Precision Grinding

Posted By Tom Stephenson on 07 May 2014

Posted in The Vintage Machinery Almanac

This article was originally published in Practical Engineering 1940 Vol No1. Information within this article is therefore correct as of 1940. The publication of this material aims to provide historical insight on the subject and its place in industry.

Grinding Wheels

Tool room precision grinding is a distinct branch of grinding practice which includes the accurate grinding of cylindrical parts, gauges and fixtures, and the sharpening of lathe and planer tools and all kinds of cutters and reamers. It calls for a high degree of skill, since the need for accuracy is great and the work varied.

Probably most tool-room grinding is done on precision tool and cutter grinders and surface grinders, in addition to floor and bench stands for off-hand grinding. Tool grinders for the automatic grinding of lathe and planer tools are frequently found. Special-purpose machines such as drill grinders, tap grinders, or combination machines, are found in many shops, especially the larger ones.

It is the aim of this series of articles to relate the grinding wheel, the work being ground, and the results obtained. To accomplish this the articles have been divided into several sections; one describing abrasives and grinding wheels in general; others dealing with methods of grinding all classes of tools, giving tool set-ups, and others describing classes of grinding, such as surface grinding, cylindrical grinding, internal grinding, and the use of portable grinders.

Abrasive Materials

There are four types of abrasives used in grinding wheels. These are marketed under various brand names, but probably one of the most familiar to the majority of engineers will be those manufactured by The Carborundum Company, Ltd. These are: Carborundum Brand Silicon Carbide; Carborundum Brand "Green-Grit" Silicon Carbide; Aloxite Brand Aluminium Oxide; Aloxite Brand "AA" Aluminium Oxide.

Carborundum Brand Silicon Carbide is made by heating a mixture of sand, coke, sawdust and salt in a resistance-type electric furnace which combines the silicon of the sand with the carbon of the coke to form silicon carbide. The crude material from the furnace is crushed, graded by screening to the required grit size, washed with acids, and alkalies to remove impurities, and magnetically treated to remove iron. It is then ready for processing into grinding wheels.

Carborundum Brand "Green-Grit" Silicon Carbide is similar in appearance to the regular silicon carbide except that it is, as the name implies, a light green in colour as compared to the almost black colour of the regular "Car-borundum." It is quite brittle and breaks with a sharp fracture. Because of its friable nature and its sharpness, it finds application in the grinding of the cemented carbides and certain of the extremely hard alloy-tool steels.

Aloxite Brand Aluminium Oxide is made by heating bauxite (a clay) in an arc-type electric furnace which drives off the chemically-combined water and forms the crystalline aluminium oxide. The crude material from the furnace is crushed and treated in a manner similar to that used in treating “Carborundum."

Aloxite Brand "AA" Aluminium Oxide is made in a manner similar to the regular "Aloxite," but from an alumina of high purity. The product has a porous and more friable structure, and is chemically of a higher degree of purity. The various types of abrasive differ materially in their characteristics. "Carborundum" is much harder than "Aloxite," but more brittle. It is also lighter in weight. "Aloxite" crystals are quite tough, and so will stand much stress. Aloxite "AA" is more brittle than the regular "Aloxite," and is used where a cool cutting action is required as in tool-room grinding.

Grain Size or Grit

The grit or grain of a precision grinding wheel refers to the size of the particles of abrasive used. The sizes are indicated by standard numbers corresponding to, the number of meshes in the screen through which they will pass. For example, a 36-grit will pass through a screen-having 36 meshes to the linear inch.

Some of the very fine grits are designated by numbers such as 280, 320, 400, 500, 600. Combinations of various grit sizes are also used in making grinding wheels, depending upon particular requirements of the grinding operation.

Grade

Grade is the tenacity with which the cutting particles are held in the body of the grinding wheel under grinding pressure. In other words, it is the resistance which the composite effect of grain and bond offers to the grinding forces which tend to break down the wheel structure. The various grades are produced by a variation in the amount and character of the bond used, as well as the shape and distribution of the pore spaces. Carborundum and Aloxite Brand vitrified wheels are graded from D (extra hard) to W (very soft).

Grinding-wheel Grading

The term "grading" applied to a grinding wheel does not mean degree of hardness of the abrasive material. It is rather, a term denoting the combination of grit, grade and bond of a grinding wheel and should in no way be confused with the term "grade " which, as before, means the tenacity with which the abrasive particles are held in the wheel under grinding stress.

The majority of grinding wheels are made by the vitrified process. This process consists essentially of -mixing the abrasive grain with the correct amount of a mixture of clays of pre-determined characteristics, moulding to approximate size in hydraulic presses or by other suitable means and burning in kilns to a high temperature to vitrify the bond. Bonds so produced are high-temperature glasses or porcelains. When vitrified, most of the wheels are dressed all over, bushed in the arbor to the correct hole size and, after speeding and inspection, are ready for use.

Wheels made by the vitrified process have an exceedingly strong bond and can be made in a variety of grits and grades. A wide range of bond structures is obtained by variations in the kinds of bond used and in manufacturing methods to give the desired porosity or density to the finished product as may be required for any particular grinding operation. Most vitrified toolroom wheels are open in bond structure and grind rapidly without heating the work.

Silicate wheels are those in which the abrasive is bonded with sodiurn silicate. Wheels produced by this method have a mild cutting action and find their field of application in the grinding of fine-edged tools, and knives of various kinds, etc. They should not be used for rough grinding. Among the advantages of the silicate process is the shortness of time involved in the making. There are several modifications of the silicate bonded wheels. One is the “L" type of wheel which gives an extreme open structure and brittle bonding, which makes it especially suited to surface-grinding operations, using the side of a cup or ring wheel where there is a large area of contact.

"Redmanol" is a phenolic resin, a synthetic organic compound. "Redmanol" bonded wheels are cool cutting and remove stock rapidly. These wheels are made in many sizes and for many purposes. As cut-off wheels they can be operated safely at speeds as high as 16,000 s.f.p.m. for cutting off all kinds of materials. Larger wheels, operated at speeds around 9,000 s.f.p.m. are used for snagging castings, and at normal speeds for finishing cams, roll-grinding and saw-gumming.

Rubber-bonded Wheels

Rubber-bonded grinding wheels are used chiefly where a good finish is required. The rubber softens under the heat of grinding and acts as a cushion for the grains of abrasive so that they do not cut as deeply as when more rigid bonds are used. The rubber also acts as a buff to polish out the grain marks. Extremely thin wheels can be made in. this bond because of its strength and toughness. For example, wheels as thin as .005in. are used for slotting pen points.

Grinding-wheels bonded with shellac are classed as elastic wheels. It gives a cool cutting and good finishing wheel. Since shellac is somewhat elastic and softens under the heat of grinding it is similar to rubber as a bond, but wheels so bonded cut more freely than rubber wheels and will take deeper cuts without burning.

Factors Affecting Grinding-wheel Action

A.    Tensile Strength.— High tensile strength materials such as carbon steel, alloy steel and high-speed steel require the use of an aluminous abrasive such as "Aloxite." Usually, materials having a tensile strength over 50,000 lbs. per square inch are ground with wheels of this abrasive. Low tensile strength materials such as cast iron, brass, bronze, aluminium, copper, etc., require a silicon. Carbide abrasive such as "Carborundum" Materials having a tensile strength less than 50,000 lbs. per square inch are usually ground with this type of abrasive.

B.    Hardness.— The harder the material, the softer the grade, and the finer the grit of the grinding wheel necessary, within the limits specified for the various types of grinding. The ordinary run of material used in the tool-room can be ground with either the Aloxite Brand vitrified wheel or Aloxite Brand "AA" wheel. Some extremely hard steels, such as a fully-hardened high carbon, high chrome steel can be ground more efficiently with a Carborundum Brand wheel.

C.    Ductility.— The more ductile the material the coarser the grit should be. The stainless irons and steels do not follow the above rules exactly, since either a Carborundum Brand wheel or an Aloxite Brand wheel may be used, depending upon the operation. The material may seem to be somewhat stringy, resulting in loading of the wheel. The nature of the chip removed often determines the more efficient wheel and this in turn is dependent upon the structure of the metal.

Hardness and Finish

The larger the amount of stock to be removed the coarser the grit, the harder the grade and the denser and tougher the bond structure within the limits specified for the various types of precision grinding. The better the finish required the finer the grit size, although in machine grinding fairly good finishes can be obtained with relatively coarse grits by proper use of the diamond dressing tool, together with careful adjustment of work speeds and wheel speeds. The larger the arc or area of contact between the wheel and the work, the coarser the grit size, the softer the grade and the more open the bond structure. The diagrams show various arcs and areas of contact between the wheel and the work. The higher the work speed the harder the grade required.

The Grinding Wheel

The larger the wheel the greater the arc of contact between the wheel and the work and the softer the grade should be. The higher the wheel-speed the harder the resulting grade action, and the softer the grade should be. The use of coolants produces finer finishes with the same size of grit in the wheel. The employment of coolant also permits the use of harder grades and higher rates of stock removal, with resulting economy. The more rigid the grinding machine and spindle, the softer the grade of the grinding wheel should be and the more accurate the work produced.

Dressing and Truing

Truing may be defined as any operation on any part of a wheel to create concentricity or parallelism or to alter the wheel shape, either before or after a grinding period. It may be accomplished with any dressing tool, provided it is rigidly fixed in relation to the point of contact with the wheel. Dressing may be defined as any operation performed on a wheel face to change the nature of its-cutting action.

All grinding wheels should be trued after mounting to make them concentric with the spindle on which they are mounted and to square off the wheel face. Dressing is necessary when the abrasive particles in the wheel face become dulled or when the wheel face becomes loaded with metal. It removes the dulled grains or metal from the wheel face and exposes fresh, sharp grains of abrasive for faster, more efficient grinding. Usually, both truing and dressing operations are performed with the same tool.

Dressing tools include the Huntington or star type, abrasive stick, abrasive wheel and diamond types. Throughout the remainder of the articles directions for dressing and truing are given for the various operations as they are described.

Standard Types of Grinding Wheels

In the interest of standardisation, and to enable the user to visualise at once the wide range of types of grinding wheels, there are shown on the previous page cross sections of the nine standard types which are representative of practically all grinding wheels used on the standard makes of grinding machines.

These nine types of wheels are numbered and each dimension designated by letter. The key to the dimension letters is shown below, with the illustrations of the types.

This classification of grinding wheels greatly simplifies the stocking of wheels wherever a quantity is kept on hand. It also enables the user to order a grinding wheel accurately by giving the type number and the complete dimensions necessary to construct such a wheel, as designated by the cross section of that type.

KEY TO LETTER DIMENSIONS.

A — Flat Spot of Bevelled Wall.

D — Diameter (Over All).

E — Centre or Back Thickness.

F — Depth of Recess  (See Type 5).

G — Depth of Recess  (See Type 7).

H — Arbor Hole Diameter.

J — Diameter of Flat or Small Diameter.

K — Diameter of Flat Inside.

M — Large Diameter of Bevel.

P — Diameter of Recess.

R — Radius.

T — Thickness (Over All).

U — Width of Face.

V — Angle of Bevel.

W — Thickness of Wall.

Care and Use of Grinding Wheels

Competent men should be assigned to the mounting, care and inspection of grinding wheels and machines. Whenever a wheel breaks, a careful inspection should be made to ensure that the hood has not been damaged, nor the flanges bent or sprung out of true or out of balance. The spindle and nuts should also be carefully inspected. After mounting a new wheel, care should be taken to see that the hood is properly replaced.

All new wheels should be run at full operating speed for at least one minute before applying work, during which time the operator should stand at one side. Work should not be forced against a cold wheel, but applied gradually, giving the wheel an opportunity to warm and thereby minimise the chance of breakage. This applies to starting work in the morning in cold rooms, and to new wheels which have been stored in a cold place. Wheels should be occasionally tested for balance, and rebalanced if necessary.

Wheels worn out of round should be trued by a competent man. Wheels out of balance through wear, which cannot be balanced by truing or dressing, should be removed from the machine.

Wheels used in wet grinding should not be allowed to stand partly immersed in the water. The water-soaked portion may throw the wheel dangerously out of balance.

All wet tool-grinders which are not so designed as to provide a constant supply of fresh water should be thoroughly drained at end of each day's work, and a fresh supply provided just before starting.

Grinding on the flat sides of straight wheels is often hazardous, and should not be allowed on such operations when the sides of the wheel are appreciably worn, or when any considerable or sudden pressure is brought to bear against the sides.

Wheel-dresserg should be equipped with guards over the tops of the cutters to protect the operator from flying pieces of broken cutters or wheel particles.

The space about the machine should be kept light, dry and as free as possible from obstructions. Care should be exercised so that the spindle will not become sufficiently heated to damage the wheel. All arbors, adaptors, or other machine parts in which wheels fit, should be periodically inspected and maintained to size.