Essential Guide to CNC Machining: Strategies and Best Practices

Due to the complexity of CNC machining (such as different machine tools, different materials, different tools, different cutting methods, different parameter settings, etc.), it takes a relatively long time to reach a certain level in CNC machining (whether machining or programming). This guide is a summary of some experiences summarized by engineers in the long-term actual production process on CNC machining technology, processes, selection of commonly used tool parameters, monitoring during machining, etc., which can be used for your reference.

1. Q: How to divide the machining process?

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

1. Tool concentration sequencing method is to divide the process according to the tools used, and use the same tool to complete all the parts that can be completed on the part. Then use the second and third tools to complete the other parts that they can complete. This can reduce the number of tool changes, compress idle time, and reduce unnecessary positioning errors.
2. Sequencing by machining parts 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, external shape, curved surface or plane. Generally, planes and positioning surfaces are processed first, and then holes; simple geometric shapes are processed first, and then complex geometric shapes; parts with lower precision are processed first, and then parts with higher precision requirements.
3. Sequential processing by roughing and finishing For parts that are prone to deformation during processing, correction is required due to the deformation that may occur after roughing. Therefore, generally speaking, all processes that require roughing and finishing must be separated.

In summary, when dividing the processes, it is necessary to flexibly grasp the structure and processability of the parts, the function of the machine tool, the amount of CNC processing content of the parts, the number of installations, and the production organization status of the unit. It is also recommended to adopt the principle of process concentration or the principle of process dispersion. It should be determined according to the actual situation, but it must be reasonable.

1. Q: What principles should be followed in arranging the processing sequence?

A: The arrangement of the processing sequence should be based on the structure and blank condition of the parts, as well as the need for positioning and clamping. The focus 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 cannot affect the positioning and clamping of the next process. The interspersed general machine tool processing processes should also be considered comprehensively.
2. Carry out the inner shape and inner cavity processing process first, and then the outer shape processing process.
3. It is best to carry out the processes that are processed with the same positioning, clamping method or the same tool in succession to reduce the number of repeated positioning, tool changes and platen movements.
4. For multiple processes carried out in the same installation, the process that causes the least damage to the workpiece rigidity should be arranged first.

1. Q: What aspects should be paid attention to when determining the workpiece clamping method?

A: The following three points should be paid attention to when determining the positioning reference and clamping scheme:

1. Strive to unify the references of design, process, and programming calculation.
2. Minimize the number of clamping times and try to process all the surfaces to be processed after one positioning.
3. Avoid using manual adjustment schemes that occupy the machine.
4. The fixture should be open, and its positioning and clamping mechanism should not affect the tool path during processing (such as collision). When encountering such a situation, it can be clamped by using a vise or adding a bottom plate to remove screws.

1. Q: How to determine the tool point more reasonably?

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

A: 1. The tool setting point can be set on the processed part, but note that the tool setting point must be the reference position or the part that has been finely processed. Sometimes the tool setting point is destroyed by processing after the first process, which will make it impossible to find the tool setting point in the second process and later. Therefore, when setting the tool in the first process, pay attention to setting a relative tool setting position at a place with a relatively fixed size relationship with the positioning reference, so that the original tool setting point can be found according to the relative position relationship between them. This relative tool setting position is usually set on the machine tool workbench or fixture. The selection principles are as follows:

1) Easy to align.

2) Convenient programming.

3) Small tool setting error.

4) Convenient inspection during processing.

2. The origin position of the workpiece coordinate system is set by the operator himself. It is determined by tool setting after the workpiece is clamped. It reflects the distance position relationship between the workpiece and the machine tool 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, during processing, the workpiece coordinate system and the programming coordinate system are consistent.

1. Q: How to choose the tool path?

A: The tool path refers to the movement trajectory and direction of the tool relative to the workpiece during CNC processing. The reasonable selection of the processing route is very important because it is closely related to the processing accuracy and surface quality of the parts. The following points are mainly considered when determining the tool path:

1) Ensure the processing accuracy requirements of the parts.

2) Facilitate numerical calculations and reduce programming workload.

3) Seek the shortest processing route and reduce the idle tool time to improve processing efficiency.

4) Minimize the number of program segments.

5) Ensure the roughness requirements of the workpiece contour surface after processing, and the final contour should be arranged for the last tool to be processed continuously.

6) The tool's entry and exit (cut-in and cut-out) route should also be carefully considered to minimize stopping at the contour (elastic deformation caused by sudden changes in cutting force) and leaving tool marks, and also avoid vertical cutting on the contour surface and scratching the workpiece.

1. Q: How to monitor and adjust during the machining process?

A: After the workpiece is aligned and the program is debugged, it can enter the automatic machining stage. During the automatic machining process, the operator must monitor the cutting process to prevent abnormal cutting from causing workpiece quality problems and other accidents.

The following aspects should be considered for monitoring the cutting process:

1. Process monitoring Rough machining mainly considers the rapid removal of excess allowance on the surface of the workpiece. During the automatic machining of the machine tool, according to the set cutting amount, the tool automatically cuts according to the predetermined cutting trajectory. At this time, the operator should pay attention to observing the changes in the cutting load during the automatic machining process through the cutting load table, and adjust the cutting amount according to the tool's bearing capacity to maximize the efficiency of the machine tool.
2. Monitoring of cutting sound during cutting During the automatic cutting process, when the cutting process begins, the sound of the tool cutting the workpiece is generally stable, continuous, and brisk, and the movement of the machine tool is stable. As the cutting process proceeds, when there are hard spots on the workpiece or the tool is worn or the tool is clamped, the cutting process becomes unstable. The instability is manifested by changes in the cutting sound, collision sounds between the tool and the workpiece, and vibration of the machine tool. 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 stopped to check the condition of the tool and workpiece.
3. 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. At this time, attention should be paid to the influence of built-up edge on the processing surface. For cavity processing, attention should also be paid to overcutting and cutting at the corners. To solve the above problems, one is to pay attention to adjusting the spray position of the cutting fluid so that the processing surface is always in the best cooling condition; the second is to pay attention to observing the quality of the processed surface of the workpiece and avoid quality changes as much as possible by adjusting the cutting amount. If the adjustment still has no obvious effect, the machine should be stopped to check whether the original program is reasonably programmed. In particular, pay attention to the position of the tool when pausing for inspection or stopping for inspection. If the tool stops during the cutting process and the spindle stops suddenly, tool marks will be produced on the surface of the workpiece. Generally, the machine should be stopped when the tool leaves the cutting state.
4. Tool monitoring The quality of the tool determines the processing quality of the workpiece to a large extent. During the automatic machining and cutting process, the normal wear and abnormal damage of the tool should be judged by sound monitoring, cutting time control, pause inspection during cutting, workpiece surface analysis and other methods. According to the processing requirements, the tool should be handled in time to prevent the occurrence of processing quality problems caused by the tool not being handled in time.

1. Q: How to reasonably choose the processing tool?

What are the major factors of cutting consumption? How many materials are there for tools? How to determine the tool speed, cutting speed, and cutting width?

A: 1. When plane milling, you should use a non-reground carbide end mill or end mill. In general milling, try to use secondary tooling. The first tooling is best to use an end mill for rough milling, and the tool is continuously moved along the surface of the workpiece. The width of each tooling is recommended to be 60%-75% of the tool diameter.

2. End mills and end mills with carbide inserts are mainly used to process bosses, grooves and box mouth surfaces.

3. Ball cutters and round cutters (also known as round nose cutters) are often used to process curved surfaces and variable bevel contour shapes. Ball cutters are mostly used for semi-finishing and finishing. Circular cutters with carbide inserts are mostly used for roughing.

1. Q: What is the role of the machining program sheet?

What should be included in the machining program sheet?

A: (1) The machining program sheet is one of the contents of CNC machining process design. It is also a procedure that operators need to follow and implement. It is a specific description of the machining program. The purpose is to let the operator know the program content, clamping and positioning methods, and the issues that should be paid attention to when using the tools selected for each machining program.

(2) The machining program sheet should include: drawing and programming file name, workpiece name, clamping sketch, program name, tool used for each program, maximum cutting depth, machining nature (such as roughing or finishing), theoretical machining time, etc.

1. Q: What preparations should be made before CNC programming?

A: After determining the processing technology, before programming, you need to understand: 1. Workpiece clamping method; 2. The size of the workpiece blank - in order to determine the scope of processing or whether multiple clamping is required; 3. The material of the workpiece - in order to select the tool used for processing; 4. What tools are in stock - to avoid modifying the program due to the lack of this tool during processing. If this tool must be used, it can be prepared in advance.

1. Q: What are the principles for setting the safety height in programming?

A: The principle for setting the safety height: generally higher than the highest surface of the island. Or set the programming zero point at the highest surface, which can also minimize the risk of tool collision.

1. Q: Why do we need to post-process after the tool path is compiled?

A: Because different machine tools can recognize different address codes and NC program formats, it is necessary to select the correct post-processing format for the machine tool used to ensure that the compiled program can run.

1. Q: What is DNC communication?

A: There are two ways of program transmission: CNC and DNC. CNC means that the program is transmitted to the memory of the machine tool through media (such as floppy disks, tape readers, communication lines, etc.) and stored. When processing, the program is called out from the memory for processing. Since the capacity of the memory is limited by size, DNC can be used for processing when the program is large. Since the machine tool reads the program directly from the control computer during DNC processing (that is, it is sent while doing), it is not limited by the size of the memory capacity. There are three major factors in cutting parameters: cutting depth, spindle speed and feed speed. The overall principle of cutting parameters selection is: less cutting, fast feed (that is, small cutting depth and fast feed speed). According to material classification, tools are generally divided into ordinary hard white steel knives (material is high-speed steel), coated tools (such as titanium plating, etc.), and alloy tools (such as tungsten steel, boron nitride tools, etc.).