Assemblies, moving assemblies, shortcuts and ~ parts

[Philippe Hurbain]

When parts are used in assemblies (provided as such by LEGO, and impossible to separate in normal use) their description begins with ~ so they don't appear in part tree. LDraw user should use assembly shortcuts instead.

It seems that things are not completely clear, as discussions around this come up from time to time on PT. Here is my opinion about this.

• For assemblies without moving parts, it's simple: all parts composing the assembly are ~, and an assembly shortcut is provided (several shortcuts may be needed to account for different color combinations). Example: NXT light sensor 55969 shortcut, composed of ~parts 54732, 55972, 55816, 55970, 55966.
  
• For assemblies with moving parts, a shortcut should be provided for each mechanical entity. This way the LDraw user can compose its own assembly in any position. An overall assembly is provided as well, to simplify user life when default position is OK, and for bill of materials usage. Example: NXT motor has a case assembly 54715c01, and an axle hub assembly 54725c01. Case assembly 54715c01 is made of ~parts 54714, 54715, 54716, 54717 and 54732. Hub assembly 54725c01 is made of two 54725 ~parts. Complete motor assembly 53787 is made of 54715c01 + 54725c01
  
• In case one of the moving assemblies is reduced to a single part (eg. axle hub of PF motors), the description of this part is not tilded, so that the user can pick it easily to create his own position combination. For example, the small linear actuator has 3 sections moving independantly:
  - the body, assembly 92693c01, composed of ~parts 92693 and 92697
  - the rotating axle, single part not tilded 92695.
  - the piston, single part not tilded 92696.
  Two complete assemblies are provided, contracted and extended (92693c02 and 92693c03)