How to improve machining precision and efficiency of workpieces?

How to improve machining precision and efficiency of workpieces?

To improve the final machining precision and efficiency of workpieces, apart from the factors related to the CNC machine tools themselves, several aspects should be considered comprehensively: setting a reasonable machining route, selecting and correctly installing cutting tools, selecting appropriate cutting parameters, programming skills, and rapid control of dimensional accuracy.

Programming Skills:

CNC programming is the fundamental task in CNC machining, and the quality of the workpiece machining program directly affects the machining precision and efficiency of the machine tool. Several aspects can be considered to enhance programming skills, such as clever use of subprograms, reducing cumulative errors in the CNC system, and flexible application of main programs and subprograms.

1.Flexible Use of Main Programs and Subprograms

In complex mold machining, it is common to machine multiple workpieces using the same mold. If there are several identical shapes on the mold, the relationship between main programs and subprograms should be flexibly utilized. Subprograms can be repeatedly called in the main program until the machining is completed. This approach not only ensures consistency in dimensional accuracy but also improves machining efficiency.

2.Reducing Cumulative Errors in the CNC System

Generally, incremental programming is used for workpiece programming, which is based on the previous point for machining. When multiple program segments are executed continuously, it inevitably results in certain cumulative errors. Therefore, in program development, absolute programming should be used as much as possible, with each program segment based on the workpiece origin. This helps to reduce cumulative errors in the CNC system and ensures machining precision.

Machining precision is primarily used to measure the degree of production and evaluate the geometric parameters of machining surfaces. However, actual parameters obtained from any machining method are never absolutely accurate. From the perspective of the functionality of the part, as long as the machining errors are within the tolerance range specified on the part drawing, the machining precision is considered to be guaranteed.

Mechanical machining precision refers to the degree to which the actual geometric parameters (dimensions, shapes, and positions) of the part after machining correspond to the ideal geometric parameters. The difference between them is called machining error, and the magnitude of the machining error reflects the level of machining precision. The larger the error, the lower the machining precision, while the smaller the error, the higher the machining precision.

Below are some methods to improve the machining precision of workpieces:

1.Adjusting the Process System

• Trial Cutting Method: Adjust the tool feed based on trial cutting, measurement of dimensions, and repeat the process until the desired size is achieved. This method has low production efficiency and is mainly used for small-batch production of individual parts.
• Adjustment Method: Pre-adjust the relative positions of the machine tool, fixture, workpiece, and cutting tool to obtain the desired dimensions. This method has high productivity and is mainly used for mass production.

2. Reducing Machine Tool Errors

• Improve the rotational precision of bearings:
• Use high-precision rolling bearings.
• Adopt high-precision hydrodynamic pressure bearings.
• Utilize high-precision hydrostatic bearings.
• Improve the accuracy of components that mate with bearings:
• Improve the machining precision of the support holes and spindle journals.
• Improve the machining precision of the mating surfaces with bearings.
• Measure and adjust the radial runout range of corresponding components to compensate for errors or cancel each other out.
• Proper preloading of rolling bearings:
• Eliminate clearances.
• Increase bearing stiffness.
• Equalize rolling element errors.
• Prevent the reflection of spindle rotation accuracy on the workpiece.

3.Reducing Transmission Chain Errors

• Fewer transmission components and shorter transmission chains lead to higher transmission accuracy.
• The principle of using speed reduction transmission is important for ensuring transmission accuracy. The closer the transmission pair is to the end, the smaller the transmission ratio should be.
• The accuracy of the end components should be higher than that of other transmission components.

4. Minimizing Tool Wear

• Sharpen the tool before it reaches the stage of rapid wear.
• Use specialized cutting fluids for sufficient lubrication.
• Select tool materials that meet the process requirements.

5.Minimizing Deformation in the Process System

• Improve the system's stiffness, especially in weak areas of the process system.
• Reduce loads and load variations.

6.Minimizing Thermal Deformation in the Process System

• Reduce heat generation and isolate heat sources.
• Balance the temperature field.
• Adopt a reasonable structure and assembly reference for machine tool components.
• Accelerate the attainment of heat transfer balance.
• Control the environmental temperature.

7.Minimizing Residual Stress

• Implement heat treatment processes to eliminate internal stresses.
• Arrange the process sequence rationally.

The above methods describe how to reduce errors in workpiece machining. Proper process planning can effectively improve workpiece precision.

Setting a Reasonable Machining Route

Setting a reasonable machining route and sequence is an important foundation for optimizing the programming of workpiece machining. Considerations can be made regarding the machining trajectory and feed-in methods.

When performing CNC milling of workpieces, the appropriate feed-in method should be selected based on the process requirements to ensure cutting precision and efficiency. For milling the outer contour of a planar workpiece, the tool's entry and exit paths should be arranged properly, preferably along the extension lines of the contour curve to avoid tool marks at the junctions. Additionally, climb milling or conventional milling should be selected based on the workpiece conditions.

Selection and Correct Installation of Cutting Tools

Whether it is CNC machining or conventional machining, the selection and installation of cutting tools are the most critical factors affecting workpiece machining precision and surface quality since the tools directly interact with the workpiece. This is especially true when machining workpieces on CNC machining centers, where the tools are stored in the tool magazine and cannot be changed randomly once the machining starts. Therefore, the general principles for selecting tools are convenience in installation and adjustment, good rigidity, high durability, and accuracy.

Reasonable Selection of Cutting Parameters

Determining the cutting parameters is an essential part of CNC machining processes. The size of the cutting parameters is crucial for the primary and feed movements of the machine tool and has a significant influence on workpiece machining precision, efficiency, and tool wear. The selection of cutting parameters includes cutting speed, depth of cut, and feed rate. The basic principle for selection is to choose a larger cutting depth for roughing to reduce the number of passes and increase workpiece productivity, while for finishing, a smaller cutting depth is typically chosen to achieve higher surface quality.