Parts don’t have revisions. Such a broad statement seems a little absurd especially considering today’s available software technology, but it’s nonetheless a long-held tenet of engineering configuration management.
While it might appear outlandish to set such a seemingly arbitrary restriction on engineering process, there are sound reasons why the advice holds true. But before you make a unilateral decision never to revise a part, let’s understand just why such thinking became an accepted standard. Then you can make judicious and informed decisions with your own engineering design.
What do we mean when we say parts don’t have revisions?
A part is identified by a Part Identification Number (PIN), aka the part number. If you were holding the part in your hand, this is what would be physically emblazoned upon it. A part is also accompanied by documentation that plainly and precisely describes the part model. In many cases, the documentation is a fully dimensioned engineering drawing, though these days it might also be Product Manufacturing Information (PMI), if you’re riding the technology wave. In the case of a drawing, the documentation also carries an identifying number. While it may be tempting to make the part and drawing numbers the same, such an approach aims to misbehave. For example, a drawing is often changed for very different reasons than the part it describes, often in a fashion that has no impact on design. In addition, drawings may describe multiple parts. In other words, drawing and part life cycles are unique, so the identification number for each must also be unique.
Documentation can be revised, but the part itself should not. If a part changes, the revised part is issued a new part number. In the case of PMI, where the "documentation" portion is integral to the part, revisions are more esoteric. Allowable PMI revisions in that case depend on whether the documentation portion is being updated or the part model is being physically changed.
Why can't parts be revised?
It’s all about how parts are consumed in manufacturing. Parts are ordered, processed, and stocked by part number, not revision. Issuing a new part number for changes addresses the following:
- Existing Stock: The prior part didn't just spontaneously cease to exist; it's still in inventory and it may continue to live on in products that may already be sold, delivered, or in use. It's important to distinguish between these to control how the change might or might not be propagated.
- Alternate Uses: If a part is used in multiple applications, then a favorable change for one application may be detrimental to the second. It's critical to keep such histories separate and distinct.
- Failure Modes: If two parts carry the same identification, but are functionally different, or are used for different purposes, confusion is inevitable. Confusion can lead to failure and liability.
Exception for Interchangeability
It might seem costly to constantly issue new part numbers for every tweak and adjustment. Especially if a change in a component part causes the next higher assembly to behave differently and propagate additional part number changes. This is where interchangeability comes into play, more commonly known as the "Three F’s:"
- Form: The physical nature of the part, including its strengths and weakness, structural or otherwise.
- Fit: How the part interacts with other parts.
- Function: What the part actually does.
The interchangeability rule states if a part modification does not appreciably affect the form, fit, and function of the part, then the modification is deemed interchangeable. Here, a new part number is not required. In other words, if you were to blindly reach in a bucket with both versions of the part, it wouldn't matter at all which you would pull out. As you can imagine, few changes are truly considered interchangeable. When traversing a change up an assembly structure, engineering judgment is used to determine the point of interchangeability (i.e. where part numbers stop changing).
Someone might guess that if a part is identified with the part number and the revision level, then the problem is solved. But that's not the case. Instead, revision level has become nothing more than an extension of the part number, so the manufacturing impact is the same as if everything was not interchangeable! Additionally, most manufacturing processes don't accept such a concept because parts are not ordered, processed, and stocked by revision level.
One permissible use of revisions is pre-production. Many changes occur while a part is in the concept stage. Without a supply chain consideration, it’s okay to revise parts, as long as you don’t intend to use the prior part revisions in production.
Deal: When to revise a part
- Interchangeable modifications
- Pre-production parts
No Deal: When to issue a new part number
- Non-interchangeable modifications
- New applications for an existing part
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