- Benchmark first
Parts processing generally starts from the processing of precision datum, and then processes other surfaces with precision datum. Therefore, the surface selected as the precision reference should be processed first in the initial process of the process, so as to provide the precision reference for the subsequent process. For example, for shaft parts, first process the center hole at both ends, and then use the center hole as the precision benchmark to rough and finish all the cylindrical surfaces. In gear machining, the inner hole and the reference end face are machined first, and then the inner hole and the end face are used as the fine reference to rough and finish the tooth surface.
- First coarse, then fine
After finishing, the machining of the whole part should be the roughing process, followed by semi finishing, finishing and finishing. According to the principle of roughing before finishing, the main surfaces with high precision are machined first, that is, roughing first, then semi finishing, and finally finishing and finishing. Before finishing the important surface, sometimes it is necessary to trim the precision datum to ensure the machining accuracy of the important surface. For example, in the high-precision grinding of the spindle, the center hole must be ground before finishing and superfinishing; in the precision gear grinding, the inner hole must also be ground.
- First, first, then
According to the function and technical requirements of the parts. First, separate the main surface and the secondary surface of the part, then arrange the processing of the main surface first, and then insert the processing procedure of the secondary surface into it. Secondary surface generally refers to keyway, screw hole, pin hole and other surfaces. These surfaces generally have certain relative position requirements with the main surface, so the secondary surface processing should be based on the main surface, so the secondary surface processing is generally placed after the semi finishing of the main surface, and the primary processing is finished before finishing. Some are placed in the final processing, but at this time should be careful not to damage the main surface has been processed.
- Face before hole
For the box, base, bracket and other parts, the plane contour size is larger, and it is more stable and reliable to use it as the precision datum to process the hole, and it is also easy to process, which is conducive to ensuring the accuracy of the hole. If the hole is machined first, and then the plane is machined based on the hole, it will be more difficult, and the machining quality will also be affected.
In the production process of compressor parts, we often come into contact with some difficult to process materials. For example, the material for manufacturing compressor impeller is a kind of high-strength structural steel containing Cr, Ni, Mo and other alloy elements. Once the steel reaches a certain hardness after quenching and tempering, it is difficult to turn. Because of the existence of Qinzhou alloy elements, Qinzhou alloy impeller brings a lot of trouble to turning. The process of carburizing and quenching of large hardened gears will cause some hard surfaces to be machined, which are difficult to turn. There are also some sets of parts made of pure metal, such as copper, which are commonly used in transportation machinery, also bring considerable trouble to turning. In order to solve the turning problems of these difficult to machine materials, we need to have a sufficient understanding of the characteristics of difficult to machine materials, and then take targeted measures to solve them.
What is difficult to process material
The so-called difficult to machine materials mainly refer to the materials with poor cutting performance. The machinability of metal materials is mainly measured from three aspects: the tool life, the quality of machined surface and the difficulty of chip formation and removal. As long as there is an obvious difference in the above three aspects, it can be considered as difficult to process materials. The common difficult to process materials are high strength steel and stainless steel
Turning temperature: when cutting hard to machine materials, the cutting temperature is generally high. The main reasons are as follows.
Low thermal conductivity: the thermal conductivity of difficult to machine materials is generally low (except for pure metal copper, etc.), the cutting heat is not easy to spread during cutting, and it is easy to concentrate on the tool tip.
High thermal strength: the tensile strength of superalloys such as nickel base alloy reaches the highest value at 500-800 ℃. Therefore, when turning this kind of alloy, the turning speed of the turning tool should not be too high, generally not more than 10m / min, otherwise the cutting resistance of the tool cutting into the workpiece will increase.
Cutting deformation coefficient and work hardening: high temperature alloy and stainless steel in difficult to machine materials, the deformation coefficient of these materials are relatively large. When the cutting speed is about 6 m / min, the chip deformation coefficient will reach the maximum value. Because of the plastic deformation of the chip formed in the turning process, the metal hardens and strengthens, which increases the cutting resistance, speeds up the tool wear, and even leads to edge collapse. For example, the austenitic structure of high temperature alloy, high manganese steel and austenitic stainless steel has a great degree of hardening and depth, which is several times larger than that of 45 steel. Due to severe hardening, high temperature and hardness of chips, good toughness and high cutting temperature (strong and tough chips), if such chips flow through the rake face, it is easy to produce sticking phenomenon such as bonding and fusion welding. Sticking is not conducive to the removal of chips, so that the chip grains are blocked and it is easy to cause cutting. It is also easy to cause the tool to wear and crack. In addition, the tough chip is serrated, which is easy to damage the cutting edge.
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