Chapter 9 Material-Removal Processes: Abrasive Machining and Finishing Operations.

Chapter 9 Material-Removal Processes: Abrasive Machining and Finishing Operations

GRINDING AND OTHER ABRASIVE PROCESSES
Related Abrasive Process

Introduction In all the machining processes described previously, the cutting tool is made of a certain material and has a clearly defined shape. Furthermore, the machining process is carried out by chip removal. However, there are many situations in manufacturing where the workpiece material is either: Too hard Too brittle Or its shape is difficult to produce with sufficient dimensional accuracy by any of the machining methods described previously. One of the best methods for producing such parts is using an abrasive.

Abrasives definition An abrasive is a small, hard particle that has sharp edges and irregular shape, unlike typical cutting tools. Abrasives are capable of removing small amounts of material from surface by cutting process that produces tiny chips

Abrasive Machining process
Material removal by action of hard, abrasive particles usually in the form of a bonded wheel that produces a tiny chips Generally used as finishing operations after part geometry has been established by conventional machining [are generally among the last operations performed on manufactured products]. They are not necessarily confined to fine or small-scale material removal from workpiece, they can indeed compete economically with some machining processes, such as milling and turning Grinding is most important abrasive process Other abrasive processes: honing, lapping, superfinishing, polishing, and buffing

Abrasive Machining process
Abrasive machining processes compete economically with some machining processes such as milling and turning: Because abrasives are hard so they are used in finishing processes for very hard or heat-treated metals and alloys Applications include: Finishing of ceramics and glasses Cutting off lengths of bars, structural shapes, masonry and concrete Removing unwanted weld beads and spatter Cleaning surfaces with jets of air or water containing abrasive particles.

Why Abrasive Machining Processes are Important
Can be used on all types of materials Some can produce extremely fine surface finishes, to m (1 -in) Some can hold dimensions to extremely close tolerances 1 micron (micrometre, µm) = millimeters (mm) = centimeters 0.025 micron (micrometre, µm) = millimeters (mm)

Unconventional Machining Processes
In addition to abrasive machining options, several advanced machining processes have been developed in the beginning of 1940s. Also called nontraditional or unconventional machining processes. These processes are based on : Electrical principles Chemical principles Fluid principles Thermal principles

Unconventional Machining Processes
The nontraditional or unconventional machining processes are attractive when one or more of the following occur: The hardness and strength of the workpiece material is very high, typically above 400 HB The part is too flexible or slender to support the machining, or parts are difficult to clamp in workpiece devices. The shape of the part is complex, such as internal and external profiles or small diameters deep holes. The surface finish and dimensional accuracy required are better than those obtainable by other processes. Temperature rise or residual stresses in the workpiece are undesirable or unacceptable

Abrasives Abrasives that are commonly used: Conventional abrasives
Aluminium oxides (Al2O3) Silicon carbide (SiC) Superabrasives Cubic boron nitride (cBN) Diamond Above abrasives are harder than conventional cutting-tool materials Cubic boron nitride and diamond are the two hardest materials known, hence the term superabrasives

Abrasives - Hardness Abrasives are significantly harder than conventional cutting-tool materials, as can be seen in the table below:

Abrasives – Friability characteristic
In addition to hardness, an important characteristic of an abrasive is Friability defined as the ability of abrasive grains to fracture (break down) into smaller pieces Friability gives abrasives self-sharpening characteristics, which are important in maintaining the sharpness of the abrasives during use. Abrasives exhibit High friability indicates low strength or low fracture resistance of the abrasives; thus, a highly friable abrasive grain fragments more rapidly under grinding forces than an abrasive grain with low friability. Aluminium oxide has lower friability than silicon carbide

Abrasives – Types Abrasive Types
Commonly found in nature are emery, corundum (alumina), quartz, garnet and diamond . However, natural abrasives contain unknown amounts of impurities and posses nonuniform properties; consequently, their performance is inconsistent and unreliable. As a result, abrasives are mad synthetically to control impurities. Abrasives that have been made synthetically: Aluminium oxide Silicon carbide Cubic boron nitride Diamond : produced synthetically, in which the case it is known as synthetic or industrial diamond Emery a grayish-black mixture of corundum and magnetite, used in powdered form as an abrasive. Quartz; a hard white or colorless mineral consisting of silicon dioxide, found widely in igneous, metamorphic, and sedimentary rocks. It is often colored by impurities (as in amethyst, citrine, and cairngorm). garnet a precious stone consisting of a deep red vitreous silicate mineral.

Abrasives – Grain size Abrasive Grain Size
Abrasives are very small when compared to the size of cutting tools and inserts Also abrasives have sharp edges, thus allowing the removal of very small quantities of material from workpiece surface. Thus, very fine surface finish and dimensional accuracy can be obtained using abrasives as tools The size of an abrasive grain is identified by a grit number, which is a function of sieve size; the smaller the sieve size, the larger is the grit number [Smaller the grain size, larger the grit number]. For example, grit number 10 is rated as coarse, 100 as fine, and 500 as very fine.

Abrasives –workpiece material Compatibility
Abrasive–workpiece-material Compatibility Affinity of an abrasive grain to the workpiece material is important The less the reactivity of the two materials, the less wear and dulling of the grains occur during grinding Recommendations when selecting abrasives: Aluminum oxide: Carbon steels, ferrous alloys, and alloy steels. Silicon carbide: Nonferrous metals, cast irons Cubic boron nitride: Steels and cast irons Diamond: Ceramics, cemented carbides, hardened steels

Abrasives - Bonded Abrasives: Grinding Wheels
Because each abrasive grain removes only a very small amount of material at a time, high rates of material removal can be achieved when large number of these grains act together. This is done by using bonded abrasives, typically in the form of grinding wheel. Physical model of a grinding wheel, showing its structure and grain wear and fracture patterns

Grinding Wheel Model Figure : Schematic illustration of a physical model of a grinding wheel showing its structure and wear and fracture patterns.

The abrasive grains are held together by a bonding material (will be described later), which acts as supporting posts or braces between the grains. In bonded abrasives, some porosity is essential to : Provide clearance for the minute ships being produced As well as to provide cooling. Otherwise, the chips would interfere with the grinding operation [it would be impossible to use a grinding wheel that is fully dense (solid) with no porosity.] Porosity can be observed by the surface of grinding wheel For non-porosity, it is fully dense and solid

Commonly used types of grinding wheels - For conventional abrasives Aluminium oxides (Al2O3) Silicon carbide (SiC)

Commonly used types of grinding wheels - For superabrasives Cubic boron nitride (cBN) Diamond Due to high cost of superabrasives, only a small portion of the periphery of the wheels consists of superabrasives

Standard marking system for Aluminium oxides & Silicon carbide bonded abrasive Bonded abrasives are indicated by the type of abrasive, grain size, grade, structure, and bond type Grade is a measure of its bond strength Because strength and hardness are directly related, The grade is also referred to as the hardness of a bonded abrasive The smaller number the lager the grain size The Structure is a measure of the porosity [ the spacing between the grains].

Standard marking system for diamond and cBN bonded abrasive Bonded abrasives are indicated by the type of abrasive, grain size, grade, structure, and bond type Grade is a measure of its bond strength 1 micron (micrometre, µm) = millimeters (mm) = centimeters Cost of grinding wheels depends on the type and size of the wheel

Abrasives - Bond Types:
Common types of bonds: Vitrified bond Resinoid bond Rubber bond Metal bond They are used for conventional abrasives [Aluminium oxides & Silicon carbide]as well as for superabrasives [Cubic boron nitride & Diamond]( except rubber bond)

Vitrified (ceramic bond): It is the most common and widely used bond The raw materials in the bond consist of feldspar [aluminosilicates] (a crystalline mineral) and various clays. These materials are first mixed with the abrasive, moistened, and then molded under pressure into the shape of grinding wheels. These ‘green’ products are then slowly fired, up to a temperature of about k, to fuse the glass to develop structural strength. The wheels are then cooled slowly to prevent thermal cracking Vitrified bonds produce wheels that are strong, stiff, porous, and resistant to oils, acids, and water; however, because the wheels are brittle, they lack resistance to mechanical and thermal shock.

Resinoid: Resinoid Bonding materials are thermosetting resins The basic manufacturing procedure consists of mixing the abrasive with liquid or powdered phenolic resins and additives. Pressing the mixture into the shape of a grinding wheel Then curing it at a temperature of about k Because the elastic modulus of thermosetting resins is lower than the glasses, resinoid wheels are more flexible than vitrified wheels When a long-chain molecules in a polymer are cross-linked in a three dimensional arrangement, the structure in effect becomes one giant molecule with strong covalent bonds. Such polymers are called thermosetting polymers or thermosets, because during polymerization, the network is completed, and the shape of the part is permanently set

Rubber bond: The most flexible bond used in abrasive wheels is rubber bond Such wheels are manufactured by mixing crude rubber, sulfur, and abrasive grains together. Rolling mixture into sheets Cutting out the sheet into circles Heating the circles under pressure to vulcanize the rubber Thin wheels can be made in this manner and are used like saws for cutting-off operations (cut-off blades) Vulcanization (or vulcanisation) is a chemical process for converting natural rubber or related polymers into more durable materials via the addition of sulfur[1] or other equivalent curatives or accelerators

Metal bond: Abrasives grains, usually diamond or cubic boron nitride (cBN), are bonded in a metal matrix to the periphery of a metal wheel, typically to depths of 6 mm or less

Metal bond: The bonding is carried out under high pressure and temperature The wheel itself (core) may be made of aluminium, bronze, steel, ceramics, or composite material, depending on special requirements for the wheel such as : Strength stiffiness

Abrasives - Wheel grade & Structure:
The grade of a bonded abrasive is a measure of its bond strength, and it includes both the type and the amount of bond in the wheel. Because strength and hardness are directly related, The grade is also referred to as the hardness of a bonded abrasive. A hard wheel has a stronger bond and/or a larger amount of bonding material between the grains than a soft wheel The Structure is a measure of the porosity [ the spacing between the grains]. The structure of bonded abrasive range from dense to open.

Abrasives - Wheel Structure:
Refers to the relative spacing of abrasive grains in wheel In addition to abrasive grains and bond material, grinding wheels contain air gaps or pores Volumetric proportions of grains, bond material, and pores can be expressed as:

Figure : Typical structure of a grinding wheel.

Measured on a scale that ranges between "open" and "dense." Open structure means that the volumetric proportion of porosity Pp is relatively large and Pg is relatively small - recommended when clearance for chips must be provided Dense structure means that the volumetric proportions of porosity Pp is relatively small and Pg is larger - recommended to obtain better surface finish and dimensional control

Abrasives - Wheel grade :
Wheel grade Indicates bond strength in retaining abrasive grits during cutting Depends on amount of bonding material in wheel structure (Pb) Measured on a scale ranging between soft and hard Soft" wheels lose grains readily - used for low material removal rates and hard work materials Hard wheels retain grains - used for high stock removal rates and soft work materials

Grinding Wheel Specification
Standard grinding wheel marking system used to designate abrasive type, grit size, grade, structure, and bond material Example: A-46-H-6-V Also provides for additional identifications for use by grinding wheel manufacturers

Bonded abrasives are indicated by the type of abrasive, grain size, grade, structure, and bond type

Standard marking system for diamond and cBN bonded abrasive Bonded abrasives are indicated by the type of abrasive, grain size, grade, structure, and bond type Cost of grinding wheels depends on the type and size of the wheel

Surface Finish Most grinding is performed to achieve good surface finish Best surface finish is achieved by: Small grain sizes Higher wheel speeds Denser wheel structure = more grits per wheel area Grits = مطحون

The Grinding Process Grinding is a chip-removal process in which the cutting tool is an individual abrasive grain To increase shear plane angle .Increase the rake angle. Higher shear plane angle means smaller shear plane . Smaller  with a corresponding larger shear plane area. Schematic illustration of chip formation by an abrasive grain. Note the large negative rake angle of the grain, the small shear angle Grinding chip being produced by a single abrasive grain. Note the large negative rake angle of the grain

Negative rake angle VERSUS Positive rake angle
To increase shear plane angle .Increase the rake angle which means smaller shear plane area . smaller  with a corresponding larger shear plane area.

Chip Formation by Abrasive Grain
To increase shear plane angle .Increase the rake angle which means smaller shear plane area which means lower shear force, cutting forces, power, and temperature. smaller  with a corresponding larger shear plane area. Figure : (a) Grinding chip being produced by a single abrasive grain: (A) chip, (B) workpiece, (C) abrasive grain. Note the large negative rake angle of the grain. The inscribed circle is mm ( in.) in diameter. (b) Schematic illustration of chip formation by an abrasive grain with a wear flat. Note the negative rake angle of the grain and the small shear angle

Grinding Wheel Surface
Figure : The surface of a grinding wheel (A46-J8V) showing abrasive grains, wheel porosity, wear flats on grains, and metal chips from the workpiece adhering to the grains. Note the random distribution and shape of the abrasive grains. Magnification: 50x. Source: S. Kalpakjian. A: Aluminum oxide, 46 : abrasive grain size (medium) , J: grade (strength of bonding [close to medium]), 8: structure (medium dense), V: Bond type (vitrified)

The Grinding Process The following are factors that differentiate the action of a single grain from of a single-point cutting tool: The individual grain has an irregular geometry and is spaced randomly along the periphery of the wheel. The average rake angle of the grains is highly negative, typically [-1.05 rad (-60o)]; consequently, the shear angles are very low. The grains in the periphery of a grinding wheel have different radial position The cutting speeds of grinding wheels are very high, typically, on the order of 30m/s To increase shear plane angle .Increase the rake angle . smaller  with a corresponding larger shear plane area.

Mechanics of Grinding Process
The mechanics of grinding and the variables involved can best be studied by analyzing the surface-grinding operation. A grinding wheel of diameter D is removing a layer of metal at a depth d, knowing as the wheel depth of cut. An individual grain on the periphery of the wheel is moving at tangential velocity V. The workpiece is moving at a velocity of v. The grain is removing a chip with an undeformed thickness (grain depth of cut), t. And an undeformed Chip length l. For the condition v <<V, the undeformed-chip length, l is An deformed chip thickness t is : C is the number of cutting points per unit area of wheel surface ( per mm2)

The Grinding Process Grinding Forces
A knowledge of grinding forces is essential for: Estimating power requirements Determining the deflections that the workpiece and grinding machine will undergo. Deflections, in turn, adversely affect dimensional accuracy of the workpiece, which is especially critical in precision grinding. Grain force is proportional to the process variables: C D = diameter of grinding wheel d= the wheel depth of cut V= tangential velocity of An individual grain on the periphery of the wheel v =The workpiece velocity C = the number of cutting points per unit areaof wheel surface

Three Types of Grain Action
The Grinding Process Three Types of Grain Action Cutting - grit projects far enough into surface to form a chip - material is removed Plowing - grit projects into work, but not far enough to cut ,instead surface is deformed and energy is consumed, but no material is removed Rubbing - grit contacts surface but only rubbing friction occurs, thus consuming energy, but no material is removed

The Grinding Process Specific Energy
Specific Energy dissipated (consumed) in producing a grinding chip consists of components : Chip formation (cutting) Plowing Friction (Rubbing) Figure : Three types of grain action in grinding: (a) cutting, (b) plowing, and (c) rubbing.

Why Specific Energy in Grinding is High
The Grinding Process Why Specific Energy in Grinding is High Size effect - small chip size causes energy to be significantly higher to remove each unit volume of material Roughly 10 times higher for grinding compared to conventional machining Individual grains have extremely negative rake angles, resulting in low shear plane angles and high shear strains Not all grits are engaged in actual cutting To increase shear plane angle .Increase the rake angle which means smaller shear plane area which means lower shear force, cutting forces, power, and temperature. smaller  with a corresponding larger shear plane area which means higher shear force, higher cutting forces, higher power.

The Grinding Process Temperature
Temperature rise in grinding is important as it can: Adversely affect the surface properties on the workpiece. Cause residual stresses on the workpiece Cause distortions due to thermal expansion and contraction of the workpiece surface When some of the heat generated during grinding and is conducted to the workpiece, the heat expands the part being ground, thus making it difficult to control dimensional accuracy.

The Grinding Process Temperature
Surface-temperature rise in grinding is The peak temperatures in chip generation during grinding can be as high as K. Experiments indicate that as much as one-half the energy dissipated in grinding (converted into the heat during grinding) is conducted to the chip, a percentage that is higher than that in machining. On the other hand, the heat generated by sliding (Rubbing) and plowing conducted mostly into workpiece (high temperature on the workpiece surface)

The Grinding Process The major effects of temperature in grinding are : Residual stresses develops in workpiece Temperature change and gradient within the workpiece are mainly responsible for residual stresses in grinding (thermal cycling induces residual stresses) Other contributing factors are the physical interactions of the abrasive grain in chip formation and the sliding of the wear flat grain along the workpiece surface, causing plastic deformation of the surface, thus creating residual stresses.

The Grinding Process Burning of workpiece surface
Excessive temperature during grinding may burn the workpiece surface A burn is characterized by a bluish color on ground steel surfaces A burn may not be objectionable in itself; however, the surface layers may undergo metallurgical transformations Heat Checking High temperatures in grinding lead to thermal stresses and may cause thermal cracking of the workpiece surface, known as heat checking

How to Reduce Grinding Temperatures
The Grinding Process How to Reduce Grinding Temperatures Temperature decreases with decreasing d, D, and V Decrease (depth of cut) d Reduce wheel speed V (tangential velocity of the grain) Increase workpiece speed vw Use a grinding fluid

The Grinding Process: Grinding wheel Wear
Grinding wheel wear is an important consideration because it adversely affects the shape and accuracy of ground surfaces. Three different mechanisms of grinding wheels wear: Attritious wear Grain fracture Bond fracture

Attritious Grain Wear The cutting edges of a sharp grain become dull by attrition (known as attritious wear), developing a wear flat. Wear is caused by the interaction of the grain with the workpiece material, resulting in complex physical and chemical reactions. These reactions involve diffusion, chemical decomposition of the grain, fracture at a microscopic scale, plastic deformation and melting. Attritious wear is low when the two materials are chemically inert with respect to each other. [The more inert the materials, the lower will be the tendency for reaction to occur between the grain and the workpiece being ground]. Selection of abrasive is based on the reactivity between the grain and workpiece.

Attritious Grain Wear

Grain fracture Because abrasive grains are brittle, their fracture characteristics in grinding are important (friability). The grain should fracture at a moderate rate So that new sharp cutting edges are produced continuously during grinding

Bond fracture The strength of the bond (grade) is a significant parameter in grinding. If the bond is too strong, dull grains cannot be dislodged so that other, sharp grains along the circumference of the grinding wheel can being to contact the workpiece and remove the chips. Thus, the grinding process becomes insufficient. On other hand, if the bond is too weak, the grains are easily dislodged, and wear rate of the wheel increases. Consequently (if too strong and too weak), maintaining dimensional accuracy of the workpiece becomes difficult. In general, softer bond are recommended for harder material and for reducing residual stresses and thermal damages to workpicec. Hard-grade wheels are used for softer materials and for removing large amount of material at high rates

Typical Wear Curve in Grinding
The Grinding Process: Grinding wheel Wear Typical Wear Curve in Grinding Figure: Typical wear curve of a grinding wheel. Wear is conveniently plotted as a function of volume of material removed, rather than as a function of time.

Grinding Ratio Grinding wheel wear is generally correlated with the amount of material grounded by a parameter called grinding ratio, G, which is defined as where G = grinding ratio; Vw = volume of work material removed; and Vg = corresponding volume of grinding wheel worn

The Grinding Process: Dressing, Truing, and Shaping of Grinding Wheels
Dressing is the process of conditioning worn grains of the surface of a grinding wheel in order to produce sharp new grains Dressing is necessary when excessive attritious wear dulls the wheel or when the wheels becomes loaded Loading occurs when the porosities on the wheel surfaces become filled or clogged with chips from the workpiece Loading can occur While grinding soft workpiece By improper selection of the grinding wheel, such as wheel with low porosity. Dressing techniques and their frequency affect grinding forces and workpiece surface finish

Dressing the Wheel Dressing - accomplished by rotating disk, or abrasive sticks (special dressed wheel) held against the wheel being dressed as the wheel rotates Functions: Break off dulled grits to expose new sharp grains Remove chips clogged in wheel Required when wheel is in third region of wear curve

Truing the Wheel Truing - use of a diamond‑pointed tool fed slowly and precisely across wheel as it rotates Very light depth is taken (0.025 mm or less) against the wheel Not only sharpens wheel, but restores cylindrical shape and insures straightness across outside perimeter

The Grinding Process: Grinding Chatter
Chatter is particularly significant in grinding because it adversely affects surface finish and wheel performance Vibrations during grinding may be caused by: Bearings Spindles Unbalanced grinding wheel As well as external sources, such as from nearby machinery. The grinding process itself can also regenerative chatter

The analysis of chatter in grinding involves: Self-excited vibration Regenerative chatter Thus, the important variables for self-excited vibration: Stiffness of the grinder and of the workholding device Damping of the system Additional factors to grinding chatter (regenerative chatter) Nonuniformities in the grinding wheel itself The dressing techniques used Uneven wheel wear. These variables produce chatter mark

Ways to reduce the tendency for chatter in grinding: Using soft-grade wheels Dressing the wheel frequently Changing dressing techniques Reducing the material-removal rate Supporting the workpiece rigidly

The Grinding Process: Grinding fluids
Importance of using a fluid: Prevent excessive temperature rise in the workpiece Improves part surface finish and dimensional accuracy Improves the efficiency of the operation by reducing wheel wear and loading and lowering power consumption. Washing away chips Reducing friction removing heat Grinding fluids are water-based emulsions for grinding and oils for thread grinding

General Recommendations for Grinding Fluids
The Grinding Process: Grinding fluids General Recommendations for Grinding Fluids

The Grinding Process: safety
Safety in Grinding Operations Grinding wheels are brittle and when rotate at high speeds, they can fracture and cause serious injury There must be care in handling, storage and usage of grinding wheels.

Application Guidelines
To optimize surface finish, select Small grit size and dense wheel structure Use higher wheel speeds (v) and lower work speeds (vw) Smaller depths of cut (d) and larger wheel diameters (D) will also help To maximize material removal rate, select Large grit size More open wheel structure Vitrified bond Aluminium oxides (Al2O3) Silicon carbide (SiC)

For steel and most cast irons, use Aluminum oxide as the abrasive For most nonferrous metals, use Silicon carbide as the abrasive For hardened tool steels and certain aerospace alloys, use Cubic boron nitride as the abrasive For hard abrasive materials (e.g., ceramics, cemented carbides, and glass) use Diamond as the abrasive

For soft metals, use Large grit size and harder grade wheel For hard metals, use Small grit size and softer grade wheel

The Grinding Process: Grinding Operations and Machines
Grinding operations are carried out with a wide variety of wheel- workpiece configurations. The selection of a grinding process for a particular application depends on Part shape, Part size, Easy of fixturing, Production rate required The basic types of grinding operations are: Surface grinding Cylindrical grinding Internal grinding Centerless grinding The relative movement of the wheel in these operations may be : Along the surface of the workpiece (traverse grinding) Or it may be radially into workpiece (plunge grinding)

Surface Grinding Surface grinding is one of the most common grinding operations and involves grinding of flat surfaces. The four types o f surface grinding Figure : (a) horizontal spindle with reciprocating worktable, (b) horizontal spindle with rotating worktable, (c) vertical spindle with reciprocating worktable, (d) vertical spindle with rotating worktable.

Various Surface-Grinding Operations
Figure : Schematic illustrations of various surface-grinding operations. (a) Traverse grinding with a horizontal-spindle surface grinder. (b) Plunge grinding with a horizontal-spindle surface grinder. (c) A vertical-spindle rotary-table grinder (also known as the Blanchard type.)

Various Surface-Grinding Operations
Figure : Surface grinder with horizontal spindle and reciprocating worktable (most common grinder type).

Cylindrical Grinding The external and internal cylindrical surfaces and shoulders of workpieces are ground, such as crankshaft bearing, spindles, pins and bearing rings The rotating cylindrical workpiece reciprocates laterally along its axis to cover the width to be ground Capable of grinding rolls with large diameter

Cylindrical Grinding [ external & internal] Figure: Two types of cylindrical grinding: (a) external, and (b) internal.

Cylindrical Grinding [shoulders]

Cylindrical Grinding Thread grinding is done on cylindrical grinders, as well as on centerless using specially dressed wheels matching the shape of the threads

Internal Grinding A small wheel is used to grind the inside diameter of the part such as bearing races Internal profiles is ground with profile-dressed wheels that move radially into the workpiece Figure : Schematic illustrations of internal grinding operations: (a) traverse grinding, (b) plunge grinding, and (c) profile grinding.

Centerless Grinding A high-production process for continuously grinding cylindrical surfaces in which the workpiece is supported not by centers. The Workpiece is supported by a blade Typical parts made by this operation include piston pins, engine valves, camshaft and similar components. Classified into: Through-feed grinding Plunge grinding Internal grinding

Centerless Grinding Through-feed grinding The workpiece is supported on work-rest blade and is grounded between two wheels. Grinding is done by the large wheel, while the smaller wheel (Rubber bonded) regulates the axial movement of the workpiece

Centerless Grinding Plunge grinding Parts with variable diameters such as bolts and distributer shaft can be grounded by plunge grinding.

Centerless Grinding internal grinding The workpiece is supported between three rolls and is internally grounded. Thread grinding can be done with centerless grinders, using special dressed wheels.

Creep Feed Grinding Creep-feed Grinding Grinding has traditionally been associated with small rate of material removal and fine finishing operation Grinding can also be used for large-scale metal-removal operations to compete with milling, broaching and planing In creep-feed grinding, the wheel depth of cut, d, is as much as 6 mm, and the workpiece speed is low

Creep Feed Grinding Depths of cut 1000 to 10,000 times greater than in conventional surface grinding Feed rates reduced by about the same proportion Material removal rate and productivity are increased in creep feed grinding because the wheel is continuously cutting In conventional surface grinding, wheel is engaged in cutting for only a portion of the stroke length

Creep Feed Grinding

Other Abrasive Processes
Honing Lapping Superfinishing

Honing Abrasive process performed by a set of bonded abrasive sticks using a combination of rotational and oscillatory motions Common application is to finish the bores of internal combustion engines Grit number (grain size) range between 30 (medium) and 600 (very fine) (the smaller grain size, the larger grit number) Surface finishes of 0.12 m (5 -in) or better Creates a characteristic cross‑hatched surface that retains lubrication 1 Micrometer (Micron) = mm, 0.12 Micrometer = 12*10^-5 mm 1 microinches = 2.45*10^-5 mm 1 Microm = × 10-5 inches

Honing Figure The honing process: (a) the honing tool used for internal bore surface, and (b) cross‑hatched surface pattern created by the action of the honing tool.

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

Lapping Uses fluid suspension of very small abrasive particles between workpiece and lap (tool) Lapping compound (fluid suspension) - fluid with abrasives, general appearance of a chalky paste Typical grit sizes (Grit number) between 300 (medium) to 600 (very fine) Applications: optical lenses, metallic bearing surfaces, gages The same as polishing samples for microscopic or electronic microscopic tests

Figure : The lapping process in lens‑making.

Lapping Figure : (a) Schematic illustration of the lapping process. (b) Production lapping on flat surfaces. (c) Production lapping on cylindrical surfaces.

Superfinishing Similar to honing - uses bonded abrasive stick pressed against surface and reciprocating motion Differences with honing: Shorter strokes Higher frequencies Lower pressures between tool and surface Smaller grit sizes

Superfinishing Figure : Superfinishing on an external cylindrical surface.

Superfinishing Figure : Schematic illustration of the superfinishing process for a cylindrical part. (a) Cylindrical microhoning. (b) Centerless microhoning.

Chapter 9 Material-Removal Processes: Abrasive Machining and Finishing Operations.

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Chapter 9 Material-Removal Processes: Abrasive Machining and Finishing Operations.

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