Metal-grinding

Grinding is one of the material removal processes by making relative motion in between work piece and abrasive surface Horizontal-spindle surface grinding operation.

Classification of grinding operation In general grinding operations are classified in to seven operations as follows

Surface Grinding Operations (a) Traverse grinding with a horizontal-spindle surface grinder. (b) Plunge grinding with a horizontal-spindle surface grinder, producing a groove in the work-piece. (c) A vertical-spindle rotary-table grinder (also known as the Blanchard type). Surface grinding is an abrasive machining process in which the grinding wheel removes material to from the plain flat surfaces of the work piece. Plunge grinding : Cylindrical wheel is fed directly into the work piece Travers grinding : Wheel is fed perpendicular to the axis of the part

Cylindrical Grinding Operations Examples of various cylindrical grinding operations. (a) Traverse grinding, (b) plunge grinding, and (c) profile grinding. In cylindrical grinding, also called center-type grinding, the external cylindrical surfaces and shoulders of the work piece are ground. Typical applications include crankshaft bearings, spindles, pins, bearing rings, rollers for rolling mills

6 Center-less Grinding Center-less grinding is a process for continuously grinding cylindrical surfaces. The work piece is place in between two wheels. The larger grinding wheel does grinding, while the smaller regulating wheel, which is tilted at an angle I, regulates the velocity Vf of the axial movement of the work piece.

Schematic illustrations of internal grinding operations: (a) traverse grinding, (b) plunge grinding, and (c) profile grinding. Internal Grinding In internal grinding a small wheel is used to grind the inside diameter of the part, such as to bushing and races.

Honing Schematic illustration of a honing tool used to improve the surface finish of the bored or ground holes. Used to improve the surface finish of the bored or holes.

Lapping (a) Schematic illustration of the lapping process. (b) Production lapping on flat surfaces. (c) Production lapping on cylindrical surfaces. Lapping is a finishing operation used on flat or cylindrical surfaces. The lap is usually made of cast iron, copper, leather or cloth. The abrasive particles are embedded in the lap, or they may be add as a slurry or powder.

Ultrasonic Machining (a) Schematic illustration of the ultrasonic machining process. (b) and (c) Types of parts made by this process. Note the small size of holes produced. The air at the tip of the tool vibrates at a frequency of 20kHz and lower amplitude. This vibration, in turn, imparts a high velocity to abrasive grains between the tool and the work piece. This vibrating particles are remove un wanted materials by producing impact with the work piece. Ultrasonic machining is best suited for materials that are hard and brittle.

Chip formation in grinding The work piece surface is machined by removing very tiny amount of material at random points where a particle contact it by using large number of particles, the effect is averaged over the entire surface and result is very good surface finish. In grinding, the formed chips are small but are formed by the same basic mechanism of metal machining

Typical structure of a grinding wheel contains abrasive grains, bond material and pores (air gaps) Volumetric proportions can be expressed as V g + V b + V p = 1 If V p relatively large and V g relatively small, it is called open structure. If V g relatively large and V p relatively small, it is called dense structure Grinding wheel structure Schematic illustration of the grinding wheel structure

Grade of a grinding The grade of a grinding wheel is a measure of how strongly the grains are held by the bond. Grain size number It is the measure of size of the abrasive particles in the grinding wheel. Grain sizes used in grinding changes between 8-250, while 8 is very coarse, but 250 is very fine. Grain size is a Important parameter in determining surface finish and material removal rate. Small grains produce better finishes, larger grains permit larger material removal rates. Also, harder materials need smaller grain sizes to cut effectively, while softer materials require larger grit size. Grade and grain size number of a grinding wheel

Aluminum Oxide (Al 2 O 3 ) known as Alundum or Aloxide. Various substances may be added to enhance hardness, toughness, etc. Plain Al 2 O 3 is white, and used to grind: steel, ferrous, high strength alloys. Silicon Carbide (SiC) known in trade as Carborundum and Crystalon. Harder than Al 2 O 3 but not as tough. Used to grind: aluminum, brass, stainless steel, cast irons, certain brittle ceramics. Boron Nitride in the forms of single-crystal cubic boron nitride (CBN) and microcrystalline cubic boron nitride (MCBN) under trade names such as Borazon or Borpax. Used for hard materials such as hardened tool steels and aerospace alloys. Diamond, a pure form of carbon, both natural and artificial. Used on hard materials such as ceramics, cemented carbides and glass. Common abrasive materials

Bonding materials Abrasive materials bonded by using organic or ingorganic materials. Inorganic bonds Vitrified bond: Clay bond melted to a porcelain or glass like consistency. It can be made strong and rigid for heavy grinding and not effected by water, oil, acids. Most grindig wheels have vitrified bonds. Slicate bond is essentially water glass hardened by baking. It holds grains more loosely than a vitrified bond and give closer cut. Large wheels can be made more easily with slicate bond. Usually used in situation where heat generation must be minimized. Metallic bond: Cubic boron nitride and diamond abrassives are usually (but not always) embeded in metallic bonds, for utmost in strength and tendency to hold the costly long- wearing grains. organic bonds Rubber bond is a flexible bond, used in cutoff wheels. Resinoid bond is a high strength bond, used for rough grinding and cutoff operations. Shellac bond is relatively strong but not rigid, used in applications that requires good finishing.

Attrition grain wear: Sharp edges of an abrasive grain become dull by attrition, developing flat areas. Grinding wheel wear Grain fracture grains:Portion of the grain breaks off. The fractured area becomes new cutting area. Bond fracture: Partin of the bond and results removing of grain from the structure allowing the new layer of grains to continue the grinding action

American National Standard Institude’s marking system for standard grinding wheels For Aluminium oxide and Silicon carbide abrasives

American National Standard Institude’s marking system for standard grinding wheels For Cubic boron nitride and diamond abrasives

Home work Read the Chapter 25 Abrasive machining and finishing operations Manufacturing Engineering and Technology By Serope Kalpakjian Steven R. Schmid