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Introducing new part numbers for design changes – FFF Rule

When engineers modify a design, they must decide between updating the original design or assigning a new part number, based on interchangeability. The Form-Fit-Function rule helps determine if designs can be swapped without issues.
Estimated reading time: 3 min
Engineers performing a change of an original design must make the right decision on whether to revise the original design or to create a new part number. If the original design and the new design are not interchangeable the new design will need to be assigned with a new part number. However, if the original and the old design are interchangeable a revision of the original part is sufficient. Making the right decision is vital, because revising designs that are not interchangeable can lead to fatal consequences.

Form-fit-function Rule

The Form, Fit and Function (FFF or 3F) rule is a common method to determine the designs interchangeability. Form – the relevant physical characteristics of item like shape and size. Mass and center of gravity are sometimes also important. Fit – the ability of an item to physically connect with other components including tolerances. Function – the action that an item is expected to perform to fulfill its purpose during its life cycle. The rule dictates that if change of one of the three characteristics triggers a new part number.

Why is a new Part number needed?

Modern PLM systems use part numbers, revisions and iterations and are in complete control of the content of assemblies etc. However, this is not the case for ERP systems. Some ERP do have revision identification, but this functionality is not implemented end-to-end. Let us look at SAP as an example. In SAP you can make orders with reference to revision (levels). However, the revision is not controlling parts on stock. The individual items are “anonymous” on stock and the part number is the only identifier. The only exception is serialization or batch controlled parts. This means that it is not possible in SAP to control which design revision you are pulling from the stock. It is therefore required that the new design is identified with a new item number if the new design cannot be replaced with the original. Some companies compensate by manually up marking components with version, but this only reduce transparency on what actually is on stock and what has been used where.


R&D should limit the use of versions and instead create new material numbers whenever the specifications are changed.   Want to learn more? Visit our blog:

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