Lathe speed selection: In which situations is it necessary to select a slow spindle speed on a lathe?

Introduction / Context:
Spindle speed affects cutting speed at the tool–work interface. Excessive surface speed causes overheating, rapid tool wear, and dimensional errors. Lathes require careful speed selection, especially during thread cutting, on large diameters, or with hard/tough materials.

Given Data / Assumptions:

• Turning with a single-point tool.
• Target is safe cutting speed and accurate geometry.
• Conventional HSS/carbide tooling.

Concept / Approach:
Cutting speed (m/min) = pi * diameter * rpm / 1000. For a fixed rpm, larger diameters generate higher surface speeds, so rpm must be reduced. Thread cutting demands coordinated tool advance with the lead; lower rpm gives control and prevents tool crash at thread roots. Hard/tough materials require lower surface speeds to limit tool temperature and preserve edge integrity.

Step-by-Step Solution:

Verification / Alternative check:
Tool data charts list recommended speeds significantly lower for hard alloys; threading procedures universally suggest slow speeds, especially for manual lathes.

Why Other Options Are Wrong:
Each individual option (a, b, c) is correct, so the comprehensive choice “all of these” is the best answer.

Common Pitfalls:
Keeping rpm fixed when changing work diameter; attempting high-speed threading; ignoring coolant and feed adjustments when speed is lowered.

Final Answer:
all of these