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Short answer: Structure the BOM the way the machine is built and serviced. The top level is the system. Below it come major modules (frame, gas box, chamber, electrical). Below those are subassemblies that are built or bought as a unit. Purchased parts and raw-material parts sit at the bottom. Each line needs a unique part number, revision, quantity per parent, unit of measure and a make/buy flag. Use phantom (non-stocked) levels only for grouping that nobody builds as a unit.
Key concepts
- Level: depth in the tree. Level 0 is the top assembly.
- Quantity per parent: quantity at that level, not the total. Total quantities are calculated by multiplying down the tree.
- Indented BOM: shows the full tree. Top-level (single-level) BOM: shows only direct children. Parts-only (flattened): shows totals per part number, which is useful for purchasing.
- Phantom assembly: a grouping that appears in engineering but isn't stocked or built as a separate item. Its children are pulled straight into the parent during manufacturing.
- Make/buy: whether the line is fabricated in-house or from a drawing (made) or bought from a catalog (purchased).
Steps to build the structure
1. Draw the module map first. List the units that are physically built, tested or swapped as one piece: frame weldment, gas box, each gas stick, chamber, pump package, electrical cabinet.
2. Make each buildable unit a subassembly with its own part number and drawing.
3. Use phantoms for convenience groupings, for example "panel hardware kit," when nobody kits or stocks them.
4. Put purchased items at their install level. A regulator belongs to the gas stick it sits in, not to a global hardware list.
5. Handle bulk items (tubing, cable, adhesives) with a unit of measure such as m or mL and an allowance policy. Don't use "1 EA."
6. Make the CAD structure match (or map it cleanly to) the BOM structure. In SOLIDWORKS, the subassembly's BOM properties ("Show," "Hide," "Promote") control how the subassembly appears in a drawing BOM.
7. Release from PDM or PLM so every line has a controlled revision.
Worked example
A gas cabinet has two identical gas sticks.
L0 GC-1000 Gas cabinet 1 EA
L1 GC-1100 Cabinet frame weldment 1 EA (made)
L1 GS-2000 Gas stick, process gas 2 EA (made subassembly)
L2 V-0101 Manual valve, VCR 1 EA (buy)
L2 R-0201 Regulator, VCR 1 EA (buy)
L2 F-0301 Filter, VCR 1 EA (buy)
L2 G-0401 Gasket, VCR, nickel 6 EA (buy)
L2 T-0501 Tube 1/4" 316L EP 0.8 M (buy, bulk)
L1 GC-1900 Cabinet hardware (phantom) 1 (phantom)
L2 S-0601 Screw M6x16 A4 24 EA (buy)Total gaskets for purchasing = 6 × 2 = 12. The flattened view does this multiplication. A common error is writing "12" at L2, which then gets multiplied by 2 again.
Gaskets have their own line, because maintenance replaces them whenever a joint is opened.
If you have to rebuild a structure like this from a legacy assembly drawing, extracting the parts list table first saves retyping. MST's free Drawing to BOM tool does that step. Check the extracted lines against the drawing before you use them.
Common mistakes
- Total quantities at child levels. This double-counts when multiplied.
- One giant flat BOM for the whole machine. Procurement can work with it, but service, build and change control can't.
- Engineering-only groupings stocked as real items. This creates phantom inventory and extra work orders.
- Missing consumables and seals (gaskets, O-rings) that are only shown in the model.
- Duplicated part numbers for the same item from different library copies.
- CAD structure drifting from the BOM after rework.
- "EA" for bulk materials.
Review checklist before release
- Every line has a part number, revision, description, quantity, unit of measure and make/buy flag.
- No part number appears with two different descriptions.
- Quantities are per parent. Spot-check one flattened total by hand.
- Each made part has a released drawing. Each purchased part has a manufacturer or approved-vendor reference.
- Seals, gaskets and other consumables are included and placed at their install level.
- Phantoms are marked as phantoms in both CAD and ERP or PLM.
- Bulk items use real units (m, kg, mL) and follow your allowance rule.
- The CAD-generated BOM and the released BOM agree. A line-by-line compare of the two exports catches drift.
- Long-lead items are flagged so purchasing sees them early.
Running this list at every release, not just the first, is what keeps a multi-level BOM trustworthy as the machine evolves through engineering changes.
FAQ
EBOM or MBOM: which one is this?
This is an engineering BOM organized for build. Many companies derive a separate manufacturing BOM that adds process items and regroups by work center.
How deep should a BOM go?
As deep as the real build and service units go. Extra levels with no build meaning add maintenance without value.
Should fasteners be in subassemblies or the top level?
Put them where they're installed, so kitting and service match the BOM.
How do I handle options?
Use configurations or variant rules in PDM or PLM, with each configured result giving a valid, separately numbered BOM where needed.