JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year ENGINEERING METROLOGY Previous Question Paper (March/April - 2021 - R16)

Interchangeability refers to the ability to produce components such that any part selected at random from a batch will assemble correctly with any other mating part without requiring manual fitting or adjustment. This is essential for mass production, reducing assembly time and costs, and facilitating easy replacement of damaged parts.

Selective Assembly is a technique used when high precision is required but manufacturing costs for extremely tight tolerances would be prohibitive. Instead of producing all parts to a very narrow tolerance, parts are manufactured to wider, more economical tolerances and then measured and sorted into groups or classes based on their actual dimensions. Mating components are then selected from corresponding groups to achieve the desired fit, ensuring high-quality assembly while maintaining cost-effectiveness.

Tolerances are the permissible variations in a dimension to ensure functional performance and ease of assembly. They are categorized primarily as follows:

1. Unilateral Tolerance: The dimension is allowed to vary in only one direction from the basic size. For example, a shaft of 50.00 +0.05/-0.00 mm. This is preferred for precision fits where the limit of one part must be strictly controlled.

2. Bilateral Tolerance: The dimension is allowed to vary in both directions from the basic size. For example, a shaft of 50.00 ±0.03 mm. This is commonly used for non-critical dimensions where the variation can be distributed on either side of the nominal value.

3. Limit Tolerance: The maximum and minimum permissible sizes are explicitly stated rather than using a basic size and deviation. For example, 50.05 mm (max) and 49.95 mm (min).

Visualizing these, a Unilateral sketch shows the tolerance zone entirely above or below the basic size line. A Bilateral sketch shows the tolerance zone split symmetrically or asymmetrically across the basic size line.