Design Challenge (DAMC) 2016

The concept “Custom Lugs” (based on the project Metal lugs), was entered to the Design for Additive Manufacturing Challenge 2016 and ranked to the top 3 in the student category.

For producing customized bicycle frames from steel, the use of lugs offers several benefits, such as reduced amount of manual work. By manufacturing the lugs by 3D printing, these advantages can be combined with a high degree of freedom in design. The final product contains several improvements over the existing solutions, such as integration of functions and optimized structures. In addition, it offers significantly improved possibilities for customisation and optimization for different uses.

Concept description

In cycling, steel frames combine the tradition of bicycle making with pleasant riding characteristics and modern manufacturing. However, if the bicycle is to be adapted for the driver a made-to-measure frame is required. For manufacturing custom and small series steel frames, the alternatives are either to build the frame by directly welding the tubes, or by brazing using lugs. Welding requires a significant amount of machining and manual finishing work, and current cast lugs cannot be easily customized.

We present the concept of manufacturing bicycle frame lugs with 3D printing. By this, we obtain at the same time easy assembly, geometric flexibility and ability to customize one-off or small series products without e.g. casting.

By the increased freedom of design offered by 3D printing, several functions can be integrated to parts. As the lugs include the shapes required for joining the tubes, assembling and welding the frame is easier and no post-weld machining operations are needed. The inherently weak welds are moved away from the highly stressed areas and the joints are smoother. This leads to improved fatigue resistance. In addition, integrated cable stops and internal cable routing, eyelets for mudguards, mounts for disc brakes etc. are included in the lugs. Parts are made as hollow thin-walled parts or sandwich structures to minimize weight. Finally, the lugs can be flexibly branded, as geometrically complex texts, logos etc. can be easily added.

Tools used

We used Altair solidThinking for topology optimization. The stiffness-to-weight ratio was maximized with load cases found in the literature. The results were used for 3D CAD in SolidWorks. We also participated in nTopology Element beta testing. Element is a software for generating lattice structures for additive manufacturing. In addition to modelling, we also
produced a prototype lug from 316L steel and successfully TIG-welded bicycle tubing to it.

The parts are designed to be printed from stainless steel using selective laser melting. The build orientation has been taken into account in the design. The need for external support is minimized and removal of internal support is made easy. In the future we hope to replace
internal support structure with low-density lattice structure. This allows thinner walls and means less post-processing, i.e. machine time is not used to produce structures that will be removed afterwards.

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