When conducting process analysis, the programmer shall have the machine tool manual, programming manual, cutting quantity table, standard tools, fixture manual and other materials, select the appropriate machine tool according to the material, contour shape and machining accuracy of the workpiece to be machined, formulate the machining scheme, determine the machining sequence of parts, tools, fixtures and cutting parameters used in each process, etc. In addition, programmers should constantly summarize and accumulate practical experience in process analysis and write high-quality NC machining programs.

1、 Rational selection of machine tools

When machining parts on CNC machine tools, there are generally two situations. The first case: if there are part drawings and blanks, select the CNC machine tool suitable for processing the part. The second case: if there is already a CNC machine tool, select the parts suitable for processing on the machine tool. In either case, the main factors to be considered are the material and type of blank, the complexity of part contour shape, size, machining accuracy, number of parts, heat treatment requirements, etc. To sum up, there are three points: ① ensure the technical requirements of processed parts and process qualified products. ② It is conducive to improving productivity. ③ Minimize production costs (processing costs).

2、 Process analysis of NC machining parts

The process analysis of NC machining involves a wide range. Here, it is only analyzed from two aspects: the possibility and convenience of NC machining.

（1） The dimension data on the part drawing shall be given in accordance with the principle of convenient programming

1. The dimensioning method on the part drawing shall adapt to the characteristics of NC machining. On the NC machining part drawing, the dimensions shall be cited with the same benchmark or the coordinate dimensions shall be given directly. This marking method is not only convenient for programming, but also convenient for mutual coordination between dimensions, and brings great convenience in maintaining the consistency of design basis, process basis, detection basis and programming origin setting. Because the part designers generally pay more attention to the use characteristics such as assembly in dimensioning, they have to adopt the local and partial marking method, which will bring a lot of inconvenience to the process arrangement and NC machining. Due to the high accuracy of NC machining and repeated positioning, the use characteristics will not be damaged due to large accumulated errors. Therefore, the local decentralized marking method can be changed to the marking method of the same datum injection size or directly giving the coordinate size.

2. The conditions of geometric elements constituting the contour of parts shall be sufficient
Base point or node coordinates shall be calculated during manual programming. In automatic programming, all geometric elements constituting the part contour shall be defined. Therefore, when analyzing the part drawing, it is necessary to analyze whether the given conditions of geometric elements are sufficient. For example, circular arc and straight line, circular arc and circular arc are tangent in the drawing, but according to the dimensions given in the drawing, when calculating the tangent conditions, they become intersected or separated. Due to the insufficient conditions of geometric elements of parts, it is impossible to start programming. In case of this situation, it shall be solved through consultation with the part designer.

（2） The structural manufacturability of each processing part of the part shall meet the characteristics of NC machining

1) The inner cavity and shape of parts should preferably adopt unified geometric types and dimensions. This can reduce the tool specification and the number of tool changes, make the programming convenient and improve the production efficiency.

2) The size of the fillet of the inner groove determines the diameter of the tool, so the fillet radius of the inner groove should not be too small. The manufacturability of parts is related to the height of the machined contour and the radius of the transition arc.

3) When milling the bottom plane of the part, the fillet radius r of the groove bottom shall not be too large.

4) Unified datum positioning shall be adopted. In NC machining, if there is no unified datum positioning, the contour position and size of the two machined surfaces will be inconsistent due to the reinstallation of the workpiece. Therefore, in order to avoid the above problems and ensure the accuracy of its relative position after two clamping processes, a unified datum positioning should be adopted.

It is better to have a suitable hole on the part as the positioning reference hole. If not, set the process hole as the positioning reference hole (for example, add the process lug on the blank or set the process hole on the allowance to be milled in the subsequent process). If the process hole cannot be made, at least the finished surface should be used as a unified benchmark to reduce the error caused by two clamping.

In addition, it shall also analyze whether the machining accuracy and dimensional tolerance required by the parts can be guaranteed, whether there are redundant dimensions causing contradictions or closed dimensions affecting the process arrangement.

3、 Selection of processing method and determination of processing scheme

（1） Selection of processing methods
The selection principle of machining method 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 actual selection should be comprehensively considered in combination with the shape, size and heat treatment requirements of parts. For example, for holes with it7 accuracy, boring, reaming, grinding and other processing methods can meet the accuracy requirements, but the holes on the box are generally bored or reamed, rather than grinding. Generally, reaming is selected for small-size box holes, and boring shall be selected when the hole diameter is large. In addition, the requirements of productivity and economy, as well as the actual situation of the production equipment of the factory shall also be considered. For the economic machining accuracy and surface roughness of common machining methods, please refer to the relevant process manual.

（2） Principles for determining processing scheme
The machining of more precise surfaces on parts is often achieved gradually through rough machining, semi finish machining and finish machining. For these surfaces, it is not enough to only select the corresponding final processing method according to the quality requirements, but also correctly determine the processing scheme from blank to final forming.

When determining the processing scheme, the processing method required to meet these requirements shall be preliminarily determined according to the requirements of the accuracy and surface roughness of the main surface. For example, for it7 precision holes with small aperture, when the final machining method is fine reaming, drilling, reaming and rough reaming are usually carried out before fine reaming.

4、 Division of processes and steps
（1） Division of processes
When machining parts on CNC machine tools, the processes can be relatively concentrated, and most or all processes can be completed as far as possible in one clamping. Firstly, according to the part drawing, consider whether the processed part can complete the processing of the whole part on one NC machine tool. If not, decide which part is processed on the NC machine tool and which part is processed on other machine tools, that is, divide the processing procedures of the parts. The general process division has the following ways: (2) division of work steps. The division of work steps is mainly considered from the two aspects of machining accuracy and efficiency. In a process, it is often necessary to use different cutting tools and cutting parameters to process different surfaces. In order to facilitate the analysis and description of more complex processes, the process is subdivided into work steps. Take machining center as an example to illustrate the principle of work step division:
1) Rough machining, semi finish machining and finish machining shall be completed for the same surface in turn, or all machined surfaces shall be processed separately according to rough machining before finish machining.
2) For parts with both milling surface and boring hole, the surface can be milled before boring. By dividing the work steps according to this method, the accuracy of the hole can be improved. Because the cutting force is large during milling, the workpiece is easy to deform. Milling the surface first and then boring, so that it can recover for a period of time, so as to reduce the impact on the accuracy of the hole caused by deformation.
3) Work steps are divided by tool. In some machine tools, the rotation time of the worktable is shorter than the tool change time. Work steps can be divided according to the tool to reduce the number of tool changes and improve the machining efficiency.
In short, the division of processes and steps should be comprehensively considered according to the structural characteristics and technical requirements of specific parts.
5、 Installation of parts and selection of fixtures
（1） Basic principles of positioning and installation

1) Strive to unify the benchmark of design, process and programming calculation.
2) Reduce the clamping times as much as possible, and process all the surfaces to be processed after one-time positioning and clamping.
3) Avoid using manual adjustment processing scheme to give full play to the efficiency of NC machine tools.

（2） Basic principles of fixture selection

The characteristics of NC machining put forward two basic requirements for the fixture: one is to ensure that the coordinate direction of the fixture is relatively fixed with that of the machine tool; The second is to coordinate the dimensional relationship between parts and machine tool coordinate system. In addition, the following four points should be considered:

1) When the processing batch of parts is small, combined fixtures, adjustable fixtures and other general fixtures should be used as far as possible to shorten the production preparation time and save the production cost.
2) Only in batch production can special fixtures be 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) All parts and components on the fixture shall not hinder the machining of all surfaces of parts by the machine tool, that is, the fixture shall be open, and its positioning and clamping mechanism elements shall not affect the tool walking in machining (such as collision, etc.).

6、 Selection of cutting tools and determination of cutting parameters
（1） Tool selection
The selection of cutting tools is one of the important contents in NC machining technology. It not only affects the machining efficiency of machine tools, but also directly affects the machining quality. When programming, the selection of 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 requires not only high precision, good stiffness and high durability, but also stable size and convenient installation and adjustment. This requires the use of new high-quality materials to manufacture NC machining tools and optimize tool parameters.
When selecting a tool, the size of the tool should be compatible with the surface size and shape of the workpiece to be machined. In production, end milling cutter is often used to process the peripheral contour of plane parts. When milling the plane, carbide blade milling cutter should be selected; When machining bosses and grooves, select high-speed steel end milling cutter; When machining rough surface or rough machining hole, corn milling cutter with cemented carbide can be selected. When selecting end milling cutter for machining, it is recommended to select the relevant parameters of the cutter according to the empirical data. Ball end milling cutter is often used in surface machining, but when machining flat parts of the surface, the cutter cuts with the end edge of the top of the ball, and the cutting conditions are poor, so ring cutter should be used. In single piece or small batch production, in order to replace the multi coordinate linkage machine tool, drum cutter or conical cutter is often used to process some variable angle parts on the aircraft, and gear insert disc milling cutter is added. It is suitable for machining some spherical surfaces on the five coordinate linkage NC machine tool. Its efficiency is nearly ten times higher than that of ball end milling cutter, and good machining accuracy can be obtained.
On the machining center, various cutting tools are installed on the tool magazine respectively, and the tool selection and tool change are carried out at any time according to the program regulations. Therefore, there must be a set of extension rods connecting ordinary 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, you should understand the structural size, adjustment method and adjustment range of the tool bar used on the machine tool, so as to determine the radial and axial dimensions of the tool during programming. At present, China's machining centers adopt TSG tool system, and its handle has straight handle (three specifications) and conical handle (four specifications), including 16 kinds of knives for different purposes.

（2） Determination of cutting parameters
The cutting parameters include the spindle speed (cutting speed), back draft and feed. For different machining methods, different cutting parameters need to be selected and compiled into the program sheet.
The principle of reasonable selection of cutting parameters is that during rough machining, the main purpose is to improve productivity, but economy and processing cost should also be considered; In semi finishing and finishing, the cutting efficiency, economy and processing cost should be considered on the premise of ensuring the processing quality. The specific value shall be determined according to the machine tool manual, cutting parameter manual and experience.

7、 Determination of tool setting point and tool changing point

During programming, the positions of "tool setting point" and "tool changing point" should be correctly selected. "Tool setting point" is the starting point of tool movement relative to workpiece when machining parts on NC machine tool. Since the program segment is executed from this point, the tool setting point is also called "program starting point" or "tool starting point".
The selection principle of tool setting point is: 1 Simplify the preparation and processing of digital programs; 2. It is easy to align on the machine tool and check during processing; 3. Small machining error caused by.
The tool setting point can be either on the workpiece or outside the workpiece (such as on the fixture or machine tool), but it must have a certain dimensional relationship with the positioning datum of the part.
In order to improve the machining accuracy, the tool setting point should be selected on the design basis or process basis of the part as far as possible. For example, for the workpiece positioned by the hole, the center of the hole can be selected as the tool setting point. The position of the tool is aligned with this hole to make the "tool location" coincide with the "tool setting point". The common alignment method in the factory is to install the dial indicator on the machine tool spindle, and then rotate the machine tool spindle to make the "tool location" consistent with the tool setting point. The better the consistency, the higher the tool setting accuracy. The so-called "tool location" refers to the tool tip of turning tool and boring tool; Drill tip of drill bit; The center of the bottom surface of the end milling cutter and end milling cutter, and the center of the ball head of the ball end milling cutter.
After the parts are installed, there is a definite dimensional relationship between the workpiece coordinate system and the machine tool coordinate system. After the workpiece coordinate system is set, the coordinate value of the first program segment from the tool setting point; The coordinate value of the tool setting point in the machine tool coordinate system is (x0, Y0). When programming according to the absolute value, it is x2 and Y2 regardless of whether the tool setting point coincides with the workpiece origin; When programming according to the increment value, when the tool setting point coincides with the workpiece origin, the coordinate values of the first program segment are x2 and Y2. When they do not coincide, they are (x1 + x2) and Y1 + Y2).
Tool setting point is not only the starting point of the program, but also the end point of the program. Therefore, in batch production, the repeated accuracy of the tool setting point should be considered, which can be checked by the coordinate values (x0, Y0) between the tool setting point and the origin of the machine tool.
The so-called "machine tool origin" refers to a fixed limit point on the machine tool. For example, for a lathe, it refers to the intersection of the rotation center of the lathe spindle and the end face of the head chuck.
When the tool needs to be changed during processing, the tool change point shall be specified. The so-called "tool change point" is the position of the 100 tool holder during tool change. This point can be a fixed point (for example, the position of the tool changing manipulator of the machining center machine tool is fixed) or any point (for example, lathe). The tool change point shall be set outside the workpiece or fixture, subject to the fact that the tool holder does not touch the workpiece and other parts during rotation. Its set value can be determined by actual measurement method or calculation.

8、 Determination of processing route

In NC machining, the path of the tool position relative to the workpiece is called machining route. During programming, the determination principles of processing route mainly include the following points:
1) The processing route shall ensure the accuracy and surface roughness of the processed parts, and the efficiency shall be high.
2) The numerical calculation is simple to reduce the programming workload.
3) The processing route should be minimized, which can not only reduce the program section, but also reduce the empty tool time. Degree, determine whether it is one-time tool walking or multiple tool walking to complete the machining, and whether forward milling or reverse milling is used in milling.

For point controlled CNC machine tools, only high positioning accuracy is required, and the positioning process is as fast as possible, while the motion route of the tool relative to the workpiece is irrelevant. Therefore, this kind of machine tools should arrange the tool path according to the shortest empty distance. In addition, the axial motion size of the tool should be determined, which is mainly determined by the hole depth of the machined part, but some auxiliary dimensions should also be considered, such as the lead-in distance and overrun of the tool.

When threading the NC machine tool, the z-direction feed along the pitch direction should maintain a strict speed ratio relationship with the rotation of the machine tool spindle. Therefore, cutting in the process of acceleration or deceleration of the feed mechanism should be avoided. For this purpose, there should be a lead-in distance δ 1 override distance δ 2。 The value of and is related to the dynamic characteristics of the machine tool driving system, the pitch of the thread and the accuracy of the thread. It is generally 2-5mm, and the larger value is taken for threads with large pitch and high precision; Generally take about 1 / 4 of. If there is no undercut at the end of the thread, the shape of the end is related to the NC system, and it is generally ended as 45o.
When milling plane parts, end mills are generally used