Bill of Materials
A bill of materials (BOM) is a hierarchical list of components used in an assembly. The bill of materials is used chiefly for cost estimates, but is also used for inventory control and tracking where parts are used. A bill of materials may or may not include labor.
Before a bill of materials can be constructed, there needs to be an inventory of parts, as well as a list of suppliers.
The inventory must contain all the necessary information for each particular part and must contain a unique no. Also assemblies and sub assemblies must contain a unique no. Each part no. would contain either a raw material or a bill of materials. The parts should have the following attributes:
- A unit, i.e. each, lb, kg, m, hours
- A supplier, if required
- Validation, the preventing of errors getting into your bill of materials. i.e. the inclusion of mandatory data and the prevention of incorrect data for the quantities.
Each part no. has a unique no. Its very common to have one or two hyphens in the part no. to separate the different categories, by just looking at the number. In some companies the part no. directly corresponds to a drawing no.
A bill of materials cannot exist without a inventory.
List of Suppliers
A list of suppliers supports the bill of materials, as this has the contact information for the vendor e.g. website, email address & physical location etc. All parts require a vendor, except those produced in house or labor.
The structure of a Bill of Materials
A bill of materials can either be single level or multi level. A single level is simpler to understand, but it is not suitable for creating a bill of materials for anything other than a very simple construction. A single level bill of materials is best compared with a supermarket shopping list.
A bill of materials may be either work in progress or live.
The bill of materials may or may not include labor as this is sometimes handled separately.
The number of levels can vary, but it is not often this goes beyond 7 levels, and extremely rare to go beyond 12 levels
Single Level Bill of Material
This is a total count of all the parts used in a construction. The parts are all listed in order of the part numbers. Though easy to read when ordering or collating parts for a construction, this type of bill of material really becomes unsuitable the larger the construction. As there is no distinction for sub-assemblies, this also becomes increasingly difficult to locate the whereabouts of parts, and the likelihood of errors increases the larger the assembly becomes. A single level bill of materials can be converted from a multi level, but not the other way round.
For an example of a single level Bill of Materials, please click here
Multi Level Bill of Material
A multi level bill of materials is far more suited for medium and larger assemblies. As a construction becomes larger, it is necessary to break this into sub assemblies and each of these sub assemblies into smaller sub assemblies and so on. This is essential for the prevention of errors and the accurate tracking of parts.
The multi level Bill of materials is indented to show each level. Each part no. must have a parent in the bill of materials, with the only exception being the final assembly.
Each part no. is either:
- A raw material
- A sub assembly, which would have children part no.s
For an example of a multi level Bill of Materials, please click here
Prevention of Errors
The two main causes of errors are:
- Parts having the wrong quantity in a bill of materials
- Parts being omitted from the bill of materials