SolveSpace

FreeCAD is powerful, but its installer approaches 500 MB, startup takes time, and the interface shuffles between workbenches. If all you want is an accurate bracket, spacer or enclosure for something, that heavyweight toolchain is disproportionate.

SolveSpace is the other extreme: a few megabytes that opens on a double-click — while doing genuine parametric constraint modeling rather than being a toy. 4,096 stars, GPL-3.0.

Its core is a constraint solver. Sketching does not require drawing accurately; you state constraints instead — these two lines are parallel, this circle is tangent to that edge, this length is 25 mm, this angle is 30 degrees — and the solver computes the unique shape satisfying all of them. Change one number and the whole model re-solves. That is what parametric modeling means.

What it does:

• 3D solid modeling: extrude, revolve and loft from sketches, with boolean operations.
• Assemblies: constrain multiple parts together and check interference.
• Mechanism motion analysis — a distinctive strength. Simulate linkage movement to see whether something binds or whether the travel is sufficient. Genuinely useful for mechanical design and for 3D-printed parts with moving components.
• Degree-of-freedom analysis telling you how many constraints a planar mechanism still needs, which helps enormously in understanding constraint systems.
• Export: STL and 3MF for slicers, DXF and PDF for 2D drawings, STEP for other CAD packages.

To be clear about:

• It is a small tool, not a FreeCAD replacement. No technical drawing workbench, no finite element analysis, no CAM, no BIM. Complex multi-part projects and formal engineering drawings belong in FreeCAD.
• The interface is minimal to the point of being spartan. There are no elaborate toolbars and much depends on keyboard shortcuts, so a first-time user needs to follow the official tutorial or will not know where to begin. Fortunately it is short — an hour or two suffices.
• It is wrong for organic surfaces. It excels at disciplined mechanical parts and struggles with flowing enclosures or artistic shapes, which are Blender's territory.
• Releases are unhurried (currently v3.2, March 2026); the project is mature and stable rather than rapidly iterating.

Who it suits: 3D printing enthusiasts making functional parts, anyone designing and validating mechanisms, and anyone who wants "change a number and the model follows" without installing 500 MB. It does not conflict with FreeCAD — plenty of people use this for small parts and FreeCAD for large projects.