Difficulty: Easy
Correct Answer: Slow speed
Explanation:
Introduction / Context:
Selecting turning speed is a balance between tool life and productivity. Materials that are hard and/or abrasive impose higher cutting forces and temperatures, which accelerate tool wear, especially with high-speed steel (HSS) or uncoated carbide tools. Cast iron, while commonly described as hard and brittle (not tough), is abrasive due to graphite and carbides, demanding conservative speeds.
Given Data / Assumptions:
Concept / Approach:
Higher speeds increase temperature at the tool–chip interface, worsening flank and crater wear. For abrasive materials, this wear is rapid, so lower speeds (relative to mild steel or aluminium) are recommended. Taylor’s tool life relation V * T^n = C implies that reducing V substantially increases T for a given n, favouring slower speeds for difficult-to-cut materials.
Step-by-Step Solution:
Verification / Alternative check:
Typical data show HSS speeds for cast iron below those for mild steel; coated carbides allow increases, but still lower than for aluminium. This confirms the slow-speed choice in general practice.
Why Other Options Are Wrong:
Common Pitfalls:
Assuming “hard” always means “tough”; cast iron is brittle yet abrasive. Confusing finish-pass speeds with roughing speeds can also mislead selection.
Final Answer:
Slow speed
Discussion & Comments