10 abrasive machining processes

8/13/2019 10 Abrasive Machining Processes

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A b r a s i v e Ma c h i n i n g

P r o c e s s e s

BR SI VE M CH I NING PROCESSES

ME202

Manu f act u r i ng

Tech no l og i es

ME 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 2


P r o c e s s e s

A b r a s i v 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 integralparts of abrasive particles.

An abrasive is a hard material that can cut or abradeother materials.


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P r o c e s s e s

Natural abrasivesSandstone was used by ancient people to sharpen tools

and weapons.

Emery, a mixture of alumina (Al2O3) and magnetite(Fe3O4), is another natural abrasive still in use today.

Corundum (natural Al2O3) and diamonds are othernaturally occurring abrasive materials.

Today, the only natural abrasives that have commercial

importance are quartz, sand, garnets, and diamonds.

A b r a s i v e s



P r o c e s s e s

Ar ti ficial abrasives

Artificial abrasives date from 1891, when Edward G.Acheson , while attempting to produce precious gems,discovered how to make silicon carbide (SiC).

Silicon carbide is made by charging an electric furnacewith 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 carbidecrystals result.

After the furnace has cooled, the mass of crystals isremoved, crushed, and graded.

A b r a s i v e s


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P r o c e s s e s

Ar ti ficial abrasivesOther artificial abrasives used today are:

Aluminum oxide (Al2O3) is the most widely used artificialabrasive.

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 producedspecifically for abrasive purposes.

Cubic Boron Nitride (CBN) is extremely hard.

It is the second-hardest material known, and is oftenreferred to, along with diamonds, as a superabrasive.It is not found in the nature, it is produced.

A b r a s i v e s



P r o c e s s e s

A b r a s i v e s


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P r o c e s s e s

Grinding Grinding Machines Work Holding Devices Tool and Cutter Grinding Snagging Mounted Wheels and Points

Coated Abrasives Honing

Superfinishing Lapping

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



P r o c e s s e s

G r i n d i n g

Grinding is the most common abrasive machining process.

Generally not su itable for bulk material removal,but used for dimensional accuracy and surfaceimprovement by removing a small amount of materialfrom the workpiece surface.

The cutting tool, which rotates during the process is called

the grinding wheel, wherein the abrasives are bondedtogether into a wheel of some shape.

There are a lot of bonding materials used for this purpose.


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P r o c e s s e s

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 otherceramic substances.

2. Resinoid, or phenolic resins can be used.

3. Silicate wheels use silicate of soda (waterglass) as thebond material.

4. Shellac-bonded wheels are made by mixing theabrasive grains with shellac in a heated mixture.

5. Rubberbonding is used to produce wheels that canoperate at high speeds that must have a considerabledegree of flexibility.

6. Superabrasive wheels are either electroplated, or a thinsegmented drum of vitrified CBN surrounds a steelcore.

ME 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 1 0


P r o c e s s e s

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


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FIGURE 28-10 Meaningof 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.

Gr i n d i n g W h e e l St r u c t u r e



P r o c e s s e s

G r i n d i n g

During the process, material is also removed from the

grinding wheel.

The G-ratio is defined as the ratio of workpiece materialremoved to grinding wheel material removed.

Ratios of 20:1 to 80:1 are

common.

Abrasives are sized tocontrol material removal

rate and resulting surfacefinish of the workpiece.


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P r o c e s s e s

A grinding wheel is made by bonding many grains with non-specific geometry together into a wheel of some shape.

Many grains have negative rake angles ().

The specific cutting energy(W/cm3/min) is much larger incomparison to turning or milling.

The volume cut is small per unit time; thus time consuming.

Hence an expensiveprocess.

G r i n d i n g



P r o c e s s e s

Used when;

A high dimensional accuracy is required,

A smooth surface, (high surface quality) is required,

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.)

w h y Gr i n d i n g ?


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P r o c e s s e s

FIGURE 28-9SEM micrograph of stainless steel chipsfrom a grinding process.

The tops (T) of the chips have the typicalshear front-lamella structure while the

bottoms (B) are smooth where they slideover the grit.

G r i n d i n g

Rake angles of abrasive particlescan be positive or negative.



P r o c e s s e s

Grinding wheels lose their geometry with use.

Truing restores the original shape by removing a smallamount of material from the wheel surface, in order toexpose new grinding media and new cutting edges onworn glazed grains.

Gr i n d i n g W h e e l T r u i n g


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P r o c e s s e s

As grinding wheels are used they tend to become loadedwith lodged metal chips in the cavities.

Dressing is used to remove the lodged metal chips.

Gr i n d i n g W h e e l D r e ss i n g



P r o c e s s e s

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

Tools used for

grinding wheel

truing

Grinding wheel

dresser


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P r o c e s s e s

Grinding is done in three ways:

Infeed, moving the wheel across the workpiece surface bymoving 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 workwhile 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.

G r i n d i n g



P r o c e s s e s

G r i n d i n g


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P r o c e s s e s

Center type Cylindrical Grinding

Chucking type Grinding

Tool-post Grinders (an attachment)

Centerless Grinding

Surface Grinding

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



P r o c e s s e s

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


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P r o c e s s e s

I . Ce n t e r Ty p e Cy l i n d r i ca l Gr i n d i n g

Commonly used for producing external cylindrical-and conical (tapered)- surfaces.

During the process, the grinding wheel revolves at anordinary cutting speed, and the workpiece rotates oncenters at a much slower speed, usually at 23-38m/min.

The grinding wheel and the workpiece move in oppositedirections at their point of contact.

The depth of cut is determined by infeed of the wheel orthe workpiece.

This motion also determines the finished diameter of theworkpiece, therefore accurate control of this movementis required.



P r o c e s s e s

Ce n t e r Ty p e Cy l i n d r i ca l Gr i n d i n g

Plain center type cylindricalgrinding machine


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P r o c e s s e s

1 - P l a in Ce n t e r T y p e

Cy l i n d r i ca l Gr 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 beswiveled approximately 10 about a vertical axis with respect tothe table carrier on which it is mounted. This permits grinding oftapered cylinders up to about 10.

For the traverse motion, the table assembly can be reciprocatedalong the ways on the main frame either manually or by power.

Infeed is provided by movement of the wheelhead at right angles tothe 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.



P r o c e s s e s

2 - U n i v e r s a l Ce n t e r Ty p eCy l i n d r i ca l Gr i n d i n g M a c h i n e s

Basically the same as plain center-type grinders except for twofeatures.

Both the headstock and thewheelhead can be swiveled aboutvertical axes to grind tapers of allangles, and to do certain othertypes of work that cannot be doneon plain center-type grinders.

Most machines have dual spindles

on the swiveling wheelhead, onefor external grinding and the otherfor internal grinding. Either spindlecan be brought into use byswiveling or tilting the wheelhead.


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P r o c e s s e s

Ce n t e r Ty p e Cy l i n d r i ca l Gr i n d i n g

P lu n g e Cu t

FIGURE 28-14Continuous

crush roll dressingand truing of a

grinding wheel

(form-truing and

dressing throughout

the process rather

than between cycles)

doing plunge cut

grinding on a cylinderheld between

centers.



P r o c e s s e s

3 - Ro l l Gr i n d e r

Roll grinders are basically plain center-type machines designedfor grinding large, cylindrical mill and calendar rolls.

Because of the weight of such workpieces, the wheelhead,instead of the work, reciprocates.


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P r o c e s s e s

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

The workpiece is held in a chuck for grindingboth external and internal cylindrical surfaces.



P r o c e s s e s

1 - Ch u ck i n g T y p e Ex t e r n a l Gr i n d i n g

Chucking type grinding machines are production-typemachines, 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 beremoved from one while another piece is being groundin the other.


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P r o c e s s e s

2 - Ch u c k i n g T y p e I n t e r n a l Gr i n d i n g

a - Re g u l a r

Infeed movement of the wheelhead is normal to the axis ofrotation of the work.

There are plain and universal regular chucking-type internalgrinders. On plain internal grinders of this type, the

workhead can be swiveled so that both straight internalcylinders and beveled holes can be ground. On universalinternal grinders, the workhead not only can be swiveled, butit also is mounted on a cross slide.

In the regular chucking-typeinternal grinders, the chuck-held workpiece revolves, arelatively small, high-speedgrinding wheel is rotated ona spindle arranged so that itcan be reciprocated in andout of the workpiece.



P r o c e s s e s

2 - Ch u c k i n g T y p e I n t e r n a l Gr i n d i n g b - P l a n e t a r y

The revolving grinding wheel also has planetary rotation aboutan axis that is coincident with the axis of the finished cylinder.

The diameter of the ground surface is controlled by adjusting theradius of the planetary rotation.

On this type of machines, the work is reciprocated past thewheel.

Planetary-typeinternal grinders areused for work that istoo large to berotated conveniently.


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P r o c e s s e s

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

The entire mechanism is mounted either on the tool post or onthe 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.

Tool-post grindersare attachments usedto permit occasionalgrinding to be doneon a lathe.



P r o c e s s e s

I I I . Ce n t e r l e s s Gr 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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P r o c e s s e s

1 - Ex t e r n a l Ce n t e r l e s s Gr 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 rotatesat approximately the same surface speed as that of the

regulating wheel.



P r o c e s s e s

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


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P r o c e s s e s




P r o c e s s e s

In thrufeed grinding, the workpiece is of constant diameterand is fed completely through between the rolls, startingat one end. This is the simplest type and can easily bemade automatic.

In infeed centerless grinding the work rest and theregulating 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.

Ex t e r n a l Ce n t e r l e s s Gr i n d i n gO p e r a t i o n s


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P r o c e s s e s

In endfeed centerless grinding, both the grinding andregulating 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 andinclined to impart random rotation.


O p e r a t i o n s



P r o c e s s e s

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.

2 - I n t e r n a l Ce n t e r l e ss Gr i n d i n g

External surface of thecylinder should befinished accuratelybefore the operation.

Centerless internalgrinding assures theconcentricity of externaland internal cylindricalsurfaces of tube-likeworkpieces.


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P r o c e s s e s

1. It is very rapid; infeed centerless grinding is almostcontinuous.

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 bythe 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 wearcan be minimized.

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



P r o c e s s e s

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

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

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

4. In external centerless grinding of tubes, there is noguarantee that the outside and inside surfaces of the

tube are concentric;

In internal centerless grinding, outside

and inside surfaces are concentric.

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


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P r o c e s s e s

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, irregularsurfaces can be produced onsome types of surface grindersby using a formed wheel.



P r o c e s s e s

1. Horizontal Spindle and Reciprocating Table

The wheelhead is given a transverse motion at the endof each table motion.

Infeed is controlled by lowering the grinding wheeltoward the work.

2. Vertical Spindle and Reciprocating Table

The wheel diameter should exceed the width of thesurface to be ground.

Usually, no transverse motion of either the table or thewheelhead is provided.

Can produce very flat surfaces, and are used primarilyfor production-type work.

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


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P r o c e s s e s

3. Horizontal Spindle and Rotary Table

Produce very flat surfaces.Usually made in rather small sizes.

4. Vertical Spindle and Rotary TableProduction-type machines.Frequently have two or more grinding heads, so bothrough and finish grinding is accomplished in one rotationof 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 fedautomatically onto work-holding devices and moved past

grinding wheels.After they pass the last grinding head, they areautomatically removed from the machine.

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



P r o c e s s e s

Su r f a c e Gr i n d i n g

FIGURE 28-1 Schematic of surface grinding, showing infeed and cross feedmotions along with cutting speeds VS , and workpiece velocity VW.


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P r o c e s s e s

Su r f a c e Gr i n d i n g



P r o c e s s e s

W o r k p i ec e H o l d in g d u r i n gSu 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 permanentmagnets.

Can only be used for ferromagnetic materials.

Electrostatic chucks can be used for any electricallyconductive material, and also for nonmetals coated with

metals.

Vacuum chucks can be used for both nonmetals andmetals.


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P r o c e s s e s

Ma g n e t i c Ch u c k s



P r o c e s s e s

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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P r o c e s s e s

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

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.

BenchGrinder



P r o c e s s e s

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


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P r o c e s s e s

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



P r o c e s s e s

S n a g g i n g

A type of rough grinding that is done in a foundry to remove fins,

gates, risers, and rough spots from castings, preparatory tofurther machining.

The primary objective is to remove substantial amounts of metal

rapidly without much regard for accuracy.


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P r o c e s s e s

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



P r o c e s s e s

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

Coated abrasives are used in finishing both metal andnonmetal products.

These are made by gluing abrasive grains onto a cloth orpaper backing.

They are available in sheets, rolls, or endless belts anddisks of various sizes.

When the abrasive particles become dull, the article mustbe replaced.


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P r o c e s s e s

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

BeltGrinder



P r o c e s s e s

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.

H o n i n g


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P r o c e s s e s

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.

H o n i n g



P r o c e s s e s

Either flat or round surfaces may be honed, but majority ofhoning is done on internal, cylindrical surfaces, such asautomobile cylinder walls.

Although honing occasionally is done by hand, as infinishing the face of a cutting tool, it is usually done byusing special equipment.

For honing single, small internal cylindrical surfaces theworkpiece is manually held and reciprocated over arotating hone.

Single and multiple-spindle honing machines are availablein both horizontal and vertical types.

In honing internal cylinders, a small rotation is combinedwith an oscillatory axial motion.

H o n i n g


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P r o c e s s e s

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.

H o n i n g

Cross-hatchedsurface helps toretain thelubricant duringthe operation ofthe component.



P r o c e s s e s

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

HoningMachine

Honing

Head


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P r o c e s s e s

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



P r o c e s s e s

S u p e r f i n i s h i n g

Variation of honing that can be applied to both flat and

cylindrical surfaces.

Improves surface finish (finishing).

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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P r o c e s s e s




P r o c e s s e s

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.



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P r o c e s s e s


Cy l i n d r i c a l Su r f a c e s



P r o c e s s e s

S u p e r f i n i s h i n gFl a t S u r f a c e s


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P r o c e s s e s

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

Crankshaft



P r o c e s s e s

L a p p i n g

Lap is a tool made by charging (causing to becomeembedded) 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 removesmall amounts of material from the harder surface.

Lap material itself does not remove material from the

workpiece surface.


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P r o c e s s e s

L a p p i n g

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 forlapping small, cylindrical parts, such as piston pins and

ball-bearing races.



P r o c e s s e s

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


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P r o c e s s e s

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

M e asu r e m e n t

( M e c h a n i c a l )



P r o c e s s e s

Polished surface RMS = 1.65 nm

Peak to valley St = 12.7 nmDiamond turned RMS = 19.8 1 (Angstrom) = 1.0 10 -10 meters

Su r f a c e R o u g h n e s sM e asu r e m e n t

( O p t i c a l )


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P r o c e s s e s

T y p i c a l

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

V a l u e s



P r o c e s s e s

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 28Abrasive MachiningProcesses

10 abrasive machining processes

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