A method for joining superposed metal sheets and a tool set for producing such joints

Abstract: A method and a tool set for joining superposed metal sheets are disclosed. In a first step, a preferably ring-shaped free space is provided by dislocating material of at least one sheet. Thereafter, material of all sheets is squeezed within the confines of the free space whereby material flows laterally into the free space producing an interengaging form lock.

Description: The present invention relates to a method for joining superposed metal sheets by local plastic deformation so that an interengaging connection is formed. A typical method of this kind is disclosed in U.S. patent 4,601,090, and in particular in Fig. 8, 9 and 11 through 15 thereof. A predetermined volume of sheet material is displaced, by means of appropriate dies, in a direction generally orthogonal to the sheet surfaces. The protuberance so formed is then squeezed causing the metal material to cold-flow laterally thereby forming the interengaging connection. The joining element so produced still projects beyond the sheet surface. Such projection is often considered as a disadvantage under functional and esthetic aspects. In an attempt to overcome this drawback, document EP-A-284 902 proposes a method and an apparatus for joining superposed metal sheets by local plastic deformation so that an interengaging connection is formed. At first, a predetermined volume of sheet material is displaced out of the planes of the sheets and squeezed to produce lateral cold-flow of the material thereby forming an interengaging connection. In a supplemental step, the interengaging connection is re-displaced into the depression formed by the first displacement. The apparatus comprises a first die including a clamping member and a punch member and a second die including a clamping member and a punch member. In use, the first die punch member displaces and squeezes a predetermined volume against the second die punch member. Thereafter, the first die punch member is retracted and the second die punch member is actuated to redisplace the squeezed material. The invention aims towards an improved aspect of the joint due to the fact that the interengagement of the sheets occurs within the volume delimited by the exposed outer sheet surfaces. In accordance with the present invention, this aim is achieved by a method according to claim 1 respectively a tool set according to claim 8. Embodiments of the invention are illustrated in the accompanying drawings and will be explained in detail hereinafter with reference thereto. It is to be noted that the drawings have been prepared using polished sections of joints of two sheets produced in accordance with the teachings of the invention and therefore closely reflect the reality. All drawing figures illustrate section views of joint areas; the tooling is only in Fig. 1 and 2 schematically indicated. Fig. 1 shows the first phase of the method in a first variant, Fig. 2 illustrates the second phase thereof, Fig. 3 shows the first phase of a second variant, Fig. 4 illustrates the second phase thereof, Fig. 5 shows the first phase of a third variant, Fig. 6 shows the second phase thereof, and Fig. 7 shows the third phase thereof. In the embodiments illustrated, two metal sheets 10, 12 are joined. The method is not restricted to two sheets, however. Further, sheets 10 and 12 are shown to have equal thickness but this is not compulsory either. Further, it is assumed that the joints to be produced are circular in plan view so that the active tool faces also must have circular section shapes. However, other joint shapes may be produced, and the free space 28 need not even be closed in itself but may consist e.g. of two short straight parallel channels. The rectangular section shape of the free space may also be modified. In all embodiments, the joint boss protrudes with very little height beyond the sheet surface or surfaces. Further, in all embodiments the very joining action occurs within a space delimited by the outer surfaces of the two metal sheets. In Fig. 1 and 2, a tool set is indicated, comprising a top die 16 and a bottom die 18. It must be kept in mind, however, that these designations simply serve the purpose to distinguish the two dies: The tool set works equally well upside down or in any other position. Further, it is assumed that the joint is formed in one single work station but this is not compulsory: Two successive stations could be provided, one each for a respective method step. The foregoing remarks are applicable for all embodiments here disclosed. Referring now to Fig. 1, a first step of a first variant is illustrated. At first, the two sheets 10, 12 are clamped between a top clamping member portion 20 of die 16 and a bottom clamping member 24 of bottom die 18. Thereafter, tubular top punch member 22 is pushed -- by means not shown -- towards sheet 10 so as to penetrate thereinto forming a circular ring-shaped depression which constitutes free space 28 when the top punch member 22 is withdrawn. The dislocated material causes bulging of the surrounding sheet portions, as shown. It is to be noted that no undercut or form lock is caused between sheets 10 and 12 at this stage. Such form lock, however, is produced in the second step illustrated in Fig. 2. A bottom punch member 20 which, in the first step, was flush with the bottom clamping member 24, is displaced towards sheet 12 and penetrates into the latter. It is to be noted that the cylindrical bottom punch member has a diameter slightly smaller than the inner diameter of tubular top punch member 22. During this second step, top members 20, 22 lie flush with their fronts facing sheet 10. Bottom clamping member 24 may yield slightly so to give way for bulging sheet 12. The volume portion of sheets 10, 12 within free space 28 is squeezed, the metal flows in lateral direction and forms the interengaging form lock. It will be noted that after withdrawal of the dies the joint will be flush with the upper or outer surface of sheet 10. In the embodiment of Fig. 3 and 4, bottom clamping member 24 is held stationary during the second step while top punch member 22 together with the portion of the top clamping member encircled by it may slightly yield so that once the joint is formed recess 32 is flush with the surrounding portions of sheet 12, the boss being formed on the side of sheet 10. The embodiment of Fig. 5, 6 and 7 comprises three steps. In a first step illustrated in Fig. 5, a cup-shaped depression is formed thereby defining the outer boundary of the to-be-formed free space. In a second step, a central portion 34 of the "cup bottom" is relocated by a bottom punch member (not shown) thereby defining the inner boundary of free space 28 (Fig. 6) until the portion of sheet 10 on top of portion 34 hits the top clamping member (not shown). Further displacement of the bottom punch member squeezes the sheets to form the interlock illustrated in Fig. 7. It is to be noted that very flat bosses remain on both sides of the joint.

Claims: A method for joining superposed metal sheets (10, 12) by local plastic deformation so that an interengaging connection is formed, the method comprising the steps: (a) a depression (28) is produced in one of the surfaces of said superimposed sheets by local plastic deformation, said depression at least partially surrounding a volume of the superimposed sheets where a connection is to be formed, in such manner, that the depression (28) delimits said volume in at least one direction, (b) squeezing said volume to cause lateral cold flow of its material into said depression whereby said interengaging connection is formed. The method of claim 1 wherein said depression is formed as a channel. The method of claim 1 or 2 wherein said depression is formed as a closed loop. The method of claim 1 wherein a plurality of separate depressions is formed. The method of claim 1 wherein said connection is formed flush with one outer sheet surface. The method of claim 1 wherein said connection is formed substantially flush with both outer surfaces of the sheets. The method of claim 3 wherein step (a) comprises: (a1) displacing a predetermined first sheet volume substantially orthogonally with respect to a plane defined by said sheets thereby defining an outer boundary of said depression, (a2) re-displacing a predetermined second sheet volume within said outer boundary thereby defining an inner boundary of said depression. A tool set for producing a joint according to the method set forth in any of the preceding claims, said tool set comprising a first die (16) and a second die (18), said first die comprising a first clamping member (20) arranged to abut a first outer surface of said sheets (10, 12) and a first punch member (22) arranged to penetrate into said sheets from from said first outer surface thereof, the first punch member (22) being suitable for producing said depression (28), said second die comprising a second clamping member (24) arranged to abut a second outer surface of said sheets opposite said first outer surface and a second punch member (26) arranged to penetrate into said sheets from said second outer surface thereof, the arrangement being such that, in use, sheet material between said second punch member and said first die clamping member and its retracted first punch is squeezed causing its flow into said depression. The tool set of claim 8 wherein said first punch member (22) is tubular whereby a substantially circular depression is formed and wherein siad second punch member (26) is cylindrical having a diameter smaller than an inner diameter of said first punch member (22).

Inventors: LIEBIG HANS PETER PROF, SACK PETER DIPL-ING, LIEBIG, HANS PETER, PROF., SACK, PETER, DIPL.-ING.
Applicants: ECKOLD VORRICHTUNG, ECKOLD GMBH & CO. KG
Application year: 1993
Main classification: B21D39
IPC codes: B23K20, B21K25, B21D39, B23P11