10 Abrasive Machining Processes

ME202 Manu f act u r i ng Tech no l o g i es Abrasive Machining Processes

A b r a s iv e M a c h i n i n g

Abrasive machining is the basic process in which chips are formed by very small cutting edges that are integral parts of abrasive particles . An abrasive is a hard material that can c ut or abrade other materials. Abrasive Machining Processes

M E 2 0 2 M a n u f a c t u r i n g T ec ec h n o l o g i e s

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Abrasives

Natural abrasives Sandstone was used by ancient people to sharpen tools and weapons.

Emery , a mixture of alumina (Al 2 O3 ) and magnetite (Fe3O4 ), is another natural abrasive still in use today.

Corundum (natural Al 2 O3 ) and diamonds are other naturally occurring abrasive materials. Today, the only natural abrasives that have commercial importance are quartz, sand, garnets, and diamonds.

Abrasive Machining Processes

M E 2 0 2 M a n u f a c t u r i n g T ec h n o l o g i e s

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Abrasives

Arti fi ci al abrasives Artificial abrasives date from 1891, when Edward G. Ac heson , while attempting to produce precious gems, discovered how to make silicon carbide ( SiC ). Silicon carbide is made by charging an electric furnace with silica sand, petroleum coke, salt, and sawdust. By passing large amounts of current through the charge, a temperature of over 4000 °F (2200 °C) is maintained for several hours, and a solid mass of silicon carbide crystals result. After the furnace has cooled, the mass of crystals is removed, crushed, and graded. Abrasive Machining Processes


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Abrasives

Arti fi ci al abrasives Other artificial abrasives used today are: Al uminum oxide (Al 2 O3 ) is the most widely used artificial abrasive. Diamonds are the hardest of all materials. Those that are used for abrasives are either natural, off-color stones that are not suitable for gems; or small, synthetic stones that are produced specifically for abrasive purposes. Cubic Boron Nitride (CBN) is extremely hard. It is the second-hardest material known, and is often referred to, along with diamonds, as a superabrasive. It is not found in the nature, it is produced. Abrasive Machining Processes


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Abrasives



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A b r a s iv e M a c h i n i n g P r o c e s s es

• Grinding

• Grinding Machines • Work Holding Devices • Tool and Cutter Grinding • Snagging • Mounted Wheels and Points • Coated Abrasives • Honing Superfinishing • Lapping Abrasive Machining Processes


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Grinding

Grinding is the most common abrasive machining process. Generally not suitable for bulk material removal , but used for dimensional accuracy and surface improvement by removing a small amount of material from the workpiece surface. The cutting tool, which rotates during the process is called the grindi ng wheel , wherein the abrasives are bonded together into a wheel of some shape. There are a lot of bonding materials used for this purpose. Abrasive Machining Processes


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B o n d i n g M a t e r i a l s f o r Gr i n d i n g W h e e l s

1. Vitrified bonds are composed of clays and other ceramic substances. 2. Resinoid, or phenolic resins can be used. 3. Silicate wheels use silicate of soda (waterglass) as the bond material. 4. Shellac-bonded wheels are made by mixing the abrasive grains with shellac in a heated mixture. 5. Rubber bonding is used to produce wheels that can operate at high speeds that must have a considerable degree of flexibility. 6. Superabrasive wheels are either electroplated, or a thin segmented drum of vitrified CBN surrounds a steel core. Abrasive Machining Processes


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G r i n d i n g W h e e l Sh a p e s



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Gr i n d i n g W h e e l St r u c t u r e FIGURE 28-10 Meaning of terms structure and grade for grinding wheels. (a) The structure of a grinding wheel depends on the spacing of the grits. (b) The grade of a grinding wheel depends on the amount of bonding agent (posts) holding abrasive grains in the wheel.



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Grinding During the process, material is also removed from the grinding wheel. The G-ratio is defined as the ratio of workpiece material removed to grinding wheel material removed. Ratios of 20:1 to 80:1 are common. Abrasives are sized to control material removal rate and resulting surface finish of the workpiece.



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Grinding

A grinding wheel is made by bonding many grains with nonspecific geometry together into a wheel of some shape. Many grains have negative rake angles ( α ). The specific cutting energy ( W/cm 3/min ) is much larger in comparison to turning or milling. The volume cut is small per unit time; thus time consuming. Hence an expensive process. Abrasive Machining Processes


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w h y Gr in d i n g ? Used when; ●

A high dim ensional accuracy is required,

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A smooth surface, ( h igh surface quality ) is required,

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Surfaces with high hardness need to be cut, where it will be difficult to remove the material by turning or milling, Very thin layers of material need to be removed from the surface.

Also used when small forces should be applied to the workpieces in order to have small deformations. (Cutting forces in grinding are relatively small.) Abrasive Machining Processes


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Grinding

Rake angles of abrasive particles can be positive or negative.

FIGURE 28-9 SEM micrograph of stainless steel chips from a grinding process. The tops ( T ) of the chips have the typical shear front-lamella structure while the bottoms ( B ) are smooth where they slide over the grit.



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Gr i n d i n g W h e e l T r u i n g Grinding wheels lose their geometry with use. Truing restores the original shape by removing a small amount of material from the wheel surface, in order to expose new grinding media and new cutting edges on worn glazed grains.



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Gr i n d i n g W h e e l D r e ss i n g As grinding wheels are used they tend to become loaded with lodged metal chips in the cavities. Dressing is used to remove the lodged metal chips.



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Gr i n d i n g W h e e l Tr u i n g a n d D r e s si n g

Tools used for grinding wheel truing

Grinding wheel dresser Abrasive Machining Processes


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Grinding Grinding is done in three ways:

Infeed , moving the wheel across the workpiece surface by moving the wheel into the work, or the work into the wheel. Then the desired surface is produced by traversing the wheel across the workpiece, or vice versa.

Plunge-cut , feeding of the wheel radially into the work while the work is rotating on centers. Usually a formed grinding wheel is used. It is similar to form cutting on a lathe.

Creep feed , the workpiece is feed past the wheel. Abrasive Machining Processes


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Grinding



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Gr i n d i n g P r o c e ss e s

• Center type Cylindrical Grinding • Chucking type Grinding Tool-post Grinders (an attachment)

• Centerless Grinding • Surface Grinding



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Gr i n d i n g M a c h i n e s



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I . Ce n t e r T y p e Cy l i n d r i c a l G r i n d i n g Commonly used for producing external cylindrical -and conical (tapered)- surfaces. During the process, the grinding wheel revolves at an ordinary cutting speed, and the workpiece rotates on centers at a much slower speed, usually at 23-38 m/min. The grinding wheel and the workpiece move in opposite directions at their point of contact. The depth of cut is determined by infeed of the wheel or the workpiece. This motion also determines the finished diameter of the workpiece, therefore accurate control of this movement is required. Abrasive Machining Processes


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Ce n t e r T y p e Cy l i n d r i c a l G r i n d i n g Plain center type cylindrical grinding machine



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1 - Pla in Ce n t e r T y p e Cy l i n d r i c a l G r i n d i n g M a c h i n e s The work is mounted between headstock and tailstock centers. The headstock and tailstock are mounted on a table, which can be swiveled approximately 10° about a vertical axis with respect to the table carrier on which it is mounted. This permits grinding of tapered cylinders up to about 10°. For the traverse motion, the table assembly can be reciprocated along the ways on the main frame either manually or by power. Infeed is provided by movement of the wheelhead at r ight angles to the longitudinal axis of the table. They contain systems for storing, filtering and circulating adequate amounts of grinding fluid. Their size is designated by the maximum diameter and length of work that can be ground between centers. Abrasive Machining Processes


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2 - U n i v e r s a l Ce n t e r T y p e Cy l i n d r i c a l G r i n d i n g M a c h i n e s Basically the same as plain centertype grinders except for two features. ●

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Both the headstock and the wheelhead can be swiveled about vertical axes to grind tapers of all angles, and to do certain other types of work that cannot be done on plain center-type grinders. Most machines have dual spindles on the swiveling wheelhead, one for external grinding and the other for internal grinding. Either spindle can be brought into use by swiveling or tilting the wheelhead.


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Ce n t e r T y p e Cy l i n d r i c a l G r i n d i n g P lu n g e Cu t

FIGURE 28-14 Continuous

crush roll dressing and truing of a grinding wheel (form-truing and dressing throughout the process rather than between cycles) doing plunge cut grinding on a cylinder held between centers. Abrasive Machining Processes


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3 - Ro ll Gr i n d e r

Roll grinders are basically plain center-type machines designed for grinding large, cylindrical mill and calendar rolls. Because of the weight of such workpieces, the wheelhead, instead of the work, reciprocates. Abrasive Machining Processes


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I I . Ch u c k i n g T y p e G r i n d i n g

The workpiece is held in a chuck for grinding both external and internal cylindrical surfaces. Abrasive Machining Processes


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1 - Ch u c k i n g T y p e Ex t e r n a l G r i n d i n g

Chucking type grinding machines are production-type machines, for use in rapid grinding of relatively short parts, such as ball bearing races. Chucks or collets can be used to hold the workpieces. Frequently they have two spindles so that the work can be removed from one while another piece is being ground in the other. Abrasive Machining Processes


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2 - Ch u c k i n g T y p e I n t e r n a l G r i n d i n g a - Re gu la r I n the regular chucking-type

internal grinders, the chuckheld workpiece revolves, a relatively small, high-speed grinding wheel is rotated on a spindle arranged so that it can be reciprocated in and out of the workpiece. Infeed movement of the wheelhead is normal to the axis of rotation of the work. There are plain and universal regular chucking-type internal grinders. On plain internal grinders of this type, the workhead can be swiveled so that both straight internal cylinders and beveled holes can be ground. On universal internal grinders, the workhead not only can be swiveled, but it also is mounted on a cross slide. Abrasive Machining Processes


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2 - Ch u c k i n g T y p e I n t e r n a l G r i n d i n g b - Pla n et a ry

Planetary-type internal grinders are used for work that is too large to be rotated conveniently. The revolving grinding wheel also has planetary rotation about an axis that is coincident with the axis of the finished cylinder. The diameter of the ground surface is controlled by adjusting the radius of the planetary rotation. On this type of machines, the work is reciprocated past the wheel. Abrasive Machining Processes


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T o o l Po s t Gr i n d e r

Tool-post g rinders are attachments used to permit occasional grinding to be done on a lathe. The entire mechanism is mounted either on the tool post or on the compound rest. The grinding wheel is attached to the tool-post grinder. The lathe spindle provides rotation for the workpiece, and the lathe carriage is used to reciprocate the wheelhead. Abrasive Machining Processes


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I I I . Ce n t e r l e s s G r i n d i n g Centerless grinding makes it possible to grind both external and internal cylindrical surfaces without the necessity of the workpiece being mounted between centers or in a chuck, which eliminates center holes in workpieces and the necessity for mounting the workpiece, and reduces the cycle time. The workpiece rests between two wheels, one providing the grinding and the other providing regulation of the grinding speed and traverse.



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1 - Ex t e r n a l Ce n t e r l e s s G r i n d i n g Two abrasive wheels are used. The larger one operates at regular grinding speeds and does the actual grinding. The smaller wheel is the regulating wheel and is mounted at an angle to the plane of grinding wheel to control the rotation and longitudinal motion of the workpiece. It usually is a plastic or rubber bonded wheel with a fairly wide face. The workpiece is held against the work-rest blade by the cutting force exerted by the grinding wheel and rotates at approximately the same surface speed as that of the regulating wheel. Abrasive Machining Processes


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Ex t e r n a l C e n t e r l e s s G r i n d i n g



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Ex t e r n a l C e n t e r l e s s G r i n d i n g



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Ex t e r n a l Ce n t e r l e s s G r i n d i n g Operations

In thrufeed grinding, the workpiece is of constant diameter and is fed completely through between the rolls, starting at one end. This is the simplest type and can easily be made automatic. In infeed centerless grinding the work rest and the regulating wheel are retracted, so that the work can be put in position and removed when grinding is completed. This arrangement permits multiple diameters and curved parts to be ground. Abrasive Machining Processes


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Ex t e r n a l Ce n t e r l e s s G r i n d i n g Operations

In endfeed centerless grinding, both the grinding and regulating wheels are tapered and thus produce tapered workpieces. The stock is fed from one side until it reaches the stop. For ball grinding, the regulating wheel is grooved and inclined to impart random rotation. Abrasive Machining Processes


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2 - I n t e r n a l Ce n t e r l e s s G r i n d i n g In centerless internal grinding, three rolls support the workpiece on its outer surface and impart rotation to it. The grinding wheel traverses into the workpiece. External surface of the cylinder should be finished accurately before the operation. Centerless internal grinding assures the concentricity of external and internal cylindrical surfaces of tube-like workpieces. Abrasive Machining Processes


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A d v a n t a g e s o f Ce n t e r l e s s G r i n d i n g

1. It is very rapid; infeed centerless grinding is almost continuous.

2. Very little skill is required of the operator. 3. It can often be made automatic (single-cycle automatic). 4. At the location of cutting, the work is fully supported by the work rest and the regulating wheel. This permits heavy cuts to be made.

5. Because there is no distortion of the workpiece, accurate size control is easily achieved.

6. Large grinding wheels can be used, thereby wheel wear can be minimized. Abrasive Machining Processes


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D i s a d v a n t a g e s o f Ce n t e r l e s s G r i n d i n g

1. Special machines are required that can do no other type of work.

2. The workpieces must be round - no flats, such as keyways, can be present.

3. Its use on workpieces having more than one diameter or on curved parts is limited.

4. In external centerless grinding of tubes, there is no guarantee that the outside and inside surfaces of the tube are concentric; In internal centerless grinding, outside and inside surfaces are concentric. Abrasive Machining Processes


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I V . Su r f a c e Gr i n d i n g Surface grinding is primarily used to grind flat surfaces. However formed, irregular surfaces can be produced on some types of surface grinders by using a formed wheel.



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Su r f a c e Gr i n d i n g M a c h i n e s

1. Horizontal Spindle and Reciprocating Table The wheelhead is given a transverse motion at the end of each table motion. Infeed is controlled by lowering the grinding wheel toward the work.

2. Vertical Spindle and Reciprocating Table The wheel diameter should exceed the width of the surface to be ground. Usually, no transverse motion of either the table or the wheelhead is provided. Can produce very flat surfaces, and are used primarily for production-type work. Abrasive Machining Processes


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Su r f a c e Gr i n d i n g M a c h i n e s

3. Horizontal Spindle and Rotary Table Produce very flat surfaces. Usually made in rather small sizes. 4. Vertical Spindle and Rotary Table Production-type machines. Frequently have two or more grinding heads, so both rough and finish grinding is accomplished in one rotation of the workpiece. By using a special rotary feeding mechanisms, machines of this type often are made automatic. Parts are damped on the rotary feeding table and fed automatically onto work-holding devices and moved past grinding wheels. After they pass the last grinding head, they are automatically removed from the machine. Abrasive Machining Processes


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S u r f a c e Gr i n d i n g

FIGURE 28-1 Schematic of surface grinding, showing infeed and cross feed motions along with cutting speeds V S , and workpiece velocity V W .



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W o r k p i ec e H o l d in g d u r i n g S u r f a c e Gr i n d i n g Magnetic, electrostatic and vacuum chucks can be used to hold the workpieces.

Magnetic chucks can use electromagnets, or permanent magnets. Can only be used for ferromagnetic materials.

Electrostatic chucks can be used for any electrically conductive material, and also for nonmetals coated with metals.

Vacuum chucks can be used for both nonmetals and metals. Abrasive Machining Processes


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M a g n e t i c Ch u c k s



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M a g n e t i c Ch u c k w / Pe r m a n e n t M a g n e t s



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E l e c t r o s t a t i c Ch u c k



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V a c u u m Ch u c k



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T o o l a n d Cu t t e r G r i n d i n g

Bench Grinder

Simple, single-point tools often are sharpened by hand on bench or pedestal grinders. More complex tools, such as milling-cutters, reamers, and single-point tools for production-type operations, require more complex grinding machines, commonly called universal tool and cutter grinders. These machines are somewhat similar to small universal cylindrical center-type grinders. Abrasive Machining Processes


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T o o l a n d Cu t t e r G r i n d e r s



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T o o l a n d Cu t t e r G r i n d e r s



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Snagging

A type of rough grinding that is done in a foundry to remove fins, gates, risers, and rough spots from castings, preparatory to further machining. The primary objective is to remove substantial amounts of metal rapidly without much regard for accuracy. Abrasive Machining Processes


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M o u n t e d W h e e ls a n d P o in t s



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Co a t e d A b r a s i v e s

Coated abrasives are used in finishing both metal and nonmetal products. These are made by gluing abrasive grains onto a cloth or paper backing. They are available in sheets, rolls, or endless belts and disks of various sizes. When the abrasive particles become dull, the article must be replaced. Abrasive Machining Processes


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Co a t e d A b r a s i v e s

Belt Grinder Abrasive Machining Processes


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Honing Honing is used to remove small amounts of material to produce an exact size and surface finish. It can be used both for finishing and sizing . The most common application is to produce precise surface finish in engine cylinder walls and hydraulic cylinder fabrication. Other applications are manufacturing of bearings, hydraulic cylinders, gun barrels, ... . Combined rotation and axial oscillation is used to produce the desired surface throughout the entire length of the hole. Abrasive Machining Processes


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Honing Honing uses fine abrasive stones to remove very small amounts of metal. It is used to size and finish bored holes, removing common errors left by boring, or remove the tool marks (scratches) left by grinding. The amount of metal removed usually is typically about 0.10 - 0.15 mm. Virtually all honing is done with stones made by bonding together various fine artificial abrasives. They are called honing stones , and usually are held in a honing head . The honing head is not guided externally but instead floats in the hole being guided by the work surface. Abrasive Machining Processes


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Honing Either flat or round surfaces may be honed, but majority of honing is done on internal, cylindrical surfaces, such as automobile cylinder walls. Although honing occasionally is done by hand, as in finishing the face of a cutting tool, it is usually done by using special equipment. For honing single, small internal cylindrical surfaces the workpiece is manually held and reciprocated over a rotating hone. Single and multiple-spindle honing machines are available in both horizontal and vertical types. In honing internal cylinders, a small rotation is combined with an oscillatory axial motion. Abrasive Machining Processes


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Honing A cutting fluid is used in virtually all honing operations. A complete honing cycle, including loading and unloading the work, is often less than 1 minute.

Cross-hatched surface helps to retain the lubricant during the operation of the component. Abrasive Machining Processes


Honing Head

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H o n i n g M a ch i n e Honing Machine



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Er r o r s t h a t c a n b e Co r r e c t e d b y H o n i n g



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Superfinishing Variation of honing that can be applied to both flat and cylindrical surfaces. Improves surface finish ( f inishing ). Leads to very uniform surfaces with repeatable smoothness. Not used to change dimensions ( not sizing ). Uses large amount of lubricant to: ● ●

Keep workpiece surface at uniform temperature, Wash away abraded metal particles so as to prevent scratching.



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Superfinishing



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Superfinishing Superfinishing is based on the phenomenon that, a lubricant of given viscosity will establish and maintain a separating, lubricating film between two mating surfaces if their roughness does not exceed a certain value, and if a certain critical pressure, holding them together, is not exceeded. Consequently, as the minute peaks on a surface are cut away by the honing stone, applied with a controlled pressure, a certain degree of smoothness is achieved. When this certain degree of smoothness is achieved, the lubricant establishes a continuous lubricating film between the stone and the workpiece and separates them so that no further cutting action occurs. Abrasive Machining Processes


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Superfinishing Cy l i n d r i c a l S u r f a c e s



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Superfinishing Fl a t S u r f a c e s



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Su p e r f i n i sh i n g M a c h i n e

Crankshaft



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Lapping

Lap is a tool made by charging (causing to become embedded) fine abrasive particles into a soft material. Lap materials range from various types of cloth, or a soft metal such as copper. Lap material should be softer than the material to be finished, being only a holder for the hard abrasive particles. As the charged lap is rubbed against a surface, the abrasive particles in the surface of the lap remove small amounts of material from the harder surface. Lap material itself does not remove material from the workpiece surface. Abrasive Machining Processes


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Lapping Lapping removes material very slowly and is typically used to remove machining or grinding marks (scratch marks), producing a polished surface. Used to obtain very flat and smooth surfaces. But, slow, thus expensive, and it should not be specified unless such a surface is absolutely necessary. Can be done by hand or special machines, in which the workpieces are placed loosely in holders and are held against the rotating lap by means of floating heads. A special type, centerless lapping machine is used for lapping small, cylindrical parts, such as piston pins and ball-bearing races. Abrasive Machining Processes


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L ap p i n g M a c h i n e



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Su r f a c e R o u g h n e s s M e asu r e m e n t (Mechanical)



Peak to vall ey St = 12.7 nm Diamond turned RMS = 19.8 Å 1 Å (Angstrom) = 1.0 × 10 -10 meters

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Su r f a c e R o u g h n e s s M e asu r e m e n t (Optical)

Polished surface RMS = 1.65 nm Abrasive Machining Processes


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Typical Su r f a c e R o u g h n e s s Values



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Re la t e d Ch a p t e r i n t h e T e x t b o o k

Chapter 28 Abrasive Machining Processes



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10 Abrasive Machining Processes

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