Hydroforming tools Toolmaking: Tailor-made for every task

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Siebenwurst makes large hydroforming tools for complex automotive parts, among other things. For the automotive supplier Benteler, the efficiency experts are currently building a tool for axle carriers in which three different components are formed. For the cylinders, they are relying on special models from their partner AHP Merkle, which has proven itself for decades.

The Siebenwurst company also has a good name for complex manufacturing tasks in the automotive industry. In addition to models, injection moulding tools for bumpers, for example, as well as forming tools, embossing tools and tools for hydroforming are produced at the Dietfurt location in Germany.

The toolmaking company from Dietfurt in the Altmühltal already has a long partnership and many successful joint projects with the automotive supplier Benteler as a client. “The tool design usually comes from Benteler,” explains Christian Stephan, design engineer at Siebenwurst. “The specifications are always very precise in the projects — they know exactly what they need. It's clear from the outset which steel is used or how large the cylinders used in the tool have to be.”

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Two tools for axle beams

One specific project involves the tool set for axle beams — the left and right side members as well as the cross member are machined in the tools at the same time. On the one hand, the tool set consists of a preforming tool, in which the parts preformed from tubes are pre-stamped for machining. The heart of the machining process, however, is the hydroforming tool, in which the axle beams are given their final shape and finished at the same time.

“This project is a first for us in several respects,” explains Stephan. “The tube of the axle beam components, which is preformed from hot-work steel, has a relatively high wall thickness, and the pressure with which we form is about two times higher than usual.” In addition, defined openings are to be punched in the IHU tool in the formed workpiece at the same time.

Close cooperation with client and suppliers

Benteler had already carried out extensive preliminary work on this tool and determined the material to be used as well as the specifications of the hydraulic cylinders to be installed in the tool design. The positions for the subsequent openings in the workpiece were also specified.

The designers at Siebenwurst filled these specifications with practical experience and used them to design tools that implement their production tasks precisely, quickly and reliably. And the design engineers at AHP Merkle took over the finishing touches and ensured that the hydraulic cylinders were also optimised in their details to suit the tool and application.

Hydraulic cylinders are used in the tools for a wide variety of tasks. The spectrum here ranges from the cylinders that have to securely close the pipe ends during the hydroforming process, to the cylinders for inserting precise punch openings, to cylinders for fixing as well as lifting and ejecting the workpieces for automated robot-assisted removal.

Cylinders move very precisely in line with actual demand

“For hydraulic cylinders, we have relied on products from AHP Merkle for years,” Stephan emphasises. “We've had very good experiences with our contacts at the Gottenheim plant right from the start; communication is excellent. Especially at the level of the design departments, a very close cooperation characterised by mutual understanding has been established. In addition, the quality of the cylinders is also very good — we receive consistently positive feedback from our customers. In addition, AHP Merkle delivers on time — even for special cylinders. And the price is also excellent.”

The blanks are first inserted into the preforming tool — these are tubes with an approximately square cross-section that have been preformed in a bending device. The preforming tool is designed as an embossing tool in which the blanks for longitudinal and transverse beams are pre-machined. It roughly forms the contours of the workpieces when they are brought together.

In this tool, block cylinders take over the fixing of the workpieces on the one hand, and on the other hand they also introduce a surface into the tube at one point. “In addition, there is a lot of sensor technology in the tool and on the cylinders — the processes are meticulously monitored and documented,” Stephan adds. "This means that the cylinders can be run very precisely in line with the actual demand.

High standards for cylinders in the hydroforming tool

The cylinders in the hydroforming tool are of a different calibre. Especially those that have to work against the internal pressure in the workpiece during the forming phase. In the case of the longitudinal members, there are four round cylinders with a diameter of 360 millimetres, while a cylinder diameter of 250 millimetres is sufficient for the cross member.

“A challenge in the hydroforming process is that the tubular elements become shorter during forming,” Stephan explains. “In this case, the cylinders that close the tubes have to readjust very carefully so that an absolutely tight seal is ensured during the entire forming process.”

During each cycle, the cylinders close the mould with their sealing mandrels. During closing, the workpiece is filled via special channels — the first thing to do here is to build up the gas volume. The high pressure itself — relatively little volume is moved — is then built up by a high-pressure unit in a separate circuit through a feed in the sealing mandrels. The wall of the workpiece is thus pressed against the mould contour and formed, and the workpiece is given its shape.

Sustainable use of existing production presses

“At Benteler, we deliberately push the limits — within safe and process-stable parameters, of course — in order to make sustainable use of the existing machinery,” Stefan explains. “It is all the more important that all the components involved are optimally coordinated in advance.”

Still under high pressure, numerous openings are made in the workpiece by further cylinders using punches. Their parameters also come from the tool design at Benteler. They are precisely matched to the material thickness and the size of the opening to be created. The special cylinders, which are operated at a pressure of 100 bar, are adapted to the very narrow installation spaces in the tool at Benteler’s request.

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Punching happens at exactly the same time

“It is important that the punching happens absolutely simultaneously everywhere,” Stephan emphasises. “Because as soon as the first punch penetrates the wall, the pressure inside the tool collapses abruptly.”

All these cylinders move back to their starting position at the end before the tool opens again on the press. Then another set of cylinders comes into play to ensure that the workpiece can be demoulded in a controlled and gentle manner after it has been demoulded on the press.

The cylinders of the upper half of the mould press the workpieces out of the mould, the ejectors of the lower half lift the parts. “This ensures safe demoulding, and the grippers of the automated parts handling system can safely grasp the components and transport them away," Stephan explains. "For absolute synchronicity, the cylinders for each nest are controlled via a separate circuit.”

Optimum cycle time for complex machining sequence

With these tools, Benteler achieves an optimum cycle time in the production of the axle carriers — they are very fast here for the complex machining sequence. “The people responsible for designing the tools at Benteler really know their job and hydraulics,” explains Stephan. “Our tool design can dock directly onto the specifications. And the designers at AHP Merkle support us in implementing all this as optimally as possible in steel and iron.”

In the IHU tool, a total of 25 core pulls are moved separately over the cylinders. A large part of the cylinders is equipped with a special anti-rotation device, which further increases process reliability in production.

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Grown cooperation based on partnership

The fact that Merkle supplies around 98 percent of the hydraulic cylinders at Siebenwurst is no coincidence. The lived values of the family-run companies fit together excellently. In addition to the high quality, short delivery times, individually configurable cylinders and fair pricing, the decades of excellent, open and trusting cooperation between the design departments is certainly one of the keys to their joint success.

“It is a great advantage here that the experts at AHP Merkle support us very effectively in this and meticulously implement the exact specifications in the cylinders,” Stephan emphasises. “Our design departments have a very short line to each other and we speak the same language when it comes to technical solutions. That's how we get the ideal hydraulic cylinders, customised for the respective application, quickly and in high quality. AHP Merkle is a valuable partner for us who fits in with us and who makes a lasting contribution to our company's success in our supplier network.”

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