Types Of Tolerances

Plus or minus dimensioning.

By specifying the basic size followed by the limits of tolerance above and / or below the basic size.

a) when the limits are equally disposed above and below the basic size.

b) when the limits are not equally disposed above and below the basic size. The upper limit should always be shown in the upper position and lower limit in the lower position. This applies to both shafts and holes

UNILATERAL SYSTEM OF TOLERANCE

This tolerance allows variation in only one direction form the basic size. This method is advantageous when a critical size is approached as material is  removed during manufacturing, as in case of close-fitting holes and shafts. A unilateral tolerance is always all plus or all minus; that is either the plus or the minus value be zero.

BILATERAL SYSTEM OF TOLERANCE

This tolerance allows variation in both  directions from the basic size. Bilateral tolerances are usually given with location dimensions or with any dimensions that can be allowed to vary in either direction.

If it is desired to specify an equal variation in both directions, the combined plus or minus symbol (±) is used with a single value.