3、 Selection of processing method and determination of processing scheme
（1） Selection of processing method
The selection principle of machining method in Dongguan CNC machining plant is to ensure the machining accuracy and surface roughness of the machined surface. Because there are many processing methods to obtain the same level of accuracy and surface roughness, the shape, size and heat treatment requirements of the parts should be considered in the actual selection. For example, boring, reaming, grinding and other processing methods can meet the accuracy requirements for the holes with it7 accuracy, but boring or reaming is generally used for the holes on the box rather than grinding. Generally, the small-size box hole should be reamed. When the hole diameter is large, boring should be selected. In addition, we should also consider the requirements of productivity and economy, as well as the actual situation of the production equipment of the factory. For the economic machining accuracy and surface roughness of common machining methods, please refer to the relevant process manual.
（2） Principle of determining processing plan
Dongguan CNC machining plant parts on the more precise surface processing, often through rough machining, semi finishing and finishing gradually achieved. For these surfaces, it is not enough to select the corresponding final processing method only according to the quality requirements, but also to correctly determine the processing scheme from the blank to the final forming. When determining the processing scheme, the processing method needed to meet these requirements should be initially determined according to the requirements of the main surface precision and surface roughness. For example, for a hole with a small diameter and an accuracy of grade it7, when the final machining method is fine reaming, it usually needs drilling, reaming and rough reaming before fine reaming.
4、 Division of working procedure and working step
（1） Division of working procedure
Dongguan CNC machining plant processes parts on CNC machine tools, the process can be more concentrated, and most or all of the processes can be completed in one clamping. First of all, according to the part drawing, we should consider whether the parts to be processed can be processed on one CNC machine tool. If not, we should decide which part of the parts to be processed on the CNC machine tool and which part to be processed on other machine tools, that is, we should divide the processing procedures of the parts.
（2） Division of work steps
The division of working steps is mainly considered from two aspects of machining accuracy and efficiency. In a process, it is often necessary to use different tools and cutting parameters to process different surfaces. In order to analyze and describe more complex process, the process is subdivided into steps. Take the machining center as an example to illustrate the principle of step division:
1) The same surface is finished by rough machining, semi finish machining and finish machining in turn, or the whole surface is processed separately by rough machining before finish machining.
2) For parts with both milling face and boring, milling face first and then boring. According to this method, the accuracy of hole can be improved. Because of the large cutting force in milling, the workpiece is prone to deformation. Milling the surface first and boring the hole later, so that it can recover for a period of time and reduce the influence on the accuracy of the hole caused by deformation.
3) Divide the work step according to the cutting tool. In order to reduce the times of tool changing and improve the efficiency of machining, the turning time of some machine tool table is shorter than that of tool changing.
In a word, the division of working procedure and working step should be considered comprehensively according to the structural characteristics and technical requirements of specific parts.
5、 Parts installation and fixture selection
（1） Basic principles of positioning and installation
1) Strive to unify the design, process and programming calculation.
2) Reduce the clamping times as much as possible, and process all the surfaces to be processed after one positioning and clamping.
3) In order to give full play to the efficiency of CNC machine tools, the manual adjustment processing scheme should be avoided.
（2） The basic principle of selecting fixture
The characteristics of NC machining put forward two basic requirements for fixture: one is to ensure that the coordinate direction of fixture is relatively fixed with that of machine tool; The second is to coordinate the dimension relationship between parts and machine coordinate system. In addition, the following four points should be considered:
1) When the processing batch of parts is small, modular fixture, adjustable fixture and other general fixtures should be used as far as possible to shorten the production preparation time and save production costs.
2) In batch production, the special fixture is considered, and the structure is simple.
3) The loading and unloading of parts should be fast, convenient and reliable, so as to shorten the pause time of the machine tool.
4) The parts on the fixture shall not hinder the machining of the surface of the parts by the machine tool, that is, the fixture shall be open to its positioning, and the clamping mechanism elements shall not affect the tool path in the machining (such as collision, etc.).
6、 Selection of cutting tools and determination of cutting parameters
（1） Tool selection
The selection of cutting tool is one of the important contents in NC machining process, which not only affects the machining efficiency of machine tool, but also directly affects the machining quality. When programming, the selection of cutting tools usually takes into account the processing capacity of the machine tool, process content, workpiece material and other factors. Compared with traditional machining methods, NC machining has higher requirements for cutting tools. It not only requires high precision, good rigidity and high durability, but also requires stable size and convenient installation and adjustment. This requires the use of new high-quality materials to manufacture CNC machining tools, and optimize tool parameters.
When selecting cutting tools in Dongguan CNC machining plant, the size of the cutting tools should be adapted to the surface size and shape of the workpiece to be processed. In production, end mills are often used to process the peripheral contour of plane parts. When milling the plane, the carbide cutter should be selected; When machining boss and groove, high speed steel end mill is selected; When machining rough surface or rough hole, corn milling cutter with cemented carbide can be selected. It is recommended to select the relevant parameters of the end milling cutter according to the empirical data. The ball end milling cutter is often used in curved surface machining, but when machining the flat part of curved surface, the cutting condition of the cutter is poor, so the ring cutter should be used. In the single or small batch production, in order to replace the multi coordinate linkage machine tool, the drum cutter or conical cutter is often used to process some variable angle parts of the aircraft with the insert gear disc milling cutter. It is suitable for machining some spherical surfaces on the five coordinate linkage CNC machine tool. Its efficiency is nearly ten times higher than that of the ball end milling cutter, and good machining accuracy can be obtained.
In the machining center of Dongguan CNC machining plant, all kinds of cutting tools are installed on the tool magazine respectively, and the tool selection and change work are carried out at any time according to the program. Therefore, there must be a set of connecting rods for common tools, so that the standard tools for drilling, boring, expanding, reaming, milling and other processes can be quickly and accurately installed on the machine tool spindle or tool magazine. As a programmer, we should know the structural dimension, adjustment method and adjustment range of the tool bar used on the machine tool, so as to determine the radial and axial dimension of the tool during programming. At present, TSG tool system is used in machining centers in China. There are two kinds of shank: straight shank (three specifications) and taper shank (four specifications), including 16 kinds of tools for different purposes.