UNIT - I THEORY OF METAL CUTTING
UNIT - I
THEORY OF METAL CUTTING
1.1 INTRODUCTION
In an industry, metal components are made into different shapes and dimensions by using various
metal working processes.
Metal working processes are classified into two major groups. They are:
Non-cutting shaping or chips less or metal forming process - forging, rolling, pressing, etc.
Cutting shaping or metal cutting or chip forming process - turning, drilling, milling, etc.
1.2 MATERIAL REMOVAL PROCESSES
1.2.1 Definition of machining
Machining is an essential process of finishing by which work pieces are produced to the desired
dimensions and surface finish by gradually removing the excess material from the preformed blank in
the form of chips with the help of cutting tool(s) moved past the work surface(s).
1.2.2 Principle of machining
Fig. 1.1 typically illustrates the basic principle of machining. A metal rod of irregular shape, size
and surface is converted into a finished product of desired dimension and surface finish by machining by
proper relative motions of the tool-work pair.
Fig. 1.1 Principle of machining (Turning) Fig. 1.2 Requirements for machining
1.2.3 Purpose of machining
Most of the engineering components such as gears, bearings, clutches, tools, screws and nuts etc.
need dimensional and form accuracy and good surface finish for serving their purposes. Preforming like
casting, forging etc. generally cannot provide the desired accuracy and finish. For that such preformed
parts, called blanks, need semi-finishing and finishing and it is done by machining and grinding.
Grinding is also basically a machining process.
Machining to high accuracy and finish essentially enables a product:
Fulfill its functional requirements.
Improve its performance.
Prolong its service.
1.2.4 Requirements of machining
The essential basic requirements for machining a work are schematically illustrated in Fig. 1.2.
DEPT. OF MECH. ENGG. 1
,MANUFACTURING TECHNOLOGY - II
The blank and the cutting tool are properly mounted (in fixtures) and moved in a powerful device
called machine tool enabling gradual removal of layer of material from the work surface resulting in its
desired dimensions and surface finish. Additionally some environment called cutting fluid is generally
used to ease machining by cooling and lubrication.
1.3 TYPES OF MACHINE TOOLS
1.3.1 Definition of machine tool
A machine tool is a non-portable power operated and reasonably valued device or system of
devices in which energy is expended to produce jobs of desired size, shape and surface finish by
removing excess material from the preformed blanks in the form of chips with the help of cutting tools
moved past the work surface(s).
1.3.2 Basic functions of machine tools
Machine tools basically produce geometrical surfaces like flat, cylindrical or any contour on the
preformed blanks by machining work with the help of cutting tools.
The physical functions of a machine tool in machining are:
Firmly holding the blank and the tool.
Transmit motions to the tool and the blank.
Provide power to the tool-work pair for the machining action.
Control of the machining parameters, i.e., speed, feed and depth of cut.
1.3.3 Classification of machine tools
Number of types of machine tools gradually increased till mid 20th century and after that started
decreasing based on group technology.
However, machine tools are broadly classified as follows:
According to direction of major axis:
Horizontal - center lathe, horizontal boring machine etc.
Vertical - vertical lathe, vertical axis milling machine etc.
Inclined - special (e.g. for transfer machines).
According to purpose of use:
General purpose - e.g. center lathes, milling machines, drilling, machines etc.
Single purpose - e.g. facing lathe, roll turning lathe etc.
Special purpose - for mass production.
According to degree of automation:
Non-automatic - e.g. center lathes, drilling machines etc.
Semi-automatic - capstan lathe, turret lathe, hobbing machine etc.
Automatic - e.g., single spindle automatic lathe, swiss type automatic lathe, CNC
milling machine etc.
According to size:
Heavy duty - e.g., heavy duty lathes (e.g. ≥ 55 kW), boring mills, planning machine,
horizontal boring machine etc.
Medium duty - e.g., lathes - 3.7 ~ 11 kW, column drilling machines, milling machines etc.
Small duty - e.g., table top lathes, drilling machines, milling machines.
Micro duty - e.g., micro-drilling machine etc.
According to blank type:
Bar type (lathes).
Chucking type (lathes).
Housing type.
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,UNIT - I THEORY OF METAL CUTTING
According to precision:
Ordinary - e.g., automatic lathes.
High precision - e.g., Swiss type automatic lathes.
According to number of spindles:
Single spindle - center lathes, capstan lathes, milling machines etc.
Multi spindle - multi spindle (2 to 8) lathes, gang drilling machines etc.
According to type of automation:
Fixed automation - e.g., single spindle and multi spindle lathes.
Flexible automation - e.g., CNC milling machine.
According to configuration:
Stand alone type - most of the conventional machine tools.
Machining system (more versatile) - e.g., transfer machine, machining center, FMS etc.
1.3.4 Specification of machine tools
A machine tool may have a large number of various features and characteristics. But only some
specific salient features are used for specifying a machine tool. All the manufacturers, traders and users
must know how machine tools are specified.
The methods of specification of some basic machine tools are as follows:
Centre lathe:
Maximum diameter and length of the jobs that can be accommodated.
Power of the main drive (motor).
Range of spindle speeds and range of feeds.
Space occupied by the machine.
Shaper:
Length, breadth and depth of the bed.
Maximum axial travel of the bed and vertical travel of the bed / tool.
Maximum length of the stroke (of the ram / tool).
Range of number of strokes per minute.
Range of table feed.
Power of the main drive.
Space occupied by the machine.
Drilling machine (column type):
Maximum drill size (diameter) that can be used.
Size and taper of the hole in the spindle.
Range of spindle speeds.
Range of feeds.
Power of the main drive.
Range of the axial travel of the spindle / bed.
Floor space occupied by the machine.
Milling machine (knee type and with arbor):
Type; ordinary or swiveling bed type.
Size of the work table.
Range of travels of the table in X - Y - Z directions.
Arbor size (diameter).
Power of the main drive.
Range of spindle speed.
Range of table feeds in X - Y - Z directions.
Floor space occupied.
DEPT. OF MECH. ENGG. 3
, MANUFACTURING TECHNOLOGY - II
1.4 THEORY OF METAL CUTTING
1.4.1 Types of cutting tools
Cutting tools may be classified according to the number of major cutting edges (points) involved
as follows:
Single point: e.g., turning tools, shaping, planning and slotting tools and boring tools.
Double (two) point: e.g., drills.
Multipoint (more than two): e.g., milling cutters, broaching tools, hobs, gear shaping cutters etc.
1.4.2 Geometry of single point cutting (turning) tools
Both material and geometry of the cutting tools play very important roles on their performances
in achieving effectiveness, efficiency and overall economy of machining.
1.4.2.1 Concept of rake and clearance angles of cutting tools
The word tool geometry is basically referred to some specific angles or slope of the salient faces
and edges of the tools at their cutting point. Rake angle and clearance angle are the most significant for
all the cutting tools. The concept of rake angle and clearance angle will be clear from some simple
operations shown in Fig. 1.3.
Fig. 1.3 Rake and clearance angles of cutting tools
Definition
Rake angle (γ): Angle of inclination of rake surface from reference plane.
Clearance angle (α): Angle of inclination of clearance or flank surface from the finished surface.
Rake angle is provided for ease of chip flow and overall machining. Rake angle may be positive,
or negative or even zero as shown in Fig. 1.4 (a, b and c).
(a) Positive rake (b) Zero rake (c) Negative rake
Fig. 1.4 Three possible types of rake angles
Relative advantages of such rake angles are:
Positive rake - helps reduce cutting force and thus cutting power requirement.
Zero rake - to simplify design and manufacture of the form tools.
Negative rake - to increase edge-strength and life of the tool.
Clearance angle is essentially provided to avoid rubbing of the tool (flank) with the machined
surface which causes loss of energy and damages of both the tool and the job surface. Hence, clearance
angle is a must and must be positive (30 ~ 150) depending upon tool-work materials and type of the
machining operations like turning, drilling, boring etc.
4
UNIT - I
THEORY OF METAL CUTTING
1.1 INTRODUCTION
In an industry, metal components are made into different shapes and dimensions by using various
metal working processes.
Metal working processes are classified into two major groups. They are:
Non-cutting shaping or chips less or metal forming process - forging, rolling, pressing, etc.
Cutting shaping or metal cutting or chip forming process - turning, drilling, milling, etc.
1.2 MATERIAL REMOVAL PROCESSES
1.2.1 Definition of machining
Machining is an essential process of finishing by which work pieces are produced to the desired
dimensions and surface finish by gradually removing the excess material from the preformed blank in
the form of chips with the help of cutting tool(s) moved past the work surface(s).
1.2.2 Principle of machining
Fig. 1.1 typically illustrates the basic principle of machining. A metal rod of irregular shape, size
and surface is converted into a finished product of desired dimension and surface finish by machining by
proper relative motions of the tool-work pair.
Fig. 1.1 Principle of machining (Turning) Fig. 1.2 Requirements for machining
1.2.3 Purpose of machining
Most of the engineering components such as gears, bearings, clutches, tools, screws and nuts etc.
need dimensional and form accuracy and good surface finish for serving their purposes. Preforming like
casting, forging etc. generally cannot provide the desired accuracy and finish. For that such preformed
parts, called blanks, need semi-finishing and finishing and it is done by machining and grinding.
Grinding is also basically a machining process.
Machining to high accuracy and finish essentially enables a product:
Fulfill its functional requirements.
Improve its performance.
Prolong its service.
1.2.4 Requirements of machining
The essential basic requirements for machining a work are schematically illustrated in Fig. 1.2.
DEPT. OF MECH. ENGG. 1
,MANUFACTURING TECHNOLOGY - II
The blank and the cutting tool are properly mounted (in fixtures) and moved in a powerful device
called machine tool enabling gradual removal of layer of material from the work surface resulting in its
desired dimensions and surface finish. Additionally some environment called cutting fluid is generally
used to ease machining by cooling and lubrication.
1.3 TYPES OF MACHINE TOOLS
1.3.1 Definition of machine tool
A machine tool is a non-portable power operated and reasonably valued device or system of
devices in which energy is expended to produce jobs of desired size, shape and surface finish by
removing excess material from the preformed blanks in the form of chips with the help of cutting tools
moved past the work surface(s).
1.3.2 Basic functions of machine tools
Machine tools basically produce geometrical surfaces like flat, cylindrical or any contour on the
preformed blanks by machining work with the help of cutting tools.
The physical functions of a machine tool in machining are:
Firmly holding the blank and the tool.
Transmit motions to the tool and the blank.
Provide power to the tool-work pair for the machining action.
Control of the machining parameters, i.e., speed, feed and depth of cut.
1.3.3 Classification of machine tools
Number of types of machine tools gradually increased till mid 20th century and after that started
decreasing based on group technology.
However, machine tools are broadly classified as follows:
According to direction of major axis:
Horizontal - center lathe, horizontal boring machine etc.
Vertical - vertical lathe, vertical axis milling machine etc.
Inclined - special (e.g. for transfer machines).
According to purpose of use:
General purpose - e.g. center lathes, milling machines, drilling, machines etc.
Single purpose - e.g. facing lathe, roll turning lathe etc.
Special purpose - for mass production.
According to degree of automation:
Non-automatic - e.g. center lathes, drilling machines etc.
Semi-automatic - capstan lathe, turret lathe, hobbing machine etc.
Automatic - e.g., single spindle automatic lathe, swiss type automatic lathe, CNC
milling machine etc.
According to size:
Heavy duty - e.g., heavy duty lathes (e.g. ≥ 55 kW), boring mills, planning machine,
horizontal boring machine etc.
Medium duty - e.g., lathes - 3.7 ~ 11 kW, column drilling machines, milling machines etc.
Small duty - e.g., table top lathes, drilling machines, milling machines.
Micro duty - e.g., micro-drilling machine etc.
According to blank type:
Bar type (lathes).
Chucking type (lathes).
Housing type.
2
,UNIT - I THEORY OF METAL CUTTING
According to precision:
Ordinary - e.g., automatic lathes.
High precision - e.g., Swiss type automatic lathes.
According to number of spindles:
Single spindle - center lathes, capstan lathes, milling machines etc.
Multi spindle - multi spindle (2 to 8) lathes, gang drilling machines etc.
According to type of automation:
Fixed automation - e.g., single spindle and multi spindle lathes.
Flexible automation - e.g., CNC milling machine.
According to configuration:
Stand alone type - most of the conventional machine tools.
Machining system (more versatile) - e.g., transfer machine, machining center, FMS etc.
1.3.4 Specification of machine tools
A machine tool may have a large number of various features and characteristics. But only some
specific salient features are used for specifying a machine tool. All the manufacturers, traders and users
must know how machine tools are specified.
The methods of specification of some basic machine tools are as follows:
Centre lathe:
Maximum diameter and length of the jobs that can be accommodated.
Power of the main drive (motor).
Range of spindle speeds and range of feeds.
Space occupied by the machine.
Shaper:
Length, breadth and depth of the bed.
Maximum axial travel of the bed and vertical travel of the bed / tool.
Maximum length of the stroke (of the ram / tool).
Range of number of strokes per minute.
Range of table feed.
Power of the main drive.
Space occupied by the machine.
Drilling machine (column type):
Maximum drill size (diameter) that can be used.
Size and taper of the hole in the spindle.
Range of spindle speeds.
Range of feeds.
Power of the main drive.
Range of the axial travel of the spindle / bed.
Floor space occupied by the machine.
Milling machine (knee type and with arbor):
Type; ordinary or swiveling bed type.
Size of the work table.
Range of travels of the table in X - Y - Z directions.
Arbor size (diameter).
Power of the main drive.
Range of spindle speed.
Range of table feeds in X - Y - Z directions.
Floor space occupied.
DEPT. OF MECH. ENGG. 3
, MANUFACTURING TECHNOLOGY - II
1.4 THEORY OF METAL CUTTING
1.4.1 Types of cutting tools
Cutting tools may be classified according to the number of major cutting edges (points) involved
as follows:
Single point: e.g., turning tools, shaping, planning and slotting tools and boring tools.
Double (two) point: e.g., drills.
Multipoint (more than two): e.g., milling cutters, broaching tools, hobs, gear shaping cutters etc.
1.4.2 Geometry of single point cutting (turning) tools
Both material and geometry of the cutting tools play very important roles on their performances
in achieving effectiveness, efficiency and overall economy of machining.
1.4.2.1 Concept of rake and clearance angles of cutting tools
The word tool geometry is basically referred to some specific angles or slope of the salient faces
and edges of the tools at their cutting point. Rake angle and clearance angle are the most significant for
all the cutting tools. The concept of rake angle and clearance angle will be clear from some simple
operations shown in Fig. 1.3.
Fig. 1.3 Rake and clearance angles of cutting tools
Definition
Rake angle (γ): Angle of inclination of rake surface from reference plane.
Clearance angle (α): Angle of inclination of clearance or flank surface from the finished surface.
Rake angle is provided for ease of chip flow and overall machining. Rake angle may be positive,
or negative or even zero as shown in Fig. 1.4 (a, b and c).
(a) Positive rake (b) Zero rake (c) Negative rake
Fig. 1.4 Three possible types of rake angles
Relative advantages of such rake angles are:
Positive rake - helps reduce cutting force and thus cutting power requirement.
Zero rake - to simplify design and manufacture of the form tools.
Negative rake - to increase edge-strength and life of the tool.
Clearance angle is essentially provided to avoid rubbing of the tool (flank) with the machined
surface which causes loss of energy and damages of both the tool and the job surface. Hence, clearance
angle is a must and must be positive (30 ~ 150) depending upon tool-work materials and type of the
machining operations like turning, drilling, boring etc.
4
