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Application of 3D Spatial Error Compensation Technology for CNC Machine Tools
The application of three-dimensional space error compensation technology in CNC machine tools has a long history, rooted in the precision compensation techniques originally used in coordinate measuring machines (CMMs). These techniques were essential for ensuring the high accuracy of CMMs as reliable measuring instruments. However, achieving the required mechanical and electrical accuracy in manufacturing is challenging. As a result, CMMs are typically calibrated to meet the demands of high-precision measurements. With the continuous advancement of CNC technology, the demand for higher machine tool accuracy has grown significantly.
Traditionally, machine tool precision was addressed through mechanical design and hardware manufacturing, which became standard industry practices. Renishaw, a leader in measurement technology, pioneered the use of "space error compensation technology" in its CMM controllers. After more than a decade of successful implementation, this technology has now been introduced into CNC systems such as Fanuc and Siemens. This development has enabled the improvement of spatial accuracy in machining centers, CNC boring and milling machines, and gantry systems.
Based on Renishaw’s advanced XL-80 laser interferometer and QC-20 ballbar, two sets of spatial error correction software—RVC-Fanuc and RVC-Siemens—have been developed and introduced to the market. These tools allow for 3D space compensation, improving the performance of CNC machines equipped with Fanuc or Siemens control systems. User feedback indicates that RVC software is flexible, easy to use, and highly effective in addressing common accuracy issues.
The core principle of this compensation involves identifying and correcting the 21 geometric errors present in a three-axis machine tool: 9 linear, 9 angular, and 3 perpendicularity errors. Accurate detection of these errors, followed by the development of compatible software, enables real-time compensation. In practice, multiple errors often combine, making it impossible to correct them all at once. Therefore, a systematic approach is necessary to ensure consistent and accurate results across the entire workspace.
Modern CNC systems, such as Siemens 840Dsl (VCS) and Fanuc 31i (3D error compensation), support spatial precision compensation as an advanced feature. These systems can generate full workspace error parameters, allowing for real-time correction of spatial positioning errors during operation. This method has proven to be one of the most effective ways to enhance machine tool accuracy.
Globally, research into spatial accuracy measurement and error compensation is ongoing. Techniques such as laser tracking, laser interferometry, and ballbar testing are widely used to detect and isolate error sources. The XL-80 laser interferometer, for instance, is popular due to its accuracy and traceability. It also offers open software interfaces, enabling users to develop custom solutions.
For implementing space error compensation, key equipment includes the XL-80 laser interferometer for measuring linear displacement, straightness, and angles, and the QC20-W ballbar for evaluating verticality and diagnosing mechanical or electrical errors. Additional tools like the RX10 turntable and electronic levels further assist in measuring corner accuracy and roll parameters.
The RVC software provides three main functions: linear error compensation, 3D spatial error compensation, and triaxial vertical error compensation. When using Fanuc or Siemens CNC systems, the appropriate ELF files must be loaded, and the compensation function activated via GUD variables. The system then calculates and applies the corrections in real time.
Before performing spatial error compensation, it is crucial to evaluate the machine's overall precision using a ballbar. If significant electrical errors exist, such as backlash or servo mismatch, compensation may not yield significant improvements. Ensuring the machine is adjusted to secondary accuracy levels before compensation is essential for optimal results.
In practical applications, the RVC software has demonstrated impressive results. For example, on a Fanuc Robodrill machine, the roundness error decreased from 9.1 μm to 5.7 μm after compensation. Similarly, on a Siemens 840D-based Flymill 1000 gantry machine, X-axis positioning accuracy improved from 68 μm to 2 μm, showcasing the effectiveness of the technology.
Overall, the integration of space error compensation into CNC systems represents a major step forward in enhancing machine tool accuracy. By leveraging advanced measurement tools and sophisticated software, manufacturers can achieve greater precision and consistency in their production processes.