Form, Fit and Function rules specify how much new revisions of a part can deviate from the original before new part numbers are created. This is key in ensuring components can be substituted without issue.

March 2, 2016

During the design process, Engineers regularly have to decide on whether a new part or change meets the Form, Fit and Functional requirements. This methodology concerns both design and BOM components. In order to determine whether to create a new part number or revise before any modification, the Engineer should consider the Form, Fit and Functional (FFF) requirements:

  • Form is the shape, size, dimensions, mass or other visual parameters which uniquely characterise an item. This defines the “look” of the part or item. Sometimes weight, balance and centre of mass are considerations in ‘form.’
  • Fit is the ability of an item to physically interface with all other components. This also includes tolerances modifications.
  • Function refers to the action or actions that an object has been designed to perform.

When an engineer assesses the proposed change of a component, they should consider how the change will affect the component. Based on the consideration relative to FFF, the engineer can:

  • Create a new component with a new Part Number: 
    • If Form, Fit or Function is affected by the proposed change of the component then the component is not interchangeable therefore a new component should be created. This may be an evolution or derivation of an existing component but should use a new Part Number. New Part Numbers should also be assigned to higher up assemblies up to the point where interchangeability is re-established.
  • Revise the component: 
    • If the proposed changes do not affect Form, Fit or Function, then it is typically sufficient to revise that component where the Part Number would not change.
    • For example a change to improve manufacturability or correcting a defect would probably warrant a revision.
    • Any new revision must be compatible with all of its previous revision.

Various real-life scenarios exist where following the rules are not always possible but the Form, Fit and Function (FFF) methodology provides a basic robust concept whenever possible.