2. Principle of metal cutting:
• Relative motion between the cutting tool and work piece.
• Tool edge comes in contact with the metal, it exerts
pressure on metal.
• Metal severely compressed , causes high temperate shear
stress in metal.
3. Principle of metal cutting: contd…
• As tool advance, stress in the work piece just ahead of
cutting tool reaches a value exceeding the ultimate
strength of metal.
• Particles of metal start shearing away and flow plastically
along the shear plane.
• It forms segment of chip which moves up alone the face of
the tool.
• Cycle of compression, plastic flow and shearing away is
repeated.
• It results into from of a continuously flowing chip.
4. Classification of Metal cutting processes:
• Classified based on the position of the cutting edge of the
cutting tool as-
1] Orthogonal cutting
• Cutting edge of the tool is perpendicular to the direction of
tool travel.
2] Oblique cutting
• Cutting edge of the tool is inclined to the direction of tool
travel.
• Most of the machining carried out in the workshop is
through Oblique cutting.
5. Classification of Metal cutting processes: contd…
1] Orthogonal cutting
• Cutting edge of the tool is perpendicular to the direction of
tool travel.
6. Classification of Metal cutting processes: contd…
2] Oblique cutting
• Cutting edge of the tool is inclined to the direction of tool
travel.
9. Types of chips:
• Chips are separated from the work piece to impart the
required sizes and shape of the work piece.
• Type of chips edge formed is basically function of the work
material and cutting conditions.
• Chips represents the behavior and quality of the process.
• Classified into three types:
1. Continuous chips
2. Continuous chips with built-up edge
3. Discontinuous or Segmented chips
10. Continuous chips
• These types of chips are produced when, machining more
ductile materials.
• Due to large plastic deformations possible with ductile
materials, longer continuous chips are produced.
Types of chips: contd…
11. Continuous chips
• This type of chip is the most desirable, since it is stable
cutting, resulting in generally good surface finish.
• On the other hand, these chips are difficult to handle and
dispose off.
• The chips coil in a helix (chip curl) and curl around the work
and the tool and may injure the operator when break
loose.
• Also, this type of chip remains in contact with the tool face
for a longer period, resulting in more frictional heat.
Types of chips: contd…
12. Types of chips: contd…
Continuous chips with built-up edge
• When machining ductile materials, conditions of high local
temperature and extreme pressure in the cutting zone and
also high friction in the tool-chip interface noticed.
• It will cause the work material to adhere or weld to the
cutting edge of then tool forming the built-up edge.
13. Types of chips: contd…
Continuous chips with built-up edge
• Successive layers of work material are then added to the
built up edge.
• When this edge becomes larger and unstable, it breaks up
and part of it is carried up the face of the tool along with
the chip
• While the remaining is left over the surface being
machined, which contributes to the roughness of the
surface.
• Although, the built-up edge protects the cutting edge of
the tool, changes the geometry of the cutting tool.
14. Discontinuous or Segmented chips
• These types of chips are usually produced when cutting
more brittle materials like grey cast iron, bronze and hard
brass.
• These materials lack the ductility necessary for appreciable
plastic chips formation.
Types of chips: contd…
15. Discontinuous or Segmented chips
• The material ahead of the tool edge fails in a brittle fracture
manner along the shear zone.
• This produces small fragments of discontinuous chips.
• Since the chips break up into small segments, the friction
between the tool and the chips reduces, resulting in better
surface finish.
• These chips are convenient to collect, handle and dispose
off.
Types of chips: contd…
16.
17.
18.
19. Chip thickness or Cutting ratio:
• Ratio of depth of cut [chip thickness prior to deformation]
to the chip thickness after deformation.
• Wedged shaped tool having rake angle α, AB line of shear
plane of shear plane with shear angle ϕ.
28. Force on the chip contd…
• Merchant established a relation between various forces
acting on the chip during orthogonal metal cutting (2D
cutting).
• The forces acting on the chip in orthogonal cutting are as a
result of the cutting force (R) applied through the tool.
• Forces are as follows –
1. Force excreted by the workpiece on the chip
2. Force excreted by the tool on the chip
3. Resultant forces
29. Force excreted by the workpiece on the chip:
• Fs = Shear force or metal resistant to shear during chip
formation, action along the shear plane
• Fn = Compressive normal force or backing up force exerted
by work piece on the chip acting normal to shear plane
30. Force excreted by the tool on the chip:
• N = Normal force excreted by tool on chip, acting normal
to the tool face
• F = Friction force or resistance of the tool against the chip
flow, acting alone tool face
F=µN, Where µ is co-efficient of friction between tool face
and chip
31. Resultant forces:
• R= Resultant force of Fs and Fn
• R’= Resultant fierce of F and N
• Fc= Cutting force, horizontal component of resultant force R
• Ft= Axial feed force or tangential force, thrust force, vertical
component of resultant force R, acting in direction to feed
Fc and Ft can be found out by Force Dynamometer
32. Merchant’s circle diagram:
• It is a graphical representation of different force with help
of circle.
• By knowing Fc, Ft, α, ∅ all the component of force acting on
the chip can be determined.
33. Merchant’s circle diagram: contd…
• Draw line AP equivalent to Fc and line PC equivalent to Ft
with convenient scale
• AP an PC are perpendicular to each other
• Join line AC which is equivalent to resultant force R
35. Merchant’s circle diagram: contd…
• From point A set off line AO making angle ϕ with Fc to cut
circle at point O
• Join OC
• The magnitude of Fs and Fn are now known
36. Merchant’s circle diagram: contd…
• From point A, set off line AB at an angle (90-α) with Fc to
cut circle at point B
• Join BC
• The magnitude of F and N are now known
• Co-efficient of friction at
chip-tool interface is µ
• µ= tan ß = F/N
• Where ß is angle of friction
of tool-chip interface
44. • Measuring equipment which measures cutting forces.
Types:
• Mechanical type
• Strain gauge type
• Electrical type
• Piezoelectric type
• Pneumatic type
• Hydraulic type
• Basic principle is same for all.
• Due to force applied, tool deflects and if its deflection is
measured, it gives force applied on cutting tool.
Tool force dynamometer:
45. • Strain gauge and piezoelectric type are used for measuring
machining forces accurately and precisely.
• Strain gauge type dynamometers are inexpensive but less
accurate and consistent.
• The piezoelectric type are highly accurate, reliable and
consistent but very expensive due to high material cost and
rigid construction.
• Capable of measuring forces in 2 or 3 dimensions.
• For ease of manufacturing and low cost strain gauge type is
used and preferably of 2 dimensions.
Tool force dynamometer: contd…
46. • Force measured are cutting force Fc and thrust force Ft
• 2 full bridges comprising 4 live strain gauges are provided
for Fc and Ft channels.
• These are connected with the strain measuring bridge for
detection and measurement of strain in terms of voltage.
• Measured voltage provides magnitude of the cutting forces
through calibration.
Tool force dynamometer: contd…
49. • The selection of proper cutting material depends on –
• Cutting operation involved
• The work piece material
• The machine to be used
• Production requirement
• Cost of tool material and operation
• Surface finish
• Accuracy desired
Cutting tool materials:
50. • Cutting tool material have to withstand extreme process
conditions specific for cutting –
• High hot hardness
• Toughness
• Wear resistant
• Low co-efficient of friction
• High thermal conductivity and specific heats
• Machinability
Cutting tool materials: contd…
51. • Carbon steels
• High speed steels [HSS]
• Cast alloys or Stellites
• Cemented or sintered carbides
• Ceramic or Oxides
• Whisker-reinforced alumina or Kyon
• Sialon
• Diamonds
• Cubic Boron Nitride [CBN]
• Cermets
• Coronite
Classification of cutting tool materials:
52. • Provides higher tool life and can achieve higher cutting
speed to increase productivity.
• Coating can positively alter-
Tool wear
Friction
Heat generation
Wear resistance to built up edge performance
Hardness
Ductility
Thermal impact resistance
Cutting tool coating:
53. • Coating materials generally used are :
• Titanium Nitride [TiN]
• Titanium Carbide [TiC]
• Titanium Carbonate [TiCN]
• Aluminium Oxide [Al2O3]
• These coating are in the thickness range of 2-15 micron
• Applied on base tool material by :
• Chemical Vapour Deposition [CVD]
• Physical Vapour Deposition [PVD]
Cutting tool coating: contd…
55. Crater wear:
• Developed on rake surface at tool face, at a small distance
for the cutting edge
• Caused by severe abrasion between chip and tool face
• Diffusion of tool material at high temperature
• Possible in ductile material due to continuous chip
• Increases the actual rake angle and make cutting easier
• Weakens the tool wedges and increase the possibility for
tool breakage
Tool wear: contd…
56. Flank wear:
• Occurs on the tool flank below the cutting edge
• Friction or abrasion between newly machined surface of
the workpiece and tool flank results into flank wear
• Increases cutting force and may cause tool failure
• Dimensional accuracy and surface finish of machined part
can be affected
Tool wear: contd…
60. • Tool life is the time period between two consecutive re-
sharpening, with which the tool cuts the material
effectively.
• Tool life is important factor in production work since
considerable time is lost whenever a tool is re-sharpened
and reset on the machine.
• Tool life is actual machining time by which a fresh cutting
tool [or point] satisfactorily works after which it needs
reconditioning or replacement.
Tool life:
61. • Tool life based upon the criterion of the volume of material
removed is derived as follows :
• Volume of material removed per minute,
= πD · t · f · N mm3/min
= πD · t · f · N · T mm3
• Were,
D=Diameter of workpiece , mm
t= depth of cut, mm
F=feed, mm/rev
N=revolution of work piece, rpm
T= time for tool failure, min
Tool life: contd…
A
A
62. • Cutting speed
V = [π D N ] / 60 m/s
= [π D N ] / 1000 mm/min
π D N = 1000 V
• Tool life [TL] or volume of material removed,
= 1000V · t · f · T mm3
Tool life: contd…
A
63. • Cutting speed is the most important parameter which
influence the tool wear and hence tool life
Taylor tool life equation:
66. • The resultant relationship is a straight-line expressed in
equation form called, Taylor tool life equation given by,
V Tn = C
V = Cutting speed, m/min
T = Tool life, min
n = Tool life index
C = Machining index
• Modified version is,
V Tn · fn1 · dn2 = C
f = feed rate, mm/rev
d = depth of cut, mm
Taylor tool life equation: contd…
67. Cutting speed
Cutting temperature
Feed and depth of cut
Tool geometry
Tool material
Workpiece material
Nature and cutting
Rigidity of machine and work
Factor effecting tool life:
