Machinability : Failure of cutting tools and tool life Mechanical Engineering Notes | EduRev

Mechanical Engineering : Machinability : Failure of cutting tools and tool life Mechanical Engineering Notes | EduRev

 Page 1


 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Module  
         3 
Machinability 
Version 2 ME IIT, Kharagpur 
 
Page 2


 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Module  
         3 
Machinability 
Version 2 ME IIT, Kharagpur 
 
 
 
 
 
 
 
 
 
 
Lesson 
14 
Failure of cutting tools 
and tool life 
 
Version 2 ME IIT, Kharagpur 
 
Page 3


 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Module  
         3 
Machinability 
Version 2 ME IIT, Kharagpur 
 
 
 
 
 
 
 
 
 
 
Lesson 
14 
Failure of cutting tools 
and tool life 
 
Version 2 ME IIT, Kharagpur 
 
 Instructional objectives  
 
At the end of this lesson, the students will be able to  
 
(i) State how the cutting tools fail 
(ii) Illustrate the mechanisms and pattern of tool wear 
(iii) Ascertain the essential properties of cutting tool materials 
(iv) Define and assess tool life 
(v) Develop and use tool life equation. 
 
 
(i)  Failure of cutting tools 
 
Smooth, safe and economic machining necessitate 
• prevention of premature and catastrophic failure of the cutting tools 
• reduction of rate of wear of tool to prolong its life 
To accomplish the aforesaid objectives one should first know why and how the 
cutting tools fail. 
Cutting tools generally fail by : 
i) Mechanical breakage due to excessive forces and shocks. Such kind of tool 
failure is random and catastrophic in nature and hence are extremely 
detrimental. 
ii) Quick dulling by plastic deformation due to intensive stresses and 
temperature. This type of failure also occurs rapidly and are quite 
detrimental and unwanted. 
iii) Gradual wear of the cutting tool at its flanks and rake surface. 
The first two modes of tool failure are very harmful not only for the tool but also for 
the job and the machine tool. Hence these kinds of tool failure need to be prevented 
by using suitable tool materials and geometry depending upon the work material and 
cutting condition. 
But failure by gradual wear, which is inevitable, cannot be prevented but can be 
slowed down only to enhance the service life of the tool. 
The cutting tool is withdrawn immediately after it fails or, if possible, just before it 
totally fails. For that one must understand that the tool has failed or is going to fail 
shortly. 
It is understood or considered that the tool has failed or about to fail by one or more 
of the following conditions : 
(a)  In R&D laboratories 
• total breakage of the tool or tool tip(s) 
• massive fracture at the cutting edge(s) 
• excessive increase in cutting forces and/or vibration 
• average wear (flank or crater) reaches its specified limit(s) 
(b)  In machining industries 
• excessive (beyond limit) current or power consumption 
• excessive vibration and/or abnormal sound (chatter) 
• total breakage of the tool 
• dimensional deviation beyond tolerance 
• rapid worsening of surface finish 
• adverse chip formation. 
Version 2 ME IIT, Kharagpur 
 
Page 4


 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Module  
         3 
Machinability 
Version 2 ME IIT, Kharagpur 
 
 
 
 
 
 
 
 
 
 
Lesson 
14 
Failure of cutting tools 
and tool life 
 
Version 2 ME IIT, Kharagpur 
 
 Instructional objectives  
 
At the end of this lesson, the students will be able to  
 
(i) State how the cutting tools fail 
(ii) Illustrate the mechanisms and pattern of tool wear 
(iii) Ascertain the essential properties of cutting tool materials 
(iv) Define and assess tool life 
(v) Develop and use tool life equation. 
 
 
(i)  Failure of cutting tools 
 
Smooth, safe and economic machining necessitate 
• prevention of premature and catastrophic failure of the cutting tools 
• reduction of rate of wear of tool to prolong its life 
To accomplish the aforesaid objectives one should first know why and how the 
cutting tools fail. 
Cutting tools generally fail by : 
i) Mechanical breakage due to excessive forces and shocks. Such kind of tool 
failure is random and catastrophic in nature and hence are extremely 
detrimental. 
ii) Quick dulling by plastic deformation due to intensive stresses and 
temperature. This type of failure also occurs rapidly and are quite 
detrimental and unwanted. 
iii) Gradual wear of the cutting tool at its flanks and rake surface. 
The first two modes of tool failure are very harmful not only for the tool but also for 
the job and the machine tool. Hence these kinds of tool failure need to be prevented 
by using suitable tool materials and geometry depending upon the work material and 
cutting condition. 
But failure by gradual wear, which is inevitable, cannot be prevented but can be 
slowed down only to enhance the service life of the tool. 
The cutting tool is withdrawn immediately after it fails or, if possible, just before it 
totally fails. For that one must understand that the tool has failed or is going to fail 
shortly. 
It is understood or considered that the tool has failed or about to fail by one or more 
of the following conditions : 
(a)  In R&D laboratories 
• total breakage of the tool or tool tip(s) 
• massive fracture at the cutting edge(s) 
• excessive increase in cutting forces and/or vibration 
• average wear (flank or crater) reaches its specified limit(s) 
(b)  In machining industries 
• excessive (beyond limit) current or power consumption 
• excessive vibration and/or abnormal sound (chatter) 
• total breakage of the tool 
• dimensional deviation beyond tolerance 
• rapid worsening of surface finish 
• adverse chip formation. 
Version 2 ME IIT, Kharagpur 
 
 
(ii)  Mechanisms and pattern (geometry) of cutting tool wear 
 
For the purpose of controlling tool wear one must understand the various 
mechanisms of wear, that the cutting tool undergoes under different conditions. 
The common mechanisms of cutting tool wear are : 
i) Mechanical wear 
• thermally insensitive type; like abrasion, chipping and delamination 
• thermally sensitive type; like adhesion, fracturing, flaking etc. 
ii) Thermochemical wear 
• macro-diffusion by mass dissolution 
• micro-diffusion by atomic migration 
iii) Chemical wear 
iv) Galvanic wear 
In diffusion wear the material from the tool at its rubbing surfaces, particularly at the 
rake surface gradually diffuses into the flowing chips either in bulk or atom by atom 
when the tool material has chemical affinity or solid solubility towards the work 
material. The rate of such tool wear increases with the increase in temperature at the 
cutting zone. 
Diffusion wear becomes predominant when the cutting temperature becomes very 
high due to high cutting velocity and high strength of the work material. 
Chemical wear, leading to damages like grooving wear may occur if the tool material 
is not enough chemically stable against the work material and/or the atmospheric 
gases. 
Galvanic wear, based on electrochemical dissolution, seldom occurs when both the 
work tool materials are electrically conductive, cutting zone temperature is high and 
the cutting fluid acts as an electrolyte. 
The usual pattern or geometry of wear of turning and face milling inserts are typically 
shown in Fig. 3.2.1 (a and b) and Fig. 3.2.2 respectively. 
 
K
T 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 3.2.1 (a) Geometry and major features of wear of turning tools 
 
 
Version 2 ME IIT, Kharagpur 
 
Page 5


 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Module  
         3 
Machinability 
Version 2 ME IIT, Kharagpur 
 
 
 
 
 
 
 
 
 
 
Lesson 
14 
Failure of cutting tools 
and tool life 
 
Version 2 ME IIT, Kharagpur 
 
 Instructional objectives  
 
At the end of this lesson, the students will be able to  
 
(i) State how the cutting tools fail 
(ii) Illustrate the mechanisms and pattern of tool wear 
(iii) Ascertain the essential properties of cutting tool materials 
(iv) Define and assess tool life 
(v) Develop and use tool life equation. 
 
 
(i)  Failure of cutting tools 
 
Smooth, safe and economic machining necessitate 
• prevention of premature and catastrophic failure of the cutting tools 
• reduction of rate of wear of tool to prolong its life 
To accomplish the aforesaid objectives one should first know why and how the 
cutting tools fail. 
Cutting tools generally fail by : 
i) Mechanical breakage due to excessive forces and shocks. Such kind of tool 
failure is random and catastrophic in nature and hence are extremely 
detrimental. 
ii) Quick dulling by plastic deformation due to intensive stresses and 
temperature. This type of failure also occurs rapidly and are quite 
detrimental and unwanted. 
iii) Gradual wear of the cutting tool at its flanks and rake surface. 
The first two modes of tool failure are very harmful not only for the tool but also for 
the job and the machine tool. Hence these kinds of tool failure need to be prevented 
by using suitable tool materials and geometry depending upon the work material and 
cutting condition. 
But failure by gradual wear, which is inevitable, cannot be prevented but can be 
slowed down only to enhance the service life of the tool. 
The cutting tool is withdrawn immediately after it fails or, if possible, just before it 
totally fails. For that one must understand that the tool has failed or is going to fail 
shortly. 
It is understood or considered that the tool has failed or about to fail by one or more 
of the following conditions : 
(a)  In R&D laboratories 
• total breakage of the tool or tool tip(s) 
• massive fracture at the cutting edge(s) 
• excessive increase in cutting forces and/or vibration 
• average wear (flank or crater) reaches its specified limit(s) 
(b)  In machining industries 
• excessive (beyond limit) current or power consumption 
• excessive vibration and/or abnormal sound (chatter) 
• total breakage of the tool 
• dimensional deviation beyond tolerance 
• rapid worsening of surface finish 
• adverse chip formation. 
Version 2 ME IIT, Kharagpur 
 
 
(ii)  Mechanisms and pattern (geometry) of cutting tool wear 
 
For the purpose of controlling tool wear one must understand the various 
mechanisms of wear, that the cutting tool undergoes under different conditions. 
The common mechanisms of cutting tool wear are : 
i) Mechanical wear 
• thermally insensitive type; like abrasion, chipping and delamination 
• thermally sensitive type; like adhesion, fracturing, flaking etc. 
ii) Thermochemical wear 
• macro-diffusion by mass dissolution 
• micro-diffusion by atomic migration 
iii) Chemical wear 
iv) Galvanic wear 
In diffusion wear the material from the tool at its rubbing surfaces, particularly at the 
rake surface gradually diffuses into the flowing chips either in bulk or atom by atom 
when the tool material has chemical affinity or solid solubility towards the work 
material. The rate of such tool wear increases with the increase in temperature at the 
cutting zone. 
Diffusion wear becomes predominant when the cutting temperature becomes very 
high due to high cutting velocity and high strength of the work material. 
Chemical wear, leading to damages like grooving wear may occur if the tool material 
is not enough chemically stable against the work material and/or the atmospheric 
gases. 
Galvanic wear, based on electrochemical dissolution, seldom occurs when both the 
work tool materials are electrically conductive, cutting zone temperature is high and 
the cutting fluid acts as an electrolyte. 
The usual pattern or geometry of wear of turning and face milling inserts are typically 
shown in Fig. 3.2.1 (a and b) and Fig. 3.2.2 respectively. 
 
K
T 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 3.2.1 (a) Geometry and major features of wear of turning tools 
 
 
Version 2 ME IIT, Kharagpur 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 3.2.1 (b)  Photographic view of the wear pattern of a turning tool insert 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
IntermediateEdge 
Average Wear ( Vb2 ) 
Intermediate 
 Cutting Edge
Main Cutting 
Edge 
Planishing 
Edge  
Main Cutting Edge 
 Average Wear 
( Vb1)
Planishing Edge 
Average Wear ( Vb3 ) 
Insert 
   (a)          (b) 
Fig. 3.2.2  Schematic (a) and actual view (b) of wear pattern of face milling insert 
 
In addition to ultimate failure of the tool, the following effects are also caused by the 
growing tool-wear : 
• increase in cutting forces and power consumption mainly due to the 
principal flank wear 
• increase in dimensional deviation and surface roughness mainly due to 
wear of the tool-tips and auxiliary flank wear (V
s
) 
• odd sound and vibration 
• worsening surface integrity 
• mechanically weakening of the tool tip. 
 
 
 
 
 
Version 2 ME IIT, Kharagpur 
 
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