Lengh

Background

Three-dimensional measuring machines are widely used in industry for controlling parts, but also - in view of their increasing accuracy - for calibration purposes.

The complexity of these machnies poses the problem of performance assessment and traceability as they are used in increasingly varied and demanding fields of application in terms of performance and measurement uncertainty.

A measuring machine comprises three measurement axes (sometimes four with a rotating plate). In general, the three measurement axes can be described by 18 functions and three parameters representing the machine's errors:

• straightness (x 2) and freedom from bias (x 1) on each axis
• rotation (x 3) on each axis
• perpendicularity (x 3) between each axis

Most measuring machines have a software application which corrects these elementary errors with a reasonable degree of precision. However, depending on the error assessment techniques and correction algorithms used by manufacturers, residual errors remain. They must be assessed, either for additional correction or for conformity and/or performance assessment.

The objective of the research project carried out at LNE is to provide all parties working with 3-D measuring machines - users, maintenance companies or manufacturers - with a simple and efficient method of:

• checking that a machine complies with specifications,
• adjusting a machine to bring it into compliance with specifications,
• assessing measurement uncertainties in specific configurations.

The method

The principle consists in measuring a standard of length in the form of a holed bar. The bar is calibrated in advance by determining the distances between the holes along the neutral plane of the bar.

This standard is then measured in different positions in the machine's measurement space. From these measurements the six error functions and relative position of each of the three axes can be deduced. More measurements are made than the minimum required for this determination, so the machine's measurement consistency and repeatability can be checked.

This method offers the following advantages over the ball plate system:

• the standard is less costly
• the machine is easier to calibrate and therefore offers better traceability (a ball plate is calibrated on a three-dimensional machine)
• it can handle a capacity of 3 or 4 m without transfer (a large ball plate is 1 sq m in size)
• by duplicating measurements it is possible to check the consistency of results and detect any operational anomalies such as incorrect bar position, abnormal behaviour of the machine or effects of temperature.

The information obtained can also be used to assess the machine's metrological behaviour (repeatability, hysteresis).

Project stages and progress

A software application has been produced for data capture and error computation.

This first stage of the project has been successfully completed. Application of the method in industrial conditions has demonstrated its validity and usefulness for all users of measuring machines.

Work is in progress on refining the methods of estimating the machine's errors and the corresponding uncertainties. Work is also being done on developing a specific calibration bench for the holed bar in order to ensure measurement traceability.

Contact

Georges Vailleau