3D printing delivers topological benefits for bike manufacturer

You could think of it as being like a game of
Jenga. The objective is to remove blocks in low stress areas, that aren’t
contributing to the overall strength of the structure, whilst maintaining the
tower’s core shape.

With traditional
methods of manufacturing, such as
subtractive and formative processing,
topological optimisation has to make certain compromises. For example, if a
part is created by machining, material layout has to compensate for tool
access. 

If a part is created by casting or moulding, considerations must be
made with regard to opening the mould and removing the part. These
manufacturing constraints limit the design freedom.

However, additive manufacturing (AM) doesn’t suffer from the same limits. As a
result of building the part up layer-by-layer, more complex designs are possible
with no need for tooling or cutting. Therefore forms can be created that are closer
to the optimum design.

When Chris Williams, managing
director of
Empire Cycles first contacted Renishaw, he had
already produced a full size 3D printed replica of his bike in plastic. However,
he desperately wanted the components built in a stronger but still light-weight
material. We did our research and originally agreed to optimise and manufacture
the
seat post bracket only.

Designs for the bracket were originally
drawn up using CAD (Computer Aided Design). Using this design as a basis the
fixed points and loads were input into the topological optimisation software. 

Much like the Jenga analogy the software splits the problem down into a number
of blocks or finite elements, this simplifies the calculation of internal
stress and strain. A series of iterative steps are then run, each one removing
material under the least load. Using this method the software evolves the most
efficient design for load bearing.

This was then remodelled in CAD,
and subtly optimised for build using Renishaw’s applications expertise. By
eliminating many of the downward facing surfaces, areas that would otherwise
have needed wasteful support structures were removed.

Empire Cycles and Renishaw collaborated to 3D print an entire bike frame and seat post bracket out of titanium alloy.

The final piece was produced in
titanium alloy on a Renishaw AM250 laser melting system. This was both light
weight and strong; much stronger than our Jenga tower and 44% lighter than an
aluminium alloy bracket. A mass-reducing design such as this also ensures a
quicker and cheaper build.

The seat post bracket was then
tested using the mountain bike standard EN 14766 where it withstood 50,000
cycles of a 1,200 N force (equivalent to a 19 stone rider). This was then repeated
to six times the standard without failure. Needless to say, we were incredibly
pleased with the results.

Because the seat bracket’s design, build and tests were so
successful, we then decided that 3D printing the whole frame was a practical
goal. In time for the December 2013 Euromold exhibition, Renishaw and Empire
Cycles were the proud parents of
the
world’s first 3D printed metal bike frame
, which weighed in 33% lighter
than its brother – the original aluminium model.

Without using topological optimisation techniques for the seat post,
it would not be as light as it is. Titanium alloys are denser than aluminium
alloys, with relative densities of around 4 g/cm
3 and 3 g/cm3 respectively.
But titanium (Ti6Al4V) has more than three times the ultimate tensile strength
(UTS), allowing thinner wall sections to be used.

It was only by significantly altering Chris’ original design,
removing any material not contributing to the overall stability of the part,
that mass was reduced whilst maintaining strength.

Without additive manufacturing, such a design would have been
impossible to produce. Traditional methods would not have been able to handle
the complexities of the topological optimisation features, such as thin walls
and hollow structures, which resulted in a lighter, stronger and more
economical build.

Earlier this year, a nearly-completely 3D printed car had its first drive at the weekend after being created on site at the International Manufacturing Technology Show in Chicago.

[David Ewing is Technical Marketing
Engineer, Renishaw Additive Manufacturing
Products Division
]