Cutting data for drilling: With a high-speed steel (HSS) twist drill, the typical recommended cutting speed range of 24 to 45 m/min corresponds to drilling which material?

Introduction / Context:
In machining, selecting the correct cutting speed for a given tool–workpiece combination is essential to balance tool life, productivity, and surface finish. For twist drilling with high-speed steel (HSS) drills, recommended cutting speeds vary substantially with material properties such as hardness, thermal conductivity, and work-hardening behaviour. This item asks which material typically matches the range 24–45 m/min.

Given Data / Assumptions:

• Tool material: HSS twist drill.
• Target cutting speed window: 24 to 45 m/min.
• Common workshop materials provided as options: mild steel, copper, aluminium, brass.
• General shop conditions; no exotic coatings or cryogenic cooling assumed.

Concept / Approach:
HSS retains hardness only up to moderate temperatures, so speeds are lower than those used with carbides. Copper machines freely and conducts heat well, permitting a mid-range HSS drilling speed higher than mild steel but typically below aluminium or free-cutting brasses. Typical handbook values for HSS drills are roughly: mild steel ~15–30 m/min; copper ~25–45 m/min; aluminium often higher, e.g., 60–90 m/min or more; many brasses also allow higher speeds than copper under similar conditions.

Step-by-Step Solution:

Verification / Alternative check:
Cross-checking with multiple machining data tables shows mild steel usually falls below the upper 40s with HSS; aluminium and many brasses are often drilled appreciably faster than 45 m/min, validating copper as the best match.

Why Other Options Are Wrong:

• Mild steel: commonly ~15–30 m/min with HSS, below the top of the 24–45 m/min band.
• Aluminium: typically allows much higher speeds than 45 m/min with HSS.
• Brass: free-cutting brasses are often drilled faster than copper with HSS.

Common Pitfalls:
Mistaking spindle speed (r.p.m.) for cutting speed (m/min); ignoring that the same cutting speed corresponds to different r.p.m. depending on drill diameter via v = (π * D * N) / 1000.

Final Answer:
Copper