2. Tool Life
⚫Useful cutting life of tool expressed in time
⚫Time period measured from start of cut to failure
of the tool
⚫Time period between two consecutive
resharpenings or replacements.
3. Ways of measuring tool life
⚫No. of pieces of work machined
⚫Total volume of material removed
⚫Total length of cut.
⚫Limiting value of surface finish
⚫Increase in cutting forces
⚫Dimensional accuracy
⚫Overheating and fuming
⚫Presence of chatter
4. Modes of tool failure
Excessive stress and mechanical chipping
Cutting force becomes excessive and/or dynamic,
leading to brittle fracture
Thermal cracking and softening
Cutting temperature is too high for the tool
material losing its hardness
Gradual wear
Sliding of the chip along the rake face
Sliding of the tool along the newly cut work
surface
5. Preferred Modes of tool failure
Fracture and temperature failures are premature
failures
Gradual wear is preferred because it leads to the
longest possible use of the tool
Gradual wear occurs at two locations on a tool:
Crater wear – occurs on top rake face
Flank wear – occurs on flank (side of tool)
6. Tool wear
⚫Tool wear causes the tool to lose its original
shape- ineffective cutting
⚫Tool needs to be resharpened
8. Locations of Tool wear
Gradual wear occurs at two locations on a tool:
⚫Crater wear – occurs on top of the rake face
⚫Flank wear – occurs on flank (side of the tool)
10. Crater wear
⚫ Direct contact of tool and w/p
⚫ Forms cavity
⚫ This consists of a concave section on the tool
face formed by the action of the chip sliding on
the surface. Crater wear affects the mechanics of
the process increasing the actual rake angle of
the cutting tool and consequently, making cutting
easier. At the same time, the crater wear
weakens the tool wedge and increases the
possibility for tool breakage.
⚫ Leads to
⚫ weakening of the tool
⚫ Increase in cutting temp
⚫ Cutting forces & friction
11. Flank Wear
⚫ Tool slides over the surface of the work piece and friction is
developed
⚫ Brittle and discontinuous chip
⚫ Increases as speed is increased.
⚫ This occurs on the tool flank as a result of friction between
the machined surface of the workpiece and the tool flank.
Flank wear appears in the form of so-called wear land and is
measured by the width of this wear land, VB, Flank wear
affects to the great extend the mechanics of cutting. Cutting
forces increase significantly with flank wear. If the amount of
flank wear exceeds some critical value (VB > 0.5~0.6 mm),
the excessive cutting force may cause tool failure.
12. Tool flank wear vs Time of Cutting
Fig.: Tool Wear as a Function of Cutting Time
13. ⚫Primary stage rapid
wear due to very high
stress at tool point
⚫Wear rate is more or
less linear in the
secondary stage
⚫Tertiary stage wear
rate increases rapidly
resulting in
catastrophic failure.
15. Abrasive wear
⚫ Due to
⚫ Presence of hard materials in w/p material.
⚫ Strain hardening induced in the chip & w/p due to plastic deformation.
⚫ Contributes to flank wear
⚫ Effect can be reduced by fine grain size of the tool & lower percentage of
cobalt
19. Minor Mechanisms of Tool Wear
⚫Oxide Wear: oxidation of tool material at the
elevated temperatures
⚫Plastic deformation due to excessive heat
(contributes to flank wear)
⚫Chemical decomposition through localized
chemical reactions
20. Tool Life
⚫ Length of cutting time that a tool can be used before
the flank wear reaches the limiting width of flank wear.
21. Measurement of tool life
⚫Time for Total destruction in case of HSS or time
to produce 0.75 mm wear for carbide tools
⚫Tool life expressed by Taylor’s eqn
⚫VTb = C
⚫V = cutting speed in cm/min
⚫T= tool life in min
⚫b= const= 0.1 for HSS
⚫C= 50 for HSS
⚫Cemented carbide : b=0.125, C=100
⚫Tool life expressed in volume of metal removed
⚫L = TVfd
22. Factors affecting tool life
1. Cutting speed
2. Physical properties of w/p
3. Area of cut
4. Ratio of feed to depth of cut
5. Shape and angles of tool
6. Tool material and its heat treatment
7. Nature and quantity of coolants
8. Rigidity of tool and wp
23. Concept of Machinability
⚫ It is generally applied to the machining properties of work material
⚫ It refers to material (work) response to machining
⚫ It is the ability of the work material to be machined
⚫ It indicates how easily and fast a material can be machined.
⚫ Machinability is defined in terms of:
Surface finish and surface integrity
Tool life
Force and power required
The level of difficulty in chip control
⚫ Good machinability indicates good surface finish and surface
integrity, a long tool life, and low force and power requirements
⚫ Machinability ratings (indexes) are available for each type of material
and its condition
24. Quantifying Machinability
Machinability can be measured or quantified
mostly in terms of :
TOOL LIFE which substantially influences
productivity and economy in machining
magnitude of CUTTING FORCES which affects
power consumption and dimensional accuracy
SURFACE FINISH which plays role on
performance and service life of the product.
25. Factors affecting machinability of metals
⚫ Material of w/p- hardness, tensile properties, strain hardenability
⚫ Tool material.
⚫ Size and shape of the tool.
⚫ Type of machining operation.
⚫ Size, shape and velocity of cut.
⚫ Type and quality of machine used
⚫ Quality of lubricant used in machining
⚫ Friction b/w chip & tool
⚫ Shearing strength of w/p material.
27. Evaluating Machinability Rating
Machinability rating (MR) =
Fig. Machinability rating in terms of cutting velocity giving 60 min tool life.
28. Influencing Parameters on Machinability
properties of the work material
cutting tool: material and geometry
levels of the process parameters
machining environments (cutting fluid, etc.)
strength, rigidity and stability of the machine
kind of machining operations done in a given
machine tool
30. Exercise problems
Pb.: A better surface finish is desired on a
workpiece. Recommend three steps without
involving tool change.
Soln.:
Increase cutting speed
Decrease feed and
Decrease depth of cut.
31. Economics of Metal cutting
• To get high return with minimum investment, in case we want to minimize
costs while increasing cutting speed.
• Efficiency for good quality parts are produced at a reasonable cost.
• Cost is affected by: Tool life; power consumed
• The production is affected by: accuracy (dimensions & surface finish); mrr
(material removal rate)
• The factors that can be modified are:
• Cutting velocity; feed & depth; work material ; tool material; tool shape ;
cutting fluid
• There are 2 conditions:
• Low cost: Low cutting speeds, low mrr, longer tool life
• High production rate: High cutting speeds, short tool life, high mrr
• For optimising cost therefore abrupt increase of cutting speed and feed rate
is not a feasible solution; rather, an optimization is necessary.
32. Economics of Metal cutting
• Ultimate objective of machining is to give intended shape, size and
finish by gradually removing material from w/p.
• Relevant steps such as removal of material, setting the job and cutting
tool and dispatching the machined job consume substantial amount of
time, which are at least not negligible.
• For effective planning of the entire production, overall machining or
cutting time must be incorporated
• In order to fulfill first faster production rate, the cutting speed & feed
rate should be increased. This may lead to reduced cutting tool life due
to faster wear rate & higher heat generation. Hence, cutting tool is
required to change frequently, which will ultimately impose a loss for
the industry as a result of idle time for changing tools. Cost of tool is
also not negligible. Therefore abrupt increase of cutting speed and
feed rate is not a feasible solution; rather, an optimization is necessary.
33. Economics of Metal cutting
Overall machining time (Tm) = (Tc) + (Tct) + (Ti)
34. In order to keep the cost low, it is necessary to adopt the parameters: The
optimum speed is regarded because if the cutting or machining speed is
high, the chances of in-accuracy increases. Because of high speed, the tool
also gets more heated up & thus more cutting fluid will be applied, thereby
increasing the cost, thus optimum speed is adopted.
The aspects adopted for maximum production rate are: Minimum
production time
Shortest time to meet the planned target
Optimal cutting parameter
If the production time is high, then the production rate decreases & thereby
affecting the market of the product. The shortest time is planned so that
one may know how much time the production will take to get maximum
profit. Optimal cutting parameters are selected because if the parameter is
very much accurate, then it will take more machining time to prepare the
w/p. On the other hand, if the dimensions are poorer than the w/p made
will become useless. Thus, optimum parameters are considered.
Economics of metal cutting
