Lecture 5

Manufacturing Processes-2SUBJECT CODE :4ME04

DEPARTMENT OF MECHANICAL ENGINEERING

SEM 4

BY MR.K.P.PAWAR

LECTURER

ANURADHA COLLEGE OF ENGINEERING

* Unit-1( Theory of Metal Cutting)

LECTURE -5

Objectives of Lecture

To study various modes of tool failure

To understand mechanisms of tool wear

Tool Failure A properly designed and ground cutting tool is expected to

perform the metal cutting operation in an effective smooth manner.

If ,however, it is not giving a satisfactory performance it is

indicative of the tool failure and the same is reflected by the

following adverse effects observed during the operation:

1. Extremely poor surface finish on the work piece

2. Higher consumption of power

3. Work dimensions not being produced as specified

4. Overheating of cutting tool

5. Appearance of a burnishing band on the work surface

MODES OF FAILURE OF CUTTING TOOL

1. Thermal cracking

2. Mechanical Chipping

3. Gradual wear

Thermal cracking & softening1.Due to high heat the tool tip and area closer to the cutting edge

becomes very hot

2.Although the cutting tool material is quite hard to withstand this ,still

every tool material has a certain limit to which it can withstand and the

elevated temp. without losing its hardness

3.If that limit crossed, the tool material starts deforming plastically at the

tip and adjacent to the cutting edge under the action of the cutting

pressure & the high temp.

4.Thus the tool loses its cutting ability and is said to have failed

due to softening

5.The main factors responsible for creating such conditions of

tool failure are :

a.Cutting speed

b.High feed rate

c.Excessive depth of cut

d.Smaller nose radius

e.Choice of a wrong tool material

6.On account of fluctuations in temperature and severe temp.

gradients the tool material is subjected to local expansion and

contraction

7.This give rise to the setting up of temp stresses or thermal stress

due to which cracks are developed in the material

8.These cracks known as thermal cracks& they extends in-words

Mechanical chipping

Mechanical chipping of the nose or the cutting edge of the tool

are commonly observed causes of tool failure.

The common reasons for such failure are

Too high cutting pressure,

Mechanical impact,

Excessive wear ,

Too high vibration & chatter,

Weak tip & cutting edge etc.

A typical form of mechanical chipping is shown in fig

This type of failure is more pronounced is carbide tipped and

diamond tool due to the high brittleness of tool material

Gradual wear

When a tool is in use for sometime it is found to have lost

some weight or mass, implying that it has lost some material

from it, which is due to wear

The following two types of wears are generally found to occur

in cutting tools:

1.Crater wear

2.Flank wear

Crater wear

Crater wear The principle region where wear takes place in a cutting tool is its

face, at a small distance from it’s cutting edge.

This type of wear generally take places while machining ductile

materials like steel & steel alloys, in which continuous chip is

produced

The resultant feature of this type of wear of a crater at the tool

chip interface

This type of wear, or the formation of crater on the tool face is

due to the pressure of the hot chip sliding up the face of the tool

The metal from the tool face is supposed to be transferred to the

sliding chip by means of the diffusion process

Flank wear Another region where an appreciable amount of wear occurs is

the flank below the cutting edge.

It occurs due to abrasion between the tool flank & the work

piece and excessive heat generated as a result of the same

The abrasion action is aided by the hard micro constituents of

the cut material provide a lot of abrasive material readily

Flank wear

The entire area subjected to flank wear is known as wear land.

This type of wear mainly occurs on the tool nose, front & side

relief faces.

The magnitude of this wear mainly depends on the relative

hardness of the work piece and tool materials at the time of

cutting operation.

When the tool is subjected to this type of wear ,the work piece

loses its dimensional accuracy, energy consumption is increased

and the surface finish is poor.

Flank wear (S-Curve)

Flank wear (S-Curve)

The total flank wear consists of three main components,drawn

between the wear land height (VB) and time(t).

The first component (A), which exits for a small duration,represents

the period during which initial wear take place at a rapid rate.

The second segment (B) a rapid rate. The second segment which

exits for a very long duration,represents the period during which the

wear progress uniformly.

The last segment(C) represents the region in which wear occurs at a

very rapid rate and results in total failure of the tool.This region is

known as the period of destructive wear

Wear Mechanisms

1.Adhesive wear mechanism

2.Abrasive wear mechanism

3.Diffusion mechanism

4.Chemical wear

2.Adhesion Wear

when two surfaces are brought together under load, asperities of the

two surfaces adhere to each other.

The conditions at the interface of these junctions are similar to those

of a cold weld.

A strong bond is formed but without much inter-diffusion of atoms

and recrystallization as would occur in a hot weld.

During sliding, these junctions are sheared. Shearing may occur at

the interface or within one of the two asperities.

Adhesive Wear

Most junctions shear at the interface, but occasionally shearing

will occur in one of the two materials. This will result in a wear

fragment being transferred from one surface to the other.

Then the break may occur in the softer material occasionally in

the harder material should it contain a local weak spot.

Some junctions may be stronger than the base metal itself

because of plastic yielding and work hardening.

In the normal process of adhesive wear, there will be some

transfer of particles from one surface to the other.

Some particles may be transferred back to the original surface or

break off as loose wear particles.

Abrasive Wear

Abrasive wear occurs when either a rough, hard surface or a

soft surface with hard particles embedded in its surface slides

over a softer material.

A plowing action takes place.

When abrasive wear is the result of loose wear particles and

contaminants, it is called three-body abrasive wear.

Abrasive Wear

•When a metal is in sliding contact with another metal the temp. at

the interface is high

•The high temperature allows the atoms of hard material to diffuse

into softer material matrix

•Hence the strength and abrasiveness of the softer material Increase.

Atoms of the softer metal may also diffuse into harder medium, thus

weakening the surface of harder material

•Diffusion phenomenon is strongly dependent on temperature

Diffusion Wear

Diffusion Wear

Chemical Wear

This type wear occurs when such a cutting fluid is used in the

process of metal cutting which is chemically active to the

material of the tool.

This is clear the result of chemical reaction tacking places

between the cutting fluid and the tool material, leading to a

change in the chemical composition of the surface material of

tool.

Question & Answer Session

Q.1.What are various types of tool wears?

Q.4.What are mechanisms of tool wears ?