Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 0220864~ 1997-06-24
A Process and an Apparatus for Fine-machining the Tooth
Flanks of a Gear Wheel on a Machining Tool
The present lnvention relates to a process for fine
machining the tooth flanks (3) of a gear wheel (2) on a
machine tool in which a machining tool (1) that ls essentlally
symmetrical about its axls is brought into contact with the
tooth flanks (3) of the gear wheeled (2) that is to be
machined, the machining tool (l) rotating about its axis and
the gear wheel (2) rotating about its axis during the
machlning process.
The present lnventlon also descrlbes an apparatus
for carrylng out the process.
When hardened gear wheels are machined, very
fre~uently hobbed and hardened gear wheels have the exact
tooth-flank profile (involutions) imparted to them by
grindlng. A honing process sometimes follows the grindlng
process ln order to minimize any meshing faults that may stlll
remain after the grinding process.
In particular, when gear wheels are mass produced,
gear wheel hones are used because only a relatively small
amount of tlme ls needed for the honlng process. In thls
process, the gear wheel that ls to be machlned rotates about
lt axls. In so dolng, lt meshes wlth a honlng tool which--in
the event that teeth to be machlned are external--ls in the
form of an internally toothed annular ring. The honing tool
also rotates about its axis. The inside teeth of the honing
tool corresponding exactly to the shape of the tooth tight
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that are to be machlned, taklng into account the rotatlng
movement between tool and workplece; thus, the meshlng
geometry of the coating tool ls so configured that lt
generates the preclse, deslred geometry of the tooth flanks
that are to be machined.
The working surface of the honing tool ls provlded
wlth abraslve partlcles ln order to remove materlal from
hardened gear wheels that are to be machlned. Thls removal ls
effected ln that, for example, the tool ls drlven and a
speclflc retardlng moment acts on the work plece. As an
alternatlve, provlslon can be made such that there ls a
posltlve coupllng between the rotatlonal movement of the tool
and that of the work plece.
It has been found that gear wheels that have been
perfectly ground and honed may not, under certaln
clrcumstances, display optlmal runnlng behavlour when ln
operatlon. Tests have shown that thls ls caused by the
surface mlcrostructure of the flanks of the gear wheel. In
the mlcroscoplc range, there are frequently "pro~ectlons" on
the tooth flanks even after grindlng and honing, whlch are
devlatlons from the ldeal form (lnvolutlon). Thls means that
the actlve proflle, whlch ls to say the area ln whlch there ls
contact between the two gear wheels that are enmeshed, ls not
maxlmal.
Experlence has also shown that some of these
lrregularltles cannot be ellmlnated even by addltlonal
abrasive machlnlng (grlndlng, honlng, lapplng).
Thus, lt ls the task of the present lnventlon to
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descrlbe a fine-machlnlng process and an approprlate apparatus
for machlnlng the tooth flanks of gear wheels that wlll help
ln achlevlng an lmprovement of the surfaces of the tooth
flanks and thus a hlgher bearlng capaclty ln the operatlon of
the gear wheels.
The solutlon to thls problem by the present
inventlon ls characterlzed ln that ln a process of the type
described ln the introduction hereto, the machinlng surfaces
(6) of the machlnlng tool (1) do not abrade material from the
tooth flanks (3) that are to be machlned. In partlcular,
provlslon ls made such that the machlnlng surfaces (6) of the
machinlng tool (1) only smooth the surface of the tooth flanks
(3) that are to be machlned, wlthout removlng materlal from
the surface of the tooth flanks (3) that are to be machlned.
The present lnventlon ls based on the knowledge that
the surface geometry cannot be improved as deslred, even by
extenslve abraslon-type machlnlng, although lmprovements ln
the sense of the task undertaken by the present lnventlon can
be effected by non-abraslve rolllng contact as a flnal stage
ln machlnlng hard surfaces.
It ls advantageous lf provlslon be made such that
the axls of rotatlon (4) of the machlnlng tool (1) and the
axls of rotatlon (5) of the gear wheel (2) subtend an angle
between 0~ and 20~, preferably between 3~ and 15~. Thls means
that ln the clrcle of contact (pltch clrcle dlameter), when
there ls usually no relatlve speed between the rolllng flanks
of the tool and the workplece when the axes subtend an angle
of 0~, there ls ln thls case (glven an angle of lntersectlon
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that is not equal to 0C).
Various drivlng concepts for this process have been
conceived:
Initially, provislon can be made such that the
machining tool (1) is driven and the gear wheel (2) is
retarded, so that there is positive contact between the
machining surfaces (6) of the machining tool (1) and the tooth
flanks (3) of the gear wheel (2) that are to be machined.
An alternative to this is that the machining tool
(1) can be retarded and the gear wheel (2) can be driven so
that there is positive contact between the machining surfaces
(6) of the machining tool and the tooth flanks (3) of the gear
wheel (2) that is to be machined.
Finally, it is also possible that the rotational
movement of the machining tool (1) and the rotational movement
of the gear wheel (2) be positively coupled.
The apparatus used to carry out this process
incorporates:
- a machining tool (1) that is essentially symmetrical
about its axis, and
- a gear wheel (2) that is to be machined,
the machining tool (1) rotating about its axls (4) and
engaging in the gear wheel (2) that is to be machined and that
also rotates about its axis (5).
According to the present invention, thls apparatus
ls characterlzed in that the machining surfaces (6) of the
machine tool (1) are not abrasive.
One development of the present invention is such
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that the axis of rotation (4) of the machlne tool (1) and the
axis of rotatlon (5) of the gear wheel (2) subtend an angle of
lntersectlon between 0~ and 20~, preferably between 3~ and
15~.
It ls lmportant for achlevlng a long servlce llfe
for the tool and for efflclent machlnlng of the workplece that
the machlnlng surfaces (6) of the machlnlng tool (1) be
hardened.
As an alternatlve to the foregolng, provlslon can be
made such that the machlnlng surfaces (6) of the machlnlng
tool (1) are coated wlth hard metal or wlth another wear-
reslstant materlal. In addltlon to hard metal, thls could be
a layer of dlamonds or a layer of boron nltrlde. Flnally, lt
ls also posslble that the machlnlng surfaces (6) of the
machlnlng tool be coated, ln partlcular plasma coated.
Materlals that are extremely hard and whlch dlsplay a hlgh
level of ductlllty are sultable for such a coatlng.
In order to reallze the varlous drlvlng concepts,
conslderatlon was flrst glven to provldlng means to drlve the
machlnlng tool (1) and means to retard the gear wheel (2), so
that there ls posltlve contact between the machlnlng surfaces
(6) of the machlnlng tool (1) and the tooth flanks (3) of the
gear wheel (2) that are to be machlned.
As an alternatlve to thls, lt ls also posslble to
provlde means to retard the machlne tool (l) and means to
drlve the gear wheel (2) so that there ls posltlve contact
between the machlnlng surfaces (6) of the machlne tool (1) and
the tooth flanks (3) of the gear wheel (2) that are to be
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machlned.
Flnally, means are provlded to produce a forced
coupllng of the rotatlonal movements of the machlnlng tool ll)
and the gear wheel (2).
The machinlng tool (1) can advantageously be formed
essentlally ln one plece and comprlse a steel body or, as an
alternatlve to thls lt, can be a ceramlc body.
The drawlng shows one embodlment of the present
inventlon and provldes a diagrammatlc view of a gear wheel
that ls engaged wlth a machlnlng tool.
The drawlng shows a gear wheel 2 ln the form of an
externally toothed spur gear that ls belng machlned flnlshed
by means of an annular machinlng tool 1. After soft machlnlng
Ihobblng) and hardenlng the gear wheel was sub~ected to
machlne flnlshlng when hard; ln thls connectlon, the gear
wheel was flrst proflle-ground and then honed or sub~ected to
the Coronier~ process.
The flne finishing process ls carrled out, for
example, by the Coronler~ process, a speclal hlgh performance
machlnlng procedure. Thls is a process developed by Knapp
GmbH, Coburg, Germany, ln whlch a steel rlng that ls coated
wlth abraslve partlcles (the arrangement ls slmllar to that
shown ln the drawlng~ engages wlth the workplece. There ls a
posltlve coupllng between the rotatlon of the Coronier~ rlng
and the workplece, and thls ls controlled electronlcally.
For purposes of slmpliflcation, the drawlng shows
the case ln whlch the axls of rotatlon 4 of the machlnlng tool
1 and the axls of rotation S of the gear wheel 2 are arranged
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so as to be parallel. This ls not so in normal practlce.
Normally, both axes 4 and 5 lntersect at an angle of up to 205
or even more, but preferably between 3~ and 15~. Thls ensures
that there is a relative speed between the machlning surface 6
of the tool l and the tooth flanks 3 that are to be machined,
even ln the circle of contact. Provlslon can be made such
that the angle at whlch the axes lntersect can be varied
within the clted llmlts durlng the machlnlng process.
Durlng the flnished machlnlng that takes place after
the grinding/honlng processes the machlning tool 1 and the
gear wheel 2 are engaged wlth each other. The tooth flanks 3
of the gear wheel 2 that are to be machlned are thus ln
rolllng engagement wlth the machlnlng surfaces 6 of the
machinlng tool 1.
Durlng the machlnlng process, the machlning tool 1
rotates about lts axls 4 ln the dlrectlon of rotatlon 7; the
gear wheel 2 rotates about lts axis 5 in the dlrectlon of
rotatlon 8.
The machlnlng surfaces (6) of the machlnlng tool (1)
are coated with hard metal or wlth another wear-reslstant
material. In addition to hard metal, this could be a layer of
dlamonds or a layer of ceramlc materlal. The machlnlng
surfaces could also be coated by a plasma-coating process.
The rotational movement of the tool 1 and that of
the gear wheel 2 are coupled by way of an "electronlc shaft",
i.e., the relative rotation of the machining tool 1 and the
gear wheel 2 corresponds to the "ldeal" correlatlon that ls
the result of error-free meshlng.
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The aim of the process according to the present
lnventlon ls that durlng machlnlng of the teeth of the gear
wheel 2 by the machinlng tool 1, protruding peaks (ln the
mlcroscoplc range), whlch constltute a devlation from the
ideal lnvolute shape of the tooth flanks 3, are elther
flattened or broken down. However, thls ls not brought about
by the abrasive actlon of the machinlng tool 1 on the tooth
flanks 3 of the work plece 2 that are being machined, but
because the machining surfaces 6 of the tool 1 have a
reshaplng smoothlng effect on the surface of the tooth flanks
3 that are to be machined. Thus, pro~ectlons on the tooth
flanks of the gear wheel are flattened and, lf needs be, any
surviving (mlcro) plt marks ln the area of the tooth flanks
are evened out.
To this end, the machlnlng surfaces 6 are covered
wlth super-hard materlals that should, as far as posslble,
dlsplay a high degree of ductllity. Such materials have been
known for a long time and requlre no addltlonal dlscusslon.
In partlcular, lt is possible that the machlning surfaces 6 be
provlded wlth CBN (cublc crystalllne boron nltrlde), wlth
diamonds, or wlth nitrides of other elements. On the other
hand, of course, hard metals such as those that are normally
used ln manufacturlng processes that remove metal can also be
used as coatlng materlal for the machlnlng surfaces 6.
As the tool 1 rolls wlth the workplece 2, especlally
ln the case of rolllng coupling, the machlnlng surfaces 6 of
the tool 1 form the counter-proflle on the tooth flanks 3 of
the gear wheel 2, and thls counter-proflle corresponds to the
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ldeal tooth flank geometry. To this end, the machining tool 1
as a whole must be a stable structure, for which reason it ls
advantageous that the tool l be a steel basic body which, with
reference to the machlnlng surfaces 6, ls machlned (ground)
corresponding to the involute profile of the gear wheel that
ls desired.
Using the process according to the present invention
it is possible to achieve an improvement of the tooth-flank
geometry 3 of the gear wheel 2, which is to say to the surface
of the running surface, so that in subsequent operatlon, the
bearing portion of to gear wheels that are meshed together is
increased. Thus, it is possible by approprlate layout of the
machlnlng surfaces 6 to process the tooth foot of the
gear-tooth system of the gear wheel 2, i.e., the area that is
located between two adiacent tooth flanks 3, during the
process according to the present invention.
A further advantage of the process described herein
is that by machining the tooth flanks 3 with the machining
surfaces 6, pressure acts on the tooth flanks 3. The result
of this ls that the residual compressive stress in the surface
area of the tooth flanks is increased. However, residual
compressive stresses have a favourable effect on the useful
life of the meshing system.
This advantage is achieved by positive coupling of
the rotatlonal movement of the tool l and the workplece 2, as
well as when tool or workplece drlven and the other wheel ln
each instance is subjected to a retardlng moment.
Even though the technology descrlbed above applies
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essentlally to processes that do not lnvolve the removal of
metal, lt ls nevertheless advantageous that the process be
carried out with the appllcatlon of oil, emulslon, or another
coollng lubrlcant.
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Parts list for drawing:
1 machining tool
2 gear wheel
3 tooth flanks of the gear wheel to be machined
4 axis of rotatlon - machlnlng tool
axis of rotatlon - gear wheel
6 machlnlng surfaces of the machlning tool
7 dlrectlon of rotatlon - machlnlng tool
8 dlrectlon of rotatlon - gear wheel
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