Centre punching is a crucial step when drilling on curved or cylindrical surfaces. This process involves creating a small indentation at the drilling point using a center punch tool. Let's explore why this step is necessary:

1. Stability and Accuracy: The primary reason for center punching is to provide a stable starting point for the drill bit. On curved surfaces, the drill bit tends to slip and skid due to the uneven terrain. By creating a small indentation, the center punch provides the drill bit with a guide to follow, minimizing the risk of slipping.
2. Increased Friction: The indentation increases the friction between the drill bit and the surface, helping to stabilize the bit as it begins to cut into the material.
3. Prevents Damage: Without center punching, the slipping of the drill bit can lead to unwanted scratches or damage on the workpiece, especially on finished surfaces.
4. Reduces Errors: A center-punched mark ensures that the hole will be drilled in the correct location, reducing the risk of errors in alignment that could result from the bit wandering.

Among the given options, the correct answer is: "Drill bit tends to slip more easily on curved surfaces." This accurately describes the challenge of drilling on curved surfaces and the reason why center punching is necessary.

The other options can be ruled out because they do not accurately describe the mechanical or physical properties of curved surfaces that affect the drilling process:

• Curved surfaces always contain soft metal layers: This is not true for all curved surfaces; the material properties are independent of the surface shape.
• Curved surfaces increase drilling speed automatically: Surface shape does not impact the speed of drilling; rather, it is impacted by tool speed and material cut.
• Curved surfaces reduce hardness of metals: The shape of a surface does not affect the inherent hardness of the material.