U.S. patent application number 15/085009 was filed with the patent office on 2016-10-06 for method for bending metal sheet to achieve high angle accuracy.
The applicant listed for this patent is Hitachi, Ltd., Shanghai Jiaotong University. Invention is credited to Hideaki TANAKA, Xingfeng ZHAO, Zhen ZHAO, Xincun ZHUANG.
Application Number | 20160288184 15/085009 |
Document ID | / |
Family ID | 56937732 |
Filed Date | 2016-10-06 |
United States Patent
Application |
20160288184 |
Kind Code |
A1 |
ZHAO; Zhen ; et al. |
October 6, 2016 |
Method for Bending Metal Sheet to Achieve High Angle Accuracy
Abstract
A method for controlling the bending of a metal sheet includes
the steps of: establishing a relational database on a relationship
among material parameters, forming angles, amounts of springback,
and amounts of press of an upper die; blanking and
reverse-engineering a material parameter of a metal sheet to be
processed; comparing the material parameters in the relational
database with the material parameter of the metal sheet to be
processed which is obtained by the reverse-engineering; and
performing an bending operation.
Inventors: |
ZHAO; Zhen; (Shanghai,
CN) ; ZHUANG; Xincun; (Shanghai, CN) ; ZHAO;
Xingfeng; (Beijing, CN) ; TANAKA; Hideaki;
(Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Shanghai Jiaotong University
Hitachi, Ltd. |
Shanghai
Tokyo |
|
CN
JP |
|
|
Family ID: |
56937732 |
Appl. No.: |
15/085009 |
Filed: |
March 30, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01N 2203/0218 20130101;
B21D 5/02 20130101; G06F 2113/24 20200101; G01N 2203/0282 20130101;
G06F 30/00 20200101; G01N 2203/0216 20130101; G06F 30/23 20200101;
B21D 5/004 20130101; G01N 3/20 20130101; G06F 30/20 20200101 |
International
Class: |
B21D 5/00 20060101
B21D005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 31, 2015 |
CN |
201510148039.2 |
Claims
1. A method for controlling bending of a metal sheet comprising the
steps of: (1) establishing a relational database on a relationship
among material parameters, forming angles, amounts of springback,
and amounts of press of an upper die, the establishing including
establishing a finite element simulation model for forming based on
a structure parameter of a die used for a bending process,
obtaining a simulated amount of springback corresponding to a
forming angle by inputting a material parameter of a metal sheet
for a simulation into the finite element simulation model for the
forming and thus simulating the bending process of the metal sheet
for the simulation, and using the simulated amount of springback as
an amount of springback to be added to the relational database, and
after measuring material parameters of different metal sheets,
simulating the bending process of each different metal sheet based
on the obtained finite element simulation model for the forming,
and for each of the different metal sheets, establishing a
relational database on a relationship among a material parameter, a
forming angle, an amount of springback, and an amount of press of
the upper die; (2) blanking and reverse-engineering a material
parameter of a metal sheet to be processed, the blanking and
reverse-engineering including blanking the metal sheet to be
processed, and thereby obtaining a blank, and obtaining an
experimental load-stroke curve of the metal sheet to be processed
during a blanking experiment by measurement, establishing a finite
element simulation model using a method for reverse-engineering an
actual physical parameter of the metal sheet, simulating the
blanking, and repeatedly reverse-engineering the material parameter
of the metal sheet to be processed, (3) comparing the material
parameters in the relational database with the material parameter
of the metal sheet to be processed which is obtained by the
reverse-engineering, if a reference material parameter, between
which and the material parameter of the metal sheet to be processed
an error is less than a second threshold value, exists in the
relational database, performing a bending operation, and if a
reference material parameter, between which and the material
parameter of the metal sheet to be processed an error is less than
a second threshold value, does not exist in the relational
database, like in the establishing the relational database,
simulating the bending process using the material parameter of the
metal sheet to be processed which is obtained by the
reverse-engineering, adding a relationship which is obtained by the
simulation among the material parameter of the metal sheet to be
processed, a forming angle, an amount of springback, and an amount
of press of the upper die to the relational database, making the
material parameter of the metal sheet to be processed which is
obtained by the reverse-engineering become the reference material
parameter of the metal sheet to be processed in the relational
database, and subsequently performing the bending operation; and
(4) performing the bending operation, the performing including
inputting a bending angle, and calculating a needed amount of press
of the upper die based on a formula representing a relation between
the forming angle corresponding to the reference material parameter
and the amount of press of the upper die of the die of a bending
machine for performing the bending operation, sending the needed
amount of press of the upper die to the bending machine, and
controlling the bending machine in a way that the bending machine
bends the blank.
2. The method for controlling bending of a metal sheet according to
claim 1, wherein the structure parameter of the die used for the
bending process includes a draw radius of the upper die and an
opening of a lower die in the die.
3. The method for controlling bending of a metal sheet according to
claim 1, wherein in order to establish the finite element
simulation model for the forming, an actual amount of springback of
the metal sheet for the simulation is obtained by performing a
bending test, and a difference between the actual amount of
springback and the simulated amount of springback is calculated,
and if the difference is greater than a first threshold value, the
simulation is repeatedly performed by refining a grid size of the
finite element simulation model for the forming until the
difference becomes less than the first threshold value.
4. The method for controlling bending of a metal sheet according to
claim 1, wherein each material parameter used to establish the
relational database on the relationship among the material
parameters, the forming angles, the amounts of springback, and the
amounts of press of an upper die includes a performance parameter
and a flow stress curve of a material measured by a tensile test
for detecting the material.
5. The method for controlling bending of a metal sheet according to
claim 1, wherein the first threshold value is 2%.
6. The method for controlling bending of a metal sheet according to
claim 1, wherein the bending process is simulated, and a
relationship between a different bending angle .theta.' and an
amount .DELTA.H of press is recorded, subsequently, an amount
.DELTA..theta. of springback corresponding to the different bending
angle .theta.' is predicted using the finite element simulation
model, and a relationship between the different bending angle
.theta.' and the amount .DELTA..theta. of springback is obtained,
and based on .theta.=.theta.'+.DELTA..theta. representing a
relationship among a forming angle .theta., the bending angle
.theta.' and the amount .DELTA..theta. of springback, a
relationship between the forming angle .theta. and the amount
.DELTA.H of press is obtained
7. The method for controlling bending of a metal sheet according to
claim 1, wherein the second threshold value is 1%.
8. The method for controlling bending of a metal sheet according to
claim 7, wherein if the reference material parameter, between which
and the material parameter of the metal sheet to be processed an
error is less than the second threshold value, does not exist in
the relational database, the bending process is simulated by
inputting the material parameter of the metal sheet to be
processed, which is obtained by the reverse-engineering, into the
finite element simulation model, thereby, a relationship between a
forming angle .theta. of the metal sheet to be processed and an
amount .DELTA.H of press is obtained, and the relationship between
the forming angle .theta. of the metal sheet to be processed and
the amount .DELTA.H of press is sent to the relational database.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a metal sheet processing
technology, and specifically to a method for increasing accuracy of
bending a metal sheet.
[0003] 2. Description of the Related Art
[0004] Bending is an important method for processing metal sheets
in the industrial production. In a process of producing metal sheet
members, the angle of the bent metal sheet increases due to the
elastic recovery of the metal material. In addition, there are
large variations in performance among raw materials with different
specifications and from different makers, making the amounts of
springback of the materials after bending differ. This makes the
accuracy of forming structural members unstable. For this reason,
an increase in the forming accuracy by effectively avoiding the
springback is very important in the forming process.
[0005] Against this background, CN203140585U discloses a method for
increasing the accuracy of forming stamped parts by designing a new
V-shaped bending die for the parts to reduce the springback.
However, a great restriction is imposed on such method. The
thus-designed bending die can be used for a bending process
requiring only one forming angle, and the products formed using the
bending die is in one size only. The same die cannot be used when
the forming angle of products is changed, or when the size of
products is increased. Thus, a set of dies for the corresponding
angle and size needs to be newly processed. This results in a very
large production cycle very and very high costs.
[0006] CN201020102617 discloses a V-shaped 90.degree.
self-supporting die capable of adjusting an amount of bending
springback and compensating for a springback angle. The die
includes a punch and a lower die. The punch includes an upper die
base. Angle-adjusting side plates capable of adjusting a stamping
angle of a lower end of the punch are attached to two ends of the
upper die base. The angle-adjusting side plates are shaft-coupled
in a bending radius section. The lower die includes a lower die
base. Left and right self-supporting plates are connected to an
upper end of the lower die base with a hinge.
[0007] According to the patent disclosed in CN201020102617, the
compensation is achieved by adjusting the formed angle. The
compensation also needs to be carried out by: performing bending
tests while repeating an angle adjustment process; testing the
forming angle; thereby predicting the amount of springback; and
eventually adjusting the die angle. This method requires so many
manual operations, and cannot be implemented with an automated
control. Furthermore, dies used for such method are complicated,
and after used for long time, the dies are deformed. Moreover,
larger dies need to be designed for larger bending materials.
However, the rigidity of the thus-designed larger dies cannot be
secured, and the dies maybe warped or deformed during the forming
processes, adversely affecting the forming quality.
[0008] With these taken into consideration, the present invention
aims at obtaining a relational database on a relationship among
amounts of springback, forming angles, material performances and
sheet thicknesses of metal sheets by simulating bending processes
using a CAE (Computer-Aided Engineering) simulation technique,
based on a principle that the amount of bending springback is
different among materials depending on properties of the materials,
without directly controlling the amounts of springback. Thus, for
each material, the amount of springback can be known before the
material is actually bent. Furthermore, for each material, accuracy
requirements can be satisfied by: compensating for the springback
in advance; and forming a bent member at one time. The forming
angle can be automatically controlled. It is no longer necessary to
adjust a grinder by manually measuring the forming angle. Both the
automation level and the forming accuracy are high. This method can
satisfy forming demands for different material performances,
different sheet thicknesses, and different forming angles, as well
as can process products in accordance with accuracy
requirements.
SUMMARY OF THE INVENTION
[0009] The present invention has been made to solve the above
problems and makes it an object thereof to provide a method for
controlling the bending of a metal sheet while capable of
accurately controlling an angle at which to bend the metal
sheet.
[0010] The present invention provides a method for controlling
bending of a metal sheet comprising the steps of:
[0011] (1) establishing a relational database on a relationship
among material parameters, forming angles, amounts of springback,
and amounts of press of an upper die,
[0012] the establishing including [0013] establishing a finite
element simulation model for forming based on a structure parameter
of a die used for a bending process, obtaining a simulated amount
of springback corresponding to a forming angle by inputting a
material parameter of a metal sheet for a simulation into the
finite element simulation model for the forming and thus simulating
the bending process of the metal sheet for the simulation, and
using the simulated amount of springback as an amount of springback
to be added to the relational database, and [0014] after measuring
material parameters of different metal sheets, simulating the
bending process of each different metal sheet based on the obtained
finite element simulation model for the forming, and for each of
the different metal sheets, establishing a relational database on a
relationship among a material parameter, a forming angle, an amount
of springback, and an amount of press of the upper die;
[0015] (2) blanking and reverse-engineering a material parameter of
a metal sheet to be processed,
[0016] the blanking and reverse-engineering including [0017]
blanking the metal sheet to be processed, and thereby obtaining a
blank, and [0018] obtaining an experimental load-stroke curve of
the metal sheet to be processed during a blanking experiment by
measurement, establishing a finite element simulation model using a
method for reverse-engineering an actual physical parameter of the
metal sheet, simulating the blanking, and repeatedly
reverse-engineering the material parameter of the metal sheet to be
processed,
[0019] (3) comparing the material parameters in the relational
database with the material parameter of the metal sheet to be
processed which is obtained by the reverse-engineering, [0020] if a
reference material parameter, between which and the material
parameter of the metal sheet to be processed an error is less than
a second threshold value, exists in the relational database,
performing a bending operation, and [0021] if a reference material
parameter, between which and the material parameter of the metal
sheet to be processed an error is less than a second threshold
value, does not exist in the relational database, like in the
establishing the relational database, simulating the bending
process using the material parameter of the metal sheet to be
processed which is obtained by the reverse-engineering, adding a
relationship which is obtained by the simulation among the material
parameter of the metal sheet to be processed, a forming angle, an
amount of springback, and an amount of press of the upper die to
the relational database, making the material parameter of the metal
sheet to be processed which is obtained by the reverse-engineering
become the reference material parameter of the metal sheet to be
processed in the relational database, and subsequently performing
the bending operation; and
[0022] (4) performing the bending operation,
[0023] the performing including [0024] inputting a bending angle,
and [0025] calculating a needed amount of press of the upper die
based on a formula representing a relation between the forming
angle corresponding to the reference material parameter and the
amount of press of the upper die of the die of a bending machine
for performing the bending operation, sending the needed amount of
press of the upper die to the bending machine, and controlling the
bending machine in a way that the bending machine bends the
blank.
[0026] With the use of the foregoing method, the present invention
obtains the relational database on a relationship among amounts of
springback, forming angles, material performances and sheet
thicknesses of metal sheets by simulating bending processes using a
CAE simulation technique, based on a principle that when bend,
materials show different amounts of springback depending on
properties of the materials, without directly controlling the
amounts of springback. Thus, for each material, the amount of
springback can be known before the material is actually bent.
Furthermore, for each material, accuracy requirements can be
satisfied by: compensate for the springback in advance; and forming
a bent member at one time. The forming angle can be automatically
controlled. It is no longer necessary to adjust a grinder by
manually measuring the forming angle. Both the automation level and
the forming accuracy are high. This method can satisfy forming
demands for different material performances, different sheet
thicknesses, and different forming angles, as well as can process
products in accordance with accuracy requirements.
[0027] The present invention provides the method for controlling
bending of a metal sheet, in which the structure parameter of the
die used for the bending process may include a draw radius of the
upper die and an opening of a lower die in the die.
[0028] The present invention can establish the finite element
simulation model for forming using the structure parameters of the
die, which include the draw radius of the upper die and the opening
of the lower die. Using this small number of parameters, the
present invention can obtain the finite element simulation model
for accurately simulating actual forming conditions. Thus, the
present invention increases the efficiency of establishing the
finite element simulation model.
[0029] The present invention provides the method for controlling
bending of a metal sheet, in which in order to establish the finite
element simulation model for the forming, an actual amount of
springback of the metal sheet for the simulation may be obtained by
performing a bending test, and a difference between the actual
amount of springback and the simulated amount of springback may be
calculated, and
[0030] if the difference is greater than a first threshold value,
the simulation may be repeatedly performed by refining a grid size
of the finite element simulation model for the forming until the
difference becomes less than the first threshold value.
[0031] With the use of the foregoing method, the present invention
can simulate the bending process using the CAE simulation technique
with 98 percent accuracy, and can makes the predicted value almost
equal to the actual value. Thereby, the present invention ensures
that the accuracy requirements are satisfied by forming the bent
member at one time. The present invention can automatically control
the forming angle. The present invention makes it no longer
necessary to adjust the grinder by manually measuring the forming
angle. The present invention improves the automation level and the
forming accuracy.
[0032] The present invention provides the method for controlling
bending of a metal sheet, in which each material parameter used to
establish the relational database on the relationship among the
material parameters, the forming angles, the amounts of springback,
and the amounts of press of an upper die may include a performance
parameter and a flow stress curve of a material measured by a
tensile test for detecting the material.
[0033] Since the present invention performs the finite element
simulation on the forming process using the flow stress curve as
the material parameter, the present invention can achieve higher
simulation accuracy and efficiency.
[0034] The present invention provides the method for controlling
bending of a metal sheet, in which the first threshold value may be
2%.
[0035] The present invention provides the method for controlling
bending of a metal sheet, in which the bending process may be
simulated, and a relationship between a different bending angle
.theta.' and an amount .DELTA.H of press may be recorded,
subsequently, an amount .DELTA..theta. of springback corresponding
to the different bending angle .theta.' may be predicted using the
finite element simulation model, and a relationship between the
different bending angle .theta.' and the amount .DELTA..theta. of
springback may be obtained, and based on
.theta.=.theta.'+.DELTA..theta. representing a relationship among a
forming angle .theta., the bending angle .theta.' and the amount
.DELTA..theta. of springback, a relationship between the forming
angle .theta. and the amount .DELTA.H of press may be obtained
[0036] The present invention provides the method for controlling
bending of a metal sheet, in which the second threshold value may
be 1%.
[0037] The present invention provides the method for controlling
bending of a metal sheet, in which if the reference material
parameter, between which and the material parameter of the metal
sheet to be processed an error is less than the second threshold
value, does not exist in the relational database, the bending
process may be simulated by inputting the material parameter of the
metal sheet to be processed, which is obtained by the
reverse-engineering, into the finite element simulation model,
thereby, a relationship between a forming angle .theta. of the
metal sheet to be processed and an amount .DELTA.H of press may be
obtained, and the relationship between the forming angle .theta. of
the metal sheet to be processed and the amount .DELTA.H of press
may be sent to the relational database.
[0038] The performing of the step (1) before the bending process
makes it possible to simulate the bending process for any of
materials produced by different makers, under different
specifications, in different batches and with different thicknesses
by testing the material. If the range of kinds, specifications and
batches of materials stored in the database is fully comprehensive,
the relationship between the forming angles and the amounts of
press of a corresponding material can be obtained directly from the
database before the bending process, and high-speed accurate
control can be achieved. If the database is incomprehensive, there
is likelihood that a material similar to a metal sheet to be
process cannot be found in the database. In that case, before the
first bending, additional time needs to be spent in simulating the
bending process of the metal sheet to be processed in order to
obtain the relationship between the forming angles and the amounts
of press.
[0039] Referring to the drawings, detailed description will be
hereinbelow provided for the present invention, citing a
particularly preferable example for the purpose of making the
foregoing contents of the present invention more easily
understood.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] FIG. 1 is a flow chart showing how a database of the present
invention is established;
[0041] FIG. 2 is a flow chart showing how a blanking process and a
bending process of the present invention are performed;
[0042] FIG. 3 shows flow stress curves of generally-used
cold-rolled steel sheets;
[0043] FIG. 4 shows a finite element simulation model for
forming;
[0044] FIG. 5 is a schematic diagram showing how the forming is
simulated;
[0045] FIG. 6 shows comparisons of simulated amounts of springback
with experimental amounts of springback;
[0046] FIGS. 7A to 7D are schematic diagrams each showing a
relationship between an amount of press of an upper die and a
bending angle during a bending process;
[0047] FIG. 8 shows a relationship between bending angles
(approximately 86 degrees to 96 degrees) and amounts of press of
the upper die which is obtained when a bending process is
simulated;
[0048] FIG. 9 shows a relationship between the bending angles
(approximately 86 degrees to 96 degrees) and amounts of springback
which is obtained when the bending process is simulated;
[0049] FIG. 10 shows a relationship between forming angles
(approximately 86 degrees to 98 degrees) and the amounts of press
of the upper die for each of materials A, B;
[0050] FIG. 11 shows a relationship between forming angles
(approximately 86 degrees to 98 degrees) and amounts of press of
the upper die for a material X; and
[0051] FIG. 12 is a schematic diagram of a bending die.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0052] Embodiments for carrying out the present invention will be
hereinafter described in detail with reference to the accompanying
drawings.
Establishing a Database
[0053] The present invention provides a method for controlling
bending of a metal sheet comprising the steps of:
[0054] (1) establishing a relational database on a relationship
among material parameters, forming angles, amounts of springback,
and amounts of press of an upper die,
[0055] the establishing including [0056] establishing a finite
element simulation model for forming based on a structure parameter
of a die used for a bending process, obtaining a simulated amount
of springback corresponding to a forming angle by inputting a
material parameter of a metal sheet for a simulation into the
finite element simulation model for the forming and thus simulating
the bending process of the metal sheet for the simulation, and
using the simulated amount of springback as an amount of springback
to be added to the relational database, and [0057] after measuring
material parameters of different metal sheets, simulating the
bending process of each different metal sheet based on the obtained
finite element simulation model for the forming, and for each of
the different metal sheets, establishing a relational database on a
relationship among a material parameter, a forming angle, an amount
of springback, and an amount of press of the upper die;
[0058] (2) blanking and reverse-engineering a material parameter of
a metal sheet to be processed,
[0059] the blanking and reverse-engineering including [0060]
blanking the metal sheet to be processed, and thereby obtaining a
blank, and [0061] obtaining an experimental load-stroke curve of
the metal sheet to be processed during a blanking experiment by
measurement, establishing a finite element simulation model using a
method for reverse-engineering an actual physical parameter of the
metal sheet, simulating the blanking, and repeatedly
reverse-engineering the material parameter of the metal sheet to be
processed,
[0062] (3) comparing the material parameters in the relational
database with the material parameter of the metal sheet to be
processed which is obtained by the reverse-engineering, [0063] if a
reference material parameter, between which and the material
parameter of the metal sheet to be processed an error is less than
a second threshold value, exists in the relational database,
performing a bending operation, and [0064] if a reference material
parameter, between which and the material parameter of the metal
sheet to be processed an error is less than a second threshold
value, does not exist in the relational database, like in the
establishing the relational database, simulating the bending
process using the material parameter of the metal sheet to be
processed which is obtained by the reverse-engineering, adding a
relationship which is obtained by the simulation among the material
parameter of the metal sheet to be processed, a forming angle, an
amount of springback, and an amount of press of the upper die to
the relational database, making the material parameter of the metal
sheet to be processed which is obtained by the reverse-engineering
become the reference material parameter of the metal sheet to be
processed in the relational database, and subsequently performing
the bending operation; and
[0065] (4) performing the bending operation,
[0066] the performing including [0067] inputting a bending angle,
and [0068] calculating a needed amount of press of the upper die
based on a formula representing a relation between the forming
angle corresponding to the reference material parameter and the
amount of press of the upper die of the die of a bending machine
for performing the bending operation, sending the needed amount of
press of the upper die to the bending machine, and controlling the
bending machine in a way that the bending machine bends the
blank.
[0069] The structure parameter of the die used for the bending
process includes a draw radius of the upper die and an opening of a
lower die in the die.
[0070] In order to establish the finite element simulation model
for the forming, an actual amount of springback of the metal sheet
for the simulation is obtained by performing a bending test, and a
difference between the actual amount of springback and the
simulated amount of springback is calculated, and if the difference
is greater than a first threshold value, the simulation is
repeatedly performed by refining a grid size of the finite element
simulation model for the forming until the difference becomes less
than the first threshold value.
[0071] Each material parameter used to establish the relational
database on the relationship among the material parameters, the
forming angles, the amounts of springback, and the amounts of press
of an upper die includes a performance parameter and a flow stress
curve of a material measured by a tensile test for detecting the
material.
[0072] The bending process is simulated, and a relationship between
a different bending angle .theta.' and an amount .DELTA.H of press
is recorded, subsequently, an amount .DELTA..theta. of springback
corresponding to the different bending angle .theta. is predicted
using the finite element simulation model, and a relationship
between the different bending angle .theta.' and the amount
.DELTA..theta. of springback is obtained, and based on
.theta.=.theta.+.DELTA..theta. representing a relationship among a
forming angle .theta., the bending angle .theta.' and the amount
.DELTA..theta. of springback, a relationship between the forming
angle .theta. and the amount .DELTA..theta. of press is
obtained
[0073] If the reference material parameter, between which and the
material parameter of the metal sheet to be processed an error is
less than the second threshold value, does not exist in the
relational database, the bending process is simulated by inputting
the material parameter of the metal sheet to be processed, which is
obtained by the reverse-engineering, into the finite element
simulation model, thereby, a relationship between a forming angle
.theta. of the metal sheet to be processed and an amount .DELTA.H
of press is obtained, and the relationship between the forming
angle .theta. of the metal sheet to be processed and the amount
.DELTA.H of press is sent to the relational database.
[0074] Descriptions will be hereinbelow provided for an example of
the present invention while referring to specific numerical values.
Subjects of an examination for the example are two low-carbon steel
sheets each with a sheet thickness of 1.2 mm, which are similar but
are produced by the respective makers. The steel sheets are
respectively denoted by reference signs A, B. A tension test is
performed on the two materials A, B using a Shimadzu's universal
tensile testing machine at room temperature. Thereby, a load
displacement curve is obtained from each of the materials A, B.
Furthermore, for each material, a flow stress curve and a
mechanical performance parameter are calculated based on a
theoretical formula, and are used as input parameters to evaluate
the mechanical characteristic of the material in simulating the
bending process. FIG. 3 shows the flow stress curves obtained from
the respective materials. It will be explained that albeit similar,
the materials produced by the respective different makers are very
different from each other in terms of the flow stress curve, and in
terms of the mechanical performance. It will be shown how much the
materials are different from each other in terms of the
performance.
[0075] One may consider that the right-angle bending process is the
most generally-used process in the actual industrial production.
For this reason, the example will be explained citing the
right-angle bending process. Parameters such as the draw radius of
the upper die and the opening of the lower die in a needed bending
die differ according to the thickness of an object to be bent. When
a thinner sheet is the object to be bent, a smaller draw radius is
selected for the upper die, and a smaller opening is selected for
the lower die. When a thicker sheet is the object to be bent, a
larger draw radius is selected for the upper die, and a larger
opening is selected for the lower die. Because the example will be
explained citing a case where the thickness of a sheet material to
be bent is 1.2 mm, generally-used parameters suitable to bend the
sheet material with the thickness of 1.2 mm are selected for the
die. As shown in FIG. 12, the included angle of the upper die is 86
degrees, the draw radius of the upper die is 1.2 mm, and the
opening of the low die is 8 mm. As shown in FIG. 4, a simplified
finite element simulation model for the bending using finite
element simulation software LS-DYNA is established based on the
parameters of the actual bending die, that is to say, 86 degrees as
the included angle of the upper die, 1.2 mm as the draw radius of
the upper die, and 8 mm as the opening of the lower die. In the
finite element simulation model, the die is defined as a rigid
body, that is to say, as a body having no possibility of deforming,
and the flow stress curve measured in the tension test is used as
an input data for the finite element simulation model. A finite
element simulation is performed on the process of the bending of
the sheet material, and thereby the sheet material bent at 90
degrees is simulated as shown in FIG. 5. Thereafter, an amount of
springback is predicted by simulating a simulated amount of
springback corresponding to the bending angle by reuse of the
finite element method. The example uses the case of the right-angle
bending process in which: the thickness of the sheet material to be
bent is 1.2 mm; the included angle of the upper die is 86 degrees;
the draw radius of the upper die is 1.2 mm; and the opening of the
lower die is 8 mm. These specific numerical values are used only
for the purpose of explaining the example. The present invention is
not limited to the specific numerical values, and uses arbitrary
numerical values depending on the actual necessity.
[0076] In order to examine the accuracy of the thus-established
finite element simulation model, the present invention calculates
actual amounts .DELTA..theta. of springback of the materials A, B
by: bending the materials A, B using the die shown in FIG. 12
according to the actual production conditions; measuring the angles
.theta. of bend of the materials A, B after bending before their
springback; removing the upper die, and measuring the angles
.theta. of bend of the materials A, B after their springback; and
subtracting the angles .theta. of bend of the materials A, B after
their springback from the angles 0' of bend of the materials A, B
before their springback. If comparison of the actual amounts of
springback with the simulated amounts of springback shows that an
error between the actual and simulated amounts of springback is
less than a first threshold value, for example 2%, the accuracy of
the established finite element simulation model for the bending can
be considered as reliable. If the error between both amounts is
greater than 2%, the grid size of the finite element simulation
model is reasonably refined until the error between the simulated
and experimental amounts of springback becomes less than 2%. As for
the refinement of the finite element simulation model in the
example, the finite element grid size in a direction of the
thickness of the sheet material is the most influential in the
simulation accuracy. Accordingly, as the grid in the direction of
the thickness becomes thinner, the accuracy thereof becomes higher.
Meanwhile, improvement in the accuracy by continually refining the
grid size is limited when the grid in the direction of the
thickness of the sheet material is refined to become equal to one
seventh of the thickness of the sheet material. However, the
simulation time becomes very long if the grid becomes too thinner.
In this context, in the example, the optimal grid size in the
direction thickness is one seventh of the thickness of the sheet
material. FIG. 6 shows comparison of the simulated result and the
experimental result for each of the materials A, B in the example.
The error in the simulated amount of springback is 1% for the
material A, and 1.5% for the material B. The simulation accuracy
satisfies the requirement that the error should be less than 2%.
For this reason, this finite element simulation model for the
bending will be used as a universal model for the following
simulation prediction of other materials. In the present invention,
the first threshold value is 2%. However, the first threshold value
is not limited to this specific numerical value, and may be
arbitrarily changed depending on the actual necessity.
[0077] In the bending process, a relationship exists between the
bending angle and the amount of press of the upper die. For this
reason, as shown in FIGS. 7A to 7D, the angle amount of bend of a
metal sheet can be controlled by controlling the amount of press of
the upper die of a servo bending machine. The relation between
different bending angles and the amounts of press of the upper die
can be obtained by: simulating the entirety of the bending process
performed on the material A based on the above-established finite
element simulation model for the bending; and recording the various
amounts of press of the upper die and the corresponding bending
angles. Since the target bent angle for the example is 90 degrees,
a focus is placed on the state of being bent at approximately 90
degrees. Thus, FIG. 8 shows an obtained relationship between
bending angles (approximately 86 degrees to 96 degrees) and amounts
of press of the upper die, as well as amounts of springback
according to angle bending. Thereby, a relationship between
different bending angles (approximately 86 degrees to 96 degrees)
and amounts of springback is obtained, and is shown in FIG. 9.
[0078] A relationship between forming angles and the amounts of
press of the upper die can be obtained based on a relationship
among the forming angles .theta., the bending angles .theta.' and
the amounts .DELTA..theta. of springback, that is to say,
.theta.=.theta.'+.DELTA..theta.. Based on a linear analysis, the
relationship between the forming angles .theta. of the material A
and the amounts .DELTA.H of press of the upper die is obtained as
.theta.=-36.719.times..DELTA.H+169.02. In FIG. 10, the relationship
for the material A is represented by a line A. Similarly, the
finite element simulation for the bending is performed on the
material B, and the relationship between the forming angles .theta.
of the material B and the amounts .DELTA.H of press of the upper
die is thereby obtained as .theta.=-36.656.times..DELTA.H+170.03.
In FIG. 10, the relationship for the material B is represented by a
line B.
[0079] If materials produced by different makers, materials under
different specification, and materials in different batches are
tested according to this method, a relational database can be
established among performances of the materials, forming angles,
amounts of springback and amounts of press of the upper die. For
each of the materials different in thickness, a finite element
simulation model can be established for the forming using the draw
radius of the upper die and the opening of the lower die which
correspond to the thickness of the object material to be bent. In
addition, the relational database can be established among the
performances of the materials, the forming angles, the amounts of
springback and the amounts of press of the upper die . In this
manner, the database can cover problems with the bending of the
materials produced by different makers, under different
specifications, in different batches, and with different
thicknesses.
Blanking
[0080] Each time a metal thin sheet X is blanked, force which the
sheet material receives from the blanking die during the blanking
process and displacement of a blanked part are measured and
recorded. Thereby, a load-stroke curve corresponding to the actual
blanking of the sheet material and the pre-bent blank are obtained.
A finite element simulation model for the blanking process is
established using a method described in a prior patent application
(its application number is 201310680718.5), and a parameter
representing a mechanical performance of a material is set for the
sheet material to be blanked. Thereby, a finite element simulation
is performed on the blanking process. The analysis is carried out
by: combining the established finite element simulations for the
blanking based on the load-stroke curve corresponding to the actual
blanking; and for each process, repeatedly reverse-engineering the
mechanical performance parameter of the sheet material X. The
result of the repetition of approximately 30 processes shows that:
simulated values representing the load-stroke curve basically
coincide with the load-stroke curve corresponding to the actual
blanking; and an error between the two curves is less than 1%.
Thereafter, the mechanical performance of the material obtained by
the reverse-engineering is outputted, and the corresponding flow
stress curve is drawn. For the detailed method for
reverse-engineering the material performance, see the prior
application whose application number is 201310680718.5.
Bending Example 1
[0081] The performance parameter of a material C obtained by the
reverse-engineering is sent to a control unit of the bending
machine. The control unit draws a flow stress curve based on the
mechanical parameter of the material to be processed, and compares
the thus-drawn flow stress curve with the flow stress curves of the
materials stored in the database. If the flow stress curve of the
material A basically agrees with the flow stress curve of the
material C, and the error between the two curves is less than a
second threshold value, for example 1%, the control unit
understands the material A in the database is similar to the
material C and selects the material parameter of the material A as
a reference material parameter of the material C. Since the bending
performance of a material depends on the mechanical performance of
the material, the bending performance of the material C should be
similar to the bending performance of the material A. Although the
present invention sets the second threshold value at 1%, the second
threshold value is not limited to this specific numerical value.
The second threshold value may be arbitrarily changed depending on
the actual necessity.
[0082] Thereby, the conditions for processing the material C can be
determined using the relationship between the forming angle and the
amount of press of the upper die obtained from the material A, that
is to say, .theta.=-36.719.times..DELTA.H+169.02. The forming angle
requirement is inputted into the control unit. In this example, the
forming angle is 90 degrees. By substituting the angle in the
relational formula, the needed amount .DELTA.H of press of the
upper die can be calculated as .DELTA.H=(169.02-90)/36.719=2.15
[mm].
[0083] Subsequently, an instruction that the amount of press of the
upper die should be set at 2.15 [mm] is sent to the bending
machine. The bending machine bends the blank of the material C
which has already been blanked, and measures the forming angle of
the material C after the springback. If the forming angle of the
material C after the springback measures 90.04 degrees, the forming
accuracy is far higher than the industrial requirement of the
process accuracy for formed products, that is to say, 90+0.5
degrees.
[0084] In a bending example 2, a case where no material to meet the
error requirements is found in the database is referred to.
Bending Example 2
[0085] The performance parameter of the material X obtained by the
reverse-engineering is sent to the control unit of the bending
machine. The control unit draws a flow stress curve based on the
mechanical parameter of the material to be processed, and compares
the thus-drawn flow stress curve with the flow stress curves of the
materials stored in the database. If the control unit finds no
material to meet the error requirements in the database, that is to
say, if a reference material parameter, between which and the
material parameter of the metal sheet X to be processed an error is
less than the second threshold value, does not exist in the
relational database, the control unit understands that the
performances of the materials stored in the relational database is
very different from the performance of the material X to be
processed. In this case, the bending process to be performed on the
material X to be processed needs to be simulated by reuse of the
finite element simulation model for the bending.
[0086] A flow stress curve of the material X to be processed
obtained by reverse-engineering is sent to the finite element
simulation model for the bending, and a bending is thus simulated.
A relationship between forming angles of the material X to be
processed and amounts of press of the upper die is obtained. FIG.
11 shows the thus-obtained relationship. In addition, a
relationship among the performance of the material to be processed,
the corresponding forming angle and the corresponding amount of
press of the upper die is sent to the database, and the contents of
the relational database is thus complemented. In a case where the
flow stress curve of a metal sheet M to be process basically agrees
with the flow stress curve of the material X, the material
parameter of the metal sheet X to be process obtained by the
reverse-engineering becomes the reference material parameter of the
metal sheet M to be processed which is stored in the relational
database. After that, the control unit determines appropriate
processing conditions based on the relationship between the forming
angle corresponding to the material X to be processed and the
amount of press of the upper die which is stored in the relational
database. Moreover, when bending the sheet material X, the control
unit can find the material X directly from the relational database
to determine the processing conditions based on the relationship
between the forming angle and the amount of press of the upper die
which both correspond to the material X. The method refers to the
forming example 1.
[0087] With this taken into consideration, as the number of
materials stored in the relational database becomes greater, a
material whose performance is in extremely high agreement with the
performance of the material to be processed can be more easily
found from the relational database. This makes the relationship
between the forming angles of the materials and the amounts of
press of the upper die in the relational database become better
matched to the materials to be processed, and increases the
accuracy of the bent products. This also makes no finite element
simulation necessary to be performed for the forming, and thus
increases the control speed. Even if a material whose performance
is in agreement with the performance of the material to be
processed cannot be found from the material database, the control
system can complement the relational database by performing a
simulation on the bending using the material parameter obtained by
reverse-engineering when performing the blanking process; and
predicting the relationship between the forming angle of the
material to be processed and the amount of press of the upper die.
Next time a similar material is bent, the control system uses data
directly from the relational database, and accordingly need not
perform a simulation on the bending.
[0088] The forgoing example is used only for the purpose of
explaining the principle and effect of the present invention using
the instance, and does not limit the present invention. Those
skilled in the art can modify or change the foregoing example
without departing from the spirit or scope of the present
invention. For this reason, all the equivalent modifications and
changes which those skilled in the art complete without departing
from the spirit or technical concept disclosed in the present
invention shall be included in the claims of the present
invention.
* * * * *