A multi-level BOM with revision control lists every assembly, sub-assembly and part of a machine in its real structure, and ties each line to a drawing number and its current revision. For a machine builder, it is the single list that design, purchase, stores, the shop and service all work from, so a change made in design reaches everyone who needs it.
Most machine builders already have BOMs. The trouble is that they live in several places: a CAD export, a purchase spreadsheet, a stores register and the assembly supervisor’s marked-up print. Each copy is correct on the day it is made. After the first design change, they start to drift.
What is a multi-level BOM?
A single-level BOM lists the parts that go directly into one item. A multi-level BOM keeps going: the machine is made of assemblies, each assembly of sub-assemblies, each sub-assembly of parts and raw material. A typical special-purpose machine might have a base frame, a spindle head, a fixture, a hydraulic power pack and an electrical panel at the first level, and several hundred lines below them.
The structure matters because each level is handled differently on the shop floor:
- Bought-out items such as motors, bearings, cylinders and controllers go to purchase.
- Made-in parts such as brackets, shafts and weldments become job cards with a routing.
- Sub-contracted parts go out for machining, grinding, heat treatment or painting on a challan.
- Raw material lines (plates, bars, sections) are calculated from the made-in parts.
When the BOM is flat, or kept as separate lists, the links between these levels are lost. Purchase cannot see which assembly a motor belongs to, and stores cannot tell whether a short bracket will hold up the fixture or the whole machine.
Why drawing revisions cause so much rework
Design changes are normal in engineer-to-order work. The customer changes a stroke length, a supplier discontinues a cylinder, or the first assembly shows that a bracket fouls a hose. The drawing is revised from B to C, and the problem starts if the change does not reach everyone at the same time.
Common results are familiar to any works manager:
- Purchase orders a part to revision B because the PO was raised from an old list.
- The shop machines to revision B because the print on the machine is old.
- Stores issues revision B parts that were made in advance for a previous order.
- The service team, months later, orders a spare to the wrong revision because nobody recorded what was actually built.
Each of these costs material, machine hours and, worst of all, time at assembly, when the machine is closest to its delivery date.
How revision control works in an ERP
Revision control in an ERP means every item carries its drawing number and current revision, and every change is kept as history instead of overwriting the old data. A practical setup has four parts.
1. One item code, many revisions
The part number stays the same across revisions, so stock, history and costs stay connected. The revision is a property of the item and of every transaction made against it: purchase orders, job cards, receipts and issues all record which revision they were for.
2. A dated change record
When design releases a change, the BOM is updated with the new revision, the date and a short reason. The old revision is kept, so anyone can see what changed and when. This record is also what you show a customer who asks why a machine differs from the original drawing set.
3. Open orders flagged
When a revision changes, the system shows the open purchase orders, job cards and stock that belong to the old revision. Purchase can amend or cancel orders, production can stop or rework jobs, and stores can decide whether old stock is still usable.
4. As-built records by serial number
When the machine is assembled, the parts issued against it, and their revisions, are recorded against the machine’s serial number. That as-built record is what the service team needs years later.
Buying and making against the BOM
Once the BOM is reliable, it can drive the work instead of being a reference document. For each machine or batch, the system compares BOM requirements with stock, open purchase orders and work in progress, and lists the shortages by the date they are needed at assembly. Purchase raises requisitions from those shortages, production releases job cards for made-in parts, and sub-contract items go out on challans tracked against the project.
This also makes the shortage list honest. Instead of a long list of everything not yet in stores, the team sees which missing parts will actually stop assembly next week and which have time.
Costing a machine from its BOM
A structured BOM also gives a better estimate. When a new enquiry is similar to a past machine, the estimator starts from that machine’s BOM and actual cost, adds the new items, and prices the difference. During the build, material, labour hours and sub-contract charges post to the project as they happen, so the variance against the estimate is visible while there is still time to act on it.
Common questions
Do we need CAD integration to keep a multi-level BOM?
No, but it saves typing. Many machine builders start by importing BOMs from Excel exports, then connect their CAD system so released BOMs move across without being retyped. Either way, the ERP holds the BOM that purchase and production use.
How deep should a BOM go?
Deep enough that every line is something you buy, make or send out. If a sub-assembly is always built and stocked as one unit, give it its own BOM level. If it is never stocked, it can stay as a group of parts under its parent.
What about standard machines with options?
For configure-to-order machines, a base BOM holds the common parts and each option adds or replaces lines. Choosing the options on the sales order generates the BOM for that machine, so stores and production get the right list for every order.
How Formezy helps
Formezy keeps multi-level BOMs with drawing numbers, revisions and change history, generates configured BOMs for standard machines, raises purchase requisitions and job cards from BOM shortages, tracks sub-contracted parts, and records the as-built configuration against each serial number. You can see how this works step by step on the industrial machinery page, or read about the Production & Purchase module. To see it on your own machines, book a demo.
