Renishaw supports Empire Cycles 3D printed bike project; WNT (UK) helps with the final cut

Empire Cycles designed the mountain bike, MX6-R, to take advantage of Renishaw's additive manufacturing technology, allowing it to create a titanium frame that would be both strong and light, using topological optimisation (material removed from areas of low stress, generating a design optimised for load bearing) - the original bike frame weighs in at 2,100 g, while the additive manufactured version weighs 1,400 g, a 33% weight saving. Titanium alloys are more dense than aluminium alloys, with relative densities of around 4 g/cm3 and 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. For example, the original aluminium alloy seat post bracket is 360 g and the hollow titanium version is 200 g, a weight saving of 44%. This is just the first iteration; with further analysis and testing it could be reduced further. By working together, Renishaw and Empire Cycles optimised the bicycle design for additive manufacture, eliminating many of the downward facing surfaces that would otherwise have needed wasteful support structures. The entire bike frame was arranged in sections, with the seat post bracket on one build plate, and fabricated in one go. Sections were then bonded together. The additive manufacturing process offers design freedom, taking in: rapid iterations; flexibility to make design improvements right up to production; the ability to make shapes derived by topological optimisation; ultimate customisation and tailoring - make one-offs as easily as production batches. In terms of construction, the benefits took in: complex shapes with internal strengthening features; hollow structures; built-in features, such as the rider's name. As for performance, this saw: the seat post bracket 44% lighter than an aluminium alloy version; high strength - tested to EN 14766; and corrosion-resistance and long life. The project's aim is 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. Testing of the completed bicycle frame will continue, both in the laboratory using Bureau Veritas UK, and on the mountainside using portable sensors in partnership with Swansea University. But the project also needed the help of some more traditional techniques to bring the project to a successful conclusion. While additive manufacturing has developed significantly, precision machining is still required to finish safety-critical and fine tolerance elements of the cycle frame. "The machining of these components was vital to the success of the project, as the additive manufacturing is not able to achieve the tolerances required for the bearing locations in areas such as the headstock and crank. Simply put, without the precision machining expertise of WNT (UK) and Merlin Engineering, there would be no bike," says Chris Williams, Empire Cycles' managing director. The use of additive manufacturing, or 3D printing, threw up its own particular challenges when it came to machining these frame components, as the whole idea was to reduce the weight of the frame, which meant that the amount of material left for machining was minimal, as little as 2 mm in some areas. This meant that WNT had to rethink some of its machining strategies as Scott Bradley, WNT's technical sales engineer working on the project, explains: "Conventional thinking when it comes to machining titanium is that climb milling is the optimum process, as this helps to get through the tough skin of the material. However, at WNT we have been carrying out in-depth cutting trials in preparation for this particular project and the conclusion was that, due to the clamping of the part and the thin wall thicknesses, conventional milling provided the best option to semi-finish the part, with climb milling used only for the finishing cuts." WNT selected its 2011 series of indexable insert cutters that have been developed as an alternative to solid carbide mills at diameters between 16 and 40 mm diameter (although the range extends to 80 mm diameter). With their soft cutting action and radial force compensation features, they are said to be the ideal solution to thin walled components that are cut on machines with high speed, low power spindles. The inserts used on the frame machined at Merlin Engineering were selected specifically for the application. For the headstock area, WNT selected an insert developed specifically for machining titanium, with an insert developed for machining super alloys being selected for the bottom bracket of the frame. "We have worked closely with Empire Cycles for many years, helping with the machining of the frames on the original MX-6 cross country bike made from solid aluminium. The creation of this world's first high performance cycle frame using additive technology is a major milestone in manufacturing. However, it is reassuring that precision metalcutting techniques are still vital to the creation of the finished article and make the whole process practical," concludes Mr Pennington, managing director, WNT (UK). There are lighter carbon fibre bikes available, but Empire Cycles' Mr Williams has researched this already and says: "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. I over-engineer my bikes to ensure there are no warranty claims."