CAD assembly

A CAD assembly is a 3D model made up of multiple parts and subassemblies whose relative positions are defined by mates (called constraints in some CAD systems). The assembly file usually holds only references to the separate part files and their positions, not the geometry itself. A change to a part therefore shows up in every assembly that uses it.

Every free body has six degrees of freedom in space (three translations and three rotations). Mates such as coincident, concentric, parallel, distance or angle remove these degrees of freedom one by one. A fully constrained assembly cannot move, while degrees of freedom left open on purpose let you simulate the motion of a mechanism, such as a hinge rotating or a piston stroking. Large products are broken down into subassemblies that match the physical assembly units, which keeps teamwork and the bill of materials structure clear.

Two approaches are combined when building an assembly. In bottom-up design, parts are modeled individually and put together afterwards. In top-down design, parts are designed directly in the context of the assembly from a layout sketch or skeleton model, so a change to an installation dimension carries through to the dependent parts. The second approach is faster in concept design, but it creates external references that must be kept under control.

A finished assembly produces the bill of materials, an assembly drawing with item numbers (balloons), an exploded view for assembly instructions, and the total mass and center of gravity. The assembly is also the basis for interference checks and for kinematic and structural simulations.

Build an assembly as soon as the product has more than one part or has to fit into its surroundings (a machine, enclosure or vehicle). Interference and clearance checks in CAD catch errors that would cost far more to fix once the parts are made: a bolt you cannot reach with a wrench, a hole that misses its mating part, or a cover that will not go on.

An assembly also pays off for smaller fixtures and devices, because it gives you the bill of materials for purchasing and an overview of bought-in components (bearings, drives, fasteners). For moving mechanisms, it lets you verify the range of motion and check for collisions at the end positions.

A common problem is over-defined or conflicting mates, and mates attached to unstable geometry (fillet edges, temporary faces) that break when the part changes. Attach mates to reference planes, axes and functional faces instead. Interference checks in CAD also work only with nominal dimensions: zero interference does not mean the assembly will go together under the worst combination of tolerances. That is what tolerance analysis is for.

When handing over data, send the assembly with all referenced part files, or it will not open at the other end. Exporting to STEP preserves part positions and the assembly structure but not the mates, so the recipient cannot edit it as easily as in the native format. For large assemblies, agree on simplified representations and file naming rules, so that two different parts with the same name never end up in one project.