1. The material of the needle file
The file is made of carbon tool steel T12 or T13, and the hardness of the cutting part reaches HRC62-72 after heat treatment. is a tool of production.

2. The structure of the file
The file consists of two parts: the file body and the file handle.
(1) The file body includes three parts: the file surface, the file edge, and the file tail.
The file surface refers to the upper and lower sides of the file, which is the main working surface. The file surface is made into a convex arc shape at the front end, and there are cut teeth on the upper and lower sides, which is convenient for filing. The effect of the file is also made into a convex arc in the longitudinal direction, which can offset the convex phenomenon on the surface caused by the swinging of both hands during filing, so as to make the workpiece flat.
The file edge refers to the two sides of the file, which are divided into toothed edge and smooth edge. The toothed edge can be used for cutting, and the smooth edge is only used as a guide. Some files have no teeth on both sides, and some have teeth on one side. The side without teeth is called the smooth side, and its function is to use the smooth side against the machined surface to file the other right-angle side when filing one face of the inner right angle, so as to prevent the machined surface from being bruised.
The file tail (tongue) is used to hold the file handle. The file tongue is not hardened.
(2) The function of the file handle is to facilitate holding and transmitting thrust during filing. Usually made of wood, there should be an iron hoop at one end of the mounting hole.

3. Cut and cut
(1) Cut teeth: Cut teeth are the tooth type used by the file for cutting. When filing, each tooth is equivalent to a chisel, cutting metal materials.
The tooth profile of the cut teeth is milled. The milling teeth are milled by the milling method. The tooth file is easy to process and low in cost, but the teeth are blunt and the response to the filing resistance is large, but the teeth are not easy to wear and can cut hard metals. Milling tooth files are time-consuming and expensive to process, but the teeth are sharp. Because the teeth are easy to wear, it is only suitable for cutting soft metals.
The thickness of the file teeth is expressed according to the pitch of the file teeth. Large pitch for rough files and small pitch for fine files. Its thickness grades are as follows:
No. 1 file is used for rough files, with a tooth pitch of 2.3 to 0.83 mm. No. 2 file is used for medium and coarse files with a tooth pitch of 0.77 to 0.42 mm. File No. 3 is used for fine files with a tooth pitch of 0.33 to 0.25 mm. No. 4 file is used for double fine files with a tooth pitch of 0.25 to 0.20 mm. No. 5 file is used for oil-gloss files, with a tooth pitch of 0.2 to 0.16 mm.
(2) File pattern: The file pattern is the pattern of the arrangement of the file teeth, and there are two kinds of single tooth pattern and double tooth pattern.
Single tooth pattern refers to the tooth pattern on the file in only one direction. Suitable for filing soft materials.
The single tooth pattern is mostly milled teeth, cutting with positive rake angle, the strength of the teeth is weak, the full tooth width participates in cutting at the same time, the chips removed by filing are not easy to break, and even have the same width as the file, so the cutting resistance is large and requires a large cutting force , so it is only suitable for filing soft materials and filing narrow workpieces.
Double tooth pattern refers to the tooth pattern arranged in two directions on the file. Suitable for filing hard materials.
Most of the double tooth patterns are teeth, the first one is the bottom tooth pattern (the tooth pattern is shallow), and the last one is the surface tooth pattern (the tooth pattern is deep). The direction and angle of the surface tooth pattern and the bottom tooth pattern are different, and the cut teeth formed in this way are inclined and regularly arranged along the center line of the file. When filing, the filing marks of each tooth are staggered without overlapping, and the filing surface is relatively smooth. The chips are broken during filing, which reduces the cutting resistance and saves effort for filing. The tooth strength is also high, so the double-tooth file is suitable for filing hard materials and wide-faced workpieces.

4. Types of files
Files are usually divided into three categories: ordinary files, special files and plastic files. According to the processing method of the teeth, the file can be divided into two kinds: tooth file and milling tooth file; according to the arrangement of the file tooth pattern, it can be divided into two kinds: single tooth file and double tooth file; according to its processing object, it can be divided into ordinary file , special files and three kinds of plastic files.
(1) Ordinary files are mainly used for the processing of general workpieces. According to their different cross-sectional shapes, they are divided into five types: flat file (plate file), square file, triangular file, semicircle file and round file. for processing of different surfaces.
Ordinary files can be divided into three types: coarse teeth (4-12 teeth), fine teeth (13-24 teeth), and glossy teeth (30-40 teeth) according to the number of tooth patterns per 10mm length.
(2) Special files are used to process special surfaces of parts. There are knife edge file, diamond file, flat triangle file, oval file, round belly file and so on.
(3) Shaping files (group files or assorted files) are mainly used for the processing of small parts, narrow surfaces, fine processing of punching dies, templates and trimming of small parts on the workpiece. The length and cross-sectional size of the plastic file are very small, and the cross-sectional shapes include circles, scalene triangles, rectangles, semicircles, etc. It gets its name from the grading of small files with a cross-sectional shape. Usually a set of 5, 6, 8, 10 or 12 sets is used.

5. Specifications of files
The size of the file is generally expressed by the length of the toothed part of the file. Commonly used plate files are 100mm, 150mm, 200mm, 250mm and 300mm. The size of the file, different files are represented by different parameters. The size of the round file is represented by the diameter, the size of the square file is represented by the square size, and the other files are represented by the length of the file body.

6, the choice of file
Reasonable selection of files has a great influence on improving filing efficiency, ensuring filing quality and prolonging the service life of files. Each file has its own purpose, so choose the right file carefully before filing. If it is not selected properly, it will not be able to give full play to its effectiveness or lose its cutting ability prematurely, and the filing quality cannot be guaranteed.
The correct selection of the file should be based on the specific conditions of the processing object and consider the following aspects:
(1) The cross-sectional shape of the file should be adapted to the shape of the workpiece.
(2) Use coarse files for roughing and fine files for finishing. Rough files are suitable for filing workpieces with large machining allowance, low machining accuracy and large surface roughness values; fine files are suitable for filing workpieces with small machining allowances, high machining accuracy and small surface roughness values; single-tooth file files Suitable for processing soft materials.
The choice of the thickness of the file depends on the nature of the workpiece material, the size of the machining allowance, the machining accuracy and surface roughness requirements, and the hardness of the workpiece material. Rough rasp (or single-tooth rasp) is suitable for filing workpieces with large machining allowance, low machining accuracy and large surface roughness value, as well as filing copper, aluminum, etc. Soft metal material; fine file is suitable for filing workpieces with small machining allowance, high machining accuracy and small surface roughness value, as well as filing steel, cast iron, etc.; glossy file is used for finishing, polishing the surface of the workpiece to increase the size precision, reduce roughness.
(3) The length of the file should generally be 150-200mm longer than the filing surface. The size of the file depends on the size of the workpiece processing surface and the size of the machining allowance. When the size of the machined surface is large and the machining allowance is large, a longer file should be used; otherwise, a shorter file should be used. The length of the file should generally be 150-200mm longer than the filing surface.