Definition
A hand file with a tooth coarseness between coarse and second-cut, used for general-purpose metal removal in aircraft maintenance. The bastard file removes material faster than a finer file but leaves a rougher surface than a smooth or second-cut file.
Plain English
A medium-rough metal file. It cuts faster than a fine file but leaves a rougher finish than a smooth one. Mechanics reach for it when they need to take metal off quickly without going to the very coarsest file available.
Context Anchor
Seen in aircraft maintenance and shop work when shaping metal parts, fitting brackets, or removing rough edges.
Derivation
In old toolmaking trade language, file grades were ranked from coarse down to dead smooth. The middle grade did not fit neatly into the named categories, so it was called the 'bastard' grade -- meaning irregular or in-between, not the modern insulting sense. The name stuck and is still standard tool terminology today.
Why Pilots Care
Pilots who do owner-permitted maintenance, or who work alongside a mechanic during inspections, will see and hear this term in the shop. Choosing the wrong file grade can damage a part or leave a finish that fails inspection.
Intuition Check
“Bastard” does not describe quality or legitimacy here. It names a coarse grade of file teeth used for faster material removal.
Example Sentence 1
The mechanic used a bastard file to quickly trim the edge of the bracket before finishing it with a smooth file.
Example Sentence 2
After rough-cutting the bracket, he switched to a bastard file to bring the part to final dimensions.