Patent Application
20090165525
The invention relates to a method for the production of stampings with an almost smooth cutting and enlarged functional surface, especially fine blanking and/or forming a workpiece out of a flat strip, wherein the flat strip at closing is clamped between an upper part consisting of a shearing punch, a pressure pad for the shearing punch, a V-shaped projection arranged on the pressure pad, and an ejector and a lower part consisting of a cutting die, an ejector and an inner form stamp, and in the cutting zone a cut is enforced by shearing at high compressive stress, wherein the V-shaped projection has been pressed into the flat strip beforehand and a compressive stress acts on the flat strip to be cut.
The invention further relates to a device for the production of stampings with an almost smooth and enlarged functional surface, especially fine blanking and/or forming a workpiece out of a flat strip, with a tool having two parts comprising at least a main shearing punch, a pressure pad for the shearing punch, a V-shaped projection arranged on the pressure pad, an ejector, and a cutting die, wherein the flat strip is clamped between pressure pad and cutting die and the V-shaped projection is pressed into the flat strip.
It is known that fine blanking at projecting contours of parts, for example toothings or corners, often leads to tears at the cutting surfaces. This phenomenon can be observed in even greater intensity the sharper an outer contour is defined, the thicker the material to be cut and the smaller is the ductility of the material. In most of the cases in fine blanking, the cutting surface acts as functional surface, wherefore tears can be the point of origin of a breaking failure of the part under load and therefore have to be avoided.
Smooth cutting surfaces at fine blanking are achieved, when in the cutting zone, by superposition of high hydrostatic pressure is enforced, a cut by shearing, i.e. a plastic deformation. The cutting surface occurs in the shearing zone, and thus, with regard to its quality, is influenced by the material properties (K. KONDO, Industrie-Anzeiger, annual volume 39, nr. 33, p. 547 to 550). At fine blanking, the V-shaped projection, before the cutting starts, is pressed into the material of the flat strip to be cut, and thus prevents continuous flow of the material during the cutting process.
Furthermore, typical features of fine blanking parts are the edge rollover and the cutting burr. Especially in corner areas, the rollover occurs and grows with decreasing corner radius and increasing sheet thickness. The depth of the rollover can be about 30% and the width of the rollover about 40% of the sheet thickness or more (see DIN 3345, Feinschneiden, Aug. 1980). Thus, the rollover depends on material thickness and quality, so that the possibility to control it is limited and often brings about a limited functionality of parts, for example due to a lack of sharp edges of the corners at toothed parts or the caused change in the functional length of the parts.
The stamping rollover thus reduces the functionality of parts and urges the manufacturer to use a thicker raw material.
A number of solutions are known for trying to produce fine and smooth shearing surfaces by cutting under pressure (DE 2 127 495 A1), re-cutting (CH 665 367 A5), shaving (DE 197 38 636 A1) or shifting of material during the cutting (EP 1 815 922 A1).
The known solutions according to CH 665 367 A5 and DE 197 38 636 A1 do not reduce the edge rollover, but largely rework the parts, so that on the one hand, significant costs for additional machining operations and tools are required and, on the other hand, a respective loss of material due to the necessity of using thicker materials occurs.
The known shearing press according to DE 2 127 495 A1 is operated at a higher hydrostatic pressure acting on the whole area of the workpiece subjected to plastic deformation. This high pressure, especially near the edges of the tool, is created by an upper jaw having a projection. This projection, so to speak, carries out the function of the V-shaped projection not existing according to DE 2 127 495 A1. But with this known method, in the first instance, the projecting stamping burr is avoided. Also with this known solution the rollover lastly is not avoided and material volume is shifted along the cutting line, which is accompanied by an increased risk of developing tears.
In the known solution according to EP 1 815 922 A1, the workpiece is machined in a single-step setup in at least two chronologically successive steps in different cutting directions, wherein during a first cutting process in vertical working direction is cut out a semi-finished product corresponding to the geometry of the workpiece with small rollover and finally cut during at least one further cutting process in the opposite working direction. The rollover of the first partial step with this shall be filled up again at least in the corner area. But with this known method, in the first instance, the projecting stamping burr is avoided. Also with this known solution, the rollover lastly is not avoided and material volume is shifted along the cutting line, which is accompanied by an increased risk of developing tears.
At this state of the art the invention has the task to avoid the inclination to tearing at cutting surfaces and the edge rollover at fine blanking parts by systematically controlling the state of stress in the cutting zone and at the same time to economically and effectively realize the fine blanking of thicker parts with high process security.
This task is solved by a method of the above mentioned kind which includes clamping a flat strip at closing between an upper part including a shearing punch, a pressure pad for the shearing punch, V-shaped projection arranged on the pressure pad and an ejector, and a lower part including a cutting die, an ejector and an inner form stamp. A cut is enforced in a cut in a cutting zone by shearing at high compressive stress, wherein the V-shaped projection has been pressed into the flat strip beforehand and a compressive stress acts on the flat strip to be cut. The method further includes adjusting a state of stress in the cutting zone to a position oriented compressive stress from the beginning of the cutting process to an end thereof by a movement slightly retarded with regard to the movement of the shearing punch additionally pressing in material in a direction almost perpendicular to the cutting direction by operation of a tool element acting with controlled force depending on a part geometry and a thickness of the workpiece parallel to the cutting line between shearing punch and cutting die.
In accordance with the invention, it is possible for the first time to economically apply the fine blanking technique for parts, for example toothed parts of greater thickness, without tears, and resultant sharp edges without finishing and material shifting along the cutting line.
This is achieved by adjusting the state of stress in the cutting zone to a position oriented compressive stress from the beginning of the cutting process to its end by a movement slightly retarded with regard to the movement of the shearing punch additionally pressing in material in a direction almost perpendicular to the cutting direction by means of a tool element acting with controlled force depending on the part geometry and the thickness of the workpiece parallel to the cutting line between shearing punch and cutting die.
It is of special advantage that the parameters for controlling the state of stress in the cutting zone, for example the volume of material to be additionally pressed in depending on the type of material, shape and geometry of the workpiece can be determined by a virtual forming simulation according to the results of which are then designed the tool elements for pressing additional material in the direction of the cutting zone.
It is of significant importance that with the method according to this invention it is possible to press additional material into the cutting zone in transversal direction, thereby significantly reducing the edge rollover at the part. By maintaining the state of stress in the cutting zone in the area where the pressure is applied, it is guaranteed that the cutting surfaces are smooth and free of tears. Further, the functional surfaces, because of the reduced edge rollover, are almost free of rollover.
Thus, the method according to this invention in high quality covers fine blanking in a wide range of dimensions, for example parts up to great thicknesses and complex part geometries, as for instance toothings of driving gears. Furthermore, with the method according to this invention it is also possible to fine blank lower steel qualities without running the risk of developing tears at the cutting surfaces.
The device according to this invention has a simple and sturdy structure.
Further advantages and details accrue from the following description with reference to the attached figures.
The invention in the following will be explained in more detail at the example of two embodiments.
The fine blanking tool has an upper part 1 and a lower part 2. The upper part 1 consists of a pressure pad 4 with a V-shaped projection 3, a shearing punch 5 guided in the pressure pad 4 and an ejector 6. The lower part 2 consists of a cutting die 7, an inner form or hollow punch 8 and an ejector 9. The flat strip 10 of alloyed stainless steel with a thickness of 12 mm, out of which, according to the method of this invention, shall be fabricated a fine blanking part 11, for example a connecting flange made of a steel strip, which, according to the state of the tool, is clamped between pressure pad 4 and cutting die 7. The V-shaped projection 3 has already penetrated the flat strip 10, whereby due to the applied force of the V-shaped projection the material is prevented from continue flow during cutting. The cutting die 7 and the inner form fall have cut about half the thickness of the fine blanking part 11.
In
In the clamped state of the flat strip 10 between cutting die 7 and pressure pad 4 with V-shaped projection 3 in the beginning of the cutting in the cutting zone a state of stress occurs characterized by a high compression stress. The deeper the shearing punch during cutting penetrates into the material, the greater the reduction of the state of compression stress in the cutting zone, so that at the end of the cutting operation, the compression stress changes into a tensile stress. This is the reason for the development of tears, especially at parts with complex geometry, for example toothings or corners, and greater thickness (R. A. Schmidt, “Umformen und Feinschneiden”, Carl Hanser Verlag München Wien, 2007).
The device according to this invention in embodiment 1 substantially corresponds to the structure of the device described according to
According to
The process parameters of the tool element 13, for example the force FW to be applied, the hydraulic pressure for creating the force FW or the value NE by which the movement is retarded with respect to the shearing punch 5, depending on type of material, shape and geometry of the workpiece are determined in a virtual forming simulation, wherein the material flow in the forming process is shown, extensions and reference stress values are analyzed to find out whether the forming can be realized and the tool elements can bear the loads. But the process parameters for determining the force FW of the active tool element 13 can be also determined at the real fine blanking part by individually measuring it. That requires a series of tests and analysis thereof to be able to respectively design the active tool element 13.
As the active tool element 13 can be used as a differential coining stamp penetrating into the workpiece with the coining side to control the state of stress, that is actively connected with a controllable hydraulic mechanism. But it is just as well possible to provide the shearing punch 5 with a shoulder or step to achieve the transversal shift of the material.
The method according to this invention is executed, so that at first, the flat strip 10 is clamped between upper and lower parts 1, respectively 2. From the beginning until the end of the cutting operation with the stud 12 and the active tool element 13 directed pressure is exerted in the area of the cutting zone by means of a controlled hydraulic mechanism. Due to this, a respective state of stress is created in the cutting zone, that during the whole cutting process, acts as compression stress.
This leads to an enhanced surface quality, especially also in the case of poor quality of the material. With the directed coining by means of the active tool element 13 a superposition of the cutting process with a transversal flow QF of part material into the cutting zone is achieved, whereby at the same time also, the stamp rollover in this area is significantly reduced. The lower V-shaped projection 3 supports the transversal flow QF of the material into the cutting zone.
In addition to the V-shaped projection 3 on the cutting die 7, a supporting platform 16 coming to lie in the free punch 17 is provided. The supporting platform 16 prevents the material from flowing into breadth. All other processes correspond with those of embodiment 1.
| Number | Date | Country | Kind |
|---|---|---|---|
| 07018892.5 | Sep 2007 | EP | regional |
