Method and device for incrementally forming a sheet

Abstract

The invention relates to a method for forming a sheet metal part (3), wherein the sheet metal part (3) has a top surface (3a) and a bottom surface (3b) and is clamped circumferentially, characterized in that a structure is introduced into the sheet metal part (3) by means of sequential processing on both sides from the direction of the top surface (3a) and the bottom surface (3b) using a forming tool. The device comprises a clamping device for clamping a sheet metal part (3) circumferentially and a forming tool for sequential processing of the sheet metal part (3) on both sides from the direction of a top surface (3a) and a bottom surface (3b).

Description

[0001] The invention relates to a method and a device for incrementally forming a sheet metal part according to the preamble of the first and tenth claims.

[0002] It is known to create shaped elements in a sheet metal by incremental forming.

[0003] In this process, the shaped elements are created in the sheet metal by movements of a mandrel, in which the mandrel penetrates the sheet metal plane and follows a predetermined path of movement ("Maschinenmarkt, 10.06.2016: Incremental sheet metal forming, Incremental forming made economical, Author / Editor: Roman Schmitz, Gerhard Hirt, Stéphane Itasse").

[0004] During metal forming, the component material is stretched, exhibiting both elastic and plastic strain. After the force required for forming is removed, the elastic strain is released, generally leading to springback of the component. In the case of uniaxial bending of a sheet metal strip, springback can be observed by the difference in bending angles during and after the removal of the bending force.

[0005] With high stiffness of the formed component, i.e., with high resistance to elastic deformation, the elastic strain components cannot be completely dissipated after the forming force is removed. As a result, in addition to partial springback, local residual stresses arise within the component. Furthermore, residual stresses develop in the immediate vicinity of plastically stretched or compressed material regions. If the component is formed on a clamping device that prevents elastic deformation after forming, the internal stresses in the component are greater than the residual stresses remaining after unclamping due to the structural stiffness.

[0006] Particularly when forming flat features (feature depth approximately 0.5 to 1.5 times the sheet thickness) into a flat, thin sheet (e.g., thin sheet metal with a thickness < 3 mm), locally varying springback causes component distortion as soon as the sheet is no longer held by the forming tool or removed from the clamping device. These flat structures include, among others, stiffening ribs and forming panels in structural components (e.g., drainboards in stainless steel sinks), customized design sheets in the advertising, construction, and decoration sectors (e.g., wall cladding such as facade elements), and flow channels in the energy and drive technology sectors (e.g., metallic bipolar plates, heat exchangers). These structures are manufactured using forming processes such as embossing or incremental sheet forming (IBU).While hollow stamping typically produces the entire structure in a single forming step, incremental sheet metal forming proceeds locally and progressively until the entire structure is formed. Incremental forming utilizes, among other things, sliding mandrels and rolling balls (process designation...). Single Point Incremental Forming, SPIF ) or roles (procedure name) Roll beads ) used.

[0007] Furthermore, the residual stresses induced in sheet metal can be used for targeted forming. The intention is to utilize the resulting geometric distortion to create sheet metal curvature. Forming based on mechanically induced residual stresses is achieved using the incremental processes of shot peening and surface hammering. Laser and plasma beam forming are other well-known methods for forming sheet metal using residual stresses, where these stresses are thermally induced.

[0008] According to patent specification DD 71 089 D1, a method for applying a decorative surface structure to thin sheet metal involves pressing solids (steel balls) into a structured sheet metal over a flexible substrate. According to the invention, a magnetic anvil can be used to fix the solids in the desired pattern and form a decorative surface structure. A flat and rigid stencil surrounding the solids is proposed for unstructured sheet metal areas.

[0009] The processes of hollow embossing and hollow embossing rolling, as well as media-based forming, can be used, particularly for the production of metallic bipolar plates, but also for plate heat exchangers.

[0010] Depending on the process, flatness deviations of between 0.5 and 1.3 mm result for a structure to be molded: "Porstmann, S.; Polster, S.; Reuther, F.; Melzer, S.; Nagel, M.; Psyk, V.; Dix, M.: Target variables and stress fields when comparing the manufacturing processes for metallic bipolar plates. In: Proceedings of the FC 3< Fuel Cell Conference, Chemnitz, 23 / 24 November 2021".

[0011] Publication EP 3 078 433 B1 describes a method for incremental sheet metal forming, in which, according to the method disclosed therein, Figure 5Two manufacturing robots are positioned on opposite sides of the sheet metal, each holding a tool that engages both sides of the sheet to create a local indentation. The opposing robots operate in such a way that at each step of the incremental sheet metal forming process, one of the opposing tools acts as the forming tool, while the other acts as a counter-holder.

[0012] A significant disadvantage of known solutions for incremental forming is that distortion occurs due to local springback.

[0013] To counteract this, countermeasures have been developed according to the state of the art, depending on the forming process. For example, in "Gösling, J.: Metamodel-supported simulation and compensation of springback in sheet metal forming, Dissertation, TU Dortmund, 2010," springback during sheet metal forming is reduced on the one hand by adjusting the blank holder force and on the other hand completely avoided by adapting the tool (crowning of the tool geometry). Increasing the blank holder force leads to tensile stress in the component material. Furthermore, springback is reduced by adapting the component geometry to increase stiffness. This prevents the complete elimination of forming-induced residual stresses, thus reducing geometric distortion.

[0014] To compensate for distortion during the hollow stamping of corrugated sheets, the following are proposed in: "https: / / 4ming.de / de / forming-handbuch / leichtbau-durch-sicken-fachbuch": adjusting the geometry of the corrugations, adapting the tool geometry, an additional forming stroke, and superimposed stretch forming. The additional forming stroke involves finishing the corrugations with large corner radii after preforming the corrugations with tight corner radii. This measure improves the residual stress state of the sheet metal with regard to reduced distortion. This approach results in increased tooling costs, as geometrically different active tool components are required for two forming stages.

[0015] A similar procedure involves subsequently reducing the bead height in a second forming step in so-called hollow embossing by counter-pressing.

[0016] See: "Liewald, M.; Schmid, P.; Helm, D.; Koch, A.; Tritschier, M.: Compensation of springback behavior during the stamping of plate-shaped components made of stainless sheet metal materials, EFB Research Report 399, IGF Project 16339 N, 2014 ".

[0017] In summary, the following compensation methods are known for the conventional single-stage forming of structures in sheet metal: Superimposing bending stress with stretching stress to increase the plastically deformed component area (e.g., by using a sheet metal blank holder), adapting the target geometry to increase component stiffness (e.g., by adding curvature or forming beads), iteratively adapting the tool geometry (e.g., creating a crown using FE calculations of the forming process), adding a second forming stage (e.g., by producing a preform), heat treatment.

[0018] Especially in incremental sheet metal forming, the kinematic shape creation process generates complex residual stress states in the formed components. The resulting geometric distortion is considered a major obstacle to the industrial application of the process.

[0019] Previous studies on geometric distortion have primarily focused on the incremental forming of three-dimensional sheet metal components (including pyramids, cones, and funnels) using a forming mandrel. Their production involves not only SPIF (forming the clamped sheet metal with a mandrel) but also forming the sheet metal into a tool shape ( Two Point Incremental Forming, TPIF ) as well as the forming of the sheet metal with two forming mandrels on both sides of the sheet metal ( Double-Sided Incremental Forming, DSIF ) known (see e.g. the aforementioned publication EP 3 078 433 B1).

[0020] Previous methods for preventing distortion, besides the use of support structures (tool or second mandrel), focus on correcting the toolpath. Toolpath adjustments have thus far required an iterative approach to reduce the distortion measured in FE calculations or forming tests.

[0021] In "Carette, Y.; Duflou, JR: Mastering the Complexity of Incremental Forming: Geometry-Based Accuracy Prediction Using Machine Learning. In: Proceedings of the 28th Saxon Conference on Forming Technology SFU and the 7th International Conference on Accuracy in Forming Technology ICAFT, Chemnitz, 2-3 November 2022," a machine learning method is applied to predict component distortion and reduce simulation or testing effort. A small number of different elliptical 3D geometries are used as the basis for model creation.

[0022] In addition to modifying the support elements (tool shape or mandrel) and adapting the toolpath, the following compensation methods are mentioned in further technical literature: Heat-assisted forming, electrical impulse treatment after forming in the clamped state by utilizing the electroplastic effect, heat treatment (annealing) after forming in the clamped state to reduce residual stresses, incremental sheet metal forming with superimposed stretching stress.

[0023] Regarding the incremental forming of flat structures (sheet metal forming with forming mandrels or rolling beads), no publications focusing on avoiding geometric distortion have been identified to date.

[0024] A thematically related invention is the local hardening and / or stress relief of a flat sheet metal structural component (backrest part of a vehicle) by laser beam application, which is known from German patent application DE 10 2013 217 969 A1. This is an additional process step after forming. According to German patent application KR 10 2020 0083770 A, a 3D sheet metal part is processed several times in succession with a forming mandrel to reduce distortion, with the component geometry being captured in real time. The processing is carried out with an industrial robot, and the sheet metal part can be turned over. Access to the back side allows the component to be processed from two sides. During processing, the component geometry is measured with a 3D scanner, and the toolpath is corrected based on a comparison with the target geometry.In this context, the sheet metal part is not a flat structure on a flat sheet. Furthermore, the back-side machining is not primarily used to prevent distortion in the clamped component. Instead, path correction is performed with reference to the component geometry in the clamped state.

[0025] Flat structures in sheet metal with shaped areas, recesses, and beads are known to be produced using die-bound tools and processes such as hollow stamping or embossing rolls. These processes are economical for large production runs.

[0026] Because a large part of the component geometry is contained in the tool geometry, the flexibility of the processes with regard to the target geometry is limited.

[0027] For the production of individually designed sheet metal parts in small quantities, incremental forming processes using a forming mandrel or roll beading are preferable.

[0028] The tools for roll beading machines offer a limited selection of roll radii, which are usually large due to the roll shape and the associated rigid axis of rotation. The greatest design freedom and scalability of the forming process are achieved when using forming mandrels with a sliding spherical surface or a rolling sphere.

[0029] Of the known measures for reducing distortion in three-dimensional sheet metal components, heat-assisted forming and post-forming heat treatment could lead to distortion reduction. Disadvantages include the associated deterioration of surface quality and the increased energy and time required for component production.

[0030] The main problems with the incremental forming of flat components with potentially insufficient stiffness, which lead to distortion, are as follows: In incremental sheet metal forming of flat structures, distortion cannot be compensated for or avoided by path planning (contour processing, surface processing); on thin sheets, distortion is more noticeable due to a greater deviation in flatness than on thick sheets.

[0031] The distortion only becomes visible after the clamping process is complete. Many compensation methods are designed for the clamped state, but they cannot detect the distortion after clamping.

[0032] The main cause of the distortion is opposing residual stresses along the formed structure on the opposite sheet metal surfaces (one side tension, one side compression), which are the cause of the unwanted sheet metal curvature.

[0033] The object of the invention is therefore to develop a method and a device for the incremental forming of a sheet metal part, with which the production of individual flat structures in a formable sheet metal semi-finished product without distortion is possible, so that the finished part retains its flat basic shape after unclamping and is not (multiaxially) curved, making it possible to produce a flat sheet metal part that is at least partially structured by forming.

[0034] This problem is solved by the features of the first and tenth patent claims.

[0035] Advantageous embodiments result from the dependent claims.

[0036] According to the invention, the method for incrementally forming a sheet metal part, which has a top and a bottom and is clamped circumferentially, is carried out by introducing a structure with a first depth into the sheet metal by means of sequential processing on both sides from the direction of the top and the bottom using at least one first forming tool, and subsequently by at least partially reshaping the introduced structure to a lesser depth.

[0037] In this process, at least one structure with a first depth is introduced into the sheet metal from the direction of the top using the first forming tool, and the structure is reshaped from the direction of the bottom using the same first forming tool or at least one further second forming tool, with a depth adjustment to a second depth.

[0038] For this purpose, after the structure has been introduced, the sheet metal is turned over from the top side using the first forming tool, and the structure is reshaped from the bottom side using the same first forming tool.

[0039] Alternatively, after the structure has been introduced with the first forming tool from the top side, the sheet metal remains in its position and the structure is reshaped from the bottom side of the sheet metal using the second forming tool.

[0040] The incremental forming is preferably carried out using a first forming tool in the form of a first forming mandrel, wherein the sheet metal is positioned between a first template and a second template, and a stack of first and second templates and the sheet metal in between is clamped in a clamping device, wherein the first template has at least one first recess corresponding to the structure to be formed, and wherein the second template has at least one second recess corresponding to the first recess, and wherein the first forming mandrel first forms a structure into the sheet metal along the first recess, then the stack is rotated, and subsequently the previously created structure in the sheet metal is at least partially reshaped with the first forming mandrel along the second recess in the second template.

[0041] Furthermore, it is possible that the incremental forming is carried out using the first forming tool in the form of the first forming mandrel and using a second forming tool in the form of a second forming mandrel, wherein the sheet metal is also positioned between a first template and a second template, and a package consisting of the first and second templates and the sheet metal in between is clamped in a clamping device, wherein the first template has at least one first recess corresponding to the structure to be formed, and wherein the second template has at least one second recess corresponding to the first recess, wherein the first forming mandrel first forms a structure into the sheet metal along the first recess, and then, with the second forming mandrel, the structure previously created in the sheet metal with the first forming mandrel is at least partially reshaped along the second recess in the second template.

[0042] Instead of introducing the structure using one or two forming mandrels, the incremental forming of the sheet metal can also be carried out using a rolling tool. The rolling tool has a roller and a counter roller, wherein the sheet metal is preferably clamped in a clamping device and moved relative to the roller and the counter roller. A structure is formed into the sheet metal in the direction of the counter roller by the roller, after which the sheet metal (possibly held in the clamping device) is rotated, and finally the previously created structure in the sheet metal is at least partially reshaped by the roller.

[0043] The shaping of the structure and the reshaping of the structure using a roller and counter-roller can be done by roller beading or roller offsetting.

[0044] By processing the sheet metal on both sides, the residual stresses are influenced during the production of flat or curved sheet metal components with flat structures by applying pressure to the opposing areas of the sheet metal on both sides using the forming tool.

[0045] By processing the opposing areas of the sheet metal structure from both sides, forming-induced residual stresses and geometric distortion are reduced or avoided. Advantageously, the sheet metal remains in its clamping position during turning. This allows for sequential forming on both sides without the intermediate release of forming-induced residual stresses.

[0046] The device according to the invention for incremental forming preferably has a clamping device for clamping a sheet metal circumferentially and at least one first forming tool for sequentially processing the sheet metal on both sides from the direction of its top and bottom.

[0047] The first forming tool is designed to introduce structural elements into the component from the direction of the top side of the component and to reshape them from the direction of the bottom side of the component.

[0048] For this purpose, the clamping device with the sheet metal clamped in it is rotatable so that first the top and then the bottom point towards the forming tool.

[0049] Alternatively, the device can have at least one second forming tool which is designed to at least partially reshape the structural elements introduced with the first forming tool.

[0050] Preferably, the first forming tool is designed in the form of a forming mandrel or a rolling tool. The second forming tool is also designed in the form of a forming mandrel.

[0051] The first forming mandrel, acting on a first side of the sheet metal and forming the sheet metal in a first direction, and the second forming mandrel, acting on the second side of the sheet metal and at least partially reshaping the previously formed sheet metal area in the opposite direction, generate unidirectional residual stresses in the sheet metal, which are in equilibrium or nearly in equilibrium on both sides of the sheet metal, thus preventing curvature of the finished part.

[0052] In this process, the sheet metal is formed in a first direction by the first forming mandrel in a forming area, then preferably turned over and then, in particular, reshaped again in the forming area using the same first forming mandrel.

[0053] Alternatively, the reshaping can be carried out without rotating the sheet metal using a second forming mandrel.

[0054] Advantageously, the device, in the case of a tool in the form of at least one first forming mandrel, has a first template with at least one first recess and a second template with at least one second recess, wherein the sheet metal can be positioned between the first template and the second template.

[0055] The first and second recesses were aligned with each other and defined the structure to be formed.

[0056] The first forming mandrel, or the first and second forming mandrels, follow the course of the recesses of the templates during the forming and re-forming of the structure.

[0057] To solve the problem of geometric distortion, a sequential processing of the top and bottom of the sheet metal is therefore planned.

[0058] The invention is explained in more detail below with reference to exemplary embodiments and accompanying drawings. These show: Figure 1: a first template 1 and a second template 2, between which a sheet metal 3 is positioned; Figure 2: the sheet metal 3 held between the templates 1, 2, on which the forming mandrel 4 acts with a first force F1 from above; Figure 3: section AA according to Figure 2 Figure 4 Section AA, however with reversed templates 1, 2 and sheet 3, Figure 5 opened templates 1, 2 with the formed sheet area 3.1` in the workpiece 3`, Figure 6 a template with a possible template pattern, Figure 7 roll forming from the direction of the top of a sheet, Figure 8 roll forming from the direction of the underside of the sheet in the area of ​​the previously created structure, Figure 9 roll beading from the direction of the top of a sheet, Figure 10 roll beading from the direction of the underside of the sheet in the area of ​​the previously created structure.

[0059] According to Figure 1Sheet metal 3 is positioned between a first template 1 and a second template 2. Sheet metal 3 is, for example, a semi-finished product that is to be shaped. Sheet metal 3 has a top surface 3a and a bottom surface 3b.

[0060] The first template 1 has a first recess 1.1 and the second template 2 has a recess 2.1 aligned with it, in order to allow access to the machining paths for the forming mandrel 4 from both sides (see Figures 2 to 4 ) make possible.

[0061] The edge of the sheet metal 3 or sheet metal blank is clamped all around in a device not shown by means of a clamping device, wherein the sheet metal 3 is located between the first template 1 and the second template 2.

[0062] The two templates 1, 2 and the sheet metal 3 located between them are preferably held together by a tension frame (not shown) and form a package.

[0063] The sheet metal 3 or the workpiece 3' produced from it and finished should be flat after being removed from the clamping frame and should have no or negligible distortion.

[0064] The package is attached to the machine table via strips with screws or quick-release fasteners (also not shown) and can advantageously be turned over with repeatable accuracy, so that first the top 3a and after turning the bottom 3b face the forming mandrel 4.

[0065] First, a structure is introduced into the sheet metal 3 from the top side 3a using the forming mandrel 4. After turning the stack over, the previously introduced structure is partially reshaped from the bottom side 3b using the same forming mandrel 4, thereby reducing residual stresses that could lead to distortion of the sheet metal. This process can be repeated multiple times. This allows the sheet metal 3 to be processed from one side (top side 3a) and then from the other side (bottom side 3b) with the same forming mandrel 4 without needing to be unclamped in between.

[0066] Alternatively, the second side, forming the underside 3b, can also be processed after temporarily unclamping the sheet 3, which has already been formed from the direction of the first side into the top side 3a. The quality of the distortion compensation depends on the positioning accuracy of the potentially distorted sheet 3. Positioning aids in the form of lateral stops can be used for this purpose (not shown). However, the distortion of the sheet 3 must be compensated for again when it is re-clamped, which can lead to positioning errors.

[0067] The forming of sheet 3 into a sheet or component with a flat local structure created by incremental forming is carried out according to Figure 2 with the aid of a forming mandrel 4.

[0068] Depending on the design, the mandrel active part 4.1, which touches and consequently shapes the sheet metal 3, can perform a rolling or sliding movement along an indicated mandrel track 5.

[0069] In doing so, it acts with a first force F1 on the top surface 3a of the sheet 3 in the area of ​​the recess 1.1 of the first template 1.

[0070] Depending on the mandrel active part 4.1, smooth or rough machining marks are produced, which determine the surface design of the finished part. Furthermore, the outer diameter 4D of the forming mandrel determines how delicate the formed structure on the sheet metal semi-finished product / sheet 3 can be. The minimum size of the outer diameter 4D is in turn limited by the lateral forces occurring during machining in conjunction with the mandrel overhang length.

[0071] The forming mandrel 4 is moved along the mandrel path 5, which determines the pattern and is preferably predefined in the machine control. The forming

[0072] Machining is carried out, for example, on a special IBU machine, a 3-axis milling machine, a table milling machine for model making, or with the aid of an industrial robot. A machine with at least three CNC-controlled axes is required for the forming process. However, it is also possible to control the forming mandrel 4 in another way and, if necessary, to move it manually.

[0073] The flat structure is created by continuous bending and stretching of the sheet metal semi-finished product / sheet 3 between mandrel active part 4.1 and supporting template 1, 2. Open and closed mandrel paths 5 can be traced, along which a depression / recess is formed in the sheet metal 3. In the case of closed mandrel paths 5, pocket-shaped structures, so-called forming fields, can be created by forming areas of the sheet metal 3 with a dimension multiple of the outer diameter 4D of the forming mandrel 4 from the semi-finished product plane / plane of the sheet metal 3 into the finished part plane.

[0074] The depth of the structure formed with the forming mandrel 4, as well as the strength of the sheet metal material, influence the number of forming increments required to produce a feature in the sheet 3. This makes it possible to produce very shallow structural features in a single pass or to create deeper structures in several passes.

[0075] In the latter case, for example, a closed structure is formed into a pocket in several passes of the mandrel track 5. The forming mandrel 4 preferably penetrates the sheet metal 3 along a helical path, whereby discontinuous feeding in several depth planes is also possible. The forming mandrel 4 can rotate freely during penetration into the sheet metal 3.

[0076] It is also possible that the mandrel is driven by a rotary drive to rotate around its own axis L and rotates during the production of the structural elements. In this case, the forming mandrel 4 rotates around its own axis L while the mandrel active part 4.1 slides or rolls.

[0077] At this point, the flexibility of form becomes apparent as a major advantage of incremental sheet metal forming. For forming the sheet metal 3, only a template 1, 2 is required as a form-bound tool component, in addition to the geometrically universally applicable forming mandrel 4. There is considerable design freedom with regard to the local structural depth, which is essentially limited by the formability of the sheet metal material.

[0078] Figure 3 shows the AA section according to Figure 2 , in the forming of the sheet 3 from the direction of its top side 3a by means of the forming mandrel 4, which acts with a first force F1 against the top side 3a of the sheet 3, so that a local depression with the first depth t1 was created in the sheet 3.

[0079] The forming of the workpiece into the shape of sheet metal 3 is expediently divided into at least two processing stages. A processing stage comprises all forming steps / increments that take place in the same clamping direction, e.g., according to Figure 2 and 3 take place.

[0080] After processing all the form elements of the flat structure, preferably first on the top side 3a of the sheet metal semi-finished product, the sheet metal 3 is preferably turned over in the package with the templates 1, 2 in the same clamping device not shown, so that the bottom side 3b can be processed with the forming mandrel 4.

[0081] Preferably, the sheet metal 3 is rotated after the first processing stage while clamped in the stack, so that forming-induced residual stresses are not released and the sheet metal 3 remains as flat as possible.

[0082] Subsequently, the structure generated from the direction of the top 3a with the first depth t1, consisting of open and / or closed form elements, is machined on the back side along the previously generated path curves 5 with the forming mandrel 4 (see Figure 4 ).

[0083] The forming mandrel 4 exerts a second force F2 against the previously formed unlabeled depression from the direction of the now upper underside 3b, so that its first depth t1 is pushed back to a second depth t2 and the structural depth is reduced to the second depth t2.

[0084] For symmetrical structures, the pathways / mandrel tracks 5 on the underside 3b are identical to the pathways 5 on the top side 3a of the sheet 3. For asymmetrical structures, the pathways 5 of the forming mandrel 4 must be around the axis L (see Figure 2 ) are mirrored, around which sheet 3 is turned.

[0085] The depth adjustment tz of the forming mandrel 4 (see Figure 4 ), perpendicular to the sheet plane, is chosen during the rear processing of the underside 3b of the sheet 3 so that after the sheet 3, which now preferably forms the finished component, is unclamped, no or almost no geometric distortion occurs.

[0086] The choice of depth feed tz = t1 - t2 during the back-side machining of the underside 3b depends on the shape of the path curves / mandrel tracks 5 of the forming mandrel 4 as well as the material strength and can be determined by simulation and / or preliminary tests.

[0087] The selection of the depth of cut tz during back-side machining using the forming mandrel 4 may be an iterative process.

[0088] If the sheet 3 or the workpiece 3' is still curved after complete processing of the structure, the size of the depth feed tz is increased or decreased according to the direction of curvature, which can compensate for the distortion.

[0089] The back-side machining alters the forming-induced residual stresses in such a way that the workpiece 3' has no curvature or no or almost no distortion after machining.

[0090] After the reverse side of all form elements has been machined using the forming mandrel 4, the now finished sheet metal 3, and thus the workpiece 3' completed from the sheet metal 3, is unclamped and the first and second templates 1, 2 are removed (see Figure 5 ) and the workpiece 3' is preferably placed on a flat surface (not shown).

[0091] It is evident that a form element 3.1` or structural element has now been introduced into workpiece 3' in the area of ​​the recesses 1.1, 2.1 of the templates 1, 2. This element is pocket-shaped here, but could also be linear, circular, semicircular, or otherwise shaped.

[0092] The underside 3b preferably forms the viewing side of the workpiece 3` when it is used as a decorative element.

[0093] It is also possible to apply more than two machining stages from the direction of the top 3a and the bottom 3b, for example to create a workpiece 3' with a flat shape and a structure in several planes.

[0094] Furthermore, with more than two processing stages, it is possible to form features with a resulting second depth t2 on both sides of the original sheet metal plane. For this purpose, the processing steps described above are repeated.

[0095] Furthermore, in addition to reshaping depressions from the direction of the underside 3b with the forming mandrel 4, further (new) depressions can be created from the direction of the underside 3b and, if necessary, reshaped from the direction of the top side 3a with the forming mandrel after turning the sheet metal 3.

[0096] After processing the underside 3b, the assembly consisting of templates 1, 2 and sheet 3 can be turned over again, and indentations are formed from the direction of the top side 3a at possibly different positions with the same or different depth along another path 5 of the forming mandrel 4 using the forming mandrel 4. After another rotation, the indentations (structures) are reshaped from the direction of the underside 3b of the sheet 3, again using the same forming mandrel 4. The number of processing steps is always even due to the preferably paired processing of the visible and underside surfaces.

[0097] Instead of back-side machining with a constant depth infeed tz along the mandrel path 5 traced by the forming mandrel 4, force-controlled machining with a changing second force F2 can be performed. If the depth infeed tz is constant along various forming elements and path contour radii, different back-pressing forces are generated at the forming element or the forming mandrel 4.

[0098] The locally generated counter-pressure force, in the form of the second force F2, depends on the local component stiffness and strength. If, instead, a constant second force F2 is applied in the opposite direction, the depth of cut tz along the path contour / mandrel track 5 adjusts according to the force value of the second force F2. This results in a constant structural depth on the finished workpiece 3' and eliminates distortion completely or almost completely.

[0099] The variant of a template 1 with a possible template pattern in the form of cutouts 1.1 with different contours is in Figure 6 The cutouts 1.1 define the path contour 5.

[0100] The minimum width b min of a recess 1.1 is shown on the far left.

[0101] This amounts to b min = 4D + 2 * s .

[0102] Here, 4D is the outer diameter of the forming mandrel 4 and s is the sheet thickness of the sheet to be formed 3 (4D and s see also Figure 3 ).

[0103] To the right of this is an elongated recess 1.1 in which the forming mandrel 4 follows a path 5 along the inner circumference of the recess 1.1.

[0104] The forming mandrel 4 preferably travels along the path 5 on the inner circumference of the respective recess 1.1 in the template 1 until the path 5 is closed. In particular, the distance between the outer circumference of the forming mandrel 4 and the inner circumference of the recess 1.1 corresponds at least to the sheet thickness s, as can be seen at the second elongated recess from the left in Figure 6 hinted at.

[0105] In addition, in the middle of the Figure 6 A recess with an even larger width b of the cutout is shown, and next to it on the right is a round recess 1.1 and on the bottom right is a recess 1.1 with different radii.

[0106] The feed of the forming mandrel 4 along the path curve 5 can be spiral.

[0107] The outer diameter 4D of the forming mandrel 4 is determined based on the geometry to be formed and the forming force expected due to the workpiece material and sheet thickness. The determined mandrel diameter can be confirmed in preliminary tests or a simulation of incremental forming.

[0108] A second template 2 is executed corresponding to the first template 1.

[0109] For example, this first template 1 is positioned on the top 3a and an identical second template 2 is placed on the bottom 3b of the sheet 3.

[0110] In the areas of the recesses 1.1, the indicated forming mandrel 4 first shapes the structure from one side and then partially reshapes it from the other side.

[0111] It is possible that the iterative procedure for determining the depth of cut during back-side machining can be shortened or eliminated by specifying a constant push-back force corresponding to the material strength.

[0112] Alternatively, the forming sequence of top and bottom surface machining can be performed with a commercially available rolling tool for beads instead of a forming mandrel 4.

[0113] In this case, the sheet metal 3 is moved on a punching and beading machine in the plane relative to the stationary tool in order to form the desired pattern, first from the direction of the top 3a and then by back forming on the bottom 3b.

[0114] This shows Figure 7The rolling process involves the placement of a sheet 3 from the top side 3a using a roller 6 and a counter-roller 7. The roller 6 has a convex contour towards the counter-roller 7, and the counter-roller 7 has a concave recess. The sheet 3 is formed into the recess of the counter-roller 7 to a first depth t1 by means of the roller 6 from the top side 3a. The roller 6 rotates freely about its axis of rotation A6, and the counter-roller 7 rotates freely about its axis of rotation A7.

[0115] The sheet metal 3 is moved between roller 6 and counter roller 7 according to the structure to be formed. Preferably, the sheet metal 3 is held in a clamping device (not shown) for this purpose.

[0116] After the structure is inserted from the direction of the top 3a of the sheet 3, the sheet is rotated so that its underside 3b points upwards (see Figure 8). Now, the rolling process is carried out from the underside 3b of the sheet 3 in the area of ​​the previously created structure, so that the depth of the structure is reduced from a first depth t1 to a smaller second depth t2 under a depth infeed not specified here. This relieves residual stresses and the sheet 3 retains its flat or substantially flat shape outside the structure.

[0117] Similarly, roll beading is carried out from the direction of the top 3a of a sheet 3, which is in Figure 9 is shown and then from the direction of the underside 3b of the sheet 3 in the area of ​​the previously created structure according to Figure 10 .

[0118] The roller 6 also has a convex contour in the direction of the counter roller 7, and the counter roller 7 has a concave recess.

[0119] Here too, the sheet metal 3 is formed into the recess of the counter roller 7 at a depth t1 from the direction of the top 3a by means of the roller 6 ( Figure 9 The roller 6 and the counter-roller 7 rotate freely around their axes of rotation A6 and A7.

[0120] The sheet metal 3 is moved in the plane between roller 6 and counter roller 7 according to the structure to be formed, whereby the sheet metal 3 is also held in a clamping device (not shown) for this purpose.

[0121] After the structure is inserted from the direction of the top 3a of the sheet 3, the sheet is rotated so that its underside 3b points upwards (see Figure 10 ). Now, with a depth infeed tz, the roll beading is carried out from the direction of the underside 3b of the sheet 3 in the area of ​​the previously created structure, so that during the roll beading the depth of the structure is reshaped from a first depth t1 to a second depth t2.

[0122] Furthermore, it is also possible to apply the inventive method for distortion prevention to single-stage formed components, e.g., by reshaping after embossing, wherein structures are embossed to a specific depth from the top surface 3a of the sheet metal and then reshaped to a defined depth from the bottom surface 3b of the sheet metal. Preferably, this is done with the sheet metal 3 clamped in the same position between the templates 1 and 2.

[0123] For example, the package in the clamping frame must be transferred from the embossing machine to the forming machine, where the forming is carried out with the forming mandrel 4. The re-forming of the embossing by the embossing tool with the forming mandrel 4 then takes place in the same clamping frame.

[0124] The workpiece 3' produced according to the invention can be used, for example, as a metal tile, wall or ceiling cladding or facade element, wherein the moldings or structures form decorative elements.

[0125] In addition, logos, house numbers and other individual markings can also be incorporated into the sheet metal.

[0126] It is also possible to produce prototypes for bipolar plates and heat exchangers (e.g. for flow analyses) by molding form elements into the sheet 3.

[0127] Another possible application is the creation of stiffening ribs to give thinner sheets greater stability.

[0128] By introducing and subsequently reshaping the structure, residual stresses are reduced and the sheet 3 retains its flat or substantially flat shape outside the structure.

[0129] The solution according to the invention is used in particular for introducing flat structures with a structure depth approximately in the range of 0.5 to 1.5 times the sheet thickness into a flat, thin sheet, e.g., thin sheet metal with a sheet thickness s less than or equal to 3 mm. The method and apparatus according to the invention completely or almost completely prevent distortion of the thin sheet.

[0130] Sheet metal is defined as sheets thinner than 3 mm. This definition is commonly used regardless of the material, the manufacturing process, or any further processing (e.g., coating such as galvanizing, tinning, copper plating, nickel plating, painting, etc.). Reference symbol list

[0131] 1. First template 1.1 Recess in first template 1 2. Second template 2.1 Recess in second template 3. Sheet metal 3a Top side of the sheet metal 3 3b Bottom side of the sheet metal 3 3'Workpiece 3.1'Forming element in the workpiece 3.1 4. Forming mandrel 4.1 Mandrel active part 4D Outer diameter of the forming mandrel 5 Mandrel track / track curve 6 Roller 7 Counter roller A6 Axis of rotation of the roller A7 Axis of rotation of the counter roller b Width of a recess b min smallest width of a recess F1first force F2second force sSheet thickness t1first depth t2second depth tzDepth feed Laxis

Claims

1. Method for incrementally forming a sheet (3), wherein the sheet (3) has a top (3a) and a bottom (3b) and is clamped circumferentially, characterized by the fact that a structure with a first depth (t1) is introduced into the sheet metal (3) by means of sequential processing on both sides from the direction of the top (3a) and the bottom (3b) using at least one first forming tool and subsequently an at least partial reshaping of the introduced structure to a depth (t2) takes place.

2. Method according to claim 1, characterized by the fact that by means of the first forming tool at least one structure with a first depth (t1) is introduced into the sheet metal (3) from the direction of the top (3a) and by means of the same first forming tool or a further second forming tool a reshaping of the structure from the direction of the bottom (3b) with a depth adjustment (tz) to a second depth (t2) is carried out.

3. Method according to claim 1 or 2, characterized by the fact that The sheet (3) is turned over after the structure has been introduced using the first forming tool from the direction of the top (3a) and the structure is reshaped from the direction of the bottom (3b) using the same first forming tool.

4. Method according to claim 1 or 2, characterized by the fact that The sheet (3) remains in its position after the structure has been introduced with the first forming tool from the direction of the top (3a) and the structure is reshaped from the direction of the bottom (3b) with the second forming tool.

5. Method according to any one of claims 1 to 4, characterized by the fact that By means of processing the sheet metal on both sides (3) the residual stresses are influenced in the production of flat or curved sheet metal components with flat structures.

6. Method according to any one of claims 1 to 5, characterized by the fact thatthe sheet (3) remains in its clamping position during turning, thus the two-sided sequential forming takes place without the intermediate release of forming-induced residual stresses.

7. Method according to any one of claims 1 to 6, characterized by the fact thatThe incremental forming is carried out using a first forming tool in the form of a first forming mandrel (4), wherein the sheet metal (3) is positioned between a first template (1) and a second template (2), and a stack of first and second templates (1, 2) and the sheet metal (3) in between is clamped in a clamping device, wherein the first template (1) has at least one first recess (1.1) corresponding to the structure to be formed, and wherein the second template (2) has at least one second recess (2.1) corresponding to the first recess (1.1), wherein the first forming mandrel (4) first forms a structure into the sheet metal (3) along the first recess (1.1), then the stack is rotated, and subsequently the previously created structure in the sheet metal (3) is at least partially reshaped with the first forming mandrel (4) along the second recess (2.2) in the second template (2).

8. Method according to any one of claims 1 to 7, characterized by the fact thatThe incremental forming is carried out using a first forming tool in the form of a first forming mandrel (4) and using a second forming tool in the form of a second forming mandrel, wherein the sheet (3) is positioned between a first template (1) and a second template (2), and a package consisting of the first and second templates (1, 2) and the sheet (3) in between is clamped in a clamping device, wherein the first template (1) has at least one first recess (1.1) corresponding to the structure to be formed, and wherein the second template (2) has at least one second recess (2.1) corresponding to the first recess (1.1), wherein the first forming mandrel (4) first forms a structure into the sheet (3) along the first recess (1.1) and then, with the second forming mandrel, forms the structure along the second recess (2.1).2) in the second template (2) the structure previously created in the sheet metal (3) with the first forming mandrel (4) is at least partially reshaped.

9. Method according to any one of claims 1 to 8, characterized by the fact that The incremental forming is carried out using a first forming tool in the form of a rolling tool in the form of a roller (6) and a counter roller (7), wherein the sheet (3) is clamped in a clamping device and is moved with the clamping device relative to the roller (6) and the counter roller (7) and a structure is formed into the sheet (3) in the direction of the counter roller (7) with the roller (6), afterwards the clamping device is rotated with the sheet (3) and subsequently the previously created structure in the sheet (3) is at least partially reshaped with the roller (6).

10. Device for carrying out the method according to claim 1, characterized by the fact thatthis clamping device for clamping a sheet (3) circumferentially and at least one first forming tool for sequentially processing the sheet (3) on both sides from the direction of a top (3a) and a bottom (3b) of the sheet (3).

11. Device according to claim 10, characterized by the fact that the at least one first forming tool is designed to introduce structural elements into the sheet (3) from the direction of the top (3a) of the sheet (3) and to reshape them.

12. Device according to one of claims 10 or 11, characterized by the fact that the clamping device with the sheet metal (3) clamped therein is rotatable so that the top (3a) and the bottom (3b) alternately point towards the forming tool.

13. Device according to one of claims 10 to 12, characterized by the fact thatthe device has at least one first forming tool which is designed to introduce structural elements into the sheet (3) from the direction of the top (3a) of the sheet (3) and that the device has at least one second forming tool which is designed to at least partially reshape the structural elements introduced with the first forming tool.

14. Device according to one of claims 10 to 13, characterized by the fact that the at least one forming tool is designed in the form of - a forming mandrel (4), - a rolling tool with a roller (6) and a counter roller (7).

15. Device according to any one of claims 10 to 14, characterized by the fact thatThe device, in the form of a tool in the form of at least one forming mandrel (4), comprises a first template (1) with at least one first recess (1.1) and a second template (2) with at least one second recess (2.1), wherein the sheet metal (3) can be positioned between the first template (1) and the second template (2), wherein preferably the first recess (1.1) and the second recess (2.1) are aligned with each other and define the structure to be formed.

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