Build123d Extension Libraries

Use this skill when a standard mechanical part should come from a pre-installed library instead of being modeled from scratch.

Hard Constraint: Printed Screw Joints

Any screw fastening into a 3D-printed part is produced by the printed screw joint kit — from screwjoint import make_screw_joint_v1. This applies even when the task reads as "add a screw boss to the enclosure" rather than "add a fastener."

• Do not hand-model a screw hole, boss thread, or lead-in.
• Do not default to a heat-set insert. It is opt-in only — for high-cycle disassembly or torque a formed plastic thread can't hold — and is not yet implemented in the kit (strategy="heat_set" raises ScrewJointNotApplicableV1); confirm insert dimensions before modeling it by hand.
• A ScrewJointNotApplicableV1 rejection is the only valid reason to model a joint by hand. Classify that connection free and say so.

Core Workflow

1. Read references/decision-guide.md to choose the library and integration pattern.
2. Read the specific library reference only when parameters or API details are needed.
3. Use library parts for standard fasteners, bearings, V-slot rails, T-nuts, wheels, and plates.
4. In CAD Workbench, publish ordinary Build123d shapes. Use export_step only in standalone generation scripts.
5. For repeated engineering connections, check the kit catalog before assembling primitives. Use a kit only when its documented topology and limits match exactly — for printed screw joints this is non-negotiable (see Hard Constraint above).
6. Treat kit and primitive recipes as optional, copy-owned source. Copy the selected recipe into the project cell so its exact logic is build-bound; if it rejects the connection or the design needs another method, classify that connection as free and model it directly.
7. Keep a project-local connection inventory and assert that every intended connection is handled by a named kit or by explicit freeform source.

For V-slot assemblies, also read references/vslot-assembly.md before placing rails, plates, T-nuts, or screws.

Reference Index