In the world of CNC machining, ensuring the precision of every part is crucial. Whether you’re manufacturing high-tech equipment or everyday items, every detail needs to be accurate. This article will take you through the core aspects of machining precision and its control, revealing the factors that affect machining errors and offering effective ways to improve machining accuracy.
Basic Concepts of Machining Precision
Machining precision mainly includes dimensional accuracy, shape accuracy, and positional accuracy. Dimensional accuracy refers to the degree to which the actual size of a part matches its designed size; shape accuracy refers to the degree to which the actual shape of a part matches its ideal shape; and positional accuracy refers to the accuracy of the relative positions of the surfaces of a part.
Machining error is the deviation between the actual geometric parameters and the ideal geometric parameters of a part. The level of machining precision is indicated by the size of the machining error. Therefore, ensuring and improving machining precision is essentially about limiting and reducing machining errors.
Reasonable Determination of Machining Precision
Generally speaking, higher machining precision results in higher machining costs and relatively lower production efficiency. Therefore, designers should reasonably specify the machining precision of parts according to their usage requirements. Process engineers should then choose appropriate machining methods based on the design requirements and production conditions to ensure that machining errors do not exceed allowable limits while maximizing productivity and minimizing costs.
Relationship Between Dimensional, Shape, and Positional Accuracy
Dimensional accuracy, shape accuracy, and positional accuracy are interrelated. Typically, shape tolerance is restricted within positional tolerance, and positional error is generally limited within dimensional tolerance. When dimensional accuracy requirements are high, corresponding shape and positional accuracy requirements are also high. However, when shape or positional accuracy requirements are high, the corresponding dimensional accuracy may not necessarily need to be high; this depends on the functional requirements of the part.
Errors in the Machining System and Their Control
In CNC machining, the machine tool, cutting tool, fixtures, and workpiece form a complete system known as the machining system. Errors in the machining system are reflected in different degrees and ways as machining errors under various specific conditions. Therefore, errors in the machining system are the root cause of machining errors.
Geometric Errors in the Machining System
Geometric errors in the machining system include machine tool guideway errors, spindle rotational errors, and transmission errors. To reduce these errors, the following measures can be taken:
- Improve the manufacturing accuracy of the machine tool, fixtures, cutting tools, and workpieces.
- Regularly inspect and maintain equipment.
- Use high-precision measuring tools for inspection.
Guideway Errors
Guideway errors mainly include the straightness errors of the guideway in the horizontal plane, vertical plane, the twist of the guideway, and the parallelism or perpendicularity of the guideway to the spindle rotational axis. These errors directly affect the machining accuracy of the workpiece.
Spindle Rotational Errors
Spindle rotational error refers to the deviation of the actual rotational axis of the spindle from the ideal rotational axis. Spindle rotational errors include radial runout, face runout, and angular wobble. These errors reduce the machining accuracy of the workpiece.
Transmission Errors
Transmission errors are caused by manufacturing errors, installation errors, and wear of transmission components such as gears and lead screws in the machine tool’s transmission system. Transmission errors result in positional deviations of the moving parts of the machine tool, affecting the machining accuracy of the workpiece.
Deformation Due to Forces in the Machining System
During machining, the machining system is subjected to cutting forces, clamping forces, and other external forces, causing deformation of the system’s components, which in turn affects the machining accuracy of the workpiece. To reduce machining errors caused by deformation due to forces, the following measures can be taken:
- Increase the stiffness of the machining system to reduce deformation.
- Choose appropriate cutting tool angles and cutting parameters.
- Use proper clamping methods to minimize clamping deformation.
Calculating the Stiffness of the Machining System
The stiffness of the machining system is defined as the ratio of the external force to the amount of deformation in the direction sensitive to machining errors. The stiffness of the machining system can be calculated through experimental methods or mechanics formulas. Increasing the stiffness of the machining system can effectively reduce machining errors caused by deformation due to forces.
Thermal Deformation in the Machining System
During machining, the machining system is affected by cutting heat, frictional heat, and environmental heat, causing thermal deformation of the system’s components, which in turn affects the machining accuracy of the workpiece. To reduce machining errors caused by thermal deformation, the following measures can be taken:
- Use coolant to reduce the temperature of the machining system.
- Arrange the machining sequence reasonably to avoid prolonged continuous machining.
- Design the machining system with a thermally symmetrical structure to balance the temperature field.
Methods to Achieve Dimensional Accuracy
There are several methods to achieve dimensional accuracy, including:
- Trial Cutting Method: The operator performs a trial cut before each machining step, then adjusts the tool position based on the trial cut results until the required size is achieved. This method has low productivity and high requirements for the operator, suitable for single-piece and small-batch production.
- Adjustment Method: By adjusting the relative positions of the machine tool, fixtures, cutting tools, and workpieces, dimensional accuracy is automatically achieved during machining. This method has high productivity but lower accuracy, mainly determined by the accuracy of the machine tool, fixtures, and adjustment errors.
- Dimensional Tool Method: Using fixed-size cutting tools such as drills, boring bars, reamers, and broaches. This method has high productivity but the size cannot be guaranteed once the tool wears out, suitable for mass production.
- Active Measurement Method: Measuring the size during machining, and stopping the machining process once the required size is achieved. This method ensures high accuracy, stable quality, and high productivity.
Statistical Analysis of Machining Errors
To effectively control machining errors, statistical analysis methods can be used to monitor the machining process. Common statistical analysis methods include histogram analysis and point chart analysis.
Histogram Analysis
Histogram analysis statistically analyzes a large amount of machining data to identify the distribution of errors. Here is a simple example:
| Sample No. | Dimensional Error (mm) |
|---|---|
| 1 | -0.02 |
| 2 | 0.01 |
| 3 | -0.03 |
| 4 | 0.00 |
| 5 | 0.02 |
| 6 | -0.01 |
| 7 | 0.03 |
| 8 | -0.02 |
| 9 | 0.01 |
| 10 | 0.00 |
Through histograms, the distribution of errors can be visually observed, helping to determine if the machining process is stable and to adjust process parameters accordingly.
Point Chart Analysis
Point chart analysis continuously monitors the dimensions of workpieces, quickly identifying and correcting machining errors. By plotting the sample mean and range chart, the stability of the machining process can be determined.
Stability Judgment
Using point chart analysis, the stability of the process can be judged. If the points do not exceed the control limits, most points fluctuate around the centerline, and a few points are near the control limits with no obvious patterns of changes, then the process is stable. If not, the process is unstable, requiring appropriate adjustments.
Methods to Improve Machining Accuracy
To improve machining accuracy, the following measures can be taken:
- Adopting Advanced Processes and Equipment: Introducing high-precision machine tools and automated machining equipment can effectively enhance machining accuracy.
- Error Prevention: Designing reasonable machining processes to reduce original errors. For example, using methods to homogenize original errors through grinding or repositioning to reduce error accumulation.
- Error Compensation: Introducing additional error factors to offset or reduce the impact of original errors. For instance, using correction devices to make micro-adjustments to the tool position, compensating for errors caused by thermal deformation.
- Online Measurement and Compensation: Real-time measurement and adjustment during machining ensure machining accuracy. For example, using digital display active measurement devices to monitor machining dimensions in real-time and adjust automatically based on the measurements.
By adopting these methods, machining accuracy can be effectively controlled and improved, ensuring that product quality meets the expected standards. I hope this article helps you better understand machining precision and its control methods, providing useful insights for your machining practices.
