Astakhov V. Tribology of Metal Cutting

Подождите немного. Документ загружается.

Cutting Tool Wear, Tool Life and Cutting Tool Physical Resource 247

and the tool cutting edge angle correlate as follows:

opt

248

0.33

. (4.15)

As for the tool cutting edge angle of the minor cutting edge (κ

), it was found that being

varied in the practically used range of 5–45

◦

, this angle has negligibly small inﬂuence

on the optimal cutting speed and tool wear rate.

The inﬂuence of the rake angle is estimated using the so-called angle of cut which is

calculated as the sum of the ﬂank angle and the angle of the cutting wedge (Appendix A)

= α +β (4.16)

When the angle of cut δ

increases, the deformation of chip, friction losses at the

tool–chip interface, and cutting temperature increase correspondingly. Therefore, when

increases, the optimal cutting speed decreases, so the optimal cutting temperature

remains invariable. 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, the optimal cutting speed

and the angle of cut δ

correlate as follows:

opt

992

0.60

. (4.17)

The cutting edge inclination angle (λ

) deﬁnes the orientation of the tool rake face with

respect to the cutting speed vector. As a result, the optimal cutting speed changes while the

optimal temperature remains invariable. The inﬂuence of the inclination angle, however,

is extremely complex because it depends on many other parameters of the cutting system

and properties (both mechanical and physical) of the work material. As discussed in

the previous chapter, the inclination angle is the major contributor to the formation

of the state of stress in the layer being removed on the one hand, and on the other,

affects signiﬁcantly the tribological processes at the tool–chip interface. Therefore, it is

impossible to determine the net inﬂuence of this angle on the tool wear rate at this stage.

Although the ﬂank angle (α) does not affect the cutting temperature directly, it does

affect the dimension wear rate. This rate increases when the ﬂank angle increases. When

the cutting speed is kept invariable, it can be correlated with the ﬂank angle as,

= C

0.40

. (4.18)

The independence of the optimal cutting temperature on the parameters of tool geometry

allows to determine the optimal cutting speed for any given tool geometry parameters

using a simple cutting test, where the relationship h

s−o

= f(ν) determined for an invari-

able uncut chip cross-sectional area. Having conducted tests for other combinations

248 Tribology of Metal Cutting

of the tool geometry parameters and determining dependences θ = f(ν) and e.m.f. =

f(ν) for these combinations, one can determine the optimal cutting speed for any of these

combinations. For example, in the turning of the custom-modiﬁed Haynes 263 alloy

(0.02% C, 20% Cr, 2% Ti, 2% Al) using a cutting tool made of carbide M10 (94% WC,

6% Co) and various combinations of cutting tool geometry parameters, eight cutting

tests were needed to obtain the relationship h

s−o

= f(ν) and about one hundred

×f ×r

×κ

) short-time tests for determining the optimal cutting speed corresponding

to the optimal cutting temperature. The experimentally obtained correlation is,

opt

8.25r

0.15

0.40

0.30

(

sin κ

)

0.4

. (4.19)

When micrograin M10 (94% WC, 6% Co) carbide was used under the same conditions,

the following correlation was obtained.

opt

8.65r

0.15

0.38

0.29

(

sin κ

)

0.38

. (4.20)

4.7 Inﬂuence of Workpiece Diameter

The diameter of the workpiece affects the cutting process in various ways as,

• The static rigidity of the machining system depends on the workpiece diameter.

In boring, the diameter of the hole being bored often determines the diameter of

boring bar or arbor and thus effects the static and dynamic stability of the machining

system.

• The workpiece diameter affects the curvature on the surface being cut, that, in turn,

affects the stressed-deformed state of the layer being removed. As a result, the ﬁnal

inclination angle and the total length of the surface of the maximum combined stress

(often referred to as the shear angle and the length of the shear plane) change with

the workpiece diameter.

• When the cutting speed is kept invariable, the rotational speed (r.p.m.) changes with

the workpiece diameter that affects the dynamics of the process.

• As discussed in Chapter 2, the interaction of the thermal and deformation waves

takes place in metal cutting. As such, if the cutting speed and feed are kept invariable,

the time of one turn of the workpiece changes with its diameter that greatly affects

the discussed interactions. In other words, less residual thermal energy left by the

previous tool pass is available at the current pass when the diameter of the workpiece

increases.

In practical testing, it is important to separate the inﬂuence of each factor. As cutting

tests are conducted with different workpiece diameters, the workpiece diameter (D

)

and its length (L

) should be selected accordingly to keep the ratio L

invariable

to exclude the inﬂuence of the workpiece diameter on the system rigidity.

Cutting Tool Wear, Tool Life and Cutting Tool Physical Resource 249

Cutting tests were carried out where two diameters of the workpiece, 15 and 29 mm,

were used. At ﬁrst, the length of the workpiece was selected to keep the same rigidity

(51×10

N/mm), then the invariable workpiece diameter was used while the length (and

thus rigidity) of the workpiece was varied. The test results are shown in Fig. 4.18(a).

As shown, when the rigidity is kept invariable (by corresponding reduction in L

decreasing the workpiece diameters leads to a certain reduction in the tool wear rate as

well in the roughness of the machined surface. However, if under the same conditions,

is not changed, the tool wear rate and surface roughness increase signiﬁcantly.

Calculations show that the total length of the surface of the maximum combined stress

(often referred to as the shear angle and the length of the shear plane) insigniﬁcantly

depends on the workpiece diameter. For example, changing this diameter from 10 to

500 mm leads to a 5–7% increase in the total length of the surface of the maximum

combined stress (shear plane) depending upon the rake angle and uncut chip thickness.

8060

n (m/min)

40200

900

700

500

opt

(N)

(µm)

040

(mm)

1.5

2.0

2.5

3.0

n=70m/min

40m/min

120m/min

40m/min

70m/min

(a) (b)

q(°C)

(µm/10

)

Fig. 4.18. Inﬂuence of cutting speed and diameter of the workpiece in turning: (a) on the cutting

temperature, tool wear rate and roughness of the machined surface. Work material – custom-

modiﬁed Haynes 263 alloy (0.02% C, 20% Cr, 2%Ti, 2%Al), tool material – micrograin carbide

M10 (94% WC, 6% Co), depth of cut d

= 0.25 mm, cutting feed f = 0.09 mm/rev: 1 − D

29 mm, L

= 230 mm, 2 −D

= 16 mm, L

= 95 mm, 3 −D

= 15 mm, L

= 230 mm Z (b)

on CCR and cutting force. Work material – Haynes 263 alloy (29% Cr, 2.5% Ti), tool material –

carbide M20 (92% WC, 8% Co), depth of cut d

= 0.25 mm, cutting feed f = 0.09 mm/rev.

250 Tribology of Metal Cutting

On this basis, one should expect some redaction in the chip compression ratio when

the diameter of the workpiece decreases. The test results, however, do not support this

hypothesis. As shown in Fig. 4.18(b), with decreasing diameter, a certain increase of

the chip compression ratio is the case. This is explained by the reduction in the energy

required for the fracture of the layer being removed due to the increased amount of

residual thermal energy (higher temperature) from the previous tool pass (see Chapter 2).

It was proved by using the water-based (great cooling ability) cutting ﬂuid for the same

test conditions. When such a ﬂuid was used, the chip compression ratio increases with

decreasing workpiece diameter although such a reduction is poorly correlated with the test

conditions. This is due to the variations in the interaction of the thermal and deformation

waves, which also depends on the workpiece diameter (Chapter 2).

The inﬂuence of the workpiece diameter shows up through the cutting temperature.

When cutting with low cutting speeds (ν = 30 m/min), increasing the workpiece diam-

eter lowers the cutting temperature bringing it down with respect to the optimal cutting

temperature, the ratio of the contact stresses and tool wear rate increases, as shown in

Fig. 4.19. When cutting with high cutting speeds (ν = 50 m/min), increasing the work-

piece diameter reduces the cutting temperature bringing it closer to the optimal cutting

temperature so that the contact stress ratio and tool wear rate are reduced. When cutting

with moderate cutting speeds (ν = 40 m/min), increasing the workpiece diameter ﬁrst

leads to decreasing the tool wear rate and contact stress ratio when the cutting temper-

ature reduces to the optimal cutting temperature. When the cutting temperature lowers

below the optimal cutting temperature, however, increasing the workpiece diameter leads

to increasing the tool wear rate and contact stress ratio.

The inﬂuence of the workpiece diameter at the optimal cutting speed can be expressed

by the following empirical relation,

opt

= C

ν−o

. (4.21)

Table 4.7 presents the value of C

ν−o

and x

ν−o

for some work conditions and materials.

The diameter of the hole being machined affects the cutting process signiﬁcantly in boring

operations. The smaller the diameter of the hole being machined (when the cutting speed

is kept invariable), the greater the chip compression ratio and thus the work of plastic

deformation. As a result, the cutting temperature increases.

The inﬂuence of the diameter of the hole being machined in boring was studied experi-

mentally. In the boring tests, stainless steel AISI 303 was used as the workpiece material,

the diameters of the bored holes were 17, 26 and 37 mm. Cutting regime: depth of cut

= 0.30 mm, cutting feed f = 0.06 mm/rev, ν = 40–160 m/mm, maximum radial

tool wear rate h

= 50 µm.

Figure 4.20 shows the inﬂuence of the cutting speed on the electromotive force (e.m.f.),

chip compression ratio and tool wear rate in boring. As shown, the optimal tool wear

rate depends on the diameter of the hole being machined (when the optimal cutting

temperature is kept invariable). As such, with increasing hole diameter, the optimal

Cutting Tool Wear, Tool Life and Cutting Tool Physical Resource 251

(mm)

400

600

700

800

θ(°C)

opt

1.0

0.8

0.6

0.4

100

300

n = 50 m/min

n = 40 m/min

n = 30 m/min

n = 40 m/min

n = 30 m/min

n = 40 m/min

n = 30 m/min

(µm/10

)

Fig. 4.19. Inﬂuence of cutting speed and diameter of the workpiece on the cutting tempera-

ture, contact stress ratio at the tool–workpiece interface and optimal tool wear rate 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

= 0.25 mm and cutting feed f = 0.09 mm/rev.

cutting speed increases and the tool wear rate and the chip compression ratio decrease.

Figure 4.21 further exempliﬁes these conclusions.

In boring of holes using cutting tools made of carbide M10 (92% WC, 8% Co) when work

material is stainless steel AISI 303, at the above-indicated cutting regime, the optimal

Table 4.7. Values of C

ν−o

and x

ν−o

in Eq. (4.21) for the depth of cut d

= 0.25 mm and feed

f = 0.09 mm/rev.

Materials Diameter of

workpiece

ν−o

Workpiece Tool

Steel AISI 1045 P20 (15% TiC, 6% Co, 79% WC) 35–130 141 0.125

Custom-modiﬁed Haynes 263

alloy (0.02% C, 20% Cr,

2% Ti, 2% Al)

Micrograin carbide M10

(94% WC, 6% Co)

20–50 20.4 0.200

Haynes 263 alloy (29% Cr,

2.5% Ti)

Micrograin carbide M10

(94% WC, 6% Co)

22–90 17.9 0.175

252 Tribology of Metal Cutting

04080 n (m/min)

120 160

e.m.f.(mV)

=37 mm

26 mm

17 mm

e.m.f.opt

(µm/10

)

Fig. 4.20. Inﬂuence of cutting speed and diameter of the hole being machined on the electromotive

force (e.m.f.) and tool wear rate. Work material – stainless steel AISI 303, tool material – carbide

M20 (92% WC, 8% Co), depth of cut d

= 0.30 mm and cutting feed f = 0.06 mm/rev.

cutting speed and optimal tool wear rate correlated with the hole diameter (D

)as

opt

= 16.6D

0.52

(

m/min

)

(4.22)

s−opt

48.8

0.22



µm/10

·sm



(4.23)

Using these equations, one can calculate the optimal cutting speed and optimal tool wear

rate for a wide range of diameters of the machined hole.

When the diameter of the machined hole increases and the cutting temperature is kept

invariant and equal to the optimal cutting temperature, the chip compression ratio, ζ

increases. Under this condition, it can be calculated as:

ζ =

0.4

. (4.24)

Cutting Tool Wear, Tool Life and Cutting Tool Physical Resource 253

(mm)

20 25 30 35

(m/min)

1.2

2.0

2.8

opt

s−o

(µm/10

)

Fig. 4.21. Inﬂuence of diameter of the hole being machined on h

s−o

, ν

opt

and ζ at the invariable

optimal cutting temperature. Turning, work material – stainless steel AISI 303, tool material –

carbide M20 (92% WC, 8% Co).

When the optimal cutting temperature is kept invariable, the dimension wear rate

correlates with the hole diameter as:

= 0.486D

0.30

(

µm/min

)

, (4.25)

The total tool life is,

T =

2.06h

0.30

(

min

)

, (4.26)

and the dimension tool life is

= 205h

0.22





. (4.27)

With the increase in diameter of the hole being machined (when θ

opt

= constant), the

total tool life decreases (the dimension wear rate increases) while the dimension tool

life increases. This apparent contradiction is readily explained by the fact that if θ

opt

constant, boring of holes of greater diameters requires higher cutting speeds to achieve

the same cutting temperature so that the tool would machine a greater area.

When boring with low cutting speeds (ν = 72 m/min), increasing the workpiece diameter

leads to a signiﬁcant increase in the tool wear rate. This happens because the cutting

temperature at ν = 72 m/min for diameter D

= 37 mm is below the optimal cutting

temperature. When boring with a moderate cutting speed (ν = 90 m/min), increasing

the diameter of the hole being machined in boring ﬁrst leads to decreasing the tool

254 Tribology of Metal Cutting

wear rate as the cutting temperature lowers and approaches the optimal cutting temper-

ature, then, reaching its minimum at the optimal cutting temperature, the tool wear rate

increases as the cutting temperature becomes lower than the optimal cutting temperature.

When machining with a high cutting speed (ν = 110 m/min), the tool wear rate reduces

monotonely with increasing hole diameter. This is because the cutting temperature is

high, so increasing the hole diameter leads to its reduction hence it becomes closer to

the optimal cutting temperature.

The foregoing analysis shows that in boring, the established optimal cutting speed for

a certain hole diameter cannot be used if the diameter of the hole being machined is

changed. For example, if a hole of 37 mm diameter is bored at the cutting speed 72 m/min

(which is optimal for a hole of 17 mm dia.), then the dimension tool life reduces by

2.36 times and the productivity of machining by 1.5 times compared to those obtained

at the cutting speed 105 m/min, which is the optimal cutting speed for the latter hole

diameter. Unfortunately, the inﬂuence of the hole diameter has never been considered as

a factor in boring operations.

4.8 Plastic Lowering of the Cutting Edge

4.8.1 Description

As discussed in Chapter 3, highest tool temperatures occur at the tool–chip interface.

When the rake face is heated to temperatures 900–1200

◦

C, a plastic ﬂow begins in vol-

umes adjacent to this face. This ﬂow takes place due to adhesion bonding between

the rake face and the chip. In the case of carbide tool materials, plastic deforma-

tion is greater in the cobalt matrix. This plastic deformation results in tearing-off of

carbide (WC and TiC) grains from “soft” cobalt layers (matrix) that undergo severe

plastic deformation, “ploughting” this “soft” layer by inclusions contacting in the work

material, and “spreading” of the tool material on the chip and workpiece contact sur-

faces. If the temperature increases even further, a liquid layer forms between tool and

workpiece due to diffusion leading to the formation of low-melting-point compound

W having a melting temperature T

= 1130

◦

C. This layer is quickly removed in

cutting [14].

At high cutting speeds, particularly in the machining of difﬁcult-to-machine work mate-

rials, in parallel with plastic ﬂow and spreading of the tool material over the chip and

tool contact surfaces (that was conclusively proven by special tests with radioactive iso-

topes[14]), the plastic lowering of the cutting edge takes place. This plastic lowering is

observed not only carbide with, but also when PCD, metaloceramic and ceramic tool

materials are used. Often, in the machining of difﬁcult-to-machine materials and in high

speed machining, the plastic lowering of the cutting edge is the predominant cause of

premature tool failure. Typical appearance of the discussed plastic lowering is shown

in Fig. 4.22.

Plastic lowering is the plastic deformation of the cutting wedge (a part of the tool between

the rake and the ﬂank contact surfaces) in a way, as shown in Fig. 4.23(a). As shown,

the rake (γ) and ﬂank (α) angles change due to plastic lowering h

of the cutting edge.

Cutting Tool Wear, Tool Life and Cutting Tool Physical Resource 255

(a) (b)

Fig. 4.22. Typical appearance of the plastic lowering of the cutting edge: (a) on a carbide insert

and (b) on a PCD insert.

Tool

Workpiece

Strain

Time

(a)

(b)

20001000200 300 400 500 800

400

500

600

800

1000

1200

1400

1800

t (MPa)

q (°C)

(c)

Fig. 4.23. Plastic lowering of the cutting edge: (a) parameters, (b) a typical engineering creep

curve, (c) creep diagram for typical components of carbide: 1 – Co, 2 – WC–Co, 3 – WC–TiC–Co,

4 – WC and 5 – TiC.

This lowering is characterized by four parameters l

, l

, h

and h

. When these param-

eters reach a certain limit, the breakage of the cutting wedge takes place. To prevent

this from happening, the transition surface between the rake and the ﬂank surface (often

referred to in machining practice as the hone) are often made as ﬁllet or chamfer instead

of the sharp cutting edge (often referred to as the cutting edge preparation).

256 Tribology of Metal Cutting

4.8.2 Cause

As conclusively proven by Makarow [14] and Talantov [24], the primary cause of the

plastic lowering of the cutting wedge is high-temperature creep. It is known that when

temperatures at the tool–chip interface reach 1000–1200

◦

C, the cutting wedge deforms

plastically. Creep is the progressive deformation of a material at constant stress. A typi-

cal engineering creep curve shown in Fig. 4.23(b) represents the dependence of plastic

deformation of a metal when constant load and temperature are applied. As shown, upon

loading of a preheated specimen, deformation increases rapidly from zero to a certain

value ε

known as the initial rapid elongation [19]. There is no need for additional energy

for this deformation because it occurs due to the thermal energy that already exists in the

specimen, so the work done by the internal forces begins with the level of energy that

has already been achieved. In other words, if the temperature is a characteristic of the

thermal energy, and deformation and stress characterize the work done by the external

forces, then the critical amount of energy accumulates in the material as the result of

their summation.

Among the phases normally present in carbides used as the tool material, the plastic

deformation is greater in the cobalt phase, as shown in Fig. 4.23(c), which generalizes

the experimental results obtained by Makarow [14]. The lines in this diagram separate

low and high temperature creep. At low temperature, plastic deformation does not exceed

1% however it reaches 300% on the whole, without fracture (due to diffusion phenomena)

at high temperature creep. The separation between low and high temperature creep is

rather conditional because the occurrence of various creep mechanisms [19] depends not

only on temperature but also on the stressed state and the level of stresses.

Plastic lowering of the cutting edge can be characterized by the ratio of radial wear, h

and with the ﬂank land, h

, i.e. h

= h



. When there is no plastic lowering, h

determined by the tool geometry and does not depend on the machining regime. Experi-

mental observations of changing h

and h

over tool life period showed that h

does not

remain invariable. Its variation and particular value are affected by the properties of the

work and tool materials, machining regime, cutting ﬂuid, etc. For example, increasing

the cutting feed leads to the reduction of h

(Fig. 4.24(a)) that is attributed to increasing

the contact stresses and temperatures. As shown in Fig. 4.24(b), the values of h

are

reduced with the increase in cutting feed when the cutting temperature is equal to the

optimal cutting temperature and kept unchanged.

The main difﬁculty in the determination of the topography of the plastic lowering is that

it takes place simultaneously with the wear of the tool ﬂank. To resolve the problem,

Talantov [24] proposed to present the experimental results on plastic lowering using

different scales for h

and l

shown in Fig. 4.25(a). The example of the results is shown

in Fig. 4.25(b). As shown, the developments of h

, h

and h

in time follow the shape

of the engineering creep curve shown in Fig 4.23(b).

As known [19], high-temperature creep is the process of plastic deformation where

two opposite-signed and equally intensive processes take place namely the growth of

stresses (causes plastic deformation) and their relaxation (caused by plastic deformation).