Open-Source CAD software How to Design with FreeCAD—A Practical Example

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Workflow for designing a mechanical part and preparing for manufacturing: Using the example of an end plate of a PEM fuel cell, we show how the design of a component works in the open-source software Freecad, what the advantages are, and how challenges can be resolved.

End plates of PEM fuel cells are part of current research and development and serve in this contribution as a practical example for the construction of components in the open-source software FreeCAD. This example is based on the publication Value chain fuel cell by NOW GmbH, NC Invention Center, Umlaut SE and Fraunhofer IPT.

Freecad is an open-source software that is becoming increasingly popular. Here, our author presents a common workflow for designing a component

FreeCAD combines CAD, CAE, and CAM and is thus used for 3D detailed design with derived technical drawings, the simulation of mechanical loads, and the generation of toolpaths for manufacturing. As a free and open-source alternative to other CAx systems, it is increasingly used in mechanical engineering companies, especially in Germany, Austria, and Switzerland.

Here's how to work with Freecad

In the presented project, a 3D CAD model of the end plate of a PEM fuel cell is first constructed in the FreeCAD work area part design. This 3D CAD model is then connected with a table from the spreadsheet workspace. Then, a manufacturing drawing of the 3D CAD model of the end plate is derived in the Techdraw workspace. Subsequently, the mechanical loads of the end plate during assembly to the whole PEM fuel cell are simulated in the FEM workspace. Finally, the toolpaths for a CNC milling machine are generated with the CAM workspace.

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Parametric design of the component (CAD)

The 3D CAD model forms the basis for the entire workflow. This is created in the Part Design workspace. A sketch and feature-based design process is used to gradually build the 3D CAD model, similar to conventional 3D CAD systems. The workflow is simplified and described in three steps below:

1. Choose an existing plane or create a new one.
2. Create a sketch of the cross section of the 3D CAD model on the selected plane.
3. Create a 3D feature, e.g. padding or rotation body (known as extrusion and revolution, each in additive and subtractive variant).

All features that are based on topology elements of the 3D CAD model are created at the end. These features include chamfers or fillets among others. In general, the number of features referencing topology elements should be minimized to obtain a stable 3D CAD model that can flexibly be modified in terms of dimensions.

In Freecad, calculations can be performed using integrated tables. The cells in the tables can be referenced in sketches, features, and other places in Freecad. This allows designers to also carry out the preliminary design of individual components and assemblies in Freecad.

Deriving the manufacturing drawing (CAD)

Based on the 3D CAD model of the end plate, the technical drawing for production is then automatically derived.

• For this, a new, blank sheet is added to the Freecad document from the sheet templates. Then the 3D CAD model is rotated into the main view and multiple projections, with the option to select the scale and projection directions, are placed on the sheet.
• Then, guidelines, dimensions with tolerances and fits, information on edges and surfaces, and other elements of technical drawings are added.
• Finally, the title block is filled out with information about the material, general tolerances, document management information, and company information.

The drawing derivation in Freecad therefore follows a similar process to other CAD systems. Consequently, designers basically don't have to learn new working methods.

The technical drawing is important for manufacturing because it is the official document that can also be referenced in contracts. A 3D CAD model alone is not decisive for manufacturing because it lacks important information.

Strength simulation with control over mesh fineness (CAE)

The parametrically created 3D CAD model of the end plate is now used to simulate the mechanical stress in the assembled state as a whole PEM fuel cell. The simulation is carried out using the finite element method in the FEM work area, which allows for thermal and electromagnetic boundary conditions in addition to the usual mechanical ones.

For the FEM model, a copy of the 3D CAD model is created, which allows additional geometries to be added that are irrelevant for a manufacturing drawing and CAM operations. These additional geometries provide greater control over local mesh refinement. They also allow a more precise definition of the component clamping and external load as a single force or surface load.

Generating the tool paths for manufacturing (CAM)

For productive manufacturing of the end plate using a CNC milling machine, tool paths are generated in the Freecad CAM workspace (formerly called Path). These tool paths are then exported as G-code and can be used directly on the 3-axis CNC milling machine for the production of the end plate.

In the CAM workspace, a tool library is created as a digital twin. The tool library represents the tools actually used in the machine tool. The tools from the library are used to simulate the travel paths.

The end plate can be manufactured in a 3-axis CNC milling machine with only two clamping operations. For this, the Freecad CAM operations Surface, Pocket, Profiles, Drilling and Helix are used.

PEM Fuel Cell Project in Freecad

Using the Freecad software, a comprehensive model (Digital Twin) of the end plate of a PEM fuel cell was developed. PEM fuel cells are used in e-mobility alongside battery-powered vehicles. Since end plates currently still offer a lot of development potential, the weight of the end plates, and thus of the entire vehicles in which the fuel cell is used, can be reduced, making the vehicles more energy efficient. In the next step, the weight of this end plate can be optimized in Freecad by using lightweight construction methods.

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Challenges when using Freecad

The project presented here of the end plate of a PEM fuel cell shows the application of Freecad using a concrete example. Considering that the software is free to use, there are a few challenges for productive usage:

• Support reliability: Freecad is maintained by a community of volunteers. Support for open-source software like Freecad is usually not always reliable and timely available. That is why Alsado has committed itself to providing long-term Freecad support in the German-speaking region, so that users can use the software reliably and efficiently.
• Documentation: Freecad has an extensive online manual in the form of a wiki page. The quality of the documentation can vary greatly because it is maintained by volunteers. In some cases, the wiki can be incomplete, outdated or non-existent. Alsado helps to close gaps in the documentation and bring explanations up to date or clarify them.
• Updates: Freecad is constantly being further developed to add new functions and improve usability. Alsado helps to keep users informed about changes and to keep the program version up to date.
• Learning process: Freecad is a 3D CAD software with many specialized functions for niche applications. The abundance of functions can be overwhelming for some users. As a result, the learning process can involve a significant time investment and a high degree of engagement. Alsado helps to learn the necessary functions and work methods quickly and safely so that productive work can be done without detours.

Those who master these challenges can look forward to saving on license costs, independence through open file formats, respect from well-trained designers, and endless expandability of the software to optimize their own design processes.