Machining engines at BMW

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Angular heads help compensate bores in crankcases at BMW’s manufacturing plant

Compensating bores or pulsation holes, as they are called at BMW in Steyr/Austria, are bores running laterally in the crankshaft bearings of 6-cylinder gasoline engines.

To machine them, additional front-end holes would have to be drilled, which afterwards would have to be closed with screw plugs. A specially developed angular head by ROMAI, adapted to the tight space available within the crankcase, as well as to the high cutting forces, and the limitations of the automatic tool changer (ATC), makes this effort redundant.

The milling of the so-called pulsation holes on the crankcase is seen as one of the most challenging machining steps when manufacturing 6-cylinder gasoline engines. Pulsation holes are pressure compensation holes, which run laterally within the crankshaft bearings. To manufacture these in the conventional way, front-end holes have to be drilled on the crankcases from both sides across the whole chain casing.

As a tool, a three-stage spiral drill would be used. Apart from the effort of three additional bores, the front-end holes would have to be closed again in a downstream bolting station.

An alternative, but considerably more efficient method to drill these pulsation holes is the use of an angular head with a milling tool. However, the very small space between the crankcases makes this variation in processing a very elaborate process.

For this, ROMAI, specializing in rotational mechanics (with a focus on the development, construction, and production of customer-specific precision transmission gears for machine tools), which manufactures multi-spindle heads, angular heads and driven tools, developed and built an angular head designed for the application.

Apart from the extremely slender and long shape, minimum specifications concerning cutting forces and the occurring breakdown torque, as well as weight constraints due to the maximum possible feeder weight of the machine were part of the functional specifications. The external structural shape, or the measurements for angular head and the milling tool is set by an immersion depth of 221 mm with a width of 70 mm. The high cutting forces of the bore, measuring 31 mm in circumference, are created by grey cast iron (GG60) and aluminum machining, mixed machining, which, in the end, is completed with a milling tool which cuts across the middle. This requires a very strong design, which is reached through rigid taper roller bearings amongst other things. The forces from machining are absorbed by a HSK63 support and an additional three-point support on the side of the spindle.

Because of construction constraints, the maximum tool change weight of 8 kg had to be considered, as the tool change would otherwise not be automatically interchangeable via the tool magazine.

“Using ROMAI angular heads in our serial production has proved successful; the machining time could be reduced to a fraction in comparison with the conventional way of processing,” says Johann Buchinger from production planning of the BMW plant in Steyr.

In addition, the downstream bolting station, and in the end also the costs for the screw plugs, are saved. “The company ROMAI has clearly prevailed in the competition for quality with other competitors and has secured the necessary technical specifications for our serial production through intensive collaboration.” Buchinger sums up.

For more information on ROMAI, contact Mittman Inc.



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