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What size is a F drill bit?

Published in Drill Bit Sizes 2 mins read

The exact size of an F drill bit is .257 inches in diameter.

Understanding Letter Size Drill Bits

Letter size drill bits are part of a standardized system used in various applications, particularly in machining and metalworking. This system, which ranges from A to Z, provides incremental drill diameters that fall between common fractional and numbered drill bit sizes. The "F" drill bit is a specific size within this sequence, offering a precise diameter often crucial for specific tapping operations or close-tolerance holes.

Here is a quick reference for the "F" drill bit and its neighboring sizes:

Letter Diameter (inches)
F .257
G .261
H .266
I .272

Practical Application

Letter drill bits like the "F" size are commonly used for tasks requiring a specific hole diameter that isn't readily available in standard fractional (e.g., 1/4", 9/32") or numbered drill sets. For instance:

  • Tapping: They are often specified as the correct pilot hole size for certain Unified National Coarse (UNC) or Unified National Fine (UNF) threads, ensuring optimal thread engagement and strength when tapping.
  • Precision Fit: When a project requires a snug fit for a shaft or pin, or a precise clearance hole, a letter drill can provide the exact diameter needed.
  • Specialty Work: In custom fabrication, tool and die making, or aerospace, where tolerances are tight, having a wide range of incremental sizes through letter drills is invaluable.

The "F" drill bit, at .257 inches, is slightly larger than a 1/4-inch (or .250 inches) drill bit, providing that crucial small increase for specific engineering requirements.