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Fundamentals of cutting

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Continuous chips are usually formed at high rake angles and/or high cutting speeds. ... ( b) View of the rake of a turning tool,showing nose radius R and crater wear ... – PowerPoint PPT presentation

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Title: Fundamentals of cutting


1
Fundamentals of cutting
  • Chapter-20

2
TOPICS
  • Introduction
  • Mechanics of chip formation
  • Types of chips produced in meta cutting
  • Mechanics of oblique cutting
  • Cutting forces and power
  • Temperature in cutting
  • Tool life Wear and failure
  • Surface finish and integrity
  • Machinability

3
Fundamentals of cutting
  • Fig 20.3 Schematic illustration of a
    two-dimensional cutting process,also called
    orthogonal cutting.Note that the tool shape and
    its angles,depth of cut,to,and the cutting speed
    are all independent variables.

Fig 20.1 Examples of cutting process
Fig 20.2 Basic principle of turning operation
4
Introduction
  • Cutting process Remove material from the
    surface of the work piece by producing chips
  • Turning operation the work piece is rotated an
    a cutting tool removes a layer of material as it
    moves to the left
  • Cutting off Cutting tool moves radially inwards
    and separated the right piece from the back of
    the blank.
  • Slab-milling rotating cutting tool removes a
    layer of material from the surface of the work
    piece
  • End-milling rotating cutter travels along a
    certain depth in the work piece and produces a
    cavity

5
Factors influencing cutting process
6
Mechanics of chip formation
  • Orthogonal cutting
  • Rake angle Alpha
  • Relief angle ( clearance angle)
  • Shear angle ( Pi)
  • Thickness of a chip Tc
  • Depth of cut- T0
  • Cutting ratio r To / Tc
  • Sin Pi / Cos ( pi-
    Alpha )

7
Mechanism of chip formation
  • Fig 20.4 (a) Schematic illustration of the basic
    mechanism of chip formation in metal cutting. (b)
    Velocity diagram in the cutting zone.

8
Mechanism of chip formation
  • Chip compression ratio 1 / r
  • Always gt unity
  • On the basis of fig 20.4-a
  • Shear strain gama
  • Gama AB/OC AO/OC OB/OC
  • Gama Cot Pi tan ( Pi Alpha )
  • Note for actual cutting operation shear strain
    gt 5

9
Mechanism of chip formation
  • Shear angle adjusts itself to minimize cutting
    force
  • Shear plane is the plane of maximum shear stress
  • Pi 45 Alpha / 2 Beta / 2
  • Beta Friction angle
  • Mu coefficient of friction
  • Mu tan beta

10
Mechanism of chip formation
  • Mass continuity has to be maintained
  • So , we have
  • V To Vc Tc
  • Vc Vr
  • Vc V Sin pi / Cos ( pi Alpha )
  • Vc Velocity of a chip
  • V Cutting Speed
  • Vs Velocity of shearing
  • From trigonometric relation
  • V / cos ( pi Alpha ) Vs / Cos ( Alpha ) Vc
    / Sin ( pi )

11
Types of chips
  • Continuous
  • Built up edge
  • Serrated or segmented
  • Discontinuous
  • Fig20.5 Basic types of chips and their
    photomicrographs produced in metal cutting (a)
    continuous ship with a narrow,straight primary
    shear zone (b) secondary shear zone at the chip
    tool interface(c) continuous chip with large
    primary shear zone (d) continuous chip with
    built-up-edge(e) segmented or nonhomogeneous
    chip and (f) discontinuous chips

12
Continuous chips
(b) Surface finish in turning 5130 steel with a
built-up edge
  • Fig 20.6 (a) Hardness distribution in the
    cutting zone for 3115 steel.Note that some
    regions in the built-up edge are as mach as three
    times harder than the bulk metal

(c) Surface finish on 1018 steel in face milling
13
Continuous chips
  • Continuous chips are usually formed at high rake
    angles and/or high cutting speeds.
  • A good surface finish is generally produced.
  • continuous chips are not always desirable,
    particularly in automated machine tools,
  • tend to get tangled around the tool
  • operation has to be stopped to clear away the
    chips.

14
Built-up edges chips
  • BUE consists of layers of material from the
    workpiece that are gradually deposited on the
    tool.
  • BUE then becomes unstable and eventually breaks
    up
  • BUE material is carried away on the tool side of
    the chip
  • the rest is deposited randomly on the workpiece
    surface.
  • BUE results in poor surface finish
  • reduced by increasing the rake angle and
    therefore decreasing the depth of cut.

15
Discontinuous chips
  • Discontinuous chips consist of segments that may
    be firmly or loosely attached to each other
  • These chips occur when machining hard brittle
    materials such as cast iron.
  • Brittle failure takes place along the shear plane
    before any tangible plastic flow occurs
  • Discontinuous chips will form in brittle
    materials at low rake angles (large depths of
    cut).

16
Serrated chips
  • Figure 20.5e
  • Segmented chips or non-homogeneous chips
  • Semi continuous chips with zones low and high
    shear strain
  • Low thermal conductivity and strength metals
    exhibit this behavior

Fig 20.5 (e)segmented or nonhomogeneous chip and
17
Chip Breakers
  • Long continuous chip are undesirable
  • Chip breaker is a piece of metal clamped to the
    rake surface of the tool which bends the chip and
    breaks it
  • Chips can also be broken by changing the tool
    geometry,thereby controlling the chip flow
  • Fig 20.7 (a) Schematic illustration of the action
    of a chip breaker .(b) Chip breaker clamped on
    the rake of a cutting tool. (c) Grooves in
    cutting tools acting as chip breakers

18
Chip Breakers
FigVarious chips produced in turning a)tightly
curled chip b)chip hits workpiece and breaks
c)continuous chip moving away from workpieceand
d)chip hits tool shank and breaks off
19
Chip Formation in Nonmetallic Materials
Fig a) cutting with an oblique tool b) Top view
showing the inclination angle, i. c) Types of
chips produced with different inclination
20
Mechanism of Oblique Cutting
  • The cutting edge is at an angle i, called
    inclination angle.
  • The chip movement is in lateral direction
  • Fig a)right hand cutting tool.Although these
    tools have traditionally been produced from
    solids tool-steel bars,they have been largely
    replaced by carbide or other inserts of various
    shapes and sizes,as shownin b).The vcarious
    angles on these tools and their effects on
    machining are described

21
Temperature In Cutting
FigPercentage of the heat generated in cutting
going into the workpiece,tool,and chip,as a
function of cutting speed.
FigTypical temperature distribution in the
cutting zone.
22
Temperature Distributions
FigTemperatures developed in turning 52100
steel a) flank temperature distributionand
b)tool-chip interface temperature distribution
23
Tool Life Wear and Failure
  • Flank wear It occurs on the relief face of the
    tool and the side relief angle.
  • Crater wearIt occurs on the rake face of the
    tool.
  • Chipping Breaking away of a small piece from the
    cutting edge of the tool .
  • Fig (a) Flank and crater wear in a cutting
    tool.tool moves to the left. (b) View of the rake
    of a turning tool,showing nose radius R and
    crater wear pattern on the rake face of the tool
    c)View of the flank face of a turning tool,sowing
    the average flank wear land VB and the
    depth-of-cut line (wear notch)

24
Wear and Tool Failures Crater wear
  • Fig (a) Schematic illustrations of types of wear
    observed on various types of cutting tools .(b)
    Schematic illustrations of catastrophic tool
    failures.A study of the types and mechanism of
    tool wear and failure is essential to the
    development of better tool materials

25
Forces acting in 2-Dimensional cutting
  • Cutting forces can be measured by using suitable
    dynamometers or force transducers mounted on the
    machine tool
  • They can also be calculated from the amount of
    power consumption,that occurs during cutting.
  • Fig Forces acting on a cutting tool in a two
    dimensional cutting .Note that the resultant
    force,R,must be collinear to balance the forces

26
THE END
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