Astakhov V. Tribology of Metal Cutting

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CHAPTER 4

Cutting Tool Wear, Tool Life and Cutting Tool

Physical Resource

4.1 Introduction

In deforming processes used in manufacturing, concern over the wear is often overshad-

owed by considerations of forces or material ﬂow. Except for hot extrusion, die life is

measured in hours and days, or in thousands of parts [1]. In metal cutting, however, tool

wear is a dominant concern because process conditions are chosen to give maximum

productivity or economy, often resulting in tool life in minutes. Central to the prob-

lem are: high contact temperatures at the tool–chip and tool–workpiece interfaces that

lead to the softening of tool material and promote diffusion and chemical (oxidation)

wear; high contact pressures at these interfaces and sliding of freshly formed (juvenile)

surfaces of the work material layers promote abrasive and adhesion wear [2]; cyclic

nature of the chip formation process which can cause cracking due to thermal fatigue.

Another tool wear mechanism is fretting wear. Fretting is a small amplitude oscillatory

motion, usually tangential, between two solid surfaces in contact. Fretting wear occurs

when repeated loading and unloading cause cyclic stresses, which induce surface or

subsurface breakup and loss of material. Vibration is a common cause of fretting wear.

The mentioned wear mechanisms (well discussed in [1]) may take place alone or, more

frequently, in combination.

The nature of tool wear, unfortunately, is not yet clear enough in spite of numerous

investigations carried out over the last 50 years. Although various theories have been

introduced hitherto to explain the wear mechanism, the complicity of the processes in

the cutting zone hampers formulation of a sound theory of cutting tool wear. Cutting tool

wear is a result of complicated physical, chemical, and thermomechanical phenomena.

Because different “simple” mechanisms of wear (adhesion, abrasion, diffusion, oxidation,

etc.) act simultaneously with predominant inﬂuence of one or more of them in different

situations, identiﬁcation of the dominant mechanism is far from simple, and most inter-

pretations are subject to controversy [1]. These interpretations are highly subjective and

based on the evaluation of the cutting conditions, possible temperature and contact stress

levels, relative velocities and many other process paramagnets and factors.

220

Cutting Tool Wear, Tool Life and Cutting Tool Physical Resource 221

As a result, experimental, or post process methods, are still dominant in the known

studies of tool wear [1–12] and only topological or simply, geometrical parameters of

tool wear are selected and thus reported in tool wear and tool life studies. Although

it might seem simple to measure and assess the topology and geometry of tool wear

using modern equipment including image processing technique, in reality it is not so.

Complicated and irregular shapes of tool wear patterns combined with complicated tool

geometry and many possible ways and methods of tool wear measurement give rise to

various methods of tool wear assessment.

4.2 Known Approaches

4.2.1 Types of tool wear

In accordance with ANSI/ASME Tool Life Testing with Single-Point Turning Tools

(B94.55M-1985), the principal types of tool wear, classiﬁed according to the regions of

the tool they affect, are:

• Rake face or crater wear (Fig. 4.1) produces a wear crater on the tool rake face.

The depth of the crater KT is selected as the tool life criterion for carbide tools. The

other two parameters, namely, the crater width KB and the crater center distance

KM are important if the tool undergoes resharpening.

• Relief face or ﬂank wear (Fig. 4.1) results in the formation of a ﬂank wear land.

For the purpose of wear measurement, the major cutting edge is considered to be

max.

Flank wear

land

Zone

SECTION A-A

Crater

Plane P

Fig. 4.1. Types of wear on turning tools according to ANSI/ASME Tool Life Testing with Single-

Point Turning Tools (B94.55M-1985).

222 Tribology of Metal Cutting

divided into the following three zones: (a) Zone C is the curved part of the cutting

edge at the tool corner; (b) Zone N is the quarter of the worn cutting edge of length

b farthest from the tool corner; and (c) Zone B is the remaining straight part of the

cutting edge between Zones C and N. The maximum VB

max and the average VB

width of the ﬂank wear are measured in Zone B, the notch wear VB

is measured in

Zone N, and the tool corner wear VB

is measured in Zone C. As such, the following

criteria for carbide tools are normally recommended: (a) the average width of the

ﬂank wear land VB

= 0.3 mm, if the ﬂank wear land is considered to be regularly

worn in Zone B; (b) the maximum width of the ﬂank wear land VB

max = 0.6mm,

if the ﬂank wear land is not considered to be regularly worn in Zone B. Besides,

surface roughness for ﬁnish turning and the length of the wear notch VB

= 1mm

can be used. However, these geometrical characteristics of tool wear are subjective

and insufﬁcient. First, they do not account for the tool geometry (the ﬂank angle,

the rake angle, the cutting edge angle, etc), so they are not suitable to compare

wear parameters of cutting tools having different geometries. Second, they do not

account for the cutting regime and thus do not reﬂect the real amount of the work

material removed by the tool during the tool operating time, which is deﬁned as the

time needed to achieve the chosen tool life criterion.

4.2.2 Wear curves and tool life

Tool wear curves illustrate the relationship between the amount of ﬂank (rake) wear

and the cutting time (τ

) or the overall length of the cutting path (L). These curves are

represented in linear coordinate systems using the results of cutting tests, where ﬂank

wear VB

max is measured after certain time periods (Fig. 4.2(a)) or after certain length

of the cutting path (Fig. 4.2(b)). Normally, there are three distinctive regions that can be

observed on such curves. The ﬁrst region (I in Fig. 4.2(b)) is the region of primary or

initial wear. Relatively high wear rate (an increase of tool wear per unit time or length

100

0.25

0.50

0.75

1.00

(a)

(b)

20 30 50 60

0.2

0.4

0.6

0.8

0.2

0.4

0.6

0.8

10000 500 1500 2000

0.25

0.50

0.75

1.00

30002500

L(m)

II III

max

(mm)

max

(mm)

(mg)

(min)

(mg)

Fig. 4.2. Typical tool rate curves for ﬂank wear: (a) as a function of time and (b) as a function of

cutting path (after Astakhov [25]).

Cutting Tool Wear, Tool Life and Cutting Tool Physical Resource 223

of the cutting path) in this region is explained by accelerated wear of the tool layers

damaged during its manufacturing or resharpening. The second region (II in Fig. 4.2(b))

is the region of steady-state wear. This is the normal operating region for the cutting

tool. The third region (III in Fig. 4.2(b)) is known as the tertiary or accelerated wear

region. Accelerated tool wear in this region is usually accompanied by high cutting

forces, temperatures and severe tool vibrations. Normally, the tool should not be used

within this region.

Tool wear depends not only on the cutting time or the length of the cutting path but also

on the parameters of tool geometry (rake, ﬂank, inclination angles, radius of the cutting

edge, etc.), cutting regimes (cutting speed, feed, depth of cut), properties of the work

material (hardness, toughness, structure, etc.), presence and properties of the cutting

ﬂuid and many other parameters of the machining system. In practice, however, the

cutting speed is of prime concern in the consideration of tool wear. As such, tool wear

curves are constructed for different cutting speeds keeping other machining parameters

invariable. In Fig. 4.3, three characteristic tool wear curves (mean values) are shown for

three different cutting speeds, ν

, ν

and ν

. Because ν

is greater than the other two, it

corresponds to the fastest wear rate. When the amount of wear reaches the permissible

tool wear VB

, the tool is said to be worn out.

Typically VB

is selected from the range

(

0.15–1.00 mm

)

depending upon the type of

machining operation, condition of the machine tool and quality requirements for the

operation. It is often selected on the ground of process efﬁciency and often called the

criterion of tool life. In Fig. 4.3, T

is tool life when the cutting speed ν

is used,

– when ν

, and T

– when ν

is the case. When the integrity of the machined surface

permits, the curve of maximum wear instead of the line of equal wear should be used

(Fig. 4.3). As such, the spread in tool life between lower and higher cutting speeds

becomes less signiﬁcant. As a result, a higher productivity rate can be achieved which

is particularly important when high-speed CNC machines are used.

Tool life affects the choice of tool, selection of the process variables, economy of

operation, and possibility of the process’s adaptive control. Standard tool-life testing

Line of

equal wear

Curve of

maximum

wear

Fig. 4.3. Flank wear vs. cutting time at various cutting speeds (after Astakhov [25]).

224 Tribology of Metal Cutting

and representation includes Taylor’s tool life formula [13]

νT

= C

, (4.1)

where T is tool life in minutes, C

is a constant into which all cutting conditions affecting

tool life must be absorbed.

Although Taylor’s tool life formula is still in wide use today and is in the very core of

many studies on metal cutting including the level of national and international standards,

one should remember that it was introduced in the year 1907 as a generalization of

many years of experimental studies conducted in the nineteenth century using work and

tool materials and experimental techniques available at that time. Since then, each of

these three components underwent dramatic changes. Unfortunately, the validity of the

formula has never been veriﬁed for these new conditions. Nobody proved so far that

the formula is still valid for any other cutting tool materials than carbon steels and high

speed steel (HSS).

Moreover, one should clearly realize that tool life is not an absolute concept but depends

on what is selected as the tool life criteria. In ﬁnishing operations, surface integrity and

dimensional accuracy are of prime concern while in roughing operations, the excessive

cutting force and chatter are limiting factors. In both, material removal rate and chip

braking could be critical. These criteria, while important from the operational point of

view, have little to do with the physical condition of the tool or physics of the cutting

process.

Analysis of the available criteria of tool wear and tool life assessment clearly indicates

that these assessments are insufﬁcient, very subjective and do not correlate with the

tribology of metal cutting. They do not account for the tool geometry (the ﬂank, rake,

cutting edge angles, for example) so they are not suitable to compare cutting tools having

different geometries. They do not account for the cutting regime and thus do not reﬂect

the real amount of the work material removed by the tool during the time over which

the measured ﬂank wear is achieved. Finally, they are passive by nature because they

are not correlated with the tribological conditions at the tool–chip and tool–workpiece

interfaces so they can hardly be used for any process improvements and optimization as

well as the process-adaptive intelligent control.

Another way to look at the problem is to correlate tool wear with some tribological

parameters at the tool–chip and tool–workpiece interfaces. To do that, the proper assess-

ment of tool wear and its correlation with the tribological parameters at the mentioned

interfaces should be established. In doing so, the inﬂuence of the geometrical and regime

parameters of the cutting process and tool wear are established and explained. These are

the objectives of this chapter.

4.3 Proper Assessment of Tool Wear

To analyze the performance of cutting tools on CNC machines, production cells and

lines, the dimension tool life, which is understood as the time period within the cutting

Cutting Tool Wear, Tool Life and Cutting Tool Physical Resource 225

tool that assures the required dimensional accuracy of the machined parts. The dimension

tool life can be represented by:

• The tool operating time (T

). The operating time is understood as that time within

which there is no dimension compensation or tool change due to tool wear.

• The number of machined parts (N

) during the tool operating time.

• The overall length of tool path (L

) during the tool operating time.

• The overall area of the machined surface (A

) during the tool operating time.

• Linear relative tool wear (h

r−s

) during the tool operating time.

All the listed characteristics of the dimension tool life are speciﬁc and thus, in general,

are not suitable for the optimization of machining operations, comparison of various

cutting regimes, assessments of various tool materials and so on. For example, it is not

possible to compare two different tool materials if two different cutting speeds (suitable

respectively for each one particularly) were used in the test. Therefore, the need is felt

to introduce new characteristics for the dimension tool life [14].

Among suitable characteristics, the following seems to be the most reasonable:

• Volumetric or mass tool wear is much versatile because it does not depend on tool

geometry and design. This parameter can be measured directly. As such, the volume

of lost tool material (V

) is obtained from the comparison of a 3D topography of

the cutting wedge with that of the new tool (presumably stored in the memory of the

image processing system). As such, the mass of worn material m

is calculated as,

= ρ

, (4.2)

where ρ

is the density of the tool material.

Figure 4.2 shows wear curves when m

is used.

Volumetric or mass tool wear can also be calculated using the results of linear measure-

ments and parameters of tool geometry. Figure 4.4 shows some standard wear patterns

and Table 4.1 presents the formulae to calculate the volumetric or mass characteristics

of tool wear using the known tool geometry and the results of linear measurements [15].

• The dimension wear rate is the rate of shortening the cutting tip in the direction

perpendicular to the machined surface taken within the normal wear period (region

II in Fig. 4.2(b)), i.e

dν

−h

r−i

T − T

νh

l−r

1000

νf h

100

(

µm/min

)

, (4.3)

where h

and h

r−i

are current and initial radial wear, respectively, T and T

are the

total and initial operating time, respectively and h

is the surface wear rate.

226 Tribology of Metal Cutting

90°-g

(a)

(b)

(d)

(c)

Fig. 4.4. Wear patterns to calculate volumetric wear.

Table 4.1. Formulas for calculating the volume and mass of material worn off the tool ﬂank

and rake.

Figure Worn volume Mass of worn material

4.4(a) V

bVB

sin α cos γ

2 cos

(

α + γ

)

= ρ

sin α cos γ

2 cos(α + γ) sin κ

where t

is the depth of cut and κ

is the cutting

edge angle [5].

4.4(b) V

sin α cos γ



(VB

+2VB

)dx

2 cos

(

α + γ

)

where VB

= VB

+cx

b sin α cos γ

2 cos

(

α + γ

)



2cVB

n + 1

2n + 1



4.4(c) V

sin α cos γ

2 cos

(

α + γ

)

b/2



−b/2



−

4VB



dx m

4ρ

bVB

sin α cos γ

15 cos

(

α + γ

)

4.4(d) V





tan

−1





−

(

180 −ε

)

360



4 cos





(

−y

)

dy m



tan

−1





−

(

180 −ε

)

360



32 cos



