Factory inspectors, laboratory testers, measurement engineers, machine tool manufacturers, medical implant manufacturers, workshops and automotive plastics moulding makers all rely on measurement in three dimensions. This is required in diverse sectors ranging from medicine to aerospace, where the need for accuracy is paramount.
Co-ordinate measurement machines (CMMs) have stolen a march over discrete conventional measurement techniques and rapid prototyping allows organisations to quickly satisfy time-critical demand for products. Classical methods of industrial and laboratory measurements have in the past relied on tools like Vernier callipers, micrometers, dial test indicators and so on.
Speeding up measurement is an essential step towards rapid product prototyping and manufacturing using additive layer techniques, including 3D printing. Barry Assheton, sales director at CRDM, argues that CMM is already becoming old hat as his company uses computerised tomography (CT) scanning, which allows engineers to see inside components.[Page Break]
"This is important if is it is a safety critical part, say in aerospace," Assheton adds. "CMM only takes external dimensions but sometimes we need more information about a part such as its precise dimensions and tolerances, whether it fits to the other parts properly and whether there are any voids inside it. The design engineer needs to ask: 'Can I take the part and create a CAD model from it?' We are finding a lot more customers asking for a CT scan as part of their initial sample reports as they then do not have to do destructive testing."
However, Ben Verduijn, account manager for metrology, spectroscopy and additive manufacturing at Renishaw Netherlands is sceptical: "What Barry is telling is nice, but exceptional at the moment. Who is using CT scan in the metal part world?" Assheton could not counter Verduijn's remarks at the time of going to press.
Some of the data fed into rapid manufacturing machines will come from measurement, but the bulk comes from desktop 3D computer aided design (CAD), according Frank Schaeflein, senior application engineer at additive layer machine manufacturer Stratasys. "I would say roughly 80 per cent is 3D CAD, although data that does not come from 3D CAD may be retrieved in many different ways," Schaeflein says.[Page Break]
"I am not just talking about classical 3D measurement but also laser and CT scanning, even in the mechanical area, and mathematical programs which are not really CAD. The approach to data generation is basically the same: you have a device that retrieves an object's geometry, and by the way 3D measuring tables are used relatively rarely because they take forever and a day."
Turning to Verduijn's assertion that CT scanning is not used for metal parts, Schaeflein rejoins: "This is partially correct. There is also magnetic resonance imaging (MRI) scanning, but in practice this is more used for soft tissue. Metal objects are represented on a CT scan clearly, but such scans do not represent the internal structure of a metal part."
The data fed to a Stratasys is typically in the form of a stereo lithography (STL) file, which Schaeflein claims is the easiest format to work with and which guarantees topologically correct surfaces or volumes. As 3D CAD technology has been around for 25 to 30 years, Schaeflein holds that that there are not many components today that do not have 3D data available.
Schaeflein deduces: "I would say that reverse engineering is of interest for measurement and quality control, but not really to redesign parts; more to verify that the output corresponds to the CAD file used for the input."
Ronen Sharon, CEO at Sharon Tuvia, an Israeli manufacturer of aerospace mechanical assemblies, parts and components, claims that his company's CMM measuring cycle has been shortened by using PAS CMM software that automates program writing time for the machine. PAS-CMM automatically creates a dimensional measuring interface specification (DMIS) program.
Sharon continues: "It possible to use an optical measuring system and compare it to the CAD file and have the deviation between the part and the CAD by using GOM optical measuring systems." Gesellschaft für Optische Messtechnik (GOM) was founded in 1990 as a spin-off of the Technical University of Braunschweig.[Page Break]
At Farnborough 2012, EADS presented the prototype of a portable unmanned aerial vehicle (UAV) produced by ALM technology with a wingspan of approximately 1.5 metres, designed by students from the University of Leeds. However in July last year, Southampton University engineers flew what they claimed to be the world's first 'printed' aircraft.
The entire structure of the Southampton University Laser Sintered Aircraft (SULSA) has been printed on an EOS EOSINT P730 nylon laser sintering machine, including wings, integral control surfaces and access hatches. The machine fabricates plastic or metal objects, building up the item layer by layer. The project was led by Professors Andy Keane and Jim Scanlan at the University's computational Engineering and design group and resulted in an electric-powered aircraft with a two metre wingspan and a top speed of nearly 100 miles per hour. It is almost silent when cruising on autopilot.
"There are four servos for the two wing surfaces and two tail surfaces, an electric motor for the engine, an autopilot and a standard aero modeller receiver," explains Professor Keane. "It is programmed to fly in a mission to different way points and then come back and land under its own steam - it does not need a ground pilot.
"The whole craft was designed from scratch in CAD. We decided how big we wanted it to be and then designed the aerodynamic shapes needed to create the parts, created the STL printer files. Each printer has a specific file format but essentially it breaks the geometry down into lots of small triangular pieces."
In the case of EADS, using ALM opened the possibility for aerodynamic optimisation such as wing twist, which would otherwise be difficult and expensive to realise for an aircraft of this scale. Different, detachable wings can be 'printed' in a relatively short time to adapt the UAV to missions with different requirements using EADS proprietary ScalmalloyRP material, which provides exceptional mechanical properties useful in the production of structures with complex shapes.
Martin Muir, research engineer at EADS acknowledges that ALM design cost can be spread more widely on larger aircrafts, and reasons that putting more complexity into the design is worthwhile because the fuel savings achieved can be justified.
Muir explains: "For a small product such as a UAV, you generally do not want to optimise for a particular environment or profile. The majority of small scale UAVs tend to have rectangular wing sections that use a single chord profile - if you take a slice through the wing and look at it end-on from the wing, you would see a 2D plan form of the wing called the chord profile.[Page Break]
Wing chord profiles
"Historically, wing design has used the US National Advisory Committee for Aeronautics (NACA) group of chord profiles, which are determined by a four-digit number like NACA 0012 for example, which specifies the thickness of the chord at a certain profile and whether it is symmetrical.
"In an Airbus A350 for example, there would be several chord profiles that make up the wing, so you would have one at the root, one a little further out, another further out still, and so on until last profile at the tip. They allow the wing to be twisted and contoured in a specific way as it moves outboard from the fuselage.
"On a UAV, you do not tend to do that - you would often have the same chord profile at the root as at the tip, in a similar as you would on a light aircraft like a Cessna for example, which has a simple wing profile. Otherwise it becomes very complex to design, and even more so to manufacture. With ALM, the cost of complexity is removed because the cost is all in the size of the product."
The STL files to drive the ALM process came in part from students using Solidworks and from EADS using CATIA, both of which come from the Dassault aerospace group. Muir describes the EADS printed drone as a static demonstrator rather than a working prototype.
Professor Keane derides EADS's Farnborough display, saying: "I think what they are doing catching us up is. We have moved on since last year and built two more UAVs. We do not use printed nylon for everything on the latest designs, but only where it is most useful.
"Printed nylon is not the best material to use for wings and we prefer numerically cut foam, which makes for a lighter, more efficient structure. We then support the wings with a carbon fibre spar, so the current process we favour most is a selective laser sintering (SLS) laser printed fuselage with foam wings linked together by carbon fibre tubing."
Southampton's latest generation UAV is designated the Mark V airframe with a 20 kg maximum takeoff weight, rapid prototyped structure and ultra-light foam core aerodynamic surfaces on a wingspan of four metres. It boasts a 5 kg payload and 12 hour endurance from a twin cylinder four stroke petrol engine with on board start and 6/12V generator. Auto take-off and landing is supported by on-board GPS and camera systems, with fully redundant dual circuit avionics.
"You do not need the same strength in wings as you do for a fuselage," Keane contends. "The trouble with printed nylon is the minimum thickness of about 1mm, which is too much material so wings are heavier than they need to be. By cutting foam, we can make the wings larger because they carry an evenly distributed load.
"If you laser sinter a wing, because of its minimum wall thickness it will have more weight than a foam wing, unless you are building a larger aircraft with a wingspan of perhaps as much as ten metres, at which point you cannot print in nylon because the printers are not big enough."[Page Break]
Next generation propulsion
Muir disputes Keane's contention that foam is the best wing material, arguing that foam unsuitable if anything is to be stored inside wings and that those on the EADS UAV is designed to hold the next generation of propulsion systems, which can be distributed in the wing profile.
"Most UAVs either use electric or petrol propulsion," reasons Muir. "Ours uses the former, currently with lithium polymer batteries but we are researching the use of hydrogen fuel cells, which provide substantially more power for less weight. Our UAV was sized to take advantage of several materials and technologies coming up on the horizon.
"In order to manufacture the UAV from the polyamide material we used, we had to beef up the structure and the static prototype was produced at 1mm wall thickness, whereas the actual design the students came up with is designed to use substantially thinner walls from a material that is less flexible in order to maintain the same weight [Page Break]characteristics."