he file can be used to process work-piece surface, inner hole, groove and various complex outer surface. Its precision can reach 0.01mm, and the surface roughness Ra can reach 0.8 μ M. In the process of automobile maintenance, the uneven wear and deformation of individual parts need to be filed. Filing is a manual operation, which is one of the main methods to assess the actual operation skills of the automobile maintenance fitter.

1. Materials for making files
The file is generally made of carbon tool steel T12 and T13 and hardened to 62 HRC ~ 67 HRC after heat treatment.
2. File composition
The file is composed of file surface, file edge, file tongue, file tail, wood handle, etc
3. Types and selection principles of files
(1) The types of files can be divided into fitter files, special files and plastic files according to their uses.

① Fitter files can be divided into five types according to their cross-section shape: flat file, round file, semi-circular file, square file and triangle file; according to the tooth pattern, it can be divided into single tooth pattern file and double tooth pattern file; according to the tooth pattern thickness, it can be divided into five kinds: coarse teeth file, medium teeth file, fine tooth file, rough oil smooth (double fine teeth) file and fine oil smooth file.

② The shaping file is mainly used for fine machining and trimming the small parts difficult to be machined on the workpiece. It is composed of several files with various cross-section shapes.
③ Special file can be used to process special surface of parts. It has two kinds of straight and curved, and its cross-section shape is many.

(2) Selection principle of file
The selection of the shape of the file depends on the shape of the workpiece to be processed; the thickness of the file is determined by the size of the machining allowance, the level of the machining accuracy and the hardness of the workpiece material. Generally, rough file is suitable for soft metal with large machining allowance, low machining accuracy and surface roughness, while fine file is the opposite.