Mechanism of Metal Cutting that will be helpful

Machine Tools

1. Mechanism of Metal Cutting

2. •Deformation of metal during machining,
• nomenclature of lathe,
•milling tools,
•mechanics of chip formation,
•built-up edges,
•mechanics of orthogonal and oblique cutting,
• Merchant cutting force circle and shear angle relationship
in orthogonal cutting,
•factors affecting tool forces.
•Cutting speed, feed and depth of cut, surface finish.
• Temperature distribution at tool chip interface.
•Numerical on cutting forces and Merchant circle.

3. Mechanism of chip formation in
machining
•Machining is a semi-finishing or finishing
process essentially done to impart required or
stipulated dimensional and form accuracy and
surface finish to enable the product to
• fulfill its basic functional requirements
• provide better or improved performance
• render long service life.

4. •Machining is a process of gradual removal of excess
material from the preformed blanks in the form of
chips.
•The form of the chips is an important index of
machining because it directly or indirectly indicates :
• Nature and behaviour of the work material under
machining condition
• Specific energy requirement (amount of energy required to
remove unit volume of work material) in machining work
• Nature and degree of interaction at the chip-tool
interfaces.

5. •The form of machined chips depend mainly upon :
• Work material
• Material and geometry of the cutting tool
• Levels of cutting velocity and feed and also to some extent
on depth of cut
• Machining environment or cutting fluid that affects
temperature and friction at the chip-tool and work-tool
interfaces.
•Knowledge of basic mechanism(s) of chip formation
helps to understand the characteristics of chips and to
attain favourable chip forms.

6. Mechanism of chip formation in
machining ductile materials
During continuous machining the uncut layer of the work material
just ahead of the cutting tool (edge) is subjected to almost all sided
compression as indicated in Fig. The force exerted by the tool on the
chip arises out of the normal force, N and frictional force, F as
indicated in Fig

7. •Due to such compression, shear stress develops,
within that compressed region, in different
magnitude, in different directions and rapidly
increases in magnitude.
•Whenever and wherever the value of the shear
stress reaches or exceeds the shear strength of
that work material in the deformation region,
yielding or slip takes place resulting shear
deformation in that region and the plane of
maximum shear stress.

8. •But the forces causing the shear stresses in the region
of the chip quickly diminishes and finally disappears
while that region moves along the tool rake surface
towards and then goes beyond the point of chip-tool
engagement.
•As a result the slip or shear stops propagating long
before total separation takes place.
•In the mean time the succeeding portion of the chip
starts undergoing compression followed by yielding
and shear.
•This phenomenon repeats rapidly resulting in
formation and removal of chips in thin layer by layer.

10. •In actual machining chips also, such serrations are
visible at their upper surface as indicated in Fig.
•The lower surface becomes smooth due to further
plastic deformation due to intensive rubbing with
the tool at high pressure and temperature.
•The pattern of shear deformation by lamellar
sliding, indicated in the model, can also be seen in
actual chips by proper mounting, etching and
polishing the side surface of the machining chip
and observing under microscope.

11. •The pattern and extent of total deformation of
the chips due to the primary and the
secondary shear deformations of the chips
ahead and along the tool face, as indicated in
Fig, depend upon
• work material
• tool; material and geometry
• the machining speed (V
C
) and feed (s
o
)
• cutting fluid application

12. Primary and secondary deformation
zones in the chip

13. •The overall deformation process causing chip
formation is quite complex and hence needs thorough
experimental studies for clear understanding the
phenomena and its dependence on the affecting
parameters. The feasible and popular experimental
methods [2] for this purpose are:
• Study of deformation of rectangular or circular grids
marked on the side surface as shown in Fig.
• Microscopic study of chips frozen by drop tool or quick
stop apparatus
• Study of running chips by high speed camera fitted with
low magnification microscope.

14. It has been established by several analytical and experimental methods including circular
grid deformation that though the chips are initially compressed ahead of the tool tip, the
final deformation is accomplished mostly by shear in machining ductile materials.
However, machining of ductile materials generally produces flat, curved or coiled
continuous chips.

15. Mechanism of chip formation in
machining brittle materials
•The basic two mechanisms involved in chip
formation are
• Yielding – generally for ductile materials
• Brittle fracture – generally for brittle materials

16. •During machining, first a small crack develops at the tool
tip as shown in Fig. due to wedging action of the cutting
edge.
•At the sharp crack-tip stress concentration takes place. In
case of ductile materials immediately yielding takes place
at the crack-tip and reduces the effect of stress
concentration and prevents its propagation as crack.
• But in case of brittle materials the initiated crack quickly
propagates, under stressing action, and total separation
takes place from the parent workpiece through the
minimum resistance path as indicated in Fig.
•Machining of brittle material produces discontinuous chips
and mostly of irregular size and shape.

18. Orthogonal and Oblique Cutting
•The two basic methods of metal cutting using
a single point tool are the orthogonal (2 D)
and oblique (3D).
•Orthogonal cutting takes place when the
cutting face of the tool is 90 degree to the line
of action of the tool.
•If the cutting face is inclined at an angle less
than 90 degree to the line of action of the
tool, the cutting action is known as oblique.

19. Oblique Cutting

21. Orthogonal Cutting

22. Mechanics of orthogonal metal
cutting
•During metal cutting, the metal is severely compressed in the area
in front of the cutting tool.
•This causes high temperature shear, and plastic flow if the metal is
ductile.
•When the stress in the workpiece just ahead of the cutting tool
reaches a value exceeding the ultimate strength of the metal,
particles will shear to form a chip element, which moves up along
the face of the work.
•The outward or shearing movement of each successive element is
arrested by work hardening and the movement transferred to the
next element.
•The process is repetitive and a continuous chip is formed.
•The plane along which the element shears, is called shear plane.

23. Assumptions in orthogonal metal
cutting
•No contact at the flank i.e. the tool is perfectly
sharp.
• No side flow of chips i.e. width of the chips
remains constant.
• Uniform cutting velocity.
• A continuous chip is produced with no built up
edge.
• The chip is considered to be held in equilibrium
by the action of the two equal and opposite
resultant forces R and R/ and assume that the
resultant is collinear.

25. Built-up-Edge (BUE) formation
•Causes of formation
•In machining ductile metals like steels with long chip-tool
contact length, lot of stress and temperature develops in
the secondary deformation zone at the chip-tool interface.
• Under such high stress and temperature in between two
clean surfaces of metals, strong bonding may locally take
place due to adhesion similar to welding.
•Such bonding will be encouraged and accelerated if the
chip tool materials have mutual affinity or solubility.
•The weldment starts forming as an embryo at the most
favourable location and thus gradually grows as
schematically shown in Fig.

26. Scheme of built-up-edge formation

27. •With the growth of the BUE, the force, F also
gradually increases due to wedging action of
the tool tip along with the BUE formed on it.
•Whenever the force, F exceeds the bonding
force of the BUE, the BUE is broken or sheared
off and taken away by the flowing chip. Then
again BUE starts forming and growing. This
goes on repeatedly.

28. Characteristics of BUE
•Built-up-edges are characterized by its shape,
size and bond strength, which depend upon:
• work tool materials
• stress and temperature, i.e., cutting velocity and
feed
• cutting fluid application governing cooling and
lubrication.

29. Different forms of built-up-edge
•BUE may develop basically in three different
shapes as schematically shown in Fig.

30. Overgrowing and overflowing of BUE
causing surface roughness
•In machining too soft and ductile metals by
tools like high speed steel or uncoated carbide
the BUE may grow larger and overflow
towards the finished surface through the flank
as shown in Fig.

31. •While the major part of the detached BUE goes away
along the flowing chip, a small part of the BUE may
remain stuck on the machined surface and spoils the
surface finish.
•BUE formation needs certain level of temperature at
the interface depending upon the mutual affinity of the
work-tool materials. With the increase in V
c

and s
o

the
cutting temperature rises and favours BUE formation.
•But if V
C is raised too high beyond certain limit, BUE will
be squashed out by the flowing chip before the BUE
grows.

32. Role of cutting velocity and feed on
BUE formation

33. •Fig. 5.14 shows schematically the role of
increasing V
C
and s
o

on BUE formation (size).
•But sometime the BUE may adhere so strongly
that it remains strongly bonded at the tool tip
and does not break or shear off even after
reasonably long time of machining.
•Such detrimental situation occurs in case of
certain tool-work materials and at speed-feed
conditions which strongly favour adhesion and
welding.

34. Effects of BUE formation
•Formation of BUE causes several harmful effects, such as:
• It unfavorably changes the rake angle at the tool tip causing
increase in cutting forces and power consumption
• Repeated formation and dislodgement of the BUE causes
fluctuation in cutting forces and thus induces vibration which is
harmful for the tool, job and the machine tool.
• Surface finish gets deteriorated
• May reduce tool life by accelerating tool-wear at its rake surface
by adhesion and flaking
•Occasionally, formation of thin flat type stable BUE may
reduce tool wear at the rake face.

35. Types of chips and conditions for
formation of those chips
•Different types of chips of various shape, size,
colour etc. are produced by machining depending
upon
• type of cut, i.e., continuous (turning, boring etc.) or
intermittent cut (milling)
• work material (brittle or ductile etc.)
• cutting tool geometry (rake, cutting angles etc.)
• levels of the cutting velocity and feed (low, medium or
high)
• cutting fluid (type of fluid and method of application)

36. •The basic major types of chips and the
conditions generally under which such types
of chips form are given below:

38. •Often in machining ductile metals at high
speed, the chips are deliberately broken into
small segments of regular size and shape by
using chip breakers mainly for convenience
and reduction of chip-tool contact length.

39. Metal cutting Terminologies
Schematic illustration of a two-
dimensional cutting
process (also called orthogonal
cutting).

41. Chip thickness ratios
The outward flow of the metal causes the chip to be
thicker after the separation from the parent metal.
That is the chip produced is thicker than the depth of
cut.

42. Chip thickness ratio

43. Velocity Relationship

44. Velocity Relationship

46. Benefit of knowing and purpose of
determining cutting forces.
•The aspects of the cutting forces concerned :
• Magnitude of the cutting forces and their
components
• Directions and locations of action of those forces
• Pattern of the forces : static and / or dynamic.

47. •Knowing or determination of the cutting forces
facilitate or are required for :
• Estimation of cutting power consumption, which also
enables selection of the power source(s) during design of
the machine tools
• Structural design of the machine – fixture – tool system
• Evaluation of role of the various machining parameters
( process – V
C
, s
o
, t, tool – material and geometry,
environment – cutting fluid) on cutting forces
• Study of behaviour and machinability characterisation of
the work materials
• Condition monitoring of the cutting tools and machine
tools.

48. Cutting forces

49. Cutting forces

53. Cutting Forces in Oblique Cutting

54. Cutting Forces in Orthogonal Cutting

55. Forces acting on Chip in two-
dimensional cutting

56. The forces acting on the chip in
orthogonal cutting

57. •Fs = Shear Force, which acts along the shear
plane, is the resistance to shear of the metal in
forming the chip.
•Fn = Force acting normal to the shear plane, is
the backing up force on the chip provided by the
workpiece.
•F = Frictional resistance of the tool acting against
the motion of the chip as it moves upward along
the tool.
•N = Normal to the chip force, is provided by the
tool.

58. Merchant’s Circle Diagram

59. The procedure to construct a
merchants circle diagram

75. Thrust Force vs Rake Angle
FIGURE Thrust force as a
function of rake angle and
feed in orthogonal cutting
of AISI 1112 cold rolled
steel. Note that at high rake
angles, the thrust force is
negative. A negative thrust
force has important
implications in the design of
machine tools and in
controlling the stability of
the cutting processes.

76. Shear and Normal Force
FIGURE Shear force and normal force as a function of the area of the shear plane
and the rake angle for 85-15 brass. Note that the shear stress in the shear plane is
constant, regardless of the magnitude of the normal stress. Thus, normal stress
has no effect on the shear flow stress of the material.

77. Shear and Normal Force
FIGURE: Schematic illustration of the distribution of normal and shear stresses at the
tool-chip interface (rake face). Note that, whereas the normal stress increases
continuously toward the tip of the tool, the shear stress reaches a maximum and
remains at that value (a phenomenon know as sticking).

78. Thermal Aspects of Chip Formation
•Machining is inherently characterized by generation of heat and
high
•cutting temperature. At such elevated temperature the cutting tool
if
•not enough hot hard may lose their form stability quickly or wear
out
•rapidly resulting in increased cutting forces, dimensional inaccuracy
of
•the product and shorter tool life. The magnitude of this cutting
•temperature increases, though in different degree, with the
increase of
•cutting velocity, feed and depth of cut, as a result, high production
•machining is constrained by rise in temperature.

79. This problem increases further with the increase
in strength and hardness of the work material.
Knowledge of the cutting temperature rise in
cutting is important, because increases in
temperature:
adversely affect the strength, hardness and wear
resistance of the cutting tool
cause dimensional changes in the part being
machined, making control of dimensional accuracy
difficult and
can induce thermal damage to the machined surface,
adversely affecting its properties and service life.

82. Temperature distribution at tool chip
interface.
FIGURE: Typical temperature
distribution in the cutting
zone. Note that the maximum
temperature is about halfway
up the face of the tool and
that there is a steep
temperature gradient across
the thickness of the chip.
Some chips may become red
hot, causing safety hazards to
the operator and thus
necessitating the use of safety
guards.

83. Temperature Distribution in Turning
FIGURE : Temperature distribution in turning: (a) flank temperature for tool shape
(b) temperature of the tool chip interface. Note that the rake face temperature is
higher than that at the flank surface.

84. Numerical Problems 1
•If, in Orthogonal cutting a tool of
the force components, Fx and Fz are
measured to be 400 N and 800 N respectively
then what will be the value of the coefficient
of friction at the tool interface at that
condition.

85. Numerical Problems 2
•Determine with out using MCD, the values of
Fs (Shear force) and Fn using the following
given values associated with a turning
operation Fz=1000 N, Fx=400 N, Fy=200N,

86. Numerical Problems 3
•During turning a ductile alloy by a tool of Fz=
400 N, Fx= 300 N and Fy = 2.5. Evaluate, using
MCD, the values of F, N and μ as well as Fs and
Fn for the above machining.

87. Numerical Problems 4
•During turning a steel rod of diameter 160
mm at speed 560 rpm, feed 0.32 mm/rev. and
depth of cut 4.0 mm by a ceramic insert of
geometry 0
o
, —10
o
, 6
o
, 6
o
, 15
o
, 75
o
, 0 (mm)
The following were observed:
Fz=1600 N, Fx=800 N and chip thickness = 1 mm.
Determine with the help of MCD the possible
values of F, N, Fs, Fn, cutting power and
specific energy consumption.