Coil Nesting

Bodies of cars are made from some hundreds of metal parts. In the production process these three-dimensional parts are cut from two-dimensional sheet metal before they are formed and welded together.

The two-dimensional sheet metal is available as long rolls which are called coils. Coils can have different widths, material properties and prices. Each metal part in the car has certain requirements regarding the quality of the metal it is made of.

In the CoilNest project we minimize the material costs of the sheet metal by assigning the two-dimensional parts to appropriate coils of sheet metal and by nesting them on these coils under consideration of a given set of production constraints.

Due to machine restrictions only a few different parts are allowed to be nested together on a coil. This is different from many other classical problems like marker making on fabric and leather where up to many hundred pieces have to be arranged. The parts that are placed on the coil form a periodically repeating pattern - see the picture below. The width of the coil is selected by the optimization algorithm.

© Photo Fraunhofer SCAI
The problem solved in the CoilNest project is not a pure packing problem like the problems arising in textile and leather manufacturing industry. In the production of car bodies a substantial part of the optimization consists in deciding which parts are assigned to the same coil. This problem belongs to the group of set partitioning problems.

Some of the supported constraints are:

  • a maximum number of parts per repeated pattern
  • free or restricted rotation of the parts
  • placing the parts upside down allowed or forbidden
  • different requirements on the type of metal for each part

The CoilNest project is tailor-made for the needs in the production process at BMW. However the arising optimization problems and constraints are very typical for the car industry in general. Moreover, the concepts developed to handle multi-coil layout, periodic placements, and layouts of very few parts can be used to solve nesting problems in other branches of industry.

Related software products are AutoNester-T and PackAssistant. AutoNester-T optimizes the automatic placement of markers onto fabric panels. PackAssistant helps to find the best possible solution of how standard containers, e.g. skeleton boxes, should be packed with individual parts of the same type.