[China Aluminum Industry Network] 1) Fast Forward and Precision Movement
In CNC machining programs, both G00 and G01 commands are used to control linear tool movement, but they serve very different purposes. Beginners often confuse them, which can lead to errors in the machining process.
The feed rate for work feed commands like G01, G02, and G03 is controlled by the F command in the program. However, for fast traverse commands such as G00 and parts of the zero return function, the speed is not set in the program itself. Instead, it is predefined in the machine's parameters and stored within the CNC system. This allows for rapid movement without affecting the cutting process.
Work feed commands must ensure that the tool reaches the correct endpoint with precise path and feed speed. This level of accuracy relies on the CNC system's micro-interpolation calculations and real-time dynamic control.
The main purpose of fast forward (G00) is to reduce non-cutting travel time, especially in complex multi-axis operations. However, due to unpredictable motion paths in such systems, G00 should never be used during actual cutting. It’s strictly for positioning.
2) Positioning Accuracy vs. Repeatability
Many machining center manufacturers highlight repeatability in their marketing materials but rarely mention positioning accuracy. This can lead users to mistakenly believe that repeatability is the key factor in part machining quality. In reality, high repeatability doesn’t guarantee high machining accuracy. Positioning accuracy is the primary factor that determines how precisely a part is machined.
Repeatability refers to the machine’s ability to return to the same position multiple times. To measure this, a dial indicator is fixed on the spindle, and after calibrating the zero point, the spindle is moved to various positions and returned. The maximum deviation recorded across multiple cycles represents the repeatability error.
With modern CNC technology, servo motors, rotary encoders, ball screws, or rack and pinion systems typically achieve repeat positioning errors of less than 2 pulse equivalents—far below 0.01mm.
Positioning accuracy, on the other hand, measures how accurately the machine can move a specific distance across its entire stroke. For large aluminum processing centers, which can have strokes up to 7 meters, laser interferometers are commonly used for measurement.
Several factors influence positioning accuracy, including guide rail shape errors, mechanical backlash, screw or rack inaccuracies, machine deformation, and electrical following errors. These errors are measured using a laser interferometer and then input into the CNC system for compensation, significantly improving overall machining precision.
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