Fits and tolerances — count of standard tolerance grades (per Indian/ISO practice) According to Indian Standards harmonised with ISO (e.g., IS 919), how many standard tolerance grades (IT grades) are recognised across the full accuracy range?

Introduction / Context:
Manufacturing drawings specify sizes as well as acceptable variation. Standard systems, such as ISO/Indian Standard tolerance grades (IT grades), define bands of tolerance quality from very precise gauge work to coarse commercial fits. Knowing how many grades exist helps you choose appropriate quality levels for cost-effective production.

Given Data / Assumptions:

• Indian Standards align with ISO tolerance system nomenclature.
• IT grades span from very fine metrology (e.g., IT01) to coarse (e.g., IT16).
• Count should include the special fine grades along with the numbered sequence.

Concept / Approach:
The well-known sequence of standard tolerance grades includes IT01, IT0, and IT1 through IT16. Counting them yields a total of 18 grades. These are applied with fundamental deviations (letters) to generate holes and shafts fits for the hole-basis or shaft-basis systems. Selecting a higher IT number increases the tolerance band, reducing machining cost but also reducing accuracy.

Step-by-Step Solution:
Enumerate: IT01, IT0, IT1, IT2, …, IT16.Compute count: 2 special + 16 numbered = 18 total grades.Relate to practice: fine gauges or precision bearings use very low IT numbers; structural parts may use higher IT numbers.

Verification / Alternative check:
Standards tables list tolerances for each grade versus basic size; cross-check any machining handbook to see the 18-grade set confirmed.

Why Other Options Are Wrong:

• 8, 12, 20, 24: do not match the complete recognised set including special fine grades.

Common Pitfalls:
Forgetting to count IT01 and IT0; learners often start at IT1 and arrive at 16, which is incomplete.

Final Answer:
18