Metrology Notes for GATE 2021
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.
This is used for general identification purposes and the size of any part is defined during its drawing.
The size of a part to which all limits of variation are applied. It is a theoretical concept.
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.
It is the lowest permissible size for a dimension.
This is the difference between the upper limit and the lower limit. This is also the maximum permissible variation in a dimension.
A straight line corresponding to the basic size and the deviations are measured from this line.
It is an algebraic difference between a size (actual, maximum) and the corresponding basic size.
It is the algebraic difference between actual size and the corresponding basic size.
The algebraic difference between actual size and the corresponding basic size.
It is the algebraic difference between minimum size and the corresponding basic size.
It is the arithmetical mean of lower and upper deviations.
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.