Turning is to change the shape and size of the blank by using the rotary motion of the workpiece and the linear or curvilinear motion of the cutter on the lathe, so as to meet the requirements of the drawing.
Turning is a method of cutting workpiece on a lathe by rotating the workpiece relative to the cutter. The cutting energy of turning is mainly provided by the workpiece rather than the cutter. Turning is the most basic and common cutting method, which plays an important role in production. Turning is suitable for machining rotary surface. Most of the workpieces with rotary surface can be machined by turning method, such as inner and outer cylindrical surface, inner and outer cone surface, end face, groove, thread and rotary forming surface, etc. the tool used is mainly turning tool.
In all kinds of metal cutting machine tools, lathe is the most widely used one, accounting for about 50% of the total number of machine tools. The lathe can be used not only for turning the workpiece, but also for drilling, reaming, tapping and knurling with drills, reamers, taps and knurling cutters. According to the different process characteristics, layout forms and structural characteristics, lathes can be divided into horizontal lathe, floor lathe, vertical lathe, turret lathe and profiling lathe, and most of them are horizontal lathes.
The ideal processing program should not only ensure the qualified workpiece in accordance with the drawing, but also make the function of CNC machine tool be reasonably applied and fully developed. CNC machine tool is a kind of high efficiency automation equipment, its efficiency is 2 ~ 3 times higher than ordinary machine tool. In order to give full play to this characteristic of CNC machine tool, we must analyze the process of workpiece before programming, and select economic and reasonable process scheme according to specific conditions. The improper consideration of turning process is an important factor affecting the machining quality, production efficiency and processing cost of CNC machine tools. It is a process of changing the overall dimension or performance of workpieces with processing machinery. According to the temperature state of the workpiece to be processed, it can be divided into cold working and hot working. Generally, it is processed at normal temperature and does not cause chemical or physical changes of the workpiece, which is called cold working. Generally, the processing above or below the normal temperature will cause the chemical or physical phase changes of the workpiece, which is called thermal processing. Cold working can be divided into cutting and pressure machining according to the difference of processing methods. Heat treatment, forging, casting and welding are common in hot working. There are two steps in the formulation of machining process specification. First of all, it is to draw up the process route of parts processing, and then determine the process size of each process, the equipment and process equipment used, as well as the cutting specifications, man hour quota, etc. These two steps are interrelated and should be analyzed comprehensively.
The main task is to select the processing method of each surface, determine the processing sequence of each surface, and the number of processes in the whole process.
General principles for drawing up process routes
- Machining datum plane first
In the process of machining parts, the surface as positioning reference should be machined first, so as to provide precise reference for subsequent processing as soon as possible. It is called “benchmark first”.
- Dividing processing stages
The surface with high machining quality is divided into three stages: rough machining, semi finishing and finishing. It is mainly to ensure the processing quality, to use the equipment reasonably, to arrange the heat treatment process, and to find the blank defects easily.
- Face before hole
For parts such as box, bracket and connecting rod, the plane should be machined first and then the hole should be processed. In this way, the hole can be processed by plane positioning, which can ensure the position accuracy of plane and hole, and the machining of hole on the plane is convenient.
- Finishing
The finishing of the main surface (such as grinding, honing, finishing grinding / rolling, etc.) shall be carried out at the last stage of the process route. The surface finish after processing shall be ra0.8um. Slight collision will damage the surface. In Japan, Germany and other countries, after finishing, flannel shall be used for protection. It is absolutely forbidden to directly contact the workpiece with hands or other objects to avoid polishing The finished surface is damaged due to transportation and installation between processes.
Machining surface quality refers to the micro unevenness of the machined surface of parts after machining, also known as roughness. It is indicated by three codes of RA / RZ / ry and numbers. There will be corresponding surface quality requirements in mechanical drawings. Generally, the surface with surface roughness Ra < 0.8um is called mirror surface. The surface quality after machining directly affects the physical, chemical and mechanical properties of the workpiece. The working performance, reliability and service life of products depend on the surface quality of main parts to a great extent. Generally speaking, the surface quality requirements of important or key parts are higher than that of ordinary parts. Machining this is because the table and good quality parts will greatly improve its wear resistance, corrosion resistance and fatigue damage resistance
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