1. The document discusses various modes of failure and wear mechanisms in cutting tools. The major modes of failure are thermal cracking, mechanical chipping, and gradual wear. 2. Gradual wear includes crater wear on the tool face and flank wear on the side of the tool. Flank wear follows an S-curve as it progresses over time from initial rapid wear to uniform wear and finally destructive wear. 3. Mechanisms of tool wear are adhesive wear from bonding between surfaces, abrasive wear from hard particles plowing surfaces, diffusion wear from transfer of atoms between materials, and chemical wear from reactions between cutting fluids and tool materials.

1. Manufacturing Processes-2
SUBJECT CODE :4ME04
SEM 4
DEPARTMENT OF MECHANICAL ENGINEERING

2. * Unit-1( Theory of Metal Cutting)
BY MR.K.P.PAWAR
LECTURER
ANURADHA COLLEGE OF ENGINEERING

3. LECTURE -5
Objectives of Lecture
 To study various modes of tool failure
 To understand mechanisms of tool wear

4. 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

5. MODES OF FAILURE OF CUTTING TOOL
1. Thermal cracking
2. Mechanical Chipping
3. Gradual wear

6. Thermal cracking & softening
1.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.

7. 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

8. 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

9. 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

10. 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

11. Crater wear

12. 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

13. 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

14. 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.

15. Flank wear (S-Curve)

16. 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

17. Wear Mechanisms
1.Adhesive wear mechanism
2.Abrasive wear mechanism
3.Diffusion mechanism
4.Chemical wear

18. 2.Adhesion Wear

19. Adhesive 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.

20.  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.

21. Abrasive Wear

22. 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.

23. Diffusion 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

24. Diffusion Wear

25. 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.

26. Question & Answer Session
Q.1.What are various types of tool wears?
Q.4.What are mechanisms of tool wears ?