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.