Teach you to understand the engraving machine

The ball bar instrument is a specialized tool used for the rapid assessment of two-axis linkage accuracy in CNC machining centers and for diagnosing machine tool performance issues. Before an engraving machine is delivered, ball bar testing is a mandatory procedure to ensure its precision. The device consists of a high-precision displacement sensor housed within a retractable fiber rod. This sensor includes two coils and a movable inner rod, operating on a principle similar to that of an LVDT (Linear Variable Differential Transformer) sensor. As the length of the bar changes, the inner rod moves within the coil, causing a change in inductance. This signal is then converted into a displacement measurement with a resolution of 0.1 micrometers and transmitted to a PC via an interface. Its accuracy has been verified using a laser interferometer, achieving a precision of ±0.5 micrometers at 20°C. Compared to traditional test cutting methods, which require complex setups, long testing cycles, and high-precision test pieces, the ball bar method offers a faster and more efficient way to evaluate machine tool accuracy. However, it's important to note that test cutting still remains a preferred choice in certain scenarios, especially when evaluating the dynamic behavior of virtual axis CNC machines. Unlike conventional systems, virtual axis machines involve nonlinear relationships between tool pose errors and rod length variations, making it necessary to develop specific workpiece testing strategies. In addition, while the laser-based inspection method is widely used for assessing traditional CNC machines, the ball bar technique is increasingly recommended as a more practical and cost-effective alternative. However, due to limitations in error diagnosis software, the ball bar cannot directly assess the performance of virtual axis machines. Therefore, in some cases, the test cutting method remains the most reliable option. For virtual axis CNC machines, the attitude accuracy during movement plays a critical role. When the machine operates in a fixed attitude, only three position variables are considered, but when the attitude changes, the moving platform continuously adjusts, significantly affecting the working space. Larger attitude angle changes can lead to greater deviations, particularly in smaller working areas. This highlights the need for appropriately sized test specimens that match the machine’s operational range. Standards such as JB/T 17421.2-2000 provide general guidelines for inspecting positioning and repeatability accuracy in CNC axes, helping to bridge gaps between manufacturers and users. Meanwhile, the ball bar instrument allows for quick, economical, and accurate evaluations of dynamic accuracy, servo control, and backlash in various types of CNC machines, including vertical and horizontal machining centers and lathes. It is easy to use, portable, and capable of delivering results in just 10–15 minutes, making it an essential tool for modern machine tool diagnostics.

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