Tool Life

Tool Life

Tool life generally indicates, the amount of satisfactory performance or

service rendered by a fresh tool or a cutting point till it is declared failed.

Tool life is defined in many ways :

In R & D : Actual machining time (period) by which a fresh cutting tool (or
point) satisfactorily works after which it needs replacement or reconditioning.

In industries or shop floor : The length of time of satisfactory service or

amount of acceptable output provided by a fresh tool prior to it is required to
replace or recondition.

Tool life means,

➢ no. of pieces of work machined
➢ total volume of material removed
➢ total length of cut.
 Three Modes of Tool Failure

Fracture failure (Mechanical chipping)

When the cutting force at tool point becomes excessive, it leads to failure
by brittle fracture.
Temperature failure (Thermal cracking and softening)
Cutting temperature is too high for the tool material, which makes the tool
point to soften, and leads to plastic deformation along with a loss of sharp
edge.
Gradual wear
Gradual wearing of the cutting edge causes loss of tool shape, reduction in
cutting efficiency and finally tool failure.
 Preferred Mode of Tool Failure:
Gradual Wear
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)

Tool Failures
 Flank wear & Crater wear

Flank Wear or wear land Crater wear

• It occurs on the tool flank as a result of • It consists of a concave section on the
friction between the machined surface tool face formed by the action of the
of the work piece and the tool flank. chip sliding on the surface.
• Due to Friction and abrasion. • Direct contact of tool and chip.
• Increases as speed is increased. • Forms cavity
 Measurement of tool wear

i) by loss of tool material in volume or weight, in one life time –generally

applicable for critical tools like grinding wheels.

ii) by grooving and indentation method – in this approximate method wear

depth is measured indirectly by the difference in length of the groove or
the indentation outside and inside the wear area.

iii) using optical microscope fitted with micrometer – very common and
effective method.
 Measurement of tool wear

iv) using scanning electron microscope (SEM) – used generally, for detailed
study; both qualitative and quantitative

v) Talysurf profilometer, specially for shallow crater wear.

 Tool Wear Curve
➢ The first is the break-in period, in which the
sharp cutting edge wears rapidly at the
beginning of its use. This first region occurs
within the first few minutes of cutting.

➢ The break-in period is followed by wear that

occurs at a uniform rate. This is called the
steady-state wear region. In our figure, this
region is pictured as a linear function of
time, although there are deviations from the
straight line in actual machining.

➢ Finally , wear reaches a level at which the

wear rate begins to accelerate. This marks
the beginning of the failure region, in which
cutting temperatures are higher, and the
general efficiency of the machining process
is reduced. If allowed to continue, the tool
Tool Wear Curve
finally fails by temperature failure..
 Taylor’s tool life equation.
➢ Wear and hence tool life of any tool for any work material is governed
mainly by the level of the machining parameters i.e., cutting velocity (Vc),
feed (f) and depth of cut (d).

➢ Cutting velocity affects the tool life is maximum and depth of cut is
minimum.

Growth of flank wear and assessment of tool life

➢ The tool life obviously decreases with the increase in cutting velocity.
 Taylor’s Tool Life equation

If the tool lives, T1, T2, T3, T4 etc are

plotted against the corresponding cutting
velocities, Vc1, Vc2, Vc3, Vc4 etc., a
smooth curve like a rectangular hyperbola
is found to appear.

Growth of flank wear and assessment of tool life

Cutting velocity – tool life relationship Cutting velocity vs tool life on a log-log scale
 Taylor’s Tool Life equation
When F. W. Taylor plotted the same figure taking both V and T in log-scale, a
more distinct linear relationship appeared as schematically shown in Fig.

Taylor derived the simple equation as,

𝑉𝑇 𝑛 = 𝐶
n-is called, Taylor’s tool life exponent
C-Constant; depends also on the limiting
value of flank wear.

Cutting velocity vs tool life on a log-log scale

Both ‘n’ and ‘c’ depend mainly upon the tool-work materials and the
cutting environment (cutting fluid application).
 Taylor’s Tool Life Equation

vT n = C

v – Cutting Speed (m/min)

T – Tool Life (min)
n, C are constants
C – intercept / Taylor’s constant
n – slope of the line / Taylor’s
exponent

log v1 − log v2
n = tan  =
log T2 − log T1
For HSS, n=0.08-0.2
For Carbides, n=0.2-0.6
For Ceramics, n=0.5-0.8
 Modified Taylor’s Tool Life equation
➢ In Taylor’s tool life equation, only the effect of variation of cutting
velocity, VC on tool life has been considered.
➢ But practically, the variation in feed (f) and depth of cut (d) also play role
on tool life to some extent.
➢ Taking into account the effects of all those parameters, the Taylor’s tool
life equation has been modified as,

V𝑇 𝑎 𝑓 𝑏 𝑑𝑐 = C

T --tool life in min

C ⎯a constant depending mainly upon the tool – work materials and the
limiting value of flank wear.
a, b and c ⎯exponents so called tool life exponents depending upon the tool –
work materials and the machining environment.
➢ Generally, a > b > c as Vc affects tool life maximum and depth of cut is
. minimum.
➢ The values of the constants, C, a, b and c are available in Machining Data
Handbooks or can be evaluated by machining tests.
 Numerical
Tool life tests in turning yield the following data: (1) when cutting speed is 100 m/min, tool life is 10 min;
(2) when cutting speed is 75 m/min, tool life is 30 min. (a) Determine the n and C values in theTaylor tool
life equation. Based on your equation, compute (b) the tool life for a speed of 110 m/min,and (c) the speed
corresponding to a tool life of 15 min.
 Numerical
A series of turning tests are performed to determine the parameters n, m, and K in the expanded version of
the Taylor equation. The following data were obtained during the tests: (1) cutting speed = 1.9 m/s, feed
= 0.22 mm/rev, tool life = 10 min; (2) cutting speed = 1.3 m/s, feed = 0.22 mm/rev, tool life = 47 min;
and (3) cutting speed = 1.9 m/s, feed = 0.32 mm/rev, tool life = 8 min. (a) Determine n, m, and K. (b)
Using your equation, compute the tool life when the cutting speed is 1.5 m/s and the feed is 0.28 mm/rev.