manufacturing lecture

5/21/2018 manufacturing lecture

1/123

Manufacturing TechnologyII

ME 307

Chapter # 25


2/1232010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Chapter 25

GRINDING AND OTHER ABRASIVEPROCESS



Abrasive Machining

Material removal by the action of hard, abrasiveparticles that are usually in the form of a

bonded wheel.

Grinding is the most important abrasive

process. Other traditional abrasive processes include

Honing,

lapping,

superfinishing,

polishing, and

buffing.

Generally used as finishing operations.



Abrasive Machining

Abrasive processes are important commerciallyand technologically for the following reasons:

They can be used on all types of materials

ranging from soft metals to hardened steels

and hard nonmetallic materials such asceramics and silicon.

can produce extremely fine surface finishes, to

0.025 mm (1 m-in).

dimensions can be held to extremely closetolerances.



What is Grinding

Abrasive material removal process

Grinding is achieved by a bonded grinding

wheel rotating at high speed

Tool i.e. Grinding wheel is usually disk shaped

Precisely balanced

Similar to Milling but with almost infinite cutting

teeth (abrasive particles) rotating at very high

speed.



After Materials and Processes in Manufacturing, by E. Paul DeGarmo, J.T. Black,

and Ronald A. Kohser, Prentice Hall of India, 2001.


7/123

2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern

Manufacturing 4/e

Workpieces and Operations Used in

Grinding

Figure 26.2 The types of workpieces and operations typical of grinding: (a) cylindrical

surfaces, (b) conical surfaces. (c) fillets on a shaft, (d) helical profiles, (e) concaveshape, (f) cutting off or slotting with thin wheels, and (g) internal grinding.



Grinding vs Milling

the abrasive grains in the wheel are muchsmaller and more numerous than the teeth on a

milling cutter;

cutting speeds in grinding are much higher than

in milling; the abrasive grits in a grinding wheel are

randomly oriented and possess on average a

very high negative rake angle; and

a grinding wheel is self-sharpeningas thewheel wears, the abrasive particles become

dull and either fracture to create fresh cutting

edges or are pulled out of the surface of the

wheel to expose new grains.


9/123



Grinding Wheel

Abrasive Material

Grain Size

Bonding Material

Wheel Grade

Wheel Structure



Abrasive Material

High Hardness

Wear Resistance

Toughness

Friability

It is the capacity of the abrasive material to

fracture when cutting edge become dull,

thereby exposing a new surface



Abrasive Material


13/123

2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Grain size

Important parameter in determining surfacefinish and material removal (MRR)

Small Grain size more finish

Large grain size better MRR

Harder work materials require smaller grain

sizes

softer materials require larger grit sizes.

Grain size is determined by Screen Mesh

Grain size varies from 8 to 250 with size 8

being very coarse.


14/123


Figure 28.3:

Typical screens for sifting abrasives into sizez. The larger the screen

number (of opening per linear inch), the smaller the grain size.

(Courtesy of Corborundum Cornpony.)




15/123


Figure 28.2:

Loose abrasive grains at high magnification, showing their irregular, sharp cutting

edges. (Courtesy of Norton Cornpony.)




16/123


Bond Material

The bonding material holds the abrasive grainsand establishes shape and structural integrity

of the grinding wheel

The bonding material should withstand

grinding forces,

high temperatures,

shock loading and

rigidly holding the abrasive grains.


17/123


Bond Material


18/123


Wheel Structure

Relative Spacing of abrasive grains in thewheel

The total structure is made up of abrasive

grains, bond material and air

Pg+Pb+Pp=1.0

Wheel may be open or dense

Open structure is one in which Ppis large,

while in dense structure Pgis Large

Dense structure is used for better surface finish


19/123


Wheel Structure

FIGURE 25.1

Typical structure of a grinding wheel.


20/123


Grinding Wheel Model

Figure 26.3 Schematic illustration of a physical model of a grinding wheel

showing its structure and wear and fracture patterns.


21/123




Figure 28.6:

The cavities or voids between the grains must be large enough to hold

all the chips during the cut.


22/123


Wheel Grade

It indicates the grinding wheels bond strengthin retaining the abrasive grits during cutting.

largely dependent on the amount of bonding

material present in the wheel structure.

ranges between softand hard. Soft

lose grains readily.

generally used for applications requiring

low material removal rates and grinding of hard work materials.

Hard

retain their abrasive grains.

Typically used to achieve

high stock removal rates and


23/123


Grinding Wheel Specification


24/123




25/123



FIGURE 25.2

Some of the standard grinding wheel shapes: (a) straight, (b) recessed two sides, (c)

metal wheel frame with abrasive bonded to outside circumference, (d) abrasive cutoff

wheel.


26/123



FIGURE 25.2

Some of the standard grinding wheel shapes: (e) cylinder wheel, (f) straight cup wheel,

and (g) flaringcup wheel.


27/123


Bonded Abrasives Used in Abrasive-Machining Processes

Figure 25.1 A variety of bonded abrasives used in abrasive-

machining processes. Source: Courtesy of Norton Company.


28/123

Figure 28.25: Examples of mountedabrassive wheels & Points. (Courtesy

of Norton Company)n


29/123


Manufacturing 4/e

Grinding Wheels

Figure 26.4 Common

types of grinding wheels

made with conventional

abrasives. Note thateach wheel has a specific

grinding face; grinding on

other surfaces is

improper and unsafe.

ANALYSIS OF THE GRINDING


30/123



PROCESS

The cutting conditions in grinding:very high speeds and

very small cut size, (compared to milling)

The peripheral speed is determined by:

v = DN

where v = surface speed of wheel, m/min (ft/min);

N = spindle speed, rev/min; and D = wheel

diameter, m (ft).



31/123



PROCESS

FIGURE 25.3

(a) The geometry of surface grinding, showing the cutting conditions; (b) assumed

longitudinal shape and (c) cross section of a single chip.



32/123



PROCESS

Infeed

Depth of cut d,

It is the penetration of the wheel below the

original work surface.

Crossfeed

the lateral feed of grinding wheel across the

surface of the work on each pass.

it determines the width of the grinding path w.

The width w, multiplied by depth ddetermines the

cross-sectional area of the cut.

the work moves past the wheel at a speed vw, so

the material removal rate is

RMR

= vw

wd



33/123


we are interested in how the cutting conditionscombine with the grinding wheel parameters to

affect

surface finish,

forces and energy, temperature of the work surface, and

wheel wear.


PROCESS


34/123


Grinding Wheel Surface

Figure 26.9 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.


35/123


Manufacturing 4/e

Abrasive Grain Plowing Workpiece Surface

Figure 26.11 Chip formation and plowing of the workpiece surface by an abrasive grain.


36/123


Grinding achieves a surface finish that is superior to

that of conventional machining.

It is affected by the size of the individual chips

formed during grinding.

One obvious factor in determining chip size is grit

size

smaller grit sizes yield better finishes.

it can be shown that the average length of a chip

is given by:

where lc is the length of the chip, mm; D = wheel

diameter, mm; and d= depth of cut, or infeed, mm.

This assumes the chip is formed by a grit that acts

throughout the entire sweep arc.

Surface Finish


37/123


The assumed cross-sectional shape istriangular

width w'being greater than the thickness t by a

factor called the grain aspect ratio rg, defined

by

Typical values of grain aspect ratio are

between 10 and 20.

Surface Finish


38/123


C= The number of active grits (cutting teeth)

per square inch on the outside periphery of the

grinding wheel.

smaller grain sizes give larger C values.

A denser structure means more grits per area.

the number of chips formed per time is

nc= v w C

where v= wheel speed, mm/min; w= crossfeed,

mm; and C= grits per area on the grindingwheel surface, grits/mm2.

surface finish improve by increase in number of

chips formed per unit time on the work surface

for a given width w.

Therefore, increasin v and/or C will

Surface Finish


39/123


The specific energy can be determined as:

where U = specific energy, J/mm3; Fc= cuttingforce, N; v = wheel speed, m/min; v

w= work

speed, mm/min; w = width of cut, mm; and d =

depth of cut, mm.

Forces and Energy


40/123


Manufacturing 4/e

Approximate Specific-Energy Requirements

for Surface Grinding


41/123


In grinding, the specific energy is much greater

than in conventional machining. because:

Size effect. The chip thickness in grinding is

comparatively much smaller.

Therefore the energy required to remove unit

volume of material is significantly higher than in

conventional machiningroughly 10 times higher.

The individual grains possess extremely

negative rake angles. (average about30o,

some values as low as60o). These result in low values of shear plane angle

and high shear strains, both of which mean higher

energy levels in grinding.

Forces and Energy


42/123


not all of the individual grits are engaged in

actual cutting.

Because of the random positions and orientations

of the grains, some grains do not project far

enough into the work surface to accomplish

cutting.

Forces and Energy


43/123


Three types of grain actions:

cutting,in which the grit projects far enough

into the work surface to form a chip and

remove material;

plowing,in which the grit projects into the work,

but not far enough to cause cutting; instead,

the work surface is deformed.

energy is consumed without any material removal;

rubbing,in which the grit contacts the surface

during its sweep, but only rubbing friction occurs,

consume energy without removing any material.

Grain Actions


44/123


FIGURE 25.4: Three types of grain action in grinding: (a)

cutting, (b) plowing, and (c) rubbing.

The size effect, negative rake angles, and ineffective grain

actions combine to make the grinding process inefficient in terms

of energy consumption per volume of material removed.


45/123

Figure 28.7:

The grits interact with the surface

in three ways: cutting, plowing,

and rubbing.




46/123


Using the specific energy relationship, and

assuming that the cutting force acting on a

single grain in the grinding wheel is

proportional to rgt,

where

F'c= the cutting force on an individual grain,K

1= constant of proportionality

Forces and Energy


47/123


Because of

size effect,

high negative rake angles, and

plowing and rubbing

the grinding process is characterized by hightemperatures.

In conventional machining most of the heat

generated is carried off in the chip

In grinding much of the energy remains in theground surface, resulting in high work surface

temperatures.

Temperatures at the Work Surface


48/123


1. The high temperatures may result in surface burnsand

cracks.

The burn marksappear as discoloration.

burns are sign of metallurgical damage immediately

beneath the surface.

The surface cracks are perpendicular to the wheelspeed direction.

They indicate an extreme case of thermal damage to

the work surface.

1. The high temperatures may result in softening of the

work surface.2. Thermal effects can cause residual stresses in the work

surface, possibly decreasing the fatigue strength of the

part.

Temperatures at the Work Surface

Factors Influencing Work Surface


49/123


Experimental observations ---- surface

temperature is dependent on

energy per surface area ground (U).

where K2= a constant of proportionality


Temperatures



50/123


high work temperatures can be mitigated by

decreasing depth of cut d,

decreasing wheel speed v, and

decreasing number of active grits per square

inch C, or

by increasing work speed vw.

In addition,

dull grinding wheels and

wheels that have a hard grade and densestructure

tend to cause thermal problems.

using a cutting fluid can also reduce grinding

temperatures.


Temperatures


51/123


1. Grain fracture,

2. attritious wear, and

3. bond fracture.

Grain fracture occurs when a portion of the

grain breaks off, but the rest of the grainremains bonded in the wheel.

The edges of the fractured area become new

cutting edges.

This tendency of the grain to fracture is calledfriability.

High friability means ---- the grains fracture

more readily.

Wheel Wear


52/123


Attritious wearinvolves dulling of the individual

grains, resulting in flat spots and rounded

edges.

analogous to tool wear in a conventional

cutting tool.

caused by

friction and diffusion,

chemical reactions between the abrasive and

the work material.

Wheel Wear


53/123


Bond fracture occurs when the individual

grains are pulled out of the bonding material.

along other factors it depends on wheel grade.

occurs because of dull grains caused by

attritious wear,

The resulting cutting force is excessive.

Sharp grains cut more efficiently with lower

cutting forces;

hence, they remain attached in the bondstructure.

Wheel Wear


54/123


Wheel Wear

FIGURE 25.5: 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. (Based on [16].)


55/123


The three mechanisms combine to cause the

grinding wheel to wear.

1. grain fracture: the grains are initially sharp,

and wear is accelerated because of grain

fracture.

2. attritious wear:the wear rate is fairly constant,

resulting in a linear relationship between

wheel wear and volume of metal removed.

Mainly characterized by attritious wear, with

some grain and bond fracture.

Wheel Wear


56/123


1. the grains become dull, and the amount of

plowing and rubbing increases relative to

cutting.

In addition, some of the chips become clogged in

the pores (called wheel loading), which impairs

the cutting action and leads to higher heat andtemperatures.

grinding efficiency decreases, and the volume of

wheel removed increases relative to the volume of

metal removed.

Wheel Wear


57/123


Grinding ratio: a term used to indicate the slope

of the wheel wear curve.

where GR = the grinding ratio, Vw= the volume of

work material removed, and Vg = the

corresponding volume of the grinding wheel

that is worn in the process.

Typical values of GR range between 95 and

125,

about five orders of magnitude less than the

analogous ratio in conventional machining.

Wheel Wear


58/123


Grinding ratio:

It generally increases by increasing wheel

speed v.

higher wheel speeds also improve surface

finish.

However, when speeds become too high,

attritious wear and surface temperatures

increase.

As a result, the GR is reduced and the surface

finish is impaired.

Wheel Wear


59/123


Wheel Wear

FIGURE 25.6: Grinding ratio and surface finish as a functionof wheel speed. (Based on data in Krabacher [14].)


60/123


Dressing:When the wheel is in the third region, it

must be resharpened by a procedure called

dressing. which consists of:

1. breaking off the dulled grits on the outside

periphery of the grinding wheel in order to expose

fresh sharp grains and2. removing chips that have become clogged in the

wheel.

It is accomplished by

a rotating disk, an abrasive stick, or

another grinding wheel operating at higher speed.

Wheel Wear


61/123


Turning:

Although dressing sharpens the wheel, it does

not guarantee the shape of the wheel.

Truing is an alternative procedure that

1. sharpens the wheel,2. restores its cylindrical shape and

3. ensures that it is straight across its outside

perimeter.

The procedure uses a diamond-pointed tool(or other truing tools) fed slowly and precisely

across the wheel as it rotates.

A very light depth is taken (0.025 mm or less).

Wheel Wear


62/123


Manufacturing 4/e

Grinding-

Wheel

Dressing

Figure 26.12 (a) Forms of grinding-wheel dressing. (b) Shaping the grinding face of a

wheel by dressing it with computer control. Note that the diamond dressing tool is normal

to the surface at point of contact with the wheel. Source: Courtesy of Okuma Machinery

Works Ltd.

APPLICATION CONSIDERATIONS IN


63/123


C O CO S O S

GRINDING

APPLICATION CONSIDERATIONS INAPPLICATION CONSIDERATIONS IN


64/123


Grinding Fluids:

The functions performed by grinding fluids are

similar to those performed by cutting fluids.

Reducing friction and

removing heat from the process.

washing away chips and

reducing temperature of the work surface.

Types of grinding fluids by chemistry include

grinding oils and emulsified oils.

GRINDINGGRINDING

General Recommendations for Grinding


65/123


Manufacturing 4/e

General Recommendations for Grinding

Fluids

GRINDING OPERATIONS AND


66/123


Grinding is traditionally used to finish parts whose

geometries have already been created by otheroperations.

In addition applications include more high speed,

high material removal operations.

The Grinding operations and machines includes thefollowing types:

1. surface grinding,

2. cylindrical grinding,

3. centerless grinding,4. creep feed grinding, and

5. other grinding operations.

GRINDING MACHINES


67/123


Normally used to grind plain flat surfaces.

It is performed using either

peripheral grinding or

face grinding.

SURFACE GRINDING

FIGURE 25.7 Four types of surface grinding: (a) horizontal spindle with

reciprocating worktable, (b) horizontal spindle with rotating worktable, (c)


68/123

reciprocating worktable, (b) horizontal spindle with rotating worktable, (c)

vertical spindle with reciprocating worktable, and (d) vertical spindle with

rotating worktable.


69/123


Manufacturing 4/e

Various Surface-Grinding Operations

Figure 26.13 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

Blanchardtype.)


70/123

FIGURE 25.8

Surface grinder with horizontal spindle and reciprocating worktable.


71/123


used for rotational parts.

divided into two basic types

(a) external cylindrical grinding and

(b) internal cylindrical grinding.

CYLINDRICAL GRINDING

FIGURE 25.9 Two types

of cylindrical grinding: (a)

external, and (b) internal.

External cylindrical grinding


72/123


performed much like a turning operation.

These grinding machines closely resemble a lathe.

The workpiece is rotated at a surface speed of 18

to 30 m/min, and the grinding wheel, at 1200 to

2000 m/min.

Two types of feed motion possible, traverse feed and

plunge-cut.

The infeed is set within a range typically from

0.0075 to 0.075 mm. used to finish parts, machined to approximate size

and heat treated to desired hardness. e.g. axles,

crank-shafts, spindles, bearings and bushings, and

rolls for rolling mills.

y g g

(center-type grinding)

External cylindrical grinding


73/123

(center-type grinding)

FIGURE 25.10

Two types of feed motion in external cylindrical grinding: (a)

traverse feed, and (b) plunge-cut.


74/123


Manufacturing 4/e

Cylindrical-Grinding Operations

Figure 26.16 Examples of various cylindrical-grinding operations. (a) Traverse grinding,

(b) plunge grinding, and (c) profile grinding. Source: Courtesy of Okuma Machinery

Works Ltd.


75/123


Manufacturing 4/e

Plunge Grinding on Cylindrical Grinder

Figure 26.17 Plunge grinding of a workpiece on a cylindrical

grinder with the wheel dressed to a stepped shape.


76/123


Manufacturing 4/e

Grinding a Noncylindrical Part on Cylindrical Grinder

Figure 26.18 Schematic illustration of grinding a noncylindrical part on a

cylindrical grinder with computer controls to produce the shape. The part

rotation and the distancexbetween centers is varied and synchronized to

grind the particular workpiece shape.


77/123


operates somewhat like a boring operation.

The work-piece is rotated at surface speeds of 20

to 60 m/min. Wheel surface speeds similar to

external cylindrical grinding.

The wheel is fed in either

traverse feed, or

plunge feed.

the wheel diameter must be smaller than the bore

hole, which necessitate very high rotational speeds

in order to achieve the desired surface speed. used to finish the hardened inside surfaces of

bearing races and bushing surfaces.

Internal cylindrical grinding


78/123


Manufacturing 4/e

Internal Grinding Operations

Figure 26.21 Schematic illustrations of internal grinding operations:

(a) traverse grinding, (b) plunge grinding, and (c) profile grinding.


79/123


It is an alternative process for grinding external and

internal cylindrical surfaces.

As its name suggests, the workpiece is not held

between centers.

This results in a reduction in work handling time;

hence used for high-production work. The workparts are supported by a rest blade and

fed through between the two wheels.

The grinding wheel rotate at surface speeds of

1200 to 1800 m/min. The regulating wheel rotates at much lower speeds

and is inclined at a slight angle I to control

throughfeed.

Centerless Grinding


80/123

Centerless Grinding

FIGURE 25.11External centerless grinding.


81/123

Centerless Grinding

Figure 28.22: Centerless grinding showing the relationship among the grinding wheel, the regulating

wheel, and the workpiece in centerless method. (Courtesy of Carborundum Company.)

Centerless Grinding


82/123


Manufacturing 4/e

Operations

Figure 26.22 Schematic

illustration of centerless

grinding operations: (a)

through-feed grinding, (b)

plunge grinding, (c) internalgrinding, and (d) a

computer numerical-control

cylindrical-grinding

machine. Source:

Courtesy of Cincinnati

Milacron, Inc.


83/123


The following equation can be used to predict

throughfeed rate:

fr= D

rN

rsin I

where fr= throughfeed rate, mm/min; Dr=

diameter of the regulating wheel, mm; Nr

= rotational speed of the regulatingwheel, rev/min; and I = inclination angle

of the regulating wheel.

Centerless Grinding


84/123


In place of the rest blade, two support rolls are

used.

The regulating wheel is tilted at a small inclination

angle to control the feed.

Because of the need to support the grinding wheel,

throughfeed is not possible. Therefore it cannot achieve the high-production

rates as in the external process.

capable of providing very close concentricity

between internal and external diameters on atubular part such as a roller bearing race.

Internal Centerless Grinding

C G


85/123

Internal Centerless Grinding

FIGURE 25.12Internal centerless grinding.

C F d G i di


86/123


It is performed at very high depths of cut and very

low feed rates; hence, the name creep feed.

Depths of cut are 1000 to 10,000 times greater than

conventional surface grinding.

the feed rates are reduced by about the same

proportion. However, material removal rate and productivity are

increased because the wheel is continuously

cutting.

Typical advantages include:1. high material removal rates,

2. Improved accuracy for formed surfaces, and

3. reduced temperatures at the work surface.

Creep Feed Grinding

C F d G i di


87/123

Creep Feed Grinding

FIGURE 25.13

Comparison of (a) conventional surface grinding and (b) creep feed

grinding.

C F d G i di


88/123


It can be applied in both surface grinding and

external cylindrical grinding.

Surface grinding applications include grinding of

slots and profiles.

Especially suited to cases in which depth-to-width

ratios are relatively large. The cylindrical applications include threads, formed

gear shapes, and other cylindrical components.

The term deep grinding is used in Europe.

Creep Feed Grinding

C F d G i di M hi


89/123


Special features for creep feed grinding:

high static and dynamic stability,

highly accurate slides,

2-3 times the spindle power of conventional

grinding machines,

consistent table speeds for low feeds,

high-pressure grinding fluid delivery systems, and

dressing systems capable of dressing the grinding

wheels during the process.

Creep Feed Grinding Machines

C F d G i di


90/123


Manufacturing 4/e

Creep-Feed Grinding

Figure 26.23 (a) Schematic illustration of the creep-feed grinding process. Notethe large wheel depth-of-cut, d. (b) A shaped groove produced on a flat surface

by creep-grinding in one pass. Groove depth is typically on the order of a few mm.

(c) An example of creep-feed grinding with a shaped wheel. This operation also

can be performed by some of the processes described in Chapter 27. Source:

Courtesy of Blohm, Inc.

T l G i di


91/123


Special grinding machines of various designs to sharpen

and recondition cutting tools. They have devices for positioning and orienting the tools

to grind the desired surfaces at specified angles and

radii.

Some are general purpose while others cut the unique

geometries of specific tool types. General-purpose grinders use special attachments and

adjustments to accommodate a variety of tool

geometries.

Single-purpose tool grinders include

gear cutter sharpeners,

milling cutter grinders of various types,

broach sharpeners, and

drill point grinders.

Tool Grinding


92/123

Figure 28.24: Three typical setups for grinding single and multiple-edge tools

on a universal tool & cutter grinder. (a) single point tool is held in a device that

permits all possible angles to be ground. (b) Edgers of a large hand reamer

are being ground. (c) Milling cutter is sharpened with cupped grinding wheel.

Ji G i di


93/123


Traditionally used to grind holes in hardened steel parts

to high accuracies.

Applications include

pressworking dies and tools.

broader range of applications in which high accuracy

and good finish are required on hardenedcomponents.

Numerical control is available on modern jig grinding

machines.

Jig Grinding

Di k G i di


94/123


Grinding machines with large abrasive disks mounted on

either end of a horizontal spindle.

The work is held (usually manually) against the flat

surface of the wheel.

Some machines have double opposing spindles.

By setting the disks at the desired separation, theworkpart can be fed automatically between the two

disks and ground simultaneously on opposite sides.

Advantages are

good flatness and

parallelism

at high production rates.

Disk Grinding

Disk Grinding


95/123


Disk Grinding

FIGURE 25.14Typical configuration of a disk grinder.

Snag Grinding


96/123


It is similar in configuration to a disk grinder.

The difference is that the grinding is done on the

outside periphery of the wheel rather than on the side

flat surface.

The grinding wheels are therefore different in design.

It is generally a manual operation, used for roughgrinding operations such as

removing the flash from castings and forgings, and

smoothing weld joints.

Snag Grinding

Abrasive Belt Grinding


97/123


It uses abrasive particles bonded to a flexible

(cloth) belt.

A platen located behind the belt provides it

support required when the work is pressed

against it. This support is by a

roll or

a flat platen (for work having a flat surface).

a soft platen if it is desirable for the abrasive

belt to conform to the general contour of the

part.

Belt speed depends on the material being

ground; typical range 750 to 1700 m/min.




98/123


traditional applications in light grinding.

Belt sanding: light grinding applications to

remove burrs and high spots, and produce an

improved finish quickly by hand.

Owing to improvements in abrasives and

bonding materials, being used increasingly for

heavy stock removal rates,




99/123



FIGURE 25.15 Abrasive belt grinding.



100/123


Belt Grinding of Turbine Nozzle Vanes


101/123


Manufacturing 4/e

Belt Grinding of Turbine Nozzle Vanes

Figure 26.26Belt grinding of turbine nozzle vanes.

RELATED ABRASIVE PROCESSES


102/123


Other abrasive processes include

honing,

lapping,

superfinishing,

polishing, and buffing.

RELATED ABRASIVE PROCESSES

HONING


103/123


It is an abrasive process performed by a set of

bonded abrasive sticks.

Applications include finishing the bores of

internal combustion engines (common).

bearings, hydraulic cylinders, and

gun barrels.

Surface finishes of around 0.12 m or slightly

better are typical. In addition, it produces a cross-hatched surface

that tends to retain lubrication, thus contributing

to its function and service life.

HONING

HONING


104/123


HONING

FIGURE 25.16

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


105/123


The tool consists of a set of bonded abrasive

sticks.

The number of sticks depends on hole size.

Two to four sticks used for small holes (e.g.,

gun barrels), and

a dozen or more used for larger diameter

holes.

The motion of the tool is a combination of

rotation and linear reciprocation, regulated in

such a way that a given point on the abrasive

stick does not trace the same path repeatedly.

This complex motion accounts for the cross-

hatched pattern on the bore surface.

HONING

HONING


106/123


HONING

HONING


107/123


Honing speeds are 15 to 150 m/min.

During the process, the sticks are pressed

outward against the hole surface to produce

the desired abrasive cutting action.

Hone pressures of 1 to 3 MPa are typical.

The honing tool is supported in the hole by two

universal joints,

causing the tool to follow the previously

defined hole axis.

It enlarges and finishes the hole but cannot

change its location.

HONING

HONING


108/123


Grit sizes range between 30 and 600.

The amount of material removed during a

honing operation may be as much as 0.5 mm,

but is usually much less than this. A

cutting fluid must be used in honing to

cool and lubricate the tool and

to help remove the chips.

HONING

Honing Tool


109/123


Manufacturing 4/e

Honing Tool

Figure 26.27 Schematic illustration of a honing tool used

to improve the surface finish of bored or ground holes.

LAPPING


110/123


An abrasive process used to produce surface

finishes of extreme accuracy and smoothness.

used in the production of

optical lenses,

metallic bearing surfaces,

gages, and

parts requiring very good finishes.

Applications

Metal parts that are subject to fatigueloading or

surfaces that must be used to establish a

seal with a mating part.

LAPPING

LAPPING


111/123


LAPPING

FIGURE 25.17

The lapping process in lens-making.

LAPPING


112/123


Instead of a bonded abrasive tool, a fluid

suspension (lapping compound) of very smallabrasive particles is used between the

workpiece and the lapping tool.

The lapping compound has the general

appearance of a chalky paste.

The fluids used to make the compound include

oilsand kerosene.

Common abrasives are

aluminum oxide and

silicon carbide

typical grit sizes between 300 and 600.

LAPPING

LAPPING


113/123


The lapping tool is called a lap,

it has the reverse of the desired shape of the

workpart.

The lap is pressed against the work and moved

back and forth over the surface

in a figure-eight or

other motion pattern,

subjecting all portions of the surface to the

same action.

sometimes performed by hand,

lapping machines accomplish the process with

greater consistency and efficiency.

LAPPING

LAPPING


114/123


Materials used to make the lap range from

steel and cast iron copper and

Lead

Wood

It is hypothesized that two alternative cutting

mechanisms are at work in lapping.

1.the abrasive particles roll and slide between

the lap and the work, with very small cuts

occurring in both surfaces.

2.the abrasives become embedded in the lap

surface and the cutting action is very similar

to grinding.

LAPPING

LAPPING


115/123


For laps made of soft materials, the embedded

grit mechanism is emphasized; and

for hard laps, the rolling and sliding mechanism

dominates.

LAPPING

Production Lapping


116/123


Manufacturing 4/e

Production Lapping

Figure 26.29 (a) Schematic illustration of the lapping process. (b) Production

lapping on flat surfaces. (c) Production lapping on cylindrical surfaces.

SUPERFINISHING


117/123


It is an abrasive process similar to honing.

Both processes use a bonded abrasive stick

moved with a reciprocating motion.

The two differs in:

1.the strokes are shorter, 5 mm;

2.higher frequencies, up to 1500 strokes per

minute;

3.lower pressures are applied between the

tool and the surface, below 0.28 Mpa

4.workpiece speeds are lower, 15 m/min or

less; and

5.grit sizes are generally smaller.

SUPERFINISHING

SUPERFINISHING


118/123

SUPERFINISHING

FIGURE 25.18Superfinishing on an external cylindrical surface.

SUPERFINISHING


119/123


The relative motion of the abrasive stick is

varied so that individual grains do not retracethe same path.

Cutting fluid is used to

cool the work surface

wash away chips

separate the abrasive stick from the work

surface after a certain level of smoothness

is achieved.

The result is mirror-like finishes (surface

roughness around 0.025 mm).

used to finish flat and external cylindrical

surfaces.

SU S G

Superfinishing Process


120/123


Manufacturing 4/e

p g

Figure 26.28 Schematic illustration of the superfinishing process for a cylindrical

part. (a) Cylindrical microhoning. (b) Centerless microhoning.

POLISHING


121/123


Used to remove scratches and burrs and to

smooth rough surfaces

abrasive grains are glued to the outside

periphery of the wheel

Rotate at high speedaround 2300 m/min.

The wheels are flexible and made of

canvas,

leather,

felt, and even paper;

POLISHING


122/123


After the abrasives have been worn down and

used up, the wheel is replenished with newgrits.

Grit sizes

20 to 80 for rough polishing,

90 to 120 for finish polishing, and

above 120 for fine finishing.

Polishing operations are often accomplished

manually.

BUFFING


123/123

Similar to polishing in appearance, but different

in function.

used to provide attractive surfaces with high

luster.

wheels materials similar to those used for

polishing wheels.

but buffing wheels are generally softer.

The abrasives are very fine and are contained

in a buffing compound.

usually done manually, (automatic machines

also available)

Date post:	12-Oct-2015
Category:	Documents
Upload:	mohtram1037
View:	7 times
Download:	0 times

manufacturing lecture

Documents