Introduction:
In drilling operations, the cutting speed refers to the speed at which the drill bit rotates while cutting through the material. The cutting speed is a crucial parameter that affects the efficiency and effectiveness of a drilling operation. Different materials have different optimal cutting speeds, and it is important to choose the appropriate cutting speed for each material to achieve the best results.

Explanation:
The average cutting speed with H.S.S. (High-Speed Steel) drills is maximum for aluminium compared to cast iron (hard) and cast iron (soft). This is because of the following reasons:

1. Properties of Aluminium:
- Aluminium is a relatively soft and lightweight metal compared to cast iron.
- It has a lower hardness and tensile strength, which makes it easier to cut.
- Aluminium has good thermal conductivity, which helps dissipate heat generated during drilling.

2. Tool Material:
- H.S.S. drills are made from high-speed steel, which has excellent cutting performance and durability.
- High-speed steel is specifically designed to withstand the high temperatures generated during cutting.
- H.S.S. drills have good wear resistance and can maintain their cutting edge for a longer duration.

3. Cutting Speed and Heat Generation:
- Cutting speed directly affects the amount of heat generated during drilling.
- Higher cutting speeds result in higher heat generation, which can lead to excessive heat buildup and cause tool wear or even damage.
- However, aluminium has good thermal conductivity, which helps dissipate the heat and prevents overheating of the tool.

4. Chip Formation:
- Aluminium has a tendency to produce long and continuous chips during drilling.
- This chip formation is more favorable for efficient cutting and chip evacuation.
- The high cutting speed with H.S.S. drills helps in creating these long chips, resulting in a smoother and faster drilling process.

Conclusion:
Considering the properties of aluminium, the tool material (H.S.S.), and the relationship between cutting speed and heat generation, it can be concluded that the average cutting speed with H.S.S. drills is maximum for aluminium. However, it is important to note that the optimal cutting speed may vary depending on the specific type and grade of aluminium being drilled. It is always recommended to refer to manufacturer guidelines or consult with experts for accurate cutting speed recommendations.