Engineering metrology is defined as the principal method of measuring and inspecting dimensions such as; length, flatness, thickness, diameter, taper, profile. Without metrology components and parts simply will not fit together. In the past metrology was normally the process of measuring after a part was produced. Now, however, we can check dimensional accuracy and tolerance from the moment raw material or bought-in components arrive. Temperature, tool-wear and tool vibration can cause dimensional change in components requiring constant checks.
Both machine tools and the parts they make need constant monitoring. With machine tools just knowing where the tool is in relation to the work piece is a challenge. This is done using a combination of lasers, stylus and sensors (and other electronic gauges). They can either touch or sense the surface of a component or tool and are used to send data to the CNC machine. This data in turn is used in numeric form to help control the tool path on the machine.
On parts manufactured the smaller the tolerance the higher the production costs. This means balancing the tolerance of parts to make them fit yet keep costs down. Simple checking involves basic measurement tools from a steel rule, micrometer or dial indicator, where line divisions can easily be read off or compared. Where large numbers of simple parts require checking for tolerance then gauges can be used; Ring Gauges, Plug Gauges featuring Go and No Go sizing and gauge blocks. For bores and holes air gauges are used.
More advanced measurement of complex forms and profiles use co-ordinate measuring machines (CMM). These can check compliance of products bought-in and parts manufactured prior to dispatch.
Use of optical devices is a fundamental part of measurement and checking of geometric features eg; Optical contour shadow gauges project enlarged images, Interference fringes (interferometry) can be used to detect trueness/flatness. Laser based scanners to capture high definition 3D features are now widely used to convert into a parametric model on screen.
Even if a part is in tolerance, variations in surface finish might make the component fail. So surface ‘roughness’ is also checked.