Measurements with coordinate-measuring machine
- Measurements of linear and axial quantities
- Measurements of shape and position deviations
- Measurements in the wide measuring range: 900x1200x700mm
- Measurement accuracy MPE_P=1.7um
Contemporary coordinate measuring machines can be equipped with different types of measuring heads – contact and non-contact, pulse or scanning. It is not easy to choose, but we don’t have use one solution for the rest of our lives. In case of the majority of measurement tasks the contact pulse measurements do not ensure sufficient repeatability or speed of collecting a large number of measuring points.
In this case, a much better solution is using contact scanning heads. However, there are some measurement tasks that cannot be performed using contact method and in such cases it is recommended to use optical measuring heads. The example of applying optical method is measuring small parts or components made of soft materials that prevent measurement using contact method. MASS system, used in the ACCURA coordinate measuring machine enables using central, active contact scanning heads, tilting rotary joints with passive contact scanning heads as well as the optical scanning heads.
Multi Application System Sensor (MASS) allows using active scanning central head and heads that can be mounted in the tilting rotary joints, operating in the Multi Probe Sensor (MPS) system. It is possible thanks to leading the power supply, control and information cables to the location of the head and the appropriate design of the head mounting bracket. Replacement of the heads cooperating with the tilting rotary joint (MPS) may be performed automatically or manually, whereas the joint can be replaced with a central head manually by the user. MASS not only facilitates applying the optimal solution at any time, even for the most advanced measurement tasks, but also allows saving costs connected with retooling the coordinate measuring machine.
Contact passive scanning heads
For the measurement tasks that require applying a large number of different angular positions of stylus (for example turbines with vanes, parts pressed from metal sheets), the best solution seems to be using tilting rotary joint. For the ACCURA machine we can use the Carl Zeiss RDS joint and passive scanning head Zeiss VAST XXT. It is possible thanks to small size and low mass of the VAST head. This way, we achieve considerable functionality and the simplicity of preparing the machine to perform the measurement task. Due to the possibility of setting the contact pressure from the software, a VAST XXT head is perfect for measuring small parts or components made of soft materials using low-rigidity, small styluses. In order to retain the specified precision of the machine and optimal speed for scanning with a VAST XXT passive head in the ACCURA machine, a modern C99N control system has been used. In terms of the measuring accuracy using the points of single contact, the passive VAST XXT head is equally effective as the active VAST XT head. For scanning purposes, however, the active head is much faster.
Contact active scanning heads
Using passive heads with a joint is not the solution optimal for all measurement tasks. One example may be measuring long, horizontal, precise holes, high-precision gear wheels or scanning in the In-Line coordinate measuring machines used in production halls. For this application, central active scanning heads will much better perform the measurement tasks both in terms of accuracy and speed. Contrary to the passive head, measuring force is generated by the electric drive (and not by the mechanical spring) and is constant within the entire measurement range of the head +/-2 mm. For this reason, controlling the coordinate measuring machines while scanning is much easier for the active scanning heads. Constant force measurement in three directions enables the right, active adjustment of the measuring force to assign constant value and exert it always in the direction perpendicular to the tangential plane on a given spot of the 3D surface. The above-mentioned properties of the active scanning heads enable us to measure with greater accuracy than using passive heads with the same scanning speed or much faster retaining the same measuring accuracy. It is particularly important in situations where both scanning time and accuracy matter. It is also crucial to use active measuring heads to measure concentricity deviations, when the measurements should be performed from two sides of the part.
Additionally, active heads have the possibility of measuring in the self-centering mode, which can be useful, for instance, for setting coordinate systems, angular positioning of the gear wheels or measuring the tooth pitch and run-out of gear wheels. The advantage of central heads is also optimal use of the machine measurement range, which enables some of the measurement tasks to be performed on the machine one number smaller than in case of the passive head with a tilting rotary joint.
VAST navigator – the third generation of scanning
The ACCURA machine can also be equipped with the latest active VAST gold or VAST XT gold scanning heads used in the VAST navigator technology. Apart from the optimized construction of the active scanning head and standard correction of static errors of the coordinate measuring machine (CAA – Computer Aided Accuracy) a new, special method of correcting the machine dynamic deformation and dynamic deflection of the stylus tip has been introduced (DCAA – Dynamic Computer Aided Accuracy). Thanks to the dynamic stylus calibration, the machine’s control system receives information about the behavior of the stylus during scanning with different speed and forces exerted from different directions. During the measurement process, the control system predicts the size of dynamic deflections of the coordinate measuring machine and stylus and adjusts the trajectory so as to minimalize the deflection retaining constant measuring force.
Owing to this, it is possible to take scanning measurements with speeds even several times bigger than in case of a standard, active scanning head and a few dozen times faster than in case of a passive head. One advantage of the dynamic correction is the scanning measurement with the same accuracy on different spots of the measuring surface of the machine without the necessity (as it happens in case of the passive head) of the troublesome calibration using the model with dimensions close to the measured element, placed in the spot of taking measurement. The CALYPSO navigator programming option assists the machine operator choosing the best scanning speed for the measurement task so that it is always possible to scan with high speed retaining the measuring accuracy. Additional benefit of using navigator option is the tangential (no stops) approach to the measured element, measuring along a spiral and tangential retracting after taking the measurement. VAST navigator also required using modern control system C99N, which must follow with the high speed machine movements, rapidly analyze the situation and enable collecting possibly large amount of points in real time.
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