SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
LESSON THREE SURFACE QUALITY
Surface quality is another aspect of machining
accuracy,It consists of the surface finish and the physic-
mechanical state of the surface layer,It affects the proper
functioning and service life of the parts.
Surface Finish
Machining accuracy is expressed quantitatively in
machining error,Surface finish is expressed quantitatively
in surface roughness,The surface roughness on a
machined workpiece surface is caused primarily by the
geometrical factors (cutting tool geometry and cutting
feed),the plastic deformation of the surface material,as
well as the vibration of the MFTW system during
machining,几何形状
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The surface roughness of a workpiece after machining
depends upon many technological factors,such as the workpiece
material,tool material,tool geometry,cutting conditions (cutting
speed and feed),rigidity of the MFTW system,cooling condition
in machining process,and so on,The surface roughness is a kind
of micro geometrical deviation or micro-irregularity of workpiece
surfaces,It appears on all machined workpiece surfaces no matter
how smooth they look to the naked eye,In regard to the criteria
for evaluating surface roughness,refer to other textbooks,ISO,
and other standards and references.
The surface roughness of machined parts influences their
performance to a great extent,In order to ensure product quality,
to improve its service life,and to reduce its production cost,the
surface roughness of machine parts has to be specified accurately
in design and carefully controlled in manufacturing.
不规则性,不平度 criterion
ISO:International Standardization Organization
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Physic-Mechanical State of Surface Layer
In machining process,the metal nearest the
workpiece surface deforms plastically,This is due to
the intrusion of the cutting tool rake to the workpiece
surface material,the friction between the tool relief
and the machined surface,and the effect of the tool
nose radius,This results in the surface layer on the
workpiece in quite a different way from the internal
material of the workpiece after machining,Until now,
the evaluation of the physic-mechanical state of the
surface layer is still in the experimental investigation
stage,Complete standards for the evaluation do not
exist,However,it is known that the variations of the
material on the surface layer from the original metal
are caused by cold hardening,metallographical
change,and residual stress.
intrusion
硬挤性
tool rake
刀具前面
刀具前角
tool relief
刀具后面
刀具后角
tool nose
刀尖
cold
hardening
冷作硬化
residual
stress
残余应力金相组织变化
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The cold hardening of the surface layer material is caused
by plastic deformation of the material,resulting in the
increase of its microhardness,The parameters which
characterize the cold hardening are shown below:
The hardened depth h,which is the depth of the surface
layer,deformed plastically in the machining process.
The microhardness H,The degree of cold hardening N,
which is the ratio of the microhardness,increases the surface
layer to the microhardness of the original metal,i.e.,
N = [(H-H0)/H0] × 100% 6.3
where H0 stands for the microhardness of the original metal.
minus
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
A large part of the energy consumed in machining is
transformed into heat,Which results in the increase of the
temperature in the cutting or grinding area,especially on the
workpiece surface being machined,In general cutting
processes,most of the heat generated is carried away by the
chips,and the temperature of the workpiece surface is not
very high,However,in the machining processes consuming a
very large amount of energy,for instance as in grinding,the
temperature of the workpiece surface may reach or exceed
the critical point of phase change of workpiece material,This
condition causes the metallographical variation of the surface
material.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The residual stress is generated in the surface layer after
machining,The causes of the generation of the residual stress are:
(i) The material of the workpiece surface expands while it is heated
by the cutting or grinding heat,and it contracts when it is cooled.
The expansion and contraction are resisted by the internal material
of the workpiece which results in the residual tensile stress in the
surface layer after machining; (ii) The surface material plastically
deforms under the extrusion and friction of the tool,while the
internal material close to the surface layer elastically deforms,After
machining,the recovery of the elastic deformation of the internal
material is restricted by the surface material,which has deformed
plastically,This results in the residual stress,usually compressive
residual stress,in the surface layer; (iii) The metallographical
variation of the surface layer leads to its volume change,either
expansion or contraction,which is restricted by the internal
material and results in the residual stress (compressive or tensile) in
the surface layer.
挤
压
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Influence of Surface Quality on the Performance of
Machined Parts
A field failure of a machined part caused by its wear,
fatigue or corrosion,etc.,starts from the part surface in most
cases,The surface quality of a machine part greatly influences
its performance,such as the fitting between parts,wear
resistance,corrosion resistance,and fatigue strength.
Influence on Fitting between Parts
The surface roughness influences directly the fitting
condition between parts,For the clearance fit,the existence of
the micro-irregularities on part surfaces leads to a rapid initial
wear,Thus the clearance increases between the mating parts,
which cause the deterioration of the fitting condition.
强度 间隙配合
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
As to the interference fit,the strength of fit
depends upon the surface roughness of the parts.
When one part is press-fitted into another,the
ridges of the micro-irregularities are extruded:
This reduces the actual interference and thus the
strength of fit,The actual interference can be
calculated by the following equation:
e = (Ds - Dh) - l.2[(Hmax)s + (Hmax)h] 6.4
where e— actual interference,
(Hmax)s,(Hmax)h— maximum height of
irregularities of shaft and hole,respectively,
Ds— diameter of shaft,
Dh— diameter of hole.
interference fit
过盈配合
ridge 波峰
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The surface roughness and surface hardening influence
greatly the wear resistance of part surfaces,When dry friction
exists between two fitting surfaces,only the ridges of the
irregularities on both surfaces are in contact with each other
at the initial stage,The pressure between the two mating parts
is concentrated on these small areas.
For example,the actual contacting area for turned or
milled surfaces is only 15-20% of the total area,and for the
finely ground surfaces,30-50%,Due to the high unit
pressure,the irregularities on the part surfaces deform
elastically and plastically and portions are sheared because of
the relative movement of the surfaces,These fallen particles
are scattered,which intensify the wear between the
contacting surfaces,shear 剪切，剪断
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The situation of the wet friction is more complicated,At the
initial stage of the wet friction the lubricant films are pierced
through by the ridges of the roughness to form a dry friction
between two fitting surfaces,The intensive initial wear changes
the proper fit of the mating parts,However,as the wear gradually
increases,the unit pressure between the surfaces reduces,and the
wear slows down,When a certain point is reached,the wear is
intensified again.
A reduction of the surface roughness can increase the wear
resistance of part surface,However,the relationship between them
is not linear,It has been proved by experiments that an optimal
value for the surface roughness exists under a given condition
with which the minimum wear can be expected,If the fitting
surfaces are too smooth,they will be in close contact with each
other,which leads to a larger affinity between the molecules of the
surfaces,This will result in an intensive wear of the surfaces.
lubricant film 润滑油膜亲合力,亲合性
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The hardening of the surface layer can greatly increase the
wear resistance of part surface,However,an excessive increase
of the microhardness may cause the peeling of the hardened
layer,Therefore,caution should be exercised when applying
microhardening.
Influence on Corrosion Resistance
The surface roughness greatly influences the corrosion
resistance of a part surface,Usually the corrosive substances
are gathered at the valleys of the surface micro-irregularities,
which will attack the part surface,The deeper and more sharply
defined the valleys between the ridges of microirregularities,
the more destructive the effect of corrosion directed into the
depth of the metal.
peel 剥皮，脱皮 valley 山谷，波谷
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The hardening of the surface layer and the existence of the
residual stress in the surface layer will reduce the corrosion
resistance of part surface,This is because the structure of the
surface layer after plastic deformation is in a non-equilibrium
state,possessing higher energy,It is more prone to corrosion.
Influence on Fatigue Strength
The destructive failure of metal parts under alternate loads
starts from part surfaces or under a certain depth of the
hardened surfaces,Consequently the fatigue strength of a part
depends upon the surface quality of the part to a great extent.
Subject to periodically alternate load,the valleys between the
ridges of surface micro-irregularities may become the points of
internal stress concentration,which may result in the failure of
the part,The higher the type of surface finish,the higher the
fatigue strength of the part.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Cold hardening of the surface layer prevents the extension
of existing microcracks,The generation of new microcracks
on a part surface reduces the harmful effect of the surface
roughness and the external defects of the part,This helps the
increase of the fatigue strength.
However,the excessive cold
hardening of the surface layer
may intensify the extension of
the microcracks,Under alternate
load or high temperature,this
decreases the fatigue strength of
the part,Therefore,cold hardening
should be controlled within certain
limits.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The influence of the residual stress in surface layer on
the fatigue strength of a part depends on the direction and
the magnitude of the residual stress,The compressive
residual stress tends to close the microcracks and thus its
existence can greatly increase the fatigue strength of the
part,On the other hand,the existence
of the tensile residual stress will
decrease the fatigue strength.
However,since the generation of
the tensile residual stress is always
accompanied with the hardening
of the surface layer,this harmful
effect is reduced.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
It can be seen from the above that the
hardening of the surface layer and the existence
of the compressive residual stress are conductive
to the increase of the fatigue strength of parts.
Appropriate methods can be adopted to generate
hardening and compressive residual stresses in
the surface layer,such as the ball peening,the
burnishing,etc.,Some heat-treatment methods,
such as carbonizing and nitriding,can also
generate the compressive residual stress in the
surface layer,In addition,some micro-finishing
methods,such as polishing and vibrated
finishing,etc.,can be used to improve the
surface finish,thus leading to the increase of the
fatigue strength of parts.
peening
喷丸硬化
burnishing
压力抛光
carbonize
渗碳
nitriding
渗氮
polishing
抛光
finishing
修整，磨光
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
In machining,factors affecting machining accuracy are
numerous and complicated,In order to improve the
machining accuracy of an operation,the causes leading to
machining errors for the specific operation must be located
through synthetical analysis and investigation,On this basis,
proper technological measures can be taken to control the
influences of these factors on the machining accuracy.
The methods for the improvement of machining
accuracy can be summarized by the reduction or elimination
of machining errors,compensation and transfer of
machining errors,and machining based on workpieces
classification or combination.
Methods for Machining Accuracy Improvement
补偿
综
合
的
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Reduction or Elimination of Machining Errors
The reduction or elimination of machining errors,whenever
possible,are effective ways to improve the machining accuracy.
It begins with identifying the root causes of the errors for the
specific machining operation,Appropriate measures can then be
taken to reduce or to eliminate the influences of these causes on
machining errors,Usually machining errors can be reduced or
eliminated through the following technological measures:
l,Improve the geometrical accuracy of the component parts of
the MFTW system.
2,Minimize the deformation of each component part of the
MFTW system,especially when machining the workpiece with
low rigidity,It is because it tends to deform under external
forces and heating and to vibrate that deteriorates the surface
finish of the workpiece,deteriorate 恶化
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
3,Properly select the machining conditions (speed and feed),
which influence both machining accuracy and surface finish.
4,Improve the machine tool set-up accuracy through using
the more accurate adjusting and measuring tools and devices,
and carefully conducting the set-up work.
5,Minimize the workpiece setting errors through the proper
selection of the locating references and the rational design of
the workholders.
6,Arrange the sequence of operations to reduce or eliminate
some of the machining errors,It also prevents the attained
accuracy from damage by subsequent operations (details will
be discussed in the later chapters).
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Compensation and Transfer of Machining Errors
When it is impossible to completely eliminate machining
errors for some technological or economical reasons,
compensation or transfer of machining errors can be used to
improve machining accuracy,Machining error compensation
is devised to counteract a machining error in the MFTW
system by substituting another error,The counteracting error
is purposely exerted on the system,It should be equal to the
existing error in magnitude and opposite in direction.
抵抗，抵消
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
For instance,the machining error caused by tool wear can
be compensated for by the automatic tool compensation
system on an NC machine,A long shaft with a small diameter
and a low rigidity would tend to deform in the turning
operation,In order to reduce the machining error caused by
the workpiece deformation,two turning tools opposite to each
other can be used to turn the shaft simultaneously,This can
reduce the radial cutting force and thus reduces the
deformation of the workpiece,A certain machining error is
caused by the deformation of a workpiece with low rigidity
under external forces in the machining operation,The error
can be minimized if the workpiece is pre-stressed in its
setting to create an elastic deformation in the direction
opposite to that of the deformation.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Machining error transfer is used
frequently in machining operations as well.
It transfers the sources of the machining
errors from one part of the MFTW system
to another,As an example,the accuracy of
a set of concentric precision holes can be
attained by using a precision boring
machine,However,if these holes are
machined on a radial drilling machine,
which cannot ensure the required accuracy,
a precision boring jig can be adopted to
ensure the accuracy of the hole series.
This means the machining errors are
transferred from the radial drilling
machine to the boring jig.
concentric
同心，同轴
boring machine
镗床
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Machining based on Workpiece Classification or
Combination
The machining accuracy of an operation also depends
upon the variation of the workpiece dimensions after
machining in the preceding operation,The variation of the
workpiece dimensions prior to the operation may lead to
machining errors in the machining process due to the
reproduction of errors or to the inaccuracy of workpiece
setting,These machining errors can be reduced through
machining based on workpiece classification.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The workpieces are classified into n groups according
to the directions and the values of the errors on the
workpieces prior to the operation,Thus the range of
workpiece errors in each group is reduced to l/n of that of
the entire batch of workpieces,Each group of workpieces
is machined under a specific machine tool set-up,which
adapts the range of workpiece errors of that particular
group,This greatly decreases the influence of the
machining errors of the preceding operations on the
current operation.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Sometimes the mating part can be combined and
machined in one operation,In this case,the workpieces
will not be affected by the inaccuracy of some component
parts of the MFTW system,For example,a higher
concentricity between the cylindrical surfaces on two
mating parts is attained when they are combined and
machined,Similarly,higher machining accuracy can be
obtained through the corrective finishing of lathe chucks
or the locating surfaces of workholders mounted on
machine tools.
LESSON THREE SURFACE QUALITY
Surface quality is another aspect of machining
accuracy,It consists of the surface finish and the physic-
mechanical state of the surface layer,It affects the proper
functioning and service life of the parts.
Surface Finish
Machining accuracy is expressed quantitatively in
machining error,Surface finish is expressed quantitatively
in surface roughness,The surface roughness on a
machined workpiece surface is caused primarily by the
geometrical factors (cutting tool geometry and cutting
feed),the plastic deformation of the surface material,as
well as the vibration of the MFTW system during
machining,几何形状
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The surface roughness of a workpiece after machining
depends upon many technological factors,such as the workpiece
material,tool material,tool geometry,cutting conditions (cutting
speed and feed),rigidity of the MFTW system,cooling condition
in machining process,and so on,The surface roughness is a kind
of micro geometrical deviation or micro-irregularity of workpiece
surfaces,It appears on all machined workpiece surfaces no matter
how smooth they look to the naked eye,In regard to the criteria
for evaluating surface roughness,refer to other textbooks,ISO,
and other standards and references.
The surface roughness of machined parts influences their
performance to a great extent,In order to ensure product quality,
to improve its service life,and to reduce its production cost,the
surface roughness of machine parts has to be specified accurately
in design and carefully controlled in manufacturing.
不规则性,不平度 criterion
ISO:International Standardization Organization
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Physic-Mechanical State of Surface Layer
In machining process,the metal nearest the
workpiece surface deforms plastically,This is due to
the intrusion of the cutting tool rake to the workpiece
surface material,the friction between the tool relief
and the machined surface,and the effect of the tool
nose radius,This results in the surface layer on the
workpiece in quite a different way from the internal
material of the workpiece after machining,Until now,
the evaluation of the physic-mechanical state of the
surface layer is still in the experimental investigation
stage,Complete standards for the evaluation do not
exist,However,it is known that the variations of the
material on the surface layer from the original metal
are caused by cold hardening,metallographical
change,and residual stress.
intrusion
硬挤性
tool rake
刀具前面
刀具前角
tool relief
刀具后面
刀具后角
tool nose
刀尖
cold
hardening
冷作硬化
residual
stress
残余应力金相组织变化
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The cold hardening of the surface layer material is caused
by plastic deformation of the material,resulting in the
increase of its microhardness,The parameters which
characterize the cold hardening are shown below:
The hardened depth h,which is the depth of the surface
layer,deformed plastically in the machining process.
The microhardness H,The degree of cold hardening N,
which is the ratio of the microhardness,increases the surface
layer to the microhardness of the original metal,i.e.,
N = [(H-H0)/H0] × 100% 6.3
where H0 stands for the microhardness of the original metal.
minus
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
A large part of the energy consumed in machining is
transformed into heat,Which results in the increase of the
temperature in the cutting or grinding area,especially on the
workpiece surface being machined,In general cutting
processes,most of the heat generated is carried away by the
chips,and the temperature of the workpiece surface is not
very high,However,in the machining processes consuming a
very large amount of energy,for instance as in grinding,the
temperature of the workpiece surface may reach or exceed
the critical point of phase change of workpiece material,This
condition causes the metallographical variation of the surface
material.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The residual stress is generated in the surface layer after
machining,The causes of the generation of the residual stress are:
(i) The material of the workpiece surface expands while it is heated
by the cutting or grinding heat,and it contracts when it is cooled.
The expansion and contraction are resisted by the internal material
of the workpiece which results in the residual tensile stress in the
surface layer after machining; (ii) The surface material plastically
deforms under the extrusion and friction of the tool,while the
internal material close to the surface layer elastically deforms,After
machining,the recovery of the elastic deformation of the internal
material is restricted by the surface material,which has deformed
plastically,This results in the residual stress,usually compressive
residual stress,in the surface layer; (iii) The metallographical
variation of the surface layer leads to its volume change,either
expansion or contraction,which is restricted by the internal
material and results in the residual stress (compressive or tensile) in
the surface layer.
挤
压
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Influence of Surface Quality on the Performance of
Machined Parts
A field failure of a machined part caused by its wear,
fatigue or corrosion,etc.,starts from the part surface in most
cases,The surface quality of a machine part greatly influences
its performance,such as the fitting between parts,wear
resistance,corrosion resistance,and fatigue strength.
Influence on Fitting between Parts
The surface roughness influences directly the fitting
condition between parts,For the clearance fit,the existence of
the micro-irregularities on part surfaces leads to a rapid initial
wear,Thus the clearance increases between the mating parts,
which cause the deterioration of the fitting condition.
强度 间隙配合
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
As to the interference fit,the strength of fit
depends upon the surface roughness of the parts.
When one part is press-fitted into another,the
ridges of the micro-irregularities are extruded:
This reduces the actual interference and thus the
strength of fit,The actual interference can be
calculated by the following equation:
e = (Ds - Dh) - l.2[(Hmax)s + (Hmax)h] 6.4
where e— actual interference,
(Hmax)s,(Hmax)h— maximum height of
irregularities of shaft and hole,respectively,
Ds— diameter of shaft,
Dh— diameter of hole.
interference fit
过盈配合
ridge 波峰
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The surface roughness and surface hardening influence
greatly the wear resistance of part surfaces,When dry friction
exists between two fitting surfaces,only the ridges of the
irregularities on both surfaces are in contact with each other
at the initial stage,The pressure between the two mating parts
is concentrated on these small areas.
For example,the actual contacting area for turned or
milled surfaces is only 15-20% of the total area,and for the
finely ground surfaces,30-50%,Due to the high unit
pressure,the irregularities on the part surfaces deform
elastically and plastically and portions are sheared because of
the relative movement of the surfaces,These fallen particles
are scattered,which intensify the wear between the
contacting surfaces,shear 剪切，剪断
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The situation of the wet friction is more complicated,At the
initial stage of the wet friction the lubricant films are pierced
through by the ridges of the roughness to form a dry friction
between two fitting surfaces,The intensive initial wear changes
the proper fit of the mating parts,However,as the wear gradually
increases,the unit pressure between the surfaces reduces,and the
wear slows down,When a certain point is reached,the wear is
intensified again.
A reduction of the surface roughness can increase the wear
resistance of part surface,However,the relationship between them
is not linear,It has been proved by experiments that an optimal
value for the surface roughness exists under a given condition
with which the minimum wear can be expected,If the fitting
surfaces are too smooth,they will be in close contact with each
other,which leads to a larger affinity between the molecules of the
surfaces,This will result in an intensive wear of the surfaces.
lubricant film 润滑油膜亲合力,亲合性
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The hardening of the surface layer can greatly increase the
wear resistance of part surface,However,an excessive increase
of the microhardness may cause the peeling of the hardened
layer,Therefore,caution should be exercised when applying
microhardening.
Influence on Corrosion Resistance
The surface roughness greatly influences the corrosion
resistance of a part surface,Usually the corrosive substances
are gathered at the valleys of the surface micro-irregularities,
which will attack the part surface,The deeper and more sharply
defined the valleys between the ridges of microirregularities,
the more destructive the effect of corrosion directed into the
depth of the metal.
peel 剥皮，脱皮 valley 山谷，波谷
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The hardening of the surface layer and the existence of the
residual stress in the surface layer will reduce the corrosion
resistance of part surface,This is because the structure of the
surface layer after plastic deformation is in a non-equilibrium
state,possessing higher energy,It is more prone to corrosion.
Influence on Fatigue Strength
The destructive failure of metal parts under alternate loads
starts from part surfaces or under a certain depth of the
hardened surfaces,Consequently the fatigue strength of a part
depends upon the surface quality of the part to a great extent.
Subject to periodically alternate load,the valleys between the
ridges of surface micro-irregularities may become the points of
internal stress concentration,which may result in the failure of
the part,The higher the type of surface finish,the higher the
fatigue strength of the part.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Cold hardening of the surface layer prevents the extension
of existing microcracks,The generation of new microcracks
on a part surface reduces the harmful effect of the surface
roughness and the external defects of the part,This helps the
increase of the fatigue strength.
However,the excessive cold
hardening of the surface layer
may intensify the extension of
the microcracks,Under alternate
load or high temperature,this
decreases the fatigue strength of
the part,Therefore,cold hardening
should be controlled within certain
limits.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The influence of the residual stress in surface layer on
the fatigue strength of a part depends on the direction and
the magnitude of the residual stress,The compressive
residual stress tends to close the microcracks and thus its
existence can greatly increase the fatigue strength of the
part,On the other hand,the existence
of the tensile residual stress will
decrease the fatigue strength.
However,since the generation of
the tensile residual stress is always
accompanied with the hardening
of the surface layer,this harmful
effect is reduced.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
It can be seen from the above that the
hardening of the surface layer and the existence
of the compressive residual stress are conductive
to the increase of the fatigue strength of parts.
Appropriate methods can be adopted to generate
hardening and compressive residual stresses in
the surface layer,such as the ball peening,the
burnishing,etc.,Some heat-treatment methods,
such as carbonizing and nitriding,can also
generate the compressive residual stress in the
surface layer,In addition,some micro-finishing
methods,such as polishing and vibrated
finishing,etc.,can be used to improve the
surface finish,thus leading to the increase of the
fatigue strength of parts.
peening
喷丸硬化
burnishing
压力抛光
carbonize
渗碳
nitriding
渗氮
polishing
抛光
finishing
修整，磨光
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
In machining,factors affecting machining accuracy are
numerous and complicated,In order to improve the
machining accuracy of an operation,the causes leading to
machining errors for the specific operation must be located
through synthetical analysis and investigation,On this basis,
proper technological measures can be taken to control the
influences of these factors on the machining accuracy.
The methods for the improvement of machining
accuracy can be summarized by the reduction or elimination
of machining errors,compensation and transfer of
machining errors,and machining based on workpieces
classification or combination.
Methods for Machining Accuracy Improvement
补偿
综
合
的
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Reduction or Elimination of Machining Errors
The reduction or elimination of machining errors,whenever
possible,are effective ways to improve the machining accuracy.
It begins with identifying the root causes of the errors for the
specific machining operation,Appropriate measures can then be
taken to reduce or to eliminate the influences of these causes on
machining errors,Usually machining errors can be reduced or
eliminated through the following technological measures:
l,Improve the geometrical accuracy of the component parts of
the MFTW system.
2,Minimize the deformation of each component part of the
MFTW system,especially when machining the workpiece with
low rigidity,It is because it tends to deform under external
forces and heating and to vibrate that deteriorates the surface
finish of the workpiece,deteriorate 恶化
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
3,Properly select the machining conditions (speed and feed),
which influence both machining accuracy and surface finish.
4,Improve the machine tool set-up accuracy through using
the more accurate adjusting and measuring tools and devices,
and carefully conducting the set-up work.
5,Minimize the workpiece setting errors through the proper
selection of the locating references and the rational design of
the workholders.
6,Arrange the sequence of operations to reduce or eliminate
some of the machining errors,It also prevents the attained
accuracy from damage by subsequent operations (details will
be discussed in the later chapters).
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Compensation and Transfer of Machining Errors
When it is impossible to completely eliminate machining
errors for some technological or economical reasons,
compensation or transfer of machining errors can be used to
improve machining accuracy,Machining error compensation
is devised to counteract a machining error in the MFTW
system by substituting another error,The counteracting error
is purposely exerted on the system,It should be equal to the
existing error in magnitude and opposite in direction.
抵抗，抵消
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
For instance,the machining error caused by tool wear can
be compensated for by the automatic tool compensation
system on an NC machine,A long shaft with a small diameter
and a low rigidity would tend to deform in the turning
operation,In order to reduce the machining error caused by
the workpiece deformation,two turning tools opposite to each
other can be used to turn the shaft simultaneously,This can
reduce the radial cutting force and thus reduces the
deformation of the workpiece,A certain machining error is
caused by the deformation of a workpiece with low rigidity
under external forces in the machining operation,The error
can be minimized if the workpiece is pre-stressed in its
setting to create an elastic deformation in the direction
opposite to that of the deformation.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Machining error transfer is used
frequently in machining operations as well.
It transfers the sources of the machining
errors from one part of the MFTW system
to another,As an example,the accuracy of
a set of concentric precision holes can be
attained by using a precision boring
machine,However,if these holes are
machined on a radial drilling machine,
which cannot ensure the required accuracy,
a precision boring jig can be adopted to
ensure the accuracy of the hole series.
This means the machining errors are
transferred from the radial drilling
machine to the boring jig.
concentric
同心，同轴
boring machine
镗床
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Machining based on Workpiece Classification or
Combination
The machining accuracy of an operation also depends
upon the variation of the workpiece dimensions after
machining in the preceding operation,The variation of the
workpiece dimensions prior to the operation may lead to
machining errors in the machining process due to the
reproduction of errors or to the inaccuracy of workpiece
setting,These machining errors can be reduced through
machining based on workpiece classification.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The workpieces are classified into n groups according
to the directions and the values of the errors on the
workpieces prior to the operation,Thus the range of
workpiece errors in each group is reduced to l/n of that of
the entire batch of workpieces,Each group of workpieces
is machined under a specific machine tool set-up,which
adapts the range of workpiece errors of that particular
group,This greatly decreases the influence of the
machining errors of the preceding operations on the
current operation.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Sometimes the mating part can be combined and
machined in one operation,In this case,the workpieces
will not be affected by the inaccuracy of some component
parts of the MFTW system,For example,a higher
concentricity between the cylindrical surfaces on two
mating parts is attained when they are combined and
machined,Similarly,higher machining accuracy can be
obtained through the corrective finishing of lathe chucks
or the locating surfaces of workholders mounted on
machine tools.
