Product Structure and Bills of Material

Adapted by DRM Associates from the CONFLOW Project

Product Structure

The product structure provides a hierarchical classification of the items which form a product. With the product structure, the understanding of the components which compose a product as well as their attributes, can be represented. The product structure shows the material, component parts subassemblies and other items in a hierarchical structure that represents the grouping of items on an assembly drawing or the grouping of items that come together at a stage in the manufacturing process.

In the very early phases of new product design, engineers begin the design by sketching out a structure for the product which identifies the major components and systems that will combine to create the desired product. For each component, existing standard parts and previously designed custom parts must be evaluated for their ability to provide the necessary function. In those cases for which no existing alternative can be identified, the engineer must specify that new parts should be created, and the details of the structure begin to emerge. Product structure management provides the mechanism to capture and manage as-designed product structures with ease. It allows for the creation and re-use of unlimited numbers of parts and assemblies, to provide for many different variations of a basic structure or the creation of complex, one-of-a-kind structures.

Later in the development cycle, engineering parts lists or drawing parts lists are used to create a version of the product structure to support manufacturing. This standalone product structure is called the bill of materials.

Bill of Materials

A bill of material is a formally structured list for an object (semi-finished or finished product) which lists all the component parts of the object with the name, reference number, quantity, and unit of measure of each component. A bill of material can only refer to a quantity greater than or equal to one of an object. It is a product data structure, which captures the end products, its assemblies, their quantities and relationships.

There are usually two kinds of bills of materials needed for a product: engineering and manufacturing BOM. The engineering BOM normally lists items according to their relationships with parent product as represented on assembly drawings. But this may not be sufficient to show the grouping of parts at each stage of the production process nor include all of the data needed to support manufacturing or procurement. These requirements may force the arrangement of the product structure to be different in order to assure manufacturability. Thus, engineering and manufacturing will usually have different valid views for the same product.

Manufacturing Requirements Planning (MRP) systems typically rely on a Bill of Materials (BOM) and the product structure for their information. In theory, the BOM can and should be produced automatically by the CAD system but in practice there is usually human intervention or even re-entry. The main reasons are:

  • The difficulty of tracking changes to the BOM (product structure) and effectivity dates and transferring this data back to the design system. Many changes, such as different suppliers for fasteners, do not affect the design form, fit and function and are therefore only made and stored in the manufacturing systems.
  • The need in manufacturing to view the product structure differently. It is often necessary to batch similar components from different products together for mass production or efficient purchasing.

Product Data Management (PDM) technology enables changes to be tracked and implemented through the design and engineering change process and then passed over to the MRP system when approved. Therefore PDM systems use BOM to represent configuration management of the product.

A Bill of Materials is a product data structure which captures the end-products, its assemblies, their quantities and relationships. The structure of a part’s list determines the accessibility of the part’s information by various departments in a company. It also helps to determine the level of burden put on the computational device in searching for product information. In many companies the BOM is structured for the convenience of individual departments. This, however, engenders problems in other departments.

In Figure 3.1, a product named Product 1 is shown graphically with the summarized products structure and the number of all items that are needed to make the parent products are enclosed in brackets.


Figure 3.1 Product structure for product 1

Figure 3.2 contains a bill of materials for Product 1 in which the total usage of each item is collected into a single list for the product. This kind of list is convenient for the master production schedule but results in the duplication assemblies. This implies that each product bill that uses assembly must be changed whenever there is a change in assembly. Furthermore, since lead times of intermediate assemblies cannot be determined, parts are ordered too early the first time they are encountered in the product structure.


Figure 3.2 Summarized Bill of materials (BOM)

Other arrangements used in arranging the bill of materials is by indenting the product data as shown in Figure 3.3. One disadvantage of this method is that all components of an assembly are repeated each time the assembly is used, resulting in massive duplication of data.


Figure 3.3 Indented bill of materials

One solution to the duplication problem is by holding each assembly only once in ‘single level’ bill of materials as shown in Figure 3.4. In this approach it identifies only the components used by one level and a required subassembly. This means that engineering changes can be made in only one place.


Figure 3.4 Single-level bill of materials

‘Where used’ lists give an easy overview of the products where a given material is contained. This happens by displaying of all BOM’s, which contain this component.

The table below shows the typical information which can be found in a bill of materials:

  • Header (Product or Higher-Level Assembly)
    • Product/Assembly Item Number
    • Product/Assembly Name
    • Product/Assembly Description
    • Unit of Measure
    • Revision Level
  • Each Component/Item
    • Item Number
    • Item Name
    • Item Description
    • Revision Level
    • Unit of Measure
    • Quantity per (each higher level assembly)
    • Effectivity (date in and date out or serial number in and serial number out)