The flatness of the product plays an important role in the processing of plate material. For this reason, the EN 10029 standard examines flatness and maximum admissible flatness deviations. This standard differentiates between normal (Class N) and restricted flatness tolerances (Class S). As a general rule, unless agreed otherwise, plates are supplied with normal flatness tolerances.
EN 10029 also defines the measuring method for determination of flatness and specifies limits at 1,000 millimetre and 2,000 millimetre measuring lengths. It does not, however, provide any data on the measurement of flatness across greater measuring lengths, to cover the entire surface of a plate. It should be noted that the plate must be located on a flat substrate (a "plane table") for measurement. The measurement is then performed at a defined distance from the edge of the plate (e.g. not less than 25 millimetre from the longitudinal edges) using a precision calibrated straightedge (a large, high-precision "ruler") and, if necessary, also spacers. The distance between the measuring straightedge and the plate surface is measured. The greatest distance between the plate and measuring straightedge (the "wave trough") or the difference between the height of the spacer and the smallest distance between the plate and the measuring straightedge (the "wave crest") is defined as the flatness deviation.
Figure 1: Flatness measurement as per EN 10029 at 1,000 millimetre and 2,000 millimetre measuring length (schematic diagram)