Metal deforming – Process – Deforming sheet metal
Reexamination Certificate
2002-09-25
2004-06-15
Crane, Daniel C. (Department: 3725)
Metal deforming
Process
Deforming sheet metal
C072S306000
Reexamination Certificate
active
06748788
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a method for forming such members made of metal plates as used for car parts and the like, and in particular, relates to a method for improving a defective shapes caused by a change of an angle of bent portion of the formed member, caused by elastic recovery when removed from a forming die.
2. Description of Related Art
In recent years, a body or a part for automobiles is increasingly stronger and lighter for mileage improvement, environmental consideration, and safety improvement. One way to achieve stronger and lighter parts is to use lighter materials such as high-tension steel or aluminum based alloy for a press-formed member of metal plates that takes a large portion of car component parts.
FIG. 1
shows a representative example of the press-formed member.
FIG. 1A
shows an L-shaped member formed by one edge of an upper flat portion
51
and a longitudinal flat portion
53
forming the member perpendicularly through a bent portion
52
.
FIG. 1B
shows a U-shaped member where the L-shaped members are placed symmetrically.
FIG. 1C
shows a Z-shaped member formed in a manner that on one edge of an upper flat portion
61
, a longitudinal flat portion
63
is formed perpendicularly through a bent portion
62
, and on the other edge thereof, a flange portion
65
is coupled through a bent portion
64
so as to be parallel to the upper flat portion
61
.
FIG. 1D
shows a hat-channel shaped member where the Z-shaped members are placed symmetrically. These formed members are formed integrally of a metal plate by relatively displacing a forming die and a forming punch (sometimes called “bending blade”) having a forming surface corresponding to a shape of the formed member.
A problem with a press forming of a metal plate of high tension steel or aluminum based alloy is that large springback caused when a plate is removed from the forming die, deteriorates dimensional accuracy of the formed member. Examples of the above are as follows:
FIG. 2A
is a diagram in which chain double dashed lines show a shape of a U-shaped member before removed from the die (i.e., a target forming shape) and solid lines show a shape of the member after removed from the die; likewise,
FIG. 2B
is a diagram in which chain double dashed lines show a hat-channel shape of a member before removed from the die (i.e., a target forming shape) while solid lines show a shape of the member after removed from the die. In both cases, an angle change defect &Dgr;&thgr; occurs at the bent portions
52
and
62
, respectively, after the member is removed from the forming die. Note that the angle change defect &Dgr;&thgr; is an angle formed between a tangent of the end of the bent portion on a longitudinal flat portion side and a longitudinal flat portion of the target forming shape. In the case of the hat-channel shaped member (the Z-shaped member), in addition to the angle change defect of the bent portion
62
, the longitudinal flat portion
63
receiving bending for bending-back force warps suffers from an outward curvature. In the figure, &Dgr; represents a maximum dividing distance between the warped portion and a line segment coupling between the end of the bent portion on the both sides of the longitudinal flat portions. The bending for bending-back force means that when a plate material bent and formed, the bent portion bends back to an opposite direction to that was bent first, and the portion that was bent back is distorted by rebounding to the first bending direction. The angle change defect and the curvature causes an opening, with an opening distance &Dgr;W, to be generated at a lower end of the longitudinal flat portion of the hat-channel shaped member. The opening distance &Dgr;W is a horizontal distance between the lower ends of the respective longitudinal flat portions of the forming shape just removed from the die and the target forming shape. The lower end of the longitudinal flat portion of the hat-channel shaped member (and the Z-shaped member) refers to an intersecting portion extended from an inner surface of the longitudinal flat portion and a lower surface of the flange portion.
Some examples of various methods of preventing angle change defect at the bent portion of the formed member are proposed as follows.
First is a method for designing a shape of a forming die so that a formed member becomes the right size when in springback;
Second is a method in which a reverse bending radius portion having an opposite direction of the bending direction is formed along a ridge line of a bent portion, described in JP-A-7-204743.
Third is a method for giving compression stress in a direction of a plate thickness of a bent portion so as to reduce the remaining stress, described in JP-A-8-174074. Incidentally, the similar methods can be basically applied to reducing the wall curvature &Dgr;.
Problems of those methods are as follows. In the first method, it is difficult to design a forming die (a tool) in an appropriate shape, requiring more time to try and learn, which lead to increase a cost of the forming die and to delay to start producing the product. In the second method, the reverse bending radius portion needs to be added to the bent portion of the product, causing to change an appearance of the product to a poor one.
Incidentally, the reverse bending radius portion is the one that is not originally needed. In the third method, there needs additional machines other than a pressing machine, such as a device to add compressive force.
SUMMARY OF THE INVENTION
Under the circumstances, the present invention aims at solving the problems and providing a method for bending a metal plate that can reduce an angle change defect of the bent portion of the formed member with no need of changing product shape or of preparing for a special forming die or a facility.
First aspect of the present invention resides in a method for bending a metal plate, comprising:
a first forming step for obtaining a first formed member having a bent portion obtained by bending a metal plate; and
a second forming step for bending the bent portion of the first formed member in the same direction as a bending direction of the bent portion, so that the bent portion of the first formed member receives bending-back force.
Second aspect resides in a method for bending a metal plate, comprising:
a first forming step for bending a metal plate by relatively moving a first forming punch from a first flat forming portion to a first forming die with a second flat forming portion formed therein, through a bent forming portion having a radius R1, to thereby obtain a first formed member in which a first flat portion and a second flat portion is coupled with each other through the thus formed bent portion by the bent forming portion; and
a second forming step for relatively moving a second forming punch from the first flat forming portion to a second forming die with the second flat forming portion formed therein, through a bent forming portion having a radius R2, to thereby bend the first formed member by the bent forming portion of the second forming die;
the second forming step further comprising a substep for placing the second flat portion of the first formed member on the second flat portion side of the second forming die, so that the bent portion of the first formed member receives the bending-back force while abutting with at least one of the bent forming portion and the second flat forming portion, both of the second forming die.
In the second aspect, it is preferable that in the second forming step, a part of the bent portion of the first formed member abuts with a bend stop point on a second flat forming portion side of the bent portion of the second forming die.
Further, it is preferable that the method performs the second forming step using following formulas (A) and (B) in which k is obtained from the formula (A), and then &Dgr;W is obtained from the formula (B) by the thus obtained k,
k
=−0.4836·ln(
R
2
/R
1)+0.1817 (A
Iwaya Jiro
Yamano Takayuki
(Kobe Steel, Ltd.)
Crane Daniel C.
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