Empire Cycles, based in Bolton, Lancashire, UK, is a small designer and manufacturer of high-end mountain bicycles. It was founded in 2004 when its MD, Chris Williams, decided to branch out from designing motorcycle parts and incorporate his obsession of mountain biking and dirt BMX riding into his job, bringing together the requirements of mountain bike frame structures with the technology already apparent in motocross motorcycles, using aluminium pieces bolted together. All Empire frame parts are designed, manufactured, and assembled in the UK and the company prides itself on its “personal touch”.
Before it contacted Renishaw, a manufacturer of laser sintering machines, the company was already aware of the benefits of 3D printed parts. Williams had already produced a full size 3D printed replica of his current bike before he approached Renishaw, so had a good idea of what he wanted to achieve.
After the companies met, Williams put together a new design for the bike frame which would take better advantage of additive manufacturing technology. The aim was to produce a titanium frame that would be both strong and light.
To begin with, Renishaw had agreed to improve and manufacture the seat post bracket only, but after this stage proved successful, decided the whole frame was a practical goal. Williams updated his design with guidance from Renishaw's applications team on what would build well, and manufacture started using the AM250 laser melting system.
The AM250 combines vacuum chamber preparation prior to re-pressurising the build chamber with Argon gas refill, to provide an oxygen free atmosphere as a key process foundation. The process uses a high power fibre laser, focussed to around 70 microns, to fuse atomised metallic powders together layer by layer to form the finished three dimensional part, a process that allows additional complexity to be ‘built in’ as the part grows in layer thicknesses between 20 and 100 microns, depending on the demands of the application.
Manageable chunks
It was decided to form the frame in sections, so that it could be manufactured using the 300 mm size of the company’s AM250 laser melting machine.
“One challenge was splitting the frame into manageable chunks – our machine has a print volume of about a cubic foot (250 mm × 250 mm × 300 mm), so the whole frame wasn’t going to fit,” David Ewing, product marketing engineer at Renishaw’s additive manufacturing products division, told Metal Powder Report.
The companies also decided to print the sections separately to increase design freedom and make it easier to make design improvements right up to production. This would also improve customisation and tailoring, so that one-offs as well as batch parts could be produced. For example, the rider’s name could be printed on the part.
Material’s “logical place”
Right from the start there was an important focus on what the companies call “topological optimisation” software – programs that are used to determine the “logical place” for material – normally using iterative steps and finite element analysis. Material is removed from areas of low stress until a design optimised for load bearing is evolved. The resulting model is both light (due to the low volume) and strong.
In the past, making such parts using conventional tooling has been problematic, due to the complex steps involved. However, according to Renishaw, this can now be overcome by using additive manufacturing. To make the bicycle parts, Renishaw and Empire Cycles worked at optimising the bicycle design for additive manufacture, eliminating many of the downward facing surfaces that would otherwise have needed wasteful support structures.
“The bike frame components built very well – this was not accidental, as they were designed for process,” commented Ewing. “It is and still remains the best way to utilise the tool in hand – additive is just another tool and to get the most out of it we made sure the geometry suited the build rules. A lot of people get excited that you can build just about anything – and it’s true there is much greater flexibility – but you are still welding metals which obey the same rules of physics. Sharp corners, for example, will still be stronger with a radius.
“When building the bike we stacked the components to fully utilise the build volume,” he added. “They were also arranged to minimise support structure (required on faces overhanging by more than 45° to 60°). We faced some challenges on the first build with rippling and buckling due to the thin flat walls being less than a millimetre thick, but these were quickly solved by adding an internal web, creating stronger T shapes.”
A lightweight design
In this case, the companies chose titanium alloy to make the frame sections. Titanium alloys have a high ultimate tensile strength (UTS) of more than 900 MPa when processed using additive manufacturing and near perfect densities of greater than 99.7% are achieved – this is better than casting and, as any porosity is both small and spherical, it has little effect on strength. Titanium alloys are denser than aluminium alloys, with relative densities of around 4 g/cm3and 3 g/cm3 respectively. Therefore, the only way to make a titanium alloy version of a part lighter than its aluminium alloy counterpart is to significantly alter the design to remove any material not contributing to the overall strength of the part.
In their final form, the companies succeeded in creating lighter weight, stronger parts, making the most of their complex shape with internal strengthening features and hollow insides. While the original bike frame weighs in at 2100 g, when it was redesigned to make use of additive manufacturing, the weight dropped to 1400 g, a 33% weight saving. The original aluminium alloy seat post bracket is 360 g and the hollow titanium version is 200 g, a weight saving of 44%.
The project's aim was to produce a fully functioning bicycle, so the seat post bracket was tested using the mountain bike standard EN 14766; it withstood 50 000 cycles of 1 200 N. Testing continued to six times the standard without failure.
The design also has the advantages of a pressed steel ‘monocoque' construction, a structural approach that supports loads through an object's external skin (similar to an egg shell) that is used in motorbikes and cars, while avoiding the tooling costs this entails.
Bonding 3D parts
Because the frame was printed in parts, it had to be bonded. “We researched bonding methods with 3M and used an epoxy developed specifically for titanium alloy,” said Ewing. Sealant manufacturer Mouldlife providing the adhesive, and technical specialists 3M providing test facilities. Adhesion of 3D printed parts is a relatively new concept and Renishaw says that it plans to look at iterative improvements in bonding methods, such as specific surface finishes.
The wheels, drive train and components required to finish the bike were provided by Hope Technology Ltd.
The whole project covered a short 20 week timescale, which highlights the capabilities of additive manufacturing as no tooling or specific material needed to be pre-ordered. Design and topological optimisation of the seat post bracket took three weeks, while in week six the companies decided to manufacture the whole bike frame. The first frame components were completed in week 16, and three of five frame sections built in week 17. The finished machine was exhibited at Euromold 2013.The final bicycle, the MX6-EVO is an endurance bike with a carefully located pivot point, full-compliment needle bearings and a 66.5° head angle. It can be run with 27.5" (650b) or 26” wheels. The bolt-through rear dropout can be set as 135 mm or 142mm and the bottom bracket has tabs designed to take both ISCG 03 and ISCG 05 standards.
Reducing weight further
“This is just the first iteration; with further analysis and testing it could be reduced further,” Renishaw explained. The companies plan to continue testing of the completed bicycle frame, both in the laboratory using health and safety compliance specialist Bureau Veritas UK, and on the mountainside using portable sensors in partnership with Swansea University.
“The potential performance has not been completely explored yet, but we hope to continue to develop the project,” the company added. “As no tooling is required, continual design improvements can be made easily; and as the component cost is based on volume and not complexity, some very light parts will be possible at minimal costs.”
Could 3D printed titanium alloy be a successful alternative to carbon fibre? There are lighter carbon fibre bikes available, but according to Williams, “The durability of carbon fibre can't compare to a metal bike; they are great for road bikes, but when you start chucking yourself down a mountain you risk damaging the frame.”
According to Ewing, a key aspect of the project is showing 3D printing’s flexibility. “This is a key advantage of additive manufacturing – its flexibility allows you to adapt designs and learn quickly,” he said. “It’s akin to digital photography – before digital the photographer had to wait for the film to be developed and it took time to learn. With the instant feedback of a LCD display there was an explosion of creativity.
“A key focus for the additive sector now is to create design software that is easy to use and will therefore make the technology more accessible to a wider audience and open up huge scope for ingenious engineering.”
However, machine size is still an issue. “As it stands the size of current machines means limitations, so lightweight components and sub-assemblies have been the first applications; but the potential is huge,” he comments. “It changes the rules of traditional manufacturing which state that more complex lighter components are generally more expensive to make. We also believe that the bike can be optimised further to make it even lighter, therefore improving performance and reducing manufacturing time.
So what about the future for printing bikes? “We really don’t foresee a proliferation of metal based additive manufacturing machines in peoples’ homes, any more than you might currently find a CNC milling machine in someone’s workshop or garage,” Ewing explains. “What the future might hold, when software allows, is an increase in industrial service providers who will print a bike to your personal design – customised to your size, weight, accessory preferences and riding style.”
