Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02773343 2012-04-05
METHOD AND DEVICE FOR INCREASING THE BEARING AREA RATIO OF A FINE-
BLANKED PART HAVING A TOOTH, TOOTH SECTION OR THE LIKE
Background of the Invention
The invention relates to a method for increasing the bearing area ratio of a
flat fine-
blanked part having a tooth, a tooth section or the like, and in particular a
parking-gear pawl for
automatic transmissions, in which a blank is integrally cut from a flat strip
clamped between the
upper part and the lower part of a tool in a fine-blanking stage, is captured
by a cross slide,
which is moved horizontally into the opened tool, and is carried, correctly
positioned, into a
forming stage in a direction opposite to the feed direction of the flat strip,
the cross slide then
moving out of the tool into the home position thereof, whereupon the tool
closes.
The invention further relates to a device for carrying out the method using a
tool
having an upper part and a lower part, comprising a fine-blanking stage
provided for cutting a
blank having at least one tooth from a flat strip, and comprising a forming
stage having an
upsetting head, a plate-shaped holder, a clamping plate, an ejector and a
control pin for partially
upsetting the blank, wherein a cross slide carries the blank, correctly
positioned, out of the fine-
blanking stage into the forming stage.
In fine-blanking and metal forming technology, primarily steels are processed.
The
diversity of materials that are used ranges from simple constructional steels
to high-strength,
fine-grained steels. The aspect of "materials" has gained considerable
importance in recent
years. The production costs of a component can be significantly influenced by
utilizing materials
in an optimal manner. High-strength steels make it possible to obtain thinner-
walled
components having the same strength behavior.
In fine-blanking, the sheared edge acts as a functional surface in most cases,
and therefore
rollover is a cost factor.
Typical features of fine-blanked parts are the edge rollover and the burr. The
rollover
notably forms in corner sections, and increases as the corner radius decreases
and the sheet
thickness increases. Rollover depth can amount to approximately 30% of the
sheet thickness
and rollover width can amount to approximately 40% or more of the sheet
thickness (see DIN
3345, Fine-blanking, Aug. 1980). Rollover is therefore dependent on the
thickness and quality of
the material, and so control thereof is limited and is often associated with
restrictions on part
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function, such as in terms of the corners not having sharp edges in the case
of fine-blanked
parts having tooth geometries, such as parking-gear pawls, or terms of changes
in the
functional length of the parts.
Thus, punched rollover reduces part function and makes it necessary for the
manufacturer to
use a thicker starting material.
An entire series of solutions is known, which attempt to either eliminate the
edge
rollover by shaving (CH 665 367 A5) or scraping (DE 197 38 636 Al), or to
compensate therefor
by shifting material during cutting (EP 1 815 922 Al), negative deformation
(EP 2 036 631 B1)
or thickening the functional surfaces (DE 102009001305A1).
The known solutions according to CH 665 367 A5 and DE 197 38 636 Al do not
reduce the
edge rollover, but rather re-machine the parts in a complex manner, thereby
requiring
considerable costs for additional machining processes and tools, and result in
a related material
loss due to the need to use thicker material.
In the known solution according to EP 1 815 922 Al, the workpiece is machined
in a single-
stage system, in at least two chronologically successive step sequences in
different cutting
directions, wherein, in a first cutting process, a semi-finished product
matched to the workpiece
geometry, which has a slight rollover, is cut in the vertical working
direction and, in at least one
further cutting process, the final cutting of the part is carried out in the
opposite working
direction. The rollover from the first partial step should be refilled, at
least in the corner section.
This known method, however, primarily avoids the protruding burr; the rollover
is ultimately not
eliminated, but rather a material volume is shifted along the cutting line,
which increases the risk
of crack formation.
The known method according to EP 2 036 631 B1 attempts to compensate for the
edge rollover
in that, before the cutting starts, deformation that is negative with respect
to the cutting direction
is carried out at the clamped, untreated flat strip, using a preforming
element opposite to the
cutting direction that corresponds to the expected edge rollover into the
cutting die, in terms of
size and geometry at cutting, including an allowance, and generates a material
volume at the
side of the rollover in a mirror-inverted form.
In the known method according to DE 10 2009 001 305 Al as well, before
cutting, the edge
region of a sheet metal part is subjected to a permanent deformation, that is,
upsetting.
Solid-blank forming carried out before fine-blanking has the fundamental
disadvantage,
however, that the dimensional stability of the finished parts can be
unfavorably influenced. This
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means that the forming devices must adhere to very narrow tolerances to ensure
plane
parallelism in the upset regions, which is indispensible for fine-blanking.
Summary of the Invention
In this state of the art, the object of the invention is to markedly increase
the bearing
area ratio of the functional surfaces at the tooth geometry of a fine-blanked
part, and
simultaneously reduce the sheet thickness, while saving material and ensuring
economic
advantages, and to further improve the dimensional stability of the parts.
The basic idea of the solution according to the invention is that of avoiding
solid-
blank forming, which influences the tolerances of the material strip, before
fine-blanking, and
performing solid-blank forming at the functional surfaces of the tooth
geometry of the finished
cut part, with the requirement of carrying out a desired partial thickening.
This is achieved by the following steps, which take place in the forming
stage:
a) clamping the blank, at the flat sides thereof, between a clamping plate and
an ejector in such
a way that the tooth of the blank is not clamped and remains accessible for a
forming operation,
b) upsetting the tooth by application of a compressive force suited to the
material of the blank
and the tooth shape in such a way that the force is directed, by an upsetting
head, horizontally
to the flat sides into the core of the tooth, whereby the tooth is
symmetrically thickened by up to
30% relative to the starting thickness of the blank.
It is advantageous for the compressive force to be introduced perpendicularly
into
the tip surface of the tooth, thereby permitting the material to flow
uniformly and symmetrically in
the tooth region.
In a preferred embodiment of the method according to the invention, the
upsetting
head is driven mechanically or hydraulically by way of a wedge drive. This is
associated with the
advantage of achieving a flat and compact design for the forming stage.
It has proven advantageous to adjust the compression at the partially
thickened
surfaces of the tooth geometry to values between 1.25 and 1.625, and to match
the upsetting
head to the tooth shape. This ensures that the thickening takes place
substantially
symmetrically at the tooth, thereby markedly increasing the thickness of the
tooth with respect to
the thickness of the remaining blank and reducing the moment of inertia.
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It is particularly advantageous that the process parameters for upsetting,
such as the
geometry and the material volume of the upsetting region, can be determined
according to the
material type, and the shape and geometry of the workpiece, by way of a
virtual forming
simulation. This results in a faster, realistic setting of the compressive
force.
The process parameters for upsetting can also be iteratively determined by
measuring real fine-
blanked parts, without the scope of the invention being limited as a result.
According to a further preferred embodiment of the invention, and in
particular of the
device according to the invention, the holder has a receiving opening matched
to the shape and
size of the blank, which encloses the blank except for the tooth, whereby the
blank is held in the
receiving opening in a clamped manner at the flat sides thereof, between a
clamping plate and
an ejector, but the tooth is not clamped, and the upsetting head is aligned
with the tooth in the
forming stage and is disposed so that the upsetting direction thereof is
oriented horizontal to the
flat sides, and perpendicular to the tooth tip surface of the non-clamped
tooth.
In a further advantageous embodiment of the device according to the invention,
the
upsetting head comprises a wedge drive, having wedge elements with two
opposing wedge
surfaces, the first wedge element being disposed so that it is displaceable
perpendicular to the
feed direction of the flat strip in the forming stage below the die plane
(ME), and the second
wedge element being disposed so that it is displaceable perpendicular and
horizontal thereto,
wherein the wedge element carries the upsetting head, which brings about the
perpendicular
motion of the upsetting head onto the tooth tip surface by way of the
displacement of the wedge
element along the wedge surfaces.
In a further embodiment of the device according to the invention, the forming
stage
comprises a mechanical and/or hydraulic drive system, which is independent of
the fine-
blanking stage.
The solution according to the invention is characterized in that it is
possible to
thicken fine-blanked parts at the functional surfaces thereof, partially, and
in a well-defined
manner, without the precision of the fine-blanked parts being influenced by
the forming process.
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Further advantages and details will become apparent from the description that
follows, with reference to the accompanying drawings.
The invention is described in greater detail in the following with reference
to an
exemplary embodiment.
Brief Description of the Drawings
Shown are:
FIG. 1 a perspective view of the blank for a parking-gear pawl according to
the prior
art;
FIG. 2 a schematic view of the rollover at a fine-blanked tooth;
FIG. 3 a perspective view of a parking-gear pawl manufactured using a method
according to the invention;
FIG. 4 a schematic view of the device according to the invention;
FIG. 5 a perspective view of the blank placed into the plate-shaped holder,
having a
tooth that is freely accessible for forming, and
FIG. 6 a schematic view of the upsetting head comprising a drive.
Detailed Description of the Invention
FIG. 1 shows a fine-blanked parking-gear pawl 1 according to the prior art,
for an
automatic transmission. The parking-gear pawl 1 has a tooth geometry in the
form of a tooth 2.
This tooth 2 is designed such that it fits into a tooth space of a parking
gear, which is not shown,
when the parking-gear pawl 1 is engaged with the parking gear.
The parking-gear pawl 1 is made of case hardening steel, of the grade 16MnCr5,
and has a
thickness s of 10 mm and a moment of inertia with respect to the centroid of
29958 kgmm2. As
shown in FIG. 1, the parking-gear pawl 1, as a component, has a uniform
thickness in the width
and length directions, and therefore the tooth 2 also has this thickness.
CA 02773343 2012-04-05
Due to the rollover produced in fine-blanking, the bearing area ratio at the
functional
surfaces F of the tooth 2 is reduced. In other words, the available thickness
is reduced by the
amount of the rollover, which results from the height h and the width b of the
rollover. These
relationships are depicted in FIG. 2, in which the bearing area ratio TA of
the functional surface
F is shaded, and the lost surface is labeled VF.
To ensure that the required bearing area ratio is available at the functional
surfaces for the
transmission of moments, the starting thickness of the strip material must be
increased
accordingly. This leads to a higher weight and all the associated
disadvantages for the technical
parameters of the transmission, as well as economic disadvantages, such as
higher costs.
In the present example, a parking-gear pawl, as shown in FIG. 3, shall be
produced
using the method according to the invention and the device according to the
invention.
The reference characters used above are retained in the following description.
The parking-gear pawl 1 should have a thickness s of 8 mm and reach a
thickness SZD of 10 mm
in the region of the tooth. The moment of inertia should be 23187 kgmm2.
FIG. 4 schematically illustrates an example of a device according to the
invention,
with which the parking-gear pawls 1 are produced. The device according to the
invention
comprises a fine-blanking stage 3 and a forming stage 4, which are coupled to
one another by a
cross slide 5, which, in the opened state of the device, moves between the
upper part and the
lower part, captures the blanks 7 that were integrally cut from the flat strip
6 in the fine-blanking
stage 3, and moves to the forming stage 4. The cross slide 5 is shown in the
non-engagement
position in FIG. 4.
The fine-blanking stage 3 primarily comprises a guide plate 8, a die 9, a
shearing
punch 10, an ejector 11, an inner form punch 12, and an ejector 13, which are
disposed in the
upper part 0 or the lower part U of the device according to the invention, in
accordance with the
function they perform.
The guide plate 8, the shearing punch 10, the ejector 11, the inner form punch
12 and the
ejector 13 are connected by way of a piston/cylinder unit, which is not shown
in greater detail, to
a hydraulic system 14, which generates the appropriate pressure forces for the
fine-blanking
operations and applies them to the operative elements. The fine-blanking stage
3 otherwise
corresponds to the prior art, and therefore a more detailed depiction can be
omitted.
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The forming stage 4 primarily comprises a holder 15 in the form of a thin
plate, a
pressure plate 16 in the form of an anvil, an ejector 17, an upsetting head 18
and a control pin
24. The pressure plate 16 and the ejector 17 are driven directly, and the
upsetting head 18 is
driven indirectly, by way of a mechanical wedge drive 21, by an auxiliary
hydraulic system 19,
which is independent of the hydraulic system 14. The control pin 24 moves
against the wedge
element 23, which moves the upsetting head 18 with the compressive force PSK
against the
tooth tip surface ZKF. The holder 15 is aligned with the die plane ME of the
fine-blanking stage
3, thereby enabling the cross slide 5 to move the blank 7 horizontally between
the fine-blanking
stage 3 and the forming stage 4. The holder 15 has a receiving opening 20,
which is matched,
in terms of size and shape, to the contour of the blank 7 for the parking-gear
pawl 1 (see FIG.
5). The receiving opening 20 is designed so that it can receive the entire
blank 7 except for the
tooth 2. The tooth 2 is therefore not enclosed by the holder 15 and is freely
accessible from the
outside for relevant forming operations.
The holder 15 has a thickness T, which is slightly greater than the thickness
s of the blank 7.
The blank 7, which is captured by the cross slide 5, is deposited into the
receiving opening 20 of
the holder 15, and therefore one flat side FSB of the blank 7 lies flat on the
ejector 17, and the
other flat side FSD faces the pressure or clamping plate 16.
When the tool is closed, the clamping plate 16 moves against the flat side FSD
of the blank, and
the ejector 17 moves against the flat side FSB of the blank 7. The blank 7 is
therefore held in
the receiving opening 20 of the holder 15, clamped between the pressure plate
16 and the
ejector 17. The clamping force applied by the auxiliary hydraulic system 19 is
lower than the
compressive force PSK applied by the upsetting head 18.
The upsetting head 18, with the wedge drive 21 thereof, is depicted
schematically in
FIG. 6. The wedge drive 21 comprises two wedge elements 22 and 23, wherein
each of the
wedge elements 22 and 23 has a wedge surface KI and K2, respectively. Wedge
surfaces K1
and K2, which are displaceable towared one another, are provided at wedge
elements 22 and
23. The wedge element 22 is disposed below the die plane ME so that it can be
displaced
vertically, and the wedge element 23 is disposed so that it can be displaced
vertically and
horizontally with respect to the strip feed direction VS of the flat strip 6.
The displacement of the
wedge element 23 is brought about by the control pin 24 moving against the
wedge element 23,
and therefore the wedge surface K2 moves along the wedge surface K1, and the
upsetting head
18, which is carried by the wedge element 23, moves in the direction of the
tooth 2, which is
exposed for forming.
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The upsetting head 18 and the compression direction thereof are oriented
parallel to the flat
sides FSB and FSD of the clamped blank 7, perpendicular relative to the tooth
tip surface ZKF
of the exposed tooth 2, and therefore the compressive force PSK is directed
into the core of the
tooth 2 in a uniform manner and causes the material in the tooth 2 to flow. A
symmetrical
thickening of the entire tooth geometry results, which is to say at the flat
sides of the tooth 2 and
at the functional surfaces, which allows the bearing area ratio at the
functional surfaces of the
tooth 2 to be increased in a well-defined manner.
The method according to the invention is carried out as follows. The blank 7,
which is
blanked in the fine-blanking stage 3, is clamped except for the tooth 2
thereof in the forming
stage 3, at the flat sides thereof, between the pressure plate 16, which is in
the form of an anvil,
and the ejector 17, and is held in the clamped state in the receiving opening
20 of the holder 15.
The tooth 2 is not clamped and remains freely accessible to a forming
operation, in a plane
disposed below the die plane ME.
The clamping force PK is established such that the blank 7 without the tooth 2
retains the shape
and form obtained in the fine-blanking operation, and deformation is prevented
in the clamped
region of the blank 7.
In the next method step, the freely accessible tooth 2 is upset by application
of a force adjusted
in accordance with the material and the tooth geometry in such a way that the
compressive
force PSK is directed by the upsetting head 18 into the core of the tooth 2,
perpendicular to the
tooth tip surface ZKF, and parallel to the flat sides of the blank 7. To this
end, the upsetting
head 18 is moved by way of the wedge drive 21 thereof in the manner described
heretofore
against the tooth 2 and creates a partial thickened region of the tooth 2.
The method according to the invention makes it possible to easily obtain
partial thickened
regions, of up to 30% relative to the starting dimension, whereby the weight
reductions reach
considerable levels. The partial compression achieved in the tooth region are
in the range of
1.25 to 1.625.
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