Volumetric Error Compensation – The Science


A new process for machine tool calibration

Since the beginning of 2010, API has been offering a new, breakthrough technology for the volumetric compensation of machine tools, known as VEC. This transformational process has been developed together with BOEING and the machine tool manufacturer MAG. Increases in the volumetric accuracy of up to one order of magnitude, as well as savings of several days in terms of the effort for calibration of machine tools have been achieved. Among others, this technology has been acknowledged by the 2010 R&D 100 Award (awarded by the R&D Magazine) as well as by the Defense Manufacturing Excellence Award (awarded by the National Centre for Advanced Technologies (NCAT).

A combination of new hardware and software

The prerequisite for the realization of the new method, which treads completely new paths in both the approach and the systems used, was the use of equipment that has previously never been used in the field of the calibration of machine tools. This applies above all to the Active Target from API, which is an active reflector; this active sensor, which looks rather insignificant (see picture above), but is equipped with amazing properties, makes it possible, for the first time ever, to continually track a moving target in free space using a conventional laser tracker. Micro-motors built into this sensor permanently align it to the tracking laser beam of the tool holder, so that the tool holder with the mounted Active Target can run through any trajectory without breaking the connection to the laser tracker. This unbroken possibility for measuring the position in space makes a completely new compensation process possible, which, unlike conventional processes, no longer has to be aligned to the axes of a machine tool – with dramatic consequences for the running of the procedure itself, and/or the required time.

The task

A basic fact of life for all machine tools (even for high-end machines, such as those that are used, for example, in the aerospace industry) is that, for various reasons, parts are not always machined according to the specifications. As a rule, this is always caused by the same components – due to wear, ball screws and racks are some of the main causes of errors in machine-related tolerances, but there are also geometric errors or problems with the foundations that could lead to deflections, thereby contributing to highly undesirable inaccuracies.

The solution to this problem has been known for many years – the volumetric compensation of machine tools. An exact knowledge of the precision of the tool path results from the complete recording of all errors with regard to the working area of a machine tool. The control of modern machine tools permits the real-time compensation of the positioning errors of this machine tool while it is moving, through the use of compensation tables.

Experience has shown that it is expensive to draw up the complete fault record for a machine tool; not only the procedure itself, but also the loss of production during the shutdown of the machine results in additional costs. Furthermore, not only expensive equipment, but also specialists with a great deal of experience are required, with the result that these tasks are frequently outsourced to service providers who specialize in the calibration of machine tools.

The real main reason why a comprehensive fault record for a volumetric compensation of machine tools is rarely carried out and, above all, is not carried out regularly, is the fact that this process consumes a great deal of time: It must be borne in mind that the complete compensation for the 360-degree tracking of the tool holder in space requires measurement procedures that can only be carried out with a great deal of outlay using conventional methods – which makes the measurement process extremely slow and complicated. In the case of very large machines with a large work volume, these procedures are extremely expensive and are practically impossible to carry out.

The Active Target – a motorised SMR

This is where the Active Target from API comes into the picture, which makes the tracking of an object in space possible without any manual intervention, regardless of its position – only a free field of vision is required for the laser measurement beam. Thanks to battery operation without any additional cabling, this creates a continuous link between the tool holder and the laser tracker – the tracker can continuously measure every position
on the support of the permanently mounted active targets within the working volume of the machine tool – without any manual intervention of any kind. It is exactly this functionality that makes it unique on the market, and makes dramatic time savings possible in the implementation of the compensation measurements.