TRIBOLOGICAL STUDY OF CUTTING TOOLS

1. TRIBOLOGICAL STUDY OF
CUTTING TOOLS
CR4102: TRIBOLOGY OF MATERIALS
M Lakshmi (118CR0131)
Department of Ceramic Engineering
Course mentor:-Prof.Debasish Sarkar

2. TRIBOLOGY
The term tribology comes from the Greek word tribos, meaning friction, and logos, meaning law.
Tribology is therefore defined as ‘‘the science and technology of interactive surfaces moving in
relation to each other.’
Metal cutting is described as a forming process that occurs in the components of the cutting
system that are arranged in such a way that the external energy provided to the cutting system
produces the intentional fracture of the layer being removed.
Goals to be achieved:
• Increased tool life,
• Improved integrity of the machined surface
• higher process efficiency and stability
CUTTING TOOL TRIBOLOGY SYSTEM
These groups are related to each other does not
work individually
The cutting tool
tribology's major goal is
to improve the
tribological parameters:
• At the tool–chip
interfaces
• Tool–workpiece
interfaces

3. PRINCIPLE:
DISTINGUISHING FEATURES OF METAL CUTTING TOOLS
 Bending moment-The bending moment creates a combined tension in the deformation
zone, greatly reducing the work material's resistance to cutting.
 Under combined stress, purposeful (micro) fracture of the layer being removed-Each
consecutive cycle of chip development results in a fracture. The fracture strain, and thus
the energy, is greatly affected by stress triaxiality in the deformation zone.
 Stress singularity at the cutting edge- when compared to other closely similar forming
procedures, the maximum combined stress does not act at the cutting edge. Rather, in
front of the cutting edge, a (micro) crack occurs.
 Cyclical nature-Metal cutting is a cyclic process by nature. As a result, each chip
manufacturing cycle produces only one chip fragment..

4. GEOMETRY
The tool-chip interface consists of the parts,
namely plastic and elastic.
The length of the plastic part of the tool-chip
interface:
TRIBOLOGICAL CONDITIONS AT THE INTERFACE
A.TOOL-CHIP INTERFACE
The basic tribological characteristics of the tool–chip interface are:
• Contact length
• Relative velocity of the counter bodies
• Friction force
• The specific frictional force/mean shear stress
• Distribution of the shear stress over the contact length
• Normal force
• Mean normal stress
• Distribution of the normal stress over the contact length
• Mean contact temperature

5. SYSTEM CONSIDERATION OF THE CHIP FORMATION PROCESS
IN METAL CUTTING
Phase 1-When the tool comes into contact with the
workpiece, the cutting force Fc causes a deformation
zone to form in front of the cutting edge.
Phase 3- When full contact is established, the condition of stress in front of the tool becomes
more complicated, involving a mix of bending and compressive pressures. The cutting force Fc
increases the size of the deformation zone and the maximum stress.
Phase 2- With a cutting speed of v, the tool goes
forward.. As a result, an elastoplastic zone forms ahead
of the tool, allowing it to penetrate deeper into the
workpiece.

6. Phase 5-The resistance to tool penetration diminishes as the deformation zone slides,
resulting in a reduction in the size of the plastic section of the deformation zone. The structure
of the work material, which has been deformed plastically and is now returning to its original
state, is, nevertheless, different from that of the original material.
interface.
Phase 6 - When a new section of the work material comes into touch with the tool rake face,
the chip fragment continues to slide until the force acting on it from the tool is reduced. Part
of the cutting force Fc is drawn to this new section. As a result, the stress along the sliding
surface decreases until it reaches the limiting stress, at which point the sliding is stopped. A
new chip fragment begins to form.
Phase 4- At the beginning of chip formation, a partially formed chip starts to slide with a
velocity vch1 relative to the tool rake face. As soon as the sliding surface forms, all the chip-
cantilever material begins to slide along this surface with a velocity vch2.

7. STRESS DISTRIBUTION AND MEAN
Fig: Dynamic analysis of the shear and normal stress distribution over the tool-chip interface
Fig a: Shear (tangential) distributions
• Stress distributions at the
beginning of a new chip formation
cycle
• The maximum normal stress
becomes higher when increasing
the rake angle.
• Moreover, the location of this
maximum shifts towards the
cutting edge
Fig b: Normal stress distributions
• Suggests the existence of two
zones (plastic and elastic)

8. CONCLUSIONS FROM THE ABOVE GRAPH AFTER
ANALYSIS:
1. The contact length is reduced when the rake angle is increased, resulting in a rise in the
maximum shear and normal stresses at this interface.
• Furthermore, the maximum normal stress is shifted to the cutting edge.
• Because the use of high positive tool geometry (high rake angles) has recently become a
new trend that has been widely adopted by leading tool manufacturers
• The use of cutting tool with high rake angles was made possible by the application of sub-
and nano-grain carbides
2. The high scattering in the results of applying various coatings can be explained by the high
frequency changes of normal and shear stresses. Obviously, these variations should be factored
into the design of any coating.
Good correlation is shown by the empirical relation :

9. CONTRADICTORY RELATION OF MEAN CONTACT
TEMPERATURE WITH SHEAR STRESS AT THE TOOL CHIP
INTERFACE
 Among other things, the independence of this stress from the mean contact temperature at
the tool-chip interface is suprising and seems contradictory.
 It was explained by mutual influence of two reverse-proportional factors:
• Strain rate
• Temperature
The strain rate effect balances the influence of temperature on the mean shear stress at the
tool-chip interface, ensuring that this stress remains constant.

10. Fig a: Typical distribution of heat in the workpiece,
the tool, and the chips with cutting speed; results
by Schmidt and Roubik
Fig b: Heat distribution between the
chip, workpiece and tool
TEMPERATURE DISTRIBUTION
 Fig b is more qualitative than quantitative nature
 In both the figures under, most of the thermal energy generated in the cutting process is
conducted into the chip ‘normal’ cutting conditions.

11. The heat balance equation is given by:
where = total thermal energy (heat)
generated in the cutting process
Qc = thermal energy transported by the chip
Qw = thermal energy conducted into the
workpiece
Qt = thermal energy conducted into the tool.
 Heat partition in metal cutting is application dependent, and the percentage of
heat that goes into the various components of the cutting system is not fixed.
 Drawback of the graphs:The partition of heat is always shown as a function of the cutting
speed. The cutting feed, thermal properties of the work and tool materials, influence of
MWF and many other ‘thermal’ particularities of a given machining operation are not
taken into consideration.

12. • The moving chip and stationary tool make up the tool-chip interaction. The plastic
deformation of the layer being removed, the material of the chip at this contact is
extremely strain-hardened work material of high temperature.
RESULTS OF TEMPERATURE CONSIDERATION
• The chip's temperature is not consistent. The severely distorted contact layer has the
maximum temperature, whereas the so-called chip free surface has a significantly lower
temperature.
• The heat trapped in this layer, along with friction at the tool-chip contact, results in a high
temperature at this point.
• At the end of the plastic section of the tool chip contact, which corresponds to the maximum
extent of the plastic deformation zone of the chip contact layer generated by the tool rake
face, the temperature on the too chip interface is the highest.

13. B.TOOL-WORKPIECE INTERFACE
Normal and shear stress distributions at the
tool–workpiece interface during lead
turning with various uncut chip thicknesses,
where x is the distance from the cutting
edge.
 The normal and shear stress distributions have
maximum values in the vicinity of the cutting
edge.
 The amount of stresses then stabilises during
the contact length, eventually reaching zero at
the contact's conclusion.
 The higher the amount of contact stresses near
the end of the contact, when both stresses
may have second maxima, the smaller the
curvature of the workpiece surface.
 The plastic section of the contact has no or a
very tiny area.
 The area next to the cutting edge experiences
adhesion, whereas the rest of the contact
experiences simple friction.
NORMAL AND SHEAR STRESS DISTRIBUTIONS

14. Tool wear is considered as a gradual process.
Two basics zones of wear in cutting tools:
 flank wear
 crater wear
TOOL WEAR
General mechanisms that cause tool wear:
(1) Abrasion
(2) Diffusion
(3) Oxidation
(4) Ffatigue
(5) adhesion
GRADES OF TOOL MATERIALS
The three general properties of a tool material are:
• Hardness
• Toughness
• Wear resistance

15. Fig a: Shows the hardness of typical tool materials
as a function of temperature
Fig b: Shows that, for tool materials,
hardness and toughness change in opposite
directions. Increased toughness while
keeping high hardness is a prominent trend
in tool material development.
a. Hardness Vs Temperature b. Hardness vs Toughness
c. Bending strength Vs Hardness
Fig c: Shows HSS has the following advantages
Great bending strength
a sharp cutting edge can be achieved
less work hardening
greater tolerance for non-rigid machining system

16. Any alteration of tribological conditions in a cutting tool that results in at least
one of the following is considered an improvement:
CUTTING TOOL TRIBOLOGICAL CONDITIONS
IMPROVEMENTS
 Improved tool life
 Increased productivity of a given operation by allowing for a larger material
removal rate, which is directly proportional to the cutting speed, feed, and
cut depth.
 Increased efficiency of a given operation