Rolling Revisions up the BOM Tree, 101: When Should You Change Revisions?
When it comes to item management and revision control, we’ve seen many product companies wrestle with the question of whether a revision to a component should trigger a new revision up the assembly or bill of materials (BOM) tree.
This answer is not as simple as it might seem. If you always allow minor changes to lower-level components or assemblies to trigger new revisions to their next-level using assemblies, you may quickly find yourself buried in a never-ending stream of product assembly changes and documentation updates in your engineering and production systems. And, with minor changes taking place continually, you may lose track of critical product-level changes while introducing more requirements to reidentify, label, track, produce, ship, and service multiple revisions of components or assemblies.
However, there are definite scenarios where you should roll revisions up the BOM tree, sometimes all the way through the finished product (or top-level assembly). If a redesign of an existing component or assembly introduces a functional, safety, or reliability improvement, you need to distinguish which product revisions contain the updated components or subassemblies and determine whether rolling revisions is better than reidentifying parts or assemblies with new part numbers instead.
To Roll or Not to Roll Revisions?
You shouldn’t create a policy that always requires rolling the revision of your next-level product assemblies whenever you make component-level changes, nor should you create a policy that never requires changing next- or top-level assembly revisions up the product tree.
In general, you should issue (or roll) a new revision of a next-level assembly for the same reason you release a new revision of a component—when the change is significant enough that you may need a way to easily distinguish “before” from “after.” Here are two simple rules to consider:
Rule #1: If the change represents a minor alteration to the component, do not release a new revision of the parent assembly.
Rule #2: If the change represents a major alteration to the component, decide whether it also represents a major revision to the parent assembly. If it does, issue and release a new revision of the parent.
When to Consider New Part Numbers
Because enterprise requirement planning (ERP) and shop floor management systems normally only track inventory by part number (and not by revision level), it’s a good practice to configure your systems so that the separate “revision” attribute is used only for minor revisions. If the form, fit, or function changes significantly, then the part number itself should change instead of simply rolling the revision of the existing part number.[i]
Form, fit, and function (FFF or F3) define the parameters that set the characteristics of a part:
- Form: The shape, size, dimensions, mass, weight, and other visual parameters that uniquely distinguish a part.
- Fit: The ability of a part to physically interface with, connect to, or become an integral part of another part.
- Function: The action or actions that a part is designed to perform.
When there is a FFF change, it is recommended to create a new item or part number and not change the revision. On the other hand, certain records that define a part—like metadata or documentation—can and should be tracked by revision.
Revisions can also alter a part as long as it does not affect FFF and it’s still compatible with older assemblies that the part resides in. For example, if material in a part was changed to make it stronger or a hole in the part was made slightly larger to make it easier to assemble.
Creating new part numbers makes it easier to stock inventory for parts with FFF changes separately on the shop floor and it clearly communicates that a given change does affect form, fit, or function.
Final Thoughts on Rolling Revisions
We’ve discussed rolling revisions of end items or components as well as using assemblies up the product or BOM tree. Whether you’re considering when to roll individual component revisions or next-level assemblies, the considerations are similar.
For major changes that impact form, fit, and function—you’ll want to reidentify the impacted parts, and often the using assemblies, if they are not backwards and forwards compatible. Minor changes, on the other hand, can be documented by simply rolling the revisions of the parts and using assemblies.
Product lifecycle management (PLM) systems were designed to help manage and track all parts and assemblies from concept through production and end of life. Any changes to a part are automatically updated in the BOM to ensure every aspect of the product is controlled and documented, in context, to each revision.
Having a PLM system as the single source of truth gives added visibility and traceability to ensure all product teams can better identify changes of any kind, at any time, to optimize your item management and engineering revision control. Let us know if you have different practices for rolling, or not rolling, your component revisions. And, consider how PLM can help.