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Generalized Model of Chip Formation 17
1.2.3 Comparison of the known solutions for the single-shear plane model with
experimental results
The next logical question is: how good is the single-shear plane model? In other words,
how far is this model from reality? Naturally, during the period of 1950–1960, when
decent dynamometer and metallographic equipment became widely available, a number
of fundamental studies were carried out to answer this question. The results of these
extensive researches are well summarized by Pugh [66] and Chisholm [67]. In the
authors opinion, the best research study and a detailed description of the experimental
methodology were presented by Pugh [66]. The results obtained by Pugh were discussed
by Bailey and Boothroyd ten years later [68]. In his study, all the possible “excuses” for
“inadequate” experimental technique were eliminated. The experimental results conclu-
sively proved that for every work material tested, there is a marked disagreement in the
ϕ vs.
(
µ γ
)
relation between the experiment and the prediction of the Ernest
and Merchant, the Merchants, and the Lee and Shafer theories (Eqs. (1.5), (1.7) and
(1.8), respectively). The examples of the experimental results obtained are shown in
Figs. 1.9–1.12.
Figure 1.9 shows the experimental results for lead as the work material. Although such
a choice of the work material might seem strange, one should realize that lead defi-
nitely has a significant advantage in cutting tests. This is because lead is chemically
passive so it does form neither solid-state solutions nor chemical compositions with
common cutting tool materials. Therefore, the use of lead as the work material allows
to carry out much more “pure” cutting tests. In Fig 1.9, line 1 graphically represents the
Ernst and Merchant solution, line 2 represents the Lee and Shafer solution, and line 3
approximates the experimental results. Figure 1.10 shows the results for various tested
20
Shear angle, ϕ(°)
Dry
0
3040 20
Lubricated
010 10
30
10
5060 40
20
101020 0 20
2
3
30
40
30 5040 60
1
j + mg/4
mg (°)
2j + mg/2
g (°):
Fig. 1.9. Relation between ϕ and
(
µ γ
)
for lead: 1 – Ernst and Merchant solution, 2 – Lee and
Shafer solution and 3 experimental results.
18 Tribology of Metal Cutting
Mild
Steel
Copper
Aluminum
Tin
Lead
30 20 100 102030405060
40
30
20
10
0
50
60
2
1
Shear angle, j(°)
j + mg/4
mg (°)
2j + mg/2
Fig. 1.10. Comparison between calculated and experimental results for tin, aluminum, mild steel,
lead and copper.
work materials. As seen, the experimental results are not even close to those pre-
dicted theoretically. Similar conclusive results were presented by Creveling, Jordon and
Thomsen [69] (an example is shown in Fig. 1.11 for steel 1113, where various cutting
fluids were used) and by Chisholm [67].
The modified Merchant solution in which the shear stress is assumed to be linearly
dependent on the normal stress through a factor k
1
c = cot
1
k
1
τ = τ
0
+k
1
σ (1.19)
(according to Merchant, τ
0
and k
1
are work material constants) has also been examined
for a wide variety of work materials. Equation (1.19) is shown plotted in Figs. 1.12(a)
and (b) for copper and mild steel, respectively, together with the experimentally obtained
values [66]. As shown, the shear stress does not increase with the normal stress at the rate
required by the modified Merchant solution, i.e. to fit the experimental results. In fact,
it would appear that the shear stress is almost independent of the normal stress on the
single shear plane.
The above conclusions were confirmed by Bisacre [66] who conducted similar cutting
experiments. The results of these experiments enabled Bisacre to conclude that if the
Merchant solution (theory) was correct, there would be a marked effect of the normal
stress on the shear stress acting along the shear plane. To support his point, Bisacre
noted that the results of tests carried out, in which the same material was subjected
simultaneously to torsion and axial compression, showed that the shear strength of the
Generalized Model of Chip Formation 19
30
Air
CCl
4
Lusol
20 25 4035
20 100 102030
0
10
50
20
30
40
2
1
g (°):
Shear angle, j(°)
j + mg = π/4
mg (°)
2j + mg = π/2
Fig. 1.11. Relation between ϕ and
(
µ γ
)
for steel SAE 1113.
280 350210
70
70
0
0 140
140
g,(°)
Nornal Stess, s (MPa)
Shear stress, t (MPa)
Shear stress, t (MPa)
210
280
350
Copper
140
210
0 280
250
280
420
560420 700 840
Mild Steel
490
560
(a)
Nornal Stess, s (MPa)
(b)
Dry Lubricated
g,(°)
Dry Lubricated
20
10
20
10
0
20
t = t
0
+k
1
s
k
1
=Cot60°
t = t
0
+k
1
s
k
1
=C
ot4
7
°
Fig. 1.12. Comparison between the estimated and experimentally obtained relationship “shear
stress–normal stress” for copper (a) and steel (b).
material was almost independent of normal stress. As a result, the difference between
the theoretical and experimental results cannot be attributed to the effect of the normal
stress on the shear strength of the work material, as suggested by Merchant.
Zorev also presented clear experimental evidences that the solutions discussed are inade-
quate [70]. He showed that the Merchant solution is not valid even in the simplest case of
cutting at low cutting speeds. Reading this, one may wonder why Zorev did not mention
20 Tribology of Metal Cutting
his findings about the single-shear plane model in his book [29] published five years
later. In the authors opinion, if he had done so, he would have recognized that there
was no model of metal cutting available. As a result, he included the above-discussed
“general solution” for the single-shear plane model “forgetting” to mention that none of
the possible particular solutions to this model is in any reasonable agreement with the
experimental results.
1.2.4 Conclusions
It is conclusively proven that there is a marked disagreement between the solutions
available for the single-shear plane model and the experimental results. Hill, one of the
founders of engineering plasticity [71], noticed [72] that “it is notorious that the extent
theories of mechanics of machining do not agree well with experiment.” Other prominent
researchers in the field conclusively proved that the experimental results are not even
close to those predicted theoretically [66,67,69,70]. Recent researchers further clarified
this issue presenting more theoretical and experimental evidences [38,73,74].
As one might expect, knowing these results, the single-shear plane model would become
history. In reality, however, this is not the case and the single-shear plane model managed
to “survive” all these conclusive facts and is still the first choice for practically all the
textbooks on metal cutting used today [10–12,42,55,62,75]. In contradiction, all the
excellent works showing complete disagreement of this model with reality are practically
forgotten and not even mentioned in modern metal cutting books, which still discuss the
single-shear plane model as the very core of the metal cutting theory. Moreover, the book
“Application of Metal Cutting Theory” [11] is entirely based on this model showing how
to apply it in practical calculations although other research works complain about the
absence of “predictive theory or analytical system which enables us, without any cutting
experiment, to predict cutting performance such as chip formation, cutting force, cutting
temperature, tool wear and surface finish” [2]. It should become clear that any progress in
the prediction ability of the metal cutting theory could not be achieved if the single-shear
plane model is still used.
It is necessary to list the major drawbacks of the single-shear plane model:
Inherent drawbacks
Infinite strain rate. Infinite deceleration and thus strain rate of a microvolume
of the work material passing through the shear plane.
Unrealistically high shear strain. The calculated shear strain in metal cutting
is much greater than the strain at fracture achieved in the mechanical testing of
materials under various conditions. Moreover, when the chip compression ratio
ζ = 1, i.e. the uncut chip thickness is equal to the chip thickness, no plastic
deformation occurs in metal cutting [49], the shear strain, calculated by the
model remains very significant without any apparent reason for that.
Unrealistic behavior of the work material. Perfectly rigid plastic work material
is assumed which is not the case in practice.
Generalized Model of Chip Formation 21
Improper accounting for the resistance of the work material to cut. The
shear strength or the flow shear stress cannot be considered as an adequate
characteristic feature with respect to this because, considered alone, the stress
does not account for the energy spent in cutting.
Unrealistic representation of the tool–workpiece contact. The cutting edge is
perfectly sharp and no contact takes place on the tool flank surface. This is in
obvious contradiction to the practice of machining where the flank wear (due
to the tool flank–workpiece contact) is a common criterion of tool life [76].
Inapplicability for cutting brittle work materials. This model is not applicable
in the case of cutting of brittle materials, which exhibit no or very little plastic
deformation by shear. Nevertheless, the single-shear model is still applied to
model the machining of gray cast iron [77], cryogenic water ice [78], etc.
Ernst and Merchant-induced drawbacks
Incorrect velocity diagram. In the known considerations of velocities in metal
cutting, the common coordinate system is not set, hence the known velocity
diagram consists of velocity components from different coordinate systems. As
a result, unrealistic velocity components are considered.
Incorrect force diagram. The bending moment due to the parallel shift of the
resultant cutting force is missed in the force diagram. As shown [25], this
missed moment is the prime cause for chip formation and thus it distinguishes
the cutting process among other deforming processes. Moreover, the state of
stress imposed by this moment in the chip root causes chip curling.
Constant friction coefficient. Because the friction coefficient at the tool–
chip interface can be thought as the ratio of the shear and normal force on
this interface, the distributions of the normal and shear stresses should be
equidistant over this interface. The available theoretical and experimental data
[12,29,43,52–56] do not conform to this assumption.
1.3 What is the Model of Chip Formation?
In metal cutting, the term “chip formation” has been used since the ninteenth century.
Its initial meaning is the formation of the chip in the primary and secondary deforma-
tion zones. Primary attention was devoted to the kinematic relationships, cutting force
and contact processes at the tool–chip interface. Later on, the chip-breaking problem
became increasingly important with increasing cutting speed and the development of
new difficult-to-machine materials. Even though the term “chip formation” is still in use,
its original meaning has been transformed. The modern sense of this term implies the
chip, which just left the tool–chip interface, is yet to be broken [79,80].
The first widely known classification of the chip was presented by Ernst [61]. Accord-
ing to this classification, there are three basic types of chips found in metal cutting:
Type 1 – discontinuous chip (segmental chip) in which initially compressed layer passes
22 Tribology of Metal Cutting
off with each chip segment. According to Ernst, this type of chip is most easily dis-
posed off; finish of the machined surface is good, when pitch of the segments is small;
Type 2 continuous chip with continuously escaping compressed layer adjacent to the
tool face. According to Ernst, this chip type is the ideal chip form from the stand-
point of quality of finish of the machined surface, temperature of the tool point and
power consumption; and Type 3 continuous chip with built-up edge adjacent to
the tool face. According to Ernst, this chip type is commonly encountered in ductile
materials. Finish is rough due to fragments of built-up edge escaping with the workpiece
[61]. Although these chip types were identified as “classical” [51] and this classifi-
cation is still widely used today in many books on metal cutting [11,42,81], no one
pays attention to either the way these chip types were obtained (cutting regime, tool
and work materials, tool geometry, etc.) or to the physical characteristics of these chip
types.
As is well known [6,12,25,29], the shape of the chip depends primarily on the work
material, cutting regime, and tool material and geometry. According to Ernst [61], these
chip types were obtained in pure orthogonal cutting at extremely low cutting speed
(2 in/min = 0.05 m/min) using very specific work materials (high lead bronze and low
carbon, medium nickel chromium steel SAE 3115) and cutting tool (rake angle 23
).
From the results of numerous experiments presented by Zorev [29], it is conclusively
proven that cutting physics and mechanics of the machining are entirely different at low
and at high cutting speeds as well as in the appearance, shape and metallurgy of the
chip formed.
Using the results of comparison of cutting at low and high cutting speeds obtained by
Zorev [29], one can conclude that the classification discussed cannot satisfy growing
theoretical and practical requirements to understand the nature of chip formation. As a
result, the national industries of the developed countries have adopted more practical
classifications of chip type. For example, in Japan, the Subcommittee “Chip Disposal”
of the Japan, Society for Precision Engineering (JSPE) adopted a revised system of chip
forms, which includes nine chip types, basically classified according to the length of
the chip. Standard ISO 3685-1977 gives a comprehensive chip-form classification based
on the size and shape of various chips generally produced in metal machining. Other
available classifications are discussed in detail by Jawahir and Luttervelt [82].
Unfortunately, the known classifications of the chip formed in machining originate only
from the differences in chip appearance, but pay no attention to the physical state of the
chip, including its state of stress and strain, hardness, texture, etc. Moreover, neither the
complete set of tool geometry parameters nor the cutting regime (for example, the true
uncut chip thickness and its width) is taken into consideration [83]. Thus, the known
classifications are of a post-process nature rather than being of help in making pre-process
intelligent decisions in process optimization and in understanding the tool–chip contact
phenomena.
A need is felt to develop a model of chip formation that can be used to analyze actual
tribological conditions at the tool–chip interface. As such, in addition to the system
concept in metal cutting, time dependence of cutting system parameters and their dynamic
interactions [25,83] and the time axis will be added to this model.
Generalized Model of Chip Formation 23
1.4 System Concept in Metal Cutting
The system concept in metal cutting was first introduced by Astakhov and Shvets [83].
According to this concept, metal cutting is considered to be taking place in a system
consisting of the following components: the tool, the workpiece and the chip. The process
of metal cutting is defined as a deforming process, which takes place in the components
of the cutting that system are so arranged through which the external energy applied
causes the purposeful fracture of the layer being removed. This fracture occurs due to
the combined stress including the continuously changing bending stress causing a cyclic
nature of this process.
The most important property in metal cutting studies is the system time. The system time
was introduced as a new variable in the analysis of the metal cutting system and it was
conclusively proven that the relevant properties of the cutting system’s components are
time dependent [83]. The dynamic interactions of these components take place in the
cutting process causing a cyclic nature of this process.
1.5 Generalized Model of Chip Formation
Although the basics of the cutting tool geometry are discussed in Appendix A, it is
necessary to set here proper definitions for the terms “cutting edge” and “cutting wedge”
that will be used in further considerations because these two are mostly misunderstood
and thus misused in the literature on metal cutting. One should clearly realize that the
cutting edge is the line of intersection of the face and the flank surfaces. Therefore,
the basic characteristics of the cutting edge should be thought of as those that can be
attributed to a line. When one talks about cutting edge geometry, it should be understood
only as the geometric characteristics of a line in terms of its length, 3D shape, etc. It is
highly improper to speak about cutting edge wear because a line cannot have any wear.
The cutting tool geometry can be discussed as attributed to the point of the cutting edge
considered. In this case, however, this line should be considered as belonging to the
corresponding rake or flank surface so that the geometry of the corresponding surface
should be meant.
The working part of the cutting tool is referred to as the cutting wedge, which is enclosed
between the tool rake and flank contact surfaces intersecting to form the cutting edge. This
cutting wedge is under the action of stresses applied on the tool–chip and tool–workpiece
contact surfaces. Moreover, due to the heat that flows into the cutting tool, this wedge
has high temperature. Therefore, when it is said that the cutting tool penetrates into the
workpiece, the penetration of the cutting wedge is meant because the configuration of
the cutting tool outside the cutting wedge does not affect the metal cutting mechanics
and physics of the cutting process (at least theoretically when the tool is considered to
be infinitely rigid as in any model of chip formation).
Consider the cutting wedge (tool) starting to advance into the workpiece under the action
of the applied penetration force P, as shown in Fig. 1.13(a). As a result, the stresses grow
in the workpiece and, as might be expected, the maximum stress occurs in front of the
24 Tribology of Metal Cutting
Cutting tool
Workpiece
P
(a)
PP
(b) (c)
Fig. 1.13. Cutting tool starting to advance into the workpiece.
cutting edge due to stress singularity at the point. When this maximum reaches a certain
limit, depending on the properties of the work material the following happens:
If the work material is brittle, a crack appears in front of the cutting edge
(Fig. 1.13(b)). Later on, this crack leads to the final fracture of the layer being
removed.
If the work material is ductile, a visible crack would not be readily observed because
of the “healing” effect of plastic deformation. Instead, a certain elastoplastic zone
forms in the workpiece in front of the tool rake face, as shown in Fig. 1.13(c). The
dimensions of the plastic and elastic parts of this zone depend on ductility of the
work material. It is understood that for a perfectly plastic work material, the elastic
zone would not form at all, while for a perfectly brittle material the plastic zone
would never form.
This simple qualitative consideration illustrates that the properties of the work material
play an extremely important role from the beginning of chip formation. Because the
general behavior of work materials in metal cutting can be classified as ductile or brittle
depending upon the degree of plastic deformation that a material exhibits in its defor-
mation, this fact should be organically incorporated in the chip formation model. As
such, a certain criterion (or criteria) should be established to classify a given material
as brittle or ductile because no perfectly brittle or perfectly ductile work materials exist
in reality.
In mechanical metallurgy, elongation or strain at fracture is usually used for such a
purpose [81]. It would be necessary to emphasize here, however, that none of the known
criteria of ductility (brittleness) can be regarded as a mechanical property of the work
material because the same material may exhibit considerably different elongations and
strain at fracture depending upon the loading conditions which include the state of stress,
strain rate, temperature, etc. Unfortunately, today the decision about ductility of the work
material is routinely made on the basis of its mechanical properties obtained in one of
the standard material tests under uniaxial stress state, room temperature and moderated
strain rate (4–5 orders lower than that found in metal cutting).
Generalized Model of Chip Formation 25
P
P
(a) (b) (c)
Fig. 1.14. Difference in deformation pattern in compression and in cutting: (a) specimen with the
scribed grid, (b) distortion of the initial grid in compression and (c) distortion of the initial grid in
cutting.
Another important issue follows from Fig. 1.14. Figure 1.14(a) shows a specimen made
of a ductile material with the grid scribed on its cylindrical surface. Figure 1.14(b) shows
grid distortion occurred in compression where simple shearing is the prime deformation
mode. Figure 1.14(c) shows grid distortion occurred in cutting. If one compares deforma-
tion patterns due to compression and cutting, one observes significant difference because
simple shearing is not the prime deformation mode in metal cutting, as suggested by the
single-shear plane model discussed earlier. This simple fact is known from mechanics of
materials and could be easily confirmed by anyone conducting a simple test similar to
that shown in Fig. 1.14. Unfortunately, the known works on metal cutting do not account
for this simple result.
1.5.1 Brittle work materials
Consider the machining of a brittle work material using the system approach in metal
cutting. Two basically different cases are possible here depending on the arrangement
of the components of the cutting system. The first one takes place when the compo-
nents of the cutting system are arranged so that the resultant force R acting on the chip
from the tool rake face intersects the conditional axis of the partially formed chip, as
illustrated in Fig. 1.15. As such, the bending moment that rose in the root of the chip
(Section 1.1) due to the action of this force, causes chip fracture along the path where
the combined stress (compression and bending) exits the fracture stress for this work
material. Figure 1.16 illustrates a system consideration of the model shown in Fig. 1.15.
Phase 1 illustrates the initial stage when the tool comes in contact with the workpiece.
26 Tribology of Metal Cutting
1
1
R
Conditional axis of the chip
Fig. 1.15. Resultant force R intersects the conditional axis of the partially formed chip (after
Astakhov [18]).
1
234
5678
PPPP
PPPP
System Time
System Time
Fig. 1.16. System consideration of the model shown in Fig. 1.15.
To advance the tool into the workpiece, the penetration force P has to be applied to the
tool, provided the workpiece is fixed rigidly. The interaction between the tool and the
workpiece results in the formation of a stressed zone ahead of the tool (Phase 2). Because
there are absolutely no brittle work materials, this zone consists of the elastic and plastic
parts although its plastic part is small compared to the elastic part. When the maximum
stress in the stressed zone reaches a certain limit, a crack forms in front of the cutting
edge (Phase 3). Further increase in the penetration force P leads to the development of
this crack formed (Phase 4). At this stage, cutting completely resembles splitting. Due to