Grinding _ Mechanics Of_ Wear

8/11/2019 Grinding _ Mechanics Of_ Wear

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9 4 I

Mechanics of Grinding

Grinding

i ba

icallya chip removal process

in

which

the

cutting

rool

s

an individ

ual abra

ive grain . The following

are

major factor that differentiate

th

action of a

single grain from

that

of a single-point curring

roo1

(see Fig. 8.2) :

1 . TI1e individual grain ha an irregular geometry and is spaced randomly along

the

periphery

of

the

wheel (Fig. 9.6).

2. The average rake angle of the grains is highJy negative, typically - 60

0

or even

lower; consequently, the hear angles are very low (see Section 8.2.4).

3.

T he grains in the periphery of a grinding wheel have di fferent radia l

position

4. The

clIffing

speed of grinding

whee ls are ery high (Table

9.2),

typically

on

the

order o 30 mI .

An example of chip formation by an

abra

ive grain is hown

In

Fig. 9 7 Note

th

negati e

rak

angle, the

low shear

ang

le,

and

the very small size

of

the

ch

ip

(see

also

Example

9.1).

Grinding hip

are ea ily collt:cted

on

a piece

of adhe

ive tape

held against the

sparks

o a grinding wheel.

From

direct observation it wilJ be

noted

that

a ariery f metal chips

can

be obtained

in

grinding.

The mechanics of grinding and the variables involved can best be studied by

analyziog the

surface-grinding operatio

n shown

in

Fig. 9.8.

In

iliis figure, a

grinding

wheel

of

diameter D i removing a layer

of

metal at a

depth d known

as the wheel

depth

of

cut An individual

grain

on tbe periphery

of

the wheel

i

moving at a tangen

tial velocity V

uP or cOl1l1ention.al,

grinding

a hown

in Fi . 9 8 see also

milling

eetion

8.10.1),

an

the

workpiece

i

moving

at

a elociry

l l.

The

grain

s

removing a

ChlP with

an Imdeformed thickness (grain depth of

cut)

t and an

undeformed

l

ength

I

Fo

r the condition

of

II V, (he

tmdeformed-d1ip length I is

approximately

1==Ji5J.

(9.1 .

FIGURE

9 6 The grinding

urface

o an

abrasive wheel

( 46-j8V), howi ng gra in ' ,

porosiry,

wear

lacs

on

grains (see a lso Fig. 9.7b),

and meta l

chJPs

from the

work piece

adhering

ro rhe

grain .

Note the

random

dist

rib Irion and shape of

the abras iv

e

gr:1in

.

Magnificarion:

SO

x .

TABLE

9 2

Typical Ranges

of

Speeds nd Feeds

for

brasive Processes
http:///reader/full/cOl1l1ention.alhttp:///reader/full/cOl1l1ention.al


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Wear Itat

Workpiece

10loLm

a)

(b)

I

WorkpIece

FIGUR 9 7 (a) Grinding chip being produced by a single abrasive grain. Note the large

negative rake angle of the grain.

(b)

~ c h e m t i c illustration of chip formation

by

an

abrasive

gra in . Note

the

negative rake

ang

le, the sma ll

shear

angle, and the

wear

flat on the grain.

Source:

(a)

After M.E.

Mercham

Grains

________

FIGUR

9 8

Basic variables in

urface grinding. lo aemaJ grindin

operations, the wheel depth of cur,

d and contact

lengtb, I

are

much

smaller

than

the wheel diameter, D .

1.

The

dImension

t

called r

he

grain

d

depth

o

cut.

~

orkpiece

For

external

(cyli11drical)

grmdillg

(see Section 9.6),

Dd

1=

(9.2)

1

+(D1Dw)

and for

internal

grillding

Dd

- I )

9.3)

/ - l-(DID

where Dw is the diameter

of

the workpiece.

The

relationship between t and

other

process variables

can

be derived as fol

lows: Let C be the number of cutting points per unit area o wheel surface,

and

and

V

the surface speeds of the workpiece and rhe wheel, respectively (Fig:

9.8).

Assuming

the width

of

the workpiece to be uniry, the number of grinding chips produced per

un it time

is VC, and

the volume

of

material removed per unit time

is vd.

Letting

be

the

ratio

of

the chip vvidtb, w

to

the average chip rhickness, the


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Ridges FIGUR 9 9

Chip

formation

and

plowing (plastic deformation

without chip removal) of the

workpiece surface by an abrasive

grain.

The

specific energy

consumed

in

producing

a

grinding chip

consists of truee

components:

u =

UclUp

+

UpJowing

+

Usliding'

(9.7)

The

quantity uchip is the specific energy required for chip formation by plastic deforma

tion,

up

!nwmg

is

the

specific energy

required

for plowing,

which

is

plast

ic

deformation

without chip

removal (Fig. 9.9),

and the

last

term

Uslidrng' can best be understood by

observing the grain

in

Fig.

9.7b

.

The

grain develops a wear flat as

a

result of the

grinding operation (similar to flank wear in cutting tools; see Section 8.3).

The

wear

flat is in

contact

with the surface being ground and because

of

friction, requires en

ergy for sliding. The l

arger tbe

wear flat, the higher is the

grinding

fo rce.

Typical specific-energy

requirements

in

grinding are

given in Table 9.1.

Note

that

these energy levels

are much

higher

than

those in

cutting operations with

single

point tools, as given in Table 8.3. This difference has been attributed

to

the follow-

ing factors:

1. Size

effect As previously

stated

the size

of grinding

chips is very smail,

as

compared with chips produced in other cutting operations, by about two orders

of magnitude. As

described in Section 3.8.3 , the smaller the size

of

a piece

of

metal. the

higher

is its strength; consequently, grinding involves higher specific

energy than

machining operations

. Studies

have

indicated that extremely high

dislocation densities

(see

Section 3.3.3)

occur

in the shear

zone

during

chip

for

mation thus

influencing

the gtindmg

energies by virtue

of

increased strength.

2 . Wear flat A wear flat (see Fig. 9.7b) requires frictional energy for sliding;

this energy

contributes

significantly

to

the total energy

consumed. The

size of

the wear flat in grinding is much larger tllan

the grinding chip

unlike in metal

cutting by a single-p

oint

tool,

where

flank

wear land

is small

compared with the

size of the chip (see Section 8.3).

3. Chip morphology Because the average rake angle of a grain is highly nega

tive (see Fig. 9.7), the

shear strains

in

grinding

are very large. This indicates

that

the

energy

required

for plastic

deformation

to

produce

a grinding chip

i

higher than

in

other machining processes.

Furthermore

note

that

plowing

consumes energy

without contributing

to chip formation (see Fig. 9 .9).

EX MPLE

9.2 Forces in surface grinding


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TABLE 9 3

Approximate Specific Energy Requirements for Surface Grinding

Wurkpiece matfriaJ

llardnes

Sp

ci f

ic

energy

(W-s/mm )

Aluminum

1)0

HE

7-

27

Casr Iron

( loss

4U)

21S

HB

12-6

low-carbon

teel (1020)

IIOHB

14-68

Tiranium all }

(0 HB 16

5

Ii 1

steel T 15)

67HR

18- 82

We first determine th material remoyal rate as follow :

MRR = dWI) = (0.04)(20)(1200) = 600 mm

3

/min.

h ~

power

con

umed

I

given

b}

P wer =

(It)(J

tRR)

where

It

is the pel.:ific ent:'rgy.

a

btaineJ from Table 9, . For low-earb In steel,

ler' eSlimate

l t bt

41 W-s/mm

1.

Hence,

(

96

ower=(41)

6

= 6.56kWor6.56kJls.

Since power

IS

defined a

Power

= Tw

where

T i

[he torqu and equal to

(Fr ) DI2)

and w is the

rotational

'peed of the

wheel in d i n ~ per minute, w al

' 0

have

w

-

2rrN.

Thus

and

therefore, F

=

174 The thrust force, f , can be calculated by nOling from

experimental

dara

In the technicalliteramre. thar it IS

abour

30% higher [han the

urting force

F,.

Consequently

II

= (1.3)( J74)

=

226 N

9 4 2

Temperature

Temperature rise

in grinding is an important consideration because it can adversely

affect surface properties and cause residual tres es

on

the workp iece. Furthermore,

temperarure gradienrs in the

wor

kpiece cau e distortions

due to

thermal expansion

and contraction.

When

orne of the heat generated during grinding i conducted into

the workpiece, the heat xpands the

part

being ground rhu m king it difficult ro


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If we introducesizeeffectand assume that u varies inverselywith theundeformed

chipthickness t, then the temperarurerise is

> (V )12

Temperaturerisex D' 4d

4 - ; ; (9.9)

TIlepeak temperatures inchip generation duringgrindingcan be as highas

1923K. However, thetime involved in producingachip is extremelysh

ort

(on the

or

derof

mi

croseconds);hencemel tingofthechip

mayor

may

not

occ

ur

.Beca use,a

in

machining,thechipscarryawaymuchoftheheat generated (seeFig. S.18),on ly

asmall fraction oftheheat generated is conductedto the workpiece.Exper iments

indicatethat asmuchasone-half theener

gy

dissipated in grinding isconducted to

thechip,apercentagethat is higher

than

t hatinmachining(seeSection8.2).Onthe

ot

her

han

d,the heatgeneratedbyslidingand plowingis conductedmostly intothe

workpiece.

Sparks

The

sparksobserved ingrindingmetalsareactua

ll

yglowingchips .The

glowi

ng

occurs becauseof theexothermic reaction of the hot chipswithoxygen in

theatmosphere;sparkshavenot beenobservedwithanyme talgroundin

an

oxygen

freeenvironment.Thecolor, intens ity, andshape

of

dlesparksdependon thecom

po

sit

ionofthe meta lbei

ng

ground.If thebeargeneratedbytheexothermic reac tion

issufficiently h igh, thechipmaymeltand,becauseof surfacetension,solidifyasa

sh iny spherical particle. Observation of these particles under scanningelectron

microscopyhasrevealedthattheyarehollowandhaveafinedendriticstructure(see

Fi

g.

5.8

,

indicatingth

at

theywereoncemolten(byexothermicoxidarionofhot chips

inair )and theyresolidiiiedrapidly. Ithasbeensuggested

that

some

of

thespherica l

particlesmayalso be produced

by

plasticdef

or

m

ation

and roilingof chipsat me

grain-

wor

kpieceinterfaceduringgrinding.

9 4 3

ffects of temper ture

Themajoreffectsoftemperature

in

grindingare

1. Tempering Excessi

ve

temperature rise caused by grind ing can temper

(Se

ct

ion 5.11.5)and soften thesurfaces

of

steelcomponents,wh ichareoften

ground in the hear-treated and hardened state.Grindingprocessparameters

mustthereforebechosencarefuIJ

}

to avoidexcessivetempera tu

re

rise.Grinding

fl uids (Section9.6.9)can effectivelycontrol temperatures.

2 . Burning If the temperatureriseisexcessive,the

workp

iece

su

rfacemayburn .

Burning prod uces a bluish color onsteels, which indicatesoxidation at high

temperatures.Aburnmaynot beobjectionable

in

itself;however, thesurface

layersmayundergometa

ll

urgical transformations, withmartensiteformation

in high -c

arb

on steels from reaustenization followed

by

rapid c

oolin

g (see

Se

ction5.11).Thiseffect

is

knownasmetallurgical

u t

, whichis

an

espec ia

ll

y

seriousconcern withnickel-basealloys.

3. Heat

cheddng

High temperaturesingrindinglead to thermalstressesand

maycausemermalcracking

of

theworkpiecesurface,

known

asheat checking.

(See al

so

Section

5.10.3.)

Cracks are usually perpen

di

cular tothegrind ing

directi on. Under severe grinding cond itions, however, para llel

cracks

may


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Soluble oil 1 :20)

Highly sulfurized oil

5 KN0

2

solution

o 0.05

0.10 0.15

Depth below surface (mm)

(a)

c

o

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Grinding _ Mechanics Of_ Wear