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February 5th, 2015

MECH 360

Principles of Manufacturing

Lecture 10: Abrasive machining and finishing operations

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Outline

Grinding

Related abrasive and finishing operations

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Abrasive Machining

Material removal by action of hard, abrasive particles

usually in the form of a bonded wheel

Typical abrasive grains

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Abrasive Machining

Abrasive machining:

Generally used as finishing operations after part geometry

has been established by conventional machining

Grinding is most important abrasive process

Other abrasive processes: honing, lapping, superfinishing,

polishing, and buffing

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Abrasive Machining

Why Abrasive Processes are Important

Can be used on all types of materials

Some can produce extremely fine surface finishes, to

0.025 m (1 -in)

Some can hold dimensions to extremely close tolerances

Grinding

Material removal process in which abrasive particles are

contained in a bonded grinding wheel that operates at very

high surface speeds

Grinding wheel is usually disk-shaped and precisely balanced

for high rotational speeds

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Grinding

Different types of grinding operations 6

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Grinding

The Grinding Wheel

Consists of abrasive particles and bonding material

Abrasive particles accomplish cutting

Bonding material holds particles in place and establishes shape

and structure of wheel

Attritious wear -- the dulling of the cutting edges of a sharp

grain caused by rubbing

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Grinding

The Grinding Wheel

Superabrasive wheels

Conventional grinding wheels

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Grinding

Grinding Wheel Parameters Abrasive material

Grain size

Bonding material

Wheel grade

Wheel structure Abrasive Material Properties High hardness harder than conventional cutting tool material

Wear resistance

Toughness

Friability - capacity to fracture when cutting edge dulls, so a

new sharp edge is exposed

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Grinding

Traditional Abrasive Materials

Aluminum oxide (Al2O3) - most common abrasive

Used to grind steel and other high-strength alloys

Silicon carbide (SiC) - harder than Al2O3 but not as

tough

Used on aluminum, brass, stainless steel, some cast irons

and certain ceramics

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Grinding

Newer Abrasive Materials Abrasive material Knoop hardness

Aluminum oxide 2100

Silicon carbide 2500

Cubic boron nitride 5000

Diamond (synthetic) 7000

Diamond Even harder, very expensive

Occur naturally and also made synthetically

Not suitable for grinding steels

Used on hard, abrasive materials such as ceramics, cemented carbides, and glass

Cubic boron nitride (CBN) very hard, very expensive

Suitable for steels

Used for hard materials such as hardened tool steels and aerospace alloys

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Grinding

Grain size Small grit sizes produce better finishes

Larger grit sizes permit larger material removal rates

Harder work materials require smaller grain sizes to cut

effectively

Softer materials require larger grit sizes

Measurement of Grain Size

Grit size is measured using a screen mesh procedure

Smaller grit sizes indicated by larger numbers in the screen

mesh procedure and vice versa

Grain sizes in grinding wheels typically range between 8 (very

coarse) and 250 (very fine)

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Grinding

Measurement of Grain Size

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Grinding

Bonding Material Properties

Must withstand centrifugal forces and high temperatures

Must resist shattering during shock loading of wheel

Must hold abrasive grains rigidly in place for cutting yet

allow worn grains to be dislodged to expose new sharp

grains

Bonding Materials

Vitrified bond Resinoid

Thermoplastic Rubber

Metal

Reinforced one or more layers of fiberglass mats

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Grinding

Wheel Structure

Refers to the relative spacing of abrasive grains in wheel

In addition to abrasive grains and bond material, grinding

wheels contain air gaps or pores

Volumetric proportions of grains, bond material, and pores

can be expressed as:

0.1 pbg PPP

Open structure small,

large

gP

pP

Dense structure large,

small

gP

pP

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Grinding

Wheel grade

Indicates bond strength in retaining abrasive grains

during cutting

Depends on amount of bonding material in wheel structure (Pb)

Measured on a scale from soft to hard

Soft wheels lose grains readily - used for low material

removal rates and hard work materials

Hard wheels retain grains - used for high stock removal rates

and soft work materials

Standard grinding wheel marking system used to designate

abrasive type, grit size, grade, structure, and bond material

Example: A-46-H-6-V

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Grinding

Wheel grade

Grinding is a chip-removal process that uses an individual

abrasive grain as the cutting tool

Grain force is proportional to the process variables:

ForceGrain

D

d

V

v

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Grinding

Grinding process

Material Removal Rate in grinding

dwvRR M

Energy dissipated in producing a grinding chip consists

of energy required from:

1. Chip formation

2. Plowing

3. Friction

Specific Energy

Specific Energy energy per unit volume of material ground

from the workpiece surface, u, Ws/mm3

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Grinding

EXAMPLE

Forces in Surface Grinding

A surface-grinding operation is being performed on low-

carbon steel with a wheel of diameter D=250 mm that is

rotating at N=4000 rpm and a width of cut of w=25 mm.

For carbon steel wheel, the specific energy is 40 Ws/mm3.

The depth of cut is d=0.05 mm and the feed rate of the

workpiece, is 1.5 m/min. Calculate the cutting force.

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Grinding

Solution

Forces in Surface Grinding

The material-removal rate (MRR) is

The power consumed is

Noting that 1 W = 1 Nm/s = 60 Nm/min

Power=1250W = 75000 Nm/min

Since power is

N 8.23400025.075000

cc

cc

FF

DNFVFPower

Wmm

smm

sWMRRuPower 1250

min1875

60

min140

3

3

min/10*875.1min/mm 187515002505.0 363 mdwvMRR

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Grinding

Temperature rise in grinding is important as it can:

1. Adversely affect the surface properties

2. Cause residual stresses on the workpiece

3. Cause distortions due to thermal expansion and

contraction of the workpiece surface

Surface-temperature rise in grinding is

Temperature increases with increasing d, D, and V

2/1

4/34/1

v

VdDT

Temperature

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Grinding

Wheel wear

Attritious Grain Wear

Similar to flank wear in cutting tools

Cutting edges become dull and develop a wear flat

Selection of abrasive is based on the reactivity of the

grain, workpiece hardness and toughness

Grain Fracture

The grain should fracture at a moderate rate

So that new sharp cutting edges are produced

continuously during grinding

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Grinding

Grinding Ratio

Grinding ratio is defined as

Higher the force, greater the tendency for the grains to

fracture

Higher the wheel wear, lower the grinding ratio

wear wheelof Volume

removed material of VolumeG

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Grinding

Dressing, Truing, and Shaping of Grinding Wheels Dressing Resharpening the wheel

Functions:

Break off dulled grits to expose new sharp grains

Remove chips clogged in wheel

Truing Use of a diamond-pointed tool fed slowly and

precisely across wheel as it rotates

Very light depth is taken (0.025 mm or less) against the wheel

Not only sharpens wheel, but restores true disk shape and insures

straightness across outside perimeter

Shaping Use of a diamond-pointed tool fed slowly and

precisely to shape the grinding wheel into the designed form

Used for forming the desired workpiece gometry

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Grinding

Surface grinding operations and machines (a) Horizontal spindle with reciprocating

worktable, (b) horizontal spindle with rotating

worktable

(c) Vertical spindle with reciprocating

worktable, and (d) vertical spindle with rotating

worktable

Surface grinder with

horizontal spindle and

reciprocating worktable

(most common grinder

type)

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Other finishing operations

Honing Abrasive process performed by a set of bonded abrasive sticks

using a combination of rotational and oscillatory motions

Common application is to finish the bores of internal combustion engines

Grit sizes range between 30 and 600

Surface finishes of 0.12 m (5 -in) or better

Creates a characteristic cross-hatched surface that retains lubrication

(a) Honing tool used for internal bore surface,

(b) cross-hatched pattern created by the

action of honing tool

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Other finishing operations

Lapping

Uses fluid suspension of very small abrasive particles between

workpiece and lap (tool)

Lapping compound - fluid with abrasives, general appearance of a chalky

paste

Typical grit sizes between 300 to 600

Applications: optical lenses, metallic bearing surfaces, gages

The lapping process

in lens-making

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Other finishing operations

Superfinishing

Similar to honing - uses bonded abrasive stick pressed against

surface and reciprocating motion

Differences with honing:

Shorter strokes

Higher frequencies

Lower pressures between tool and surface

Smaller grit sizes

Superfinishing on an

external cylindrical

surface

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