FreeCAD Help - Granet JP - Limoges - France - FreeCAD Help _ jpg87
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The workbench used to create assemblies of mechanical parts :                                   

Assembly Design

I chose for the creation of classic assemblies Assembly3 workbench (version 0.11 of 04.12.2021), built by Realthunder, because it was the first to correspond to my working approach :
it allows a relative positioning of objects by contact, embedding, alignment of surfaces, as a fitter does with real objects.
It's up to you to test the other workbenches offered and make your choices.

My personal experience leads me to think that there are at least two different situations depending on our need :

1- The project is a set in which all the parts are completely interconnected :

Solution 1 :
The parts can each be built in a PartDesign Body, the whole being in a single FreeCAD file. In this case, the bodies can be drawn relative to each other using master sketches, expressions, the ShapeBinder tool, ... (see example and list of "How to do").

Solution 2 :
Parts can be built in separate files or no. In this case, an Assembly WorkBench is welcome. It allows to put in relative position the different objects in order to realize the complete product.

Note that I do not voluntarily talk about sketches positions, objects, or others, which assume numerically known the desired positions as well as the dimensions of the objects.
This is not always possible in team work, or even when you use older components in a new project whose details are no longer known (also true for standard elements from component libraries).

2- The project is a mechanism in which certain parts retain the mobility necessary for their work.

It is then composed of classes of kinematic equivalence, that is to say groups of parts completely related to each other. These groups are sub-assemblies that will be made according to the solutions 1 and 2 above.

These kinematic equivalence classes will be collected in a general assembly and the Assembly WorkBench provides the constraints to create the specific kinematic links (joints) necessary for the proper functioning of the mechanism.

This assembly can be used in simulation programs of kinematic or dynamic behavior of technical solutions before proceeding to real tests on prototypes.