Industrial quality inspection often relies upon coordinate measuring
machines or CMMs. However, until recently, there was no way to determine
the possible inaccuracy - or uncertainty - of the measurements.
A project funded under the Measurements and Testing programme has
developed a system which CMM operators can use to perform easy,
on-site checks to ensure that their machines are performing within
reasonable error limits.
Coordinate measuring machines (CMMs) are devices that measure the dimensions of all sorts of products. They are mainly used in industrial quality inspection and about 40,000 exist around the world. For CMMs to be useful, industry must be able to rely on the accuracy of the measurements that the instrument makes.
Until recently, however, the uncertainty, or degree of error, in CMM measurements has often been little more than guesswork. This meant that the CMM user could not thoroughly determine whether a CMM used to make certain measurements would give results that are traceable and useful. As a result, some users end up taking overly elaborate measurements with expensive apparatus. On the other hand, some companies may unknowingly obtain data that does not meet the requirements because the uncertainty is too high.
A coordinated effort
Three research laboratories, two CMM manufacturers and five CMM users worked together to solve this problem. They have developed the "On-line Virtual CMM-
technique" which assesses the error characteristics of individual CMMs using artefact-based techniques. Once the error parameters are known, the computer programme simulates the measurement task to determine the uncertainty for that particular machine. The system gives normal CMM operator a relatively easy way to automatically assess uncertainties associated with the nearly infinite number of different tasks that can be performed with a coordinate measuring machine.
The partners' work shows that the uncertainties predicted with the help of the new tool are reasonable for a variety of measured features - they are consistent with the errors observed. To achieve this, the Virtual CMM was integrated into two CMM manufacturers' regular evaluation software. The users then carried our measurements of previously calibrated objects. The tool predicted reasonable uncertainties.
The Virtual CMM is a fast and cost effective technique to verify the stability of the geometry of CMMs which drift over time, especially in harsh operating conditions. As the machines themselves cost between DM 30,000 and DM 2 million (the average is DM 200,000) companies and laboratories need to make sure that their equipment is giving them reliable and useful measurements.
Previously, the machine had to be thoroughly calibrated from time to time and this is costly. The Virtual CMM gives users a fast and reliable way to perform interim checks on their machines. While it does not entirely replace the need for complete calibration, it does mean that machine can operate for longer periods between full-scale recalibrations.
Guiding the way
To help CMM users, CMM calibration laboratories and CMM manufacturers, the team also wrote a set of informal guidelines specifying procedures for the Virtual CMM. This way, all relevant players' tasks are described in detail. These could go on to form the basis for written standards and technical guidelines. Because there is a tool with guidelines to check the quality of the CMM measurements, measurement laboratories that use CMMs can gain accreditation status for the first time.
With the new system, it is possible to verify the geometric tolerances, or permitted measurement errors, of nearly any product or part from ships, cars, electronic components, fuel injection nozzles, watches, lenses to aircraft, satellites and space craft boosters. Many sectors stand to benefit from the Virtual CMM.
Popular new product
User and manufacturer involvement was key to the success of the project which has given manufacturers a new product and users more confidence that they are making optimal measurements at the right price. Users can now state the uncertainty in measurements and this reduces disputes between part suppliers and buyers. Any CMM manufacturer will be able to offer the On-Line Virtual CMM technique as a new product in their range.
There is already much interest in the system. At a workshop held in October 1997, 150 experts and CMM users attended lectures and demonstrations about the new tool. Further information about this workshop is available from the coordinator. Already, there are six laboratories interested in becoming accredited.