Mechanical Integration with FreeCAD

• The board outline usually belongs to mechanical, not electricalIn any product with an enclosure, the board shape, mounting hole positions and connector positions are determined by the mechanical design. Drawing them freehand in the PCB tool and hoping is how boards arrive that do not fit. The correct flow is mechanical-defines-outline, electrical-imports-it.
• STEP is the interchange formatSTEP (ISO 10303, AP214/AP242) carries solid geometry between CAD systems and is supported by KiCad and FreeCAD, both open source. DXF carries 2D outlines and is the usual format for the board outline itself. Both are open standards, which is why this whole workflow can be done without proprietary tools.
• Datums and originsBoth models must agree on where the origin is and which way is up. Establish a single datum — usually a mounting hole or a board corner — and use it in both tools. Most fit failures are not geometry errors but origin errors.
• Component height and keep-out volumesEvery component has a height, and the enclosure has an internal clear volume. Tall parts (electrolytics, connectors, crystals, shields) need explicit keep-out volumes in the mechanical model. Check the worst case: a connector with its mating cable attached is much taller than the connector alone.
• Connector accessibility and panel cut-outsA connector must not only fit; it must be reachable through a cut-out with the mating connector's shell, latch and strain relief. Cut-out dimensions come from the mating connector, not the board-mounted one, and tolerance stack-up between the board position, the mounting hole and the panel matters.
• Tolerance stack-upBoard outline tolerance (typically +/-0.15 mm), hole position tolerance, standoff tolerance, enclosure tolerance and connector position tolerance all accumulate. Add them in the worst-case direction and check the result still fits. A design that fits at nominal and fails at worst case will fail on some fraction of units.
• Thermal and airflow in the enclosureThe mechanical model is where you decide whether heat gets out. Vent placement, component orientation relative to convection, and the thermal path from a hot part to the case are mechanical decisions with electrical consequences, and they must be made before the board is laid out.
• Drawings still matterA 3D model is not a manufacturing instruction. A dimensioned 2D drawing with tolerances, datums and notes is what a machine shop or a 3D print service works from. FreeCAD's TechDraw workbench produces these from the model.

ProblemA board must sit on four M2 standoffs inside a 3D-printed enclosure, with a USB-C receptacle accessible through a side panel cut-out. Determine the outline, hole positions and cut-out dimensions.

1. 1
   Start in FreeCAD, not KiCad. Model the enclosure interior first: internal cavity dimensions, wall thickness, and the standoff positions.
2. 2
   Choose the datum: the front-left internal corner of the cavity at the standoff seating plane. Every dimension in both tools references this point.
3. 3
   Board outline: cavity internal size minus clearance. For a 3D-printed enclosure with typical FDM tolerance of +/-0.3 mm, allow 0.5 mm clearance per side. A 51 x 41 mm cavity therefore takes a 50 x 40 mm board.
4. 4
   Mounting holes: M2 clearance hole is 2.2 mm diameter. Position them 3.5 mm in from each corner so the M2 screw head (typically 3.8 mm across) and washer clear the board edge and any components.
5. 5
   Add a keep-out annulus around each hole in the mechanical model — 4 mm diameter is a reasonable default — and carry this into KiCad as a courtyard-level keep-out so no component is placed under a screw head.
6. 6
   USB-C receptacle: find its datasheet's recommended panel cut-out. A typical vertical mid-mount or through-hole receptacle specifies a cut-out around 9.2 x 3.4 mm with radii, positioned relative to the board surface.
7. 7
   Now the tolerance stack for the cut-out. Contributions: board outline tolerance +/-0.15 mm, board position within the cavity +/-0.5 mm (the clearance you allowed), connector placement tolerance on the board +/-0.1 mm (assembly), connector's own dimensional tolerance +/-0.1 mm, enclosure print tolerance +/-0.3 mm.
8. 8
   Worst case sum: 0.15 + 0.5 + 0.1 + 0.1 + 0.3 = 1.15 mm in each direction. So the cut-out must be oversized by at least 1.15 mm per side relative to the connector body: 9.2 + 2.3 = 11.5 mm wide, 3.4 + 2.3 = 5.7 mm tall.
9. 9
   That is a visibly loose cut-out. If cosmetics matter, reduce the stack: locate the board with dowel pins rather than clearance holes (removes the 0.5 mm), tighten the print tolerance, or make the cut-out part of a separate press-fit bezel that references the connector directly.
10. 10
    Export the board outline and mounting holes from FreeCAD as a DXF, referenced to the chosen datum.
11. 11
    In KiCad, File > Import > Graphics, import the DXF onto Edge.Cuts. Verify the imported size with the measurement tool — DXF unit errors (mm versus inches) are common and produce a board ten or twenty-five times the wrong size, which is obvious, or subtly wrong, which is not.
12. 12
    Place the mounting holes at the imported positions and place the USB-C connector at its specified position relative to the datum.
13. 13
    Lay out the board. Then export it: File > Export > STEP, with the origin set to the same datum.
14. 14
    Back in FreeCAD, import the board STEP into the enclosure assembly. Now check: does every component clear the lid? Does the USB-C receptacle align with the cut-out? Do the screw heads clear the components? Use FreeCAD's measurement tools to find the minimum clearance and record it.
15. 15
    Iterate. Expect at least two rounds. Then produce a TechDraw drawing of the board outline with dimensions and tolerances for the record.

AnswerMechanical defines the outline; KiCad imports it as DXF and exports the populated board as STEP for checking. The USB-C cut-out must be 11.5 x 5.7 mm to survive a 1.15 mm worst-case tolerance stack, which is loose enough that a dowel-pin location scheme or a separate bezel is worth considering if appearance matters.

Tool: FreeCAD with KiCad STEP export and DXF import

• Install FreeCAD (LGPL, free and open source, all platforms).
• In FreeCAD, use the Part Design workbench. Create a body, sketch the enclosure's internal cavity on the XY plane, and pad it to the internal height. Then shell it or model the walls explicitly.
• Add standoffs: sketch four circles at your chosen positions and pad them to the standoff height. Add M2 tapped or self-tapping holes.
• Create the board outline: on the standoff seating plane, sketch the board rectangle and the four mounting holes. Keep this sketch clean and fully constrained — it is the master geometry.
• Export the outline: select the sketch, File > Export, choose DXF (or use the Draft workbench's DXF export for better control). Note the units.
• In KiCad's PCB editor: File > Import > Graphics. Select the DXF, set the layer to Edge.Cuts, set the units to match the export, and set the placement origin.
• Verify immediately: use the measurement tool to check the imported outline's width and height against the intended dimensions. Do not skip this.
• Place mounting holes as footprints (MountingHole_2.2mm_M2 or similar) at the imported hole positions, and add keep-out zones around them.
• Complete the layout. Ensure every footprint has a 3D model attached (this is why the library skill mattered).
• Export the board: File > Export > STEP. In the dialog, choose the origin (drill/place origin, or user-defined) and ensure it matches your FreeCAD datum. Enable 'Substitute similar models' off if you want exactly your models.
• In FreeCAD, File > Import the board STEP into the enclosure document. Position it on the standoffs using the Assembly workbench, or place it by absolute coordinates if your datums align.
• Interference check: in the Part workbench, use Part > Boolean > Intersection between the board assembly and the enclosure solid. A non-empty result is an interference. Alternatively use the Assembly workbench's collision detection.
• Measure clearances: use the measurement tools to find the minimum distance between the tallest component and the lid, and between screw heads and nearby components. Record the numbers.
• Produce a drawing: switch to the TechDraw workbench, create a page, insert a view of the board outline, add dimensions with the dimensioning tools, add a title block, and add tolerance notes. Export to PDF.
• Iterate: change the board, re-export, re-import, re-check. Establish that this loop takes you five minutes, because you will run it many times.

• FreeCAD documentation and tutorialsopen licencedoc~180 minFree and open (the wiki is CC BY 3.0). Work through the Part Design and TechDraw tutorials. The Assembly workbench documentation is worth reading for the interference checking.
• KiCad documentation - 3D models and STEP exportopen licencedoc~45 minRead the 3D viewer and export sections, including how the origin is chosen and how 3D model paths are resolved.
• FreeCAD KicadStepUp workbenchopen licencetool~60 minAn open-source FreeCAD add-on purpose-built for the KiCad round trip: sync board outlines, constraint areas and 3D models between the two tools. Install it from FreeCAD's Addon Manager.
• Connector datasheets with recommended panel cut-outsfreedatasheet~30 minFind the recommended PCB footprint and panel cut-out drawings for the connectors you actually use. These drawings are the input to your mechanical model.

Take a board you have designed and integrate it into an enclosure you model yourself, completing the full round trip and producing both a verified 3D assembly and a dimensioned drawing.

Deliverables

• A FreeCAD enclosure model with an internal cavity, standoffs, at least two panel cut-outs for real connectors, and any vents or features required, with a clearly stated datum.
• A board outline and mounting hole sketch in FreeCAD, exported as DXF and imported into KiCad on Edge.Cuts, with evidence (a measurement screenshot) that the imported dimensions are correct.
• A KiCad board with the imported outline, mounting holes with keep-out zones, and 3D models attached to every footprint.
• A STEP export of the populated board imported back into the FreeCAD assembly, with a screenshot of the assembled model from at least three angles.
• An interference report: a boolean or collision check with the result stated, plus measured minimum clearances between the tallest component and the lid, between screw heads and components, and between every connector and its cut-out.
• A tolerance stack-up analysis for one connector cut-out: every contributing tolerance listed with its source, the worst-case sum, and the resulting cut-out dimensions. If the result is unacceptably loose, a proposed change with the new stack computed.
• At least two documented iterations: the interference or fit problem found in round one, the change made, and the verified result in round two.
• A TechDraw dimensioned drawing of the board outline with mounting hole positions, tolerances and a title block, exported to PDF.
• A thermal and airflow note: where the heat-generating parts sit relative to any vents, and whether the enclosure's internal ambient assumption used in your thermal calculations is still credible.

Acceptance criteria — how you know it is good enough

• A single datum is defined and used consistently in both tools.
• The DXF import is verified dimensionally, not assumed.
• Every footprint has a 3D model so the STEP export is meaningful.
• The interference check is performed with a tool, not by eye, and the result is stated.
• The tolerance stack lists every contributor with its source and sums in the worst case.
• At least one real fit problem was found and resolved across iterations.
• The drawing carries dimensions with tolerances and a datum reference.

If you want to push further

• Install the KicadStepUp workbench and redo the round trip using it, comparing the workflow to the manual DXF/STEP process.
• 3D print the enclosure and physically fit a board (or a printed board mock-up) into it, and report where the model and reality disagreed.

• I let mechanical CAD define the board outline and import it rather than drawing it freehand.
• I use a single datum consistently across electrical and mechanical tools.
• I verify DXF imports dimensionally before relying on them.
• I check interference with a tool, not by looking at a render.
• I compute worst-case tolerance stacks for panel cut-outs.
• I produce dimensioned drawings with tolerances, not just 3D models.
• I can complete a KiCad/FreeCAD round trip in minutes and iterate on fit.