Tool path

1. Basic concept of tool path

The tool path refers to the tool’s movement relative to the workpiece during machining. 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 tool rotation matches the feed direction, providing stable cutting for fine machining; in up milling, the tool rotates opposite to the feed direction, ideal for rough machining but causing more tool wear. 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, the tool returns to the starting position before proceeding to the next, ensuring high accuracy but low efficiency due to the return step.

 

• 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:  A circular tool path follows a circular or arc trajectory, ideal for processing circular or annular parts like gears and shafts. It evenly distributes cutting force, reduces tool wear, and suits symmetrical parts.

 

• Compound tool path: Compound tool path combines multiple tool paths and is usually used for processing workpieces with complex shapes. During compound cutting, the tool selects the best method based on the workpiece geometry for optimal results. 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 workpiece’s shape and geometric elements, such as the processing domain’s geometry and island size and position, are inherent and unchangeable, but they fundamentally determine the cutting method.

2. Process route

The process route directly determines the cutting method, processing order, island merging and splitting, and the division of rough, semi-finishing, and finishing stages. 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 doesn’t directly affect the cutting method but influences tool material, size, and processing method, indirectly impacting the cutting method.

The shape and size of the workpiece blank affect the even distribution of the machining allowance. For workpieces with optional blanks, these factors influence clamping, machining domain redistribution, and 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 contact area and frequency between the tool and workpiece, affecting the cutting volume and machine 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. Tool diameter selection affects the residual area size and alters the tool path.

6. Processing domain selection

During milling, multiple bosses in a complex plane cavity create inner contours, causing extra tool lifts for line cutting and lengthening the trajectory for circular cutting.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 selection of processing domains reduces tool lifting and maintains a shorter processing trajectory while optimizing tool movement for improved efficiency.

Reasonable selection of tool walking method

1. Basic selection principles

Two factors should be considered when selecting a tool walking method: processing time and uniformity of the allowance. The circular cutting method, based on the workpiece shape, generally provides a more uniform allowance. The machining allowance of the line-cutting method is relatively uneven. To leave a uniform allowance after line cutting, add a circular tool path around the boundary.

Ignoring allowance unevenness results in a shorter line-cutting tool path. However, when considering allowance unevenness with a circular cutting path, the circular path around long boundaries (e.g., multiple islands) increases total processing time. Line-cutting paths are typically longer, easier to calculate, and use less memory but require more tool lifts. Circular paths require multiple offset adjustments and clearing of self-intersecting loops.

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. Most workpieces, like boxes and bases, are milled from a blank with a large machining allowance.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 typically processed by circular cutting due to uneven allowance distribution, high surface accuracy requirements, and the ability to better match the true shape compared to line cutting.

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, the boundaries of each stage are clear due to the single function of the machine tool, whereas in CNC milling, these boundaries are blurred, with some stages blending (e.g., roughing may include finishing).

There may also be traces of rough processing in the finishing stage). To ensure processing quality, dividing stages in CNC processing is essential. However, to reduce clamping time and simplify tool movement, determining the content of each stage differs from traditional methods.

The goal of rough processing is to maximize the material removal rate and prepare 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 flatten the workpiece contour and ensure uniform surface allowance. Therefore, the circular cutting method is often used. The goal of finishing is to achieve the required geometric dimensions, shape accuracy, and surface quality. Line cutting should be used for the interior, and circular cutting for the edges and joints.

4. Choose programming according to programming strategy

The cutting method should ensure machining accuracy and surface roughness, minimize tool idle time, and simplify calculations and programming.

For plane cavities, line cutting divides the machining domain to reduce tool lifts, while free-form surfaces are approximated by circular cutting.The size of the blank shape will affect the programming choice. Increasing the blank shape can convert difficult-to-clamp processing into easier line cutting, or replace circular cutting of free-form surfaces with line cutting to remove large allowances and improve efficiency.

Conclusion

In CNC machining, selecting the right cutting method is crucial for accuracy, efficiency, and tool life. By considering the workpiece shape, material, and requirements, using strategies like one-way, reciprocating, circular, or compound cutting can optimize the 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.