Selecting cutting speed with HSS tools When machining with a high-speed steel (HSS) tool, for which of the following materials is the recommended cutting speed typically the lowest?

Introduction / Context:
Choosing cutting speed depends on tool material and work material. HSS has lower hot hardness than carbides, so speeds must be conservative, especially on abrasive or hard materials.

Given Data / Assumptions:

• Conventional dry or lightly lubricated turning/milling.
• Standard HSS grades and average setups.

Concept / Approach:
Cast irons (especially grey cast iron) contain free graphite and hard carbides that abrade edges and generate discontinuous chips. To preserve tool life with HSS, lower cutting speeds are used than for ductile, free-cutting nonferrous metals like aluminium or brass.

Step-by-Step Solution:
Rank typical speeds (HSS): Aluminium >> Brass > Mild steel > Cast iron (lowest).Therefore, select cast iron for minimum recommended cutting speed among listed choices.

Verification / Alternative check:
Machining data tables confirm that HSS speeds for aluminium can be several times those for grey cast iron; brass also allows higher speeds than steels.

Why Other Options Are Wrong:

• Mild steel, brass, aluminium permit higher speeds with HSS due to better machinability and less abrasion.

Common Pitfalls:
Confusing feed or depth of cut with cutting speed; underestimating abrasive wear in cast iron.

Final Answer:
Cast iron