The design of the CNC machining process route must be thoroughly considered. It is essential to focus on the proper division of operations, the rational sequencing of steps, and the integration of CNC machining with conventional processes.
**1. Process Division**
Compared to traditional machine tools, CNC machining typically involves more concentrated processes. Based on the characteristics of CNC machines, the division of machining operations can be done in several ways:
1) **Division by Clamping Positioning**
This method is suitable for small batch production, where parts are divided into segments based on clamping positions. For example, when machining a shape, the inner cavity is used for clamping; when machining the inner cavity, the outer surface is clamped.
2) **Division by Tool Used**
To reduce tool changes and idle time, the principle of tool concentration is applied. A single tool is used in one setup to complete all possible machining tasks before switching to the next tool. This method is commonly used on specialized CNC machines or machining centers.
3) **Division by Rough and Finish Machining**
For parts prone to deformation, it is advisable to separate rough and finish operations. After the initial rough machining, the workpiece is repositioned for final finishing to ensure accuracy and minimize distortion.
When dividing processes, flexibility is key, taking into account the workpiece’s structure, installation method, processing technology, CNC machine capabilities, and factory organization. The goal is to achieve a reasonable and efficient workflow.
**2. Arrangement of Processing Sequence**
The sequence of operations should be planned based on the workpiece's structure and blank condition, ensuring that the workpiece remains rigid and minimizes deformation. Key principles include:
1) The previous process should not interfere with the positioning or clamping of the next step.
2) Machining the inner cavity first before working on the outer contour.
3) Minimize repeated positioning and tool changes.
4) In multi-step setups, processes that may damage the workpiece’s rigidity should be performed first.
**3. Integration of CNC and Conventional Processes**
Since CNC machining is often integrated throughout the entire production cycle, each stage must define clear state requirements, such as machining allowance, precision, tolerance of positioning surfaces and holes, and technical specifications from other processes. Heat treatment stages must also be carefully considered before and after machining.
**4. Design of CNC Machining Process and Route**
The main task in designing a CNC machining process is to determine the specific machining content, cutting parameters, equipment, positioning methods, and tool path to prepare the program. Among these, defining the tool path is particularly important. The tool path refers to the movement of the cutting tool relative to the workpiece during machining. It includes the machining content and reflects the sequence of operations, making it a critical basis for programming.
**Principles for Determining the Tool Path:**
1) Ensure the accuracy and surface finish of the machined part.
2) Reduce idle travel to improve efficiency.
3) Simplify calculations and minimize program sections to ease programming.
4) Consider the workpiece’s shape, stiffness, and machine system stiffness when determining the number of passes.
5) Plan the tool approach and direction to avoid errors caused by machine backlash.
6) Choose between climb milling and conventional milling based on machine capabilities. Most CNC machines use ball screws, which have minimal clearance, so climb milling is usually preferred.
**CNC Machining Routes:**
1) **CNC Lathe Turning Route**
As shown in Figure 1, the turning path for a CNC lathe includes points like the tool change point (A), entry point (B), cutting path (C-0p), exit point (0p), and retraction point (D).
Figure 2 illustrates the turning path for a cylindrical part, with points A (tool change), B (entry), CDE (cutting path), E (exit), and F (retraction).
2) **CNC Milling Machine Route**
When milling the outer contour, the tool should enter and exit along the tangent of the curve to avoid surface defects. Similarly, when machining inner contours, the tool should move tangentially to ensure smooth transitions. Figures 3 and 4 illustrate these paths.
3) **Hole Machining Route**
Properly arranging the hole machining path improves positional accuracy. As shown in Figure 5, when machining multiple holes, using a unidirectional approach helps avoid positioning errors caused by machine backlash, ensuring consistent accuracy between holes.
**5. Workpiece Installation and Fixture Selection**
1) **Workpiece Installation**
- Align the design reference, process standard, installation datum, and workpiece coordinate system.
- Minimize the number of setups to machine all required surfaces in one go.
- Use special fixtures to reduce setup and adjustment time.
2) **Fixture Selection**
- For small batch production, use modular, adjustable, or universal fixtures.
- For mass production, choose special fixtures for easy loading and unloading.
- Ensure the fixture does not interfere with the tool’s movement.
- Make sure the loading and unloading of parts is convenient and reliable, especially for large-scale production, where pneumatic, hydraulic, or multi-station fixtures can be used.
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