*Interchangeability**The term interchangeability is normally employed for the mass production of indentical items within the prescribed limits of sizes. A little consideration will show that in order to maintain the sizes of the part within a close degree of accuracy, a lot of time is required. But even then there will be small variations. If the variations are within certain limits, all parts of equivalent size will be equally fit for operating in machines and mechanisms. Therefore, certain variations are recognised and allowed in the sizes of the mating parts to give the required fitting. This facilitates to select at random from a large number of parts for an assembly and results in a considerable saving in the cost of production. In order to control the size of finished part, with due allowance for error, for interchangeable parts is called limit system.*

**1. Nominal size.**It is the size of a part specified in the drawing as a matter of convenience.

**2. Basic size.**It is the size of a part to which all limits of variation (i.e. tolerances) are applied to arrive at final dimensioning of the mating parts. The nominal or basic size of a part is often the same.

**3. Actual size.**It is the actual measured dimension of the part. The difference between the basic size and the actual size should not exceed a certain limit, otherwise it will interfere with the interchangeability of the mating parts.

**4. Limits of sizes.**There are two extreme permissible sizes for a dimension of the part. The largest permissible size for a dimension of the part is called upper or high or maximum limit, whereas the smallest size of the part is known as lower or minimum limit.

**5. Allowance.**It is the difference between the basic dimensions of the mating parts. The allowance may be positive or negative. When the shaft size is less than the hole size, then the allowance is positive and when the shaft size is greater than the hole size, then the allowance is negative.

**6. Tolerance.**It is the difference between the upper limit and lower limit of a dimension. In other words, it is the maximum permissible variation in a dimension. The tolerance may be unilateral or bilateral.

**a)**When all the tolerance is allowed on one side of the nominal size, then it is said to be unilateral system of tolerance. The unilateral system is mostly used in industries as it permits changing the tolerance value while still retaining the same allowance or type of fit.

**b)**When the tolerance is allowed on both sides of the nominal size, then it is said to be bilateral system of tolerance.

**8. Zero line.**It is a straight line corresponding to the basic size. The deviations are measured from this line. The positive and negative deviations are shown above and below the zero line*respectively.*

**9. Upper deviation.**It is the algebraic difference between the maximum size and the basic size. The upper deviation of a hole is represented by a symbol ES (Ecart Superior) and of a shaft, it is represented by es.

**10. Lower deviation.**It is the algebraic difference between the minimum size and the basic size. The lower deviation of a hole is represented by a symbol EI (Ecart Inferior) and of a shaft, it is represented by ei.

**11. Actual deviation.**It is the algebraic difference between an actual size and the corresponding basic size.

**12. Mean deviation.**It is the arithmetical mean between the upper and lower deviations.

**13. Fundamental deviation.**It is one of the two deviations which is conventionally chosen to define the position of the tolerance zone in relation to zero line.

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