Measuring up

The rapid advance in technologies to measure and inspect finished products has led to a manufacturing quality undreamt of a decade or two ago. Even in relatively mundane products the need for quality, and methods of measuring during and after production to ensure that quality, is obvious.

The dramatic fall in the cost of lasers, as well as the increase in precision possible from laser beams, has in led to a growth in their use for monitoring and inspection. For example, Blum Novotest laser technologies are used by Alstom Aerospace for precision tool setting in the manufacture of complex gas turbine jet engine components. Alstom uses a 5-axis Hermle CNC machine tool, for which the laser tool setting system has to position three separate 29-position tool carousels.

The laser positioning system is claimed to have reduced tool setting times to one sixth of when the setting was done off-machine, and repeatability tolerances of about a micron (0.00005in) have been achieved in practice.

Laser profile sensors are also used to ensure the UK's rail system runs on safe tracks. Non-contact laser-based profile measurement sensors produced by Micro-Epsilon measure and monitor wear on tracks here and around the world — wear being an indicator of the track reliability and condition of the rails.

Russia's rail network uses a wagon inspection system where an array of Micro-Epsilon laser sensors detects back-scattered light from the laser using a CMOS array. This can detect the profile of the track in any condition or state of corrosion.

Past systems had been unable to cope with differences between shiny and corroded rails and their different reflection characteristics, making manual inspection the most cost-effective solution.

The sensors are also designed to measure the track profiles quickly and accurately — they can be designed for train speeds of between 74 and 80mph (120 and 130kph) and to measure 4,000 profiles/second.

Measurement of size and body parts is also an essential feature of Formula One racing cars. Faro UK has supplied one of its Platinum FaroArms to the Super Aguri team to measure bodywork parts and ensure the cars comply with the stringent size rules imposed on F1 cars.

The FaroArm is used to reverse engineer, perform alignments and inspect parts to high accuracy — all feeding in to Mitutoyo MCOSMOS-3 inspection software.

Midland F1 is another company involved in Formula One racing. It uses an optical co-ordinate measuring machine with a linear CCD (charge coupled device)-based camera that detects an array of LEDs mounted on a touch probe to inspect cars.

With a hand-held laser scanner, the system can digitise parts and produce 3D point clouds for reverse engineering or rapid prototyping — an essential feature in F1 to get ahead of the competition.

The system, supplied by Metris, can also be used for dynamic measurements and can measure car components under load, for example in the flexible wings of an F1 car. The legality checks every car has to go through before a race begins are also catered for.

Co-ordinate measuring machines are also used for production car inspection. For example, Aston Martin Vantage body shells are subjected to something like 400 separate point inspections using a pair of LK machines as part of the production process. The machines are integrated into the production line, unlike those for the DB9, which are in a separate room.

The LK control software (CAMIO Studio 4.2 in this case) identifies the model, and irrespective of whether the car is a right or left-hand drive version it begins the inspection cycle.

Touch-trigger probes record positional information from the shells — inform- ation that is processed into a form where SPC (statistical process control) can provide direct feedback to the shop floor to alert engineers when the manufacturing processes are drifting towards being 'out of tolerance'. It is claimed that the measurements are accurate to within 10µ over the length of the cars.

One inspection task undertaken recently demonstrates the need for accurate measurement and machining over a large scale. ITP Group has built a 7.3m co-ordinate measuring machine to measure and machine satellites for the Beijing Space Authority — a machine the company claims may be the tallest of its kind in the world.

Even though the satellites are large, they are made to tight tolerances and the machines are capable of measuring all of their faces to an accuracy of 0.15mm. The system uses Renishaw touch probes as well as an indexable milling insert for light machining, and can be controlled manually, by a joystick or under full CNC control using ITP Group's software.

For space vehicles, monitoring doesn't end when the craft has been manufactured. Infrared camera systems made by FLIR Systems have been specified for the Space Shuttle and International Space Station programmes to inspect and report damage to the craft.

The importance of the condition of the surface is essential for safety — the 2003 Columbia Space Shuttle disaster was caused when heat insulation on the leading edge of one of the wings was damaged. This structural failure led to the craft burning up upon re-entry.

NASA's Langley Research Centre and FLIR have produced a camera able to survive the extreme temperatures, radiation shocks and vibration resistance of space. The system first came into use with the International Space Station this summer and proved its worth when taken on a 'spacewalk' where heat shield repair techniques were tested.

The hand-held cameras transferred images to a flash card, which dumped the data on to a laptop for analysis. If the device is certified by NASA, it will accompany future missions in space and could help to avert future Columbia-like disasters.

These examples highlight just a few applications where high-precision measurement, mostly at the extreme ends of the spectrum, are critical.

One thing is clear — accurate measurement taking is never going to go out of fashion. Indeed, we can expect more and more sophisticated solutions to appear in years to come — on Earth and in space.