Abstract : Based on the comparison of CNC lathe processing technology and traditional processing technology, the key problems in CNC lathe processing technology are deeply analyzed and the process design method of CNC lathe is summarized. It is proved by examples that the correct analysis and design of CNC lathe processing technology is helpful to improve the processing quality and production efficiency of parts.

Keywords : process design ; process analysis ; CNC programming

  The first thing to consider in machining parts on CNC lathe is the process problem. How to select the fixture to facilitate the installation on the lathe, to facilitate the coordination of parts and lathe coordinate system size ; how to select the tool setting point to make the programming simple and processing convenient ; how to choose the tool path to improve the production efficiency and how to reasonably arrange the process, tool and determine the cutting parameters to ensure the machining accuracy and surface roughness of the parts are the problems that must be solved before writing the program.

1 The advantages of CNC lathe processing technology

  Compared with the traditional processing technology, the numerical control processing technology has changed in many aspects.Benchmark selection and positioning error are more prominent problems. Understanding these differences is helpful to the smooth progress of numerical control processing and the guarantee of processing quality.

  In CNC machining, the problem that the positioning datum and the design datum do not coincide still exists, but the measurement datum and the process datum can be consistent with the design datum to avoid the dimension chain solution and its error. When the NC machining program is compiled, the coordinate method with controllable precision is used to determine the shape and size of each section, and the positioning accuracy of the servo system is also very high ( 0.001 mm or more ), which can easily ensure the dimensional tolerance and shape and position tolerance of the product. Therefore, even if the datum does not coincide or the datum is not uniform, the influence on the accuracy of the workpiece can be controlled very small.

  The positioning error consists of two parts: the non coincidence error of the reference and the reference error. When using fixtures for mass production in traditional machining, the benchmark error ‚ The impact on machining parts is very prominent ‚ But the frequent use of fixtures in CNC machining is no longer common ‚ The parts need to be aligned with the tool before processing ‚ It is often directly aligned with the actual surface of the workpiece ‚ During the processing, there is also little need for re clamping or repositioning ‚ The impact of fixtures has been greatly reduced ‚ So compared to traditional processing techniques ‚ The influence of positioning error in CNC machining is no longer the main factor ‚ This is also one of the advantages of CNC machining technology.

2 CNC lathe processing technology analysis

  The analysis of CNC lathe processing technology involves a wide range of aspects. This paper only analyzes from two aspects of possibility and convenience.

1. The size data in the part drawing should be given to ensure that the programming is convenient. It is best to mark the size with the same benchmark or directly give the coordinate size on the part drawings of CNC machining. In the manual programming to calculate the coordinates of each node ; all geometric elements that constitute the contour of the part should be defined during automatic programming.
2. The structure of the parts of the processing parts should be in line with the characteristics of CNC machining. On the premise of meeting the requirements of use, it is best to adopt a unified geometric type and size to reduce the tool specifications and the number of tool changes to simplify programming. CNC machining should try to use a unified reference positioning, otherwise it will cause the shape and position error of the workpiece due to the multiple installation and positioning errors of the workpiece.

The required machining accuracy of the parts is analyzed, including whether the dimensional tolerance, shape and position tolerance, surface roughness, etc.can be guaranteed to have redundant dimensions that cause contradictions or closed dimensions that affect the process arrangement.

3 CNC lathe machining process design

A.Design of the machining process route for CNC lathes. The machining process of a CNC lathe is divided into multiple stages. Rough machining stage: The main task is to cut off most of the machining allowance, so that the blank is close to the finished part in shape and size. Semi precision machining stage: achieve a certain degree of accuracy on the main surface, prepare for the precision machining of the main surface, and complete some minor surface machining. Precision machining stage: Ensure that all major surfaces meet the quality requirements specified in the drawings. Finishing stage: For surfaces with high quality requirements (IT6 and above, Ra ≤ 0 ∙ 32m), finishing is necessary, mainly used to further improve dimensional accuracy and reduce surface roughness values.

  B. CNC lathe machining fixture and tool design. Combination fixture, general fixture or adjustable fixture should be preferred in single small batch production. Special fixture can be used in batch production. It is also required that the fixture is installed on the CNC machine tool to accurately coordinate the size relationship between the workpiece and the machine tool coordinate system. Generally, standard tools are preferred, and various composite tools and other special tools can also be used. Can also choose a variety of advanced tools such as indexable tool cemented carbide tools, ceramic coated tools, etc. The type, specification and precision grade of the tool should meet the processing requirements. The tool material should be compatible with the part material.

C. CNC lathe machining tool path design. To determine the tool path should be considered to ensure that the processing quality as much as possible to shorten the tool path programming calculation to the number of simple program segments and ' less tool change ', ' less empty tool ' and so on.'

D. The design of CNC lathe machining cutting parameters. The cutting parameters mainly include cutting depth, spindle speed and feed rate. The cutting speed has the greatest influence on the tool life, followed by the feed cutting depth. Considering the relationship between cutting parameters and tool durability, a large cutting depth should be given priority when selecting the cutting parameters of rough machining, followed by a large feed rate and finally a reasonable cutting speed. Tool tip wear is often an important factor affecting machining accuracy during finishing. Therefore, tool materials with good wear resistance should be selected and worked within the optimal cutting speed range as much as possible.

4 CNC parts turning process analysis and design examples

  The 45 steel bar with a diameter of Φ70mm and a total length of 98mm is processed into the parts shown in Fig.1 by CNC lathe.

  Process analysis and design : The programming zero point of the part is set at the intersection of the right end face of the workpiece and the central shaft.

  Process route : clamping left end-car right end-rough machining contour-finishing contour-slotting

  The choice of tool : No.1 is rough machining cylindrical cutter No.2 is finishing cylindrical cutter No.3 is slotting cutter.

Fig.1 Parts drawing

5 Conclusion

  The process design in CNC machining is an important part of CNC programming. Whether the process design is reasonable or not will be directly related to the efficiency of CNC lathe, the processing quality of parts, the number of tools and the economy. Therefore, sufficient and comprehensive process analysis should be carried out to design the process flexibly and reasonably when formulating the NC machining process of parts. It has been proved by practice that the selection of reasonable and efficient process methods and processing routes is of great significance for the preparation of high quality NC programs to improve processing quality, production efficiency and economic benefits.

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