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Ensuring the integrity in clinching process
Journal of Materials Processing Technology 174 (2006) 277–285
Ensuring the integrity in clinching process Juha Varis ∗ Department of Mechanical Engineering, Lappeenranta University of Technology, Lappeenranta, Finland Received 8 September 2004; received in revised form 31 January 2006; accepted 1 February 2006
Abstract Clinching is a mechanical joining method, especially for sheet metal parts. The process is perceived as being a simple method that is based on only the accurate movement of a punch into a die. During the process, sheet metals are deformed locally without the use of any additional elements. Precisely selected tools are a requirement for an acceptable joint. The two most frequently used geometries in clinching tools are round and square. When round tools are used, the joint has a uniform shear load capacity in all the horizontal directions. Many papers have been written on the clinching process itself, including the tool geometries, parameter optimisation, joint strengths, simulation and FE analysis of the process, but few articles discuss the significance of anticipatory maintenance or continuous follow-up while using clinching in a mass-production process. This paper points out several problems encountered in the long-term use of a clinching process. Both the lack of systematic maintenance and continuous follow-up are discussed. The significance of changes in the construction are also evaluated in this presentation that is based on a real case study. This paper also offers proposals for improvements. © 2006 Elsevier B.V. All rights reserved. Keywords: Clinching; Quality; Mass-production
1. Introduction 1.1. Clinching Clinching is a joining method in which sheet metal parts are deformed locally without the use of any additional elements [1]. Clinching equipment includes a tooling set (a punch and a die), with which joints can be made with or without cutting [2]. The principle of clinching is shown in Fig. 1. Overlap joints are necessary due to the principle of the method. The thickness of the joint assembly normally varies between 0.4 and 8 mm for mild steel when using commercial machines and tools. The sheet thickness typically varies between 0.2 and 4 mm and both sheets are not required to be of equal thickness. A clinched joint is characterized by a pit on the punch side, and a rise on the die side. The force needed for joining two materials depends on the materials to be joined and the size of the tools, and normally varies between 10 and 100 kN [3]. Because of the relatively simple principle of the method, clinching has many advantages, such as rapidity, cleanliness and low noise. The process does not produce any heat, splashes,
flashes or harmful light. It is not necessary to use specialized personal protectors. 1.2. Clinching machines and tools Several types of machines are commercially available for clinching, and have been described in detail by Varis [4]. The choice of the suitable machine depends on the product to be made and the batch size, as well as on the limitations imposed by the working geometry. Unit designs include hand-powered machines, portable machines powered by hydraulic or pneumatic systems, self-standing hydraulic or pneumatic machines, C-frames with an additional press, and multi-tools. Generally, tens of thousands (30,000–100,000) of clinches have been reported for one pair of tools. The lifetime, no doubt, correlates with the materials to be clinched; there is a big difference when clinching, for instance, stainless steel in comparison to normal mild steel. Also, the bending of tools during the process, as well as other unstable circumstances, shorten the lifetime of tools remarkably. However, it should be possible to produce tens of thousands of joints with one pair of tools. 1.3. Modes of failure
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In general, three basic modes of failure can be observed when a joint is loaded. Two of these modes are shown in Figs. 2 and 3
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Fig. 1. The principle of clinching [TOX® GmbH].
The third failure mode, C, is a combination of failure modes A and B. In this mode, one edge of the joint fails, whilst the other is deformed in the first stage. Subsequently, the sheets separate without any further fracture [4]. Mode C has been evaluated for the best mode by Varis [15], because of the comprehensive braking process in comparison to modes A or B. 1.4. The strength of the joint
Fig. 2. Failure mode A.
[5]. In the first mode, A, there is an insufficient amount of material in the neck of the joint and loading will result in the failure of the neck. There are two apparent reasons for failure mode A: either the clearance of the tool diameters is too small or the die is too deep, which leads to excessive elongation in the region of the joint neck, causing the formation of a crack. In the second mode, B, illustrated in Fig. 3, the sheets are deformed, and the joint subsequently opens as a press-stud. A typically insufficient deformation (an excessive parameter X) produces a minor interlocking of the sheets (the “S” shape of the joint is small, Fig. 3) and leads to this mode of failure. A high shear load can be accommodated with this mode of failure if there is a good balance between the tool diameters and the parameter X.
Fig. 3. Failure mode B.
The shear strength of the joint is based on the material thickness in the neck of joint (Fig. 4). A sufficient “S” shape is needed in order to keep the sheets together while the joint is subjected to an axial force (exists). The “S” shape is created when pressing the material into the bottom of the die. The real strength of the joint can only be defined by the use of destructive testing. The test specimen used for single fastener connections is described in the AS/NZS 4600 standard [6]. Shear strength testing is based on obtaining both the displacement of the joint and the existing load. The results of the test are normally presented using a diagram with an axes displacement (X) and force (Y). The experimental method described above is still the most widely used one, even though the potential of finite element (FE) analysis has been realised, and the data has been reported openly by Budde and Klasfauseweh [7], Budde and Klemens [8], W¨ossner et al. [9] and Klasfauseweh and Hahn [10]. However, the time required for FE analysis and computation is rarely less than several hours, which when combined with the requirement for verification testing, reduces the attractiveness of FE methods. Lepist¨o et al. [11] have successfully shown the accuracy of FE analysis while carefully verifying the material properties and friction coefficients. On the other hand, for limited cases,
Fig. 4. Characteristic for an ideal joint. The strength of the clinched joint depends on the neck of joint and the “S” shape (interlocking).
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Fig. 5. The measurement of the thickness at the base of the joint (parameter X) [TOX® GmbH].
simple equations for calculating the strength of clinched joint or several joints have been generated by Pedreschi et al. [12,13] and Davies [14]. The main problems encountered in the mathematical analyses of clinch joints are the difficulties related to the accuracy of predicting the behaviour of the joined materials, the exact material properties and the dimensions at different locations of the joint area. The accurate wall thickness should be available when calculating the strength of a joint. 1.5. An ideal joint An ideal joint presented by Varis [15] has a totally filled die, without leading to counter-piping or the excessive stretching of the upper sheet (Fig. 4). In addition, the sheets are completely connected. Counter-piping occurs if too much material is pushed into a die. 1.6. Evaluating a clinched joint The quality of a joint made with a fixed die is evaluated by measuring the material thickness in the middle of the base of the joint (Fig. 5). This measure is known as the X parameter and has a generally accepted tolerance of ±15% [16]. An automatic real-time quality control system was developed and presented mainly during the nineties [17–19]. The information on the displacement of the punch and an existing force are collected simultaneously. A wide range of data can be collected
into a PC connected to the control system, or a limited amount of data can be stored in the memory of the control system. The collected data can be illustrated as shown in Fig. 6, or only the maximum force and displacement can be shown on the display as output values. An average value for tens (even hundreds) of prepared joints is illustrated in Fig. 6 as a bold average curve. The grey area below and above the average curve defines the tolerance area. Any deviation from the tolerated area causes the process to stop. Several typical cases are shown in Fig. 6: (C) the material is too soft or the tool is unsuitable, (D) the material is too hard or the punch too large, or the material adhered to the surface of the punch, (E) the punch is too short, or (F) the tool is damaged. Even if it is possible to measure and observe minor changes, it is not easy to give an exact explanation for the main reason. A minor deviation from the measured force can be caused by many factors, such as the wear and tear or the eccentricity of tools, changes in the material properties or the impurity of the material surface of tools. Without much experience, a real-time process control system is a tool for explaining main problems of the process only. As described above, even if automated systems do exist, there is plenty of use for visual inspection and for analysing the crosssections of joints. Visual inspection should focus on searching for minor cracks or breaks in the area of the joint neck. On the die side, the die should be fully filled and symmetric; bevelling in any direction is not allowed.
Fig. 6. A real-time process control system collects data from the punch sensor (displacement) and on the existing force [TOX® GmbH].
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real-time process monitoring system will kept on all the time. 2. Objective of the study
Fig. 7. A recommendation for quality control methods while ramping up a new product [TOX® GmbH]: (A) visual inspection (non-destructive); (B) destructive testing of the joint; (C) analysis of cross-sections; (D) real-time process monitoring.
Using the cross-section, a more detailed analysis can be performed. The observed aspects of the joint are its symmetry, material thickness over its whole area, internal cracks and breaks, as mentioned above, the behaviour of the formed material and the eccentricity of the tools. No general values have been presented for eccentricity; instead, the maximum crossing of tools is limited to three degrees. Crossing can be caused by the improper guidance of tools, the wear and tear of the guiding elements or the deflection of the C frame, which is typically restricted to a value of 0.6 mm. When ramping up a new product, all the previously described quality inspection methods should be used as shown in Fig. 7. During the development of the process, the amount of visually based inspection and destructive testing can be decreased. As a next step, quality control based on cross-sections can be decreased as well, but crosssections should be analysed later in all the cases while problems continue in the process or before any changes are made to the process or product. It is also possible to specify the analysis of the cross-section in internal standards, for instance, once every 50,000 joints. The
Clinching seems to be a simple joining method with few process parameters. However, great care must be taken in the quality control of clinching in many ways, and detailed information should be available on the process in order to assist in the analysis of existing problems. The case used in this study is based on a mass product of a Finnish company. Several existing problems pertaining to clinching will be presented, analysed and solved during the study. The information presented in this paper is widely applicable to all kinds of clinching cases. 3. Case 3.1. A product and a joint The case product consists of 100 clinched joints. One batch of the product consists of a total of approximately 200,000 joints. The material on the punch side is Alz DX51D AZ150 with a thickness of 1.0 mm and a tensile strength of 500 N/mm2 , and the material on the die side Alz DX52D AZ150, with a thickness of 1.0 mm and a tensile strength of 420 N/mm2 . During transportation and assembly, the joints are typically subjected to bending forces. Only minor (5 N) dynamic forces act on each joint during the process. The product is an essential part of a huge assembly and requires a lot of work if it collapses and has to be repaired. No unexpected downtime is allowed. A clinched joint has a diameter of 6.0 mm on the die side. The joint will be placed in a narrow area (14 mm), as shown in Fig. 8. The available space limits the size of the tools. The diameter of
Fig. 8. The location of a joint on a narrow roll-formed area.
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Fig. 9. The clinched materials are not orthogonally set the punch and the die.
the punch is 4.2 mm. The forms of the edges on both sheets have been roll-formed. 3.2. The equipment and quality control The clinching equipment was originally designed for use with two sheets that are set orthogonally between the punch and the die; in any case, this should be the basis for the process. The equipment consists of two similar clinching devices that move and clinch simultaneously on both sides of the whole product. No axial movement or swinging can occur during clinching, because of an assembly of clinching devices (a C frame) that is firmly attached to the assembly. The process can be run unmanned. A control measure, X, has been defined by a toolmaker, and its value is 0.55 mm. Measure X is measured regularly using samples of the original material. However, measurements of joints from the final product are made infrequently. 3.3. Existing problems It was possible to make only 4000–5000 clinches with one pair of tools. The punch was often broken at the end of the peak. What is worse is that the unstable and inaccurate clinching process led to broken joints in the whole product. Normally, one or two broken joints do not impead the use of the product, but more extensive repairs have to be made if more than three sequential joints fail. In this case, the problems in the product were observed quite soon after the start of the process. There is good reason to suspect that some cracks were already caused during assembly. The good news is that the long-term use of the products has not lead to damages in the joints.
Fig. 10. The shape of a joint when the punch and die have crossed.
The device was equipped with a pair of leading and bending rolls. It was observed that there was continuous contact between the sheets and the rolls and that the bending force was not sufficient to straighten the sheets orthogonally against the clinching tools. The clinching device had been in used for a long time, and hundreds of thousands joints had been made using the same machine; therefore, the tool guides were worn, and it was no longer possible to ensure the accurate movement of the tools. 3.4.2. Shapes of the joints When making an ideal joint, the material moves straight against both sides of the die, as is shown in Fig. 4. There is no additional bending in the material exists, as can be seen in Figs. 10 and 11. Using accurate measuring equipment, the problem related to the tool heads not being parallel can be observed when comparing measures X1 and X2 (Fig. 10). As was explained earlier on, many problems can be observed in a longitudinal cross-section of the product (Fig. 11). Fig. 12 shows, instead, a perpendicular cross-section, and the joint appears to be a typical clinched joint. When verifying an unexpected joint and a section (5) in an ideal process, shown in Fig. 6 by Liebig et al. [20], the main difference can be observed at the end of the pressing process. In
3.4. Analysing problems 3.4.1. Position of the tools Visually, it was easy to verify that the sheets to be joined were not set orthogonally between the punch and the die (Fig. 9). The clinching equipment and the whole process were designed based on a certain degree of orthogonality. The roll-forming of the sheet edges was altered at some point because of formability problems in the upper material. Nevertheless, no changes were made to the clinching process or devices.
Fig. 11. The cross-section of a joint separated from a real product. There is additional upward bending on the left side of the joint.
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Fig. 12. The joint looks like a typical clinched joint when observed in a direction perpendicular to the edge of the product.
an ideal case, the material is extruded uniformly and horizontally straight to both sides. However, in the case of non-orthogonal tools, the process starts just like an ideal process, but only until the material reaches the bottom of the die (the left corner (A) in Fig. 13). Since corner A is fully filled with the material to be joined, the other corner (B) is still unfilled, because the tools are bevelled. Since the punch has not reached the lower dead centre, more material is pushed into the die. Both the upper and lower sheets deform together and move against corner B; the result of such a case has been observed as the above-mentioned unexpected joint. 3.4.3. Tool failures During the clinching process, the edges of sheets are bent by the clinching force. The clinching force increases linearly, and the maximum bending force is achieved at the end of process. The joined sheets slide against each other until just before the completion of the joint, which means that the bending force causes an increasing horizontal force against the punch until the joint is capable of carrying the forces acting on it. The horizontal force bends the peak of the punch; in this case, the maximum penetration is 3.75 mm. As the clinching force is removed, the second bending impulse occurs against the peak. At this point, the duration of the impact is very short. The punch has a peak diameter 4.2 mm, the diameter of the frame of the tool is 14 mm and the peak length is 10 mm. The maximum strain is exerted on the base of the punch peak, which explains the tool failures.
Fig. 14. The contact between the sheets is not tight, and additional bending has occurred in the joining area.
3.4.4. Failure modes of the joints The failed joints of real products have been thoroughly investigated. Failure modes A, B and C, which were discussed earlier, were all observed. By analysing the failed joints, it was possible to observe that the force that led to a joint failure acted in the longitudinal direction of the product. Some joints, which did not open but which could not carry any load, were heavily tilted in the direction of the exerted force. The joints, which had completely failed, could be analysed on both sides, and it was possible to make the same observation as above. When the surrounding of joints were analysed, it was possible to observe scratches in the vicinity of the joint. The scratches led to the assumption that the gripper of the tool was not been used as planned. The unexpected bending occurred on the area of joint, and no doubt, the accuracy of the tool positioning did not reach an acceptable level. The joints, which exhibited the A failure mode, “a broken neck”, were thoroughly investigated. As described earlier, failure mode A occurs typically in cases, in which the diameter between the punch and the die is too small; this leads to too little material in the neck region. In this case, the joining circumstances are constant with respect to the materials, tools and X measure. Therefore, further reasons for the failure mode were investigated. Minor deformations were observed on both sheets in the joint area, as shown in Fig. 14. The deformations make it impossible to achieve continuous contact between the sheets, which leads to a situation in which the total thickness of joint area is greater than expected (two times the sheet thickness). The chosen tool combination is, therefore, not suitable for this type of joining task. The sheet on the die side will be deformed more than planned, which typically means that it will be over-stretched in the region of the neck, and failure mode A is obvious. The main reasons for this type of failure (such kind of existing case) are that the sheets are not held firmly enough during clinching or the tools are positioned inaccurately, as described earlier. 4. Solving the problems 4.1. Changes to the clinching device
Fig. 13. A case in which material is pressed into a die, which is not parallel to the head of the punch.
The problems encountered in clinching can be summarized as being based on the non-orthogonality of the sheets and
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analysis of the cross-section, the shear strength of the joint, the tool life and failure mode, and finally, an analysis of the whole process.
Fig. 15. Additional elements, which allow swinging and linear movement in the clinching device, have been installed.
tools. Therefore, the clinching equipment has been modified as described below. An automatic real-time process control system has also been installed in order to fully ensure the quality of the process. The problems related to bending sheets while clinching were discussed earlier on. Therefore, very sophisticated clinching equipment has been modified with additional swinging and linear movements, as shown in Fig. 15. The swinging movement has a sufficiently wide range of movement, which takes care of changes in the bending of the edges. With the previously described modifications, the clinching device is now able to set itself easily to the required position before the punch penetrates the sheets. The punch and die are kept orthogonally positioned around the joint area throughout the whole process. No bending of sheet edge takes place, as a result of which there are no horizontal shear or bending forces against the punch. 4.2. Automated process control system An automated process control system was installed at same time as the modifications were made to the clinching device. Both clinching guns will be separately controlled in real time with clinching. The force being exerted will be measured using a strain gauge installed onto the C-frame. The maximum clinching force will be collected onto the memory of the control system and will be easy to analyse afterwards. As described earlier, the control system has an alarm range that leads to the immediate interruption of the process if a remarkable overflow or underswing occurs in the pressing force in comparison to the movement of the punch. In addition to automated process control, the instructions for controlling the process were also written as listed below. The control list consists of five headlines; the outlook of the joint, an
(1) The outlook of the joint: • Does the value of the X measure vary within an accepted range? ◦ A deviation of ±15% is generally accepted. ◦ Measuring X on both sides of the joint will indicate how parallel the tools are. • Does the joint look symmetrical when looked at from the die side? ◦ No bevelling is allowed in any direction. ◦ The die must be fully filled with a round edge. • The grippers are not allowed to touch any other part of the product. • The traces of the grippers should be symmetrical. (2) Cross-section: • The cross-sections in both directions (the longitudinal and the direction perpendicular to the longitudinal) should look symmetrical. ◦ Is the amount of material on both sides of the joint equal? ◦ Does the joint look symmetrical? • Are the sheets in full contact with each other? • The sheet on punch side is not allowed to be stretched in the area of the joint neck. ◦ The joint neck must be sufficiently thick to be able to carry the forces acting on the joint. • Does the joint have a nicely shaped interlocking? ◦ Additional downwards bending is not allowed. (3) The shear strength based on destructive testing: • Does the joint achieve the expected strength? ◦ In this case, the shear strength should be more than 1800 N. ◦ The joint should achieve a shear strength of at least 1400 N in the axial direction. (4) The lifetime of tools and failure modes: • It should be possible to manufacture at least tens of thousands of joints using one pair of tools. • Analysis of the failure mode of the tool. ◦ Has the punch failed on the base of the punch peak because of bending? ◦ Has the punch split as a result of an excessive compressive force? ◦ Does the tool (especially the punch) bear natural marks of wear? (5) The clinching device: • Does the whole process function properly? ◦ Is there continuous contact between the leading and bending rolls? 5. Results of the modification During the ramp-up time, more than 19,000 clinches were made using one pair of tools. This amount is 4–5 times more than the starting value. The goal is 30,000 clinches, once the
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process has been fully adjusted. The most remarkable change is the so-called flowing C-frame, which makes the positioning of the clinching device more accurate than with the former construction. A little cutting oil is spread onto the location of the joint just before clinching. Hahn et al. [21] have been reported the advantages of a thin oil film in terms of the tool life and decreased pressing force. The force needed for clinching is 36 kN, which is 3 kN less than that reported by a toolmaker. The result was not distinct, because the toolmaker in question has used small individual test samples, and the value of 36 kN was measured while clinching a real product. Previously, the importance of the construction for the pressing force was analysed, and it the pressing force should be higher in comparison to the values obtained from tests with individual samples. The decreased pressing force can be explained with the above-mentioned lubricating system; also, some minor changes are explained by the change of the material properties. Some deviation was expected, because of differences in the measuring devices. As a whole, the process seems to work properly. The reliability of the process has increased greatly, because of the total process control, which has also increased customer confidence. While having the possibility to monitor every single joint, the whole construction will be fully checked totally with care. The importance of a complete check is undeniable, as mentioned earlier with respect to the occurrence of problems.
6. Conclusions Continuous quality control is a necessary and important part of the total process. Both visual and simple methods, as well as automated real-time processes, are suitable quality control systems. There are no unimportant observations and deviations; all changes to and details of the clinching process affect it. The tools and sheets must be positioned correctly. No bending of the tools is permitted during the clinching process. The eccentricity of the tools must be controlled regularly. The lifetime of clinching tools should be normally tens of thousands joints for a material in this case. If a failure occurs exists earlier, the whole process must be thoroughly investigated. The failure mode of a tool correlates with the existing types of problems. The condition of the tools, glides and whole clinching device must be controlled regularly. Wear in any sliding element leads to the inaccurate movement of tools and to problems in the strength of the joint. An automated process control system makes it possible to implement full control. Destructive testing plays a major role as an additional testing method. In particular, the coexistent use of several analysis systems is recommended when ramping up the system. The significance of changes to the process conditions or the design of the product must be analysed carefully from the manufacturing point of view. Changes in any area must be taken into account in the whole process.
7. General suggestions for further research An automated process control system makes it possible to easily collect huge amounts data. Commercial analysis software can or could be used more efficiently for analysing this data. The questions that require further study are related to the prediction of the wear of tools based on the analysis of data from an extended period. The currently available data would also be useful for the proactive maintenance of the clinching device.
References [1] H.P. Liebig, J. Bober, R. Beyer, Connecting sheet metal by press joining, in: B¨ander Bleche Rohre, vol. 9, No. 1, Vogel-Verlag, W¨urzburg, 1984, p. 9. [2] O. Hahn, L. Budde, Analyse und systematische Einteilung umformtechnischer F¨ugeverfahren ohne Hilfsf¨ugeteil, in: Blech Rohre Profile, vol. 37, No. 1, 1990, pp. 29–32 (in German). [3] J. Varis, Ohutlevyjen Puristusliitt¨aminen (Clinching of Sheet Metals), Federation of Finnish Metal, Engineering and Electrotechnical Industries, MET, Helsinki, 1997, p. 55 (in Finnish). [4] J. Varis, Kuumasinkityn lujan rakenneter¨aksen puristusliitt¨aminen (Clinching of zinc-coated high strength structural steel), Research Report No. 27, Lappeenranta University of Technology, Department of Mechanical Engineering, Lappeenranta, 1998a, 122 p. (in Finnish). [5] B. Nilsson, E. Sagstr¨om, Handbok f¨or Stuknitning Nr 1, 1995. Fakta Rapport: Bearbetning och fogning av pl˚at, Sveriges Verkstadsindustrier, Stockholm, 1995, p. 87 (in Swedish). [6] AS/NZS 4600, Cold-formed steel structures, Australian/New Zealand Standard, Homebush, Australia/Wellington, New Zealand, 1996, pp. 111–115. [7] L. Budde, U. Klasfauseweh, Analysen der Beanspruchungen durchsetzgef¨ugter Strukturen, in: B¨ander Bleche Rohre, vol. 33, No. 8, 1992, pp. 46–48 (in German). [8] L. Budde, U. Klemens, Aufbau eines Expertensystems f¨ur mechanische F¨ugetechnik, in: B¨ander Bleche Rohre, vol. 33, No. 8, 1992, pp. 40–51 (in German). [9] J.F. W¨ossner, F. Herkommer, A. Dick, In den punkt geschaut, in: Blech Rohre Profile, vol. 93, No. 4, 1992, pp. 315–318 (in German). [10] U. Klasfauseweh, O. Hahn, Numerische Simulation nichtschneidender Durchsetzf¨ugevorg¨ange, in: Blech Rohre Profile, vol. 41, No. 5, 1994, pp. 328–333 (in German). [11] J. Lepist¨o, J. Varis, TOX® -puristusliitoksen simulointi elementtimenetelm¨aa¨ k¨aytt¨aen. Ter¨asrakenteiden tutkimus—ja kehitysp¨aiv¨at 2000. 18–19.1.2000, Espoo, 2000, 8 p. (in Finnish). [12] R.F. Pedreschi, B.P. Sinha, The potential of pre-joining in coldformed steel structures, Construct. Build. Mater. 10 (4) (1996) 243– 250. [13] R.F. Pedreschi, B.P. Sinha, R. Davies, Advanced connection techniques for cold-formed steel structures, J. Struct. Eng. 123 (2) (1997) 138– 144. [14] R. Davies, R. Pedreschi, B.P. Sinha, The shear behaviour of pressjoining in cold-formed steel structures, Thin-Wall. Struct. 25 (3) (1996) 153–170. [15] J. Varis, A novel procedure for establishing clinching parameters for high strength steel sheet. Dissertation, in: Acta Universitatis Lappeenrantaensis, vol. 105, Lappeenranta University of Technology, Finland, 2000, 84 p., ISBN 951-764-490-6, ISSN 14564491. [16] J. Haller, Personal Communication 7–9.4.1999, Headquarters of TOX® , GmbH, Weingarten, Germany, 1999. [17] J. Bober, H.P. Liebig, Prozessanalyse beim druckf¨ugen, in: B¨ander Bleche Rohre, No. 10, 1990, pp 143–146 (in German).
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Ensuring the integrity in clinching process Juha Varis ∗ Department of Mechanical Engineering, Lappeenranta University of Technology, Lappeenranta, Finland Received 8 September 2004; received in revised form 31 January 2006; accepted 1 February 2006
Abstract Clinching is a mechanical joining method, especially for sheet metal parts. The process is perceived as being a simple method that is based on only the accurate movement of a punch into a die. During the process, sheet metals are deformed locally without the use of any additional elements. Precisely selected tools are a requirement for an acceptable joint. The two most frequently used geometries in clinching tools are round and square. When round tools are used, the joint has a uniform shear load capacity in all the horizontal directions. Many papers have been written on the clinching process itself, including the tool geometries, parameter optimisation, joint strengths, simulation and FE analysis of the process, but few articles discuss the significance of anticipatory maintenance or continuous follow-up while using clinching in a mass-production process. This paper points out several problems encountered in the long-term use of a clinching process. Both the lack of systematic maintenance and continuous follow-up are discussed. The significance of changes in the construction are also evaluated in this presentation that is based on a real case study. This paper also offers proposals for improvements. © 2006 Elsevier B.V. All rights reserved. Keywords: Clinching; Quality; Mass-production
1. Introduction 1.1. Clinching Clinching is a joining method in which sheet metal parts are deformed locally without the use of any additional elements [1]. Clinching equipment includes a tooling set (a punch and a die), with which joints can be made with or without cutting [2]. The principle of clinching is shown in Fig. 1. Overlap joints are necessary due to the principle of the method. The thickness of the joint assembly normally varies between 0.4 and 8 mm for mild steel when using commercial machines and tools. The sheet thickness typically varies between 0.2 and 4 mm and both sheets are not required to be of equal thickness. A clinched joint is characterized by a pit on the punch side, and a rise on the die side. The force needed for joining two materials depends on the materials to be joined and the size of the tools, and normally varies between 10 and 100 kN [3]. Because of the relatively simple principle of the method, clinching has many advantages, such as rapidity, cleanliness and low noise. The process does not produce any heat, splashes,
flashes or harmful light. It is not necessary to use specialized personal protectors. 1.2. Clinching machines and tools Several types of machines are commercially available for clinching, and have been described in detail by Varis [4]. The choice of the suitable machine depends on the product to be made and the batch size, as well as on the limitations imposed by the working geometry. Unit designs include hand-powered machines, portable machines powered by hydraulic or pneumatic systems, self-standing hydraulic or pneumatic machines, C-frames with an additional press, and multi-tools. Generally, tens of thousands (30,000–100,000) of clinches have been reported for one pair of tools. The lifetime, no doubt, correlates with the materials to be clinched; there is a big difference when clinching, for instance, stainless steel in comparison to normal mild steel. Also, the bending of tools during the process, as well as other unstable circumstances, shorten the lifetime of tools remarkably. However, it should be possible to produce tens of thousands of joints with one pair of tools. 1.3. Modes of failure
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In general, three basic modes of failure can be observed when a joint is loaded. Two of these modes are shown in Figs. 2 and 3
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Fig. 1. The principle of clinching [TOX® GmbH].
The third failure mode, C, is a combination of failure modes A and B. In this mode, one edge of the joint fails, whilst the other is deformed in the first stage. Subsequently, the sheets separate without any further fracture [4]. Mode C has been evaluated for the best mode by Varis [15], because of the comprehensive braking process in comparison to modes A or B. 1.4. The strength of the joint
Fig. 2. Failure mode A.
[5]. In the first mode, A, there is an insufficient amount of material in the neck of the joint and loading will result in the failure of the neck. There are two apparent reasons for failure mode A: either the clearance of the tool diameters is too small or the die is too deep, which leads to excessive elongation in the region of the joint neck, causing the formation of a crack. In the second mode, B, illustrated in Fig. 3, the sheets are deformed, and the joint subsequently opens as a press-stud. A typically insufficient deformation (an excessive parameter X) produces a minor interlocking of the sheets (the “S” shape of the joint is small, Fig. 3) and leads to this mode of failure. A high shear load can be accommodated with this mode of failure if there is a good balance between the tool diameters and the parameter X.
Fig. 3. Failure mode B.
The shear strength of the joint is based on the material thickness in the neck of joint (Fig. 4). A sufficient “S” shape is needed in order to keep the sheets together while the joint is subjected to an axial force (exists). The “S” shape is created when pressing the material into the bottom of the die. The real strength of the joint can only be defined by the use of destructive testing. The test specimen used for single fastener connections is described in the AS/NZS 4600 standard [6]. Shear strength testing is based on obtaining both the displacement of the joint and the existing load. The results of the test are normally presented using a diagram with an axes displacement (X) and force (Y). The experimental method described above is still the most widely used one, even though the potential of finite element (FE) analysis has been realised, and the data has been reported openly by Budde and Klasfauseweh [7], Budde and Klemens [8], W¨ossner et al. [9] and Klasfauseweh and Hahn [10]. However, the time required for FE analysis and computation is rarely less than several hours, which when combined with the requirement for verification testing, reduces the attractiveness of FE methods. Lepist¨o et al. [11] have successfully shown the accuracy of FE analysis while carefully verifying the material properties and friction coefficients. On the other hand, for limited cases,
Fig. 4. Characteristic for an ideal joint. The strength of the clinched joint depends on the neck of joint and the “S” shape (interlocking).
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Fig. 5. The measurement of the thickness at the base of the joint (parameter X) [TOX® GmbH].
simple equations for calculating the strength of clinched joint or several joints have been generated by Pedreschi et al. [12,13] and Davies [14]. The main problems encountered in the mathematical analyses of clinch joints are the difficulties related to the accuracy of predicting the behaviour of the joined materials, the exact material properties and the dimensions at different locations of the joint area. The accurate wall thickness should be available when calculating the strength of a joint. 1.5. An ideal joint An ideal joint presented by Varis [15] has a totally filled die, without leading to counter-piping or the excessive stretching of the upper sheet (Fig. 4). In addition, the sheets are completely connected. Counter-piping occurs if too much material is pushed into a die. 1.6. Evaluating a clinched joint The quality of a joint made with a fixed die is evaluated by measuring the material thickness in the middle of the base of the joint (Fig. 5). This measure is known as the X parameter and has a generally accepted tolerance of ±15% [16]. An automatic real-time quality control system was developed and presented mainly during the nineties [17–19]. The information on the displacement of the punch and an existing force are collected simultaneously. A wide range of data can be collected
into a PC connected to the control system, or a limited amount of data can be stored in the memory of the control system. The collected data can be illustrated as shown in Fig. 6, or only the maximum force and displacement can be shown on the display as output values. An average value for tens (even hundreds) of prepared joints is illustrated in Fig. 6 as a bold average curve. The grey area below and above the average curve defines the tolerance area. Any deviation from the tolerated area causes the process to stop. Several typical cases are shown in Fig. 6: (C) the material is too soft or the tool is unsuitable, (D) the material is too hard or the punch too large, or the material adhered to the surface of the punch, (E) the punch is too short, or (F) the tool is damaged. Even if it is possible to measure and observe minor changes, it is not easy to give an exact explanation for the main reason. A minor deviation from the measured force can be caused by many factors, such as the wear and tear or the eccentricity of tools, changes in the material properties or the impurity of the material surface of tools. Without much experience, a real-time process control system is a tool for explaining main problems of the process only. As described above, even if automated systems do exist, there is plenty of use for visual inspection and for analysing the crosssections of joints. Visual inspection should focus on searching for minor cracks or breaks in the area of the joint neck. On the die side, the die should be fully filled and symmetric; bevelling in any direction is not allowed.
Fig. 6. A real-time process control system collects data from the punch sensor (displacement) and on the existing force [TOX® GmbH].
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real-time process monitoring system will kept on all the time. 2. Objective of the study
Fig. 7. A recommendation for quality control methods while ramping up a new product [TOX® GmbH]: (A) visual inspection (non-destructive); (B) destructive testing of the joint; (C) analysis of cross-sections; (D) real-time process monitoring.
Using the cross-section, a more detailed analysis can be performed. The observed aspects of the joint are its symmetry, material thickness over its whole area, internal cracks and breaks, as mentioned above, the behaviour of the formed material and the eccentricity of the tools. No general values have been presented for eccentricity; instead, the maximum crossing of tools is limited to three degrees. Crossing can be caused by the improper guidance of tools, the wear and tear of the guiding elements or the deflection of the C frame, which is typically restricted to a value of 0.6 mm. When ramping up a new product, all the previously described quality inspection methods should be used as shown in Fig. 7. During the development of the process, the amount of visually based inspection and destructive testing can be decreased. As a next step, quality control based on cross-sections can be decreased as well, but crosssections should be analysed later in all the cases while problems continue in the process or before any changes are made to the process or product. It is also possible to specify the analysis of the cross-section in internal standards, for instance, once every 50,000 joints. The
Clinching seems to be a simple joining method with few process parameters. However, great care must be taken in the quality control of clinching in many ways, and detailed information should be available on the process in order to assist in the analysis of existing problems. The case used in this study is based on a mass product of a Finnish company. Several existing problems pertaining to clinching will be presented, analysed and solved during the study. The information presented in this paper is widely applicable to all kinds of clinching cases. 3. Case 3.1. A product and a joint The case product consists of 100 clinched joints. One batch of the product consists of a total of approximately 200,000 joints. The material on the punch side is Alz DX51D AZ150 with a thickness of 1.0 mm and a tensile strength of 500 N/mm2 , and the material on the die side Alz DX52D AZ150, with a thickness of 1.0 mm and a tensile strength of 420 N/mm2 . During transportation and assembly, the joints are typically subjected to bending forces. Only minor (5 N) dynamic forces act on each joint during the process. The product is an essential part of a huge assembly and requires a lot of work if it collapses and has to be repaired. No unexpected downtime is allowed. A clinched joint has a diameter of 6.0 mm on the die side. The joint will be placed in a narrow area (14 mm), as shown in Fig. 8. The available space limits the size of the tools. The diameter of
Fig. 8. The location of a joint on a narrow roll-formed area.
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Fig. 9. The clinched materials are not orthogonally set the punch and the die.
the punch is 4.2 mm. The forms of the edges on both sheets have been roll-formed. 3.2. The equipment and quality control The clinching equipment was originally designed for use with two sheets that are set orthogonally between the punch and the die; in any case, this should be the basis for the process. The equipment consists of two similar clinching devices that move and clinch simultaneously on both sides of the whole product. No axial movement or swinging can occur during clinching, because of an assembly of clinching devices (a C frame) that is firmly attached to the assembly. The process can be run unmanned. A control measure, X, has been defined by a toolmaker, and its value is 0.55 mm. Measure X is measured regularly using samples of the original material. However, measurements of joints from the final product are made infrequently. 3.3. Existing problems It was possible to make only 4000–5000 clinches with one pair of tools. The punch was often broken at the end of the peak. What is worse is that the unstable and inaccurate clinching process led to broken joints in the whole product. Normally, one or two broken joints do not impead the use of the product, but more extensive repairs have to be made if more than three sequential joints fail. In this case, the problems in the product were observed quite soon after the start of the process. There is good reason to suspect that some cracks were already caused during assembly. The good news is that the long-term use of the products has not lead to damages in the joints.
Fig. 10. The shape of a joint when the punch and die have crossed.
The device was equipped with a pair of leading and bending rolls. It was observed that there was continuous contact between the sheets and the rolls and that the bending force was not sufficient to straighten the sheets orthogonally against the clinching tools. The clinching device had been in used for a long time, and hundreds of thousands joints had been made using the same machine; therefore, the tool guides were worn, and it was no longer possible to ensure the accurate movement of the tools. 3.4.2. Shapes of the joints When making an ideal joint, the material moves straight against both sides of the die, as is shown in Fig. 4. There is no additional bending in the material exists, as can be seen in Figs. 10 and 11. Using accurate measuring equipment, the problem related to the tool heads not being parallel can be observed when comparing measures X1 and X2 (Fig. 10). As was explained earlier on, many problems can be observed in a longitudinal cross-section of the product (Fig. 11). Fig. 12 shows, instead, a perpendicular cross-section, and the joint appears to be a typical clinched joint. When verifying an unexpected joint and a section (5) in an ideal process, shown in Fig. 6 by Liebig et al. [20], the main difference can be observed at the end of the pressing process. In
3.4. Analysing problems 3.4.1. Position of the tools Visually, it was easy to verify that the sheets to be joined were not set orthogonally between the punch and the die (Fig. 9). The clinching equipment and the whole process were designed based on a certain degree of orthogonality. The roll-forming of the sheet edges was altered at some point because of formability problems in the upper material. Nevertheless, no changes were made to the clinching process or devices.
Fig. 11. The cross-section of a joint separated from a real product. There is additional upward bending on the left side of the joint.
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Fig. 12. The joint looks like a typical clinched joint when observed in a direction perpendicular to the edge of the product.
an ideal case, the material is extruded uniformly and horizontally straight to both sides. However, in the case of non-orthogonal tools, the process starts just like an ideal process, but only until the material reaches the bottom of the die (the left corner (A) in Fig. 13). Since corner A is fully filled with the material to be joined, the other corner (B) is still unfilled, because the tools are bevelled. Since the punch has not reached the lower dead centre, more material is pushed into the die. Both the upper and lower sheets deform together and move against corner B; the result of such a case has been observed as the above-mentioned unexpected joint. 3.4.3. Tool failures During the clinching process, the edges of sheets are bent by the clinching force. The clinching force increases linearly, and the maximum bending force is achieved at the end of process. The joined sheets slide against each other until just before the completion of the joint, which means that the bending force causes an increasing horizontal force against the punch until the joint is capable of carrying the forces acting on it. The horizontal force bends the peak of the punch; in this case, the maximum penetration is 3.75 mm. As the clinching force is removed, the second bending impulse occurs against the peak. At this point, the duration of the impact is very short. The punch has a peak diameter 4.2 mm, the diameter of the frame of the tool is 14 mm and the peak length is 10 mm. The maximum strain is exerted on the base of the punch peak, which explains the tool failures.
Fig. 14. The contact between the sheets is not tight, and additional bending has occurred in the joining area.
3.4.4. Failure modes of the joints The failed joints of real products have been thoroughly investigated. Failure modes A, B and C, which were discussed earlier, were all observed. By analysing the failed joints, it was possible to observe that the force that led to a joint failure acted in the longitudinal direction of the product. Some joints, which did not open but which could not carry any load, were heavily tilted in the direction of the exerted force. The joints, which had completely failed, could be analysed on both sides, and it was possible to make the same observation as above. When the surrounding of joints were analysed, it was possible to observe scratches in the vicinity of the joint. The scratches led to the assumption that the gripper of the tool was not been used as planned. The unexpected bending occurred on the area of joint, and no doubt, the accuracy of the tool positioning did not reach an acceptable level. The joints, which exhibited the A failure mode, “a broken neck”, were thoroughly investigated. As described earlier, failure mode A occurs typically in cases, in which the diameter between the punch and the die is too small; this leads to too little material in the neck region. In this case, the joining circumstances are constant with respect to the materials, tools and X measure. Therefore, further reasons for the failure mode were investigated. Minor deformations were observed on both sheets in the joint area, as shown in Fig. 14. The deformations make it impossible to achieve continuous contact between the sheets, which leads to a situation in which the total thickness of joint area is greater than expected (two times the sheet thickness). The chosen tool combination is, therefore, not suitable for this type of joining task. The sheet on the die side will be deformed more than planned, which typically means that it will be over-stretched in the region of the neck, and failure mode A is obvious. The main reasons for this type of failure (such kind of existing case) are that the sheets are not held firmly enough during clinching or the tools are positioned inaccurately, as described earlier. 4. Solving the problems 4.1. Changes to the clinching device
Fig. 13. A case in which material is pressed into a die, which is not parallel to the head of the punch.
The problems encountered in clinching can be summarized as being based on the non-orthogonality of the sheets and
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analysis of the cross-section, the shear strength of the joint, the tool life and failure mode, and finally, an analysis of the whole process.
Fig. 15. Additional elements, which allow swinging and linear movement in the clinching device, have been installed.
tools. Therefore, the clinching equipment has been modified as described below. An automatic real-time process control system has also been installed in order to fully ensure the quality of the process. The problems related to bending sheets while clinching were discussed earlier on. Therefore, very sophisticated clinching equipment has been modified with additional swinging and linear movements, as shown in Fig. 15. The swinging movement has a sufficiently wide range of movement, which takes care of changes in the bending of the edges. With the previously described modifications, the clinching device is now able to set itself easily to the required position before the punch penetrates the sheets. The punch and die are kept orthogonally positioned around the joint area throughout the whole process. No bending of sheet edge takes place, as a result of which there are no horizontal shear or bending forces against the punch. 4.2. Automated process control system An automated process control system was installed at same time as the modifications were made to the clinching device. Both clinching guns will be separately controlled in real time with clinching. The force being exerted will be measured using a strain gauge installed onto the C-frame. The maximum clinching force will be collected onto the memory of the control system and will be easy to analyse afterwards. As described earlier, the control system has an alarm range that leads to the immediate interruption of the process if a remarkable overflow or underswing occurs in the pressing force in comparison to the movement of the punch. In addition to automated process control, the instructions for controlling the process were also written as listed below. The control list consists of five headlines; the outlook of the joint, an
(1) The outlook of the joint: • Does the value of the X measure vary within an accepted range? ◦ A deviation of ±15% is generally accepted. ◦ Measuring X on both sides of the joint will indicate how parallel the tools are. • Does the joint look symmetrical when looked at from the die side? ◦ No bevelling is allowed in any direction. ◦ The die must be fully filled with a round edge. • The grippers are not allowed to touch any other part of the product. • The traces of the grippers should be symmetrical. (2) Cross-section: • The cross-sections in both directions (the longitudinal and the direction perpendicular to the longitudinal) should look symmetrical. ◦ Is the amount of material on both sides of the joint equal? ◦ Does the joint look symmetrical? • Are the sheets in full contact with each other? • The sheet on punch side is not allowed to be stretched in the area of the joint neck. ◦ The joint neck must be sufficiently thick to be able to carry the forces acting on the joint. • Does the joint have a nicely shaped interlocking? ◦ Additional downwards bending is not allowed. (3) The shear strength based on destructive testing: • Does the joint achieve the expected strength? ◦ In this case, the shear strength should be more than 1800 N. ◦ The joint should achieve a shear strength of at least 1400 N in the axial direction. (4) The lifetime of tools and failure modes: • It should be possible to manufacture at least tens of thousands of joints using one pair of tools. • Analysis of the failure mode of the tool. ◦ Has the punch failed on the base of the punch peak because of bending? ◦ Has the punch split as a result of an excessive compressive force? ◦ Does the tool (especially the punch) bear natural marks of wear? (5) The clinching device: • Does the whole process function properly? ◦ Is there continuous contact between the leading and bending rolls? 5. Results of the modification During the ramp-up time, more than 19,000 clinches were made using one pair of tools. This amount is 4–5 times more than the starting value. The goal is 30,000 clinches, once the
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process has been fully adjusted. The most remarkable change is the so-called flowing C-frame, which makes the positioning of the clinching device more accurate than with the former construction. A little cutting oil is spread onto the location of the joint just before clinching. Hahn et al. [21] have been reported the advantages of a thin oil film in terms of the tool life and decreased pressing force. The force needed for clinching is 36 kN, which is 3 kN less than that reported by a toolmaker. The result was not distinct, because the toolmaker in question has used small individual test samples, and the value of 36 kN was measured while clinching a real product. Previously, the importance of the construction for the pressing force was analysed, and it the pressing force should be higher in comparison to the values obtained from tests with individual samples. The decreased pressing force can be explained with the above-mentioned lubricating system; also, some minor changes are explained by the change of the material properties. Some deviation was expected, because of differences in the measuring devices. As a whole, the process seems to work properly. The reliability of the process has increased greatly, because of the total process control, which has also increased customer confidence. While having the possibility to monitor every single joint, the whole construction will be fully checked totally with care. The importance of a complete check is undeniable, as mentioned earlier with respect to the occurrence of problems.
6. Conclusions Continuous quality control is a necessary and important part of the total process. Both visual and simple methods, as well as automated real-time processes, are suitable quality control systems. There are no unimportant observations and deviations; all changes to and details of the clinching process affect it. The tools and sheets must be positioned correctly. No bending of the tools is permitted during the clinching process. The eccentricity of the tools must be controlled regularly. The lifetime of clinching tools should be normally tens of thousands joints for a material in this case. If a failure occurs exists earlier, the whole process must be thoroughly investigated. The failure mode of a tool correlates with the existing types of problems. The condition of the tools, glides and whole clinching device must be controlled regularly. Wear in any sliding element leads to the inaccurate movement of tools and to problems in the strength of the joint. An automated process control system makes it possible to implement full control. Destructive testing plays a major role as an additional testing method. In particular, the coexistent use of several analysis systems is recommended when ramping up the system. The significance of changes to the process conditions or the design of the product must be analysed carefully from the manufacturing point of view. Changes in any area must be taken into account in the whole process.
7. General suggestions for further research An automated process control system makes it possible to easily collect huge amounts data. Commercial analysis software can or could be used more efficiently for analysing this data. The questions that require further study are related to the prediction of the wear of tools based on the analysis of data from an extended period. The currently available data would also be useful for the proactive maintenance of the clinching device.
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