With the continuous development of modern manufacturing and processing technology, CNC machining equipment and its supporting CAM system have been widely used and developed. The machining tool trajectory (i.e., tool path) generated by the CAM system is the core of controlling the machining operation of the equipment. It directly affects the accuracy of the workpiece, surface roughness, overall processing time, service life of machine tool tools, and other aspects, and ultimately determines the production efficiency.

Tool path

1. Basic concept of tool path

The tool path refers to the movement trajectory and method of the tool relative to the workpiece during the machining process. It determines how the tool contacts the material, thereby affecting the machining effect. Common tool paths include down milling and reverse milling. In down milling, the direction of rotation of the tool is consistent with the feed direction of the workpiece, and the cutting is relatively stable, which is suitable for fine machining; in reverse milling, the direction of rotation of the tool is opposite to the feed direction of the workpiece, which is suitable for rough machining, because it can effectively remove materials but causes greater wear on the tool. The choice of tool path directly affects the quality of the machined surface, machining efficiency, and tool life.

2. Classification of tool paths

• One-way tool path: In the one-way tool path, the tool processes the workpiece along a fixed direction. After each tool path is completed, the tool returns to the starting position and then proceeds to the next tool path. This method is suitable for processing simple geometric shapes and can ensure high processing accuracy, but the efficiency is relatively low because it needs to return to the starting point after each tool path.

 

• Reciprocating tool path: In the reciprocating tool path, after completing a tool path, the tool directly changes direction for the next tool path without returning to the starting point. This method can improve processing efficiency and is suitable for long-term continuous processing, but it has high requirements for tool stability and workpiece fixation.

 

• Circular tool path: Circular tool path is a method in which the tool processes along a circular or arc trajectory. It is usually used to process circular or annular workpieces, such as gears, shaft parts, etc. This tool path can evenly distribute cutting force, reduce tool wear, and is suitable for processing parts with strong symmetry.

 

• Compound tool path: Compound tool path combines multiple tool paths and is usually used for processing workpieces with complex shapes. During the compound cutting process, the tool will adopt the most appropriate cutting method according to the geometric shapes of different parts of the workpiece to achieve the best processing effect. This method can improve the flexibility and accuracy of processing, but requires more complex programming and control.

 

Factors affecting the cutting method

1. The shape and geometric elements of the workpiece itself

The shape and geometric elements of the workpiece itself include the geometric shape of the processing domain, the size and position of the island, etc. This is an inherent characteristic of the workpiece itself and is an unchangeable factor, but it is the fundamental factor that determines the cutting method.

2. Process route

The process route is the direct process to achieve the processing purpose and is the direct basis for the selection of the cutting method. The process route determines the order of the processing domain, the merging and splitting of the island, the division of rough processing, semi-finishing, and finishing. There are many process routes to achieve the goal, which determines the different choices of the cutting method.

3. Workpiece material

The workpiece material is also one of the factors that determine the cutting method. The workpiece material is the direct processing object and does not directly affect the cutting method, but it will affect the selection of tool material, size, processing method, etc., thereby indirectly affecting the cutting method.

The shape and size of the workpiece blank will affect whether the machining allowance of each part of the workpiece is evenly distributed. At the same time, for workpieces with optional blanks, the size and shape of the blanks will change the clamping method, redistribution of the machining domain, etc., which will affect the machining strategy and lead to different tool paths.

4. Workpiece clamping and fastening method

The clamping and fastening method of the workpiece also indirectly affect the tool path, such as the impact of the new “island” generated by the pressure plate, the impact of the fastening force on the cutting amount and the change of the tool path, and the impact of vibration on the tool path.

5. Tool selection

The selection of tools includes tool material, tool shape, tool length, tool teeth number, etc. These parameters determine the area and frequency of contact between the tool and the workpiece, and thus determine the volume of the cutting material and the machine tool load per unit time. Its wear resistance and tool life determine the length of the cutting time. Among them, the tool size (i.e. diameter) has a direct impact on the tool path. Since the selection of tools with different diameters will affect the size of the residual area, the processing trajectory will change, resulting in different tool paths.

6. Processing domain selection

During the milling process, when a complex plane cavity has multiple bosses and thus forms multiple inner contours, additional tool lifting actions are often generated for line cutting; for circular cutting, the processing trajectory is lengthened. This additional tool lifting action or lengthened processing trajectory will seriously reduce the efficiency of cutting processing. Therefore, how to minimize the number of such situations is a major issue we are concerned about.

The entire cutting area is divided into several sub-areas according to processing needs, and each sub-area is processed separately. The tool lifting occurs between each sub-area. At the same time, these processing sub-areas are merged or divided according to the tool walking method, or even ignored. This different selection of processing domains not only reduces the number of tool lifting but also does not make the processing trajectory relatively longer. At the same time, the most reasonable tool walking method can be used for the new area, which improves the processing efficiency.

Reasonable selection of tool walking method

1. Basic selection principles

Two points should be considered when selecting the tool walking method: one is the length of processing time, and the other is whether the processing allowance is uniform. Generally speaking, the circular cutting method is a tool walking method based on the shape of the workpiece, and the processing allowance is relatively uniform. The machining allowance of the line cutting method is relatively uneven. If you want to leave a relatively uniform allowance after the line cutting process, you usually need to add a circular cutting tool path around the boundary.

If the allowance unevenness requirement is ignored, the length of the line cutting tool path is usually relatively short; if the circular cutting tool path is added considering the unevenness of the allowance, when the boundary of the processing area is long (such as multiple islands), the circular cutting tool path around the boundary has a more obvious impact on the total processing time, and the line cutting tool path is generally longer than the circular cutting tool path. The line cutting tool position is easy to calculate and occupies less memory, but the number of tool lifts is more. When using a circular tool path, it is necessary to offset the ring boundary multiple times and clear the self-intersecting loop.

2. Select according to the shape characteristics

The shape characteristics of the workpiece determine the machining method. According to the different machining objects, the workpiece can be simply divided into plane cavity type and free surface type. Plane cavities are generally processed by line cutting. Since most of these workpieces are formed by milling the blank as a whole, such as boxes, bases and other parts, the machining allowance is large. The line cutting method is conducive to the maximum feed speed of the machine tool and improves the machining efficiency. At the same time, its cutting surface quality is better than that of circular cutting.

Free-form surfaces are generally processed by circular cutting, mainly because the surfaces are mostly castings or formed by regular shapes, the allowance distribution is uneven, and the surface accuracy requirements of the surfaces are high; secondly, compared with the line cutting method, the circular cutting method has good surface processing characteristics and can be closer to the true shape of the surface.

3. Choose according to the processing strategy

The processing of parts is often divided into three processing stages: rough processing, semi-finishing, and finishing. Sometimes there is also a finishing stage. Reasonable division of processing stages is necessary to ensure processing accuracy. In traditional processing methods, the boundaries of each stage can be clearly seen in the process route because the machine tool function is relatively single, but in CNC milling processing methods, this boundary is relatively vague, and there may be some blending (such as the rough processing stage has the content of finishing.

There may also be traces of rough processing in the finishing stage). In order to ensure the processing quality, the division of processing stages in CNC processing is also necessary, but in order to reduce clamping time and simplify tool movement, how to determine the processing content of each stage, the issues considered may be somewhat different from traditional processing technology.

The main goal of rough processing is to pursue the material removal rate per unit time and prepare the geometric contour of the workpiece for semi-finishing. Therefore, the line cutting method or the composite method is often used for layer cutting. The main goal of semi-finishing is to make the contour shape of the workpiece flat and the surface finishing allowance uniform. Therefore, the circular cutting method is often used. The main goal of finishing is to obtain a workpiece with geometric dimensions, shape accuracy and surface quality that meet the requirements. According to the geometric characteristics of the workpiece, the line cutting method should be used for the interior, and the circular cutting method should be used for the edge and joint.

4. Choose programming according to programming strategy

The main principles for determining the cutting method during programming are: it should be able to ensure the machining accuracy and surface roughness requirements of the parts; the machining route should be shortened as much as possible to reduce the idle movement time of the tool; the numerical calculation should be simple and the number of program segments should be small to reduce the programming workload.

Generally speaking, for plane cavities, the line cutting method is used to divide the machining domain to reduce the number of tool lifts; the free-form surface type is approximated by the circular cutting method. The size of the blank shape will affect the programming choice. By increasing the blank shape, the difficult-to-clamp shape processing can be converted into the easy-to-clamp line cutting method cavity processing; or the free-form surface processed by circular cutting can be changed to the line cutting method to remove the large allowance to improve the processing efficiency.

Conclusion

In CNC machining, the correct selection of the cutting method is a key step to ensure machining accuracy, improve efficiency and extend tool life. By combining the shape, material properties and machining requirements of the workpiece, the flexible application of different strategies such as one-way cutting, reciprocating cutting, circular cutting or compound cutting can greatly optimize the machining process. Reasonable cutting methods can not only improve product quality, but also effectively reduce production costs and achieve higher economic benefits. Therefore, it is very important for every CNC machining engineer to deeply understand and master these skills.