In understanding the preconditions of precision shaft machining technology, we must first have a deep understanding and understanding of its functions, structural features and technical requirements, and then perform process analysis on different blank materials. Then we will introduce them to you. Precise machining of precision shaft machining process!
First, the function, structural characteristics and technical requirements of precision metal shaft parts
Precision metal shafts are one of the typical parts that are often encountered in machines. It is mainly used to support transmission parts, transmit torque and bear load. Shaft parts are rotating parts, whose length is greater than the diameter, generally consisting of the outer cylindrical surface, conical surface, inner bore and threads of the concentric shaft and the corresponding end face. According to different structural shapes, shaft parts can be divided into optical axis, stepped shaft, hollow shaft and crankshaft.
Shafts with an aspect ratio of less than 5 are called minor axes, and those with diameters greater than 20 are called elongated shafts, with most of the axes in between.
The precision metal shaft is supported by a bearing and the shaft section that matches the bearing is called a journal. Shafts are the assembly reference for shafts. Their accuracy and surface quality are generally higher. Their technical requirements are generally based on the main functions and working conditions of the shaft. There are usually the following:
(a) dimensional accuracy
The bearing journals that are used to support the shaft position usually have high dimensional accuracy (IT5~IT7). The precision of the journal size of the assembled transmission is generally lower (IT6~IT9).
(b) Geometrical accuracy
Accuracy of precision metal shaft geometry mainly refers to the roundness, cylindricity, etc. of the journal, outer cone, Morse taper, etc. Generally, its tolerance should be limited within the range of dimensional tolerance. The surface of the inner and outer circles that require high precision shall be marked on the drawing to allow deviations.
(three) mutual position accuracy
The position accuracy of the precision metal shaft is mainly determined by the position and function of the shaft in the machine. It is usually necessary to ensure the coaxiality of the shaft journal of the assembly transmission to the supporting journal. Otherwise, it will affect the transmission precision of the transmission (gear, etc.) and generate noise. For an axis with normal accuracy, the radial runout of the supporting shaft journal for the matching shaft section is generally 0.01-0.03 mm, and the high-precision shaft (such as the main shaft) is usually 0.001-0.005 mm.
(d) Surface roughness
The surface roughness of the shaft diameter that is generally matched with the transmission member is Ra2.5-0.63 μm, and the surface roughness of the bearing shaft that matches the bearing is Ra0.63-0.16 μm.
Second, precision metal shaft blanks and materials
(I) Precise metal shaft blanks
Precision metal shafts can be based on the use of requirements, production type, equipment conditions and structure, choose blanks, forgings and other rough form. For the shaft with a small difference in the diameter of the outer circle, the bar material is generally used; for a stepped shaft or an important axis with a large difference in the diameter of the outer circle, forgings are often used, which saves material and reduces the amount of machining work. Improve mechanical properties.
According to different production scales, blank forging methods are free forging and die forging. Small and medium-sized batch production uses free forging, and large batch production uses forging.
(II) Materials for precision metal shafts
Precision metal shaft should be based on different working conditions and use requirements of different materials and use different heat treatment specifications (such as quenching, normalizing, quenching, etc.) to obtain a certain strength, toughness and wear resistance.
45 steel is a commonly used material for shaft parts. It is cheaper and has better cutting performance after being tempered (or normalized), and it can obtain high mechanical strength such as high strength and toughness. The surface hardness after quenching can be Up to 45~52HRC.
40Cr and other alloy structural steels are suitable for shaft parts with medium precision and high speed. After quenching and tempering, these steels have better comprehensive mechanical properties.
Bearing steel GCr15 and spring steel 65Mn, after quenched and tempered and surface high-frequency quenching, the surface hardness can reach 50 ~ 58HRC, and has a higher fatigue resistance and better wear resistance, can produce a higher accuracy of the shaft.
Spindles for precision machine tools (eg grinder shafts, coordinate boring machine spindles) can be made of 38CrMoAIA nitriding steel. After tempering and surface nitriding of this steel, not only can obtain a high surface hardness, but also can maintain a relatively soft core, so the impact toughness is good. Compared with carburized and quenched steel, it has the characteristics of small heat treatment deformation and higher hardness