JNTU B.Tech III ENGINEERING METROLOGY Previous Question Paper (October - 2020 - R13)

The Indian Standard (IS) system of limits and fits, based on the ISO system, provides a systematic approach to ensure functional compatibility between mating parts. Its principle features include:

• Tolerance Grades: The system defines 18 grades of tolerance, denoted as IT01, IT0, and IT1 through IT16. These represent the magnitude of the tolerance zone, where a lower number indicates a smaller, more precise tolerance.
• Fundamental Deviations: These are represented by letters (A-ZC for holes and a-zc for shafts). They define the position of the tolerance zone relative to the zero line (basic size).
• Hole and Shaft Basis Systems: The system allows for either a hole-basis system (where the hole is kept constant and the shaft size is varied) or a shaft-basis system (where the shaft is kept constant and the hole size is varied).
• Standardization: By using a common set of symbols and tables, the system ensures that parts manufactured in different workshops can be assembled together without further fitting.

Interchangeability refers to the ability to replace a component in an assembly with any other identical component from a different batch without any modification or adjustment. It relies on strict adherence to defined tolerances and is essential for mass production, reducing assembly time and maintenance costs.

Selective Assembly is a process used when the required fit is so precise that it cannot be economically achieved through standard manufacturing tolerances. Components are produced within a wider tolerance range and then measured and sorted into groups (classes) based on their actual dimensions. Parts from corresponding groups are then selectively paired to achieve the desired fit, which is more cost-effective than high-precision machining.