The bills of material, or BOMs, for products or assemblies, allow engineering to design and guide the manufacturing of individual material, parts, or assemblies. A bill of material specifies the required materials to complete an assembly or product. In this article, you will learn the basic function of a bill of material, how bill of materials are used in production, and how they form the basis of supply chain management.
Basic Function of a Bill of Material
A bill of material provides a comprehensive list of products and sub-assemblies required to build a product or assembly. In a fabrication environment, bills of material generally appear in two places: the ordering system of record and on drawings. General assembly, piping drawings, or structural drawings are examples of drawings that receive a bill of material as these drawings require the assembly of various components and sub-assemblies. By contrast, bills of materials are found less frequently on high-level engineering drawings such as PFDs and P&IDs.
A bill of material provides the parts to build a component and assembly detail. As such, multiple levels appear as seen in the schematic below.
After the generation of a manufacturing drawing, the drawing creator or designated support staff enters the bill of material into an order management system. Order management systems range from simple Excel sheets to large enterprise resource planning tools such as SAP and Oracle.
BOM Use in Production
During manufacturing, engineers often require changes to a part or assembly. However, it is undesirable for those changes to proliferate to a product in manufacturing until an engineer completes and fully checks the revision. Smaller organizations may have the ability to make changes directly to the manufacturing BOM without affecting production.
As an organization grows in size and as products grow in complexity, the use of multiple BOMs may be required. In these scenarios, ERP systems provide engineering BOMs and manufacturing BOMs. Upon engineering BOM completion, a materials planner copies the engineering BOM to the manufacturing BOM. This manual process enables engineers to make changes to the engineering BOM without disrupting fabrication activities. After checking and releasing the engineering BOM, changes proliferate to the manufacturing BOM. Material planners may then purchase or initiate the fabrication of the necessary goods.
Differences Between Manufacturing and Engineering BOMs
Manufacturing BOMs appear similar to the engineering BOMs with several key differences. The structure of a manufacturing BOM supports product assembly, rather than an engineering BOM that supports a product’s design. Whether a part is listed as “make” or “buy” in the engineering BOM informs the manufacturing BOM’s part visibility. For instance, let’s take an example of a binder clip. Binders noted as “make” in the BOM allow the shop to see all of the components that make up the binder clip. If that same binder clip lists as a “buy” part, the manufacturing BOM lists only the top-level part.
Operations that require joining raw material into a finished product operate from a drawing or document that includes a bill of material. Often this raw material requires processing prior to assembly. In such cases, engineers provide operators with a single level bill of material, known as a cut or a material list. This gives the individual technician full visibility into the required material. Examples of operations that require a cut list include the plasma table, saw, and press brake.
Supply Chain Considerations
Consumables, such as weld wire, solder, and cleaner may appear in the bill of materials. In other cases, BOM builders exclude consumables from the BOM. The preferred method depends on the application. For repeatable, job shop type of work, purchasers derive consumable use from an overall estimate. As a result, purchasers buy in bulk according to overall estimates. For unique projects or off-site jobs, BOM builders may list consumables on each individual BOM. No matter the approach, a consistent approach to estimation provides solid guidance for material quantities.
Bills of materials form the basis of modern supply chain planning. Supply chain professionals determine raw material and buy-out needs by looking at aggregated material demands. The Manufacturing BOM imposes demand upon the system. Most mid to large sized companies automate the demand planning process by considering operational constraints and setting minimum and maximum levels of product inventory. The BOM allows for supply chain professionals to determine demands on inventory whether there’s merely a call on that inventory (allocated) or whether it has been consumed.