The concept of punching designates a slitting process in which a sheet is severed in one stroke. Shapes such as round holes are created in the part, and external contours are cut with single strokes.
A punching machine works like a hole punch for paper. The punch presses the paper against the support of the hole punch and finally into a round opening. The scrap from the punching collects in the hole punch container.
Punching works exactly the same way: the sheet is positioned between the punch and the die. The punch moves downward and plunges into the die. The edges of the punch and the die move past each other in parallel, cutting the sheet.
Observed in detail, the punching process proceeds in four phases. When the punch touches the sheet, the sheet is deformed. Then it is cut. Finally, the tension within the material is so great that the sheet breaks along the contour of the cut. The cut-out piece of sheet - the so-called punching slug - is ejected downward. When the punch travels upward again, it can happen that it pulls the sheet along. In that case, the stripper releases the sheet from the punch.
The higher the fraction of cut on the sheet edge, the better the edge quality. For precise fits, for example, preliminary holes are punched and then the final diameter is punched out with a slightly larger tool. The fraction of cut along such an edge is then as high as 100%.
In nibbling, the punch holes are set down to overlap. In this way, openings and contours of any shape can be created. Nibbling is used for large radii or irregular shapes. The cut marks of the individual strokes remain visible at the nibbled edge. The marks can be felt by running a finger along the edge. The more the holes overlap, the smoother is the edge. To achieve this effect, the punching machine must deliver more strokes along the nibbled stretch.