Astakhov V. Tribology of Metal Cutting

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Cutting Tool Wear, Tool Life and Cutting Tool Physical Resource 237

(a)

800

200

600

400

n (m/min)

3020128

300

100

40 60

HRC65

HRC55

HRC45

100

300

400

600

200

800

(b)

400 600 800

q(°C)

HRC65

HRC55

HRC45

(µm/10

) h

(µm/10

)

Fig. 4.11. Inﬂuence of cutting speed (a) and cutting temperature (b) on tool wear rate in turning

of tool steel HVG (1% C, 1% Cr, 1.5% W, 1% Mn). Tool material – carbide M20 (92% WC,

8% Co), depth of cut d

= 0.25 mm and cutting feed f = 0.10 mm/rev.

4.4.3 Methods of determining the optimal cutting temperature

The optimal cutting temperature can be determined experimentally using a tool-life test,

where the tool wear rate h

is measured as a function of the cutting temperature. The

temperature corresponding to the minimum tool wear rate is considered as the optimal

cutting temperature. However, a complete tool life test is expensive and time consuming

although it has to be carried out only once for a given combination of the work and tool

materials. It has been suggested, therefore, that such a test be conducted at a constant

depth of cut and cutting feed, varying the cutting speed and measuring tool wear rate h

As it follows from the data shown in Fig. 4.6, the cutting force and surface ﬁnish can

also be measured to establish the optimal cutting temperature. As shown in this ﬁgure,

the optimal cutting temperature was determined to be 875

◦

Conducting a number of tests with difﬁcult-to-machine materials, Silin [21] showed that

the optimal cutting temperature can be determined as corresponding to the minimum

stabilized cutting force, as shown in Fig. 4.12. As shown, the optimal cutting temperature

does not depend on a particular cutting regime.

The optimal cutting temperature can also be determined if the correlation curves of the

“temperature–hardness” are known for the work and tool materials. As such, the optimal

238 Tribology of Metal Cutting

0 n (m/s)0.1

400

1000

(N)

0.2 0.3 0.4 0.5 0.6

1200

1600

500

400

600

700

800

(°C)

Fig. 4.12. Experimental determination of the optimal cutting temperature by the minimum stabi-

lized cutting force. Workpiece materials: nickel-based alloy (0.08% C, 1% Cr, 56% Ni, 1% Co,

1% Al), tool material – carbide M20 (92% WC, 8% Co), tool geometry: γ = 12

◦

, α = 12

◦

= 45

◦

, κ

= 45

◦

, r

= 1 mm, depth of cut d

= 1.0 mm, cutting feeds as corresponding to

curves: (1) f = 0.074, (2) 0.11, (3) 0.15, (4) 0.25, (5) 0.30, (6) 0.34 and (7) 0.39 mm/rev.

cutting temperature is determined as the temperature corresponding to the maximum

difference of these hardnesses, as shown in Fig. 4.13.

Conducting a similarity study of metal cutting parameters, Silin suggested the following

theoretical equation for determining the optimal cutting temperature for carbide tools [21]

opt

= 0.6 T

mp−c





0.13



P−w

P−cf



0.27



1 + e



0.05

, (4.8)

where T

mp−c

is the melting temperature of cobalt, T

mp−c

= 1490

◦

C, k

and k

are

thermoconductivities of the tool and work materials, respectively, C

P−ct

and C

P−w

Cutting Tool Wear, Tool Life and Cutting Tool Physical Resource 239

= f(q)

opt

Fig. 4.13. Determination of the optimal cutting temperature using temperature–hardness curves.

Table 4.6. Coefﬁcient m in Eq. (4.8).

Group of work materials m Group of work materials m

Carbon steels 0.925 Nickel-based heat-resistant steels 1.100

Chromium and tool steels 0.800 Titanium alloys 0.6–0.7

Chromium–molybdenum and

chromium–tungsten steels

0.970 Aluminum high alloys 0.95

Stainless and heat-resistant steels 1.300 Copper, brass 1.25

Bronze 1.05

are speciﬁc heats of the tool and work materials, respectively, ρ

and ρ

are the densities

of the tool and work materials, respectively, e

is the elongation at fracture of the work

material and m is a coefﬁcient in Table 4.6.

4.5 Inﬂuence of Cutting Feed and Depth of Cut

4.5.1 Inﬂuence of cutting feed in a wide range of cutting parameters

The uncut chip thickness or the cutting feed has direct inﬂuence of the quality, productiv-

ity and efﬁciency of machining. It is believed that tool life decreases (and thus tool wear

increases) with the cutting feed [5,20,22,23]. Such a perception is based on a generally

adopted equation for tool life. For example, generalizing experimental data, Gorczyca

proposed (Eq. (5.9) in [23]) the following equation,

T =

48.36 × 10

1.6

0.48

(4.9)

240 Tribology of Metal Cutting

As shown, if the cutting speed ν = constant and the depth of cut d

= constant, then

the tool life increases when the cutting feed decreases and vice versa.

A great body of data to support the discussed point and thus the structure of Eq. (4.9)

can be found in the literature on metal cutting, although some researches starting with

Taylor [13] did not include the cutting feed in tool life equations because this parameter

was not considered as having a signiﬁcant inﬂuence on tool life while others found that

the experimentally obtained relation h

(

)

has a distinctive minimum. Such a great

scatter in the experimental results can be explained by the fact that the cutting tests were

carried out under invariable cutting speeds that resulted in different cutting temperatures

when the cutting feed was varied.

To gain an understanding of the true inﬂuence of the cutting feed on tool wear, this

inﬂuence should be considered in the context of other parameters of the cutting process

that make contributions to the cutting temperature. As such, the inﬂuence of the cutting

feed (the uncut chip thickness) on the surface wear is of prime interest while keeping

the area of the machined surface constant (or the volume of the removed work material)

in contrast to the length of the cutting path. This is because the area of the machined

surface (or the volume of the removed work material) does not change with the cutting

feed while the length of the tool path does.

Studying the inﬂuence of the cutting feed on the surface wear, Makarow [14] considered

the following factors,

Factor 1: When cutting feed increases (and ν = constant), the length of the tool path

decreases if the machined area is kept constant. As a result, the cutting (contact) time as

well as the tool wear rate decreases. Therefore, the surface wear rate decreases.

Factor 2: Any change in the cutting feed leads to the corresponding change in the cutting

temperature, so the cutting feed should inﬂuence the tool wear rate. As such, there are

three basic scenarios: (a) if the current machining takes place using relatively low cutting

speed so that the cutting temperature is lower than the optimal cutting temperature, then

an increase in the cutting feed leads to a decrease in the tool wear rate, (b) if the current

machining takes place using an “average” cutting speed so that the cutting temperature

passes its optimum with an increase in the cutting feed, then the relation h

(

)

has a distinctive minimum. In other words, increasing the cutting feed until the cutting

temperature remains below the optimal cutting temperature reduces the tool wear rate

while any further increase would increase this wear rate, (c) if the current machining

takes place using a high cutting speed, i.e. when the cutting temperature is higher than

the optimal cutting temperature, then any increase in the cutting feed should lead to an

increase in the tool wear rate.

Factor 3: The tool actually cuts the transient surface (the surface being cut by the major

cutting edge, see Appendix A). Because in most practical machining operations the tool

cuts part of the transient surface formed on the previous tool pass, the amount of cold

working imposed by this tool on the previous pass affects the cutting conditions on the

current pass. Among other characteristics, the depth of cold working (d

) with respect

to the uncut chip thickness (t

) is of prime concern. When the cutting feed (the uncut chip

Cutting Tool Wear, Tool Life and Cutting Tool Physical Resource 241

thickness) is small then it can so happen that d

and the major cutting edge cuts the

cold worked work material characterized by a greater strength and hardness compared

to those of the original work material. As such, the tool wear rate increases. If this is

the case, one increases the cutting feed so that the uncut chip thickness becomes greater

than d

, the tool wear rate decreases.

Factor 4: As discussed in Chapter 3, increasing the cutting feed leads to the correspond-

ing increase in the normal contact stress at the tool–chip interface and in the tool–chip

contact area (length). However, the contact area increases in a much smaller rate com-

pared to the normal contact stress [14]. When the level of the normal contact stress

reaches a certain tool-material speciﬁc limit, the chipping of the cutting edge takes place

that eventually leads to tool breakage. Such a limit can be referred to as the breaking

feed. Normally, the cutting feed used in machining common work materials is below the

breaking feed as the this feed is limited by the requirements to the surface integrity and

power of the feed drive. However, in hard turning, the operation that attracts more and

more attention in the automotive and aerospace industries, the breaking feed is normally

well below those allowed by the surface ﬁnish of machined parts and by the power of

the feed drive used, so that the working cutting feed can be in close proximity to the

breaking feed.

Factor 5: Often, the intensity of vibrations that take place in machining reduces with

the cutting feed. When it happens, the tool wear rate reduces. Moreover, increasing the

cutting feed changes the ratio of the radial, F

and the axial (feed), F

forces (Fig. A1.5)

that increases the dynamic rigidity of the machine tool.

Summarizing the above considerations, one should realize that when the cutting feed

increases, the cumulative effect of the discussed factors may affect the tool wear rate in

considerably different ways depending upon many parameters and characteristics of a

particular cutting system. Makarow [14] found that the effect of the cutting feed becomes

more apparent with difﬁcult-to-machine work materials having a great number of alloying

components.

As an example, consider the inﬂuence of the cutting feed on the surface wear rate,

in machining AL 610 alloy. AL 610 is a low-carbon (less than 0.015 wt.%),

silicon-containing (up to 4.3 wt.% Si), chromium (up to 18.5 wt.% Cr), nickel (up

to 15.5 wt.% Ni) austenitic stainless steel. This alloy is typically used for applica-

tions in the chemical industry. The high silicon content provides very good resistance

to oxidizing environments, such as concentrated nitric acid, over a wide range of

temperatures.

In the tests, the feed rate was selected to be in the range of 0.2–0.4 mm/rev. As such, the

uncut chip thickness is greater than the depth of cold working, there were no noticeable

vibrations, no chipping of the cutting edge and tool breakages so Factors 3–5 did not

affect the tool wear rate as the cutting feed was increased. Therefore, the relation h

(

)

was determined only by Factors 1 and 2.

Factor 1 always reduces the tool wear rate with increasing the cutting feed. To

study the inﬂuence of Factor 2, the cutting temperature was determined as a function

242 Tribology of Metal Cutting

750 800 850 900 950 1000 1050 1100

(°C)

opt

f=0.20mm/rev

f=0.30mm/rev

f=0.40mm/rev

n=75m/min

n=130m/min

n=160m/min

(µm/10

)

Fig. 4.14. The inﬂuence of cutting temperature on the tool wear rate in turning AL 610 alloy.

Tool material – carbide P10 (14% TiC, 8% Co). Depth of cut d

= 1 mm.

of the cutting regime. The result is shown in Fig. 4.14. The different cutting temperatures

were obtained by variation of the cutting speed.

Consider the change in the cutting temperature and tool wear rate when the cutting

feed changes from 0.2 to 0.4mm/rev at three different cutting speeds, 75, 130 and

160 m/min. As shown in Fig. 4.14, when the cutting feed increases in a zone where

the resulting cutting temperature is less than θ

opt

, this increase leads to the reduction of

the tool wear rate. If the opposite happens, then the cutting temperature exceeds θ

opt

When the cutting speed is ν = 75 m/min, an increase in the feed from 0.2 to 0.4mm/rev

leads to the increase in the cutting temperature within the left branch of curve h

(

)

As such, the higher the cutting feed, the higher the cutting temperature and the lower

the tool wear rate (points a

and a

). Therefore, Factors 1 and 2 reduce the tool

wear rate with increasing the cutting feed. When the cutting speed is ν = 130 m/min,

increasing the feed from 0.2to0.4mm/rev causes the cutting temperature to pass its

optimum. As such, an increase of the cutting feed from 0.2to0.3mm/rev leads to an

increase in the cutting temperature and reduction of the tool wear rate while the increase

of the cutting feed from 0.3to0.4mm/rev leads to the increase of the tool wear rate

(points b

and b

). In the latter transition, Factors 1 and 2 work simultaneously but

in an opposite manner in terms of their inﬂuence on the tool wear rate. The inﬂuence of

Factor 2 is stronger that causes an increase in tool wear rate. When the cutting speed is

ν = 160 m/min, any increase of the cutting feed leads to an increase of the tool wear

rate. Points c

and c

(Fig. 4.14) show what happens when the cutting feed increases

from 0.2 to 0.3mm/rev and then to 0.4mm/rev, respectively.

As it follows from the above consideration, the inﬂuence of the cutting feed on the tool

wear rate is different at different cutting speeds. In the considered case, the major factor

affecting tool wear rate is the cutting temperature.

Cutting Tool Wear, Tool Life and Cutting Tool Physical Resource 243

Factor 3 is extremely important but practically always ignored in metal cutting theory and

practice. As discussed above, the tool actually cuts the transient surface (the surface being

cut by the major cutting edge, see Appendix A). Because in most practical machining

operations, the tool cuts the part of the transient surface formed on the previous tool

pass, the amount of cold working imposed by this tool on the previous pass affect the

cutting conditions on the current pass. As mentioned, the depth of cold working (d

)

with respect to the uncut chip thickness (t

) is of prime concern (Fig. 4.15(a)). This is

particularly important when cutting at low feeds, i.e. when the uncut chip thickness is

smaller than the depth of cold working, i.e. when d

. When it happens, the major

cutting edge cuts the cold worked work material characterized by greater strength and

hardness. As such, the tool wear rate increases. Figure 4.15(b) illustrates this point. When

the feed is 0.1mm/rev, the depth of cold working is greater than the uncut chip thickness

so the cutting wedge cuts the cold worked work material that results in a greater tool

wear rate. In the feed range of 0.1–0.2 mm/rev, the inﬂuence of Factor 1 leads to the

reduction of the tool wear rate. When the feed increases further, the inﬂuence of Factor 2

becomes predominant that increases the tool wear rate.

4.5.2 Inﬂuence of cutting feed under the optimal cutting temperature

Understanding inﬂuence of the cutting feed under the optimal cutting temperature is

important in the selection of the optimal cutting regime since the optimal combination

of cutting speeds and feeds should be used in the practice of metal cutting.

0.1

f (mm/rev)

0.2 0.3

n=120m/min

n=90m/min

n=60m/min

(a) (b)

Transient surface

(µm/10

)

Fig. 4.15. Depth of coldworking of the transient surface: (a) model, (b) inﬂuence of the feed on

tool wear rate. Work material – stainless steel AISI 303, tool material – carbide M10 (97% WC,

3% Co), depth of cut d

= 0.5 mm.

244 Tribology of Metal Cutting

Makarow [14] proved that the correlation between the optimal cutting speed and feed as

well as between the optimal wear rate and feed can be established as,

opt

, (4.10)

s−opt

, (4.11)

where C

and C

are constants determined by the properties of the work material, x

are the powers determined by the speciﬁcs of the machining operation.

The dimension tool life T

can be represented as the product of the tool radial wear (h

)

and the speciﬁc dimension tool life (T

)

= h

×10





. (4.12)

As discussed above, the lower the wear rate (h

), the higher the speciﬁc dimension tool

life, the greater number of parts can be machined without correction/compensation of

the tool. The speciﬁc dimension tool life corresponding to the optimal surface wear can

be referred to as the optimal speciﬁc dimension tool life (T

UD−o

). Therefore, the optimal

tool life can be represented as

D−o

= h

UD−o

(4.13)

Because C

= constant then, when h

= constant, the dimension tool life is proportional

to power x

. This power is in the range of 0.31–0.75 and is always positive. Therefore,

in machining, if the optimal temperature is kept constant, an increase in the cutting

feed leads to an increase in the dimension tool life. The greater the x

, the stronger

the inﬂuence of the feed on the dimension tool life, and the greater the increase of the

dimension tool life with the cutting feed. For example, a fourfold increase in the cutting

feed (from 0.1 to 0.4mm/rev) in turning stainless steel AISI 303 using M20 (94% WC,

6% Co) carbide tool (power x

= 1.3) led to the increase in the dimension tool life

by 6.2 times while a 3.28-time increase was achieved in the same operation when P10

(30% TiC, 66% WC, 4% Co) tool was used (power x

= 0.88 ) [14].

4.5.3 Inﬂuence of depth of cut

When the depth of cut increases and the uncut chip thickness is kept the same, the speciﬁc

contact stresses at the tool–chip interfaces, chip compression ratio and average contact

temperature remain unchanged. Therefore, an increase in the depth of cut should not

change the tool wear rate if the machining is carried out at the optimum cutting regime.

Cutting Tool Wear, Tool Life and Cutting Tool Physical Resource 245

(a) (b)

0.5

1.0 1.5

(mm)

0.5 1.0 1.5

(mm)

f=0.10 mm/rev

f=0.20 mm/rev

f=0.40 mm/rev

f=0.12 mm/rev

f=0.23 mm/rev

f=0.47 mm/rev

(µm/10

) h

(µm/10

)

Fig. 4.16. Inﬂuence of depth of cut on the tool wear rate: (a) cutting with the invariable cutting

speed optimal for d

= 0.5 mm, work material – stainless steel AISI 303, tool material – carbide

M10 ((97% WC, 3% Co), (b) cutting with the invariable optimal cutting temperature determined

for d

= 1.0 mm, work material – AL 610 alloy, tool material – carbide M20 (92% WC, 8% Co).

Figure 4.16(a) shows the inﬂuence of the depth of cut on the tool wear rate. In the test,

the cutting speed was determined to be optimal for the depth of cut d

= 0.5 mm and

was kept invariable for the other depth of cut. As shown, the depth of cut has very little

inﬂuence on the tool wear rate. In another series of tests, the optimal cutting temperature

determined for d

= 1.0 mm was kept invariable in the test. The test results are shown

in Fig. 4.16(b). As shown, the depth of cut has little inﬂuence on the tool wear rate.

4.6 Inﬂuence of Tool Geometry

The tool geometry and signiﬁcance of its components are discussed in Appendix A.

It is shown that the uncut chip thickness is an important parameter in metal cutting and

it directly correlates with tool geometry. The inﬂuence of the tool nose radius on the

uncut chip thickness is also discussed and the shape of the uncut chip is correlated with

this radius and other geometrical parameters of the cutting tool.

The shape, mean uncut chip thickness and width as well as the cutting temperature are

greatly affected by the tool nose radius, r

. Therefore, this radius should signiﬁcantly

affect the shape of curves h

s−o

= f(ν). Figure 4.17(a) presents an experimental support

to this point. For the considered case, the following empirical equation is valid.

opt

= 23.2r

0.25

. (4.14)

When cutting with low cutting speeds (ν = 16 m/min), tool wear rate increases signif-

icantly with the nose radius, as shown in Fig. 4.17(b). The change of this radius from

= 0.5 to 4 mm led to the increase of tool wear rate h

from 32 to 120 µm/



×sm



246 Tribology of Metal Cutting

10 20 30

n (m/min)

(a)

(mm)

100

120 700

750

800

850

900

opt

40 50 60

=4.0 mm

2.0 mm

1.0 mm

0.5 mm

4.0 mm

2.0 mm

0.5 mm

1.0 mm

234

100

650

600

700

750

800

850

900

n=40 m/min

25 m/min

16 m/min

25 m/min

16 m/min

40 m/min

(b)

θ(°C) θ(°C)

(µm/10

)

(µm/10

)

Fig. 4.17. Inﬂuence of cutting speed on the tool wear rate and cutting temperature in turning.

Work material – Haynes 263 alloy (29% Cr, 2.5% Ti), tool material – micrograin carbide M10

(94% WC, 6% Co) depth of cut d

= 1 mm, cutting feed f = 0.20 mm/rev: (a) for different tool

nose radii and (b) inﬂuence of the nose radius.

i.e. by four times. This is because the cutting temperature decreases with the nose

radius, so it departures further from the optimal cutting temperature when the nose

radius increases. When cutting with moderate cutting speeds (ν = 25 m/min), the

tool wear rate ﬁrst decreases with the nose radius and after reaching a certain min-

imum, then increases. This is because the cutting temperature ﬁrst decreases and

approaches the optimal cutting temperature at which the tool wear rate is minimum.

Further increase in the nose radius leads to further decrease in the cutting tempera-

ture. As this temperature departs from the optimal cutting temperature, the tool wear

rate increases. When cutting with high cutting speeds (ν = 40 m/min), the tool

wear rate decreases monotonely and continuously increases with the nose radius. This

is because the cutting temperature decreases, thus approaching the optimal cutting

temperature.

The tool cutting edge angle (κ

) affects the uncut chip thickness, its width as well as

the cutting temperature. Conducting a great number of cutting tests, Makarow found

[14] that relationships h

s−o

= f(ν) have different minima at various tool cutting edge

angles although the optimal cutting temperature does not change. For example, in turning

AL 610 alloy using carbide M20 (92% WC, 8% Co), depth of cut d

= 0.25 mm,

feed f = 0.09 mm/rev and tool nose radius r

= 0.10 mm, the optimal cutting speed