Tool life - The actual machining time between two successive sharpenings of cutting tool is called Tool life.

Tool life is used to calculate the tool material performance and machinability of workpiece material.

Methods to specify tool life :

1) Time period in minutes between two successive grindings.

2) No. of components machined between two successive grindings.

3) Volume of metal removed between two successive grindings.

Tool life expressions :

1) Volume of metal removed per minute = $\pi$ . D .t . f . N $mm^3$ / min

2) Total volume of metal removed for tool failure = $\pi$ . D. t . f . N . T $mm^3$

where,

D = workpiece dia in mmp

t = Depth of cut in mm

f = Feed rate in mm/rev

N = no. of revolutions of workpiece per minute.

3) Total vol. of metal removed for tool failure = failure = V x 1000 x t x f x T $mm^3$

Factor affecting tool life :

a) cutting speed

b) feed and depth of cut

c) tool geometry

d) tool material

e) work material

f) nature of cutting

g) rigidity of machine tool and work

h) cutting fluids

i) process parameters

Cutting speed :

It is major for affecting tool life.

It varies inversely with tool life which leads to parabolic curve as shown.

Tool life relation is invented by F.W.Taylor.

$VT^n$ = c

V = cutting speed in m/min, C = machining constant

T = Tool life in min, n = Tool life index

Tool geometry : As a tool geometry is a variable parameter there is no quantitative relationship is available between tool geometry and tool life.

The relationship depends on the rake angle of tool

Generally as rake angle increases, the tool life also increases, But if rake angle is too large results in reduced strength of tool geometry.

Cutting fluid : when cutting fluid is used during machining, it is acting as a lubricant at friction region and carrying away the heat generated during machining.

with the use of cutting fluid, the tool life increases approximately by 25 to 40 %

Process parameters : General process parameters are speed, feed, depth of cut etc.

Because of uniqueness in process parameters their are the attempts showing the relationship between process parameters and tool life.

Taylor has assumed that cutting velocity is major parameter which influences tool life. Hence he derived $eq^n$

$VT^n$ = constant

where ,

V = cutting velocity in m/min

T = volume of material removed

C = Taylors constant

M = Taylors exponent (Depend on cutting tool material)

= 0.05 to 0.1 (HCS)

= 0.1 to 0.2 (HSS)

= 0.2 to 0.4 (Carbide)

= 0.4 to 0.6 (Ceramic)

= 0.7 to 0.9 diamond