Insights in Metal Printing
AM Yields New Breeds of Bikes
The first two-wheeled human-powered vehicles likely were the velocipedes of the early 19th century. They typically consisted of a wooden frame with two wheels—the rider would sit astride the frame and push with his legs. Wood was used as an easy-to-work-with, readily available material. Later on, after pedals were added, frames often were made from metal and took on some interesting shapes such as the well-known “penny-farthing.”
The emergence of the modern bicycle can be traced back to the “safety bicycle” designs of the 1880s and 1890s, which introduced a gear ratio between the pedals and the rear wheel. For the next 100 years, bicycle-frame design remained fairly consistent—the 1893 design shown in Fig. 1 would not have looked too out of place in a 1980s bicycle shop. Steel-tubing frames have been the construction material of choice, but other metals gained popularity in recent decades. This came as a result of their higher strength-to-weight ratios, particularly true with aluminum, but also titanium alloys.
New Materials Enable More Radical Designs
AM Takes It to Another Level
Polymer AM offers lower cost and larger build envelopes. The first complete bike frames built using AM were made with polymer materials. Some of these frames were very similar to designs of metal frames, whereas others were more radical in their shape. An early example of a nylon frame printed via laser sintering is the Airbike from the Airbus Group (formerly EADS) in 2011 (Fig. 3).
Many other designs have followed, some of them departing from the usual solid appearance and moving toward a lattice structure for part of or the entire frame. An excellent example of this is a stereolithography-printed bike frame (Fig. 4) by James Novak of Griffith University in Australia. Such open lattices can take advantage of lattice-element-optimization algorithms, which will add more material to elements where strength is needed while using minimal material from less-stressed regions to save weight.
Whole bike frames made using 3D metal printing recently have emerged. An early example from 2014 is the titanium-alloy frame made by Renishaw for Empire Cycles. Selective laser melting was used to make several components in a single setup (Fig. 5), which then were removed, finished and welded together (Fig. 6). Topological optimization was used to reduce weight, but the overall design still looks similar to a conventional mountain-bike frame.
Stronger, Lighter and Custom: Thank AM for That
Since this early foray into metal-AM bike frames, several more radical designs have appeared, some making extensive use of the lattice-structure method seen in the polymer frame from Griffith University. One such design comes from the Technical University of Delft (Fig. 7). The frame was built as a single stainless-steel component using a robotic-welding process. The frame weighs about the same as a standard steel frame, but weight potentially could be reduced using lattice-element optimization.
In the future, the use of optimized lattice structures or topology optimization could yield metal frames that are as light as CFRP structures. This is desirable because the material properties and manufacturing processes associated with metals are better understood and more mature than those for CFRP. This is probably one of the reasons that Airbus subsidiary APWorks invested in the design and manufacture of a metal-AM motorcycle frame. Known as the Light Rider, the motorcycle (Fig. 8) takes frame design a stage further than for bicycles because the forces transmitted are much higher. Topological optimization was used to create an extremely light frame (about 13.2 lb.) in laser-sintered aluminum. Unlike most of the AM bicycle designs discussed previously, the Light Rider is commercially available as a short-range, electrically powered urban vehicle.
The application of 3D metal printing to bike frames presents an interesting design challenge. In addition to a reduction in material and weight, coupled with optimized strength, AM offers the potential to achieve stunning styling and aesthetics. It also makes the production of custom bike frames economically viable. The Robot Bike Co. produces the R160 bike frame through a combination of selective laser melting from Renishaw and CFRP tubing, as shown in Fig. 9. Each bike is designed and manufactured in accordance with the rider’s body measurements. The product is priced at around $5800.
Transferable to Other Industries
The AM lessons learned in bike design and manufacture are transferable to industries such as aerospace and automotive, and could offer an additional boost to the use of metals in manufacturing. As the prices of AM machines and materials decline, and machine build times improve, more bicycles will benefit from AM’s unique capabilities. 3DMP
See also: Renishaw, Inc.
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