20 years of CNC machining practice, 12 experience summaries

CNC machining, also called CNC machining, refers to machining with CNC machining tools. Because CNC machining is programmed and controlled by a computer, CNC machining has the advantages of stable machining quality, high machining accuracy, high repeatability, complex surface machining, and high machining efficiency. In the actual processing process, human factors and operating experience will affect the final processing quality to a large extent. Let’s take a look, one with twenty CNC machining experience in the old master , summed up twelve valuable experience ….


How to divide the processing procedure?

The division of CNC machining processes can generally be carried out according to the following methods:

  • The tool centralized ordering method is to divide the processes according to the tools used, and use the same tool to process all the parts that can be completed on the part. Use the second knife and the third knife to complete other parts they can complete. This can reduce the number of tool changes, reduce idle time, and reduce unnecessary positioning errors.
  • According to the processing part order method, for parts with a lot of processing content, the processing part can be divided into several parts according to its structural characteristics, such as inner shape, shape, curved surface or plane. Generally, planes and positioning surfaces are processed first, and then holes are processed; simple geometric shapes are processed first, and then complex geometric shapes; the parts with lower precision are processed first, and then the parts with higher precision requirements are processed.
  • For parts that are prone to deformation by roughing and finishing in sequence, due to the deformation that may occur after roughing, they need to be adjusted. Therefore, in general, all the rough and finishing processes must be separated. In summary, when dividing the process, we must flexibly grasp the structure and manufacturability of the parts, the function of the machine tool, the number of parts CNC machining content, the number of installations and the production organization of the unit. Was built in principle proposed using the procedure using the principle of centralized or decentralized procedure, to be determined according to the actual situation, but must strive to be reasonable.



What principles should be followed in the arrangement of processing sequence?

The arrangement of the processing sequence should be considered according to the structure of the part and the condition of the blank, as well as the needs of positioning and clamping. The important point is that the rigidity of the workpiece is not destroyed. The sequence should generally be carried out according to the following principles:

  1. The processing of the previous process should not affect the positioning and clamping of the next process, and the processing of general machine tools should also be considered comprehensively.
  2.  Carry out the inner cavity adding procedure first, and then the contour processing procedure.
  3. It is best to connect the processes of processing with the same positioning, clamping method or the same tool to reduce the number of repeated positioning, the number of tool changes and the number of moving the platen.
  4. For the multiple processes carried out in the same installation, the process should be arranged with less rigid damage to the workpiece.


Which aspects should be paid attention to when determining the clamping method of the workpiece ?

The following three points should be paid attention to when determining the positioning datum and clamping plan:

  1. Strive to unify the design, technology, and programming calculation datum.
  2. Minimize the number of clamping times , and try to achieve all the surfaces to be processed after one positioning.
  3. Avoid using manual adjustment schemes.
  4. The fixture should be opened smoothly, and its positioning and clamping mechanism should not affect the cutting during processing (such as a collision). When encountering such a situation, use a vise or add a bottom plate to draw a screw.


How to determine the tool setting point is more reasonable?

What is the relationship between the workpiece coordinate system and the programming coordinate system?

  • The tool setting point can be set on the part to be processed, but note that the tool setting point must be a reference position or a part that has been processed. Sometimes the tool setting point is destroyed after the first process , which will cause the second process and after step unable knife point lookup, therefore toolsetting to set up in place relative to a relatively fixed size relationship between the positioning reference in the first procedure of the knife position, so that you can find the relative position relationship between them Back to the original tool setting point. This relative tool setting position is usually set on the machine table or fixture. The selection principles are as follows:

1) It is easy to align .
2) Easy to program.
3) The tool setting error is small.
4) Convenient and checkable inspection during processing.

  • The origin position of the workpiece coordinate system is set by the operator. It is determined by tool setting after the workpiece is clamped . It reflects the distance position relationship between the workpiece and the machine zero point. Once the workpiece coordinate system is fixed, it is generally not changed. The workpiece coordinate system and the programming coordinate system must be unified, that is, the workpiece coordinate system and the programming coordinate system are consistent during processing.


How to choose the knife route?

The path of the tool is the path and direction of the tool relative to the workpiece in the index control process. The reasonable choice of processing route is very important, because it is closely related to the processing accuracy and surface quality of the parts . In determining the path of the tool, the following points are mainly considered:

  1. Ensure the machining accuracy requirements of the parts.
  2. Convenient numerical calculation and reduce programming workload.
  3. Seek the shortest processing route, reduce the empty tool time to improve processing efficiency.
  4. Minimize the number of program segments.
  5. To ensure the roughness requirements of the workpiece contour surface after processing, the final contour should be processed continuously in the last pass.
  6. The advance and retreat (cut-in and cut-out) route of the tool should also be carefully considered to minimize the tool stop at the contour (a sudden change in cutting force causes elastic deformation) and leave tool marks, and also avoid vertical downward on the contour surface Knife and scratch the workpiece.


How to monitor and adjust during processing?

After the workpiece is aligned and the program is debugged, it can enter the automatic processing stage. In the automatic machining process, the operator should monitor the cutting process to prevent abnormal cutting from causing workpiece quality problems and other accidents. The monitoring of the cutting process mainly considers the following aspects:

  • Machining process monitoring The main consideration for rough machining is the rapid removal of excess allowance on the surface of the workpiece. In the automatic machining process of the machine tool, according to the set cutting amount, the tool automatically cuts according to the predetermined cutting path. At this time, the operator should pay attention to observe the cutting load changes in the automatic machining process through the cutting load table , and adjust the cutting amount according to the bearing capacity of the tool to maximize the efficiency of the machine tool.
  • through the cutting time of the cutting process in the automatic cutting sound monitoring process generally begins cutting tool cuts the workpiece sound is stable, continuous, light, motion at this time the machine is stationary. As the cutting process progresses, when there are hard spots on the workpiece, tool wear, or tool clamping , the cutting process becomes unstable. The unstable performance is that the cutting sound changes and the tool and the workpiece collide with each other. Sound, the machine tool will vibrate. At this time, the cutting amount and cutting conditions should be adjusted in time. When the adjustment effect is not obvious, the machine tool should be suspended to check the condition of the tool and workpiece.
  • Finishing process monitoring Finishing is mainly to ensure the processing size and surface quality of the workpiece, the cutting speed is high, and the feed rate is large. In this case should focus attention BUE influence on the processing surface, for the type of cavity, in the corner is also noted that the cut-processed with a flank. To solve the above problems, one is to pay attention to adjusting the spray position of the cutting fluid to keep the machined surface in a cooling condition at all times; the other is to observe the quality of the machined surface of the workpiece , and adjust the cutting amount to avoid quality changes as much as possible. If the adjustment still has no obvious effect, the machine should be shut down to check whether the original program is reasonable. Special attention should be paid to the position of the tool when the inspection is suspended or stopped. If the tool stops during the cutting process, a sudden spindle stop will cause tool marks on the surface of the workpiece. Generally, consider stopping the tool when it leaves the cutting state.
  • Tool monitoring The quality of the tool largely determines the processing quality of the workpiece. In the process of automatic machining and cutting, it is necessary to judge the normal wear condition and abnormal damage condition of the tool through methods such as sound monitoring, cutting time control, pause inspection during cutting, and workpiece surface analysis. According to the processing requirements, the cutting tools should be processed in time to prevent the processing quality problems caused by the cutting tools not being processed in time.


How to choose processing tools reasonably?

What are the major elements of cutting amount? How many kinds of materials are available? How to determine the speed, cutting speed and cutting width of the tool?

  • When plane milling, you should choose non-reground carbide end mills or end mills. In general milling, it is best to use the second pass to process. The first pass is best to use end milling cutter for rough milling, and continuous pass along the surface of the workpiece. The width of each pass is recommended to be 60%-75% of the tool diameter.
  • End mills and end mills with carbide inserts are mainly used for processing bosses , grooves and box mouth surfaces.
  • Ball knives and round knives (also known as round nose knives) are often used to process curved surfaces and variable-bevel contours. The ball knife is mostly used for semi-finishing and finishing. Insert carbide tools disc knife used for roughing.


What is the function of the processing procedure sheet?

What should be included in the processing procedure sheet?


  • The processing procedure sheet is one of the contents of the CNC machining process design, and it is also a procedure that needs to be observed and executed by the operator. It is a specific description of the processing procedure. The purpose is to let the operator clarify the content of the procedure, the clamping and positioning method, and each processing The tool selected by the program should be noted for issues.
  •  In the processing program list, it should include: drawing and programming file name, workpiece name, clamping sketch , program name, tool used in each program, maximum cutting depth, processing nature (such as roughing or finishing) , Theoretical processing time, etc.


What preparations should be made before CNC programming?

After determining the processing technology, you must understand before programming:


  1. Workpiece clamping method ;
  2. The size of the workpiece blank—-in order to determine the processing range or whether multiple clamping is required;
  3. The material of the work piece — -In order to choose which tool to use for processing;
  4. Which tools are in stock-avoid modifying the program during processing because there is no such tool. If you must use this tool, you can prepare in advance.


What are the principles for setting the safety height in programming?

The principle of setting safety height: generally higher than the highest surface of the island. Or set the programmed zero point on the highest surface, so that the risk of knife collision can be avoided to the greatest extent .


After the tool path is compiled, why post-processing is necessary?

Because the address code and NC program format that can be recognized by different machine tools are different, the correct post-processing format must be selected for the machine tool used to ensure that the compiled program can run.


What is DNC communication?

The way of program delivery can be divided into CNC and DNC. CNC means that the program is delivered to the memory of the machine tool through media media (such as floppy disk, tape reader , communication line, etc.) for storage, and the program is called from the memory during processing. Processing. Because the memory capacity is limited by the size, when the program is large, the DNC method can be used for processing. Because the machine tool directly reads the program from the control computer during DNC processing (that is, it is done while sending), it is not affected by the memory capacity Limited by size.

  1. There are three major elements of cutting parameters: depth of cut, spindle speed and feed rate . The general principle of selecting cutting parameters is: less cutting, fast feed (that is, the cutting depth is small, and the feed speed is fast)
  2. Classified by material, tool Generally divided into ordinary hard white steel knives (material is high-speed steel), coated knives (such as titanium plating, etc.), alloy knives (such as tungsten steel, boron nitride knives, etc.).

Post time: Jan-05-2021