The term Mill Tolerances is used to describe permissible deviations from the published dimensions of cross-sectional profiles listed in mill catalogs, and from the thickness or lengths specified by the purchaser. Some of the variations are negligible in smaller shapes, but tend to increase and must be taken into consideration in detailing and fabricating connections for members made up from larger shapes. Other mill tolerances permit some variation in area and weight, ends out-of-square, and camber and sweep. Factors that contribute to the necessity for mill tolerances are:
•The high speed of the rolling operation required to prevent the metal from cooling before the rolling process has been completed
•The varying skill of the operators in adjusting the rolls for each pass, particularly the final pass
•The deflection (springing) of the rolls during the rolling operation
•The gradual wearing of the rolls, which can result in some weight increase, particularly in the case of shapes
•The warping of steel in the process of cooling
•The subsequent shrinkage in length of a shape that has been cut while still hot.
The steel detailer should be familiar with the several tolerances, especially those of camber, sweep, depth of section and length. A more exhaustive presentation of these tolerances is found in the ASTM A6 Specification.
An important factor for the steel detailer to understand clearly is the effect of mill tolerances. The steel detailer must know when to take tolerances into account, particularly in ordering mill material and in detailing connections, especially those involving heavy rolled shapes. For instance, when detailing a moment connection the steel detailer must be cognizant of the permissible variations in the depth of the beam and out-of-square of the beam flanges in order to locate the connection material shop welded to the column.