3D printing: from virtual to real

Source: RTM Aug/Sept 2018

Like so many other technological advancements, 3D printing has moved from being the stuff of science fiction, through niche industrial application, to increasingly becoming a common industry tool and even a reality in the home. David Shipman, innovations engineering manager in Infrastructure Projects Signalling at Network Rail, explores how 3D printing has helped to tackle problems on the railway infrastructure.

Axle counters have become well established as the preferred method of train detection, offering a number of advantages over the various track circuit types. A key component is the detection head, which fits either side of the rail and generates a magnetic field through which a train’s wheel passes, disturbing the field and allowing both a wheel count and direction to be registered.

Naturally, the position of the detection head on the rail is a fine balance between being high enough for the magnetic field to align with the wheels of passing trains, but low enough that it is physically clear of the wheels as they pass. Therefore, there is a narrow range within which the height of the detection head must be set.

As well as the passing of passenger and freight trains, track maintenance activities see the regular passing of the fleet of rail grinding trains that maintain the integrity of the rail head and prevent the development of cracks that could, if left, have catastrophic results. The grinding train has a set of variously-angled grinding wheels which trim the rail profile. These further constrain the available space in which the axle counter detection head can fit.

Despite being fitted within specified tolerances, a small but not insignificant number of instances have occurred where the detection heads were getting struck by the grinding trains, causing damage and subsequent delays to the following day’s services. So, teams from various parts of Network Rail, including the Signalling Innovations Group (SIG), came together to try to understand the causes of these axle counter ‘strikes’ and establish a way to resolve them.

At this point the SIG team started to generate computer models of the region, relative to the rail, that the grinding trains could impact – and the space that was therefore safe for the axle counter detection head to be fitted into. This is where 3D printing comes in: these virtual models can be turned into tangible reality by producing a ‘printed’ gauge that fits around the contour of the rail head and physically occupies the space in which the detection head is at risk. Placed over a rail at the detection head, this simple-looking gauge will sit comfortably if the detection head is outside of the “exclusion zone,” and if not, then remedial action can be taken before a grinding train proves the point in a more destructive manner!

Despite the apparent simplicity of the finished gauge, a complex series of calculations and refinements were required to ensure the rail profile was fitted to correctly and that the limits of the grinding wheels were correctly modelled. Using the in-house 3D printer meant that these refinements could be rapidly prototyped and tested, and a small number of the final prototype rolled out for testing out in the field.

The result of this testing showed that even where the specification of the detection head positioning was correct, a slight flaw in the method of measuring the maximum height led to the detection head sitting higher than the specification allowed. This explained why “correctly” fitted detection heads could on occasion still be struck by the grinder.

Following successful trials, the gauge is now in a position to be manufactured on a production facility capable of producing a larger number of these to a fine tolerance, enabling installers and maintainers around the country to have access to the most accurate method of positioning an item of infrastructure that is critical to the safety of railway operation, while enabling the ongoing maintenance of the infrastructure on which the trains run.


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