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6.13 - The Bill of Materials [BOM]


The Bill of Materials [BOM] is one of the most important information resources in a manufacturing company. In its most basic form it is simply a list of parts that go into making an end product. Complex products have more than one level in their BOM’s. The levels represent that products are made from assemblies, sub assemblies, configuration modules, parts and raw materials. It is more than a list of parts – it is a model that represents a very valuable real world object – the product – which is the source of company profit.

The importance of the product Bill of Material

In a modern manufacturing company most of the activity is driven from the product or part (item) and the components that are used to make it. The Bill of Materials is used to:-

  • Calculate the cost of a product
  • Schedule component items for manufacture or assembly
  • Order parts and raw materials from suppliers
  • Layout the manufacturing and assembly operations in the factory.
  • Produce the product service manuals and parts lists.
  • Tell sales what products are available for customers.

World class manufacturer’s say that BOM accuracy must be 98% or better to enable them to conduct their business competitively.

Bill of Material Responsibilities

Work on creating the BOM begins with the product’s conceptual design when the engineer lays out the product’s main functions. S/he will then develop solutions for each of the product’s functional characteristics each requiring either standard or specially designed parts to fulfil the function.

When the engineers have put together the complete product definition, including manufacturing information, assembly operations and process sequences the BOM is released for manufacture. When the product is shipped to the customer a form of the BOM is available to him so the product can be supported throughout its service life.

The BOM is a dynamic representation of the product that defines how it is made and what is needed to support it in use. Therefore most of a company’s departments contribute to, or make significant use of, the BOM in one form or another.

It is the responsibility of engineers to ensure that the definition of the product retains the level of accuracy appropriate to the product and industry at all times. This is usually done via the BOM management modules of MRP or PLM software.

How the BOM is Represented in Software

To represent products in computer software two main information sources are required – the item (or parts) master file and the BOM structure file. The item master file (or table) describes the individual items and the BOM file (or table) describes how the items relate to one another to form the end product.

An example BOM structure is shown:-

Bill of Materials - Graphical Representation

The company has 2 end products A and K each made up of assemblies, sub-assemblies and parts.

The products are represented in the BOM software as shown below:-

Bill of Materials - In Computer Software

A bill of materials system has a number of presentation view options that are useful to users. Components, by default, are presented in part number sequence. By entering numbers in the Prefix field the components will be presented in prefix sequence – this could be used to hold the balloon number, item number or find number from a drawing. The suffix can be used to hold a reference designator as used on electronic assemblies for components – R1, R2, R3 etc. It can also be used for separate instances of the same mechanical part.

3D CAD systems use a more complex form of the BOM which essentially holds the same part information with the addition of a mathematical orientation matrix for each component in 3D space.

How product changes are handled

New parts are added to the Item Master table and changes are made to the BOM structure table under change management controls that specify when the new parts are to be phased in and the old parts phased out. See guides 2.04 and 6.14.

BOM – The Cost of Non-Conformance

Bill of material problems are difficult to diagnose. When BOM’s are not accurate, it’s hard to make or buy the right items in the right quantities at the right time. The effects translate into big problems for any company:-

Missed customer deliveries – You cannot service your customer if you cannot make and deliver the product.

Too Much Inventory – The fear that the only way to keep the production running is stockpiling materials, which means cash sitting in the stores.

Inefficient Production – When the wrong items, or the wrong number of items, are delivered to the shop floor, production stumbles and falters.

The Cost of doing business is too high – Extra money is being spent for unnecessary inventory; expeditors dashing about trying to overcome shortages and get product out of the door; increased costs of maintaining both engineering and manufacturing bills. Not the best way to do business!

Lack of teamwork and communication – Who wants to work hard for the team when some of the other team members are idiots? They can’t even deliver the right material to run the equipment…!

Frustration and poor morale – People basically want to do a good job. They want to meet their targets, and they want their company to be successful. Nothing is more frustrating than NOT having the right items to do the job. It doesn’t take many frustrations like that to sour morale at a basically good company

Wasted resources – In business we have are five critical resources – people, materials, money, capacity, and the constraint of time. These are all limited, so you have to use them carefully. Wasting capacity or wasting time is the same as wasting money. You’ve squandered a portion of your shareholders assets.

BOM Quality Assurance

One of the basic steps toward making good use of company resources is careful structuring of the Bill of Material. A suggested bill of materials policy for manufacturing excellence:-

  • All end items, subassemblies and manufactured components must be identified with a part number and must have a corresponding bill of material. For accurate costing, include raw materials & consumables.
  • Bills of materials must be entered into the system within five working days of a part numbers assignment.
  • All changes to bills of material must be documented via the engineering change policy
  • Bill of material changes and additions will be audited as per the BOM administrator’s job description
  • Bill of material accuracy must be maintained at 98% or better.
  • Bill of material accuracy is to be audited by the tracking of unscheduled issues and receipts, random sampling of single levels of bills of material, reviewing stores pick lists and by disassembling by Quality Assurance.

Bill of Materials maintenance is the responsibility of engineering design or manufacturing engineering, with input from ALL users.