Engineering tolerance

System Terminologies

• Shaft: The term ‘shaft’ used in this standard has a wide meaning and serves for specification of all outer elements of the part, including those elements, which do not have cylindrical shapes.
• Hole: The term ‘hole’ can be used for the specification of all inner elements regardless of their shape.
• Basic Size: The basic size or normal size is the standard size for the part and is the same both for the hole and its shaft. This is the size which is obtained by calculation of strength.
• Actual Size: Actual size is the dimension as measured on a manufactured  part. As already mentioned, the actual size will never be equal to the basic size  and it is sufficient if it is within predetermined limits.
• Limits of Size: These are the maximum and minimum permissible sizes of  the part (extreme permissible sizes of the feature of the part).
• Maximum Limit: The maximum limit or high limit is the maximum size  permitted for the part
• Minimum Limit: The minimum limit or low limit is the minimum size  permitted for the part.
• Zero Line: In a graphical representation of limits and fits, a zero line is a  straight line to which the deviations are referred to. It is a line of zero deviation  and represents the basic size. When the zero line is drawn horizontally, positive  deviations are shown above and negative deviations are shown below this line.
• Deviation: It is the algebraic difference between a size (actual, limit of a size,  etc.) and the corresponding basic size.
• Upper Deviation: It is designated as ES (for hole) and es (for shaft). It is the algebraic difference between the maximum limit of the size and the corresponding basic size. When the maximum limit of size is greater than the basic size, it is a positive quantity and when the maximum limit of size is less than the basic size then it is a negative quantity.
• Lower Deviation: It is designated as EI (for hole) and ei (for shaft). It is the algebraic difference between the minimum limits of size and the corresponding basic size. When the minimum limit of size is greater than the basic size, it is a  positive quantity and when the minimum limit of size is less than the basic size then it is a negative quantity.
• Fundamental Deviations (FD): This is the deviation, either upper or the lower deviation, which is the nearest one to the zero line for either a hole or a  shaft. It fixes the position of the tolerance zone in relation to the zero line.
• Actual Deviation: This is the algebraic difference between actual size and the corresponding basic size.
• Mean Deviation: It is the arithmetical mean between the upper limit and the lower limit.

• Tolerance: It is the difference between the upper limit and the lower limit of a  dimension. It is also the maximum permissible variation in a dimension.
• Tolerance Zone: It is a function of basic size. It is defined by its magnitude and its position about the zero line. It is the zone bounded by the two limits of the size of a part in the graphical presentation of tolerance.
• Tolerance Guide: It is the degree of manufacturing. It is designated by the letters IT (stands for International Tolerance). Numbers, i.e., IT0, IT01, IT1,  follow these letters up to IT16; the larger the number, the larger the tolerance.
• Tolerance Class: The term is used for a combination of fundamental deviation and tolerance grade.
• Allowance: It is an intentional difference between the maximum material limits of mating parts. For a shaft, the maximum material limit will be its high limit and for a hole, it will be its low limit.
• Fits: The relationship existing between two parts, shaft and hole, which are to be assembled, concerning the difference in their sizes is called fit.

Tolerance

• Tolerance is the total amount that a specific dimension is permitted to vary.
• It is the difference between the maximum and the minimum limits for the dimension.
• For Example: A dimension is given as 1.625 ± .002 means that the manufactured part may be 1.627” or 1.623”, or anywhere between these limit dimensions.
• How to decide tolerance?
  • Functional requirements of mating parts
  • Cost of production
  • Available manufacturing process
    • Choose  as  coarse  tolerance  as  possible  without  compromising
    • functional requirements
    • Proper balance between cost and quality of parts

UNILATERAL TOLERANCE

Tolerances on a dimension may either be unilateral or bilateral. When the two limit dimensions are only on one side of the nominal size, (either above or below) the tolerances are said to be unilateral. For unilateral tolerances, a case may occur when one of the limits coincide with the basic size.

BILATERAL TOLERANCE

When the two limit dimensions are above and below nominal size, (i.e. on either side of the nominal size) the tolerances are said to be bilateral. Unilateral tolerances, are preferred over bilateral because the operator can machine to the upper limit of the shaft (or lower limit of a hole) still having the whole tolerance left for machining to avoid rejection of parts

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