Definition
A rotary cutting tool used in a drill press or milling machine to cut large-diameter holes or to machine flat surfaces. It consists of a body that holds a single cutting bit on an adjustable arm, allowing the cutting radius to be set to the size needed.
Plain English
A spinning tool with one cutter on an arm. As it turns, the cutter sweeps a circle and cuts a hole or smooths a surface. You can slide the cutter in or out on the arm to change how big a circle it cuts.
Context Anchor
Seen in aircraft maintenance and sheet-metal repair when a technician needs to cut a controlled round opening in a panel or part.
Derivation
Called a 'fly' cutter because the single cutting bit 'flies' around in a wide circle as the tool rotates, sweeping out the cut rather than drilling straight down like a normal drill bit.
Why Pilots Care
Mechanics and builders use fly cutters when a hole is too large for a standard twist drill or hole saw, such as cutting instrument panel openings or access holes. Knowing the term helps when reading maintenance instructions or kit-build manuals.
Analogy
It works a little like a compass drawing a circle: one point stays near the center while the outer point travels around. On a fly cutter, the outer point cuts the material instead of drawing on it.
Intuition Check
“Fly” does not mean the tool is used in flight. Here it means the cutting point swings around the center as the tool turns.
Example Sentence 1
The mechanic used a fly cutter in the drill press to cut a clean three-inch hole in the new instrument panel.
Example Sentence 2
A fly cutter produced a clean circular recess without requiring an oversized end mill.