In the last project (Sterling Helicopter) all of the parts were created and assembled in one file. Although convenient in many respects it also gets to be quite cumbersome when the number of parts exceeds about twenty or so since the tree is longer than the screen is tall. An alternative to this is to divide the parts up into sub-assemblies in their own file and then assemble them into a top-assembly file. Fortunately, Assembly4 accommodates this quite nicely while still allowing design to be done in-context. You can work either in the top-assembly or sub-assembly files.
The work process starts with creating the top-assembly and sub-assembly files using "Create a new Assembly4 model" for each and then assembling the sub-assembly in the top-assembly Model using "Insert a link to a part" before creating any body or feature. Each of the sub-assembly files have to be saved and opened at this point. In this manner, you can begin modelling in either the sub-assembly files or the top-assembly file links in-context as the assembly design evolves. I use the default LCS in the Assembly4 body to locate it in the sub-assembly. That way you can either work on the body in the Part folder of the sub-assembly at the global axis or in-context in the sub-assembly link. Fantastic!
All the parts in the assembly shown in the first image can be found in the restoration video: https://www.youtube.com/watch?v=e5vCJz3mK6w
One convenient aspect of putting all of the rotating parts into one of the sub-assembly pays off if you want to animate the moving parts to check for proper operation and clearances as shown in the above video. Variables for the cutting reel and the wheel sub-assemblies were defined in the top-assembly Model. The independent variable is the rotation angle of the cutting reel about its axis and the dependent variable is the wheel rotation which is 12/56 of the reel rotation. You can probably argue that the wheel angle should be the independent variable and the reel, dependent, since the wheel turns the reel not vice versa.
The attachment offset angle of the reel sub-assembly link about its X-axis is defined with an Expression and the variable rotation_angle. The LCS_1_wheel attachment offset angle was used to animate the wheel sub-assembly with an Expression and the dependent variable ring_gear.
I will continue to work on the bodies for the frame sub-assemblies and report progress in additional posts if there seems to be continuing interest. Hopefully, by now you can already see how the use of sub-assembly files simplifies the top-assembly and adds more flexibility to the in-context modelling process and animation.
OS: Windows 10 (10.0)
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.19.22894 (Git)
Build type: Release
Python version: 3.6.8
Qt version: 5.12.1
Coin version: 4.0.0a
OCC version: 7.3.0
Locale: English/United States (en_US)