Metal stamping parts size tolerance

Metal stamping parts of size tolerance of the GB/T13914 - 2002 stamping size tolerance 'regulation of stamping parts size tolerance. According to the regulations of the flat stamping and forming stamping parts, respectively, the dimensional tolerance. Stamping parts size tolerance value and stamping parts related to the size, size and thickness of two on the other hand, related to the accuracy level. Flat stamping size tolerance: divided into 11 levels, expressed in ST1 to ST11, including ST said flat stamping size tolerance, tolerance grade code expressed with Arabic numerals. From the ST1 ST11 precision grade lower in turn. Forming size tolerance: pressing forming precision stamping is divided into ten levels, expressed with FT1 to FT10, the FT said forming stamping size tolerance, tolerance grade Arabic numerals. From the FT1 FT10, in turn, reduce the accuracy level. Metal stamping parts limit deviation: press the hole size deviation is 0, deviation of lower deviation on dimensional tolerance; Axis deviation deviation, as the basic rule value is 0, the deviation of deviation on minus tolerance of size. For hole center distance, aperture margin, bending, deep drawing, such as height and the length of the deviation rules for half of the size tolerance. Inspection methods touch with clean gauze to clean the surface of the outer covering parts. Inspector need to wear the touch gloves along the longitudinal pressing close to touch on the surface of the stamping, the inspection method depends on the checker's experience. Abrasive grinding if necessary by detection of suspicious areas and verified, but this method is an effective method of quick inspection. A: stamping parts in the punch as material, what is the cause of the distortion?