Choosing the simple way to design deep-drawing tools for complex sheet metal parts
“For us, car body panels are just like paper,” says Stanislaw Barczycki, Chief Executive Officer at Barczycki & Bernardi GmbH. The German tool-making and engineering company produces complex deep-drawing and follow-on composite tools used to form heat resistant stainless sheets to design, among others, manifolds and mufflers for exhaust systems. Tools are designed with MISSLER's TopSolid, an associative 3D CAD system, which facilitates and shortens the development process considerably.
There is a real art to producing deep-drawing tools for sheet metal forming. The tool-maker's know-how and experience cannot be replaced by computer systems – no matter how powerful they are and how they might facilitate work. Whether a tool works as planned sometimes depends on small details, as one of Barzczycki's anecdotes shows: “We had a problem with a tool that was working well at the customer’s site. Finally, we found out that our drawing emulsion was too old – the lubricating properties of the water/oil mixture had simply changed.”
During the forming process, Barczycki & Bernardi try the material's strength and elasticity to their limits which is, of course, a reason why the seemingly irrelevant circumstances described above do have an impact on the products. Situated in Homburg/Saar near the French border, the company specializes in the development of tools for the mass production of complex inner body panels and sheet metal parts for exhaust systems, which are both very hard to form. Even if the strict mathematical rules of Finite Element simulation would not allow for forming certain sheet metal parts, the tool-making company often succeeds in finding an appropriate solution.
In addition to follow-on composite tools, the company develops and manufactures progressive die tools weighing up to seven tons. These tools allow to perform the entire forming process of sheet metal parts on a transfer press and are utilized by major automobile manufacturers and their premier tier suppliers, i.e. system suppliers like ThyssenKrupp Automotive or exhaust system manufacturers such as Boysen and Eberspächer. “While the Audi A8 was changed to include rear-seat airbags, we have carried out – under strict supervision by Audi – the design of the complete tool series required to create the new aluminum parts of the inner car body,” says Stanislaw Barczycki.
The beginning
The business started rather small: In 1990, Barczycki founded his own company. This was a risky decision, for even at the time, the minimum investment for basic tool-making machinery was several million deutschmarks, and Barczycki's seed capital was more or less limited to the experience he had gained as senior designer with his previous employer. In the first few years, the company managed to live on repairing and improving tools that were mostly produced abroad. To save costs, the German automotive industry decided in the early 1990s to produce tools in Spain, Portugal, Italy and Eastern Europe. This decision turned out to be a naïve fallacy that ruined many German tool-makers and, according to Barczycki, turned out to be very costly for the manufacturers themselves.
“Very often, the tools had to be improved later on due to poor part quality. All in all, the automobile manufacturers would have paid less if they had produced their tools in Germany in the first place. It also happended that the tools needed a lot of repair and therefore caused high maintenance costs. It goes without saying that whenever a tool breaks, manufacturers do everything they can to repair it as soon as possible to avoid production downtimes. Sadly, downtimes are billed on another cost center, so the actual cause is not always as transparent as it should be.”
In the mid 1990s, the automobile manufacturers improved their tool quality standards – and so did business for Barczycki & Bernardi. With its 25 employees, the company’s annual turnover meanwhile amounts to more than 5 million deutschmarks, and the company is working at full capacity. According to Barczycki, who would welcome additional capacities, this is a prerequisite for the company to operate profitably in times of high cost pressure, especially since the industry is facing ever increasing time pressure: ten years ago, the development and manufacturing of a tool set could be spread over twelve months, whereas today, three or four months must suffice.
Each form consists of several parts which are processed and assembled by experienced tool-makers. (Photo: Wendenburg)
At the same time, the sheet metal parts to be processed become increasingly complex, as designer Wolfgang Doerr explains. “Take an exhaust system, for instance. Car engines become more powerful and regulations for exhaust emission control and sound absorption grow increasingly strict. At the same time, the available space underneath the floor pan becomes less with every new model. Therefore, sheet metal parts sometimes look peculiar. The customer's goal, however, is to form these parts with as few operations as possible, for each forming operation adds to the cost of personnel, manufacturing and maintenance.”
Tool design
To ensure a fast and flexible response to all customer requirements, it became imperative to use a powerful 3D CAD system for tool design. Already in the early 1990s, Barczycki & Bernardi replaced traditional drawing boards with a surface-based CAD/CAM system to facilitate the design and milling of tool forms with the aid of free-form surfaces. This was when the enterprise made first contact with the West system house in Sulzbach near Homburg/Saar. In the mid 1990s, West added MISSLER's associative 3D system TopSolid to its range of distributed products and quickly succeeded in pointing out the benefits of solid modeling to Barczycki & Bernardi.
The TopSolid Pro software comprises both the associative solid modeler and the TopSolid'Draft 2D design tool. The current version 6.4 also contains the TopFold module for the unfolding of sheet metal models. MISSLER’S software is installed on all four of Barczycki & Bernardi's CAD workstations. On one computer, the previous version of the system is still installed. Thus, when older tools need to be repaired, employees can quickly access archived surface models and rework faulty forms in a few milling passes. “The tools are used to produce spare parts even for vehicle models that have not been built for decades,” explains Doerr.
The tool-makers can never be sure about the strategy they have chosen until they have tested the first forms on the press. (Photo: Wendenburg)
The computers used are high-performance PCs with 1,024 MB of RAM and SCSI bus technology that allows the processing of the large volume of data needed for large tool models in an acceptable amount of time. “A memory of one GB is quite a small capacity for us,” says Doerr. All workstations are interconnected through an NT server on which all model data and 2D drawings are stored and can be accessed by all users. The network environment is very advantageous as designers often work on a tool simultaneously (e.g. one models the form while another creates the drawings of the whole tool).
Combining 2D and 3D technologies
At Barczycki & Bernardi, only forms are created as 3D designs whereas the entire tool structure is done in 2D – the only exception being tailored 3D models. Complex devices, however, are entirely generated as 3D designs so as to avoid collisions. According to Doerr, the tool structure can be created much faster when using a 2D design.
“You have to bear in mind that our tools are unique – they are only manufactured once since each customer has specific requirements for standard parts to be used, for instance. Thus, it is quite difficult for us to come up with a uniform tool structure. To do so we would either have to integrate 3D libraries of all standard parts suppliers – which would require a dedicated database server – or we would have to build a parametric library, which is very costly and time-consuming.”
Forms are modeled from the 3D geometries of the part to be formed. In general, these are CATIA models that can be imported in TopSolid via a direct interface. Doerr is quite satisfied with the CATIA interface, although it supports data import only. Data export is possible, however, by using the bidirectional STEP interface that is included in the new version. Furthermore, importing and exporting CAD data as ACIS files is supported by TopSolid version 6.4 and more recent versions.
Imported model data of the parts to be manufactured serve as a basis for creating a 3D design of the tool forms. (Photo: Wendenburg)
From Doerr's point of view, the current version of TopSolid incorporates improved and more powerful functions for the processing of imported data. The software also analyzes the geometry and then automatically converts B-spline cylinders into round bodies that require less memory. Surfaces are smoothed during import, which further reduces data volumes.
However, one of the most striking features of the system is its ability to undo fillets or chamfers on imported models and to restore sharp edges by trimming the respective surfaces. “This function is really amazing since it greatly facilitates the processing of imported geometries,” says Doerr. “You can remove a fillet and quickly insert a draft. Subsequently, you can fillet the modified geometry with the same radius.”
Working with solids
The position of part geometries received by the tool designers usually corresponds to the position where the parts are installed in the vehicle. Thus, the part geometries must be repositioned to avoid gouges within the tool. To facilitate further processing of the form, the base data are converted into a solid. “We frequently use fillets, which can be created in a solid model with a simple click of the mouse. In a surface model, you need to define boundaries and intersections for every single surface, which is much more complicated,” explains Doerr.
Once the basic geometry has been defined, designers and tool-makers come together to determine the workflow of the deep-drawing process. The number of forming steps or drawing operations depends on the shape of the part – parts with steep edges or sharp corners cannot be deep-drawn in a single drawing operation because the sheet metal would break. Before the first drawing operation is performed, it may therefore be necessary to slightly lift the original form so that more material is available for corner and edge processing. For creating crowns, Doerr either uses the patchwork function, allowing him to place a surface on top of several auxiliary curves, or he places a small solid on the part geometry and simply fillets the body with the rest of the form. The current version of TopSolid incorporates filleting functions with an unmatched level of versatility and ease of use.
The use of solid models has accelerated the milling process because each part of the form can be milled separately. (Photo: Wendenburg)
Tool-makers can never be sure about the strategy they have chosen until they have milled the first forms and tested them on the press, for only very expensive FEA programs that place high demands on computing power can perform simulations of the deep-drawing process and may deliver results that are more or less accurate. In contrast to small tool manufacturers, automobile manufacturers can afford such programs. That is why the first forms are milled in steel of minor quality. These forms are often used to build 50 or 100 prototype parts for the manufacturer, which are then subject to comprehensive load tests.
Depending on the test results, the manufacturer needs to modify the part design, the consequence being that the tool-maker needs to adapt the forms accordingly. “We have no choice but to generate a tool structure that allows for a certain degree of flexibility,” says Doerr. “If the dimensions of a part are modified, our flexibility is dramatically restricted because there are often several stages on one tool that need to be adapted accordingly.”
Flexible use of parametrics
Thanks to TopSolid's parametric and associative design environment, changes can be made in no time. While a designer is working in associative mode, the program records all design steps in order and displays it in a tree structure. To change the radius of a fillet or the angle of a draft, the designer simply needs to move to the respective node in the tree and then alter the original parameter value.
While creating the structure of the model, however, the user must not forget the order in which operations are performed. “Associative links may cause the entire part to change if you modify a single parameter,” explains Doerr. The order of these links is not final, though. One of the new key functions of the current version of TopSolid enables the user to subsequently change the tree structure. Fillets, for instance, can be moved to another location in the model tree by simple drag & drop functionality.
In addition, the user can always turn off the model tree by switching to the non-associative mode. Individual model versions can be stored as a non-associative model, and the user can continue to work with the non-intelligent geometry model using the associative design features. There is yet another advantage to deleting parametric information: The data volume of the models is reduced dramatically. As the system is capable of detecting design elements such as fillets or chamfers of non-parametric models as well, the combination of associative and non-associative design ensures maximum flexibility.
Starting the milling process much earlier
The use of solids considerably speeds the process of creating forms required for deep-drawing operations. “If only a particular portion of the basic form is required for a certain drawing phase, you can simply cut and mill this portion with a few mouse clicks. Surface models cannot be processed in the same way because they cannot identify the part context. In a solids-based system, the CAD model exactly matches the form of the part,” explains Doerr.
As milling programs only need to be generated for a particular section of the form, machine runtimes in the workshop have significantly shortened. It is difficult, however, to determine the amount of time that is actually saved because it depends on the complexity of the part or the form, respectively. “The more complex the parts, the more time can be saved,” says Stanislaw Barczycki. “For some parts we save up to 90 percent of runtime.”
Working with solid models without parametric information that have much smaller data volumes than similar surface models also facilitates data exchange with customers. And Doerr is right – it does make a difference whether 200 MB or 2 MB are sent via an ISDN connection. “We usually archive the CAD data of the tools for our customers. In case of a crash, the customer must have quick access to this data in order to mill the faulty part. Today, we supply our customers with a STEP model as it looks like in the tool.”
By Michael Wendenburg, Sevilla
Photos BG Concept Marketing Communication
