Your assembly structure is your BOM structure: build it deliberately

Two design engineers, the same job: a conveyor of roughly two hundred parts. The machines they deliver are nearly identical, the BOMs are anything but. With one, purchasing sees six orderable build groups with the details underneath; with the other, two hundred loose lines with no order and no build sequence to be found. Same machine, same parts, different tree.

The difference is not in a BOM setting, but in the browser. Inventor does not invent a list: it reads your assembly out loud. Your assembly structure is your BOM structure: every subassembly becomes a line with its own sub-list, every level in the tree a level on the list. Once you see that, you stop building the tree for your own convenience and start building it for everyone who has to buy and build from it. In this article: guidelines to make the structure match how things are built and bought, and how a toolbox like Thundercad then gets the list to its recipient without copy and paste work.

The browser is not private territory

While modelling, the tree feels like a personal workspace: a subassembly because it makes mirroring a pattern easier, a group called "misc" for whatever has no home yet. Harmless, until the BOM has to go out. Because that list is not a separate document you tidy up afterwards: it is a printout of that same tree. Purchasing, work preparation and assembly look straight at your modelling decisions through the BOM.

If you want a different list, the model has to change. The view you pick does not fundamentally alter that: structured, parts only and first-level sort and count the same structure, each in their own way. Which view answers which question is covered in Parts only, structured or first-level: which BOM view do you use when?; here we deal with the structure underneath.

A subassembly is an orderable unit

The most useful rule of thumb when setting up a tree: only create a subassembly for something that somebody builds, buys or outsources as one unit. A welded group that goes to the welding shop as a whole. A hydraulic set a supplier delivers complete. A module the assembly crew builds up and tests as a unit. Each of those groups deserves its own line with its own sub-list, and usually its own drawing.

When in doubt, turn the question around: who will receive this as one line on their list? If nobody needs that line, the subassembly exists only for you, and it should not end up as a level in the BOM. That is what phantoms are for, more on those below. In practice you end up with five standard flavours:

• welded or pressed into one unit: a subassembly set to Inseparable, so the group counts as one position and the individual members live on its own sub-list;
• bought complete, whatever is inside: Purchased, so purchasing sees one line instead of the internals;
• built and tested as a module: a normal subassembly, visible one level deep for planning and assembly;
• grouped only for modelling convenience: Phantom, so the grouping itself disappears from the count;
• surroundings, the customer's installation or the neighbouring machine: Reference, counted nowhere.

Phantoms and references: convenience that does not count

Phantom and Reference are the two markings that keep a tree readable without polluting the list. A phantom is for you: a group of fasteners you want to place in one go, an in-between level that makes mirroring easy. On the BOM the level dissolves and its content moves one floor up, which is exactly what you want: the bolts must be counted, the grouping must not.

Reference is the opposite: present in the model, absent from every list. It is meant for the world your machine has to fit into: the customer's existing frame, the infeed line you connect to. Useful on the layout drawing, invisible to purchasing.

The pitfalls are the reversed cases. A real part accidentally set to Reference vanishes silently from every count; you find out when the fitter is one bracket short. And a phantom with its own drawing and its own article number is a sign it has quietly become an orderable unit after all: promote it to a normal subassembly.

How deep do you nest?

Every extra layer in the tree is an extra floor in the list, and at some point the value stops. Too flat is easy to spot: assembly gets two hundred lines without sequence and has to work out what belongs together. Too deep is just as bad: purchasing clicks through four levels to find one bracket and adds up quantities by hand. The rule of thumb: the tree follows the physical build, and the depth stops where nobody receives a unit any more. A level that only exists to keep the screen calm should be a Phantom, or should not exist.

Note the difference with splitting to keep the model itself workable: large assemblies are sometimes cut up for entirely different reasons, for example to let more people work on them at the same time. When that makes sense is covered in Growing assemblies: when do you split into subassemblies?; feel free to mark such technical in-between levels as Phantom, so the BOM stays clear of them.

The counting test before you release

You do not verify a structure by staring at it, but by counting. Pick three parts with a story: a bracket that appears in several build groups, a bought part with internals, and a member of a welded group. Follow them through the list and ask three questions per part: does it sit on the level where it is bought or built, is the total quantity right, and is there nothing in between that nobody can order?

Then, certainly on the first project of a new machine type, have purchasing and assembly read along once. Ten minutes with the list on the table beats an afternoon of internal polishing: they spot immediately which line cannot be ordered and which build group does not match how the machine actually goes together. Process their remarks in the model, not in Excel afterwards, or you will repeat the same manual work at every revision. With Export BOM the checked list then lands in your own Excel template, one template per recipient, and the export becomes the most boring step of the project. Exactly as it should be.

Frequently asked questions

Should my tree follow the assembly sequence or the purchasing structure?

Follow the physical build: whatever is built or delivered as one unit is a subassembly. Purchasing is served automatically afterwards, because a flat order list across all levels is a matter of picking the right view. The other way round does not work: a tree arranged purely for purchasing gives assembly no build sequence.

Can I still change the structure afterwards?

Yes, Inventor can promote and demote components between levels without remodelling. Keep in mind that position numbers shift and that drawings with BOM tables and balloons need a check afterwards. So restructure before the drawing round, not after it.

How do I get the list to purchasing without retyping?

Export the BOM straight to Excel in the template purchasing is used to, with Export BOM from Thundercad. Structure right, template fixed: the list is out the door within a minute. You can try it with the free trial month, on your own assemblies.