Machines that test themselves replace hours of careful calibration

Traditionally, machine tools were carefully calibrated when they were built.

Recalibration was only carried out after errors were observed in the parts they produced. But this approach can’t achieve the right-first-time, zero-defect aspirations of modern manufacturers. Many manufacturers now use ballbars and laser interferometers to perform regular checks and recalibrations of machines. 

A ballbar test can be carried out in around 20 minutes to detect any issues with machine tool accuracy. The ballbar measures the distance between two steel spheres thousands of times a second. Magnetic cups attached to the machine tool support the steel spheres and allow them to rotate. One end is attached to the machine bed and the other to the spindle. The machine is moved through a series of circular paths and any deviations are detected. 

Such tests indicate whether the machine is able to achieve the required accuracy and indicate the source of any issues. Certain errors, such as backlash, will need to be compensated for, while many others will require further measurement using a laser.

These methods are a big step forward but still require significant time and skilled human intervention. This means that checks are typically carried out at monthly or weekly intervals, leaving a significant risk of non-conforming parts being produced. 

Now some cutting-edge manufacturers want to go further, gaining complete confidence that components are within tolerance, without the need for post-production measurements. Self-calibrating production systems are emerging that integrate laboratory-grade metrology into the machines themselves. These systems are able to perform full calibrations in minutes with no human intervention.

Frequency scanning interferometry (FSI) is a state-of-the-art laser measurement technique. It was developed by researchers at the University of Oxford to measure the tiny movements of detectors in the Large Hadron Collider at CERN, the European particle physics laboratory. 

Industrial measurement systems achieve traceability to international standards through a chain of calibrations linking them back to national metrology institutes such as the UK’s National Physical Laboratory (NPL). It is no longer necessary for labs such as NPL to calibrate their national standards against the international meter in Paris since the meter is now defined in terms of the speed of light. 

FSI also has this direct traceability to the meter definition and therefore requires no external calibration. It achieves this by checking its laser frequency against spectral absorption lines of an internal gas cell. Every time it takes a measurement it scans the cell, meaning that all measurements are fully traceable.

FSI uses a central laser source and control electronics to supply many laser measurement devices via fibre-optic cables. When installed throughout a factory, this allows large numbers of highly accurate and traceable measurements to become affordable. 

Etalon Multiline is a commercially available FSI system that can be fitted to machine tools. It has an uncertainty of 0.5 parts per million over distances of between 20mm and 30m, validated by NPL.

Each machine is fitted with at least four laser measurement heads. Each of these sends out several measurement beams in different directions. When a machine needs to check or recalibrate, it simply loads the target, using its automated tool changer, and then moves through a programmed path, following one beam after the other.  

The Etalon software is directly installed on the Siemens 840D controller. It automatically switches the measurement beams and combines the length measurements to calculate errors of the end effector at multiple positions along each measurement line. These are then compared with the nominal NC control data to give deviations at all of these positions. 

Finally, the deviations are substituted into kinematic equations for the machine tool so that all of the machine’s error parameters can be determined and updated on the machine tool’s controller. The error assessment is completed in 30 minutes without any human intervention.

Self-calibrating machines provide a route to higher quality. They should also cut costs by eliminating scrap, reducing downtime and removing the need for final product verification.