How the team measured up to eradicating errors

Machine tools suffer from three basic types of error, explains Derek Ford, director of the Precision Engineering Centre at Huddersfield University. These are :

geometric or rigid body errors, caused by the machine’s structure not being perfectly rigid, and by imperfections and misalignments;

non-rigid errors, due to factors such as the effect of cutting forces; thermal errors these do not apply in the case of the BAe machines as they are to be used in a climate-controlled area.

Geometric errors are the most straightforward to correct: they depend on characteristics built into the machine, so are repeatable rather than random.

Even so, a three-axis machine has 21 sources of error. Each axis will have linear errors in three dimensions, plus angular errors of pitch, yaw, and roll caused by flexure about three axes. These are repeated for each of the three machine axes, giving 18; additional squareness errors between the three axes give a total of 21 error sources.

Angular errors can be highly significant: four seconds of arc over a height of a metre equals a 20 m linear error.

Some machines have a limited compensation package which may correct linear errors in forward travel for each axis. Ford says such systems are usually based on interpolations between a limited number of points along the axis.

The Huddersfield system measures and corrects each error continuously along each axis. A universal algorithm run on a PC corrects for geometric errors for any possible configuration of three-axis machine.

The machine is put through a specifically devised calibration procedure to isolate each error so it can be measured individually.

This process can be repeated at intervals during the machine’s life, allowing errors due to the foundations settling or wear and tear to be compensated for.

By decoding signals from the positional sensors, the algorithm can work out where the machining head is and, from the calibrated errors, work out the required correction at that point. This correction is fed back to the position transducers.

This set-up means the correction system is invisible to the machine’s operator and independent of the controller used for its programming hence it does not have to be interfaced to the controller.

Huddersfield worked with BAe and DS Technology to expand the algorithm to compensate for errors in five axes.

In theory, the package could be fitted to existing machines, but a limiting factor would be the machine’s inherent repeatability. ‘If it was more than five years old it might need mechanical work to bring it in line on repeatability,’ says Ian Percival, DS Technology’s UK managing director.

Ford’s team is now working on ways of compensating for non-rigid and thermal errors.