Mechanical Engineering for Custom Parts
Mapping the gap between knowing FreeCAD and actually designing functional parts — what mechanical engineering knowledge is needed to go from idea to printed object.
Knowing FreeCAD means knowing how to describe a shape. That’s not the same as knowing what shape to describe. This note maps the gap — what mechanical engineering understanding is actually needed to go from a blank sketch to a part that works.
The Real Workflow
Requirement → Sketch by hand → CAD model → Slicer → Print → Test → Iterate
FreeCAD covers one step. Everything before and after it is mechanical engineering.
Design for 3D Printing (DFM)
3D printing has specific constraints that change how you design:
Print orientation — FFF/FDM printers build layer by layer. Layers bond well horizontally but are weak under tension perpendicular to the layer lines. A part that will take stress should be oriented so the load runs along the layers, not across them.
Overhangs — geometry that extends more than ~45° from vertical needs supports. Supports waste material and leave rough surfaces. Good design avoids or minimizes them by orienting the part or chamfering overhanging faces.
Wall thickness — too thin and walls don’t print cleanly or break easily; too thick and print time balloons. A minimum of 2–3 perimeter lines (typically 0.8–1.2mm) is the floor for structural walls.
Tolerances for fit — printers aren’t precise. A 10mm hole in FreeCAD prints as ~9.7mm. For parts that must mate (shaft in hole, snap fit, bolt clearance), you need to add tolerance — typically 0.2–0.4mm clearance on each mating surface. This has to be learned empirically for your specific printer.
Infill — the internal structure of a solid-looking part. 15–20% infill is fine for visual parts; 40–60% for structural loads. Gyroid infill is isotropic (equal strength in all directions); rectilinear is fast but directional.
Material Selection
| Material | Use case | Notes |
|---|---|---|
| PLA | Prototypes, display parts | Easy to print, brittle, low heat tolerance (~60°C) |
| PETG | Functional parts | Tougher than PLA, slightly flexible, good layer adhesion |
| ABS | Heat-resistant parts | Warps badly, needs enclosure, fumes |
| TPU | Gaskets, grips, flex parts | Flexible, slow to print |
| ASA | Outdoor parts | UV-resistant ABS alternative |
For most first functional parts: PETG is the right default. Tougher than PLA, easier than ABS.
Structural Basics
To design a part that won’t fail under load, you need to think about three things:
Type of load — tension (pulling apart), compression (pushing together), shear (sliding faces), bending (moment applied at a distance), torsion (twisting). FDM parts handle compression best and tension across layers worst.
Cross-section — more material = more strength, but also more weight and print time. For bending loads, an I-beam or U-channel cross-section is much stronger per gram than a solid rectangle — the material at the extremes resists bending, the middle contributes little.
Stress concentrations — sharp internal corners are where cracks start. Add fillets everywhere. The stress at a sharp corner can be 2–3× the nominal stress. FreeCAD’s Fillet tool exists for a reason.
Fasteners and Joints
Clearance holes — a bolt through a part needs a hole slightly larger than the bolt diameter so it passes through freely. M3 bolt → 3.2–3.4mm hole.
Heat-set inserts — threaded brass inserts pressed into plastic with a soldering iron. Far stronger than threading directly into plastic. Standard for any part that will be bolted together more than once.
Snap fits — a cantilevered tab that deflects and snaps into a recess. Elegant but requires understanding the deflection math (beam bending) to get the right thickness and gap. TPU makes forgiving snap fits; PLA makes brittle ones.
Press fits — a shaft slightly larger than the hole, forced in. Interference of 0.1–0.2mm is typical for plastic. The part holds by friction. Doesn’t survive repeated disassembly.
The Iteration Loop
No part is right on the first print. The actual process is:
- Model in FreeCAD
- Export as STL, slice in Cura / PrusaSlicer — check layer preview for problems
- Print a small test piece or just the critical feature (a hole, a snap fit, a mating face) before committing to a full print
- Measure with calipers, check fit, note what needs to change
- Back to FreeCAD — this is why parametric modeling matters, changing a dimension is one number
The FreeCAD feature tree exists precisely for this loop. A model built with named constraints and a coherent feature order survives iteration; one built by pushing geometry around does not.
What to Learn Next
- Beam bending and deflection — to size structural members under load
- GD&T basics — to specify tolerances properly when designing for external fabrication
- Slicer settings in depth — Cura / PrusaSlicer have significant impact on part quality independent of the model
- Calipers and measurement — physical verification of printed parts