METROLOGY, MEASUREMENT & TESTING
Equipment / Manufacturers
In the manufacturing world there is a requirement to know that products are dimensionally and structurally made to specification. The design will imply and impose tolerances and requirements which may be geometric, define surface finish, or require a test regime. The testing can be visual or physical, there is a range of non-destructive testing (NDT) and destructive testing (DT) techniques. Destructive testing will be specified for strength critical components, typically the ‘first off’ and ‘last off’ for a batch of components will be subjected to the testing and if successfully achieving specified requirements the batch is signed off as compliant.
As tolerances and variance have tightened, and complexity increased, the mechanisms for metrology measurement and testing have become more sophisticated and accurate. Early measurement systems were contact based physical devices. The range of devices now available includes remote measurement using scanners and vision techniques. The non-contact data capture metrology techniques can be typically classified as follows: –
- 3D laser scanning
- 3D rendering
- Blue light scanning
- CT scanning
- X-ray tomography
The digitization of coordinate measurement allows users to generate a 3-dimensional image of the component which can be overlaid on the designers’ model and evaluated for errors. This is increasingly done employing the use of artificial intelligence or expert systems to remove any human error and speed up the process.
A Coordinate Measuring Machine (CMM) is the centre piece of most metrology laboratories. These CMMs allow users to take single measurements or generate a cloud of points to define a feature or entire product taking a framework of reference points to position the product ‘touch points’ on an XYZ axis. This digitised data can then be checked against the design.
Measuring arms are an essential part of modern metrology laboratories. They provide accessibility to critical areas otherwise difficult to reach, including re-entrant features and deep narrow internal features. Pressure sensitive contact probes provide highly accurate pinpoints referenced back to the central coordinate system. It is usually necessary to use a system or set of metrology tools to fully check the component against specification.
Most engineering and manufacturing companies have enough metrology equipment in-house for routine work. The more sophisticated equipment is expensive and difficult to justify purchasing if it will be underutilised. This creates a commercial opportunity for outsourcing of metrology services. Some Clients place a requirement for independent verification of authenticity of compliance, necessitating the use of an accredited 3rd party. Modern metrology equipment will output a record of measurement which may be as digital data or in printed form.
Testing and Calibration
The maintenance and accurate calibration of metrology is an essential and certifiable requirement. The calibration must be performed at specified intervals using reference data. The reference data may be a physical item of known geometry or a closed-loop routine to identify closing or cumulative error. The metrology device and/or the reference data may require it being sent to an independent test laboratory to be certified as accurate. This is covered by a range of internationally agreed standards to ensure single source of truth or reference.