Brophy, Jere E.

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 The Knowledge Bank at The Ohio State University Ohio State Engineer Title:  Grinding - Tools and Technique Creators:  Brophy, Jere E.  Issue Date:  1942-06  Publisher:  Ohio State University, College of Engineering  Citation:  Ohio State Engineer, vol. 25, no. 7 (June, 1942), 14-15, 26-27.  URI:  http://hdl.handle.net/1811/35883  GrindingTools and TechniqueJERE BROPHY, Engr.ITo remove stock, to produce a finer finish and topermit closer tolerance on jobs, grinding has super-seded many types of machining. Sometimes a job maybe done quicker as well as more accurately by grindingthan by turning or shaping; plain bronze bearings maybe finished quicker by grinding (thus producing amore economical job) and also produce a better finishand a smaller tolerance.Plate glass is ground to produce a fine finish, billetsare ground to remove imperfections, bearings areground to conform to a specified size within close limitsand cemented carbides are ground since it is the solepractical method of machining them.A grinding wheel is a true self-sharpening tool sincein "perfect use" the grains of abrasive will break offand new grains will come into contact with the workbefore the effectiveness of the grains is lost throughdullness. On termination of the useful life of thegrains they are ripped from the bond to present a newgrain into contact with the work. The wheel stressesare the factors determining the above "perfect use".The stresses should be close to the shearing stress ofboth the bond and the grain. If a slight increase inthe stress breaks the grain or tears it from the bond,the wheel is said to be "soft''. If the grain stays inthe wheel and grows dull, the wheel is said to be"hard".The stresses are due to impact, shear and compres-sion. They are governed by the physical properties ofthe work being ground, the contact area between thegrinding wheel and the work, the wheel speed and thework speed, the rate of feeding the wheel into the workand the cross speed or traverse rate. For example,since copper offers much less resistance to penetrationthan chilled cast iron it produces lower stresses in thegrinding wheel.Compressive stresses are affected by the contact areabetween the wheel and the work. In surface or cylin-drical grinding, a large wheel will have a larger areaof contact than a small wheel on the same work. Withwork of the same diameter and the same wheel size,internal grinding has a larger arc of contact and agreater compressive strength.The ratio of wheel speed to work speed partiallydetermines the shearing stress in the wheel. If thewheel operates at a given peripheral speed the individ-ual grains will remove chips of a certain size. If theperipheral speed of the work is increased a greateramount of metal is sheared off in each chip. Thisincreases the stress on each grain. The grains thentend to break away from the wheel or the wheel acts"soft". If the peripheral speed of ythe work is de-creased the wheel grains have less metal to remove ineach chip. This produces a lower shearing stress andthe grains tend to stay in the wheel until they becomedull. Thus, the wheel acts "hard". This is a methodof controlling the grinding action since a "soft" wheelmay be made to act "hard" and vice versa.The in-feed rate or the rate of moving the wheeldirectly into the work affects both the shear and com-pressive stresses. If the in-feed is large a large com-pression on the grain results, also the amount of metalcontained in each chip is large which causes a largeshearing stress. Plunge-cut grinding or grinding bymoving the wheel in to the desired depth, backing outand repeating tends to cause high stresses. They areminimized by decreasing the work speed.The cross speed or traverse rate causes lateral shearand forces the edges of the wheel to break down. Thisin turn affects the finish produced on the work. Thismay be partially compensated for by slowing the tra-verse speed to produce a good finish.In deciding on the kind of abrasive wheel to be usedthe above factors plus the type of abrasive, grain size,structure, type of bond and hardness of bond or grademust be considered.There are three important types of abrasive used ingrinding wheels. They are aluminum oxide, siliconcarbide and industrial diamonds or "bort".Most aluminum oxide crystals are tough and neteasily broken. It is best suited for grinding a hightensile strength or hard material which can break thecrystals before they get too dull to do effective work.This places new, sharp cutting grains in contact withthe work. This grain is effective on such material asannealed malleable iron, tough bronze, high speedsteel, high silicon steel, alloy steel and high carbonsteel.Silicon carbide is harder than aluminum oxide, morebrittle and not as tough. Since it is not broken untilit begins to dull it is suited to two extremes of mate-rials; those of low tensile strength and those of veryhard materials from aluminum, rubber, leather, cop-per, soft bronze, lead and brass to cemented carbides,Page 14 The Ohio State Engineercobalt and chromium alloys, gray and chilled iron,marble and granite.Diamond wheels were first manufactured to grindextremely hard materials which could not be groundin any other manner. They are composed of industrialdiamonds or "bort" set in a matrix, usually an alloy.Recently, a metal bonded diamond wheel has been de-veloped for off-hand grinding of cemented carbidetools. It is made of industrial diamonds from the Bel-gian Congo embedded in an alloy under high pres-sure. This gives the wheel a high heat conductivity,small bond disintegration, low wheel wear and a rea-sonably high cutting rate. In placing one of these dia-mond wheels on the grinder, it is first indicated be-fore truing since the cost of the wheel is so high.For other grades of work the "special" aluminumoxide has been developed. This special grade breakseasier than the regular aluminum oxide. In the caseof internal grinding a "soft" wheel is required. If anoperator were to grind a hole in some high speed steelhe would like to have a wheel made of "regular" alumi-num oxide but since he is to grind internally the wheelshould also have the characteristics of silicon carbide.In internal grinding of a material of high tensilestrength a small wheel could be used; this would pro-vide a small arc of contact but the stresses would begreat, therefore, a wheel of aluminum oxide wouldhave to be used. Since it would not be economical touse small wheels, large wheels are used which increasethe arc of contact and lower the stresses. This maylower the stresses so much that the regular wheel wouldnot be self-sharpening but would become dull and re-fuse to cut or "load". Here the special aluminum oxidewould work effectively.In selecting grain sizes the amount of material to beremoved and the finish desired are of paramount im-portance. If the amount of material to be removed islarge, or if the material is soft, coarse-grained wheelsshould be used. This is particularly applicable to handgrinding operations such as snagging castings, wherefinish is of little importance and rapid stock removalis essential. However, in cylindrical grinding withcoarse wheels there is a strong tendency to spring thework. The use of fine-grained wheels to produce afine finish is desirable since large grains do not pene-trate well. This is also the reason for using fine-grained wheels on hard materials.The structure of the wheel or spacing between grainsis dependent on the material to be ground. Widegrain spacing is required for soft work since the chipsare larger and this gives each chip more clearanceand lowers the tendency to "load". Close grains areneeded for hard materials, formed wheels and goodfinishes. Highly variable pressures such as cup wheelsand snagging wheels endure need wide spacing of thegrains to permit each grain to carry its share of theload.The type of bond partially determines how long thecutting particles will be retained in the wheel. Thebond must release the cutting particles when they growdull but it must be strong enough to prevent centri-fugal force from destroying the wheel. There are fivemajor types of bonds: vitrified, silicate, shellac, resin-oid and rubber.The vitrified bond is composed of ceramic claysburned at high temperatures. It is virtually unchangedby heat or cold and can be made in a greater rangeof hardness than any other type of bond. It does notcompletely fill the void between grains, and, therefore,has more grain clearance than any other. Wheels withthis type of bond are adaptable to all types of grind-ing except where the wheel is not thick enough towithstand lateral pressure. There is no elasticity inthis bond. The wheels are strong enough to do struc-ture and also heavy work, can remove stock quicklyand produce fine finishes and close accuracy. Grind-ing wheels with this type of bond have a maximumperipheral speed of 6500 feet per minute and are usedin greater quantities and for more purposes than awheel with any other bond.The silicate bond is composed of clays using silicateof soda and fired at low temperatures. It is more sen-sitive to changes of the moisture content of the airthan the vitrified bond, gives less grain clearance andis softer than most vitrified bonds. It also has no elas-ticity and is not safe to use for thin wheels. This bondcuts with a lower temperature than vitrified and isused for grinding thin edges as required on knivesand tools.The shellac bond or elastic bond is composed ofshellac and other gums. It completely fills the inter-stices between grains, has a high tensile strength andelasticity and is used in making very thin wheels.Wheels made with shellac bonds are used for grindingchilled iron mill rolls and hardened parts such as cams.Resinoid bonded wheels are tough and resilient.They can withstand severe lateral pressure and shocksas are found in snagging. They produce fine finishes.Cut-off wheels have a maximum surface speed of 1600feet per minute, whereas, snagging wheels are success-fully operating at 9500 feet per minute. They are oftenused for billet and ingot grinding.Rubber bonds have many of the characteristics ofthe shellac bonds but they cannot be manufactured inthe same grades of hardness. Rubber wheels produceexceptionally fine finishes and almost all regulatingwheels on centerless grinders are rubber bonded.The grade of a wheel is the degree of resistance thebond presents to prevent the grains from being tornout. The ideal grade would hold the grains until theyhave become dulled through use and then releasethem. A hard grade holds the grains strongly. Hardgrades are used when grinding soft materials since it(Continued on Page 26)June, 1942 Page 15The metallurgy of the noble and preciousmetals has shared in the corresponding ad-vances in the fields of research and engi-neering. The story of a laboratory curiositybecoming an every-day raw material is nowdevoid the element of startling newness . . .Bronzes of alloy containing aluminum, mag-nesium, berylium, etc., lend variety—changethe copper dominating hues1 to golden rich-ness. Take the nickle from our familiar"white gold" and substitute palladium wehave a silver-white metal precious enoughfor the most exacting debutante to covet foran engagement or wedding ring and as de-lightfully workable to the jeweler as thetried and true platinum.When attention is on the subject or in needof counsel on your idea of a creation in analloy of noble metals consultTHE SHOP AROUND THE CORNERB A S C O M12 E. 11th Ave.B R O T H E R SWA. 5213MEN'S SLAX• Gabardine• Poplinat #4.98TWO LEGSPANTS37 N. HighJACKETS SWEATERSDINNER-TIME AT HENNICK'S..• THE LIGHTS ARE "TURNED UP1'• THE MUSIC IS 'TONED DOWN"• THE FOOD IS VERY GOOD• YOU WILL ENJOY YOUR DINNERHENNICK'SR E S T A U R A N TAt the Gate of the Campus for Over 30 YearsGRINDING(Continued from Page 15)takes a long period to dull the grains with soft mate-rial. A soft grade is used when grinding hard mate-rials since the grains are quickly dulled and shouldthen be released.In cylindrical grinding, the work is supported oncenters and driven by a grinder dog. To prevent thepiece from being tapered at the ends the grindingwheel is allowed to run off only half of the width ofthe wheel. Tapered, straight or form grinding iseasily performed on this machine. If a great amountof stock is to be removed it can be most easily accom-plished by in-feeding or plunge cutting. The finalsize can then be ground with the conventional trans-verse feed. Often the grinder operator will put hishand up to the wheel and let it grind off a bit ofskin in the feeling of the wheel to determine whetheror not the wheel is rough or loaded.German manufacturers have developed severalmethods of grinding threads that have not been verywidely adopted in this country yet. They use thethin "V" shaped wheel that moves axially just as athreading tool in a lathe moves. This method isalso used here. It is good for grinding threads ina hardened shaft of steel. It requires moving thewheel in and taking a new cut after each pass. Twogrinding methods, very little use here, were developedto speed up the process. The wide, tapered wheelhas multiple "V's" "trued" into its face. This wheelis axially fed into the work the same as the single"V" wheel but with the tapered wheel the full depthof cut can be taken in one pass. This is possiblesince the first "V" of the wheel takes part of the cutand each succeeding "V" takes more of the cut withthe last "V" grinding the final size of the thread.With this method the wheel must be able to travel adistance equal to the width of the wheel farther thanthe desired length of thread. To remove this diffi-culty and still decrease machining time the straight-faced, multiple "V" wheel was developed. With thiswheel a thread the full width of the wheel face maybe cut in one revolution of the work. This is accom-plished by plunging the wheel directly into the workand withdrawing it after a complete revolution. Thishas the added advantage of giving a precise locationof the start of the thread with respect to the rest ofthe threaded piece. Its biggest disadvantage is thedifficulty of holding close tolerance by this method.Centerless grinding is on the increase because iteliminates the operation of centering both ends ofthe piece; the work is completely supported in thegrinding zone which permits a higher efficiency forthe grinding wheel; idle or non-grinding time is re-duced in the grinding cycle; and work may be "trued"up with approximately half the grinding stock re-Page 26 The Ohio State Engineerquirement of a cylindrical grinder, thus reducing thewheel cost per piece.The centerless grinder also sizes on the diameterof the piece, and not on the radius as is done by thecylindrical grinder. The grinder design also lendsitself to automatic and hopper feeds.Essentially, a centerless grinder consists of a slowlymoving regulating wheel and opposed, high speedgrinding wheel forming a grinding throat. A workrest of cemented carbide is placed between the wheelsfor the work to ride upon. The grinding action of thelarge wheel forces the work against the regulatingwheel which drives the work at the same peripheralspeed.Recent developments in grinding machinery in-clude the grinding jig borer. It is nearly the same asthe conventional jig borer except the spindle hashigher speeds and is equipped to take a grindingwheel instead of a flybar or boring tool. With thismachine distances between holes in hardened piecescan be maintained within close limits. Formerly, thepiece was jig bored and hardened. During hard-ening the work often warped and destroyed both thedistance between holes and the hole sizes. The tool-maker then had to laboriously stone the hole backinto shape and position. This new machine eliminatesthis waste time. Grinding methods are constantlybeing improved to save machining time; in presenttimes this assumes paramount importance.June, 1942 Page 27

Grinding - Tools and Technique

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