MARGINS AND PERFORATED AREASA major advantage of perforated metal plates is that they can be produced with unperforated or plain margins on 4 sides, or with perforated areas of various shapes alternating with unperforated areas. This is particularly important if the metal plates must subsequently be finished by bending and/or flanging or by adding fixing holes. The designer can exploit the capabilities of modern punching presses to obtain a perforated plate that can be directly transformed into a finished product, without the need for additional components. A drawing is obviously needed to define the shape and size of a metal sheet with irregular full areas alternating with perforated areas. The full areas and margins are always measured from the outside edge of the holes. Technical drawings show perforated metal plates from the punch entry side. It is in any case advisable to specify the burr side for asymmetrical perforated plates. This becomes mandatory if the plates are to undergo additional operations such as bending. We should note that wide margins or large unperforated areas make
it difficult to obtain perfectly flat sheets. The mechanical stress of perforation causes bulging and buckling that, in extreme cases, cannot be corrected even with highly sophisticated leveling rolls. Large unperforated areas can also produce dimensional changes during perforation and leveling. A final trimming operation therefore becomes necessary when tolerances are tight. Because of the substantial irregularities that result from perforation, trimming causes the size of the margins to differ from the nominal values.