The present invention relates to a sheet metal forming apparatus and method for preventing surface defects in a perimeter area around a local depression in a larger forming area. Such an apparatus is defined in the preamble of claim 1 (see for example JP-A-58 157 531).
Current methods of sheet metal forming may result in minor surface defects around local depressions or abrupt form changes. These defects are particularly undesirable in so called surface critical areas, which may for example be found on automobile body panels. An example of such a surface critical area is a door handle depression in an outer car door panel. Any surface defects in this area will lead to either a rejection of the panel or extensive and costly surface refinishing work.
The surface defects occur as a result of substantial local variations in tension around the depression as the metal is formed, which in current methods of production often leads to several permanent defects, in the form of uneven surfaces, left after the sheet metal springs back as the formed panel is released and ejected from the forming apparatus.
The above mentioned problem is solved by the invention providing a sheet metal forming apparatus for preventing surface defects in a perimeter area around a local depression in a larger forming area with the features of claim 1.
In an advantageous embodiment of the invention, the forming of said local depression is adapted to be completed when the second main forming body is at a distance not exceeding 10 mm from its final abutment position.
In a well adapted embodiment, the forming of said local depression is adapted to be completed when the second main forming body is at a distance not exceeding 4 mm from its final abutment position.
In a suitable embodiment, the first main forming body is a stationary die and the second main forming body is a movable punch.
Advantageously, the sub-forming member is adapted to remain in its retracted position during ejection of a finished sheet metal part.
In a well functioning embodiment, the protrusion distance of the sub-forming member in its protruding position is adjustable by means of an exchangeable spacer element seated between the first main forming body and an abutment flange provided on the sub-forming member.
In an advantageous embodiment, the spring element is a gas spring assembly. However, it may alternatively be of another suitable kind, such as a coil spring assembly.
The invention also includes a sheet metal forming method for preventing surface defects in a perimeter area around a local depression in a larger forming area. The method includes the use of a forming apparatus comprising a first main forming body and a second main forming body, at least one of said main forming bodies being movable towards the other for forming a sheet metal blank there-between into a desired shape, whereby the method comprises
Further features and advantages of the invention will be described in the detailed description of embodiments below.
The invention will now be described in greater detail by way of example only and with reference to the attached drawings, in which
In Fig. 1, part of a car door panel 1 is shown as an example of surface defects 2 around a door handle depression 4, as produced with current, prior art production methods. The surface defects 2 are drawn with dashed lines in Fig. 1 and - as described initially - frequently occurs as a result of substantial local variations in tension around the depression as the metal is formed. More particularly, the undesired surface defects 2 are in the form of uneven surfaces which are left after the sheet metal springs back as the formed car door panel 1 is released and ejected from the prior art forming apparatus (not shown). It is the general object of the present invention to prevent such surface defects 2 from occurring in a perimeter area 6 around a local depression 4 - in this case the door handle depression - in a larger forming area 1 (i.e. in this case the car door panel). The perimeter area 6 is loosely shown in Fig. 1 with bold face dash-dotted lines.
Fig. 2 shows a schematic cross-sectional view of a sheet metal forming apparatus 8 according to an exemplifying embodiment of the present invention. The view in Fig. 2 shows the apparatus 8 in an initial position, wherein a sheet metal blank 10 to be formed has been inserted between a first main forming body 12 and a second main forming body 14. In the widest meaning, at least one of said main forming bodies 12, 14 is movably arranged in a known manner by means of hydraulic or pneumatic actuators (not shown) towards the other for forming the sheet metal blank 10 there-between into a desired shape. However, in the shown exemplifying embodiment, the first main forming body 12 is a stationary die and the second main forming body 14 is a movable punch positioned directly above the die. Thus, in this case, the forming movement is vertical, which is generally the most common arrangement due to the weight and size of the forming apparatus 8. In the automotive industry, the most common material for car body panels is still steel sheet material, but it should be understood that other metals, such as aluminium may alternatively be used as the sheet metal blank 10. The thickness of the sheet metal blank 10 is preferably uniform and commonly ranging between 0.5 - 1.5 mm, depending on application.
The skilled man in the art will recognise that the shown orientation and labelling of constituent parts, such as for example the first main forming body 12 and the second main forming body 14, are not used here in a limiting sense. Alternatively - where appropriate - the first main forming body 12 may thus be referred to as a lower die, whilst the second main forming body 14 may be referred to as an upper punch.
In a known manner, the sheet metal blank 10 is held in place during the forming by means of a blank holder 16 which is retractable after the forming of the sheet metal blank 10 is finished. Since this is a well-established feature, it will not be discussed in further detail here.
According to the invention, a separate, movable sub-forming member 16 is provided in the first main forming body 12 at a position corresponding to the local depression 4. In Figs. 2-4, the local depression 4 constitutes a door handle depression in a car door panel 1, such as the one shown in Fig. 1. It should be noted that the apparatus and method is specially suited for application on so called surface critical areas, such as automobile body panels (in this case) or surfaces on other products where high demands on an even finished surface apply.
For forming the local depression 4, a form recess 20 is formed in the second main forming body 14 (i.e. the punch in this case), which recess 20 is adapted for engagement with a corresponding projection 22 in a form surface 24 on the movable sub-forming member 18.
The sub-forming member 18 is movably arranged between a protruding position, as shown in Figs 2 and 3, and a retracted position, as shown in Fig. 4. The significance of these two positions will be made readily apparent below.
In the protruding position, the form surface 24 on the sub-forming member 18 protrudes relative to a surrounding form surface 26 of the first main forming body 12, i.e. upwards in Fig. 2.
In the retracted position, the form surface 24 on the sub-forming member 18 is level with the surrounding form surface 26.
A further feature of the invention is that the sub-forming member 18 is biased towards its protruding position by means of a spring element 28. The spring element will be further described below.
In a first phase according to the invention, the forming of said local depression 4 is completed - i.e. fully formed - before the second main forming body 14 reaches a final abutment position and before the sub-forming member 18 reaches its retracted position. Then, in a second phase, the sheet metal blank 10 within said perimeter area 6 is stretched radially outwards around the local depression 4 due to continued travel of the sub-forming member 18 immediately before the second main forming body 14 reaches its final abutment position and before the sub-forming member 18 reaches its retracted position. The term immediately here corresponds to the last few millimeters - or thereabout - of the sub-forming member's 18 downward travel.
The sheet metal forming method for use with the above-described apparatus 8, thus involves:
In Fig. 3, the forming of the local depression 4 has just been completed - thus marking the end of the first phase - whereby the sub-forming member 18 begins to move downwards whilst the spring element 28 is compressed - urged by the second main forming body 14.
In Fig. 4, the second main forming body 14 has reached its final abutment position and the sub-forming member 18 has reached its retracted position, in which the spring element 28 is now fully compressed. Thus, the second phase, which may also be referred to as a stretching phase, is now completed. The sheet metal blank 10 within said perimeter area 6 is - as described above - stretched radially outwards around the local depression 4, resulting in a more even tension around the local depression 4, in comparison with prior art forming apparatuses. This even tension prevents the undesired forming of surface defects 2 during spring back in the perimeter area 6 around the local depression 4 in the larger forming area 1 (In this case the car door panel, as illustrated in Fig. 1).
Suitably, the forming of said local depression 4 is completed when the second main forming body 14 is at a distance not exceeding 10 mm from its final abutment position. Succesful tests have been performed at distances ranging from 1-10 mm. More preferably, said distance does not exceed 4 mm from its final abutment position. In an advantageous embodiment in the shown door handle depression application, this distance is preferably in close proximity of 3 mm.
In the shown embodiment, the spring element 28 is a gas spring assembly, known per se. However, it should be noted that the spring element 28 may alternatively be a coil spring assembly or an assembly comprising an elastically compressible cushion element (not shown). The spring element 28 comprises a cylinder housing 30 and a piston 32, which piston is arranged in the cylinder housing 30 for movement in the axial direction of the same, i.e. vertically in the shown exemplifying Figs. 2-4. The cylinder housing 30 is fixedly attached to the first main forming body 12, whilst a distal end 34 of the piston is attached to a rear face 36 of the sub-forming member 18. Preferably, the spring element 28 is of a controllable type, so that the sub-forming member 18 is adapted to remain in its retracted position (as seen in Fig. 4) during ejection of a finished sheet metal part. This is achieved by locking the piston 32 in a bottom position and controlling the return stroke of the piston 32. The spring element 28 is controlled by a control valve unit 44, which is connected to the cylinder housing 30 via gas & air control conduits 46. The control valve unit 44 is further connected sources of pressurized air and gas (not shown) via gas & air supply conduits 48. For the sake of clarity, the control valve unit 44, the gas & air control conduits 46 and the gas & air supply conduits 48 are simplified in Figs. 2-4, and are drawn schematically.
In one embodiment of the invention, the spring element 28 biases the sub-forming member 18 with a force (upwardly directed in Figs. 2-4) which is sufficient to hold the sub-forming member 18 in a fixed axial position during the first phase until the forming of the local depression 4 is completed and which is then overcome by the second main forming body 14, thus initiating continued travel in the second phase when the stretching action takes place. In an alternative embodiment, the biasing force is slightly lower, so as to allow continuous downward travel of the sub-forming member 18 also during the first phase, i.e. during the forming of the local depression 4.
With reference again to Fig. 2, the sub-forming member 18 is axially movable within a bore 38 in the first main forming body 12. In the shown embodiment, the protrusion distance D of the sub-forming member 18 in its protruding position may be adjusted by means of an exchangeable annular spacer element 40 seated between the first main forming body 12 and an abutment flange 42 provided on the sub-forming member 18. The spacer element 40 is preferably made of steel, but may also be made of other suitable materials. By the term protrusion distance, D, is meant the distance between the form surface on the sub-forming member 18 and the surrounding form surface 26 of the first main forming body 12, in the axial direction of the sub-forming member 18. If the bias-force is adjusted so as to allow continuous downward travel of the sub-forming member 18 also during the first phase, i.e. during the forming of the local depression 4, the sub-forming member 18 correspondingly has an initial protrusion distance which will accommodate for sufficient axial travel both for the first phase and the second phase of operation..
Finally, it is to be understood that the invention is by no means limited to the embodiments described above, and may be varied freely within the scope of the appended claims. For example, the first main forming body may be provided with several separate sub-forming members 18 in conjunction with respective local depressions 4 in one large forming area 1.