Difficulty: Easy
Correct Answer: 22 m/min
Explanation:
Introduction / Context:
Choosing the correct cutting speed for a material–tool pair is a core machining skill. For cast iron machined with high-speed steel (HSS) tools, shops commonly use moderate speeds to balance tool life and productivity. This item checks familiarity with a representative “average” value from standard recommendations.
Given Data / Assumptions:
Concept / Approach:
Grey cast iron is free-cutting due to graphite flakes but is abrasive. HSS tools, while tougher than carbides, lose hardness at elevated temperature. Hence, typical practice uses speeds in the low-to-mid tens of m/min, depending on grade, hardness, rigidity, and coolant. A value near 22 m/min is a widely cited “average” starting point.
Step-by-Step Solution:
Verification / Alternative check:
Handbook ranges may extend lower for hard CI grades or higher with excellent cooling/rigidity. A practical starting value of about 22 m/min is consistent with conservative shop practice, optimizing tool life for HSS.
Why Other Options Are Wrong:
Common Pitfalls:
Using carbide speed data for HSS; ignoring diameter effects (surface speed changes with rpm and diameter); not adjusting for hardness, rigidity, or coolant presence.
Final Answer:
22 m/min
Discussion & Comments