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22 ISCAR GENERAL DRILLING HANDBOOK %u00bd%u0009Hole Accuracy and Surface Roughness by DrillingHole AccuracyDmaxmaximum allowable hole diameterDhole diameterDminminimum allowable hole diameterHole accuracy is determined by the width of the tolerance field, that is, the permissible deviation from the nominal dimensions.The nominal dimensions - the dimension that is a starting point for deviations and relative to which the limited dimensions are determined, that is, the dimensions that defined in the drawing.Width of the tolerance %u2013 the difference between the maximum allowable size and the minimum allowable size. Width of the tolerance is unsigned absolute value, for which it is generally accepted to use the IT Grade (International Tolerance Grade) according to the ISO 286 standard. The width of tolerance according IT Grade tables defined by number that placed after IT letters and depended on nominal dimensions, i.e., the same accuracy value according IT Grade table for different nominal dimensions will define a different range of tolerance. With an increase in the IT Grade number, the tolerance range increases, that is, the accuracy decreases.