Fundamentals of

Grinding

• Grinding is a process of material removal in the form of chips by the mechanical action of regularly shaped Abrasive Particles bonded together in the form of a wheel

• Grinding is required to achieve the following

– SIZE

– FORM

– FINISH

WHAT IS GRINDING & WHY IS IT REQUIRED?

• Grinding Wheel is a Multipoint cutting tool composed of Selectively Sized Abrasive Grains held together by a Bonding Material.

• A typical Grinding wheel consists of the following

>ABRASIVES - 40 - 45 %

>BONDING MATERIAL - 13 - 18 %

>VOIDS - 40 - 45 %

WHAT IS A GRINDING WHEEL ?

SLIDING , PLOWING, CHIP FORMATION

HOW GRINDING TAKES PLACE

WHEEL MARKING

SYSTEM

Abrasive

Type

Alu. Oxide

A, AA, RA

SA, DA

SiC

Black C

Green GC

Zirconia ZA

Abrasive

Type

Alu. Oxide

A, AA, RA

SA, DA

SiC

Black C

Green GC

Zirconia ZA

Grit Size

8, 10, 12, 14

16, 20, 24, 30

36, 46, 54, 60

80, 100, 120

150, 180, 220

240, 280, 320

400,600,800

Grit Size

8, 10, 12, 14

16, 20, 24, 30

36, 46, 54, 60

80, 100, 120

150, 180, 220

240, 280, 320

400,600,800

Grade

F G H I

J K L M

N O P Q

R S T U

V X Y Z

Grade

F G H I

J K L M

N O P Q

R S T U

V X Y Z

Grit

Combination

1

3

5

7

Grit

Combination

1

3

5

7

STRUCTURE

1 2 3 4 5

6

..............Upto

.....................3

5

STRUCTURE

1 2 3 4 5

6

..............Upto

.....................3

5

Bond System

V - Vitrified

B -

Resinoid

R - Rubber

Bond System

V - Vitrified

B -

Resinoid

R - Rubber

Cumi

Symbol

VG VVR V6

V7

V30 V10 VL

V8

V2018 VF8

VMPA VMSA

BN BR BFW

RT R RB

Cumi

Symbol

VG VVR V6

V7

V30 V10 VL

V8

V2018 VF8

VMPA VMSA

BN BR BFW

RT R RB

A 46 3 K 5 V 10

WHEELS MARKING SYMBOLS IN A CUMI GRINDING WHEEL

• A Mineral Crystal with Hardness much higher than that of work piece

WHAT IS AN ABRASIVE ?

TYPES OF ABRASIVES

• Natural

– Corundum, Emery, Diamond

• Synthetic

>Aluminium Oxide - Conventional Abrasive

>Silicon Carbide - Conventional Abrasive

>Cubic Boron Nitride - Super Abrasive

>Diamond - Super Abrasive

PROPERTIES OF ABRASIVES

• Friability - Ability to Fracture under load

• Thermal Conductivity - Ability to Conduct Heat

• Hardness - Resistance to Penetration

• Toughness - Ability to Withstand Load

Application of Conventional Abrasives

• Aluminium Oxide - For Ferrous Materials

• Silicon Carbide - For Non Ferrous Materials

TYPES OF ALUMINIUM OXIDE

Marking

• Brown Alumimium Oxide A

• While Aluminium Oxide AA

• Semifriable SA

• Combination of A & AA DA

• Pink Aluminium Oxide RA

• Sharp Crystalline Abrasive 80A

• Sintered Aluminium Oxide MSB

• Zirconia AZA

TYPES OF SILICON CARBIDE

Marking

• Black Silicon Carbide C

• Green Silicon Carbide GC

• Mixture of C & GC CGC

BROWN ALUMINIUM OXIDE(A)

• Brown in colour

• Discovered in 1899

• 95% pure

• Most commonly used abrasive

• Titanium di oxide imparts toughness

WHITE ALUMINIUM OXIDE(AA)

• White in colour

• 99% pure

• More friable than “A”

• Cooler cutting

• Better holding with the bond because of higher grain porosity

SEMIFRIABLE (SA)

• Brown in colour

• % Of titanium oxide is less than 2%

• Friability index falls between A & AA

PINK ALU. OXIDE (RA)

• Pink in Colour

• Alloyed Abrasive of Alu. Oxide and Chromic Oxide

• Retains the Sharpness for Longer time

ZIRCONIA ALU. OXIDE

• Produced by fusing zircon sand and alumina at about 1900 deg. C

• Alu. Oxide - 75% & zirconia - 23%

• Hard, very tough and sharp abrasives

SINTERED ALU. OXIDE(MSB)

• Micro crystalline abrasive

• Produced by sintering process

• Hard & tough

• Controlled fracture

BLACK SILICON CARBIDE(C)

• Black in colour

• Harder than alumina

• Chemically reactive with iron and steel alloys leading to poor attrition resistance

GREEN SILICON CARBIDE (GC)

• Green in colour

• Purer than black sic

• Cooler cutting

• More friable than black

GRAIN (GRIT) SIZE

• The size of screen opening thru which the grain pass

• Ex. 60 grit means there will be 60 openings per linear square inch of the screen

GRAIN COMBINATION

• Combination of different grit sizes in predetermined percentages

• Ex. A463 l5 V10 - combination of 46,54 & 60 grits. Primary grit 46 will be at higher percentage.

GRADE & STRUCTURE

• Grade

– Grade is the measure of the bonding strength of the wheel, usually referred to as hardness

• Structure

– Spacing of abrasive grains, measured in terms of volume of abrasives in grinding wheels

BOND

BINDING MATERIAL WHICH HOLDS THE ABRASIVE GRAINS TOGETHER IN A WHEEL.

BOND TYPES

• Vitrified bond (V)

• Resinoid bond (B)

• Rubber bond (R)

• Shellac bond (S)

• Oxychloride bond (Oxy)

• Magnesite bond

VITRIFIED BOND

Vitrification

• Bond mix melts /partly wets the abrasive

• Abrasive grain pulled together by surface tension

• Titanium rich phase from ‘A’ disbursed in to the surrounding matrix - it turns blue

VITRIFIED BOND

• More strength

• Water resistance

• Chemical resistance

• No shelf life

• Bond fractures induced by mechanical forces during grinding

• Very brittle

• Decreased bending strength

• Better form holding stability

VITRIFIED BONDS IN CUMIPrecision

V10/V10C/V45

V15/V1092/V2016

V8/VF8/V206/VMSA

V2018/VMTA/VMPA/

V60/V80/VL/VU869/

VMC/V204

VG/VMKRS/VB

VMKRP/VMKRC/V91

VMKRT

Non Precision

V6/V30/V7/V1734

VR/VVR

RESINOID BONDS

• Grains mixed with phenolic thermo-setting resins and plasticizers

• Baking at 150 to 200 deg. C

• High strength

• Ability to withstand shock loads

• Shelf life 1 - 2 years

• Low chemical & thermal resistance

RESINOID BONDS IN CUMI

PRECISION

BRT/TD924/TD1020/TD1071/B14/B14X/B8/BHK/BYZ/ BHP/RBG

NON PRECISION

BR/BM4/BM6/B754/BC4/BFW/BSN821/ TD995/TD722/BN/TD1035B/RS500/TDR014/ TDR015/BYZ

PROPERTIES OF RUBBER BONDS

• Better resiliency

• Absorbs shocks

• Produces good finish

RUBBER BONDS IN CUMI

RT1

R

RB

R188M

RCS

A) Grain Fracture

B) Bond Fracture

C) Attritious Wear

WHEEL WEAR

TYPES OF WHEEL WEAR

• GRAIN FRACTURE

TYPES OF WHEEL WEAR

• BOND FRACTURE

ATTRITIOUS WEAR

• Dulling of Abrasive Grains and Growth of Wear Flats by Rubbing against the Work Piece

TYPES OF WHEEL WEAR

WHEEL SELECTION

WHEEL SELECTION

FACTORS AFFECTING WHEEL SELECTION

�MATERIAL TO BE GROUND

�STOCK TO BE REMOVED

�WHEEL SPEED

�AREA OF GRINDING

�TYPE OF OPERATION - WET (OR) DRY

�GRINDING MACHINE

�SURFACE FINISH

GRIT SIZE SELECTION

• SURFACE FINISH

• STOCK REMOVAL

• FORM ACCRACY

GRIT SIZE SELECTION

GRIT SIZE Vs SURFACE FINISH

CLA Ra 60 80 120 180 240 320 400

24.0 0.6

20.0 0.5

16.0 0.4

12.0 0.35

10.0 0.25

8.0 0.2

6.3 0.16

4.0 0.1

3.2 0.08

2.0 0.05

1.2 0.03 osc osc

0.8 0.02 osc

GRIT SIZE SELECTION

GRIT SIZE Vs FORM

GRIT SIZE 60 80 120 180 240 320 400

CORNER RADIUS 0.5 0.4 0.2 0.13 0.1 0.08 0.05

�MATERIAL HARDNESS

�ARC / AREA OF CONTACT

�TYPE OF OPERATION

�FORM HOLDING

�WHEEL SPEED

�WHEEL WIDTH

�TYPE OF DRESSER

�TYPE OF COOLANT

GRADE SELECTION

GRADE SELECTION

MATERIAL HARDNESS

Wheel Grade

M

L

K

J

I

Soft Medium Hard50 Rc 50 - 57 Rc 58 - 63 Rc

STRUCTURE SELECTION

• STOCK REMOVAL

• THERMAL CONDUCTIVITY OF WORK MATERIAL

• MATERIAL REMOVAL RATE

STRUCTURE SELECTION

• STOCK REMOVAL

STRUCTURE SELECTION

• MATERIAL REMOVAL RATE

BOND SELECTION

• SEVERITY OF OPERATION

• WHEEL SPEED

• FORM HOLDING

• SURFACE FINISH

• MATERIAL REMOVAL RATE