**Metrology Notes for GATE 2021**

**Metrology Definition**

**What do you understand by Metrology?**

- It is basically the science of measurement.
- The purpose of this discipline is to establish means of determining physical quantities.

**Important Terminologies**

**Nominal Size**

This is used for general identification purposes and the size of any part is defined during its drawing.

**Basic Size**

The size of a part to which all limits of variation are applied. It is a theoretical concept.

**Actual Size**

It is the actually measured dimension of a part.

*Note: The difference between basic and actual size should not exceed certain limits otherwise it will interfere with the interchangeability of the mating parts.*

**Upper or High or Maximum Limits**

It is the largest permissible size for a dimension, out of the two extreme permissible sizes for a dimension.

**Lower Limit**

It is the lowest permissible size for a dimension.

**Tolerance**

This is the difference between the upper limit and the lower limit. This is also the maximum permissible variation in a dimension.

**Zero Line**

A straight line corresponding to the basic size and the deviations are measured from this line.

**Deviation**

It is an algebraic difference between a size (actual, maximum) and the corresponding basic size.

**Actual Deviation**

It is the algebraic difference between actual size and the corresponding basic size.

**Upper Deviation**

The algebraic difference between actual size and the corresponding basic size.

**Lower Deviation**

It is the algebraic difference between minimum size and the corresponding basic size.

**Mean Deviation**

It is the arithmetical mean of lower and upper deviations.

**Fundamental Deviation**

This is the deviation, either the upper or the lower deviation, which is the nearest one to the zero line for either a hole or a shaft.

**Unilateral and Bilateral Limits**

Unilateral limits occur when both maximum limit or minimum limit is either above or below the basic size, but in the case of **Bilateral limits** maximum limit is above and the minimum limit is below the basic size.

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