SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
LESSON SIX GEAR
MANUFACTURING METHODS
There are two basic methods of
manufacturing gear teeth,the
generating process and the forming
process,When a gear tooth is
generated,the workpiece and the
cutting or grinding tool are in
continuous mesh and the tooth form is
generated by the tool,In other word,
the work and the tool are conjugate to
each other,Hobbing machines,shaper
cutters,shaving machines,and many
grinders use this principle.
generating process
展成法
forming process
成形法
grinding 磨削
mesh 啮合，网格
conjugate 共轭的
hobbing 滚削，滚刀
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Shaping
Shaping is inherently similar to planing but uses a
circular cutter instead of a rack and the resulting reduction in
the reciprocating inertia allows much higher stroking speeds;
modern shapers cutting car gears can run at 2,000 cutting
strokes per minute,The shape of the cutter is roughly the
same as an involute gear but the tips of the teeth are rounded.
When a gear tooth is formed,the tool is in the shape of
the space that is being machined out,Some grinding
machines use this principle with an indexing mechanism,
which allows the gear teeth to be formed tooth by tooth.
Broaches are examples of form tools that machine all the
gear teeth simultaneously.
成形，插齿
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The generating drive between cutter and workpiece does
not involve a rack or leadscrew since only circular motion is
involved,The tool and workpiece move tangential typically
0.5 mm for each stroke of the cutter,On the return stroke the
cutter must be retracted about l mm to give clearance
otherwise tool rub occurs on the backstroke and failure is
rapid,The speed on this type of machine is limited by the
rate at which some 50kg of cutter and bearings can be moved
a distance of 1 mm,The accelerations involved require forces
of the order of 5000 N (0.5 Ton) yet high accuracy must be
maintained.
leadscrew 丝杆
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The advantages of shaping are that production rates are
relatively high and that it is possible to cut right up to a
shoulder,Unfortunately,for helical gears,a helical guide is
required to impose a rotational motion on the stroking
motion; such helical guides cannot be produced easily or
cheaply so the method is only suitable for long runs with
helical gears since special cutters and guides must be
manufactured for each different helix angle,A great
advantage of shaping is its ability to cut annular gears such
as those required for large epicyclic drives.
annular gear 内齿轮
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When very high accuracy is of importance the inaccuracies
in the shaping cutter matter since they may transfer to the cut
gear,It is obvious that profile errors will transfer but it is less
obvious than an eccentrically mounted or ground cutter will
give a characteristic,dropped tooth”,There are several causes
for "dropped tooth" but it occurs most commonly when the
diameter of the workpiece is about half,one and a half,two
and a half,etc,times the cutter diameter,If the cutter starts on a
high point and finishes on a low point during the final finishing
revolution of the gear the peak to peak eccentricity error in the
cutter occurs between the last and first tooth of the final
revolution of the cut gear; as the cumulative pitch error of the
cutter may well be over 25 microns there is a sudden pitch
error of this amount on the cut gear,The next gear cut on the
machine may however be very good on adjacent pitch if the
final cut happened to start in a favorable position on the cutter.
偏
心
的
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Various attempts have been made to prevent this effect,in
particular by continuing rotation without any further cutter
infeed but if the shaping machine is not very rigid and the
cutter very sharp then no further cutting will occur and the
error will not be removed.
Hobbing
Hobbing,the most used metal cutting method,uses the
rack generating principle but avoids slow reciprocation by
mounting many "racks" on a rotating cutter,The "tacks" are
displaced axially to form a gashed worm,The "racks" do not
generate the correct involute shape for the whole length of
the teeth since they are moving on a circular path and so the
hob is fed slowly along the teeth either axially in normal
hobbing or in the direction of the helix in "oblique" hobbing.
gash 深砍，划开 worm 蜗杆 helix 螺旋线 oblique 倾斜的
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Metal removal rates are high since no
reciprocation of hob or workpiece is required
and so cutting speeds 0f 40 m/min can be
used for conventional hobs and up to 150
m/min for carbide hobs,Typically with a 100
mm diameter hob the rotation speed will be
100 rpm and so a twenty tooth workpiece
will rotate at 5 rpm,Each revolution of the
workpiece will correspond to 0.75 mm feed
so the hob will advance through the
workpiece at about 4 mm per minute,For car
production roughing multiple start hobs can
be used with coarse feeds of 3 mm per
revolution so that 100 rpm on the cutter,a
two-start hob and a 20 tooth gear will give a
feed rate of 30mm/minute.
carbide 硬质合金
roughing 粗加工
coarse 大的，粗的
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The disadvantage of a coarse feed rate is that a clear
marking is left on the workpiece,particularly in the root,
showing a pattern at a spacing of the feed rate per
revolution,This surface undulation is less marked on the
flanks than in the root and is not important when there is a
subsequent finishing operation such as shaving or grinding.
When there are no further operations the feed per revolution
must be restricted to keep the undulations below a limit,
which is usually dictated by lubrication conditions,The
height of the undulations in the root of the gear is given by
squaring the feed per revolution and dividing by four times
the diameter of the hob,l mm feed and 100 mm diameter
gives 2.5 micron high undulations in the root,On the gear
flank (at 70oon a large gear) the undulation is roughly cos
70oas large,i.e.,about 0.85 micron.
波
纹 剃
齿
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Accuracy of hobbing is normally high for pitch and for helix,
provided machines are maintained,involute is dependent solely
on the accuracy of the hob profile,As the involute form is
generated by as many cuts as there are gashes on the hob the
involute is not exact,but if there are,say,14 tangents generating
a flank of 20 mm radius of curvature about 4 mm high the
divergence from a true involute is only about half a micron,hob
manufacturing and mounting errors can be above 10 microns.
Use of twostart hobs or oblique hobbing gives increased error
levels since hob errors of pitching transfer to the cut gear.
Broaching
Broaching is not usually used for helical gears but is useful
for internal spur gears; the principal use of broaching in this
context is for internal splines,which cannot easily be made by
any other method,As with all broaching the method is only
economic for large quantities since setup costs are high.
周
节
拉削 spline 花键
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The major application of broaching techniques to helical
external gears is that used by Gleasons in their G-TRAC
machine,This machine operates by increasing the effective
radius of a hobbing cutter to infinity so that etch tooth of the
cutter is traveling in a straight line instead of on a radius.
This allows the cutting action to extend over the whole
facewidth of a gear instead of the typical 0.75 mm feed per
revolution of hobbing,The resulting process gives a very
high production rate,more suitable for U.S.A,production
volumes than for the relatively low European volumes and
so,despite a high initial cost,is very competitive.
Broaching gives high accuracy and good surface finish
but like all cutting processes is limited to,soft” materials,
which must be subsequently casehardened,or heat treated,
giving distortion.
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Shaving And Rolling
Shaving and rolling are both used as
finishing processes for gears in the,soft”
state,The objective is to improve surface
finish and profile by mating the roughed-
out gear with a "cutter" which will improve
form.
A shaving cutter looks like a gear which
has extra clearance at the root (for swarf and
coolant removal) and whose tooth flanks
have been grooved to give cutting edges,It
is run in mesh with the rough gear with
crossed axes so that there is in theory point
contact with a relative velocity along the
teeth giving scraping action.（ c）
rolling 滚压
swarf 切屑
coolant 冷却液
切削液
scrape 刮削
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（ c） The shaving cutter teeth are relatively
flexible in bending and so will only operate
effectively when they are in double contact
between two gear teeth,The gear and cutter
operate at high rotational speeds with traversing
of the workface and about 100 micron of
material is removed,Cycle times can be less
than half a minute and the machines are not
expensive but cutters are delicate and difficult to
manufacture,It is easy to make adjustments of
profile at the shaving stage and crowning can be
applied,Shaving can be carried out near a
shoulder by using a cutter,which is plunged in
to depth without axial movement,this method is
fast but requires more complex cutter design.
crowning
鼓形齿
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Finish rolling,unlike shaving,does not remove metal but
redistributes it by imposing sufficient local contact stresses to
give plastic deformation,Surface finish is good and accuracy
is good with a long tool life but the tools are expensive and a
rigid machine is required,There is debate as to whether the
severe cold working of the surface gives a better or worse
gear life,The principal disadvantage of the process is that the
roughing,usually hobbing,process must be held to tight
limits since rolling is less tolerant than shaving of errors in
the rough gear,Economies at the finish rolling stage can
easily be negated by extra costs at the hobbing stage,in
general shaving is more economical for shorter and rolling for
larger runs.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Rolling can also be used to form gears or splines
from bar,bypassing the roughing process,Small
teeth may be formed in ductile materials by this
method,which is popular for high production on
small gears such as the gears used in epicyclics in
automatic transmissions,For the larger gears used in
manual transmissions,with teeth 5 mm or more high,
cutting methods are normally used,The production
rates in rolling are high but the machines must be
very rigid and the tools are expensive,In a single
revolution of the working pair of tools,hundreds of
teeth successively form the spaces; each tooth
differs as the gear is formed and so each must be
individually ground to shape,Splines are formed
similarly but the formers may be racks instead of
rotary tools.
ductile
易拉长的
可塑的
epicyclic
周转圆的
外摆线的
transmission
传递
变速器
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Shaving and rolling both give high accuracy,usually
after some "cut and try" at the initial design stage,however,
with gears of very high helix angles,shaving gives errors
due to deflections and shaving leaves existing pitch errors.
Subsequent heat treatment distortion must be allowed for at
this stage but is often variable,When high productivity is
required the cutters must run true to within 5 microns and
the shaving machines should be designed to be extremely
rigid but to,break” and deflect easily when a critical force
level is exceeded.
Grinding
Grinding is extremely important because it is the main
way hardened gears are machined,When high accuracy is
required it is not sufficient to pre-correct for heat treatment
distortion and grinding is then necessary.
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The simplest approach to grinding is form
grinding,often termed the Orcutt method,The
wheel profile is dressed accurately to shape using
single point diamonds,which are controlled by
templates cut to the exact shape required; 6:l
scaling with a pantograph is often used,The
profiled wheel is then reciprocated axially along
the gear,which rotates to allow for helix angle
effects; when one tooth shape has been finished,
involving typically 100 micron metal removal the
gear is indexed to the next tooth space,This
method is fairly slow but gives high accuracy
consistently,Setting up is lengthy because
different dressing templates are needed if module,
number of teeth,helix angle,or profile correction
are changed.
template 样板
(切向推进磨 )
导板
pantograph
比例绘图仪
dressing
修整
module
模数
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The fastest grinding method uses the same principle as hobbing
but replaces a gashed and relieved worm by a grinding wheel,
which is a rack in section,Since high surface speeds are needed
the wheel diameter is increased so that wheels of 0.5 m diameter
can run at over 2000 rpm to give the necessary 1000 m/min,Only
single start worms are cut on the wheel but gear rotation speeds are
high,100 rpm typically,so it is difficult to design the drive system
to give accuracy and rigidity,Accuracy of the process is
reasonably high although there is a tendency for wheel and
workpiece to deflect variably during grinding so the wheel form
may require compensation for machine deflection effects.
Generation of a worm shape on the grinding wheel is a slow
process since a dressing diamond or roller must not only form the
rack profile but has to move axially as the wheel rotates,Once the
wheel has been trued,gears can be ground rapidly until redressing
is required,This is the most popular method for high production
rates with small gears and is usually called the Reishauer method.
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Large gears are usually generated by the
Maag method,which is similar to planing in its
approach but uses cup grinding wheels of large
diameter to form the flanks of the theoretical
mating rack,Gears of very large diameter
cannot easily be moved so the gear is essentially
stationary while the grinding wheel carriage
reciprocates in the direction of the helix,The
wheel is only in contact over a small part of the
facewidth in helical gears so only a part of two
flanks is being ground at any one time,The
process is very slow but this is not important
when only a few gears of this size are made in a
year,As with form grinding,after grinding a
pair of flanks the gear is indexed to the next pair.
planing 刨削
flank 侧面
rack 齿条
reciprocate
互换,往复运动
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A similar method used for medium size gears has
stationary wheels,(which are effectively mating rack
flanks) while the rough gear is traversed under the wheels.
Corresponding rotational movement of the gear is
controlled by steel bands unwrapping from a cylinder of
pitch circle diameter so that the motion of gear relative to
"rack" is correct.
Another method,the Niles approach,uses a wheel,
which is formed to give the,theoretical mating rack”
instead of using two cup wheels as in the Maag method.
This approach is best suited to medium precision work on
smaller gears and is intermediate in speed between the
Reishauer and Maag methods.
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All grinding processes are slow and costly compared
with cutting processes and so are only used when accuracy is
essential,A rough rule of thumb is that grinding will increase
gear-cutting costs by a factor of 10 but the cost of the teeth is
often only a small part of the total cost of a gearbox,The
accuracies attainable
are surprisingly not very dependent
on size of gear; whether a gear is
5 m or 50 mm diameter the pitch
involute and helix accuracies
attainable are of the order of 5
microns or better and more
dependent on the skill and patience
of the operator and inspectors than
on any other factors.
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It is often assumed that grinding will remove all error
generated at the roughing stag,Unfortunately,grinding machines
are relatively flexible and so the grinding wheel has a tendency
to follow previous errors,The errors will thus be reduced but not
completely eliminated unless very many cuts are used; whenever
a grinding process is giving in consistent results it is advisable to
check the accuracies at the rough-cut stage,The only exception
is the form grinding process,which will not follow involute
errors though it will still allow helix and pitch errors.
Lapping And Honing
As a final process gears in the hard state may be lapped or
honed,mainly to improve surface finish and remove odd burrs,In
lapping,loose abrasive is introduced into the mesh as a pair of
gears is run together under light load.
珩磨
研磨
abrasive 磨料，研磨剂 burr 毛边
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Gears may be run either as pairs or individually with a
master,which is often cast iron; if gears are lapped as a pair
they should subsequently be kept as a pair,It is not advisable to
attempt to use lapping for profile correction as this is liable to
do more harm than good and it is unlikely that lapping will be
able to help much on very heavily loaded gears since contact is
over a very small area when lapping is occurring but is over
nearly the full tooth flank under load,Heavy lapping of a pair
of highly loaded gears would destroy the profile corrections and
give high vibration levels under load.
Honing is a process akin to shaving but uses a bonded
abrasive mating Bear with coolant,More metal is removed than
with lapping so that profile can be corrected as well as surface
finish improved but helix and pitch are not usually much
improved,Maintaining a sufficiently accurate form on an
abrasive,gear” is not easy.
LESSON SIX GEAR
MANUFACTURING METHODS
There are two basic methods of
manufacturing gear teeth,the
generating process and the forming
process,When a gear tooth is
generated,the workpiece and the
cutting or grinding tool are in
continuous mesh and the tooth form is
generated by the tool,In other word,
the work and the tool are conjugate to
each other,Hobbing machines,shaper
cutters,shaving machines,and many
grinders use this principle.
generating process
展成法
forming process
成形法
grinding 磨削
mesh 啮合，网格
conjugate 共轭的
hobbing 滚削，滚刀
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Shaping
Shaping is inherently similar to planing but uses a
circular cutter instead of a rack and the resulting reduction in
the reciprocating inertia allows much higher stroking speeds;
modern shapers cutting car gears can run at 2,000 cutting
strokes per minute,The shape of the cutter is roughly the
same as an involute gear but the tips of the teeth are rounded.
When a gear tooth is formed,the tool is in the shape of
the space that is being machined out,Some grinding
machines use this principle with an indexing mechanism,
which allows the gear teeth to be formed tooth by tooth.
Broaches are examples of form tools that machine all the
gear teeth simultaneously.
成形，插齿
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The generating drive between cutter and workpiece does
not involve a rack or leadscrew since only circular motion is
involved,The tool and workpiece move tangential typically
0.5 mm for each stroke of the cutter,On the return stroke the
cutter must be retracted about l mm to give clearance
otherwise tool rub occurs on the backstroke and failure is
rapid,The speed on this type of machine is limited by the
rate at which some 50kg of cutter and bearings can be moved
a distance of 1 mm,The accelerations involved require forces
of the order of 5000 N (0.5 Ton) yet high accuracy must be
maintained.
leadscrew 丝杆
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The advantages of shaping are that production rates are
relatively high and that it is possible to cut right up to a
shoulder,Unfortunately,for helical gears,a helical guide is
required to impose a rotational motion on the stroking
motion; such helical guides cannot be produced easily or
cheaply so the method is only suitable for long runs with
helical gears since special cutters and guides must be
manufactured for each different helix angle,A great
advantage of shaping is its ability to cut annular gears such
as those required for large epicyclic drives.
annular gear 内齿轮
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When very high accuracy is of importance the inaccuracies
in the shaping cutter matter since they may transfer to the cut
gear,It is obvious that profile errors will transfer but it is less
obvious than an eccentrically mounted or ground cutter will
give a characteristic,dropped tooth”,There are several causes
for "dropped tooth" but it occurs most commonly when the
diameter of the workpiece is about half,one and a half,two
and a half,etc,times the cutter diameter,If the cutter starts on a
high point and finishes on a low point during the final finishing
revolution of the gear the peak to peak eccentricity error in the
cutter occurs between the last and first tooth of the final
revolution of the cut gear; as the cumulative pitch error of the
cutter may well be over 25 microns there is a sudden pitch
error of this amount on the cut gear,The next gear cut on the
machine may however be very good on adjacent pitch if the
final cut happened to start in a favorable position on the cutter.
偏
心
的
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Various attempts have been made to prevent this effect,in
particular by continuing rotation without any further cutter
infeed but if the shaping machine is not very rigid and the
cutter very sharp then no further cutting will occur and the
error will not be removed.
Hobbing
Hobbing,the most used metal cutting method,uses the
rack generating principle but avoids slow reciprocation by
mounting many "racks" on a rotating cutter,The "tacks" are
displaced axially to form a gashed worm,The "racks" do not
generate the correct involute shape for the whole length of
the teeth since they are moving on a circular path and so the
hob is fed slowly along the teeth either axially in normal
hobbing or in the direction of the helix in "oblique" hobbing.
gash 深砍，划开 worm 蜗杆 helix 螺旋线 oblique 倾斜的
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Metal removal rates are high since no
reciprocation of hob or workpiece is required
and so cutting speeds 0f 40 m/min can be
used for conventional hobs and up to 150
m/min for carbide hobs,Typically with a 100
mm diameter hob the rotation speed will be
100 rpm and so a twenty tooth workpiece
will rotate at 5 rpm,Each revolution of the
workpiece will correspond to 0.75 mm feed
so the hob will advance through the
workpiece at about 4 mm per minute,For car
production roughing multiple start hobs can
be used with coarse feeds of 3 mm per
revolution so that 100 rpm on the cutter,a
two-start hob and a 20 tooth gear will give a
feed rate of 30mm/minute.
carbide 硬质合金
roughing 粗加工
coarse 大的，粗的
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The disadvantage of a coarse feed rate is that a clear
marking is left on the workpiece,particularly in the root,
showing a pattern at a spacing of the feed rate per
revolution,This surface undulation is less marked on the
flanks than in the root and is not important when there is a
subsequent finishing operation such as shaving or grinding.
When there are no further operations the feed per revolution
must be restricted to keep the undulations below a limit,
which is usually dictated by lubrication conditions,The
height of the undulations in the root of the gear is given by
squaring the feed per revolution and dividing by four times
the diameter of the hob,l mm feed and 100 mm diameter
gives 2.5 micron high undulations in the root,On the gear
flank (at 70oon a large gear) the undulation is roughly cos
70oas large,i.e.,about 0.85 micron.
波
纹 剃
齿
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Accuracy of hobbing is normally high for pitch and for helix,
provided machines are maintained,involute is dependent solely
on the accuracy of the hob profile,As the involute form is
generated by as many cuts as there are gashes on the hob the
involute is not exact,but if there are,say,14 tangents generating
a flank of 20 mm radius of curvature about 4 mm high the
divergence from a true involute is only about half a micron,hob
manufacturing and mounting errors can be above 10 microns.
Use of twostart hobs or oblique hobbing gives increased error
levels since hob errors of pitching transfer to the cut gear.
Broaching
Broaching is not usually used for helical gears but is useful
for internal spur gears; the principal use of broaching in this
context is for internal splines,which cannot easily be made by
any other method,As with all broaching the method is only
economic for large quantities since setup costs are high.
周
节
拉削 spline 花键
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The major application of broaching techniques to helical
external gears is that used by Gleasons in their G-TRAC
machine,This machine operates by increasing the effective
radius of a hobbing cutter to infinity so that etch tooth of the
cutter is traveling in a straight line instead of on a radius.
This allows the cutting action to extend over the whole
facewidth of a gear instead of the typical 0.75 mm feed per
revolution of hobbing,The resulting process gives a very
high production rate,more suitable for U.S.A,production
volumes than for the relatively low European volumes and
so,despite a high initial cost,is very competitive.
Broaching gives high accuracy and good surface finish
but like all cutting processes is limited to,soft” materials,
which must be subsequently casehardened,or heat treated,
giving distortion.
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Shaving And Rolling
Shaving and rolling are both used as
finishing processes for gears in the,soft”
state,The objective is to improve surface
finish and profile by mating the roughed-
out gear with a "cutter" which will improve
form.
A shaving cutter looks like a gear which
has extra clearance at the root (for swarf and
coolant removal) and whose tooth flanks
have been grooved to give cutting edges,It
is run in mesh with the rough gear with
crossed axes so that there is in theory point
contact with a relative velocity along the
teeth giving scraping action.（ c）
rolling 滚压
swarf 切屑
coolant 冷却液
切削液
scrape 刮削
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（ c） The shaving cutter teeth are relatively
flexible in bending and so will only operate
effectively when they are in double contact
between two gear teeth,The gear and cutter
operate at high rotational speeds with traversing
of the workface and about 100 micron of
material is removed,Cycle times can be less
than half a minute and the machines are not
expensive but cutters are delicate and difficult to
manufacture,It is easy to make adjustments of
profile at the shaving stage and crowning can be
applied,Shaving can be carried out near a
shoulder by using a cutter,which is plunged in
to depth without axial movement,this method is
fast but requires more complex cutter design.
crowning
鼓形齿
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Finish rolling,unlike shaving,does not remove metal but
redistributes it by imposing sufficient local contact stresses to
give plastic deformation,Surface finish is good and accuracy
is good with a long tool life but the tools are expensive and a
rigid machine is required,There is debate as to whether the
severe cold working of the surface gives a better or worse
gear life,The principal disadvantage of the process is that the
roughing,usually hobbing,process must be held to tight
limits since rolling is less tolerant than shaving of errors in
the rough gear,Economies at the finish rolling stage can
easily be negated by extra costs at the hobbing stage,in
general shaving is more economical for shorter and rolling for
larger runs.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Rolling can also be used to form gears or splines
from bar,bypassing the roughing process,Small
teeth may be formed in ductile materials by this
method,which is popular for high production on
small gears such as the gears used in epicyclics in
automatic transmissions,For the larger gears used in
manual transmissions,with teeth 5 mm or more high,
cutting methods are normally used,The production
rates in rolling are high but the machines must be
very rigid and the tools are expensive,In a single
revolution of the working pair of tools,hundreds of
teeth successively form the spaces; each tooth
differs as the gear is formed and so each must be
individually ground to shape,Splines are formed
similarly but the formers may be racks instead of
rotary tools.
ductile
易拉长的
可塑的
epicyclic
周转圆的
外摆线的
transmission
传递
变速器
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Shaving and rolling both give high accuracy,usually
after some "cut and try" at the initial design stage,however,
with gears of very high helix angles,shaving gives errors
due to deflections and shaving leaves existing pitch errors.
Subsequent heat treatment distortion must be allowed for at
this stage but is often variable,When high productivity is
required the cutters must run true to within 5 microns and
the shaving machines should be designed to be extremely
rigid but to,break” and deflect easily when a critical force
level is exceeded.
Grinding
Grinding is extremely important because it is the main
way hardened gears are machined,When high accuracy is
required it is not sufficient to pre-correct for heat treatment
distortion and grinding is then necessary.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The simplest approach to grinding is form
grinding,often termed the Orcutt method,The
wheel profile is dressed accurately to shape using
single point diamonds,which are controlled by
templates cut to the exact shape required; 6:l
scaling with a pantograph is often used,The
profiled wheel is then reciprocated axially along
the gear,which rotates to allow for helix angle
effects; when one tooth shape has been finished,
involving typically 100 micron metal removal the
gear is indexed to the next tooth space,This
method is fairly slow but gives high accuracy
consistently,Setting up is lengthy because
different dressing templates are needed if module,
number of teeth,helix angle,or profile correction
are changed.
template 样板
(切向推进磨 )
导板
pantograph
比例绘图仪
dressing
修整
module
模数
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The fastest grinding method uses the same principle as hobbing
but replaces a gashed and relieved worm by a grinding wheel,
which is a rack in section,Since high surface speeds are needed
the wheel diameter is increased so that wheels of 0.5 m diameter
can run at over 2000 rpm to give the necessary 1000 m/min,Only
single start worms are cut on the wheel but gear rotation speeds are
high,100 rpm typically,so it is difficult to design the drive system
to give accuracy and rigidity,Accuracy of the process is
reasonably high although there is a tendency for wheel and
workpiece to deflect variably during grinding so the wheel form
may require compensation for machine deflection effects.
Generation of a worm shape on the grinding wheel is a slow
process since a dressing diamond or roller must not only form the
rack profile but has to move axially as the wheel rotates,Once the
wheel has been trued,gears can be ground rapidly until redressing
is required,This is the most popular method for high production
rates with small gears and is usually called the Reishauer method.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Large gears are usually generated by the
Maag method,which is similar to planing in its
approach but uses cup grinding wheels of large
diameter to form the flanks of the theoretical
mating rack,Gears of very large diameter
cannot easily be moved so the gear is essentially
stationary while the grinding wheel carriage
reciprocates in the direction of the helix,The
wheel is only in contact over a small part of the
facewidth in helical gears so only a part of two
flanks is being ground at any one time,The
process is very slow but this is not important
when only a few gears of this size are made in a
year,As with form grinding,after grinding a
pair of flanks the gear is indexed to the next pair.
planing 刨削
flank 侧面
rack 齿条
reciprocate
互换,往复运动
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
A similar method used for medium size gears has
stationary wheels,(which are effectively mating rack
flanks) while the rough gear is traversed under the wheels.
Corresponding rotational movement of the gear is
controlled by steel bands unwrapping from a cylinder of
pitch circle diameter so that the motion of gear relative to
"rack" is correct.
Another method,the Niles approach,uses a wheel,
which is formed to give the,theoretical mating rack”
instead of using two cup wheels as in the Maag method.
This approach is best suited to medium precision work on
smaller gears and is intermediate in speed between the
Reishauer and Maag methods.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
All grinding processes are slow and costly compared
with cutting processes and so are only used when accuracy is
essential,A rough rule of thumb is that grinding will increase
gear-cutting costs by a factor of 10 but the cost of the teeth is
often only a small part of the total cost of a gearbox,The
accuracies attainable
are surprisingly not very dependent
on size of gear; whether a gear is
5 m or 50 mm diameter the pitch
involute and helix accuracies
attainable are of the order of 5
microns or better and more
dependent on the skill and patience
of the operator and inspectors than
on any other factors.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
It is often assumed that grinding will remove all error
generated at the roughing stag,Unfortunately,grinding machines
are relatively flexible and so the grinding wheel has a tendency
to follow previous errors,The errors will thus be reduced but not
completely eliminated unless very many cuts are used; whenever
a grinding process is giving in consistent results it is advisable to
check the accuracies at the rough-cut stage,The only exception
is the form grinding process,which will not follow involute
errors though it will still allow helix and pitch errors.
Lapping And Honing
As a final process gears in the hard state may be lapped or
honed,mainly to improve surface finish and remove odd burrs,In
lapping,loose abrasive is introduced into the mesh as a pair of
gears is run together under light load.
珩磨
研磨
abrasive 磨料，研磨剂 burr 毛边
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Gears may be run either as pairs or individually with a
master,which is often cast iron; if gears are lapped as a pair
they should subsequently be kept as a pair,It is not advisable to
attempt to use lapping for profile correction as this is liable to
do more harm than good and it is unlikely that lapping will be
able to help much on very heavily loaded gears since contact is
over a very small area when lapping is occurring but is over
nearly the full tooth flank under load,Heavy lapping of a pair
of highly loaded gears would destroy the profile corrections and
give high vibration levels under load.
Honing is a process akin to shaving but uses a bonded
abrasive mating Bear with coolant,More metal is removed than
with lapping so that profile can be corrected as well as surface
finish improved but helix and pitch are not usually much
improved,Maintaining a sufficiently accurate form on an
abrasive,gear” is not easy.
