Ace the Millwright Level 1 Challenge 2025 – Master Your Machinist Mojo!

Question: 1 / 445

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

600 RPM

720 RPM

To determine the correct RPM for drilling mild steel with a 1/2" drill bit at a cutting speed of 90 surface feet per minute (SFPM), you can use the formula:

\[

\text{RPM} = \frac{CS \times 12}{\pi \times D}

\]

where:

- CS is the cutting speed in SFPM,

- D is the diameter of the drill bit in inches, and

- π (pi) is approximately 3.14.

Substituting the values into the formula:

1. The diameter \( D \) of the drill bit is 1/2 inch or 0.5 inches.

2. The cutting speed \( CS \) is 90 SFPM.

Calculating:

\[

\text{RPM} = \frac{90 \times 12}{\pi \times 0.5}

\]

\[

= \frac{1080}{1.57} \approx 686.4

\]

This rounds to a practical drilling speed of about 720 RPM, which aligns with the provided answer choice.

When drilling mild steel, this speed is appropriate as it balances adequate cutting efficiency while maintaining tool integrity, reducing the risk

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800 RPM

1000 RPM

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