The document provides an overview of the theory of metal cutting. It discusses the mechanics of chip formation, types of chips, cutting tools and their components/angles. It also describes the metal cutting process, orthogonal vs oblique cutting, thermal aspects of cutting, tool wear and life, factors affecting surface finish and machinability. Cutting fluids, their functions and types are also summarized.
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Metal Cutting Theory and Processes Explained
1. UNIT 1
Theory of metal cutting
Mechanics of chip formation, single point cutting tool,
forces in machining, Types of chip, cutting tools –
nomenclature, orthogonal metal cutting, thermal aspects,
cutting tool materials, tool wear, tool life, surface finish,
cutting fluids and Machinability.
2. INTRODUCTION
Components are made into various shapes and sizes by using metals.
Depending the types of tools and operations.
During the metal removal process, various forces act on the cutting
tool and work piece.
3. Metal Removing Process
Non-cutting process (or) Chipless process
Forging, Drawing, Spinning, Rolling, Extruding
Cutting process (or) Chip process
Turning, Drilling, Milling, Planer, Shaping
6. Mechanism of Chip Formation
The type of chip formed during metal cutting depends upon the
machining condition and material to be cut.
The following variables are influencing in producing the type of
chip such as
Mechanical properties of material to be cut in particular ductility and
brittleness.
Depth of cut
Various angles of tool especially rake angle
Cutting speed
Feed rate
Type of cutting fluid
Surface finish required on work piece
8. Nomenclature of Single Point Cutting Tool
Parts of a single point cutting tool
Angles of single point cutting tool
Effects of Back rake angle
Effects of Side rake angle
9. Parts of a single point cutting tool
Shank
Face
Flank
Base
Nose
Cutting Edge
10. Shank: Main body of tool, it is part of tool which is gripped in tool
holder
Face: Top surface of tool b/w shank and point of tool. Chips flow
along this surface.
Flank: Portion tool which faces the work. It is surface adjacent to &
below the cutting edge when tool lies in a horizontal position.
Base: Bearing surface of tool on which it is held in a tool holder.
Nose radius: Cutting tip, which carries a sharp cutting point. Nose
provided with radius to enable greater strength, increase tool life &
surface life.
Cutting edge: It is the junction of face and flank.
13. When will be the positive rake angle used?
To machine the work hardened materials
To machine low strength ferrous and non-ferrous metals
To turn the long shaft of smaller diameters
To machine the metal having lesser recommended cutting speeds
When will be the negative rake angle used?
To machine high strength alloys
The feed rates are high
To give heavy and interrupted cuts
14. Effects of side rake angle
During the cutting process, the amount of chip bend depends on side
rake angle.
15. Tool Signature
Tool angles given in a definite pattern is called tool signature. The tool
angle have been standardized by the American Standards Association
(ASA).
Back rake angle
Side rake angle
End relief angle
Side relief angle
End cutting edge angle
Side cutting edge angle
Nose radius
16. Types of chip formation
Continuous chip
Discontinuous chip
Continuous chip with built-up edge
17. Continuous chip
During cutting of ductile material, a continuous ribbon such as chip is
produced due to pressure of the tool cutting edge in compression and
shear.
It gives the advantage of,
Good surface finish
Improving tool life
Less power consumption
However, the chip disposal is not easy and the surface finish of the
finished work get affected.
18.
19. The following condition favors the formation
of continuous chips
Ductile material such as low carbon steel, aluminum, copper etc.
Smaller depth of cut
High cutting speed
Large rake angle
Sharp cutting edge
Proper cutting fluid
Low friction between tool face and chip interface.
20. Discontinuous chip
Discontinuous chip produced while machining brittle materials such as
grey cast iron, bronze, high carbon steel at low cutting speeds without
fluids.
During machining the brittle material lacks its ductility which results
for plastic chip formation.
21.
22. The following condition favors the formation
of discontinuous chips
Machining of brittle material
Small rake angle
Higher depth of cut
Low cutting speeds
Excess cutting fluid
Cutting ductile material at very low feeds with small rake angle of the
tool.
23. Continuous chip with built-up edge
During the cutting process, the interface temperature and pressure are
quite high and also high fiction between tool-chip interface.
It causes the chip material to weld itself to the tool face near the nose is
called “built-up edge”.
This process gives the poor surface finish on the machined surface and
accelerated wears on the tool face.
24.
25. The following condition favors the formation
of discontinuous chips with built-up edge.
Low cutting speed
Small rake angle
Coarse feed
Strong adhesion between chips and tool interface
Insufficient cutting fluid
Large uncut thickness
26. Types of metal cutting process
Orthogonal cutting process (Two – dimensional cutting)
Cutting edge of the tool is perpendicular to the cutting velocity
vector.
Oblique cutting process (Three dimensional cutting)
Cutting edge is inclined at an acute angle with the normal to the
cutting velocity vector.
27.
28. Sl
No
Orthogonal cutting process Oblique cutting process
1 The chip flows over the tool face and
the direction of chip flow velocity is
normal to the cutting edge.
The chip flows on the tool face
making an angle with the normal
on the cutting edge.
2 The maximum chip thickness occurs
at its middle.
The maximum chip thickness may
not occur at the middle.
3 Tool is perfectly sharp and it contacts
the chip on rake face only.
Frequently, more than one cutting
edges are in action.
4 Tool life is less Tool life is more
29. Thermal aspects
The heat is generated in three region such as shear zone, chip tool
interface region and tool work interface region.
Shear zone
The zone which is affected by the energy required to shear the chip or to
separate the chip and work is called shear zone. The heat generation range
is 80-85%
Chip - tool interface region
The energy used to overcome the friction completely is the source of the
heat. The heat generation range is 15-20%
Tool - work interface region
The energy is supplied to overcome the rubbing friction between flank
face of the tool and work piece is the source of the heat. The heat
generation range is 1-3%
30.
31. The tool temperature increases due to the following factors such as
Cutting speed
Feed
Properties of tool materials etc.
34. Classification of tool wear
Flank wear
Feed < 0.15 mm/revolution
Crater wear
Nose wear
35.
36. TOOL LIFE
Tool life is defined as the cutting time required for reaching a tool life
criterion or time elapsed between two consecutive tool resharpening.
The following are some of ways of expressing tool life.
Volume of metal removed per grind
Number of work pieces machined per grind
Time unit
37. Factors affecting tool life
Cutting speed
Feed and depth of cut
Tool geometry
Tool material
Cutting fluid
Work material
Rigidity of work, tool and machine
41. SURFACE FINISH
Generally, the surface finish of any product depends on the following
factors.
Cutting speed
Feed
Depth of cut
42. CUTTING FLUIDS
Cutting fluids are used to carry away the heat produced during the
machining. At the same time, it reduces the friction between tool and chip.
43. Functions of cutting fluids
It prevents the work piece from excessive thermal distortion
It improves the surface finish
It causes the chips to break up into small parts. It protects the finished
surface from corrosion.
It washes away the chip from the tool.
It prevents the corrosion of work and machine.
44. Properties of cutting fluids
It should have the high heat absorbing capacity
It should be odourless
It should be non-corrosive to work and tool
It should have high flash point
It should have low viscosity
It should be economical to use
45. Types of cutting fluids
Basically two main type of cutting fluids
Water based cutting fluids
Straight (or) heat oil based cutting fluids
46. Water based cutting fluids
To improve the cooling and lubricating properties of water, the soft soap
or mineral oils are added to it. These oils are known as soluble oils.
47. Straight (or) heat oil based cutting fluids
Straight oil based cutting fluids means pure oil based fluids. Most of the
oils are not directly used but it is mixed with other oils.
It is classified into the following subgroups
Mineral oils
Straight fatty oils
Mixed oils
Sulphurised oils
Chlorinated oils
48. Methods of applying cutting fluids
Cutting fluids are used in many ways such as
Drop by drop under gravity
Flood under gravity
Form of liquid jet
Atomised form with compressed air
Through centrifugal action
49. MACHINABILITY
Machinability is defined as the ease with which a material can be
satisfactorily machined. It can also be defined as follows
The life of tool before tool failure
The quantity of the machined surface
The power consumption per unit volume of material removed.
51. Evaluation of machinability
Tool life per grind
Rate of metal removal per tool grind
Surface finish
Dimensional stability of the finished work
Chip hardness
Shape and size of chips
52. Advantages of high machinability
Good surface finish can be produced
Higher cutting speed can be used
Less power consumption
Metal removal rate is high
Less tool wear
53. Machinability index
Machinability index
I =
𝐶𝑢𝑡𝑡𝑖𝑛𝑔 𝑠𝑝𝑒𝑒𝑑 𝑜𝑓 𝑚𝑒𝑡𝑎𝑙 𝑖𝑛𝑣𝑒𝑠𝑡𝑖𝑔𝑎𝑡𝑒𝑑 𝑓𝑜𝑟 20 𝑚𝑖𝑛𝑢𝑡𝑒𝑠 𝑡𝑜𝑜𝑙 𝑙𝑖𝑓𝑒
𝐶𝑢𝑡𝑡𝑖𝑛𝑔 𝑠𝑝𝑒𝑒𝑑 𝑜𝑓 𝑠𝑡𝑎𝑛𝑑𝑎𝑟𝑑 𝑠𝑡𝑒𝑒𝑙 𝑓𝑜𝑟 20 𝑚𝑖𝑛𝑢𝑡𝑒𝑠 𝑡𝑜𝑜𝑙 𝑙𝑖𝑓𝑒
I =
𝑉𝑖
𝑉𝑠
The machinability index for some common materials is given by
Low carbon steel - 55 - 60%
Stainless steel - 25%
Aluminium alloy - 390 - 1500%