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Renishaw has collaborated with a leading British bicycle company to create a 3D-printed metal bike frame. Empire Cycles, located in Northwest England, designed the mountain bike to be stronger and lighter, using a process called topological optimization and employing Renishaw’s AM250 additive manufacturing system. The additive process offers design, construction and performance advantages that include: blending complex shapes or hollow structures with internal strengthening features, flexibility to make design improvements right up to the start of production, and the convenience of making one-off parts as easily as batches, which allows for customization. The new titanium alloy frame, about 33% lighter than the original, was manufactured in sections and bonded together.

The two companies originally agreed to optimize and manufacture only the bike’s seat post bracket, but after the part’s successful production, improvement of the whole frame became the new goal. Empire started with a full-size 3D printed replica of its current aluminium alloy bike and the frame was sectioned into parts that could be formed in the AM250’s 12-in. (300-mm) build height. The design was updated with guidance from Renishaw’s applications team and an optimized design – one that eliminates many of the downward facing surfaces that require wasteful support structures – was created using topological optimization.

Topological optimization software programs use iterative steps and finite element analysis to determine the “logical” material placement. Material is removed from areas of low stress until a design optimized for load bearing is created, resulting in a model that is light and strong. Historical challenges in manufacturing these computer-generated shapes are overcome through the additive manufacturing process.

The AM250 uses a high-powered fiber laser to produce fully dense metal parts direct from 3D CAD data. Parts are built layer by layer, in thicknesses ranging from 20 to 100 microns, using a range of fine metal powders melted in a tightly controlled atmosphere. A fully welded vacuum chamber and ultra-low oxygen content in the build atmosphere allow processing of reactive materials, including titanium and aluminum.
    
The key benefit to Empire Cycles is the performance advantages derived from the additive process. The design has all of the advantages of a pressed steel “monocoque” construction used in motorbikes and cars, without the investment in tooling that would be prohibitive for a small manufacturer. “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,” said Dave Bozich, Business Manager, Renishaw.

The original aluminium alloy seat post bracket is 12 oz. (360 g) and the first iteration of the hollow titanium version is 7 oz. (200 g), a weight savings of 44%. Comparison of the entire frame has the original bike frame weighing in at 4.6 lbs. (2100 g), with the redesigned additive-made frame at only 3.1 lbs. (1400 g), a 33% weight savings. “There are lighter carbon fiber bikes available, but the durability of carbon fiber can’t compare to a metal bike,” said Chris Williams, Managing Director at Empire Cycles. “They are great for road bikes, but when you start chucking yourself down a mountain you risk damaging the frame. We over-engineer our bikes to ensure there are no warranty claims.”

Titanium alloys have more density than aluminium alloys, with relative densities of around .14 lb/in3 (4 g/cm3) and .11 lb/in3 (3 g/cm3), respectively. Therefore, the only way to make a titanium alloy part lighter than its aluminium alloy counterpart is to significantly alter the design and remove any material not contributing to the overall strength of the part. The companies believe further analysis and testing it could result in further weight reduction.

In addition to durability and corrosion-resistance, titanium alloys have a high Ultimate Tensile Strength (UTS) of more than 900 MPa, when processed using additive manufacturing. With near perfect densities – greater than 99.7 percent – the process is better than casting and the small, spherical nature of additive-part porosity has little negative effect on strength. The seat post bracket was tested using the mountain bike standard EN 14766, and it withstood 50,000 cycles of 270 lb ft (1200 N). Testing continued to six times the standard without failure.

Empire is passionate about partnering with top British engineering companies to create elite products. Research into bonding methods resulted in Mouldlife providing the adhesive, which was tested by technical specialists at 3M test facilities. The wheels, drivetrain and components required to finish the bike, were provided by Hope Technology Ltd.

Empire and Renishaw plan to continue testing the completed bicycle frame in the laboratory, using Bureau Veritas UK, and in the field, using portable sensors in partnership with Swansea University. “We plan to develop this further, in partnership, to look at iterative improvements in bonding methods, such as specific surface finishes,” said Bozich. “This project demonstrates that excellent results can be achieved through close customer collaboration.”

For more information, visit: www.renishaw.com/empire

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