2005 Pearson Education South Asia Pte Ltd Machining Processes and Machine Tools Manufacturing Engineering and Technology 1. Fundamentals of Machining 2. Cutting-Tool Materials and Cutting Fluids 3. Machining Processes used to Produce Round Shapes: Turning and Hole Making 4. Machining Processes used to Produced Various Shapes: Milling, Broaching, Sawing and Filing; Gear Manufacturing 5. Abrasive Machining and Finishing Operations
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Chapter 5Machining Processes and Machine Tools
Manufacturing Engineering and Technology
Cutting-Tool Materials and Cutting Fluids
Machining Processes used to Produce Round Shapes: Turning and Hole Making
Machining Processes used to Produced Various Shapes: Milling, Broaching, Sawing and Filing; Gear Manufacturing
Abrasive Machining and Finishing Operations
*
Chapter Objectives
Factors involved in temperature rise and its effects.
How cutting tools wear and fail.
Surface finish and integrity of parts produced by machining.
Machinability of materials.
Chapter Outline
*
Introduction
Cutting processes remove material from the surface of a workpiece by producing chips. Common cutting processes are as follow:
Turning, in which the workpiece is rotated and a cutting tool removes a layer of material as it moves to the left.
Cutting-off operation, where the cutting tool moves radially inward and separates the right piece from the bulk of the blank.
*
Introduction
*
Introduction
showing various features.
Introduction
*
Mechanics of Cutting
Mechanics of Cutting
influenced by changes in the independent variables
listed above, and include:
(b) force and energy dissipated during cutting
(c) temperature rise in the workpiece, the tool, and the chip
(d) tool wear and failure
(e) surface finish and surface integrity of the workpiece.
*
Mechanics of Cutting
if
poor and unacceptable
(c) the workpiece becomes very hot
(d) the tool begins to vibrate and chatter.
*
Mechanics of Cutting
The simple model shown in Fig. 21.3a (referred as the M.E. Merchant model, developed in the early 1940s) is sufficient for our purposes.
This model is known as orthogonal cutting, because it is two dimensional and the forces involved are perpendicular to each other.
*
Mechanics of Cutting
Microscopic examination of chips obtained in actual machining operations have revealed that they are produced by shearing , similar to the movement in a deck of cards sliding against each other.
*
Mechanics of Cutting
mechanism of chip formation by shearing. (b) Velocity
diagram showing angular relationships among the
three speeds in the cutting zone.
*
Mechanics of Cutting
Cutting Ratio
*
Mechanics of Cutting
Cutting Ratio
The reciprocal of r is known as the chip-compression ratio or factor and is thus a measure of how thick the chip has become compared to the depth of cut; hence, the chip-compression ratio always is greater than unity.
*
Mechanics of Cutting
Shear Strain
The shear strain, , that the material undergoes can be expressed as
*
Mechanics of Cutting
Shear Strain
The shear angle has great significance in the mechanics of cutting operations.
It influences force and power requirements, chip thickness, and temperature.
This analysis yielded the expression
*
21.2 Mechanics of Cutting
Shear Strain
Among the many shear–angle relationships developed, another useful formula that generally is applicable is
*
Mechanics of Cutting
Since mass continuity has to be maintained,
*
Mechanics of Cutting
Velocities in the cutting zone
where Vs is the velocity at which shearing take place in the
shear plane.
*
The four main types are:
Continuous
Continuous chips
Continuous chips usually are formed with ductile materials, machined at high cutting speeds and/or high rake angles.
The deformation of the material takes place along a narrow shear zone called the primary shear zone.
*
Continuous chips
*
Built-up edge chips
A built-up edge (BUE) consists of layers of material from the workpiece that gradually are deposited on the tool tip—hence the term built-up.
Built-up edge commonly is observed in practice.
*
Built-up edge chips
The tendency for BUE formation can be reduced by one or more of the following means:
Increase the cutting speeds
Increase the rake angle
Use a sharp tool
Use an effective cutting fluid
*
Built-up edge chips
Serrated chips are semicontinuous chips with large zones of low shear strain and small zones of high shear strain, hence the latter zone is called shear localization.
*
Discontinuous chips
Discontinuous chips consist of segments that may be attached firmly or loosely to each other.
Discontinuous chips usually form under the following conditions:
Brittle workpiece materials, because they do not have the capacity to undergo the high shear strains involved in cutting.
Workpiece materials that contain hard inclusions and impurities or have structures such as the graphite flakes in gray cast iron.
Very low or very high cutting speeds
Large depths of cut.
Discontinuous chips
Lack of an effective cutting fluid.
Low stiffness of the toolholder or the machine tool, thus allowing vibration and chatter to occur.
*
(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
(f) discontinuous chips
Chip Curl
In all cutting operations performed on metals, as well as nonmetallic materials such as plastics and wood, chips develop a curvature (chip curl) as they leave the workpiece surface.
Among factors affecting the chip curl are:
The distribution of stresses in the primary and secondary shear zones.
Thermal effects.
Cutting fluids.
Chip Curl
Generally, as the depth of cut decreases, the radius of curvature decreases; that is, the chip becomes curlier.
*
Chip Breakers
Chip Breakers
Chip breakers have traditionally been a piece of metal clamped to the tool’s rake face, which bend and break the chip.
However, most modern cutting tools and inserts now have built-in chip-breaker features of various designs.
*
Chip Breakers
breakers generally are not necessary, since the chips already
have finite lengths.
Controlled contact on tools
Cutting tools can be designed so that the tool–chip contact length is reduced by recessing the rake face of the tool some distance away from its tip.
This reduction in contact length affects chip-formation mechanics.
Primarily, it reduces the cutting forces and, thus, the energy and temperature.
*
Oblique cutting
The majority of machining operations involve tool shapes that are three-dimensional, thus the cutting is oblique.
Whereas in orthogonal cutting, the chip slides directly up the face of the tool, in oblique cutting, the chip is helical and at an angle i, called the inclination angle.
*
*
Single-point Tool geometry
Oblique cutting
Note that the chip in Fig. 21.9a flows up the rake face of the tool at angle (chip flow angle), which is measured in the plane of the tool face.
Angle αi is the normal rake angle, and it is a basic geometric property of the tool.
This is the angle between line oz normal to the workpiece surface and line oa on the tool face.
The effective rake angle, αe is
*
Oblique cutting
Fig 21.10(a) shows the schematic illustration of a right-hand cutting tool. The various angles on these tools and their effects on machining.
*
Oblique cutting
Oblique cutting
Shaving and skiving
Thin layers of material can be removed from straight or curved surfaces by a process similar to the use of a plane to shave wood.
*
Cutting Forces and Power
reasons:
Data on cutting forces is essential so that:
Machine tools can be properly designed to minimize distortion of the machine components, maintain the desired dimensional accuracy of part and help select appropriate toolholders and workholding devices.
b. The workpiece is capable of withstanding these forces without excessive distortion.
• Power requirements must be known in order to enable the
selection of a machine tool with adequate electric power.
*
Cutting Forces and Power
*
Cutting Forces and Power
The thrust force, acts in a direction normal to the cutting speed.
These two forces produce the resultant force, R, as can be seen from the force circle.
*
Cutting Forces and Power
Note also that the resultant force is balanced by an equal and opposite force along the shear plane and is resolved into a shear force, and a normal force.
*
Cutting Forces and Power
Because the area of the shear plane can be calculated by knowing the shear angle and the depth of cut, the shear and normal stresses in the shear plane can be determined.
The ratio of F to N is the coefficient of friction, μ, at the tool–chip interface, and the angle β is the friction angle.
The magnitude of μ can be determined as
*
Cutting Forces and Power
Thrust force
A knowledge of the thrust force in cutting is important because the tool holder, the workholding devices, and the machine tool must be sufficiently stiff to support this force with minimal deflections.
*
Cutting Forces and Power
Power
Power is the product of force and velocity. Thus, the power input in cutting is
This power is dissipated mainly in the shear zone (due to the energy required to shear the material) and on the rake face of the tool (due to tool–chip interface friction).
The power dissipated in the shear plane is
*
Cutting Forces and Power
Power
Letting w be the width of cut, the specific energy for shearing, us, is given by
Similarly, the power dissipated in friction is
and the specific energy for friction, uf is
*
Cutting Forces and Power
The total specific energy, ut thus is
Because of the many factors involved, reliable prediction of cutting forces and power still is based largely on experimental data, such as those given in Table 21.2.
*
Cutting Forces and Power
Cutting Forces and Power
Measuring cutting forces and power
Cutting forces can be measured using a force transducer (typically with quartz piezoelectric sensors), a dynamometer or a load cell (with resistance-wire strain gages placed on octagonal rings) mounted on the cutting-tool holder.
*
Example 21.1 Relative energies in cutting
*
Example 21.1 Relative energies in cutting
Solution
*
(
)
Manufacturing Engineering and Technology
Cutting-Tool Materials and Cutting Fluids
Machining Processes used to Produce Round Shapes: Turning and Hole Making
Machining Processes used to Produced Various Shapes: Milling, Broaching, Sawing and Filing; Gear Manufacturing
Abrasive Machining and Finishing Operations
*
Chapter Objectives
Factors involved in temperature rise and its effects.
How cutting tools wear and fail.
Surface finish and integrity of parts produced by machining.
Machinability of materials.
Chapter Outline
*
Introduction
Cutting processes remove material from the surface of a workpiece by producing chips. Common cutting processes are as follow:
Turning, in which the workpiece is rotated and a cutting tool removes a layer of material as it moves to the left.
Cutting-off operation, where the cutting tool moves radially inward and separates the right piece from the bulk of the blank.
*
Introduction
*
Introduction
showing various features.
Introduction
*
Mechanics of Cutting
Mechanics of Cutting
influenced by changes in the independent variables
listed above, and include:
(b) force and energy dissipated during cutting
(c) temperature rise in the workpiece, the tool, and the chip
(d) tool wear and failure
(e) surface finish and surface integrity of the workpiece.
*
Mechanics of Cutting
if
poor and unacceptable
(c) the workpiece becomes very hot
(d) the tool begins to vibrate and chatter.
*
Mechanics of Cutting
The simple model shown in Fig. 21.3a (referred as the M.E. Merchant model, developed in the early 1940s) is sufficient for our purposes.
This model is known as orthogonal cutting, because it is two dimensional and the forces involved are perpendicular to each other.
*
Mechanics of Cutting
Microscopic examination of chips obtained in actual machining operations have revealed that they are produced by shearing , similar to the movement in a deck of cards sliding against each other.
*
Mechanics of Cutting
mechanism of chip formation by shearing. (b) Velocity
diagram showing angular relationships among the
three speeds in the cutting zone.
*
Mechanics of Cutting
Cutting Ratio
*
Mechanics of Cutting
Cutting Ratio
The reciprocal of r is known as the chip-compression ratio or factor and is thus a measure of how thick the chip has become compared to the depth of cut; hence, the chip-compression ratio always is greater than unity.
*
Mechanics of Cutting
Shear Strain
The shear strain, , that the material undergoes can be expressed as
*
Mechanics of Cutting
Shear Strain
The shear angle has great significance in the mechanics of cutting operations.
It influences force and power requirements, chip thickness, and temperature.
This analysis yielded the expression
*
21.2 Mechanics of Cutting
Shear Strain
Among the many shear–angle relationships developed, another useful formula that generally is applicable is
*
Mechanics of Cutting
Since mass continuity has to be maintained,
*
Mechanics of Cutting
Velocities in the cutting zone
where Vs is the velocity at which shearing take place in the
shear plane.
*
The four main types are:
Continuous
Continuous chips
Continuous chips usually are formed with ductile materials, machined at high cutting speeds and/or high rake angles.
The deformation of the material takes place along a narrow shear zone called the primary shear zone.
*
Continuous chips
*
Built-up edge chips
A built-up edge (BUE) consists of layers of material from the workpiece that gradually are deposited on the tool tip—hence the term built-up.
Built-up edge commonly is observed in practice.
*
Built-up edge chips
The tendency for BUE formation can be reduced by one or more of the following means:
Increase the cutting speeds
Increase the rake angle
Use a sharp tool
Use an effective cutting fluid
*
Built-up edge chips
Serrated chips are semicontinuous chips with large zones of low shear strain and small zones of high shear strain, hence the latter zone is called shear localization.
*
Discontinuous chips
Discontinuous chips consist of segments that may be attached firmly or loosely to each other.
Discontinuous chips usually form under the following conditions:
Brittle workpiece materials, because they do not have the capacity to undergo the high shear strains involved in cutting.
Workpiece materials that contain hard inclusions and impurities or have structures such as the graphite flakes in gray cast iron.
Very low or very high cutting speeds
Large depths of cut.
Discontinuous chips
Lack of an effective cutting fluid.
Low stiffness of the toolholder or the machine tool, thus allowing vibration and chatter to occur.
*
(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
(f) discontinuous chips
Chip Curl
In all cutting operations performed on metals, as well as nonmetallic materials such as plastics and wood, chips develop a curvature (chip curl) as they leave the workpiece surface.
Among factors affecting the chip curl are:
The distribution of stresses in the primary and secondary shear zones.
Thermal effects.
Cutting fluids.
Chip Curl
Generally, as the depth of cut decreases, the radius of curvature decreases; that is, the chip becomes curlier.
*
Chip Breakers
Chip Breakers
Chip breakers have traditionally been a piece of metal clamped to the tool’s rake face, which bend and break the chip.
However, most modern cutting tools and inserts now have built-in chip-breaker features of various designs.
*
Chip Breakers
breakers generally are not necessary, since the chips already
have finite lengths.
Controlled contact on tools
Cutting tools can be designed so that the tool–chip contact length is reduced by recessing the rake face of the tool some distance away from its tip.
This reduction in contact length affects chip-formation mechanics.
Primarily, it reduces the cutting forces and, thus, the energy and temperature.
*
Oblique cutting
The majority of machining operations involve tool shapes that are three-dimensional, thus the cutting is oblique.
Whereas in orthogonal cutting, the chip slides directly up the face of the tool, in oblique cutting, the chip is helical and at an angle i, called the inclination angle.
*
*
Single-point Tool geometry
Oblique cutting
Note that the chip in Fig. 21.9a flows up the rake face of the tool at angle (chip flow angle), which is measured in the plane of the tool face.
Angle αi is the normal rake angle, and it is a basic geometric property of the tool.
This is the angle between line oz normal to the workpiece surface and line oa on the tool face.
The effective rake angle, αe is
*
Oblique cutting
Fig 21.10(a) shows the schematic illustration of a right-hand cutting tool. The various angles on these tools and their effects on machining.
*
Oblique cutting
Oblique cutting
Shaving and skiving
Thin layers of material can be removed from straight or curved surfaces by a process similar to the use of a plane to shave wood.
*
Cutting Forces and Power
reasons:
Data on cutting forces is essential so that:
Machine tools can be properly designed to minimize distortion of the machine components, maintain the desired dimensional accuracy of part and help select appropriate toolholders and workholding devices.
b. The workpiece is capable of withstanding these forces without excessive distortion.
• Power requirements must be known in order to enable the
selection of a machine tool with adequate electric power.
*
Cutting Forces and Power
*
Cutting Forces and Power
The thrust force, acts in a direction normal to the cutting speed.
These two forces produce the resultant force, R, as can be seen from the force circle.
*
Cutting Forces and Power
Note also that the resultant force is balanced by an equal and opposite force along the shear plane and is resolved into a shear force, and a normal force.
*
Cutting Forces and Power
Because the area of the shear plane can be calculated by knowing the shear angle and the depth of cut, the shear and normal stresses in the shear plane can be determined.
The ratio of F to N is the coefficient of friction, μ, at the tool–chip interface, and the angle β is the friction angle.
The magnitude of μ can be determined as
*
Cutting Forces and Power
Thrust force
A knowledge of the thrust force in cutting is important because the tool holder, the workholding devices, and the machine tool must be sufficiently stiff to support this force with minimal deflections.
*
Cutting Forces and Power
Power
Power is the product of force and velocity. Thus, the power input in cutting is
This power is dissipated mainly in the shear zone (due to the energy required to shear the material) and on the rake face of the tool (due to tool–chip interface friction).
The power dissipated in the shear plane is
*
Cutting Forces and Power
Power
Letting w be the width of cut, the specific energy for shearing, us, is given by
Similarly, the power dissipated in friction is
and the specific energy for friction, uf is
*
Cutting Forces and Power
The total specific energy, ut thus is
Because of the many factors involved, reliable prediction of cutting forces and power still is based largely on experimental data, such as those given in Table 21.2.
*
Cutting Forces and Power
Cutting Forces and Power
Measuring cutting forces and power
Cutting forces can be measured using a force transducer (typically with quartz piezoelectric sensors), a dynamometer or a load cell (with resistance-wire strain gages placed on octagonal rings) mounted on the cutting-tool holder.
*
Example 21.1 Relative energies in cutting
*
Example 21.1 Relative energies in cutting
Solution
*
(
)
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