1.0 fundamentals of machining (b) (1)
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fundamental of machiningTRANSCRIPT
Chapter 5Temperatures in cutting
Temperature rise is a very important factor in machining because of its major adverse effects such as:
Excessive temperature lowers the strength, hardness, stiffness, and wear resistance of the cutting tool; tools also may soften and undergo plastic deformation; thus tool shape is altered.
Increased heat causes uneven dimensional changes in the part being machined, making it difficult to control its dimensional accuracy and tolerances.
*
Temperatures in cutting
A comprehensive expression for the mean temperature, in orthogonal cutting is
An expression for the mean temperature in turning on a lathe is given by
where V is the cutting speed, and f is the feed of the tool.
a= 0.2 and b= 0.125 for carbide tools
a= 0.5 and b= 0.375 for HSS
*
Temperatures in cutting
Temperature Distribution
Because the sources of heat generation in machining are concentrated in the primary shear zone and at the tool–chip interface, it is to be expected that there will be severe temperature gradients in the cutting zone.
*
Temperatures in cutting
Temperatures in cutting
Temperature Distribution
*
Temperatures in cutting
Temperature Distribution
*
Temperatures in cutting
Techniques for measuring temperature
Temperatures and their distribution in the cutting zone may be determined from thermocouples embedded in the tool and/or the workpiece.
This technique has been used successfully, although it involves considerable effort.
*
Temperatures in cutting
Techniques for measuring temperature
*
Tool life: Wear and failure
We have seen that cutting tools are subjected to
(a) high localized stresses at the tip of the tool
(b) high temperatures, especially along the rake face
(c) sliding of the chip along the rake face
(d) sliding of the tool along the newly cut workpiece surface.
Wear is a gradual process, much like the wear of the tip of an ordinary pencil.
*
Tool life: Wear and failure
Tool wear and the changes in tool geometry during cutting generally classified as flank wear, crater wear, nose wear, notching, plastic deformation of the tool tip, chipping, and gross fracture.
*
Tool life: Wear and failure
*
Flank wear
Flank wear occurs on the relief (flank) face of the tool.
It generally is attributed to
(a) rubbing of the tool along the machined surface, causing adhesive and/or abrasive wear
(b) high temperatures, which adversely affect tool-material properties
*
Flank wear
Cutting speed is the most important process variable associated with tool life, followed by depth of cut and feed, f.
Tool-life curves
*
Flank wear
Fig 21.6 shows the effect of workpiece hardness and
microstructure on tool life in turning ductile cast iron. Note the
rapid decrease in tool life (approaching zero) as the cutting
speed increases. Tool materials have been developed that
resist high temperatures, such as carbides, ceramics, and
cubic boron nitride.
Flank wear
Tool-life curves
*
Flank wear
Allowable wear land
Similarly, cutting tools need to be replaced (or resharpened) when (a) the surface finish of the machined workpiece begins to deteriorate, (b) cutting forces increase significantly, or (c) temperature rises significantly. The allowable wear land for various machining conditions is given in Table 21.4.
*
Flank wear
Flank wear
Optimum cutting speed
We have noted that as cutting speed increases, tool life is reduced rapidly.
On the other hand, if the cutting speed is low, tool life is long, but the rate at which material is removed is also low.
Thus, there is an optimum cutting speed.
*
Crater wear
Crater wear occurs on the rake face of the tool.
Fig 21.18(a) shows the Schematic illustrations of types of wear observed on various cutting tools. (b) Schematic illustrations of catastrophic tool failures. A wide range of parameters influence these wear and failure patterns.
Crater wear generally is attributed to a diffusion mechanism, that is, the movement of atoms across the tool–chip interface.
*
Crater wear
Crater wear
Crater wear
*
2005 Pearson Education South Asia Pte Ltd
Other types of wear, chipping and fracture
Nose wear is the rounding of a sharp tool, due to mechanical and thermal effects. It dulls the tool, affects chip formation, and causes rubbing of the tool over the workpiece, raising its temperature and possibly inducing residual stresses on the machined surface.
*
2005 Pearson Education South Asia Pte Ltd
Other types of wear, chipping and fracture
The notch or groove observed on cutting tools has been attributed to the fact that this region is the boundary where the chip is no longer in contact with the tool.
*
2005 Pearson Education South Asia Pte Ltd
Other types of wear, chipping and fracture
In addition to wear, tools also may undergo chipping, in which a small fragment from the cutting edge of the tool breaks away.
*
Two main causes of chipping are:
*
Tool-conditioning monitoring
With computer-controlled machine tools and automated manufacturing, the reliable and repeatable performance of cutting tools is a critical consideration.
*
Tool-conditioning monitoring
*
Tool-conditioning monitoring
A similar indirect tool-condition monitoring system consists of transducers that are installed in original machine tools or are retrofitted on existing machines.
The design of transducers must be such that they are: (a) nonintrusive to the machining operation, (b) accurate and repeatable in signal detection, (c) resistant to abuse and shop-floor environment, and (d) cost effective.
*
Surface finish and integrity
Surface finish influences not only the dimensional accuracy of machined parts but also their properties and their performance in service.
*
Surface finish and integrity
*
Surface finish and integrity
A dull tool has a large radius along its edges, just as the tip of a dull pencil or the cutting edge of a knife.
*
Surface finish and integrity
Surface finish and integrity
In a turning operation, as in other cutting processes, the tool leaves a spiral profile (feed marks) on the machined surface as it moves across the workpiece.
We can see that the higher the feed, f, and the smaller the tool-nose radius, R, the more prominent these marks will be.
*
Surface finish and integrity
Fig 21.23 shows the schematic illustration of feed marks on a surface being turned (exaggerated).
Factors influencing surface integrity are:
Temperatures generated during processing and possible metallurgical transformations.
Surface residual stresses.
*
Surface finish and integrity
Machinability
The machinability of a material is usually defined in terms of four factors:
Surface finish and surface integrity of the machined part.
Tool life.
The level of difficulty in chip control.
*
Machinability
In machining practice, tool life and surface roughness generally are considered to be the most important factors in machinability.
*
Thermally assisted machining
*
Machining processes are often necessary in order to impart the desired dimensional accuracy, geometric features, and surface-finish characteristics to components, particularly those with complex shapes that cannot be produced economically or with other shaping techniques.
Commonly observed chip types in machining are continuous, built-up edge, discontinuous, and serrated. Important process variables in machining are tool shape and tool material; cutting conditions such as speed, feed, and depth of cut; use of cutting fluids; and the characteristics of the workpiece material and the machine tool.
Concept Summary
2005 Pearson Education South Asia Pte Ltd
Temperature rise is an important consideration, since it can have adverse effects on tool life as well as on the dimensional accuracy and surface integrity of the machined part.
Two principal types of tool wear are flank wear and crater wear. Tool wear depends on workpiece and tool material characteristics; cutting speed, feed, depth-of-cut, and cutting fluids; and the characteristics of the machine tool. Tool failure also may occur by notching, chipping, and gross fracture.
Concept Summary
2005 Pearson Education South Asia Pte Ltd
Surface finish of machined components can affect product integrity adversely. Important variables are the geometry and condition of the cutting tool, type of chip produced, and process variables.
Machinability generally is defined in terms of surface finish, tool life, force and power requirements, and chip control. Machinability of materials depends on their composition, properties, and microstructure.
Concept Summary
Temperature rise is a very important factor in machining because of its major adverse effects such as:
Excessive temperature lowers the strength, hardness, stiffness, and wear resistance of the cutting tool; tools also may soften and undergo plastic deformation; thus tool shape is altered.
Increased heat causes uneven dimensional changes in the part being machined, making it difficult to control its dimensional accuracy and tolerances.
*
Temperatures in cutting
A comprehensive expression for the mean temperature, in orthogonal cutting is
An expression for the mean temperature in turning on a lathe is given by
where V is the cutting speed, and f is the feed of the tool.
a= 0.2 and b= 0.125 for carbide tools
a= 0.5 and b= 0.375 for HSS
*
Temperatures in cutting
Temperature Distribution
Because the sources of heat generation in machining are concentrated in the primary shear zone and at the tool–chip interface, it is to be expected that there will be severe temperature gradients in the cutting zone.
*
Temperatures in cutting
Temperatures in cutting
Temperature Distribution
*
Temperatures in cutting
Temperature Distribution
*
Temperatures in cutting
Techniques for measuring temperature
Temperatures and their distribution in the cutting zone may be determined from thermocouples embedded in the tool and/or the workpiece.
This technique has been used successfully, although it involves considerable effort.
*
Temperatures in cutting
Techniques for measuring temperature
*
Tool life: Wear and failure
We have seen that cutting tools are subjected to
(a) high localized stresses at the tip of the tool
(b) high temperatures, especially along the rake face
(c) sliding of the chip along the rake face
(d) sliding of the tool along the newly cut workpiece surface.
Wear is a gradual process, much like the wear of the tip of an ordinary pencil.
*
Tool life: Wear and failure
Tool wear and the changes in tool geometry during cutting generally classified as flank wear, crater wear, nose wear, notching, plastic deformation of the tool tip, chipping, and gross fracture.
*
Tool life: Wear and failure
*
Flank wear
Flank wear occurs on the relief (flank) face of the tool.
It generally is attributed to
(a) rubbing of the tool along the machined surface, causing adhesive and/or abrasive wear
(b) high temperatures, which adversely affect tool-material properties
*
Flank wear
Cutting speed is the most important process variable associated with tool life, followed by depth of cut and feed, f.
Tool-life curves
*
Flank wear
Fig 21.6 shows the effect of workpiece hardness and
microstructure on tool life in turning ductile cast iron. Note the
rapid decrease in tool life (approaching zero) as the cutting
speed increases. Tool materials have been developed that
resist high temperatures, such as carbides, ceramics, and
cubic boron nitride.
Flank wear
Tool-life curves
*
Flank wear
Allowable wear land
Similarly, cutting tools need to be replaced (or resharpened) when (a) the surface finish of the machined workpiece begins to deteriorate, (b) cutting forces increase significantly, or (c) temperature rises significantly. The allowable wear land for various machining conditions is given in Table 21.4.
*
Flank wear
Flank wear
Optimum cutting speed
We have noted that as cutting speed increases, tool life is reduced rapidly.
On the other hand, if the cutting speed is low, tool life is long, but the rate at which material is removed is also low.
Thus, there is an optimum cutting speed.
*
Crater wear
Crater wear occurs on the rake face of the tool.
Fig 21.18(a) shows the Schematic illustrations of types of wear observed on various cutting tools. (b) Schematic illustrations of catastrophic tool failures. A wide range of parameters influence these wear and failure patterns.
Crater wear generally is attributed to a diffusion mechanism, that is, the movement of atoms across the tool–chip interface.
*
Crater wear
Crater wear
Crater wear
*
2005 Pearson Education South Asia Pte Ltd
Other types of wear, chipping and fracture
Nose wear is the rounding of a sharp tool, due to mechanical and thermal effects. It dulls the tool, affects chip formation, and causes rubbing of the tool over the workpiece, raising its temperature and possibly inducing residual stresses on the machined surface.
*
2005 Pearson Education South Asia Pte Ltd
Other types of wear, chipping and fracture
The notch or groove observed on cutting tools has been attributed to the fact that this region is the boundary where the chip is no longer in contact with the tool.
*
2005 Pearson Education South Asia Pte Ltd
Other types of wear, chipping and fracture
In addition to wear, tools also may undergo chipping, in which a small fragment from the cutting edge of the tool breaks away.
*
Two main causes of chipping are:
*
Tool-conditioning monitoring
With computer-controlled machine tools and automated manufacturing, the reliable and repeatable performance of cutting tools is a critical consideration.
*
Tool-conditioning monitoring
*
Tool-conditioning monitoring
A similar indirect tool-condition monitoring system consists of transducers that are installed in original machine tools or are retrofitted on existing machines.
The design of transducers must be such that they are: (a) nonintrusive to the machining operation, (b) accurate and repeatable in signal detection, (c) resistant to abuse and shop-floor environment, and (d) cost effective.
*
Surface finish and integrity
Surface finish influences not only the dimensional accuracy of machined parts but also their properties and their performance in service.
*
Surface finish and integrity
*
Surface finish and integrity
A dull tool has a large radius along its edges, just as the tip of a dull pencil or the cutting edge of a knife.
*
Surface finish and integrity
Surface finish and integrity
In a turning operation, as in other cutting processes, the tool leaves a spiral profile (feed marks) on the machined surface as it moves across the workpiece.
We can see that the higher the feed, f, and the smaller the tool-nose radius, R, the more prominent these marks will be.
*
Surface finish and integrity
Fig 21.23 shows the schematic illustration of feed marks on a surface being turned (exaggerated).
Factors influencing surface integrity are:
Temperatures generated during processing and possible metallurgical transformations.
Surface residual stresses.
*
Surface finish and integrity
Machinability
The machinability of a material is usually defined in terms of four factors:
Surface finish and surface integrity of the machined part.
Tool life.
The level of difficulty in chip control.
*
Machinability
In machining practice, tool life and surface roughness generally are considered to be the most important factors in machinability.
*
Thermally assisted machining
*
Machining processes are often necessary in order to impart the desired dimensional accuracy, geometric features, and surface-finish characteristics to components, particularly those with complex shapes that cannot be produced economically or with other shaping techniques.
Commonly observed chip types in machining are continuous, built-up edge, discontinuous, and serrated. Important process variables in machining are tool shape and tool material; cutting conditions such as speed, feed, and depth of cut; use of cutting fluids; and the characteristics of the workpiece material and the machine tool.
Concept Summary
2005 Pearson Education South Asia Pte Ltd
Temperature rise is an important consideration, since it can have adverse effects on tool life as well as on the dimensional accuracy and surface integrity of the machined part.
Two principal types of tool wear are flank wear and crater wear. Tool wear depends on workpiece and tool material characteristics; cutting speed, feed, depth-of-cut, and cutting fluids; and the characteristics of the machine tool. Tool failure also may occur by notching, chipping, and gross fracture.
Concept Summary
2005 Pearson Education South Asia Pte Ltd
Surface finish of machined components can affect product integrity adversely. Important variables are the geometry and condition of the cutting tool, type of chip produced, and process variables.
Machinability generally is defined in terms of surface finish, tool life, force and power requirements, and chip control. Machinability of materials depends on their composition, properties, and microstructure.
Concept Summary