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
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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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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
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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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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
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A b r a s i v e s
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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
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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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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.
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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
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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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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
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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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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.
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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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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
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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 s i n g
Tools used for
grinding wheel
truing
Grinding wheel
dresser
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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
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G r i n d i n g
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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
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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 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.
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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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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.
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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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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.
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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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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.
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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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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.
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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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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.
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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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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.
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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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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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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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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.
Ex t e r n a l Ce n t e r l e s s Gr i n d i n g
O p e r a t i o n s
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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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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
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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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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.
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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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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
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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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Su r f a c e Gr i n d i n g
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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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Ma 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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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
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
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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
T o o l a n d Cu t t e r Gr i n d e r s
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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
T o o l a n d Cu t t e r Gr i n d e r s
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 5 8
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
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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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
M o u n t e d W h e e l s a n d P o in t s
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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
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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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
Co a t e d A b r a s i v e s
BeltGrinder
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 6 2
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
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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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
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
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 6 4
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
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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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
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.
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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
H o n i n g M a ch i n e
HoningMachine
Honing
Head
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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
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
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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
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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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
S u p e r f i n i s h i n g
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 7 0
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
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.
S u p e r f i n i s h i n g
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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
S u p e r f i n i s h i n g
Cy l i n d r i c a l Su r f a c e s
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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
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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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
Su p e r f i n i sh i n g M a c h i n e
Crankshaft
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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
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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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
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.
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 7 6
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
L ap p i n g M a c h i n e
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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
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 )
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 7 8
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
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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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
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
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 8 0
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
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