SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
LESSON TWO MACHINING ACCURACY
Product quality is always a matter of prime
importance in manufacturing,It should be given
the highest priority in process planning,The
quality of a machined part can be expressed in
geometrical parameters (dimension,shape,
coordination and surface finish,etc.),physical
parameters (electrical conductivity,thermal
conductivity,magnetic conductivity,etc.),
chemical parameters (corrosion resistance,etc.)
and mechanical parameters (strength,hardness,
etc.),The requirements of these parameters of a
part are determined by the designer,according to
the condition under which the product is to
function.(c)
coordination
相互位置
surface finish
表面光洁度
parameter
参数，参量
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
(c)The degree of coincidence of the real parameters
obtained after a part is manufactured,with the
parameters defined in part design,represents the
manufacturing quality of the part,For the convenience
of analysis,the degree of coincidence between the real
macro-geometrical parameters (dimension,shape and
coordination) of a part obtained after machining,and
those specified in part design is defined as machining
accuracy,The degree of coincidence between the real
micro-geometrical parameters (surface finish) and the
parameters of the physic-mechanical properties of part
surfaces,obtained after machining a part,with those
specified in part design is defined as surface quality.
Other physical and chemical parameters are involved
only in some special cases.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Machining accuracy is expressed quantitatively by the
magnitude of machining error,Machining error is defined as
the difference between the parameters of a machined part and
that of a perfect (absolutely accurate) part specified in the part
drawing,The machining error varies from part to part even in
the same batch of production,Although machining errors
always exist in machining processes,a part can be considered
acceptable,provided the magnitude of machining errors do not
exceed the tolerance limits,The tolerance represents the
maximum allowable machining error,The required machining
accuracy of a part is specified by the designer,while the
machining accuracy requirements of the part in each
machining stage are determined in accordance with the needs
and accuracy of the process by the process planner.
公
差
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Parameters of Machining Accuracy and
Surface Quality
Machining accuracy is associated with a
number of macro-geometrical parameters and
can be classified as follows:
l,Machining accuracy of part surfaces.
- Accuracy of dimensions of part surfaces (for
example,accuracy of the diameters of
cylindrical surfaces,accuracy of the diameters
of spherical surfaces,and accuracy of the
angles at the top of the cones,etc.).
- Accuracy of shapes of part surfaces (for
example,flatness of a plane,roundness,and
cylindricity of a cylindrical surface,etc.).
flatness
平直度,平面度
roundness
圆度，球度
cylindricity
圆柱度
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
2,Machining accuracy of relative positions
between part surfaces.
- Accuracy of dimensions between part
surfaces (for example,the distance between two
parallel planes or the centers of two holes,etc.).
- Accuracy of positional relations between
part surfaces (for example,the parallelism and
perpendicularity of two planes or two axes,etc.).
The surface quality of a part indicates the
surface characteristics of the part,It includes
either geometrical or physic-mechanical
parameters,which are listed below:
l) Surface roughness.
2) Physic-mechanical state of surface layer.
parallelism
平行度
perpendicularity
垂直度
surface roughness
表面粗糙度
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Among the various parameters of machining accuracy,the
shape accuracy of part surfaces relies on the cutting tools,and
the relative motions between cutting tools and workpieces.
The dimensional accuracy,including accuracy of dimensions
of part surfaces themselves and dimensions between part
surfaces,can be achieved through different method.
Methods for Obtaining Required Dimensional Accuracy
In machining,different measures can be adopted to attain
the required dimensional accuracy,There are basically two
different methods,trial cutting (or grinding) and automatic
dimension maintenance.
Trial Cutting
Trial cutting achieves the required dimensional accuracy in
several trial cutting (or grinding) passes,A certain length of the
surface to be machined is first cut and measured.(c)
试切法
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
(c) The cutting tool position with respect to the workpiece is
then adjusted in the next cutting pass,The process repeats until
the machined dimension falls into the specified tolerance limits.
After that,the entire length of the surface is machined in a
single pass and the dimensional accuracy can be ensured,For
example,when turning a shaft on a lathe,in order to obtain the
diameter d and length l with their required accuracy,several
trial cutting passes are first performed on a certain length of the
cylindrical surface,After each cut,the diameter is measured
and the turning tool is then adjusted,When the measured
diameter enters its tolerance limits,the entire length of the
cylindrical surface is turned with the automatic feed,Before the
tool reaches the stepped face,the continuous feed should be
stopped,and the trial cutting and measuring are performed until
the dimension of length l enters its tolerance limits.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Trial cutting is characterized by its low production
efficiency and is not suitable for large volume production.
However,using this method,the machine tool does not need
to be preset in advance,Moreover,the effect of dimensional
variation of the blanks (shape,sizes and hardness of the
material,etc.) can be reduced in machining operations,For
these reasons,it has been widely used in traditional small-
batch production.
The machining accuracy of the trial cutting method
depends mainly upon the minimum possible thickness of the
chip removal in the last trial cutting pass and the skill level of
the operator,That is the reason why it is easier to attain higher
machining accuracy by using a grinding operation than a
turning operation.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Automatic Dimension Maintenance
This method automatically ensures the
attainment of the required machining
accuracy without the need of trial cutting,
and measuring and correction of the cutting
tool position.
There are different ways to automatically
obtain the required machining accuracy.
l,Use of fixed dimension cutting tools,The
required machining accuracy is obtained by
using cutting tools with fixed shape and
dimensions,such as reamers,broaches and
taps.
reamer 绞刀
tap 丝锥，螺丝攻
broaches 拉刀
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
2,Machining on preset machine tools,The positions
of cutting tools and/or stoppers relative to the
workpiece are preset and fixed on the machine tool.
These positions remain unchanged in machining an
entire batch of workpieces,Before machining,the
turning tool and the stopper are preset according to
the required diameter d and length l with their
tolerances,Keeping the positions of the turning tool
and the stopper unchanged,the machining accuracy
of the batch of workpieces can be achieved in the
operation.
Many types of machine tools have been designed
for this purpose,such as multi-cut lathes,turret lathes,
planer-type milling machines and some automatic
machine tools.
preset
预调,预置
stopper
定程器
挡块
multi-cut
lathes
多刀车床
turret lathes
转塔车床
planer-type
milling
machines
龙门铣床
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The machining accuracy,which this approach can
achieve,relies upon the preset accuracy of the machine
tool and various factors affecting the machining process.
3,Use of guiding devices,The guiding devices in
workholders can often be used to ensure the required
machining accuracy on workpieces,For instance,the
drill bushings in drilling jigs and the tool setting blocks
in milling fixtures,In some cases,the guiding templates
and cams can also play this role.
4,Use of NC machine tools,The cutter paths on NC
machine tools are under the control of the machine
control systems,The machining accuracy of such
machine tools depends on the accuracy of the machine
tools,the machine control systems and the NC
programs,which issue the machining instructions to the
control systems.
bushing
衬套,导套
jig 夹具，
钻模
template
模板,导板
cam 凸轮
靠模仿形
板
tool setting
blocks
对刀块
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The method of automatic dimension maintenance is
characterized by high production efficiency and provides the
possibility of rational allocation of labor on the complicated
machine tool set-up work and the uncomplicated machine tool
operation,Also,the application of this method is the
prerequisite for the realization of machining process
automation.
Even though human factors are not taken into account in
machining processes,there still exist many factors causing
machining errors,The study of machining accuracy is aimed
at analyzing various factors causing machining errors,
estimating the magnitude of the errors,and finding ways to
eliminate or reduce the machining errors.
先决条件
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Basic Causes of Machining Errors
There are numerous factors that can lead to machining
errors in metal cutting,These factors can be divided into
three basic groups.
l,Inaccurate machine tool set-up,The objective of machine
tool set-up is to establish the correct positions of cutting
tools relative to workpieces on the machine tool,For
attaining this,first,there must be accurate positions of the
workpieces relative to the machine tool,It is achieved
through proper design,manufacture and installation of the
workholder,Secondly,there must be accurate positions of
the cutting tools relative to the machine tool,which are
obtained by precise adjustments of the cutting tools in
machine tool set-up,In such a way the relative positions
between the cutting tools and the workpieces can be
indirectly established.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The inaccuracy of machine tool set-up will cause
machining errors in machining processes,For example,in the
turning operation,if the position of the turning tool in the
radial direction is not correctly preset,it will cause
machining errors of the diameters of the workpiece after
machining,Machining error caused by the inaccurate
machine tool set-up is referred to as machine set-up error.
The magnitude of machine tool set-up error depends on
the employed machine tool set-up method,the accuracy of
the devices used for the adjustment of cutting tool position,
the measuring accuracy of the workpiece in trial machining
during the machine tool set-up process,and the skill level of
the set-up operator.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
When a trial cutting method is used to attain the required
machining accuracy,the cutting tool does not have to be
preset in advance,As a matter of fact,the adjustment of the
cutting tool position and the machining of the workpiece are
performed in turn,That is,the cutting tool position is
adjusted before each workpiece is machined.
2,Inaccurate workpiece setting,In machining,the workpiece
must have a definite and correct position on a machine tool.
This is achieved through workpiece setting,and in most cases,
through the aid of a workholder,For a variety of reasons,
such as the inaccuracy of the workholder,inaccuracy of the
workpiece locating surface,etc.,the workpiece may be
improperly set on the machine tool or the workholder,which
results in machining error in the machining operation,The
machining error caused by the inaccurate setting of the
workpiece is referred to as workpiece setting error.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
3,Machining processes,The attainment of the required shape,
dimensions and accuracy of a workpiece in machining is the
results of the accurate relative machining motions between
the cutting tool and the workpiece,However,because of the
effects of numerous factors in various machining processes,
the actual relative motions between cutting tools and
workpieces will deviate from a perfect situation,This leads to
machining errors,The machining error caused by various
factors in machining process is referred to as machining
process error.
The occurrence of machining process errors depends on
the nature of the machining system composed of machine
tool,workholder,tool and workpiece,It is not directly related
to the operation of the workman.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Factors Affecting Machining Process Accuracy
There are many factors causing machining process errors.
They are explained below:
l,Theoretical Inaccuracy,The theoretical inaccuracy is
caused by the incorrectness of the kinematic scheme of the
machine tool or the tool shape,In machining a workpiece
surface with a simple shape,such as plane or cylindrical,this
situation rarely exists,However,in machining a complicated
surface,in order to simplify the design and manufacture of the
machine tool or the tool,approximate kinematic schemes or
tool shapes are employed,which cause machining errors to
occur,This class of machining errors is called the theoretical
error.
kinematics scheme 运动系统kinematic 运动学
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The magnitude of the theoretical error is
relatively small,In most cases,it should not
exceed 1020% of the tolerance,For instance,
in gear-hobbing,the hobbing cutter is in the
form of a basic worm with a straight contour
instead of that with an evolute contour in
normal direction so that the cutter can be
easily manufactured,This causes machining
errors on the hobbed gear teeth,Moreover,
since there are limited cutting edges on the
gear-hobbing cutter,the contours of the gear
teeth generated after hobbing are comprised of
straight lines as opposed to smooth contours.
hob 滚刀，滚齿
worm 蜗杆
contour 轮廓
evolute 渐开线
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
In general,the application of the approximate machining
processes causes theoretical errors,However,since the
approximate processes are usually much simpler and easier
than the perfect ones,the use of them will be greatly
advantageous to the production efficiency and economics,
provided that the magnitude of the theoretical errors are
relatively small enough compared to the tolerance.
2,Geometrical Inaccuracy of Machine Tools and Cutting
Tools
When machining on numerically controlled machine tools,
the theoretical errors occur,if approximate algorithms are used
to approach the curved contour of the workpiece surface in
NC programming,Also,the theoretical errors exist,when
linear or circular interpolation is adopted for approaching a
contour curve.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
For example,the unparalleled longitudinal
bed ways with respect to the axis of rotation of
the spindle in the horizontal direction on a lathe
leads to tapers on the machined cylindrical
surfaces,while the lack of parallelism between
them in the vertical direction causes the
machined cylindrical surfaces hyperboloid errors.
When inaccuracy exists in the straightness of the
bed ways,the perfect cylindrical surface can not
be obtained in the turning process.
No matter how precisely a machine tool is
made,there must exist inaccuracy,which is the
causes of errors in operations using the machine
tool,Machine tool inaccuracy is a result of the
tool's manufacture,improper installation,and/or
its wear in application.
spindle 主轴
taper 圆锥形
hyperboloid
双曲面
straightness
平直度
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Runout of the running centers of a lathe leads to runout
of the machined surfaces of the workpiece,with respect to
the axis of the center holes,If the semi-finished workpiece is
transferred to another lathe with a different runout of its
centers,the misalignment of the surfaces may occur,since
they are machined under different conditions,This will not
happen in machining if the workpiece is held between two
dead centers,even if it is machined in several settings or
operations.
Runout of the rotational axis of the spindle of a lathe
results in out-of-roundness on the machined cylindrical
surface of the workpiece.
misalignment 不同轴，不同心runout 径向跳动 center 顶尖
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The magnitude of the machine tool inaccuracy is not always
equal to that of the machining error it caused,In many cases,the
machining error is a function of the machine tool inaccuracy and
should be determined through calculation,For instance,due to
the wear of a lathe in application,the concavities emerge on the
bed ways,This results in the inclination of the tool carriage,The
displacement of the tool tip in the horizontal direction can be
calculated from the following formula.
P = a × h/b 6.l
where p—displacement of the tool tip in horizontal direction,mm.
h—vertical distance between the running center of the
workpiece and the bed way,mm.
a—maximum depression of the bed way,mm.
b—width of the bed way,mm.
concavity 凹状，成凹形multiply divide
millimeter
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The unparallelism between bed ways on a lathe also
affects the position of the tool carriage with respect to the
workpiece and therefore causes shape error of the workpiece
machined,The displacement of the tool tip in the horizontal
direction resulting from the "twist" of the bed ways can be
calculated from the following equation.
Q = δ × h/b 6.2
where Q—displacement of tool tip in horizontal direction,mm.
h—vertical distance between the running center of
workpiece and the bed ways,mm.
δ—amount of twist of bed,mm.
b—width of the bed way,mm.
tool carriage 拖板
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Since the h/b ratios of most machine tools are between 0.6
and 1.0,the impact of "twist" of the bed ways cannot be under-
estimated,In addition,“twist” of the bed ways also leads to
errors in geometrical form of the workpiece over a length
corresponding to the length of the,twist” in the bed ways.
It is not that the inaccuracy of all units of the machine tool
leads to machining errors for a specific machining operation.
For instance,the unparallelism between the bed ways and the
running center of the spindle in the horizontal direction on a
lathe causes taper on the machined cylindrical surface of the
workpiece held by the chuck,because it does not affect the
machining accuracy if the workpiece is held between two dead
centers,Thus it can be seen that in order to improve the
machining accuracy of a machine tool,only the units related to
the machining conditions are required to be improved.
卡
盘
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Machining errors depend as well upon the inaccuracy
of the cutting tool manufactured,The inaccuracy of the
cutting tool is directly related to the machining errors of
workpieces in the following situations:
l,For machining using cutting tools with fixed sizes,such
as drills,reamers,grooving tools,broaches,etc.,the
inaccuracy in the dimensions of the tools directly reflects
upon the dimension errors on workpiece surfaces.
2,For machining using formed tools,such as formed
turning tools,formed milling cutters,formed grinding
wheels,etc.,the inaccuracy in the form of the tools
directly reflects upon the shape errors on the machined
surfaces of workpieces.
groove 切槽，铣槽
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Although the manufacturing inaccuracy of the standard
cutting tools,such as standard turning tools and plane milling
cutters,is not directly concerned in the machining errors of
workpieces,it is indirectly related in some cases,For example,
if the parameters of the geometry or the form of a cutting tool
are not properly manufactured,the tool wear will be
intensified in machining,which leads indirectly to machining
errors.
The tool wear is inevitable in machining,When
machining a long shaft,the wear of the turning tool causes the
taper on the machined cylindrical surface of the shaft,The
heterogeneous wear of the form-grinding wheel directly
affects the shape accuracy of the ground surface.
非均匀的
LESSON TWO MACHINING ACCURACY
Product quality is always a matter of prime
importance in manufacturing,It should be given
the highest priority in process planning,The
quality of a machined part can be expressed in
geometrical parameters (dimension,shape,
coordination and surface finish,etc.),physical
parameters (electrical conductivity,thermal
conductivity,magnetic conductivity,etc.),
chemical parameters (corrosion resistance,etc.)
and mechanical parameters (strength,hardness,
etc.),The requirements of these parameters of a
part are determined by the designer,according to
the condition under which the product is to
function.(c)
coordination
相互位置
surface finish
表面光洁度
parameter
参数，参量
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
(c)The degree of coincidence of the real parameters
obtained after a part is manufactured,with the
parameters defined in part design,represents the
manufacturing quality of the part,For the convenience
of analysis,the degree of coincidence between the real
macro-geometrical parameters (dimension,shape and
coordination) of a part obtained after machining,and
those specified in part design is defined as machining
accuracy,The degree of coincidence between the real
micro-geometrical parameters (surface finish) and the
parameters of the physic-mechanical properties of part
surfaces,obtained after machining a part,with those
specified in part design is defined as surface quality.
Other physical and chemical parameters are involved
only in some special cases.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Machining accuracy is expressed quantitatively by the
magnitude of machining error,Machining error is defined as
the difference between the parameters of a machined part and
that of a perfect (absolutely accurate) part specified in the part
drawing,The machining error varies from part to part even in
the same batch of production,Although machining errors
always exist in machining processes,a part can be considered
acceptable,provided the magnitude of machining errors do not
exceed the tolerance limits,The tolerance represents the
maximum allowable machining error,The required machining
accuracy of a part is specified by the designer,while the
machining accuracy requirements of the part in each
machining stage are determined in accordance with the needs
and accuracy of the process by the process planner.
公
差
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Parameters of Machining Accuracy and
Surface Quality
Machining accuracy is associated with a
number of macro-geometrical parameters and
can be classified as follows:
l,Machining accuracy of part surfaces.
- Accuracy of dimensions of part surfaces (for
example,accuracy of the diameters of
cylindrical surfaces,accuracy of the diameters
of spherical surfaces,and accuracy of the
angles at the top of the cones,etc.).
- Accuracy of shapes of part surfaces (for
example,flatness of a plane,roundness,and
cylindricity of a cylindrical surface,etc.).
flatness
平直度,平面度
roundness
圆度，球度
cylindricity
圆柱度
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
2,Machining accuracy of relative positions
between part surfaces.
- Accuracy of dimensions between part
surfaces (for example,the distance between two
parallel planes or the centers of two holes,etc.).
- Accuracy of positional relations between
part surfaces (for example,the parallelism and
perpendicularity of two planes or two axes,etc.).
The surface quality of a part indicates the
surface characteristics of the part,It includes
either geometrical or physic-mechanical
parameters,which are listed below:
l) Surface roughness.
2) Physic-mechanical state of surface layer.
parallelism
平行度
perpendicularity
垂直度
surface roughness
表面粗糙度
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Among the various parameters of machining accuracy,the
shape accuracy of part surfaces relies on the cutting tools,and
the relative motions between cutting tools and workpieces.
The dimensional accuracy,including accuracy of dimensions
of part surfaces themselves and dimensions between part
surfaces,can be achieved through different method.
Methods for Obtaining Required Dimensional Accuracy
In machining,different measures can be adopted to attain
the required dimensional accuracy,There are basically two
different methods,trial cutting (or grinding) and automatic
dimension maintenance.
Trial Cutting
Trial cutting achieves the required dimensional accuracy in
several trial cutting (or grinding) passes,A certain length of the
surface to be machined is first cut and measured.(c)
试切法
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
(c) The cutting tool position with respect to the workpiece is
then adjusted in the next cutting pass,The process repeats until
the machined dimension falls into the specified tolerance limits.
After that,the entire length of the surface is machined in a
single pass and the dimensional accuracy can be ensured,For
example,when turning a shaft on a lathe,in order to obtain the
diameter d and length l with their required accuracy,several
trial cutting passes are first performed on a certain length of the
cylindrical surface,After each cut,the diameter is measured
and the turning tool is then adjusted,When the measured
diameter enters its tolerance limits,the entire length of the
cylindrical surface is turned with the automatic feed,Before the
tool reaches the stepped face,the continuous feed should be
stopped,and the trial cutting and measuring are performed until
the dimension of length l enters its tolerance limits.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Trial cutting is characterized by its low production
efficiency and is not suitable for large volume production.
However,using this method,the machine tool does not need
to be preset in advance,Moreover,the effect of dimensional
variation of the blanks (shape,sizes and hardness of the
material,etc.) can be reduced in machining operations,For
these reasons,it has been widely used in traditional small-
batch production.
The machining accuracy of the trial cutting method
depends mainly upon the minimum possible thickness of the
chip removal in the last trial cutting pass and the skill level of
the operator,That is the reason why it is easier to attain higher
machining accuracy by using a grinding operation than a
turning operation.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Automatic Dimension Maintenance
This method automatically ensures the
attainment of the required machining
accuracy without the need of trial cutting,
and measuring and correction of the cutting
tool position.
There are different ways to automatically
obtain the required machining accuracy.
l,Use of fixed dimension cutting tools,The
required machining accuracy is obtained by
using cutting tools with fixed shape and
dimensions,such as reamers,broaches and
taps.
reamer 绞刀
tap 丝锥，螺丝攻
broaches 拉刀
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
2,Machining on preset machine tools,The positions
of cutting tools and/or stoppers relative to the
workpiece are preset and fixed on the machine tool.
These positions remain unchanged in machining an
entire batch of workpieces,Before machining,the
turning tool and the stopper are preset according to
the required diameter d and length l with their
tolerances,Keeping the positions of the turning tool
and the stopper unchanged,the machining accuracy
of the batch of workpieces can be achieved in the
operation.
Many types of machine tools have been designed
for this purpose,such as multi-cut lathes,turret lathes,
planer-type milling machines and some automatic
machine tools.
preset
预调,预置
stopper
定程器
挡块
multi-cut
lathes
多刀车床
turret lathes
转塔车床
planer-type
milling
machines
龙门铣床
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The machining accuracy,which this approach can
achieve,relies upon the preset accuracy of the machine
tool and various factors affecting the machining process.
3,Use of guiding devices,The guiding devices in
workholders can often be used to ensure the required
machining accuracy on workpieces,For instance,the
drill bushings in drilling jigs and the tool setting blocks
in milling fixtures,In some cases,the guiding templates
and cams can also play this role.
4,Use of NC machine tools,The cutter paths on NC
machine tools are under the control of the machine
control systems,The machining accuracy of such
machine tools depends on the accuracy of the machine
tools,the machine control systems and the NC
programs,which issue the machining instructions to the
control systems.
bushing
衬套,导套
jig 夹具，
钻模
template
模板,导板
cam 凸轮
靠模仿形
板
tool setting
blocks
对刀块
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The method of automatic dimension maintenance is
characterized by high production efficiency and provides the
possibility of rational allocation of labor on the complicated
machine tool set-up work and the uncomplicated machine tool
operation,Also,the application of this method is the
prerequisite for the realization of machining process
automation.
Even though human factors are not taken into account in
machining processes,there still exist many factors causing
machining errors,The study of machining accuracy is aimed
at analyzing various factors causing machining errors,
estimating the magnitude of the errors,and finding ways to
eliminate or reduce the machining errors.
先决条件
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Basic Causes of Machining Errors
There are numerous factors that can lead to machining
errors in metal cutting,These factors can be divided into
three basic groups.
l,Inaccurate machine tool set-up,The objective of machine
tool set-up is to establish the correct positions of cutting
tools relative to workpieces on the machine tool,For
attaining this,first,there must be accurate positions of the
workpieces relative to the machine tool,It is achieved
through proper design,manufacture and installation of the
workholder,Secondly,there must be accurate positions of
the cutting tools relative to the machine tool,which are
obtained by precise adjustments of the cutting tools in
machine tool set-up,In such a way the relative positions
between the cutting tools and the workpieces can be
indirectly established.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The inaccuracy of machine tool set-up will cause
machining errors in machining processes,For example,in the
turning operation,if the position of the turning tool in the
radial direction is not correctly preset,it will cause
machining errors of the diameters of the workpiece after
machining,Machining error caused by the inaccurate
machine tool set-up is referred to as machine set-up error.
The magnitude of machine tool set-up error depends on
the employed machine tool set-up method,the accuracy of
the devices used for the adjustment of cutting tool position,
the measuring accuracy of the workpiece in trial machining
during the machine tool set-up process,and the skill level of
the set-up operator.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
When a trial cutting method is used to attain the required
machining accuracy,the cutting tool does not have to be
preset in advance,As a matter of fact,the adjustment of the
cutting tool position and the machining of the workpiece are
performed in turn,That is,the cutting tool position is
adjusted before each workpiece is machined.
2,Inaccurate workpiece setting,In machining,the workpiece
must have a definite and correct position on a machine tool.
This is achieved through workpiece setting,and in most cases,
through the aid of a workholder,For a variety of reasons,
such as the inaccuracy of the workholder,inaccuracy of the
workpiece locating surface,etc.,the workpiece may be
improperly set on the machine tool or the workholder,which
results in machining error in the machining operation,The
machining error caused by the inaccurate setting of the
workpiece is referred to as workpiece setting error.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
3,Machining processes,The attainment of the required shape,
dimensions and accuracy of a workpiece in machining is the
results of the accurate relative machining motions between
the cutting tool and the workpiece,However,because of the
effects of numerous factors in various machining processes,
the actual relative motions between cutting tools and
workpieces will deviate from a perfect situation,This leads to
machining errors,The machining error caused by various
factors in machining process is referred to as machining
process error.
The occurrence of machining process errors depends on
the nature of the machining system composed of machine
tool,workholder,tool and workpiece,It is not directly related
to the operation of the workman.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Factors Affecting Machining Process Accuracy
There are many factors causing machining process errors.
They are explained below:
l,Theoretical Inaccuracy,The theoretical inaccuracy is
caused by the incorrectness of the kinematic scheme of the
machine tool or the tool shape,In machining a workpiece
surface with a simple shape,such as plane or cylindrical,this
situation rarely exists,However,in machining a complicated
surface,in order to simplify the design and manufacture of the
machine tool or the tool,approximate kinematic schemes or
tool shapes are employed,which cause machining errors to
occur,This class of machining errors is called the theoretical
error.
kinematics scheme 运动系统kinematic 运动学
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The magnitude of the theoretical error is
relatively small,In most cases,it should not
exceed 1020% of the tolerance,For instance,
in gear-hobbing,the hobbing cutter is in the
form of a basic worm with a straight contour
instead of that with an evolute contour in
normal direction so that the cutter can be
easily manufactured,This causes machining
errors on the hobbed gear teeth,Moreover,
since there are limited cutting edges on the
gear-hobbing cutter,the contours of the gear
teeth generated after hobbing are comprised of
straight lines as opposed to smooth contours.
hob 滚刀，滚齿
worm 蜗杆
contour 轮廓
evolute 渐开线
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
In general,the application of the approximate machining
processes causes theoretical errors,However,since the
approximate processes are usually much simpler and easier
than the perfect ones,the use of them will be greatly
advantageous to the production efficiency and economics,
provided that the magnitude of the theoretical errors are
relatively small enough compared to the tolerance.
2,Geometrical Inaccuracy of Machine Tools and Cutting
Tools
When machining on numerically controlled machine tools,
the theoretical errors occur,if approximate algorithms are used
to approach the curved contour of the workpiece surface in
NC programming,Also,the theoretical errors exist,when
linear or circular interpolation is adopted for approaching a
contour curve.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
For example,the unparalleled longitudinal
bed ways with respect to the axis of rotation of
the spindle in the horizontal direction on a lathe
leads to tapers on the machined cylindrical
surfaces,while the lack of parallelism between
them in the vertical direction causes the
machined cylindrical surfaces hyperboloid errors.
When inaccuracy exists in the straightness of the
bed ways,the perfect cylindrical surface can not
be obtained in the turning process.
No matter how precisely a machine tool is
made,there must exist inaccuracy,which is the
causes of errors in operations using the machine
tool,Machine tool inaccuracy is a result of the
tool's manufacture,improper installation,and/or
its wear in application.
spindle 主轴
taper 圆锥形
hyperboloid
双曲面
straightness
平直度
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Runout of the running centers of a lathe leads to runout
of the machined surfaces of the workpiece,with respect to
the axis of the center holes,If the semi-finished workpiece is
transferred to another lathe with a different runout of its
centers,the misalignment of the surfaces may occur,since
they are machined under different conditions,This will not
happen in machining if the workpiece is held between two
dead centers,even if it is machined in several settings or
operations.
Runout of the rotational axis of the spindle of a lathe
results in out-of-roundness on the machined cylindrical
surface of the workpiece.
misalignment 不同轴，不同心runout 径向跳动 center 顶尖
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The magnitude of the machine tool inaccuracy is not always
equal to that of the machining error it caused,In many cases,the
machining error is a function of the machine tool inaccuracy and
should be determined through calculation,For instance,due to
the wear of a lathe in application,the concavities emerge on the
bed ways,This results in the inclination of the tool carriage,The
displacement of the tool tip in the horizontal direction can be
calculated from the following formula.
P = a × h/b 6.l
where p—displacement of the tool tip in horizontal direction,mm.
h—vertical distance between the running center of the
workpiece and the bed way,mm.
a—maximum depression of the bed way,mm.
b—width of the bed way,mm.
concavity 凹状，成凹形multiply divide
millimeter
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The unparallelism between bed ways on a lathe also
affects the position of the tool carriage with respect to the
workpiece and therefore causes shape error of the workpiece
machined,The displacement of the tool tip in the horizontal
direction resulting from the "twist" of the bed ways can be
calculated from the following equation.
Q = δ × h/b 6.2
where Q—displacement of tool tip in horizontal direction,mm.
h—vertical distance between the running center of
workpiece and the bed ways,mm.
δ—amount of twist of bed,mm.
b—width of the bed way,mm.
tool carriage 拖板
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Since the h/b ratios of most machine tools are between 0.6
and 1.0,the impact of "twist" of the bed ways cannot be under-
estimated,In addition,“twist” of the bed ways also leads to
errors in geometrical form of the workpiece over a length
corresponding to the length of the,twist” in the bed ways.
It is not that the inaccuracy of all units of the machine tool
leads to machining errors for a specific machining operation.
For instance,the unparallelism between the bed ways and the
running center of the spindle in the horizontal direction on a
lathe causes taper on the machined cylindrical surface of the
workpiece held by the chuck,because it does not affect the
machining accuracy if the workpiece is held between two dead
centers,Thus it can be seen that in order to improve the
machining accuracy of a machine tool,only the units related to
the machining conditions are required to be improved.
卡
盘
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Machining errors depend as well upon the inaccuracy
of the cutting tool manufactured,The inaccuracy of the
cutting tool is directly related to the machining errors of
workpieces in the following situations:
l,For machining using cutting tools with fixed sizes,such
as drills,reamers,grooving tools,broaches,etc.,the
inaccuracy in the dimensions of the tools directly reflects
upon the dimension errors on workpiece surfaces.
2,For machining using formed tools,such as formed
turning tools,formed milling cutters,formed grinding
wheels,etc.,the inaccuracy in the form of the tools
directly reflects upon the shape errors on the machined
surfaces of workpieces.
groove 切槽，铣槽
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Although the manufacturing inaccuracy of the standard
cutting tools,such as standard turning tools and plane milling
cutters,is not directly concerned in the machining errors of
workpieces,it is indirectly related in some cases,For example,
if the parameters of the geometry or the form of a cutting tool
are not properly manufactured,the tool wear will be
intensified in machining,which leads indirectly to machining
errors.
The tool wear is inevitable in machining,When
machining a long shaft,the wear of the turning tool causes the
taper on the machined cylindrical surface of the shaft,The
heterogeneous wear of the form-grinding wheel directly
affects the shape accuracy of the ground surface.
非均匀的
