U.S. patent application number 14/006212 was filed with the patent office on 2014-01-02 for method of cold forming a piece of sheet metal by bending or press moulding.
The applicant listed for this patent is Peter Alm. Invention is credited to Peter Alm.
Application Number | 20140000336 14/006212 |
Document ID | / |
Family ID | 46879615 |
Filed Date | 2014-01-02 |
United States Patent
Application |
20140000336 |
Kind Code |
A1 |
Alm; Peter |
January 2, 2014 |
METHOD OF COLD FORMING A PIECE OF SHEET METAL BY BENDING OR PRESS
MOULDING
Abstract
A method of cold forming a piece of sheet metal (1) by bending
or press moulding, in which method:--the piece of sheet metal is
bent over a bevelled edge (12, 13) of a tool (10) so that the piece
of sheet metal in the bending area (B1, B2) is subjected to
compressive stress, on the inner side and tensile stress on the
outer side of the neutral layer. A gap is left between the sheet
metal and an edge surface (14, 15) of the bevelled
edge.--compressive force is then exerted on the piece of sheet
metal outside the bending area, which is compressed transversally
to the longitudinal direction of the bend, whereupon compressive
force is exerted on the outer side of the bending area so that the
bending area is subjected to tensile stress on the inner side and
compressive stress on the outer side of the neutral layer.
Inventors: |
Alm; Peter; (Vansbro,
SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Alm; Peter |
Vansbro |
|
SE |
|
|
Family ID: |
46879615 |
Appl. No.: |
14/006212 |
Filed: |
March 20, 2012 |
PCT Filed: |
March 20, 2012 |
PCT NO: |
PCT/SE2012/050302 |
371 Date: |
September 19, 2013 |
Current U.S.
Class: |
72/379.2 |
Current CPC
Class: |
B21D 5/01 20130101; B21D
22/21 20130101; B21D 37/08 20130101 |
Class at
Publication: |
72/379.2 |
International
Class: |
B21D 5/01 20060101
B21D005/01 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 21, 2011 |
SE |
1150251-5 |
Claims
1. A method of cold forming a piece of sheet metal (1) by bending
or press moulding, in which method the piece of sheet metal (1) is
bent over a bevelled edge (12, 13) of a tool (10) so that the piece
of sheet metal in the area (B1, B2) of the bend thus formed, being
denominated bending area, is subjected to compressive stress on the
side of the neutral layer (8) facing the inner side (3) of the
bending area and tensile stress on the side of the neutral layer
(8) facing the outer side (4) of the bending area, a gap being left
between the piece of sheet metal (1) and an edge surface (14,15) of
the bevelled edge (12, 13) on the inner side (3) of the bending
area; and compressive force is thereafter exerted on the piece of
sheet metal (1) outside said bending area (B1, B2) in such a manner
that the bending area is subjected to compression in the extension
direction of the piece of sheet metal transversally to the
longitudinal direction of the bend, whereupon compressive force is
exerted against the outer side (4) of the bending area so that the
bending area (B1, B2) is pressed inwards towards said edge surface
(14, 15) so that the bending area is subjected to tensile stress on
the side of the neutral layer (8) facing the inner side (3) of the
bending area and compressive stress on the side of the neutral
layer (8) facing the outer side (4) of the bending area.
2. A method according to claim 1, wherein the method comprises the
following consecutive steps: A) the piece of sheet metal (1) is
placed with a first part (1A) of the piece of sheet metal received
between a first tool (10) and a second tool (20), and with second
and third parts (1B, 1C) of the piece of sheet metal extending out
over bevelled edges (12, 13) of the first tool (10) on either side
of the first part (1A) of the piece of sheet metal; B) the first
tool (10) and the third tool (30) are thereafter subjected to a
mutual displacement in the direction towards each other so that the
second and third parts (1B, 1C) of the piece of sheet metal are
bent over the bevelled edges (12, 13) of the first tool (10) under
the effect of the third tool (30) and thereby are bent in relation
to the first part (1A) of the piece of sheet metal to form a first
bending area (B1) on the piece of sheet metal at the first bevelled
edge (12) and a second bending area (B2) on the piece of sheet
metal at the second bevelled edge (13), while the first and second
tools (10, 20) are kept at a distance from each other so that the
first part (1A) of the piece of sheet metal is bulged in connection
with this bending of the second and third parts (1B, 1C) of the
piece of sheet metal, a gap being left in the respective bending
area (B1, B2) between the piece of sheet metal (1) and an edge
surface (14, 15) of the bevelled edge (12, 13) on the inner side
(3) of the bending area and between the piece of sheet metal (1)
and the third tool (30) on the outer side (4) of the bending area;
C) the first and second tool (10, 20) are thereafter subjected to a
mutual displacement in the direction towards each other so that the
first part (1A) of the piece of sheet metal is flattened between
these tools while the third tool (30) retains the second and third
parts (1B, 1C) of the piece of sheet metal in the bent state,
whereby the bulge formed in step B in the first part (1A) of the
piece of sheet metal is flattened so that the piece of sheet metal
in the respective bending area (B1, B2) is subjected to compression
in the extension direction of the piece of sheet metal
transversally to the longitudinal direction of the bend; D) the
first and third tools (10, 30) are thereafter subjected to a
continued mutual displacement towards each other, while the first
part (1A) of the piece of sheet metal is retained in the flattened
state between the first and second tools (10, 20), so that the
respective bending area (B1, B2) of the piece of sheet metal is
pressed inwards by the third tool (30) towards the adjacent edge
surface (14, 15) of the first tool so that the respective bending
area (B1, B2) is subjected to tensile stress on the side of the
neutral layer (8) facing the inner side (3) of the bending area and
compressive stress on the side of the neutral layer (8) facing the
outer side (4) of the bending area; and E) the thus formed piece of
sheet metal (1) is thereafter released from said tools (10, 20,
30).
3. A method according to claim 2, wherein the first tool (10) is
kept stationary in steps B and D, while the third tool (30) is
displaced towards the first tool.
4. A method according to claim 2, wherein the first tool (10) is
kept stationary in step C, while the second tool (20) is displaced
towards the first tool.
5. A method to claim 1, wherein the piece of sheet metal (1) by
bending is bent along two rectilinear and mutually parallel bending
lines.
6. A method according to claim 5, wherein the piece of sheet metal
(1) is bent into U-shape.
7. A method according to claim 1, wherein the piece of sheet metal
(1) by press moulding is bent along one or more curved bending
lines.
8. A method according to claim 3, wherein the first tool (10) is
kept stationary in step C, while the second tool (20) is displaced
towards the first tool.
9. A method according to claim 8, wherein the piece of sheet metal
(1) by bending is bent along two rectilinear and mutually parallel
bending lines.
10. A method according to claim 2, wherein the piece of sheet metal
(1) by bending is bent along two rectilinear and mutually parallel
bending lines.
11. A method according to claim 3, wherein the piece of sheet metal
(1) by bending is bent along two rectilinear and mutually parallel
bending lines.
12. A method according to claim 4, wherein the piece of sheet metal
(1) by bending is bent along two rectilinear and mutually parallel
bending lines.
13. A method according to claim 12, wherein the piece of sheet
metal (1) is bent into U-shape.
14. A method according to claim 11, wherein the piece of sheet
metal (1) is bent into U-shape.
15. A method according to claim 10, wherein the piece of sheet
metal (1) is bent into U-shape.
16. A method according to claim 9, wherein the piece of sheet metal
(1) is bent into U-shape.
17. A method according to claim 2, wherein the piece of sheet metal
(1) by press moulding is bent along one or more curved bending
lines.
18. A method according to claim 3, wherein the piece of sheet metal
(1) by press moulding is bent along one or more curved bending
lines.
19. A method according to claim 4, wherein the piece of sheet metal
(1) by press moulding is bent along one or more curved bending
lines.
Description
FIELD OF THE INVENTION AND PRIOR ART
[0001] The present invention relates to a method of cold forming a
piece of sheet metal by bending or press moulding.
[0002] In order to achieve a permanent deformation of a piece of
sheet metal by bending or press moulding, the piece of sheet metal
has to be subjected to such a force that the yield point of the
material is reached in the parts of the piece of sheet metal that
are bent in connection with the bending/press moulding operation.
When a piece of sheet metal is bent, the piece of sheet metal will
be subjected to compressive stress at the inner side of the bend
and tensile stress at the outer side of the bend. Between the area
with compressive stress and the area with tensile stress inside the
piece of sheet metal, there is a layer, the so-called neutral
layer, which constitutes a boundary between these areas. There is
compressive stress in the sheet metal material on one side of the
neutral layer and tensile stress in the sheet metal material on the
opposite side of the neutral layer. When a piece of sheet metal is
subjected to pure bending with such a force that the yield point of
the sheet metal material is reached in the bending area, a first
plastic zone in which the sheet metal material has compressive
stress that has reached the yield point will be developed at the
inner side of the bend, whereas a second plastic zone in which the
sheet metal material has tensile stress that has reached the yield
point will be developed at the outer side of the bend. The sheet
metal material is plasticized in these plastic zones. The neutral
layer and an elastic zone in which the sheet metal material is not
plasticized are located between these plastic zones. The sheet
metal material in a first part of the elastic zone, between the
neutral layer and the first plastic zone, has compressive stress
lower than the yield point, whereas the sheet metal material in a
second part of the elastic zone, between the neutral layer and the
second plastic zone, has tensile stress lower than the yield point.
When the bending force on the piece of sheet metal is released, the
compressive stress in said first part of the elastic zone will be
released and strive towards a dilation of the piece of sheet metal
transversally to the longitudinal direction of the bend and thereby
act for a straightening of the bend, i.e. an increase of the bend
radius. In the corresponding manner, the tensile stress in said
second part of the elastic zone will be released and strive towards
a contraction of the piece of sheet metal transversally to the
longitudinal direction of the bend and thereby act for at
straightening of the bend. Furthermore, when the bending force on
the piece of sheet metal is released, the material at the neutral
layer, which essentially has no tensile stress or compressive
stress, strives to spring back to the original shape that this
material had before being subjected to the bending force. Under the
effect of the spring-back tendency of the last-mentioned material
and the released stresses in the elastic zone, there will
consequently be a certain straightening of the bend and a return of
the piece of sheet metal in the direction towards its shape before
the bending operation, i.e. the piece of sheet metal will spring
back.
[0003] A high-strength metal material has a higher yield point than
a softer metal material, which implies that the above-described
problem with spring-back is more severe when it comes to bending or
press moulding of a high-strength metal material as compared to a
corresponding bending or press moulding of a softer metal
material.
[0004] The desired final shape of a piece of sheet metal after
bending or press moulding may for instance be achieved by bending
the piece of sheet metal during the bending/press moulding
operation so far that the piece of sheet metal will spring back to
the desired final shape after being released from the forming
tool.
[0005] However, it is very difficult to determine how a piece of
sheet metal is to be bent in order to assume a desired final shape
after spring-back, and extensive and time-consuming trials are
therefore normally required before the correct forming geometry is
achieved. Furthermore, even very small property variations between
apparently identical pieces of sheet metal may cause a different
spring-back and consequently result in varying final shapes of the
pieces of sheet metal after bending/press moulding thereof.
Furthermore, it is not possible to bend a piece of sheet metal by a
bending angle larger than 90 degrees when using a conventional
forming machine with forming tools that are linearly movable in
relation to each other in mutually parallel directions, which
entails that the largest bending angle after spring-back will be
smaller than 90 degrees. In order to achieve a final bending angle
of 90 degrees after spring-back in a piece of sheet metal by means
of such a forming machine, the piece of sheet metal has to be
formed in several steps, which may be complicated and
time-consuming.
[0006] Another alternative is to counteract said spring-back by
more or less completely plasticizing the material in the bending
area of the piece of sheet metal and consequently eliminate the
above-mentioned elastic zone. This may for instance be achieved by
exerting such a powerful compressive action against the surface of
the piece of sheet metal in the bending area that the yield point
is reached through the entire piece of sheet metal. However, in
order to achieve such an extensive plasticizing when forming pieces
of sheet metal of high-strength metal materials with a high yield
point, very high compressive forces are required, which may be
difficult and sometimes impossible to achieve with a conventional
forming machine.
OBJECT OF THE INVENTION
[0007] The object of the present invention is to achieve a new and
favourable solution to the above-described problem with spring-back
in connection with the cold forming of a piece of sheet metal.
SUMMARY OF THE INVENTION
[0008] According to the present invention, said object is achieved
by means of a method having the features defined in claim 1.
[0009] According to the invention, the piece of sheet metal is bent
over a bevelled edge of a tool so that the piece of sheet metal in
the area of the bend thus formed, which area here being
denominating bending area, is subjected to compressive stress on
the side of the neutral layer facing the inner side of the bending
area and tensile stress on the side of the neutral layer facing the
outer side of the bending area, a gap being left between the piece
of sheet metal and an edge surface of the bevelled edge on the
inner side of the bending area. A compressive force is thereafter
exerted on the piece of sheet metal outside said bending area in
such a manner that the bending area is subjected to compression in
the extension direction of the piece of sheet metal transversally
to the longitudinal direction of the bend. After this compression,
compressive force is exerted against the outer side of the bending
area so that the bending area is pressed inwards towards said edge
surface so that the bending area is subjected to tensile stress on
the side of the neutral layer facing the inner side of the bending
area and compressive stress on the side of the neutral layer facing
the outer side of the bending area.
[0010] According to the invention, the bending area is consequently
influenced in such a manner that the zones with compressive stress
and tensile stress that are initially developed in the bending area
in connection with the bending of the piece of sheet metal are
shifted so that tensile stress instead of compressive stress is
developed at the inner side of the bend and compressive stress
instead of tensile stress is developed at the outer side of the
bend. When the bending force on the piece of sheet metal is
released, the compressive stress in the elastic zone on the side of
the neutral layer facing the outer side of the bend will be
released and strive towards a dilation of the piece of sheet metal
transversally to the longitudinal direction of the bend and thereby
act for a decrease of the bend radius. In the corresponding manner,
the tensile stress in the elastic zone on the side of the neutral
layer facing the inner side of the bend will be released and strive
towards a contraction of the piece of sheet metal transversally to
the longitudinal direction of the bend and thereby act for a
decrease of the bend radius. In this case, the released stresses in
the elastic zone will consequently counteract the previously
mentioned spring-back tendency of the material at the neutral
layer, which implies that the spring-back of the piece of sheet
metal can be reduced and even completely eliminated in dependence
on the relationship between the bend radius decreasing effect of
the released stresses in the elastic zone and the bend radius
increasing spring-back effect of the material at the neutral
layer.
[0011] Other favourable features of the method according to the
invention will appear from the dependent claims and the description
following below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The invention will in the following be more closely
described by means of embodiment examples, with reference to the
appended drawings. It is shown in:
[0013] FIGS. 1a-1d schematic illustrations of different steps in a
method according to an embodiment of the present invention of cold
forming a piece of sheet metal, and
[0014] FIGS. 2b-2d schematic diagrams over the distribution of the
stresses, at the different steps according to FIGS. 1b-1d, in a cut
through one of the bending areas of the piece of sheet metal.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0015] Different steps in a method according to an embodiment of
the present invention of cold forming a piece of sheet metal 1 of
metallic material are schematically illustrated in FIGS. 1a-1d. In
the illustrated example, the piece of sheet metal 1 is bent into
U-shape by bending along two rectilinear and mutually parallel
bending lines. However, the method according to the invention can
also be used for bending a piece of sheet metal along one or more
curved bending lines by press moulding.
[0016] The piece of sheet metal 1 may for instance be of
high-strength metallic material, such as steel or aluminium.
[0017] The piece of sheet metal 1 is cold formed by means of a
forming machine 2. In the illustrated example, the forming machine
2 comprises a first tool 10, over which the piece of sheet metal 1
is intended to be bent, and a second tool 20 which is arranged
opposite the first tool 10. The first tool 10 has a support surface
11 facing a corresponding support surface 21 on the second tool 20.
The first and second tools 10, 20 are mutually movable in the
direction towards each other so as to allow a piece of sheet metal
1 received in the space between the tools to be clamped between the
support surface 11 on the first tool and the support surface 21 on
the second tool. The first tool 10 is provided with a first
bevelled edge 12 in order to form a first bending line in the piece
of sheet metal to be formed, and a second bevelled edge 13 in order
to form a second bending line in the piece of sheet metal to be
formed. These bevelled edges 12, 13 extend in parallel with each
other on either side of the support surface 11. The respective
bevelled edge 12, 13 has an incline edge surface 14, 15 extending
between the support surface 11 and an adjacent lateral wall 16, 17
of the first tool 10. The transition between the respective lateral
wall 16, 17 and the adjacent edge surface 14, 15 is rounded and the
transition between the respective edge surface 14, 15 and the
support surface 11 is also rounded.
[0018] The forming machine 2 also comprises a third tool 30, which
comprises a first part 30a located on a first side of the second
tool 20 and a second part 30b located on the other side of the
second tool 20 opposite the first part 30a. These first and second
parts 30a, 30b are consequently located on either side of the
second tool 20. The third tool 30 is linearly displaceable in
relation to the first tool 10 and in relation to the second tool
20.
[0019] The piece of sheet metal 1 to be formed is placed with a
central first part 1A of the piece of sheet metal bearing against
the support surface 11 on the first tool 10 and with lateral second
and third parts 1B, 1C of the piece of sheet metal extending out
over the bevelled edges 12, 13 of the first tool 10 on either side
of the first part 1A of the piece of sheet metal, as illustrated in
FIG. 1a. Consequently, said first part 1A of the piece of sheet
metal is at a first side connected to a second part 1B of the piece
of sheet metal that extends out over the first bevelled edge 12 of
the first tool and is at an opposite second side connected to a
third part 1C of the piece of sheet metal that extends out over the
second bevelled edge 13 of the first tool. The first part 1A of the
piece of sheet metal is received between the support surface 11 on
the first tool and the support surface 21 on the second tool.
[0020] In the illustrated example, said support surfaces 11, 21 on
the first and second tools are straight.
[0021] When the piece of sheet metal 1 has been placed between the
first tool 10 and the second tool 20 in the above-mentioned manner,
the first tool 10 and the third tool 30 are subjected to a mutual
displacement in the direction towards each other so that the second
part 1B of the piece of sheet metal is bent over the first bevelled
edge 12 of the first tool 10 under the effect of the first part 30a
of the third tool 30 and thereby bent in relation to the first part
1A of the piece of sheet metal to form a first bending area B1 on
the piece of sheet metal, and so that the third part 1C of the
piece of sheet metal is bent over the second bevelled edge 13 of
the first tool 10 under the effect of the second part 30b of the
third tool 30 and thereby bent in relation to the first part 1A of
the piece of sheet metal to form a second bending area B2 on the
piece of sheet metal, as illustrated in FIG. 1b. During this mutual
displacement between the first and third tools 10, 30, the first
and second tools 10, 20 are kept at a distance from each other so
that the first part 1A of the piece of sheet metal, in connection
with the bending of the second and third parts 1B, 1C of the piece
of sheet metal, is bulged outwards from the support surface 11 on
the first tool into contact with the support surface 21 on the
second tool. At this stage of the forming process, the piece of
sheet metal 1 consequently extends in a bow between the two
bevelled edges 12, 13 of the first tool, a gap provided between the
first part 1A of the piece of sheet metal and the support surface
11 on the first tool. In connection with this initial bending of
the piece of sheet metal, compressive stress is developed in the
respective bending area B1, B2 on the side of the neutral layer 8
facing the inner side 3 of the bending area and tensile stress is
developed in the respective bending area B1, B2 on the side of the
neutral layer 8 facing the outer side 4 of the bending area, as
illustrated in FIG. 2b.
[0022] Tensile stress is indicated with a dash-patterned area 5 and
tensile stress is indicated with a dot-patterned area 6 in FIGS.
2b-2d. Furthermore, the centre plane 7 of the piece of sheet metal
is indicated with a broken line in FIGS. 2b-2d.
[0023] In connection with said initial bending of the piece of
sheet metal, a gap is left between the piece of sheet metal 1 and
the edge surface 14 of the first bevelled edge 12 on the inner side
3 of the first bending area B1 and a gap is left between the piece
of sheet metal and the first part 30a of the third tool on the
outer side 4 of the first bending area B1. In the corresponding
manner, a gap is left between the piece of sheet metal 1 and the
edge surface 15 of the second bevelled edge 13 on the inner side 3
of the second bending area B2 and a gap is left between the piece
of sheet metal and the second part 30b of the third tool on the
outer side 4 of the second bending area B2.
[0024] The first and second tools 10, 20 are then subjected to a
mutual displacement in the direction towards each other so that the
first part 1A of the piece of sheet metal is flattened between the
support surfaces 11, 20 on these tools, while the third tool 30
retains the second and third parts 1B, 1C of the piece of sheet
metal in the bent state by clamping these parts of the piece of
sheet metal between the first tool 10 and said first and second
parts 30a, 30b of the third tool 30, as illustrated in FIG. 1c.
Hereby, the above-mentioned bulge of the first part 1A of the piece
of sheet metal is flattened so that the piece of sheet metal 1 in
the respective bending area B1, B2 is subjected to compression in
the extension direction of the piece of sheet metal transversally
to the longitudinal direction of the bend. Material is pressed from
the first part 1A of the piece of sheet metal into the respective
bending area B1, B2 by this flattening of the bulge of the first
part 1A of the piece of sheet metal. After this flattening of the
first part 1A of the piece of sheet metal, the piece of sheet metal
1 extends in a first bow over the edge surface 14 of the first
bevelled edge 12 at the first bending area B1 and in a second bow
over the edge surface 15 of the second bevelled edge 13 at the
second bending area B2, while leaving a gap between the piece of
sheet metal and the edge surface 14 of the first bevelled edge 12
on the inner side 3 of the first bending area B1 and a gap between
the piece of sheet metal and the edge surface 15 of the second
bevelled edge 13 on the inner side 3 of the second bending area
B2.
[0025] The first and second tools 10, 30 are thereafter subjected
to a continued mutual displacement in the direction towards each
other, while the first part 1A of the piece of sheet metal is
retained in the flattened state between the first and second tools
10, 20, so that the respective bending area 61, B2 on the piece of
sheet metal is pressed inwards by the third tool 30 against the
adjacent edge surface 14, 15 of the first tool, as illustrated in
FIG. 1d, so that tensile stress is developed in the respective
bending area B1, B2 on the side of the neutral layer 8 facing the
inner side 3 of the bending area and compressive stress is
developed in the respective bending area on the side of the neutral
layer 8 facing the outer side 4 of the bending area, as illustrated
in FIG. 2d. During this stage of the forming process, the piece of
sheet metal is in the first bending area B1 pressed in the
direction towards the edge surface 14 of the first bevelled edge 12
under the effect of a shoulder 31a arranged on the first part 30a
of the third tool, whereas the piece of sheet metal in the second
bending area B2 is pressed in the direction towards the edge
surface 15 of the second bevelled edge 13 under the effect of a
shoulder 31b arranged on the second part 30b of the third tool. In
the illustrated example, the respective shoulder 31a, 31b is
inclined with an inclination corresponding to the inclination of
the underlying edge surface 14, 15 of the first tool 10.
[0026] The formed piece of sheet metal 1 is finally released from
the tools 10, 20, 30.
[0027] The above-mentioned mutual displacements between the first
and third tools 10, 30 may for instance be achieved by keeping the
first tool 10 stationary while the third tool 30 is displaced in
the direction towards the first tool, or alternatively by keeping
the third tool 30 stationary while the first tool 10 is displaced
in the direction towards the third tool.
[0028] The above-mentioned mutual displacement between the first
and second tools 10, 20 may for instance be achieved by keeping the
first tool 10 stationary while the second tool 20 is displaced in
the direction towards the first tool, or alternatively by keeping
the second tool 20 stationary while the first tool 10 is displaced
in the direction towards the second tool.
[0029] The invention is of course not in any way limited to the
embodiments described above. On the contrary, several possibilities
to modifications thereof will be apparent to a person with ordinary
skill in the art without thereby deviating from the basic idea of
the invention as defined in the appended claims.
* * * * *