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FreeCAD Assembly workbench

Summary

The FreeCAD Assembly workbench (Assembly 4 in FreeCAD 0.21; integrated Assembly in FreeCAD 1.0+) assembles separately modelled parts into a single document for fit verification, clearance checking, and export. In the libdrone workflow, assembly serves one function: confirming that printed parts fit together before committing to a full production run. Key checks are box-in-box rod clearance, backplane corner post alignment, and GPS mast vertical clearance above the prop plane. Assembly does not replace physical coupon testing but catches modelling errors that coupons cannot.

Concept

Assembly as verification, not design

In the libdrone parametric workflow, parts are designed individually in the Part Design workbench, driven by the central spreadsheet variables. The assembly is the proof that the variables are internally consistent — that the rod channel positions in the arm match the rod channel positions in the body, that the GX12 chimney height clears the backplane boss, that the GPS mast does not intersect the prop arc.

This is different from an assembly-first workflow where geometry is created in context of other parts. libdrone parts are fully defined by their own sketches and the spreadsheet — the assembly is the final check, not the origin of dimensions.

Joint types in FreeCAD Assembly

FreeCAD's Assembly workbench constrains parts relative to each other using joints. The relevant joint types for libdrone:

Fixed joint: eliminates all 6 DOFs between two parts. Used to place the body at the origin — one part in every assembly must be fixed as the reference.

Cylindrical joint: constrains 4 DOFs, leaving rotation and translation along a single axis free. Used to represent the CF rod in its channel — the rod can slide along its axis and rotate, but cannot translate radially. This correctly models the pre-tensioned rod fit.

Planar joint: constrains 3 DOFs (one translation + two rotations), leaving translation in a plane free. Used for the backplane resting on the attachment posts — it can slide in the XY plane until the bolt holes align.

After joints are applied, parts that are correctly constrained should snap to their geometric position without residual DOFs. A part that "floats" after jointing has unconstrained DOFs — check which joint is missing.

What to check in the libdrone assembly

Rod clearance (box-in-box): The CF rods must clear the electronics stack inside the body. In top view, verify the rod paths do not intersect any component footprint. The rods run diagonally through the body corners; the clearance to the FC stack edges is typically 1–2 mm and is the tightest tolerance in the assembly.

Backplane corner post engagement: The backplane's boss rings must seat fully on the platform's chimney posts. Check that the boss ring inner diameter matches the chimney post outer diameter and that the seating depth is correct.

GPS mast height: In side view, verify the GPS mast top (compass position) is above the prop plane by the design margin (≥10 mm above prop tip arc).

Reference

FreeCAD version note: In FreeCAD 1.0+, the Assembly workbench is integrated into the main application. In FreeCAD 0.21, install the Assembly 4 add-on via the Add-on Manager. The joint names differ slightly between versions but the concepts are the same.

Assembly file organisation: libdrone_V245.FCStd └── Assembly ├── body_top.FCStd (linked) ├── body_bottom.FCStd (linked) ├── arm_×4.FCStd (linked) ├── platform.FCStd (linked) ├── backplane.FCStd (linked) └── gps_mast.FCStd (linked)

Use linked parts (File → Link) rather than copying geometry into the assembly document. Linked parts update automatically when the source file changes — a dimension change in arm.FCStd propagates to the assembly on next open.

Procedure

Create the libdrone assembly

  1. Open a new FreeCAD document. Switch to Assembly workbench.
  2. Insert the body bottom as the fixed reference part: Insert → Link → select body_bottom.FCStd. Apply a Fixed joint to the world origin.
  3. Insert the arm: Insert → Link → select arm.FCStd. Apply a Cylindrical joint between the arm's rod channel axis and the body's rod channel axis. Repeat for all four arms.
  4. Insert the body top. Apply a Planar joint between the body top's sandwich surface and the body bottom's mating surface.
  5. Insert the platform. Apply appropriate joints to the attachment post geometry.
  6. Insert the backplane. Apply joints to the chimney posts.
  7. Run the solver (Solve in the Assembly toolbar). All parts should snap to their constrained positions.
  8. Perform clearance checks: toggle visibility of individual parts to inspect rod paths, backplane engagement, and mast height.

Rationale

The assembly step was added to the libdrone workflow after two production runs where modelling errors (a rod channel 0.3 mm off-position, a chimney post 0.5 mm too short) were not detected until physical assembly. Both errors required reprinting the affected parts. Assembly verification in FreeCAD costs 30 minutes; reprinting a body panel costs 4–6 hours. The assembly step is mandatory before any production run of more than two units.

Connections

requires: - freecad-document-setup - freecad-workbenches - prep-and-parametrics related: - topological-naming-problem - arm-shaft - coupon-validation - freecad-parametric-scaling - stl-export-and-slicer-setup leads_to: - coupon-validation - stl-export-and-slicer-setup