Millwright Level 1 Practice Exam

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At which RPM should a 1/2" drill turn when drilling mild steel with a cutting speed of 90 sfpm?

600 RPM
720 RPM
800 RPM
1000 RPM

The correct answer is: 720 RPM

To determine the correct RPM for a 1/2" drill when drilling mild steel at a cutting speed of 90 surface feet per minute (sfpm), it’s essential to use the formula that relates cutting speed, drill diameter, and RPM. The formula is: RPM = (Cutting Speed (sfm) × 12) / (π × Diameter (inches)) In this case, the diameter of the drill is 1/2" (0.5 inches). Applying the values to the formula: 1. First, convert the cutting speed from sfpm to inches per minute by multiplying by 12 (since there are 12 inches in a foot): 90 sfpm × 12 = 1080 inches per minute. 2. Now, substitute these values into the formula: RPM = (1080 in/min) / (π × 0.5 in) Approximating π as 3.14: RPM = 1080 / (3.14 × 0.5) RPM = 1080 / 1.57 RPM ≈ 688.5. Since the question asks for the nearest value among the provided options, 720 RPM is the