3D measuring probes prove to be extremely suitable for a quick check of the kinematics of a five-axis milling machine. The principle is based on measuring an object, by means of an accurate measuring probe, according to pre-programmed measuring cycles. The measurement results are presented graphically and allow the machine accuracy to be monitored over time.

Kinematic errors in machine tools can roughly be identified in two ways: directly and indirectly. An overview with different measurement principles has been published before.

Direct measuring principle

The basic ingredients for this direct measuring principle are an integrated 3D measuring probe (non-branded) and an accurate measuring ball with a known diameter on a (standard) tripod, mounted on the machine table. Depending on the size of the five-axis machine tool, different sizes of ball stands are available as standard.

Figure 1 3D measuring probe and measuring ball on tripod

For the machine kinematics of the five-axis precision milling machine at Sirris, the entire measurement procedure is defined in 7 pre-programmed, yet standard measurement cycles. However, depending on the machine construction, the number of measurement cycles required may vary. In total, NC Checker therefore counts with about 30 predefined measuring cycles. Before carrying out these measuring cycles, the position of the ball centre (X Y Z) should be determined as precisely as possible and stored in a known zero point or preset value of the machine control.

Calibration

Since the 3D probe is responsible for the quality of the measurements, care should be taken to ensure that the stylus motion is minimal at first (mechanically adjustable with control screws) and that the probe is correctly calibrated, using the standard calibration cycle. As long as no collision occurs or a new stylus is fitted, the last calibration performed can remain in effect. A next step in the measurement procedure involves checking the performance of the measuring probe, also by means of standard measurement cycles. This includes the measurement accuracy, the repeat accuracy (repeat the same cycle a number of times) and the measurement accuracy after a number of tool changes. Finally, as long as there are no striking anomalies here, the kinematics of the rotary axes can be checked, which is what it is actually about. The rotation axis in question will rotate to a number of pre-programmed angles (e.g. every 15°) and the position of the measuring ball will be measured at each rotation angle. The set of measurements then gives an indication of the accuracy of the kinematics of the rotary axes.

Figure 2 - Measurement of the A-axis

All results are presented graphically and show the state of the machine kinematics and the probe at a glance, while giving an indication if further actions are needed. The closer to the nominal circle, the more accurate the result. By clicking on a specific measurement, the underlying details of the measurement in question are shown. All measurements take a maximum of half an hour. By not performing certain measurements each time, the measuring time can be reduced.