Sheet material bending angle detector

Abstract: A bending angle detector for a sheet material which comprises first and second links (11, 12), each being disposed in line symmetry with respect to the other and being fitted pivotally at one of the ends thereof to a detector main body (21); and third and fourth links (13, 14), each being disposed in similar line symmetry with respect to the same line of symmetry, and being fitted pivotally at one of the ends thereof to the other end of each of the first and second links (11, 12); wherein the other end of each of the third and fourth links are supported by the detector main body (21) in such a manner as to be capable of moving along the line of symmetry or along parallel lines existing on both sides of the line of symmetry; the first and second links (11, 12) are brought into contact with the inner surfaces of the sheet material (5) defining an angle between them; and the position or displacement of the other end of each of the third and fourth links (13, 14) is measured.

Description: This invention relates to a sheet material bending angle detector and to an operation method of a press machine using the bending angle detector. In a press machine such as a press brake, when an upper die pushes a sheet material into a rectangular groove of a lower die, bends it and then releases the sheet material, a phenomenon referred to as "spring-back" occurs, in which the sheet material recovers slightly when released due to flexibility of the sheet material. Therefore, after the bending operation is completed, the bent sheet material is withdrawn and its bending angle is measured so as to judge acceptance of the finish angle. To eliminate this time consuming procedure, a proposal has been made (JP-A-60-247415) wherein a sensor for measuring the distance to an outer surface of the bent sheet or to a point below an apex is disposed in the lower die, and a bending angle is computed from this distance. DE-B-2044199, which is considered to be the nearest prior art discloses a similar arrangement having both mechanical and photoelectric measurement sensors. Since dedicated sensors must be provided individually to different lower dies, these arrangements are expensive and moreover, since measurement is indirect measuring accuracy is not very high. Another proposal has been made (JP-A-57-202928) wherein a gauge is disposed by the side of the lower die, and a bending angle is computed from a horizontal distance and a vertical distance from groove edges at a specific point of the portion of the sheet material protruding diagonally upwards from a groove and forming one of the sides of the angle. However, if the thickness of the sheet material is not constant or in other words, if the inner and outer surfaces of the sheet material are not precisely parallel and moreover, if the attitude of the sheet material inside the groove changes, measurement can no longer be made accurately. Furthermore, measurement cannot be made at all for those sheet materials which do not protrude from the groove up to the gauge. The present invention seeks to provide a bending angle detector which is relatively simple in construction, and can directly measure a bending angle of a bent sheet material irrespective of its attitude. Measurement and re-bending can be repeated automatically until a desired bending angle is obtained, by the use of this bending angle detector. According to a first aspect of the present invention there is provided a bending angle detector for a sheet material, comprising: first and second links, each being disposed in line symmetry with respect to the other; and third and fourth links, each being disposed in similar line symmetry with respect to the same line of symmetry, and each being fitted pivotally at one of the ends thereof to an end of a respective one of said first and second links, each of the first and second links being fitted pivotally at the other end thereof to a detector main body, the other end of each of said third and fourth links being supported by said detector main body in such a manner as to be capable of moving along said line of symmetry or along parallel lines existing on both sides of said line of symmetry, whereby, in use, the first and second links are brought into contact with the inner surfaces of said sheet material, defining an angle between them means being provided for measuring the displacement of the other ends of said third and fourth links, and means being provided for calculating the inclined angle between the contact surfaces of the first and second links in accordance with the said measured displacement. According to a second aspect of the present invention, there is provided a bending angle detector for a sheet material, comprising the features of claim 2. To operate a press machine having a movable male die or a female die and capable of changing the depth of a groove of the female die by the use of such a bending angle detector, there is provided an operation method comprising the combination of features according to claim 3. The bending angle is measured by bringing the first and second links of the bending angle detector into contact with the inner surfaces of both sides of the sheet material defining the angle between them. Therefore, measurement can be made without being affected by the attitude of the sheet material and by the change of the thickness of the sheet material, and moreover, without being affected by the dimension of the sides of the sheet material defining the angle between them. Moreover, this measurement can be carried out under the state where the load applied to the sheet material is removed completely, and hence, without any influences of spring-back. The bending operation is first carried out at an angle corresponding to insufficient bending of the sheet material, and measurement of the bending angle, the correction of the stroke of the movable die or the groove depth, and the bending operation, are automatically repeated for the sheet material in the press machine. In this way, it is possible to rapidly achieve a precise bending angle. It is also possible to use the bending angle detector according to the second aspect of the present invention in the above described operating method. For a better understanding of the present invention and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:- Fig. 1 is front and side views of a press brake according to an embodiment of the present invention; Fig. 2 is a structural view useful for explaining the principle of a bending angle detector; Fig. 3 is front and partially cut-away side views of the bending angle detector; Fig. 4 is a front view showing the bending angle detector when it is in use; Fig. 5 is a front view of another bending angle detector; Fig. 6 is a block diagram of a bending angle control system; Fig. 7 is a block diagram of a data processor in Fig. 6; and Fig. 8 is a flowchart of bending of a sheet material carried out by the bending angle detector. Fig. 1 depicts a press brake equipped with a bending angle detector for a sheet material according to the present invention. A lower die 2 consisting of a plurality of split die portions each having a rectangular groove of a variable depth is mounted onto a bed 1. A ram 3 is allowed to move up and down relative to this bed 1, and an upper die 4 is fixed to the lower end of this ram 3. A sheet material 5 to be bent is placed on a link mechanism support device 6 positioned on the front side of the lower die 2, is supported on the support device 6 by suitable means, whenever necessary, and is pushed until it strikes a gauge 7 which is at the back of the lower die 2 and decides the dimension of one of the sides of a bending angle. The ram 3 is lowered, and the upper die 4 pushes the sheet material 5 into the rectangular groove of the lower die 2 and bends it. As a result,the front side of the sheet material 5 is lifted up from the support device 6 as indicated by two-dot-chain line. The upper die 4 is again moved up and returned by the ram 3, and a bending angle detector is then fitted into the angle formed within the bend of the sheet material 5 by a feed mechanism 8 fitted to the ram 3. The theoretical construction of the bending angle detector will now be explained with reference to Fig. 2. It will be hereby assumed, as shown in Fig. 2(a), that a fixed node O at the lower end of a 4-node link mechanism consisting of four links 11, 12, 13 and 14 pivotally connected to one another at nodes O, P, Q and R is positioned inside the apex of an angle described by the bent sheet material 5 (whereby the node O is shown a little spaced apart from the apex for ease of understanding), and that the opposed node P is movable on a Y - Y axis which is a bisector of this angle. When this link mechanism extending elongatedly along the Y - Y axis is contracted along the Y - Y axis and the links 11 and 12 come into contact with both sides of the angle described by the bent sheet 5, the angle α between these links 11 and 12 is equal to the bending angle of the sheet 5. The change of the angle defined by the links 11 and 12 is related to the displacement S of the node P. Therefore, the bending angle of the sheet material can be determined by measuring this displacement S. As shown in Fig. 2(b), the upper and lower nodes P and O of the link can be divided into two nodes P', P'' and O', O'' which are symmetrical with respect to the Y - Y axis. Furthermore, as shown in Fig. 2(c), it is also possible to employ a 3-node link mechanism having only two links 12 and 14 positioned on one of the sides of the Y - Y axis. - It will be assumed in this case that the nodes O and P existing at the other end of each of the two links 12 and 14 pivotally connected to each other through the node R exist on the Y - Y axis parallel to one of the sides of the angle defined by the bent sheet material 5, that the node O is fixed and positioned inside the apex of the angle defined by the sheet material 5 (the node O being shown at a position spaced apart a little from the apex for ease of understanding), and that the node P is movable on the Y - Y axis. When the link 12 is moved in parallel with the other side of the angle, the angle a defined between the link 12 and the Y - Y axis is equal to the bending angle of the sheet material 5. The displacement S of the node P from the initial position is related with the change of the angle β. Therefore, the bending angle of the sheet material 5 can be determined from the angle β by measuring this displacement S. Fig. 3 shows a practical construction of the bending angle detector 20 corresponding to Fig. 2(b). The links 11 and 12 of the link mechanism are pivotally connected to a horizontal arm 21a of a detector main body 21 by pins functioning as the fixed nodes O', O'' at the lower end. The upper end each of the links 13 and 14, which are pivotally connected to these links 11 and 12 by pins as the nodes Q and R, is fitted to a slider 22 by pins as the nodes P' and P''. This slider 22 is so guided by a guide plate 21b fixed to the detector main body 21 as to be capable of linearly moving up and down. A rack 23 is fitted to the side of this slider 22, and a pinion 24 engaging with the rack 23 is transmission-coupled with a rotary encoder 27 through gears 25 and 26 meshing with the pinion 24. Obviously, the links 11, 12 and 13, 14 can be fixed to the horizontal arm 21a and to the slider 22 by using one each of the pins O and P in place of the two pins O', O'' and P', P'', as in Fig. 2(a). Contact sensors for confirming sufficient contact of the links 11 and 12 with the surfaces of the bent sheet 5 defining the angle between them can be provided to these links 11 and 12. The detector main body 21 is supported by a fitting member 30 through attitude regulation devices 28 and 29 so that the links 11 and 12 can come into reliable contact with the sheet surfaces in conformity with the attitude of the sheet material 5. The attitude regulation device 28 comprises a belt plate 28b accommodated in a vertical groove 28a at the lower end of the main body 21 and having a width a little smaller than the groove width, a pin 28c loosely fitting to the fitting member 30 and supporting the belt plate 28b so as to support the belt plate 28b swingable a bit inside the groove 28a, and a pin 28d fitting loosely to the belt plate 28b and embedded in the main body 21. The attitude regulation device 29 comprises a pin 29b so fixed to the main body 21 as to penetrate through the fitting member 30 with a ring-like gap 29a therebetween. When the upper die 4 is lowered towards the lower die 2, the sheet material 5 is bent inside the rectangular groove of the lower die 2 as is well known in the art, and the upper die 4 is again moved up. Next, the detector 20 is inserted (Fig. 4) into the angle within the bending range of the sheet material 5 through the support mechanism 8 (Fig. 1), and is stopped at a predetermined position. Then, the slider 22 is pushed down by a spring 31 until the links 11 and 12 come into reliable contact with the surfaces of the sheet material 5 defining the angle between them. Displacement of the nodes P' and P'' and hence, the displacement of the slider 22, is amplified by the rack 23, the pinion 24 and the gears 25 and 26, is converted to a rotating quantity by them, and is thereafter converted to electric signals such as pulses by the rotary encoder 27. The detector 20 shown in Fig. 5 is suitable for detecting the bending angle of the sheet material 5 to be bent into an L shape, and this detector corresponds to Fig. 2(c). Here, the linear displacement of the slider 22 is converted to electrical signals by a linear encoder 27 or by a differential transformer. When the differential transformer, is employed, its analog output signal is converted to a pulse signal by A/D conversion. Next, the bending operation of the sheet material using such a bending angle detector will be explained. Referring to Fig. 1, a control panel 40 is disposed by the side of the press brake, and includes a display 41, a data processor 42, a data input device 43 and a controller 44. Bending of the sheet material 5 is carried out in accordance with the principle of 3-point bending, as described, for example, in JP-U-62-13696. In other words, the bending angle of the sheet material is determined by the gap between both groove edges of the rectangular groove 2a of the lower die 2 and by the groove depth, provided that no spring-back exists. The depth of the rectangular groove 2a can be adjusted by a plurality of pan head pins 2b which are movably guided inside a plurality of vertical holes communicating with the rectangular groove 2a and which define the groove bottom. The bending angle of the sheet material is calculated and stored (45) inside the data processor 42 from the signal of the encoder 27 of the bending angle detector, and is compared (47) with a target value (46), and the groove depth of the lower die and hence, the height of the pan head pins 2b, is adjusted by the controller 44 in accordance with the difference obtained by the comparison. Fig. 7 shows the data processor 42. The output signal of the encoder 27 of the bending angle detector is counted by a counter 51, is processed by a CPU 53, an ROM 54 and an RAM 55 connected to one another by a data bus 52, and is supplied to the controller 44 and to the display 41 through interfaces 56 and 57, respectively. A method of bending the sheet material using the bending angle detector according to the present invention will be explained with reference to Fig. 8. First of all, it will be assumed hereby that the sheet material 5 is to be bent 90°. A target value of 90° cannot be obtained immediately due to the dimension and materials of the sheet material, spring-back, and so forth. Therefore, the groove depth of the lower die 2 corresponding to a bending angle 90° + 1° is first set in consideration of over-bending (Step 105). The sheet material 5 is bent by lowering the upper die 4 by the ram 3 (Step 106). While the sheet material 5 is kept held on the support mechanism 6, the upper mold 4 is moved up. The detector 20 is inserted by the feed mechanism 8 into the range of the angle of the bent sheet material 5, that is, the product, to be measured (Step 107) and the bending angle is measured. The displacement of the node P or P' and P'' of the link mechanism is transferred to the data processor 42 (Step 108). The bending angle is computed, is supplied to the controller 44 and is thereafter stored in the memory (Step 109). In the mean time, the detector 20 is removed by the feed mechanism 8 from the product. The bending operation is completed if the measured bending angle (actual value) is within the positive allowable limit of the difference from the target value 90°, such as 30' (Step 110) and furthermore, if it is within a negative allowable limit such as -30' (Step 111). A product having the desired bending angle can be obtained by bending the sheet material with the groove depth of the lower die 2 at that time. If the measured bending angle does not fall within the positive allowable limit of the error from the target value, a correction value of the groove depth is calculated (Step 112). The correction value is set to the controller 44 and the groove depth is automatically set on the basis of this correction value (Step 105'). The procedures 106 to 111 are repeated until the desired bending angle can be obtained. If the measured bending angle does not fall within the negative allowable limit of the error from the target value, this means over-bending. Accordingly, measurement is stopped (Step 114), and the sheet material is withdrawn.

Claims: A bending angle detector for a sheet material, comprising: first and second links (11, 12), each being disposed in line symmetry with respect to the other; and third and fourth links (13, 14), each being disposed in similar line symmetry with respect to the same line of symmetry, and each being fitted pivotally at one of the ends thereof to an end of a respective one of said first and second links (11, 12), characterised in that: each of the first and second links are fitted pivotally at one of the ends thereof to a detector main body (21); the other end of each of said third and fourth links (13, 14) is supported by said detector main body (21) in such a manner as to be capable of moving along said line of symmetry or along parallel links existing on both sides of said line of symmetry; in use, the first and second links (11, 12) are brought into contact with the inner surfaces of said sheet material (5) defining an angle between them; means are provided for measuring the displacement of the other ends of said third and fourth links; and means are provided for calculating the inclined angle between the contact surfaces of the first and second links in accordance with the said measured displacement. A bending angle detector for a sheet material, comprising: a first link (12); and a second link (14) pivotally connected at one of the ends thereof to an end of said first link (12), characterised in that: the first link (12) is pivotally fitted at the other of the ends thereof to a detector main body (21); the other end of said second link (14) is supported by said detector main body (21) in such a manner as to be capable of moving linearly along a guide line connecting the other end of said first link (12) and the other end of said second link (14); in use, said first link (12) and a surface of said detecting main body (21) which is parallel with said guide line are brought into contact with the inner surfaces of sheet material, defining an angle between them; means are provided for measuring the displacement of the other end of said second link; and means are provided for calculating the inclined angle between the contact surface of the first link and the said surface of the detector main body in accordance with the said measured displacement. An operation method of a press machine for bending a sheet material, said press machine having mutually approachable and separable male and female dies and capable of regulating a groove depth of said female die, said method comprising: effective bending of said sheet material to an angle having a certain specific positive error outside an allowable limit of a target value of a bending angle; inserting a bending angle detector according to claim 1 or 2 movably supported by said press machine into the angle defined by said sheet material to measure the bending angle, whilst keeping the material in the bent state; correcting a stroke of said movable die or the depth of said female die groove and effecting bending when the measured bending angle is outside the allowable limit of the positive error; and repeating measurement of the bending angle, the correction of the stroke or the groove depth and the bending operation until a bending angle within the allowable limit of the positive error is obtained.

Inventors: HONGO TOSHIO, HONGO, TOSHIO
Applicants: MARU KIKAI KOGYO KABUSHIKI KAI, MARU KIKAI KOGYO KABUSHIKI KAISHA
Application year: 1993
Main classification: B21D5
IPC codes: B21D5, G01B5