The Bastard File
A bastard file or a mill bastard file is a file whose teeth configuration is in between a rough or coarse file and a 'second cut' file. In precise terms therefore a bastard file is one which 'is one cut finer than that of a coarse file'. Bastard files are excellent for general purposes such as sharpening, smoothing and polishing.
How did the Bastard File get its name?
The bastard file is an unusual name for a tool. While no, the bastard file is not a file that was born out of wedlock, it may have gotten its name for being somehting unusual. It is a different kind of file. As a file, it can not be technically called a coarse file or a second cut file.
Bastard definition: "Of an unusual or irregular make or proportion; as, a bastard musket; a bastard culverin". [Obs] [1913 Webster]
File definition: A steel instrument, having cutting ridges or teeth, made by indentation with a chisel, used for abrading or smoothing other substances, as metals, wood, etc. [1913 Webster]