Community workshop – The Owl’s Nest

A sharp steel edge and a bundle of cables

At Paul Krugerstraat, several cables had to pass through a thick steel I-beam forming part of the ceiling structure. Mains power, network and KNX cables all pass through the same opening. The hole itself was large enough, but without protection the cable jackets would run directly against the sharp edge of the steel.

A cable grommet was therefore needed that not only fitted the opening but also accommodated the thickness of the steel section. We did not have a suitable standard grommet for this situation, making it a useful small custom problem for the 3D printer.

Holding itself in place without screws

The grommet had to remain securely in the several-millimetre-thick steel without requiring additional holes or fasteners. The model therefore has ribs around its outside that can move slightly inwards while the part is being inserted.

As the grommet is pushed into the opening, these ribs temporarily deflect inwards. Once the part is fully seated, they can move outwards again behind the steel edge. The grommet then holds itself in place while keeping plastic between the cable bundle and the steel.

This mechanical detail is exactly the kind of situation in which designing a part ourselves can be useful. The outside dimensions, fit and thickness of the material into which the part is fitted all interact. A difference of only a few millimetres can determine whether a part fails to fit, sits loosely or clips into place correctly.

Measure first, then test one layer

Much of the time involved in 3D printing does not have to be spent printing. Accurate measurement before modelling can avoid a succession of complete prototypes. Once the model had been created, we therefore printed only a single layer first to check the dimensions and geometry.

That test was sufficient. The measurements proved correct and there was no need for a series of full prototypes before making the final part. This is the approach we generally try to use for this kind of component: measure accurately, carry out a quick and simple check, and only then print the complete object.

From design to a part in actual use

After printing, the cable grommet was installed in the I-beam at Paul Krugerstraat. Its ribs retained the part in the opening, allowing the cable bundle to pass through plastic rather than directly across the cut steel edge.

The grommet was put into actual service and remains in use. The project therefore did not end with a model or a successful print: the design became part of the infrastructure for which it had been created.

Designing with free software

We used FreeCAD for the design. It is free and open-source software that allows us to create the model on our own computers and retain the source files ourselves. Confidentiality may not be particularly important for a cable grommet, but the same way of working also applies to designs where ownership, long-term availability or sensitive information matters much more.

For ASK-Solutions, that choice is also part of the purpose of the work. We want to make knowledge and practical capabilities as accessible as possible and encourage the use of free and open-source software. That means not only using end products created with free software, but also showing that tools such as FreeCAD are useful for real design work: from a problem in a workshop to a component ultimately fitted into steel.

Because the original FreeCAD file remains available, someone who wants to reuse the design later does not have to start from an STL file in which much of the design logic has been lost. The model itself can be studied, modified and developed further.

A design intended to be adapted

The cable grommet was built as a parametric model. Different dimensions in the model are therefore related and can be changed when another opening or steel section requires different measurements. The original design does not have to be rebuilt from scratch.

Exactly which values were exposed as separate parameters in this first version is no longer fully documented. The principle was nevertheless deliberate from the start: rather than making only one part for one hole, the aim was to create a model that could serve as a starting point for other dimensions and applications.

This is also why the source file was shared. We are sharing not only the final printable object, but also some of the design work behind it. Someone else can study the model, change dimensions and make their own variation.

From a local problem to a shared solution

The design and print files for the cable grommet are available on Printables and GrabCAD. Anyone adapting the model for another situation should of course check the measurements, fit and suitability themselves and generate print instructions for their own printer and material. G-code should not simply be copied from another printer without checking it.

We used a similar way of working for the mounting plate of a temporary doorbell. That project also began with a specific existing situation that needed a fitting component, and we made the FreeCAD design available there as well.

Our video from 19 December 2023 shows the design, printing process and result. More about the practical possibilities of different printing techniques can be found under 3D printing.