For planning purposes, the BOM is important because it acts as a bridge between manufacturing’s production requirements and order requirements.
In a nutshell, the BOM defines the structure of components that collectively make up a final product, assembly, or sub-assembly. BOM views may be single level or multi-level. A single-level BOM shows only those components that are directly required to make the assembly. A multi-level, or indented, BOM shows multiple levels of breakdown. So, an assembly is broken down into components, some of which may be sub-assemblies that are themselves broken down into components. An example of a multi-level BOM is set out in the Appendix below.
Now that we have defined a general BOM, it becomes important to distinguish between an engineering bill of materials (EBOM) and a production bill of materials (PBOM). The distinction is important because the PBOM – and not the EBOM – is relevant for manufacturing and planning purposes.
Let’s start with the EBOM. When engineers design products, they explode the product into its component parts. This explosion is captured in an EBOM. For various reasons, EBOMs can be revised before the design makes its way to production and after it is already in production. Once a new revision is complete, the EBOM is handed off to production and becomes the new PBOM.
Manufacturing engineers then review the PBOM and adapt it to support the physical manufacturing process. The resulting PBOM is effectively a supplemented EBOM. Items included in a PBOM and that would be omitted in an EBOM can include shop supplies such as screws, nuts and paint as well as other items (see the below Appendix for a comparison of a sample EBOM and PBOM).
At a minimum, the PBOM should include the following line items:
|Quantity Per:||The quantity of the component required to build the parent item|
|Scrap Factor||The amount of the component item lost during the manufacturing process|
|Routing Operation:||The step in the manufacturing process (routing) where this component is required|
|Phantom Flag:||For manufactured sub-assemblies, it determines whether the sub-assembly is built independently from or together with the parent assembly|
The point of the story is this: a PBOM must accurately reflect the product design, build requirements and routings. From a planning perspective, the PBOM represents the actual demand requirements that the planning engine relies upon to generate supply recommendations.
Too often, companies feed inaccurate PBOMS into their planning engines. The result is poor recommendations that are either:
- Or, if acted upon, lead to problems with inventory, lead times and scheduling.
In next week’s tip, we will discuss the final element of planning data accuracy – the manufacturing routing.
Appendix 1: Comparison of Multi-Level EBOM and PBOM for Bicycle
*this simplified example does not include more detailed BOM fields
|EBOM: 1 Bicycle||PBOM: 1 Bicycle (Red)|
|2 Frame QTY 1 EA||2 Frame QTY 1 EA|
|2 Wheel Assembly QTY 2 EA
||2 Wheel Assembly QTY 2 EA
|2 Gear Assembly QTY 1 EA
||2 Gear Assembly QTY 1 EA
|2 Brakes QTY 2 EA||2 Brakes QTY 2 EA|
|2 Handlebars QTY 1 EA||2 Handlebars QTY 1 EA|
|2 Seat QTY 1 EA||2 Seat QTY 1 EA|
|2 Red Paint QTY 0.5 GAL|
|2 Bolts QTY 20 EA|
|2 Screws QTY 10 EA|
Your POV (post comments below)
- What organizational measures do you take to ensure that your BOMS remain accurate?
- How has your organization transitioned from using manual planning tactics to using MRP effectively?
- What’s your organization’s biggest challenge to maintaining accurate BOMs?
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