While much of the hype in manufacturing is often attributed to breakthroughs in materials, machining technology or tools, industrial metrology plays a quieter but equally pivotal role in driving developments.
Without it, manufacturers are hard pressed to produce components that fit perfectly in the complex jigsaw puzzle-like assembly processes of today. To put it simply, it is essential for quality assurance.
For starters, sectors like aerospace, oil & gas, automotive and medical technology all require a strict adherence to tolerances for safety reasons. This focus has been amplified as parts and processes increase in sophistication and complexity, resulting in a demand for greater precision and accuracy. External drivers, such as the tight labour market, have also driven the ease of use factor in tools.
In addition to existing demands, technological developments in each verticals are also bringing their own set of challenges. Some of the main challenges include catering to the rise of additive manufacturing, measuring large parts in the aerospace industry and the increasingly stringent standards in the medical technology sector.
Driven By Additive Manufacturing
There has been an explosion of additive manufacturing in recent years, particularly in the areas of rapid prototyping and reverse engineering. Boeing is already using 3D printing to produce more than 20,000 military aircraft parts, while GE Aviation has announced its plans to produce more than 100,000 components for its LEAP and GE9X jet engines using additive manufacturing by 2020.
According to a Gartner forecast of the consumer and enterprise 3D printer (3DP) market, worldwide shipments of 3DPs priced less than US$100,000 will grow 49 percent in 2013 to reach a total of 56,507 units. That number is expected to further increase in 2014, growing 75 percent to 98,065 units, followed by a near doubling of unit shipments in 2015.
“Companies that do rapid prototyping appreciate 3D printing, because it positively impacts their product design process by way of two very important factors — time and confidentiality. With 3D printing, designers are able to manufacture a mock-up quickly for analysis and evaluation, and it eliminates the need for model fabrication at an external vendor’s facility, both of which are attractive attributes,” explained Quah Beng Chieh, head of marketing at Faro Asia Pacific.
This in turn, has resulted in the demand for 3D laser scanning capabilities, which is used to digitise components for additive manufacturing and allows checks to be performed on the prototype, thereby closing the loop in the 3D process. It is also essential in helping suppliers to compete for speed and attain a shorter time to market.
For this reason, GE has signed an agreement with Sigma Labs to advance in-process inspection technologies for its additive manufactured components. The focus is to verify the quality and geometry of components during the additive building process, which may ultimately ramp production speeds up by 25 percent.
As additive manufacturing continues its boom, Mr Quah is optimistic about metrology’s role in this area. “In 2014, we believe that non-contact optical measurement technology will likely continue to gain more ground,” he said. Such technology includes laser scanners, white light scanners and video systems.
Laser Scanners
The main purpose of a 3D scanner is to capture the geometry of physical objects by collecting large amounts of surface data in a point cloud, which is then used to construct digital 3D models. Extremely useful in contamination-critical environments, non-contact measurement tends to edge out traditional methods in terms of speed and precision.
These are benefits that are shared among all non-contact measurement tools, but that is not to say that differences do not exist. Laser scanners alone come in a plethora of options, which widens exponentially when other types of systems are included. Nonetheless, laser scanners are by far the equipment of choice in 3D scanning.
A laser scanner collects data by scanning a single point of illumination (either a spot or a line) across the workpiece in the X, Y and Z directions, where the signals are then passed back to a detector.
Depending on the focal range required, this can either be triangulation-based, structured light, time-of-flight or phase-shift 3D scanners. For short ranges with a focal distance of less than one metre, triangulation and structured light scanners would be the most suitable.
Triangulation based scanners operate by using a sensor (eg: a camera) to locate a laser line or dot that is shone onto the object. Meanwhile, structured light scanners work on a similar principle, but instead of a single point, patterned light is projected onto the object and the deformation of the pattern is measured.
Among the two, the former offers more portability and flexibility — it is available as an area scanner, handheld scanner and as a portable arm. Less preparation for the part is needed and it is also less sensitive to ambient light. When paired with passive, visible light sensors, surface textures and colours can be captured to create a full 3D model as well. However, they have a lower resolution and higher noise, and are generally less accurate.
Due to its ability to scan multiple points or the entire field of view at once, structured light scanners generate profiles that are much more precise than laser triangulation, often with a higher resolution and lower noise. It also reduces the problem of distortion from motion. Nonetheless, it is more sensitive to surface finish and is limited to only area scanner types.
Meanwhile, pulse-based (time-of-flight) scanners serve as a better option for mid to long range applications. These scanners get their measurements by calculating the time it takes for the laser light to be reflected back.
A similar alternative is a phase-shift system. It works on the same principle, but also modulates the power of the beam. In turn, the scanner compares the phase of the laser that is sent out and returned. This option is faster and more accurate, but is only suitable for medium range applications, unlike pulse-based systems whose range spans between 2 to 1,000 m.
Surface Metrology Tools
Apart from 3D laser scanners, other types of non-contact measurement used for surface metrology include interferometry, video systems, as well as autocollimators.
An interferometer uses the effect of interference for calculations; it generally starts with an input beam that is split into two using a beam splitter, before recombining the beams on another beam splitter and measuring the power of the resulting beam.
Depending on the type of interferometer, as the use of light sources and beam splitters differs, this technology can resolve minute differences in surface characteristics to the nanometre scale. This is particularly useful for applications where surface finish is of utmost importance.
Meanwhile, video measuring obtains accurate dimensions by comparing and contrasting the relationship between surface edges and features after magnifying the images. This technique can generate rich information and is mostly used for measuring the size and shapes of work pieces.
One of the best options for the precise angular adjustments of components is the autocollimator. It combines both the collimator and the telescope by using a single objective lens, and separates both beam paths by using a beam splitter. Measurement results are also independent from the distance to the object due to the collimated beam.
This can be automated by replacing the eyepiece with a digital camera with discrete sensor pixels, thereby enabling PC assisted measurement and eliminating the need for a skilled operator. Automated versions generally provide greater accuracy and repeatability since it is not dependent on the operators’ expertise. By eliminating operator error, sample throughput is increased.
In addition, it provides flexibility and convenience to the measurement of large parts as well, which has traditionally been a cumbersome process. “Large parts are best measured by laser trackers because they provide high accuracy, portability and a large measuring volume,” said Mr Quah.
As large parts tend to be bulky and heavy, portability serves as a boon because it eliminates the need to move parts to a measuring room. “With a large measuring range, users also do not have to worry about fitting the part onto a measuring table,” he added.
Influences On Material Properties
According to Dr Xu Jian, principal research engineer II at the Singapore Institute of Manufacturing Technology (SIMTech), the top two needs in additive manufacturing are rapid modelling of solid objects for reverse engineering, and 3D internal and external dimensional measurement for quality assurance.
Despite an emphasis on geometry repeatability, part to part repeatability of material properties is equally important and contributes to an integral part of inspection processes. One way to go about this is obtaining internal dimensional measurements to detect residual stresses that may have been caused by microstructural changes, transitions in phases and thermal issues.
For instance, warping occurs due to temperature differences between the material deposition point and its surrounding areas. This is an issue commonly seen in systems that build components one layer at a time. In laser beam systems, this problem is exacerbated as the surrounding material is not heated. However, warping is not the only byproduct of poor thermal gradients, as it affects residual stresses and porosity as well.
This has led to the push for in-process measurements that measure temperatures across the printing surface as the component is being built. Not only will it enable intelligent path planning so that thermal gradients can be reduced, it also allows a layer to layer approach so that greater control can be held over the workpiece.
Using real time sensors and a closed loop control, this approach can be used to reduce droplet size variability and rectify mistakes made in earlier layers. Some of the possibilities include using infrared cameras and high-speed measurements of droplet size.
Subsurface Measurements
Especially for additive manufactured parts that are used in sensitive applications like medical technology or aerospace, it is integral to ensure that the part has favourable internal stresses.
Some manufacturers have circumvented this problem by making parts with an extra stock envelope. Others have resorted to non-destruction methods like neutron imaging, thermal imaging and x-ray CT systems to test and ensure that internal measurements are up to scratch.
Both neutron and x-ray imaging are non-destructive techniques that are based on measuring attenuating properties of the imaged object. While x-rays are attenuated based on the material’s density, neutrons are attenuated according to material types, which either absorb or scatter neutrons.
Because of its impartiality to density, neutron imaging tends to be more comprehensive. Depending on the material, it can penetrate up to several centimetres to non-destructively measure residual strains and stresses. Another benefit of neutron imaging is that neutrons are allowed to pass through many commonly used metals, which might make it a better alternative for checking metal components.
Yet, x-ray imaging is not without its merits. According to Dr Xu, “subsurface/internal structure 3D inspection using x-ray CT” is one of the current developments in 3D inspection. He and his team at SIMTech are using high energy x-ray CT systems to reconstruct 3D internal structure of objects, with a maximum penetration length equivalent to 70 mm of steel or 250 mm of aluminium.
This focus on non-destructive methods is credited to the growing need for automated inspection, and the measurement of parts/surfaces dimensions, defects in composite structures, as well as the structure integrity of remanufactured items.
At the end of the day however, there is no single best inspection equipment or process. Choosing the right tools and approach depends on the context of each scenario. “In order to choose the most suitable measurement system, users should consider their needs for measuring range/volume, accuracy/precision (tolerance levels), portability, as well as total costs involved (eg: cost of device, dedicated measuring room, foot print of equipment, maintenance, etc),” said Mr Quah.
As with everything else in life, the key is attaining the right balance for each specific set of needs. For inspection in additive manufacturing, this could mean turning to surface metrology for geometry inspection, implementing in-process measurements, using non-destructive radiographic systems for internal dimensions, or simply dabbling in all of the above. The exact configuration does not matter, what does is reaching the end goal of quality assurance.
