Manufacturing truck engines, such as those produced at the German Mercedes-Benz plant in Mannheim, requires the highest level of precision in order to ensure the best possible engine properties. The cylinder blocks have complex geometries and numerous ‘key characteristics’, for which the highest tolerance classes must be reliably maintained.
Depending on the engine type, these include multiple H7 fits, various features with accuracy requirements of less than 15µm, and 20deg chamfers with an angle tolerance of ±0.025deg. All these must be achieved while meeting demanding requirements relating to surface finish and cost-effective manufacturing.
Mercedes-Benz commissioned a machining line for its modern truck engines in 2011. When planning a second line, the company’s engineers drew on the experience gained with the first line and identified the machining of the bores for the cylinder liners as particularly challenging; they decided to split it over two stations — one for ‘pre-machining’ and one for ‘fine machining’.
They also asked Mapal, which has a UK subsidiary in Rugby (
www.mapal.com), to develop the optimum tooling solution.
The tooling specialist developed complex actuating tools that are equipped with up to 20 indexable inserts, including ISO inserts and custom inserts. The actuating tools feature a monoblock-type tool body with an HSK-160 connection and a 200mm-diameter interface.
A single tool weighs some 60kg; and despite a tool length of 380mm, it must have a radial run-out not exceeding 3µm.
The slides are actuated by a drawbar (via an additional NC axis in the machine controller) that had to be integrated within the HSK connection, while also allowing automatic tool change.
This challenge was solved with a high-precision bayonet that couples with the drawbar automatically during a tool change. Inside the tool, the movements of the drawbar are transferred to the slides via carbide ramps; and in order to achieve the required production rate, three spindles machine three bores simultaneously.
Furthermore, each spindle is assigned three tools (the one in use, a ‘sister’ tool that is in reserve, and one that is being maintained at Mapal). These tools have been manufactured to extremely high standards; for example, the slide shafts in the tool body are lapped by hand.
Maximising throughputWith machining operations like these, the cutting edges of such tools have historically been replaced and adjusted in the working area of the machine; this is not generally a hospitable environment, and the machine cannot operate at this time.
Mapal consequently developed a concept for an automatic tool change and for adjusting the tools outside the machine; while this is a normal procedure with machining centres, the dimensions of these tools meant that automatic exchange was particularly challenging.
To ensure that external adjustment would be as simple as possible, Mapal developed a number of devices for transporting, adjusting and maintaining the tools, including a handling solution.
The tools are inserted into the machining system while hanging, as the effort of ‘turning over’ a tool twice manually would have put the personnel under a lot of strain — and risked the tool being damaged in the process.
This is why Mapal also designed setting fixtures into which the tools are inserted in a hanging position, along with a cart for ‘hanging transport’ between the machining system and the setting fixture.
The setting data established on the setting fixtures are transmitted directly to an RFID ‘chip’ on the tool, which the machining system controller reads when the tools are loaded.
Furthermore, the machine can adjust key dimensions during machining using the actuating slides — a capability that allows closed-loop quality control, which was a high priority for Mercedes-Benz.