The present invention relates to a press die for bending and rounding U-shaped blanks. Longitudinally seam-welded pipes of medium diameter or larger are usually made by uniformly rounding a blank over its entire length. Usually, a three-roller bending or folding machine rounds sheet stock, used as blank, to provide a split tube; the resulting straight joint between the abutting sheet edges are then welded, resulting in a correspondingly straight seam. Alternatively, sheet stock is first bent to assume U-shaped profile, and in a second step, a press die machine is used as tool to presswork this blank into a split tube; the resulting joint is welded as before.
Considering the second alternative more in particular, the press die is usually constructed to include two or three die members as working components. These members are usually solid cast or welded tool elements. The die members are to be as rigid as possible, so that large forces can be taken up during blank bending and rounding, without incurring any significant deformation of the die itself. Even though, breaking of the die is one point to be considered, more importantly, variations in the resulting pipe diameter, at differing pipe wall thickness are to be avoided.
Press dies of known variety usually have working surface contour (in section profile) that deviates from a circle as such tool configuration is deemed necessary to obtain a pipe with circular cross section. Nevertheless, it has been deemed unavoidable that the pipe cross section as produced deviates always to some extent from circular contour. Thus, pipes so made are usually somewhat oval in cross section, the long axis (transverse to the pipe axis) being either vertically or horizontally oriented as the pipe leaves the die; either case may lead to difficulties upon further working and processing of the pipe.
It is an object of the present invention to provide a press die for working blanks into pipes of circular cross section. The invention is based on the discovery that achieving the objective is possible neither by means of conventional dies, nor by merely modifying the construction of known dies; rather, the basic construction principle must be subjected to a new approach, considering particularly tension that may be set up within the die members during pressworking.
The invention maintaines the principle of a plural die member construction for bending and rounding a U-shaped blank into a split tube with a straight joint for welding. The plural die member arrangement may particularly include a set of upper and lower dies to work on an axial section of a blank, and plural sets may be arranged along the axis of the pipe to be made for rounding a correspondingly long sheet blank.
In accordance with the preferred embodiment of the invention, each die member has configuration of a tunnel arch with an inner contour corresponding to the outer contour of the pipe to be made (e.g., circular) and with abutments that are extended rearwardly, approximately tangentially, in side members into which act the pressworking forces. The tunnel arch is provided by means of a correspondingly bent plate, establishing, e.g., one half of a cylinder. The plate may have uniform thickness. Unlike known dies, this novel press die member has only limited bending resistance and flexural stiffness, but still, it does not require re-enforcing ribs or other local stiffening means.
The effect of the novel press die is to be seen in that its inner, e.g., circular cross section, can readily be made (machined or otherwise) and still it provides for uniform rounding of the blank, so that a pipe with circular cross section is obtained after welding of the joint that resulted from pressworking. A press die constructed in accordance with the present invention is lighter, and just because stiffening means are lacking breakage is more easily avoidable while press force action and reaction relation is subject to control to a greater extent.
The sides extend preferably rearwardly beyond the arch apex. Tension means may be provided to act on and between these side members, essentially transversely to the press forces. Moreover, these tension means act as far away from the tunnel arch abutments as possible so as to more fully control the contour of the working surface and to correct, e.g., deviations from circularity.
While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention, the objects and features of the invention and further objects, features and advantages thereof will be better understood from the following description taken in connection with the accompanying drawings in which:
The FIGURE illustrates a cross section through a die in accordance with the preferred embodiment of the invention.
Proceeding now to the detailed description of the drawing, the die illustrated, has basically similar upper and lower die members, so that only one thereof needs to be described in greater detail. The upper die member, therefor, has a plate 1 that has been bent into a tunnel arch. The apex (line) of the tunnel is at 2, and the plate thickness increases therefrom towards the sides only insignificantly but merges in abutments 3 on each side. Actually, arching plate 1 may even have uniform thickness.
The working surface 4 of the upper die member has, thus, semicylindrical contour with circular section profile as shown. The arch-shaped die member has finite, relatively low bending strength or flexural stiffness. However, even during pressworking, the circularity of the inner contour is retained as the arch is subjected to stress uniformly along its periphery.
Longitudinal walls or side members 5 and 6 extend rearwardly from the abutments 3, and somewhat tangentially thereto in relation to the tunnel arch. The sides 5 and 6 extend beyond the tunnel apex 2. Flanges 7 and 8 are respectively provided along the ends of side members 5 and 6 to be connected to the press force providing means. Pressworking forces 9 and 10 are applied to these flanges 7 and 8 accordingly.
Depending upon the length of the die (axially, transversely to the plane of the drawing) tension means may be provided where indicated by dashed line 11 (such as tie rods, turnbuckles or the like) so as to tension bias the die member for correcting the contour thereof. This control of contour, particularly of working surface 4, may become necessary in case the die member underwent some resilient relaxation upon release from the clamping arrangement in which the particular die member was held during making and shaping thereof initially. Also, controlled tension bias of the die member may be needed in case the tangential force adjustment is not sufficiently accurate. It should be noted, that upon application of pressworking forces, no inwardly directed component should arise. As shown in the drawing, the slightly outwardly directed slant of sides 6 and 7 inhibits tendency of the die member to undergo an inwardly directed squeeze action.
During working, the U-shaped blank 12 sits in the lower die member, and as the upper die member is lowered (as is usually the case), the legs of the U are gradually bent to form a split tube with a joint at 13, while upon further lowering the tube is deformed to assume circular cross section. The press forces act on and in line with side walls 5 and 6 as well as with abutments 3 while the de-forming forces are transmitted to the blank near the apex of the tunnel arch. The external press forces themselves do not act directly on the apex 2, and the tunnel arch is not deformed as to its contour. In order to bend and to round a long sheet blank, plural sets,each having an upper and a lower die member of the type described, may be provided along the axis of the pipe to be made.
The invention is not limited to the embodiments described above but all changes and modifications thereof not constituting departures from the spirit and scope of the invention are intended to be included.