Nesting of Flat Patterns (Blanks)

Nesting of flat patterns is important in sheet metal CAPP as it directly governs material utilization, productivity and cost of production. Nesting is the placement of several similar/dissimilar flat patterns (blanks) on a larger sheet/strip to maximize material utilization and minimise scrap.

Apart from material utilization, many technological constraints need to be taken into consideration during nesting [4][6]. Important among them are as under:

• Manufacturing process, Machine
• Blanking press, Flame cutting, Laser/Water jet cutting
• No. of dies for blanking Presses
• Sheet/Strip size - (Length, Width)
• Orientation of bend with grain direction
• Clearances (bridge) between band sheet edges.
• Types of blanks - similar/different geometry.
• Preferred shop practices.

Given fig shows a typical blank placement on sheet indicating the clearances (Bridge) between blanks, sheet edges, pitch of the layout and orientation of blanks. The bridge allowances are selected by the CAPP system database created from handbook recommendation and heuristics. The values are dependent upon the material of the part, hardness of sheet, grain orientation and sheet thickness.

The problem of optimal placement of parts on sheet (Nesting) is a very complex mathematical problem. Researchers world wide have developed several computer algorithms to solve this problem using various techniques like heuristics, knowledge based (Expert System), statistical techniques and recently the genetic algorithms.

                     Figure  : Blank Layout Parameters

Problems of nesting rectangular sheet is relatively simple but nesting of irregular geometry blank parts onto rectangular sheet is very complex.

Given the parts to be nested, their CAD models, number and sheet size, the CAPP system automatically designs and displays optimal nesting layout. Various cases can broadly fall into four categories.

• Single Product, Single Row (SPSR)
• Single Product, Multi Row (SPMR)
• Multi Product, Single Row (MPSR)
• Multi Product, Multi Row (MPMR)

    Tilting Angle : angle = 20^o         Tilting Angle : angle = 204^o

      Slitting Width = 157.79           Slitting Width = 157.254

       Pitch = 105.326        Pitch = 110.300

        Utilisation Ratio = 0.4272         Utilisation Ratio = 0.4094

Tilting Angle : angle = 60^o

Slitting Width = 119.663

Pitch = 155.020

Utilisation Ratio = 0.3828

                       Figure : Typical Part and its Layouts

Given figure shows a typical part and its various layouts generated with reported material utilization ratio. The user can select the desired layout based on the machine and shop constraints.