If your company manufactures something, whether it is a camera or coffee maker, satellite or slot machine, chances are it uses a bill of materials or parts list of some kind along the way. A bill of materials (often referred to by the acronym BOM) is simply a very specific shopping list that tells somebody elsewhere in the company exactly what to purchase and how much.
Steps
- Understand what you're building. You can start to compile a BOM as you go during the early phases of a design, especially if some parts will take a very long time to procure. You will complete the bill of materials as the design and documentation becomes more certain.
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List the parts in your assembly. Be thorough and complete, even if it seems obvious what goes in or the job is boring.
- Assign part numbers to each item in your assembly. If your company keeps a database of part numbers, it is often worthwhile to reuse standard parts such as fasteners, connectors, resistors and other small, purchased items. Avoid creating multiple part numbers for the same part.
- Make sure each of the part numbers you will use has some specification, such as a drawing or data sheet, to allow it to be purchased or fabricated. Often these specifications will be loaded in a database so that they are associated with the part numbers they describe.
- Assign item numbers. Many database programs require you to assign numbers to the items in a BOM, or they may assign item numbers for you.
- The item number may also be referred to as BOM ID, and should be a unique identifier.
- If there is an assembly drawing, the item numbers on it should match the item numbers in the database.
- List exact quantities of each item on your BOM. Be sure the quantity is consistent with the unit of measure in which an item is purchased (each, inch, feet, meters, ounces, etc.)
- Some systems allow things to be listed as "bulk items." Examples of bulk items are things like adhesive or thermal compound. Bulk items might specify "as required" for the quantity.
- A BOM can have multiple levels. The levels represent subassemblies inside of higher level assemblies.
- When creating multiple levels in your BOM, be sure to do so in a way consistent with your company's conventions.
- Consider the manufacturing process and purchasing process when deciding what is a subassembly. Is the subassembly going to be purchased or handled as a unit? What level of assembly needs to be purchased? What needs to be sold?
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Include reference designators if appropriate. The L201 alongside the component in this photo is an example of a reference designator.
- Reference designators are used mostly for components on circuit boards, but may occasionally be used for valve manifolds and other similar assemblies. Not all assemblies need reference designators.
- Make sure the BOM corresponds exactly to the assembly documentation, whether that is an assembly drawing, schematic, or other document.
- Include the assembly documentation or a link to it in the bill of materials. How this happens, exactly, should depend on your database and document control system. It may show as an attachment to the assembly, or you may add a document to the assembly BOM.
Tips
- While it is not the direct purpose of a BOM, a BOM can also be used to track the weight, materials cost, and other attributes of an assembly. Just remember that you get out information that is no more accurate than what you put in.
- Find out how your company handles the purchase of an inseparable assembly. For example, if you send a manufacturer a weldment drawing and get back a welded frame, you may need to list the raw materials for the frame only on the drawing and only in such detail as is necessary to weld the frame.
- Consider over-kitting small parts. If you don't know exactly how many wire ties an assembly will require, you could specify a few extras. Ask somebody in purchasing, document control, or manufacturing how best to handle your extras. Your company may prefer to add a note, add extras directly to the quantity, specify an over-kitting percentage, or simply keep spares in stock. When you figure it out, be consistent. An assembler who can't figure out where to put the three extra screws you specified may get confused.
- Don't forget such items as labels, packaging, and silkscreens.
- Depending on your company's processes, you may be required to have a BOM with released part numbers before parts can be ordered. Even if purchasing and documentation are not your job, it's worth understanding the basics, so you can monitor the process and do your part to keep it going smoothly.
Warnings
- Don't expect the people purchasing or making parts for you to read minds. The more clearly you document your assembly, the more likely you are to get all the right parts ordered and assembled correctly.
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