Due to the complexity of CNC machining (such as different machine tools, different materials, different cutting tools, different cutting methods, different parameter settings, etc.), it is determined that to reach a certain level from working in CNC machining (whether machining or programming) to reach a certain level, it must go through a relatively long period of learning and practice. This manual is a summary of the experience of the engineers in the long-term actual production process, such as NC machining process, process, the selection of commonly used tool parameters, monitoring and control in the process, for your reference.

Q: How to divide the process?

Answer: the division of numerical control working procedure can be carried out according to the following methods generally.

1)   Cutter centralized sequencing method: To divide the process according to the tool used, with the same cutter processing all parts can be completed. Same way, then use the second cutter and the third to complete other parts they can. By this way, it can reduce the number of cuter change, compresses the idle time and reduces unnecessary positioning error.

2)    Process dividing based on process the deferent part of workpiece: For parts with a lot of machining content, the machining part can be divided into several parts according to its structural characteristics, such as internal shape, outside shape, curved surface or flat surface, etc. Generally, machining flat surface and position surface first, then the holes; First processing simple geometry, then processing complex geometry; Firstly machining the parts with lower precision, then the parts with higher precision.

3)     Process dividing based on rough and precision process: For parts that are easy to deformation after machining, it has been to done correction after rough machining, so generally, for parts which rough and final machining have to be separated, the process should be divided.

 

To sum up, in the division and formulation of the process, we must be flexible in accordance with the structural characteristics and workmanship of parts, the function of machine tools, the number of parts processing content, installation times and the production organization of the unit. In addition, it is suggested that the principle of centralized process should be adopted or the principle of decentralized process should be adopted, which should be determined according to the actual situation, but must be based on the principle of rationality.

2.  Q: What principles should be followed in the arrangement of processing sequence?

Answer: Processing sequence should be arranged according to the structure characteristic of the parts and workblank status, as well as the need to consider for positioning and clamping, the emphasis is that the rigidity of the workpiece is not damaged.

2.1. The machining of the last process cannot affect the positioning and clamping for the next process, and if it has the interspersed process by general machine tool, it should also be considered comprehensively.

2.2   The internal shape and chamber should be machined firstly, followed by the outside surface machining process.

2.3   Processing with the same positioning, clamping or the same cutter should be carried out continuously in order to reduce the number of repeated positioning, tool changing and moving plate times.

2.4   If there are multiple processes under the same installation, the process which has less damage to the rigid of the workpiece shall be arranged first.

 

3. Q: what aspects should be paid more attention in workpiece clamping?

Answer: the following three points should be noted when determining the positioning datum and clamping scheme.

3.1   Try to unify the datum of design, process, and benchmark.

3.2   Minimize clamping times, and try to machine all the surfaces under one clamping and position.

3.3   Avoid manual adjustment program on the machine tool.

3.4   The fixture should be open, and its positioning and clamping mechanism can not affect the tool traveling (such as collision) in machining. When such a situation occurs, the clamping can be carried out by means of vise or bottom plate screw pulling.

 

4. Question: how to determine the cutter point is more reasonable? What is the relationship between the workpiece coordinate system and the programming coordinate system?

4.1. The cutter point can be set on the part which have been machined, but note that the cutter point must be the reference position or the part that has been done final machining. Sometimes the cutter point is destroyed after the first process, which will lead that cutter point for the second process and late on process cannot be found, so in the cutter alignment of first process, attention should be paid to a cutter point position where must have relative fixed position relationship with the positioning datum, so that the original point can be found according to the relative position relationship between them. This relative cutter location is usually located on machine tool worktable or fixture. The selection principles are as follows:

1)  Easy for alignment

2)  Convenient programming

3)  Small error in cutter alignment

4)  Easy checking during processing

 

4.2 The origin position of the workpiece coordinate system is set by the operator himself, it is determined by the cuter alignment after the workpiece clamping; it reflects the distance between the workpiece and the zero point of the machine tool. Once the workpiece coordinate system is fixed, it will not change normally. The workpiece coordinate and the programming coordinate system must be unified, means the workpiece coordinate and the programming coordinate systems are the same.

 

5. Q: how to choose the path of cutter moving?

The cutter moving path refers to the motion path and direction of the cutter relative to the workpiece in CNC machining process. The reasonable selection of processing route is very important because it is related to the machining accuracy and surface quality of parts. The following are the main points for determining the route.

(1)    Ensure the accuracy of parts machining.

(2)    Convenient in calculation, reduce programming workload.

(3)    Seek the shortest processing route and reduce the blank time to improve machining efficiency.

(4)    Minimize the number of program segments.

(5)    Guarantee the roughness requirement of the workpiece contour surface after machining, the final contour should be arranged for the last tool continuous machining.

(6)    The cutter's feeding and withdrawing (cutting and cutting) routes should also be carefully considered in order to minimize tool stopping at the contour (sudden change in cutting force resulting in elastic deformation) and leave a knife mark, but also to avoid vertical cutting on the contour surface and scratch the workpiece.

 

6. Q: how to monitor and adjust during the process?

The workpiece can enter the automatic processing stage after the alignment and program debugging. In the process of automatic machining, the operator should monitor the cutting process to prevent abnormal cutting which can cause quality problems and other accidents.

The following aspects are mainly considered in monitoring the cutting process:

6.1. Monitoring rough machining. The main purpose of rough machining is the rapid removal of redundant allowance on the workpiece surface. In the process of automatic machining, the cutter automatically cuts according to the predetermined cutting path and cutting dosage. At this point, the operator should observe the change of cutting load through the performance of cutting load, adjust the cutting amount according to the bearing condition of the cutter, and give play to the maximum efficiency of the machine tool. 3 x In the process

6.2 Monitoring sound during cutting. In the process of automatic cutting, normally in the begins of cutting, the sound of the cutting is stable, continuous and light, at this time the movement of the machine tool is stable. With cutting continuation, when there are hard spots on the workpiece or cutter wear or tool clamp etc. reasons, the cutting will appear unstable, the performance of unstable cutting is that the sound of cutting changes and the impact will occur between cutter and workpiece, machine tool appear vibration. At this time, the cutting parameters and cutting conditions should be adjusted in time，when the effect after adjustment is not obvious, the machine tool should be suspended and to check the cutters and work conditions. 6 At this time, the cutting parameters and

 

6.3 Finish cutting monitoring. Finish cutting is mainly to ensure the workpiece dimension and surface quality, normally cutting speed is higher, the feed is larger.  At this time, we should pay attention to the influence of chip to the machined surface. For inner chambering, we should also pay attention to overcutting and cutter back-off at the corner. To solve the above problems, one is to adjust the spraying position of cooling and keep the machined surface in the best cooling condition. Second, we should pay attention to the quality of the machined surface, as far as possible to avoid quality changes by adjusting the machining parameters. If the adjustment still has no obvious effect, machine should be stopped to check if the program is correct. 

Special attention should be paid to the location of the cutter when stopping machine tools. If the cutter stops during the machining, the sudden stop of spindle will cause cutter mark on the surface of the workpiece. Generally, cutter should be moved away from the cutting surface before stop machining tools.

6.4 Cutter monitoring. To a large extent, the quality of the cutting tool determines the quality of the workpiece. In the process of automatic machining, the normal wear and abnormal damage of cutting tools should be judged by means of sound monitoring, cutting time control, cutting pause for inspection and surface analysis and so on. According to the machining requirements, the cutters should be treated in time to prevent the quality problems caused by the damaged cutter.

7. Q: how to choose machining tools reasonably? What are the key elements of cutting? How many kinds of materials of cutting tool? How to determine the rotational speed, cutting speed and cutting width of the cutter?

7.1 In face milling, non regrinding cemented carbide face milling cutter or end mills should be used. General milling, as far as possible to use twice processing, the cutter of first process is best to use end milling cutter to do rough milling continuously along the surface of the workpiece. The width of each knife is recommended to be 60%--75% of the tool diameter.

7.2 The end mills and end mill with carbide inserts are mainly used for machining boss, grooves and box faces.

7.3 Ball cutter and circular cutters (also called round nose cutters) are often used for machining curved surfaces and variable oblique profiles. Ball cutters are mostly used for semi finishing and finishing, the circular knives with carbide inserts are mostly used for rough machining.

 

8. Q: what is the function of the processing program card? What should be included in the process sheet?

Answer:

(1) Processing program sheet is one of the contents of CNC machining process program design, also is the regulations that requires operators to comply with and implement, is the specific description of processing procedures, the purpose is to let the operator clear the content of the program, clamping and positioning methods, the cutter selection in deferent processing procedures (the points which need to pay attention to).

(2) In the process sheet, it should include: drawing and programming file name, workpiece name, clamping sketch, program name, the tool used in each program, the maximum depth of cutting, processing properties (such as rough or finish), theoretical processing time and so on.

9. Q: what preparation needs to be done before CNC programming?

Answer: after determining the processing route, you should know before programming.

1)      Workpiece clamping method

2)      The size of the workpiece embryo, so that determine the scope of processing or whether multiple clamping is necessary.

3)      The material of the workpiece, so that choose cutter used.

4)      Available cutters in stock, Avoid modifying the program because there is no cutter available in the process, if the cutter must be used, it can be prepared in advance.

 

10. Q: what are the principles for setting the height of safety in programming?

Answer: The principle in setting up safe height: Generally it should be higher than the highest face of the island, or setting the programming zeros on the highest surface, which can also minimize the risk of hitting cutters.、

 

11. Q: why do we need to do post-processing after the tool path is designed?

Answer: Because different machine tools can recognize deferent address code and NC program format, so choose the correct post-processing format based on deferent machining tools to ensure that the program can run.

 

12. Q: What is DNC communication?

Answer: The way of program transmission can be divided into CNC and DNC, CNC is to transport program to the memory of machine tools for store up through media (such as floppy disk, tape reader, communication lines, etc.), when processing the program will transfer from the memory of machine tolls. Because the limited capacity of memory , so when the program is large, it cannot be process anymore, but DNC mode can be used for processing, because the DNC processing machine directly from the control computer to read the program (that is, transferring while doing), so the memory capacity is not limited by the size.

(1)    There are three main factors of cutting parameters: cutting depth, spindle speed and feed speed. The general principles of cutting parameters selection are: less cutting, fast feed (that is, cutting depth is small, feed speed is fast).

(2)    According to material classification, cutting tools are generally divided into high-speed steel materials, coated cutting tools (such as titanium plating, etc.), alloy cutting tools (such as tungsten steel, boron nitride cutting tools, etc.).