This chapter explains how to properly establish multi-level product structures using subassembly BOMs.
What is a product structure?
A product structure is a bill of manufacturing and all the lower level BOMs that contribute to it. The Indented View and Tree View tabs in the Bills of Manufacturing screen display each BOM’s product structure.
Routings, not bills of material, determine product structures
In light manufacturing systems, the bill of material governs product structures and is often “flattened” out so that multiple levels can be represented within a single parent item. When you use routings, however, this is not possible or desirable. Each assembly has its own routing processes and therefore must be given its own BOM. The associated materials support the routing processes within which they are used. In a non-light manufacturing system, routings determine product structures, not bills of material.
What is a subassembly?
A subassembly is any BOM that is used as a component in another BOM. Subassemblies are commonly used as components in multiple BOMs, in which case they are members of multiple product structures.
Product structures occur automatically
Product structures occur automatically whenever you add a BOM parent component to another BOM as a subassembly. The use of subassemblies therefore is what creates product structures.
Shop control requires proper use of subassemblies
Shop control is dependent on the time-phased nature of the master schedule. Jobs are released to production in the correct order of multi-level assembly and job sequences are schedule to be performed in sequential order. For shop control to work properly, you must use subassembly BOMs when required.
When is a subassembly BOM required?
A subassembly should be given its own BOM when either of these two conditions is met:
When it is used in multiple BOMs
Whenever a subassembly is used as a component in more than one BOM, it must be defined with its own BOM. This enables MRP to generate jobs based on interdependent demand, which results in fewer jobs with more efficient run sizes. It also enables the subassembly to be eligible for forecast and supply days planning. Subassembly maintenance benefits as well because any process or component changes are only applied to the one subassembly BOM instead of to each BOM within which it is used.
When its quantity differs from the parent quantity
Whenever a subassembly has a different quantity than the quantity of the parent item within which it is used, it must be defined with its own BOM. This is because the BOM routing is based on the parent quantity for scheduling and costing calculations. If you mix subassembly sequences in with parent item sequences, the wrong quantity gets used, which makes scheduling and costing calculations nonsensical.
Subassembly BOMs are a necessity with MRP and shop control
Using subassembly BOMs is a necessity with MRP and shop control. They enable MRP to generate subassembly jobs when needed in response to interdependent demand. They enable jobs to be released in the correct order of multi-level assembly as subassembly jobs are finished. They enable each subassembly to get its own traveler and to have its routing sequences scheduled in sequential order within work centers.
Creating subassembly BOMs
If you are currently defining subassembly specifications within other BOMs and need to give them their own BOMs to conform to the above guidelines, take the following steps:
•Create a BOM parent item to represent the subassembly.
•Create a BOM header for the subassembly parent item, assigning it a BOM type of ‘BOM’.
•In the new BOM, enter the subassembly routing sequences and components.
•In the BOMs within which the subassembly was previously defined, delete the routing sequences and components associated with the subassembly.
•In the BOMs within which the subassembly was previously defined, add the subassembly parent item as a component.
Do not use phantom assemblies as subassemblies
Phantom assemblies are sets of components that are typically used with custom manufacturing to represent product options. A phantom assembly does not have revision or routing specifications. Phantom assemblies are not suitable substitutes for subassemblies for these reasons.
•A phantom assembly does not have a routing and therefore any processes associated with the phantom must be incorporated into each parent BOM in which it is used.
•A phantom assembly can never be made on its own job or be stocked. This eliminates any possibility of using forecast and supply days planning with the phantom to reduce lead times for the BOMs in which it is used.
•A phantom assembly cannot be released to production in the correct order of assembly relative to higher level jobs because it is never made on its own job.
To change a phantom assembly to a subassembly, take the following steps:
•Change its BOM Type from ‘Phantom’ to ‘BOM’.
Enter the routing sequences required to assemble the parent item.
