If I understand you correctly, this is how I accomplish that:
1. Each distinct part has its own PartNumber and corresponding Item (number) in the BOM/PartsList.
Note: I made a Custom iProperty called 'MARK' instead of using Inventor's out-of-the-box Item (number).
I believe that is the key to solving your inquiry.
I added that Custom iProperty to the part and assembly templates, so it exists in every part and assembly.
If you don't have it in your assembly or part files yet, then you need to add it to every one of them.
The simplest way to do that is to open teh top level assembly and then open its BOM.
Then add the Custom iProperty and RMB on the column and pick AddToAllParts.
That way when you assign an identification number in the BOM to each part, it will never change unless you manually change it.
Also, the MARK number will will show up in every assembly and sub-assembly as the same number.
2. Each sub-assembly (weldment or assembly) has all the parts that belong to it, and that sub-assembly has it's own PartNumber and corresponding Item (number in BOM/PartsList). Note: I did the same thing with the MARK number for assemblies for the same reason.
3. The top-level assembly has all the sub-assemblies in it, as well as the 'stray' single parts that may or may not also exist in some sub-assemblies (or there may be no single parts at all in the top-level assembly).
4. Then in the drawing, I make a drawing view + PartsList for:
a. the top-level assembly (PartsList = Structured)
b. each sub-assembly (PartsList = PartsOnly)
c. each single part (no PartsList - see 'total parts bom' below)
Each PartsList will 'roll-up' all the parts with and identical PartNumber iProperty, and will also show one Item (number = MARK) for that group in the PartsList.
Your result: A number of assemblies each with one corresponding PartsList, and a number of single part views of all the parts you want to detail. You can show the Item (number = MARK) in the ViewLabel for each assembly and single part.
The key to the 'roll-up' is to make sure that you have the exact same text in the PartNumber field in the BOM.
In order to see all the single parts in a PartsList for the top-level assembly, I do this:
1. Create an empty assembly and Insert the top-level assembly into it, then save it with a new name (I use the same name as the top-level assembly, but add suffix 'total parts bom').
2. Make another view in the drawing of that new assembly (total parts bom), and place a corresponding PartsList for it. Make sure it's a PartsOnly PartsList.
You can do the same thing for a Summary PartsLilst by using Filtering, but that's another explanation that's off point here.
Hopefully I understood your inquiry.
... Chris
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