Bending angle correction method and press brake

Details Bending angle correction method and press brake Bending angle correction method and press brake

1999-04-20

2001-02-20

Jones, David (Department: 3725)

Metal deforming

Overbending to compensate for springback

C072S031100, C072S031110, C072S389400, C072S389500

Reexamination Certificate

active

06189364

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a bend angle correction method for correcting the bend angle of a sheet-like workpiece which is bent by the cooperative movement of a movable die (punch) and a fixed die (die). The movable die is supported by a ram having three or more drive shafts, while the fixed die being supported in an opposing relationship with the movable die. The invention also relates to a press brake for use with the above method which provides high accuracy bending.
BACKGROUND ART
As such a conventional bending machine, the press brake
51
shown in
FIG. 22
is known. In the press brake
51
, a ram
52
and a fixed bed
53
are disposed, facing each other and a pair of side frames
54
,
55
are formed so as to be integral with the ends of the fixed bed
53
, respectively. Hydraulic cylinders
56
, which are positioned on the respective upper ends of the side frames
54
,
55
, raise or lower the ram
52
. Attached to the lower end of the ram
52
is an upper die (punch)
57
. Mounted on the upper face of the fixed bed
53
is a lower die (die)
58
. A sheet-like workpiece is interposed between these upper and lower dies
57
,
58
and pressed with these dies
57
,
58
by operating the hydraulic cylinders
56
, so that the workpiece can be bent to a desired angle.
When bending a workpiece with such a press brake
51
, there will be a difference between bend angles at the center and ends of the workpiece because of the deflections of the ram
52
and the fixed bed
53
. This problem is called a “boat form” phenomenon (i.e., a belly of the workpiece at its center). To avoid the boat form phenomenon, a deflection compensating mechanism is provided for the ram or bed in conventional press brakes. Representative examples of the deflection compensating mechanism are a crowning mechanism with wedges as disclosed in Japanese Patent Publication Gazette (KOKAI) No. 2-55622 (1990) and a crowning mechanism with hydraulic cylinders as disclosed in Japanese Patent Publication Gazette (KOKAI) No. 6-55218 (1994).
The present applicant proposes, in Japanese Patent Publication Gazette (KOKAI) No. 7-265957 (1995), a method for producing an accurate bend through one bending operation. According to this publication, the bend angle of a workpiece is measured at a point (provisional punch penetration point) just before a target bend angle during bending operation, and a final punch penetration point is obtained based on this measured value, springback data and a workpiece bend angle to punch penetration relationship. The punch is then driven to the final punch penetration point thereby performing high accuracy bending unaffected by variations in the material of the workpiece.
Japanese Utility Model Publication Gazette (KOKAI) No. 6-54416 (1994) discloses a deflection detector unit for detecting the deflection of the ram in a press brake having three or more drive shafts. According to a detected value of this deflection detector unit, each of the drive shafts is controlled.
In the crowning system with wedges such as disclosed in Japanese Patent Publication No. 2-55622, it is difficult in view of its structure to make alterations to crowning while the workpiece is under pressure. Although crowning can be altered during the application of pressure to the workpiece in the crowning system with hydraulic cylinders as disclosed in Japanese Patent Publication No. 6-55218, a punch penetration amount associated with crowning is not directly controlled but hydraulic pressure alone is controlled. Therefore, a punch penetration amount is obtained from open loop control, so that the punch cannot be correctly penetrated into the die.
In the case of the method of Japanese Patent Publication No. 7-
2
65957, the accuracy of bending is greatly dependent on whether or not the penetration of the punch can be corrected with high accuracy and it is therefore important to accurately control the amount of punch penetration. Even though the method is designed to calculate and feed back a punch penetration amount associated with crowning, it involves complicated control for the following reasons. First, since the shafts which apply pressure to the workpiece differ from the crowning system in terms of mechanism as well as the way of controlling, a corrective punch penetration amount obtained through bend angle measurement should be converted into different control amounts for the shafts and for the crowning system. Secondly, the delay between the control operations of the shafts and the crowning system must be taken into account, which makes the method more complicated.
In the method disclosed in Japanese Utility Model Publication No. 6-54416, angular errors such as a boat-formed belly due to mechanical deflection can be compensated in the course of bending operation. However, even if punch penetration can be correctly performed to produce a uniform desired bend angle throughout the length of the workpiece, it is impossible to overcome bend angle variations due to variations in the properties of the workpiece in its lengthwise direction. In other words, the problem of non-uniform bending resulting from gradual variations in the thickness and plastic properties of the workpiece in its lengthwise direction cannot be solved.
For effectively bending a short workpiece with a long press brake like the press brake
51
shown in
FIG. 22
, the so-called “step bending” method is usually taken, according to which two to four sets of bending dies are mounted so as to align in a lateral direction of the machine and the short workpiece is stepwise moved in the lateral direction to be bent. In the case of step bending, bending accuracy may deteriorate because of offset load, the inclination of the ram, or the deflections of the ram and the fixed bed. For achieving accurate bending, there are usually adopted the following methods. One is a method in which the inclination of the ram is avoided by driving the ram with two drive shafts disposed at the right and left, while compensating for the deflections of the ram and the bed with the hydraulic cylinders for use in crowning. Another bend angle correction method is to compensate for the inclination and deflections with a plurality of wedges disposed in a lateral direction of the machine.
When the ram driving method disclosed in Japanese Patent Publication Gazette No. 7-265957 is used in bending a short workpiece with a long machine driven by two drive shafts, there arises the following problem. Generally, the long machine is subjected to relatively greater deflection and has a greater distance between the position of the workpiece and a punch position corresponding to each drive shaft, compared to a short press brake. Therefore, when driving the ram with a value obtained by converting a corrective penetration amount on a workpiece position basis into a corrective amount on a punch position basis, mechanical deflection between these positions varies significantly owing to a change in load before and after punch penetration for compensation. As a result, accurate punch penetration for compensation cannot be performed.
This is further explained with reference to FIG.
23
. FIG.
23
(
a
) illustrates a case where a short workpiece W is bent with a long machine
61
and FIG.
23
(
b
) illustrates a case where the same workpiece W is bent with a short machine
71
. In these figures, solid line A represents the deflections of the beds
62
,
72
and solid line B the deflections of the rams
63
,
73
during bend angle measurement. Broken line A′ represents the deflections of the beds
62
,
72
and broken line B′ the deflections of the rams
63
,
73
after punch penetration for compensation. Generally, while heavy weight is objectionable for the rams (movable members)
63
,
73
, the beds (fixed members)
62
,
72
are designed to have greater rigidity. Therefore, the relationship described by b (the deflection of the ram)>a (the deflection of the bed) holds. Where increases in the deflection of the bed and in the deflection of the ram due to punch penetra

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