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.
What is a subassembly?
A subassembly is a manufactured item, consisting of its own processes and components, that is incorporated into a higher-level manufactured item.
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.
