Comparing drilling cutting speeds: materials and tool type The recommended cutting speed for drilling aluminium, brass, and bronze using carbon steel drills is _________ the cutting speed for drilling mild steel using high-speed steel (HSS) drills.

Introduction / Context:
Cutting speed selection depends on both the work material and the tool material. Although aluminium and brasses are softer than steels, the thermal limits of carbon steel drills are much lower than those of high-speed steel (HSS), which constrains allowable speed.

Given Data / Assumptions:

• Carbon steel (CS) drills have poor hot hardness; HSS has superior hot hardness.
• Work materials: aluminium/brass/bronze versus mild steel.
• Comparison requested: CS-on-soft metals vs HSS-on-mild steel.

Concept / Approach:
Allowable cutting speed is limited by tool material’s ability to maintain hardness at temperature. Even though non-ferrous materials often permit high speeds, carbon steel drills overheat quickly. HSS on mild steel commonly runs at higher speeds than CS on non-ferrous metals due to HSS’s thermal capability.

Step-by-Step Solution:
Set baseline: typical HSS drilling of mild steel uses higher surface speeds than CS can sustain on any material.Account for material softness: aluminium/brass would allow high speeds, but tool constraint (CS) dominates.Therefore, CS-on-Al/Br/Brz speed is less than HSS-on-MS speed.

Verification / Alternative check:
Typical charts: HSS on mild steel may allow substantially higher m/min than CS on aluminium; CS recommendations are conservative to avoid tempering loss.

Why Other Options Are Wrong:
(A) Equal ignores tool material effect; (C) More than misreads tool limitation; (D) and (E) contradict fundamental machining principles.

Common Pitfalls:
Assuming work material softness alone decides speed; overlooking tool material thermal limits; ignoring coolant and feed interactions.

Final Answer:
less than