Question 50 3AE02 - Third Assistant Engineer (Alt)

The Correct Answer is B **Why option B ("rapidly dull") is correct:** When a drill bit rotates too fast (the speed is too great) for the material being drilled, several negative effects occur: 1. **Increased Friction and Heat:** Excessive speed dramatically increases the friction between the cutting edges of the bit and the workpiece. This friction generates a significant amount of heat. 2. **Thermal Softening:** Drill bits (typically made of High-Speed Steel or Carbide) rely on their hardness to maintain a sharp edge. The high heat generated by excessive speed can exceed the material's tempering or operating temperature, causing the steel to lose its hardness (a process called annealing or thermal softening). 3. **Edge Failure:** Once the cutting edges soften, they are no longer hard enough to slice through the workpiece material. They collapse, chip, or wear away almost instantly, leading to the bit becoming rapidly dull. Often, you will see a blue discoloration on the bit, which is a sign of overheating. **Why the other options are incorrect:** * **A) not cut:** While the drill will *eventually* stop cutting effectively once it is dull, excessive speed initially *attempts* to cut faster. The immediate effect of high speed is heat generation and dulling, not an initial failure to cut. If the speed is too high, it might smear the material instead of properly chip-forming, but it is still attempting to perform the cutting action. * **C) cut slower:** High rotational speed is intended to increase the rate of material removal (cutting speed). If the speed is too high, the process fails due to heat and dulling, but the initial motion is geared toward cutting *faster*, not slower. A dull bit will cut slower, but the *cause* (excessive speed) leads to dulling first. * **D) cut faster:** While the initial intent of increasing the speed is to cut faster, once the maximum effective speed is exceeded, the drill rapidly dulls (B). A dull bit cannot maintain a fast cutting rate and will quickly require far more force to advance, meaning the effective cutting rate drops dramatically. Therefore, while it tries to cut faster, the immediate consequence of overheating/dulling (B) prevents this.