What is an assembly ?

In a modeler, an "assembly" represents a grouping of interrelated parts, that remains or not relative movements between these pieces.

A component of an assembly can be :

• - a single piece (in the sense of "mechanical part"), 
• - a kinematic equivalence class (kinematically linked subset),
• - a library element (hardware ...).

Note :

It is possible to create a part directly in the assembly, so as to build it in its correct relative position. In this case, there may or may not be links between parts.
 

Construct by kinematic equivalence classes

A kinematic equivalence class is a set of parts integral with each other, or animated with the same movement with respect to a given referential (same nature of movement, same direction or axis, same speed, same acceleration, ...).

1- Construct each kinematic equivalence class

Each kinematic equivalence class is itself an assembly (which can sometimes be reduced to a single part).
The principle of creation or construction of an assembly in 3D can be defined by a repetitive or sequential procedure shown  by the graph on the following page.

Primordial step, the choice of the reference element :

The reference element of your assembly (or under assembly) will be immobilized; it is therefore the other elements of the assembly that will be put in place with respect to this one (whether they remain mobile or not).

2- Build the main assembly

The same procedure as above applies for the construction of the main assembly.
We must choose the kinematic equivalence class of reference, then fix it, and finally position (constrain) the other classes of kinematic equivalence in order to be able to simulate the complete system.