Question

With respect to limits, fits and tolerances of machine components, how many grades of tolerances are there?

• A. 14
• B. 20
• C. 16
• D. 18

Hint:

To remember the number, think of the sequence: IT01 and IT0 (the first two) followed by 1 through 16 ($2 + 16 = 18$). This is the standard IT grade count used in almost all competitive engineering exams.

Answer 1

The Correct Option is D

In mechanical engineering, the standardization of limits and fits is essential for ensuring the interchangeability of parts manufactured in different locations. The magnitude of the tolerance—which is the permissible variation in a dimension—is standardized through a system of "Fundamental Tolerances." 1. The IT Grade System: The International Tolerance (IT) grades define the precision of a manufacturing process. According to the Bureau of Indian Standards (BIS) and International Organization for Standardization (ISO) systems, there are exactly

18 grades of fundamental tolerances.

2. Classification of IT Grades: These 18 grades are designated as IT01, IT0, and IT1 through IT16. They are categorized based on their application:

• IT01 to IT4: These are extremely high-precision grades used for gauge manufacturing and high-precision instruments.

• IT5 to IT11: These are standard precision grades used for general engineering fits (shatfs and holes in machinery).

• IT12 to IT16: These are coarse tolerance grades used for manufacturing processes like casting, forging, or stamping where high precision is not required.

3. Mathematical Basis: The value of each fundamental tolerance for a particular grade is calculated using a standard tolerance unit ($i$), which depends on the geometric mean diameter ($D$) of the size range. For example, the tolerance for IT6 is $16i$, while for IT16, it is $1000i$. Therefore, the total count of recognized fundamental tolerance grades in this standardized system is 18.