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Form-Fit-Function (FFF) Definition and Rules

What is form-fit-function?

Form Fit FunctionThe phrase form-fit-function (FFF) is used in manufacturing to describe the identifying characteristics of a part (a single component that goes into the final build of your product, typically kept on an item master). Form-fit-function can be defined as:

  • Form – the shape, size, dimensions, mass, weight and other visual parameters that uniquely distinguish a part. For example, you might describe a screw that will be used in your product as ‘SCREW, PAN HEAD, M3 x 0.5, 2mm Lg, 316 SS.’
  • Fit – the ability of a part to physically interface with, connect to, or become an integral part of another part. For example, if we want our screw to fit to correctly in the final product, it must adhere to the rules set by engineering in the design phase. This might include specifications for the space around the screw relative to a faceplate hole or the location of the screw’s top position relative to the product surface.
  • Function – the action or actions that a part is designed to perform. In our example, the screw is intended to hold other parts of the product together.

Set, enforce and embrace form-fit-function rules to improve manufacturing efficiency

Have you ever had to stop your assembly line because a part doesn’t fit? Or halted your manufacturing line because a part did not perform as designed? In both cases, adoption and adherence to form-fit-function rules could have prevented these delays to the production schedule and allowed your assembly line to work without incident.

Form-fit-function rules specify how far new versions of a part can deviate from the original before new part numbers are created. Typically, if the change to a part is significant enough that the new part cannot be used in place of the previous version (for example — if the size of a screw changes) then a new part number must be created. Keep in mind that the amount of acceptable deviation will vary from organization to organization, product to product, and part to part. For example, if you replace a part that has a shiny finish with a new part that has a matte finish, the change is technically a form-fit-function change, but if the two parts can be used interchangeably your form-fit-function rules may not require a new part number. Follow the suggestions below to help ensure that when a form-fit-function change is required, the process is handled as smoothly as possible.

Implement a strictly enforced part numbering scheme

A properly implemented and strictly enforced part numbering scheme coupled with well-understood form-fit-function rules will ensure that the changes required and initiated by manufacturing or engineering are properly tracked and easily followed by the entire team.

Typically upon receiving new part specifications (whether in the initial build or due to a change), manufacturers will assign each newly specified part a unique number based on a predetermined part numbering scheme and enter the information into an item master. This ensures that purchasing, engineering and design teams all have the correct form-fit-function requirements for each part. (For the benefits and disadvantages of part numbering schemes see our article “Part Numbering Schemes—Intelligent vs. Non-Intelligent.”)

Is Your Part Numbering Scheme Costing You Millions?

Is Your Part Numbering Scheme Costing You Millions WP

Manufacturers of complicated electronic products must manage, track, and store hundreds—if not thousands—of parts in their product development processes. And every time a new part is added to a company's library, it can cost $15k in time and effort to qualify and rollout.

That's serious money.

And that's why choosing the proper part numbering scheme to reference your parts is critical to your manufacturing business's success.
In Arena's "Part Numbering ebook," you'll learn how to choose the best part numbering scheme for your business.

  • Pros and Cons of Intelligent & Non-Intelligent Part Numbering
  • Considerations for Choosing Your Part Numbering Scheme
  • Part Numbers in Your Business
  • How Arena Handles Part Number with Real-World Examples

In fifteen years of working with manufacturers of all sizes, Arena has developed deep insights into the different approaches and best practices of part numbering. These findings can be yours. Download the Part Numbering ebook today.


Adopt a system for revision control

If form, fit, or function has changed enough to require a new part number, a formal change process (usually in the form of an engineering change order (ECO) or a manufacturing change order (MCO) must be initiated. If you have a form-fit-function change and a new revision isn’t created and tracked, your assembly line team members could get a changed part with the same number as the original part that doesn’t look right, fit or perform properly. This can cause your assembly line to come to a halt, increase costs to create your product, and delay your product getting to market.

When it comes to determining what changes require a new part number or a formal revision, your organization may not deem all part characteristics to be equal. For example, while it may not matter if the new stock of plastic buttons isn’t exactly the same chartreuse color that’s currently in the bin if the new button is 2.5 inches wide and the old buttons are 2 inches wide, you will have a problem when it’s time to go to production. This is one reason why creating good form-fit-function rules that are known and followed by all teams, determining the characteristics that govern a new revision, and ensuring your team operates by these rules will lead to more efficient manufacturing processes.

How Arena PLM helps

With Arena PLM, cloud-based bill of materials (BOM) and change management software, you can control part revisions and verify that your parts meet the necessary form-fit-function requirements.

Creating a new part number in Arena PLM is easy. Arena PLM lets you leverage part information previously captured in the system so all you need to do is duplicate the part, add the new part number and enter any attributes that identify the part as a new revision. You can maintain any of the metadata (shape, weight, supplier information, and other) that hasn’t changed to avoid time and errors associated with re-entering data.

Slashing Engineering Cycle Delays by 90%

Affirmed Networks is transforming mobile.With Arena's cloud-based PLM solution, Affirmed eliminated silos and increased team visibility to keep all teams on the same page.

Using streamlined ECO processes, engineering change cycle times were reduced by 90%.

By providing controlled access to the product record, Arena reduced product design confusion and versioning.

In addition, Arena produced the following results:

  • Reduced engineering cycles by 90%
  • Eliminated need for costly five-person IT management team
  • Boosted sales enablement processes


Revision control in Arena PLM allows you to mark steps in a part’s development—from unreleased to in-production to obsolete. By creating different revisions of a part and keeping a record of each stage you develop a trail for the part that you can easily track. Anyone can then identify working revisions, superseded revisions, and the effective revision of a part, and lock the specification from an unauthorized change with ease.

If you are just getting started with part numbering and revision control, the templates we offer below are a great place to begin. But there are some disadvantages to using templates, as Excel can be cumbersome and it’s easy for paper files to get lost in the shuffle. Consider taking Arena PLM for a test drive to see how BOM and change management can help your organization.