Factors Influencing CNC Machining Accuracy and Control Methods
Machining accuracy is crucial in CNC programming and processing. This article explores key factors like programming origin, data processing, machining routes, and system errors, offering effective control measures to enhance precision and efficiency.
Key Factors Influencing CNC Machining Accuracy and Effective Control Measures
1.1 Programming Origin Selection’s Impact on Machining Accuracy and Control
In CNC programming, the first factor to consider is the determination of the programming origin.
Generally speaking, the determination of the programming coordinate system needs to be considered by the relevant operators according to the characteristics of the parts to be processed as well as the drawings, etc. In this case, the determination of the programming origin is directly related to the final machining accuracy of the whole part. In practice, the programming coordinate system should be programmed to determine the benchmark, design benchmarks, process benchmarks for the principle of unity, the only way to minimize the dimensional tolerance conversion error, the actual operation can be operated in accordance with the following:
(1) Programming origin should try to match the drawing of the benchmark. In the design of parts will have the corresponding design benchmarks, processing will have the corresponding process benchmarks, programming origin should try to coincide with these benchmarks.
(2) in the calculation of the relevant data, try to simplify, avoid unnecessary dimensional chain calculation process, to reduce the generation of errors.
(3) Programming coordinate system should be selected as far as possible in some of the higher accuracy of the part surface.
1.2 Data Processing Impact on Accuracy and Control in Programming
In CNC programming, data processing mainly affects contour accuracy, especially unknown programming nodes and dimensional tolerance calculations. Some advanced CNC machines calculate node coordinates automatically. However, most still require manual calculation. Manual calculation accuracy usually deviates 0.01 to 0.03 mm compared to computers. To improve accuracy, retain intermediate data with more than four decimals during manual calculation. If using computers, keep all decimals. Size adjustment programming depends on the pulse equivalent, which is the smallest CNC control unit. For example, with a 0.001 mm pulse equivalent, calculations should keep at least three decimals.
(1) Some advanced CNC equipment can calculate node coordinates automatically. However, most machines require manual calculation. The main challenge is controlling calculation accuracy. Manual and computer calculations usually differ by 0.01–0.03 mm. To improve accuracy, retain intermediate data with more than four decimals in manual calculation. If using a computer, keep all decimals. Programming size adjustment relies on the CNC pulse equivalent, which is the smallest movement unit. For pulse equivalent of 0.001 mm, keep at least three decimals in calculations.
(2) When parts have different or asymmetrical dimensional tolerances, program dimensions manually. If the same tool is used throughout machining, select the middle tolerance value to reserve space for errors and ensure accuracy. When tolerances are symmetrical, the nominal size can be used for programming, simplifying calculations.
1.3 Influence of Machining Route on Accuracy and Control Measures
In CNC programming, the machining route greatly affects accuracy and efficiency. Selecting machining routes should start from key aspects.
(1) The tool’s entry and exit method influences the part’s contour. When feeding or retreating directly on the surface, tool error or speed may cause small dents. Therefore, for high-precision machining, avoid feeding or retreating on the surface. If necessary, use arc cutting in and out. Also, make the arc radius larger than the tool radius.
(2) To ensure surface quality meets accuracy requirements, use smooth milling generally. However, use reverse milling in some cases. First, if the part surface has hardened layers or sand, choose reverse milling. Smooth milling can damage the tool by cutting into these layers, while reverse milling lifts them off, protecting the tool. Second, for some non-metallic, especially fibrous materials, reverse milling is preferred. Smooth milling may not cut fibers effectively, causing burrs. Reverse milling cuts fibers cleanly, resulting in a smooth, flat surface.
CNC Machine Tool System Errors: Impact on Machining Accuracy and Control Measures
2.1 Pitch error and compensation
The so-called error compensation technology is to say that in the process of CNC machine tool processing, the actual position of the movement of a particular axis is recorded, and the recording results are compared with the measurement results of precision instruments. In practice, a certain number of measurement points are selected on the axis, and the error during operation is recorded and input into the control system of the machine tool, through which the control system is able to control the error at different points and times during the movement of the axis. The greater the number of these measuring points, the more obvious the implementation of pitch error compensation technology, which should focus on the following aspects when utilizing this technology:
(1) When the repeated positioning accuracy of the CNC machine tool is not high, this technology is difficult to be implemented;
(2) The technology is based on the premise of the coordinate system of the CNC machine tool;
3) The reference point is an important parameter for determining the coordinate system of the CNC machine tool, so the selection error of the reference point should be zero.
2.2 Reverse Clearance Error in CNC Machine Tools
During CNC machining, the transmission chain has many gaps. These gaps, called reverse clearance, may cause the machine to lag when moving backward. The servo motor runs, but the machine does not move. This causes errors or vibration in the machine tool. Therefore, designers must fully consider backlash. However, no improvement can eliminate all gaps completely.
2.3 Compensation Methods for Reverse Clearance Error
In semi-closed-loop CNC systems, pitch error compensation records backlash at various points. The control system uses this data to automatically correct errors during reverse motion. In full closed-loop systems, backlash compensation occurs through parameter settings. In summary, many factors affect machining accuracy, including process design, tooling, and operator skill. Therefore, one must consider all these factors to effectively improve machining accuracy.