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Difficulty: MediumPower Drills and Drivers

A mechanic needs to drill a 1/2-inch1/2\text{-inch} diameter hole through a 1/4-inch1/4\text{-inch} thick mild steel bracket using a variable-speed power drill and a high-speed steel (HSS) twist drill bit. Which operating combination of rotational speed and feed pressure should be applied to cut the metal efficiently without overheating or dulling the cutting edges?

  1. A
    High rotational speed combined with light, intermittent feed pressure
  2. Low rotational speed combined with steady, firm feed pressureAnswer
  3. C
    Maximum rotational speed combined with heavy, continuous feed pressure
  4. D
    Variable high-speed pulsing combined with zero feed pressure

Answer

Low rotational speed combined with steady, firm feed pressure
When drilling ferrous metals such as mild steel, especially with larger bit diameters like 1/2-inch1/2\text{-inch}, the standard rule is 'slow speed and firm pressure.' Lower rotational speeds keep frictional heat within acceptable limits, while steady, firm feed pressure forces the chisel edge and cutting lips into the metal to shave off continuous chips without rubbing.

Step-by-Step Solution

1
Evaluate the relationship between workpiece material hardness, drill bit diameter, and cutting speed (RPM).
Dense metals like steel require significantly lower surface feet per minute (SFM) and lower RPM than softer materials like wood or plastic, especially as bit diameter increases.
The perimeter of a larger bit travels at a higher linear velocity for any given RPM, generating rapid frictional heat.
2
Determine the appropriate feed pressure to maintain continuous cutting action.
Steady, positive pressure is required so the cutting edges bite into the metal and produce continuous curly chips rather than rubbing against the workpiece.
Rubbing without cutting causes friction, work-hardens the steel surface, and ruins the cutting lips.
3
Select the correct combination of speed and feed settings.
Operating the power drill at low speed with steady, firm feed pressure optimizes cutting rate and tool longevity.
This balance prevents thermal breakdown while sustaining continuous material removal.

Key Concept

Speed and Feed Dynamics in Metal Drilling
Estimated Time:1m 0s
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Topics

Power Drills and Drivers

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