Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~69~
The invention relates to a hobbing cutter for pro-
ducing large teeth up to approximately module 40. Its object
is generally to provide a hobbing cutter of highest economy.
A survey over the rotary gear cutting tools avail-
able at present leads to the following state of the art:
For the single-part method, in particular for
performing roughing cuts on large-module gear teeth, the
side milling cutter is used. Such side milling cutters
are also available with a hard-facing in the form of hard
metal reversible plates arranged in a staggered manner
which render re-grinding of the tool superfluous~ Since
in a side milling cutter all the teeth undergo the same
wear, the use of expensive hard metal reversible plates,
such as they are known per se in the technique of cutting,
is sensible.
Since the single-part method is time consuming
and is therefore mainly employed for cutting teeth on tooth
racks, the hobbing cutter is used throughout for cutting
teeth on wheels. The hobbing cutter exists as a solid
milling cutter of heavy duty high-speed steel, or as so-
called rail rnills the base body of which consists of normal
work-material, whereas the cutting edges consist of heavy
duty high-speed steel. Such rail mills permit repeated
re-equipment with hard-facing.
In the past, the further development of the
hobbing cutter stood obviously under the grinding problem,
in particular when the practice was introduced to provide
hobbing cutters with blades inserted in a staggered arran-
gement and intended in particular for very large divisions
and cutter dimensions.
-2-
13,.~6~
Because of the staggered arrangement of the blades such
milling cutters were not reground in front of the tooth
face, as in the case of backed-off milling cutters, but at
the outer surfaces of the teeth. For this purpose a spe-
cial machine was necessary (German Patent Specification
576 843, page 1, lines 14-19).
The re-grinding problem led to the creation of
the multiple-part hobbing cutter which consists of a plura-
lity of discs provided with plano-parallel end faces and
with helical tooth rims corresponding to the milling cutter
pitch (Germand Patent Specification 576 843).
The idea of the multiple-part hobbing cutter has
recently been revived again, likewise under the point of
view of improved facility for repair, hard-faced equipment
with securely clamped hard metal blades in a staggered
arrangements being proposed (German Offenlegungsschriften
1 752 707 and 1 918 200).
The present invention provides a gear hob compris-
ing a body comprising at least one disc-shaped body segment,
a helix of successive teeth extending around the body and
formed by the periphery of the body, each tooth having a
leading face, a top and two flanks, ahd having a seat cut
into the leading face and extending along one of the flanks
at the top, and first cutter plates replaceably mounted
at the top of respective tooth flanks for cutting both
with a top-cutting edge and a single side-cutting edge.
The first cutter plates comprise interchangeable hard
metal reversible plates having straight edges and being
mounted in respective seats in the tooth flanks, each plate
having a broad face and a narrow face, the narrow face
serving as leading face defining the top-cutting and side-
cutting edges.
--3--
i"3~(~
In a preferred embodiment of the invention, the
gear hob further comprises second hard metal reversible
cutter plates replaceably mounted at a radial distance
inwardly from the top of respective opposing tooth flanks
for cutting with a side-cutting edge only, the first and
second cutter plates alternating along the helix from left-
hand to right-hand tooth flanks and having overlapping
cutting paths as to their side-cutting edges.
According to another preferred embodiment, at
least one tooth serves to cut on the normal to the cutter
axis which extends through the center of the gear wheel,
the at least one tooth being provided in the region of the
apex thereof with a hard-metal reversible plate having
an arc-shaped cutting edge for rounding-off the tooth
base, the radius of the arc-shaped cutting edge being de-
termined by the desired rounding-off in the tooth base.
This novel construction of a hobbing cutter is
based on the following consideration:
As stated already, the expensive hard-faclng with
hard metal reversible plates can be justified in a side
milling cutter for the single-part method, because there
all the teeth are subjected to the same wear. Although
now the concept of the known multiple-part hobbing cutter
would permit without difficulty, as to the manufacture, the
transfer of the reversible plate principle, this has not
been considered seriously heretofore for two reasons.
Firstly, the production costs of such a hobbing cutter
would increase considerably because of the multitude of
its teeth. Furthermore it has been
~-4-
9~S
assumed in an analogous manner that all the reversible plates
of such a hobbing cutter must be reversed from time to time.
A feature of the invention resides in that during each rotation
of a hobbing cutter always only one tooth goes over "centre",
i.e. cuts on the perpendicular to the cutter axis extending
through the centre of the gear wheel and thus must provide
the highest cutting power. This tooth wears most, and thus
it is unnecessary in a hobbing cutter according to the invention,
to turn all the reversible plates from time to time, but only
that of a single tooth or of a few teeth the reversible plates
of which are simultaneously side cutters and head cutters. No
measuring pass is required, since the immersion depth of such
a head cutting reversible plate remains unchanged after it has
been turned. The tool need not be interchanged in order to
turn one or a few reversible plates, and accordingly fresh
adjustment of the tool is also unnecessary.
-4a-
~1~6~0
In spite of the high investment costs of a hobb-
ing cutter according to the invention, all these advantages
are the basis of its high economy not previously attained
in operation.
The top-cutting and side-cutting plates are in-
serted on edge, i.e. with a narrow limiting face as leading
face. Therefore, they are not subjected to any bending
loads during milling, for which reason working may be effec-
ted with large cutting speeds and feed speeds. Experiments
have shown that in comparison with commercially available
single-part hobbing cutters, the machining time required
for a large gear wheel can be reduced to one seventh and
less. Moreover it was found surprisingly that the teeth
receive finishing quality in a single work pass, that is
to say they are finished unless ground teeth are required
for heavy duty gears.
For large gear wheels, a hobbing cutter having at
least three compound helical gear rims is necessary. For
the treatment of gear wheels of smaller diameter, i.e.
small module, it may be sufficient to employ only one or
two discs with helical gear rims.
The invention deals also with the problem of the
rounding-off of the tooth base, the so-called protuberance,
by means of which the grinding dimension of the tooth flank
is determined. It is known that for the hobbing process
the engagement conditions of the respective kind of teeth
apply, e.g. involute teeth, because each tooth of a hobbing
cutter may be regarded as a tooth of a counter gear wheel as
to the engagement conditions. In the same way as upon en-
gagement of two gear wheels the engagement path extendsever only a portion of the base flank length - in addition
to the entire head flank length-
-5~
~L16~0
a hobbing cutter can produce the grip length of the -tooth
flanks only up to the point of the base flanks which results
from the point of intersection of the top cixcle of the hob and
the engagement line (involute teeth) or the rolling circle of
the teeth to be produced ~cycloidal teeth), The remaining
extent of the base flanks in the region of the tooth base con-
tinues necessarily by way of a rounding-off as far as the tooth
base, and it is just this rounding-off which must be enlarged
as rounding-off or protuberance, in order that the supporting
tooth flanks - and only these - can be correctly ground. The
production of the protuberance involves so to speak a "backing-
off milling" of the supporting tooth flanks, in order to render
free running-out possible for a tool - in the present case
the grinding disc - during a further treatment process.
Heretofore it was necessary to produce the protube-
rance by means of a special protuberance hobbing cutter in a
second operating step. The stock of tools had to comprise for
each tooth module not only a special protuberance hob which
are more expensive than the normal hobbing cutters, but also a
plurality thereof with different roundiny-off cutting edges,
in order to permit the extent of grinding to be irifluenced by
means of a more or less extensive rounding-off of the tooth
base dependently upon the distortion of the teeth of the gear
wheel which may have occurred during hardening.
It has been found that the separate protuberance
hobbing cutters and the second hobbing process for producing
the protuberance may be omitted in a hobbing cutter equipped
with hard metal reversible plates according to the invention
when at least the tooth which runs "over the centre", i.e. the
tooth which cuts on the normal to the cutter axis which extends
through the centre of
Cj -6-
the gear wheel, is provided in the region of the point of
the tooth with a hard metal reversible plate which possesses
a circular arc-shaped cutting edge for rounding-off the
tooth base, the radius of the cutting edge being adjusted
to the desired rounding-off (protuberance) in the tooth
base.
Apart from the considerable advantage as to costs
the invention imports the possibility to be able to select
from case to case the magnitude of the protuberance simply by
interchanging the rounding-off reversible plate or plates of
a different diameter. This necessity arises when - as stated
already ~ it is found after hardeniny of the toothed gear
wheel that one or more teeth have been particularly strongly
deformed. As a remedy it is then required that the grinding
extent of all tooth flanks is increased, in order that also
the tooth flanks of distorted teeth are still rendered
attainable by the grinding disc. This adjustability upon
demand of the protuberance was possible heretofore only by
the selection of a special protuberance hob, but only in
the gradation -~hich is predetermined by exi~ting protub~rance
milling cutters of one and the same tooth module.
The rounding-off reversible plate or plates may
consist of circular discs which are inserted in the tooth
face of the tooth or teeth of the hobbing cutter, which run
exactly or approximately "over the centre". Such circular
reversible plates have the advantage that they may be rotated
simply through 180 in order to bring a fresh circularly
shaped cutting edge into operation. However, the teeth
provided with rounding-off plates must be treated differently
in respect of the seats for the top and side cutters inserted
on edge.
In order to permit all teeth of the hobbing cutter
to be produced with unchanged characteristic of the success-
ive seats for reversible plates, the invention provides a
succession of at least two mirror-simmetrically equal hard
metal reversible plates inserted as right-hand and left-
hand cutting top and side cutters the straight side-cutting
edges of which extend under an obtuse tangential angle into
a respective circular-arc-shaped rounding-off edge which
overlap during the rotation of the hob. These mirror-symme-
trical reversible plates must be fixed in the seats in
the teeth, such as they are suitable for teeth without
rounding-off plates for the top and side cutters, so that
during the machine treatment of the teeth of the hobbing
cutter no account must be taken of which teeth are to be
fitted with rounding-off plates.
Three constructional examples of a hobbing cutter
according to the invention are illustrated in the drawing,
and there are shown in :
Fig. 1 a hobbing cutter consisting of three ring
discs, in a view from above, partly in
section on the line I-I in Fig. 2,
Fig. 2 one half of a side view from the lefthand
side,
Fig. 3 an exploded illustration of the hobbing
cutter illustrated in Fig. 1 in a view from
above, partly in section, with a circular
disc as rounding-off plate,
Fig. 4 a side view of the middle ring disc in
Fig. 3, approximately one half, from the
lefthand side,
~ -8-
1~L6~
Fig. 5 an illustration as Fig. 3 with mirror-
symmetrically equal roundi.ng-off plates,
Fig.5a a pair of mirror-symmetrical rounding-off
plates on an enlarged scale, and
Fig. 6 approximately one half of a side view from
the left
-8a-
of the middle ring disc in Fig. 5.
The novel multiple-part hobbing cutter consists in
all constructions examples of three discs 1, 2 and 3 provided
with helical gear rims, the discs being placed together by
means of their central bores 4 on a mill mandrel not illustrat-
ed, mounted thereon secured against rotation and mutually
clamped together. The individual ring discs lie against each
other by their plano-parallel side faces, only the outer side
face 5 of the disc 1 and the outer side face 6 of the disc 3
being provided with reference numerals (Fig. 1). The assembled
set of gear rims secured on the mill mandrel is limited by
ring discs 7 and 8, the rings and gear rims being provided
with a plurality of bores 9 through which dowel pins are pushed
in order to fix the helical gear rims la, 2a and 3a to form an
uninterrupted screwthread corresponding to the module-dependent
pitch of the hobbing cutter.
The principle of construction of the hobbing cutter
in respect of its equipment with hard metal reversible plates
is explained with reference to Figs. 1 and 2, as follows:
The gear rims la, 2a and 3a are equipped with hard
metal reversible plates in a staggered arrangement. In the
staggered arrangement, a distinction is made between the
reversible plates 10 mounted for cutting with a side-cutting
edge only and the plates 11 which are mounted for cutting both
with a top-cutting edge and a single side-cutting edge and are
fixed on their milled out seats in the gear rims by means of
screws not illustrated. They are inserted on edge, i.e. with
a narrow limiting face as leading face defining the top-cutting
and side-cutting edges. The hard metal reversible plates are
additionally secured against rotation by adjacent filler
members 12, 13 and 14.
_g_
In the succession of the teeth the reversible
plates are arranged in a righthand cutting and a lefthand
cutting manner. If, for example, the top and side cutter
11 illustrated in Fig. 2 must be considered at the direction
of rotation R of the hob as a lefthand cutting reversible
plate, the next following tooth supports a righthand cutting
top and side cutter 11'. However, this later-cutting tooth
supports a lefthand and side cutting reversible plate 10 at
a larger radial spacing from the periphery of the teeth than
that of the top and side cutter 11 of the preceding tooth.
These alternating radial spacings import the effect that the
side cutting reversible plate edges overlap during the
rotation of the hob. Since in Fig. 1 the top and side
cutting reversible plate 11 forms part of the last tooth of
the gear rim la of the disc 1 which is visible in the side
view according to Fig. 2, the subsequent tooth with the
lefthand cutting reversible plate illustrated in Fig. 2
is the first tooth of the gear rim 2a of the disc 2 in Fig.
1. ~hus in the side view according to Fig. 2 the preceding
portion of the gear rim la is omitted from the lower quarter.
In the case of large-module hobbing cutters more
than one lefthand cutting or righthand cutting side cutter
10 will have to be provided, in which case however the
alternating radial spacing of the reversible plates must
always be adjusted, in order that successive side cutting
reversible plate edges overlap each other during the rotation.
In the constructional examples reversible plates
10 and 11 of rectangular shape are inserted. Obviously
square reversible plates may alternatively be used the eight
cutting edges of which may be put into operation by turning
in contrast to four cutting edges in the case of rectangular
reversible plates.
--10--
9~0
In the constructional example according to Figs. 3
and 4, the gear rims la, 2a and 3a are equipped with hard metal
reversible plates 10 and 11 in a staggered arrangement, as
previously described.
One tooth of the gear rim 2a of the middle toothed
disc 2 is provided according to a preferred feature of the
invention with a circular hard metal reversible plate 16 which
is inserted in the face area of the tooth in such a manner
that it is capable of shaping the rounding-off in the tooth
base in a right-hand cutting as well as a left-hand cutting
manner. It is that tooth of the hobbing cutter which runs
"over the centre", this being adjusted prior to milling by an
axial adjustment of the hobbing cutter. Also one or more teeth
adjacent this tooth may be provided with a circular rounding-
off plate 16, in order to distribute the cutting work.
As may be seen from Fig. 3, the circular reversible
plate 16 displaces the top and side cutter 11 which is normally
provided for this tooth and which becomes a pure side cutter
10 on the tooth provided with the circular reversible plate 16.
Even for the tooth with the circular reversible plate 16 the
rule applies that the side cutting reversible plate edges
overlap each other during the rotation of the hob. Since the
circular reversible plate 16 has a right-hand cutting as well
as a left-hand cutting effect, the gaps between the circular
reversible plate 16 and the side cutters 10 of the same tooth
must be as small as possible in order to be overlapped by top
and side cutters 11 and 10 of the subsequent teeth.
Admittedly, the possibility to be able to render
operative a plurality of fresh cutting edges of a hard metal
reversible
Cl -11-
0
plate by turning and rotation, exists for the circular
reversible plate 16, but is not of overriding importance. It
is more important that the hard metal reversible plate 16
may be interchanged whenever desired with a reversible plate
having a different diameter, in order to be able to select
the size of the rounding-off or the protuberance in the tooth
base and thereby also the grinding dimension from case to
case in any desired manner.
The hobbing cutter according to Fig. 5 corresponds
to the constructional example according to Figs. 3 and 4 as
to its multiple-part construction. As described already,
the gear rims la, 2a and 3a are equipped with hard metal
reversible plates 10 inserted on edge, as side cutters, and
11 as top and side cutters in a staggered arrangement, with
the exception of two successive teeth 2a' and 2a" of the
middle toothed disc 2. As may be seen from Fig. 5, a
rounding-off plate 17 inserted on edge is substituted on
the first tooth 2a' for a top and side cutter 11, and accord-
ins to Fig. 5a it has a side cutting edge 17a which transfers
under an obtuse tangential angle ~ to a circular cutting
edge 17b by means of which the rounding-off in the tooth
base can be treated partially. The next following tooth
2" carries a rounding-off plate 18 which is mirror-symmetri
cal to the rounding-off plate 17, at the location at which a
top and side cutter would be inserted in the staggered
arrangement, which rounding-off plate 18 is likewise provided
with a side cutting edge 18a and a circular cutting edge
18b for rounding-off the tooth base. During the rotation of
the hob the two circular cutting edges 17b and 18b overlap,
so that the tooth base is rounded-off in the same manner
as with a circular reversible plate 16 according to Fig. 3.
-12-
111~9~
The advantage of this arrangement resides in the fact that
the seats for the top and side cutters 11 milled into the
toothed disc 2 may be used without difficulty as seats
for the rounding-off plates 17 and 18.
For the constructional example according to
Figs. 5 and 6, there applies also that the extent of the
desired rounding-off in the tooth base can be produced by
interchanging the rounding-off plates 17 and 18, in order to
adjust a more or less large grinding dimension. When the
rounding-off plates 17 and 18 are to be turned round in order
to bring fresh cutting edges into operation, they are simply
interchanged with each other, i.e. the righthand cutting
rounding-off plate becomes the lefthand cutting rounding-off
plate, and vice versa.
-13-
