when better glues and cements were available, which would hold the grain throughout the whole of ita useful life, then the artificial abrasives were found to be superior. Before the advent of Duco the 6rst coats of paint on automob& bodies were finished with sandpaper and the varnish coats were hnd-rnbbed with pumice and oil. But Duco necessitated changed methods. The fine sandpaper was not enough to smooth the surface Properly, and artificial abrasives were required. Furthermore, a high-prade, waterproof the the pspr so thtadhesive the psSr be to after in water Or in oil, as well as dry, in order to diminish the inconvenience
M a n y grinding wheels are used in the d r y c o n d i t i o n , but where i t is necessary to keep the work cool, ES in grinding hardened steel, a heavy stream of water is direeted between the work being g r o u n d pigura T-TMn s e c f i ~ n ~ f a n ~ l ~ n d ~ m a n d t h e wheel. A Viaieed Wheei of Medium Gmin and Grade: Trenamitted Light. X 18 mild alkali or soap is u s u a ~ yadded to the waterto diminish mstingof the grilldmachine. ~~t wheels wet are libely to act somewhat differently, ,,wing to the lubricating of the water, and that brings us to the effect of lubricants upon action. It has been found that, when grinding aluniiuuni just ae when machining it, lubrication m t h a mixture of kerosene and hrd gives a smoother and more action. With dry-grinding a lubricating effect may be ob-
* “Gntr and Gnnds,” publrshed by N ~ r f ~Company. n September. 1828. ‘Some Preusion Methods and Apparatus Employed in the Study and Development of Gnndmg Whech.” PYblshhed by the staff of Norton Com
pwy Research bboratoriea
October, 1927
INDUSTRIAL A N D EiVGlNEERING CHEMISTRY
Analytical chemistry and scientific methods of control are, of course: extremely important in enabling the manufacturers of grinding wheels to secure uniform products. The raw materials for abrasive manufacture are analyzed; chemical, as well as electrical, control is exercised over the operation of the electric furnaces; bonds are mixed according to the analyses of their ingredients; and pyrometers and ceramic cones control the temperatures in ovens and kilns. Kew materials are continually being developed and tried for abrasives and bonds. Old materials are being studied and modified to meet demands for different grinding action. hlethods of manufacture are being bettered in order to produce more uniform products. Mechanical designs of wheels and machines are being improved. Abrasives are being introduced into new fields. The future of grinding is bright, and science points the way toward new achievements.
tained by impregnating the grinding wheel itself with materials that soften a t the surface of the wheel under the heat of grinding. Wheels used for dry-snagging of aluminum often cut faster and wear less when impregnated with a proper mixture of fats and waxes. Rosin is sometimes used as a filling treatment for steel-snagging wheels, but its use has largely been developed empirically. A thorough scientific investigation of the general subject of filling treatments and lubrication as applied to grinding should yield valuable results. Achievements through Scientific Methods The petrographic microscope and microscopic methods in general have proved invaluable tools for the study of abrasives and grinding wheels. Examples have already been given of photomicrographs showing a few of their many applications. [ENDOF
1131
SYMPOSIUM]
Colorimeter for Precise Matching of Solutions in Nessler Tubes’ By John H. Yoe UNIVERSITY OF VIRGINIA,
I
S DEVELOPIXG a quantitative colorimetric method
for determining small quantities of aluminum in salts, water analysis, etc., by means of the dye aurin tricarboxylic acid (Aluminon),* recently described by Hammett and Sottery3 as a new qualitative reagent for aluminum, a simple colorimeter was devised to assist in securing a precise matching of colors in Nessler tubes. This colorimeter (Figure 1) is similar to the well-known Kennicott-Campbell-Hurley ~olorimeter,~ a series of Nessler tubes and a rack replacing the stand carrying the two comparison cylinders and reservoir tube. The light (north sky) is reflected by a small mirror placed on the rack at a n angle (not shown), just below the “unknown” and comparison tubes. The light passes upward through the two Kessler tubes, impinges on the two mirrors, A and B, 32 by 19 111111. and 32 by 35 mm., respectively, which are fastened to the wooden box a t an angle of 45 degrees, and is reflected horizontally through the metal observation tube. Half of the circular field of light from the right-hand tube is cut off by mirror A, the vertical edge of which serves as a dividing line between the two halves of the circular field. The image of half of the right-hand tube is then observed in juxtaposition to the opposite half of the image of the lefthand tube. The juxtaposed images are observed through a thin metal tube, 170 mm. long and 25 mm. in diameter, painted dull black inside and out and provided with an eyepiece having a hole 1.5 mm. in diameter. At the other end of the tube is a diaphragm having an aperture 8 mm. in diameter. By having the apertures in the eyepiece and diaphragm properly proportioned only the image of the bottoms of the Nessler tubes can be seen, thus preventing interference of light reflected from the vertical sides of the tubes. Upon looking through the eyepiece the observer sees a single circular field divided by an almost imperceptible line when the two solutions have the same intensity. The colorimeter is athched to the Nessler rack by means of a metal tube support which slides snugly down over a 1 Received May 14, 1927. Contribution No. 40.
CHARLOTTESVILLB, VA.
vertical rod securely fastened to the rack. It can thus be quickly and easily raised and turned on its horizontal axis, permitting interchange of the Nessler tubes in the series of standard solutions until a match with the “unknown” is obtained. I n practice, the approximate match is first obtained in the usual way by looking down vertically through the tubes in the rack and then the final match made by swinging the colorimeter into place. The colorimeter box is painted dull black inside and out. It is fitted with a removable cover, which permits easy access to the mirrors for the purpose of adjusting and cleaning. .Nibor A or B
Figure 1-Colorimeter
for Nessler Tubes, Vertical Cross Section
A screen made of a piece of s t 8 cardboard and painted dull black may be interposed between the Nessler tubes and the source of light. A colorheter lamp is recommended if the highest precision of matching is required. In such a case a white glass plate makes a better reflector than a mirror.
New Argentine Chemical and Metallurgical DevelopmentA company, capitalized at 5,000,000 paper pesos, has been organized in the Province of Mendoza to develop Argentine copper mines by the iodide process, by which the company ex* TBe finally adopted method and the results of an extensive study of pects t o produce hydriodic acid, sulfuric acid, copper sulfate, and it will be published later by J. H. Yoe and W. L. Hill ( J . Am. Chem. Soc., chemically pure metallic copper. Apparently, the promoters of in press). the project have based their calculations on the results obtained a J . Am. Chem. Soc., 47, 142 (192.3. a t the iodide plant in Santiago, Chile, which was inaugurated on 4 Kennicott and Sargent, Chem. Eng., I , 213 (1906-7); Campbell and February 26, 1926. Further details are available in the Chemical Hurley, I A m . Chem. .Soc.. 33, 1111’ (19111: Smeaton, Ibid., ‘28, 1433 (19061. Division, Department of Commerce.
