Question:
When is a part number change required, and when is a revision change required?
Discussion:
Unfortunately, avoiding part number changes has been seen as a method for improving documentation system efficiency. Because a revision change is always more expedient than a part number change, a part number change usually is something most people want to avoid.
The rules for governing revision and part number changes are called the rules of configuration or interchangeability. A good configuration system must be able to differentiate between parts in order to for it to be effective. Because there are degrees of interchangeability between parts, there are many different opinions as to what those rules should be. The determining principle is driven by the need to differentiate parts. The greater the differentiation, the more efficient the overall documentation and associated manufacturing systems can be.
Good configuration management is fundamental. Many problems become hidden when the rules used for interchangeability are not strictly followed. The effects are subtle, but pervasive, creating confusion and reducing efficiency of all systems within the company. Problems are more difficult to find, and recovery may be next to impossible. Poor configuration can, and will, consume a company in an administrative, bureaucratic quagmire if given enough time.
Revisions are interchangeable and must be completely interchangeable for revisions from "01"to "99". Revisions do not drive a change to the configuration of the product. Part number changes force a change in the product's configuration, and force all other systems to acknowledge that change. Most systems, MRP included, are designed to treat revisions changes and parts changes as such.
Our recommendation:
Our recommendation is to enforce the rules of interchangeability absolutely. If a part is different in form, fit, or function, in any degree, change the part number. If the part is not interchangeable with all previous revisions, change the part number. If the proposed part and existing part might not be interchangeable in future applications, change the part number. If in doubt, change the part number.
Interchangeability of components is far more critical to a cardiac pacemaker manufacturer than it is for someone manufacturing dental equipment, so you might have some leeway. Not considering future applications is the most likely thing to cause trouble. Just because a 20 volt and 30 volt rated capacitor both work in a 16 volt circuit does not mean they will work interchangeably in the next application, if it operates at 24 volts.
Perhaps the best reason we can give for chancing a part number is that it eliminates confusion. Everybody knows that parts are different when they have different part numbers. This includes your suppliers, who have to live with differing interpretations of interchangeability. We were able to significantly reduce the rejection rate of labeling by renumbering labels when the text changed. A part number change made it easier for the supplier to recognize the labeling was different. It also provided us with greater control over implementation on the manufacturing floor.
When using the above rules, it does not take one long to conclude that there are not many parts that are truly interchangeable, but that is the real world. These rules are tough to live with at times, but it guarantees the document system is a reflection of reality. If two parts are different, and you want to make them interchangeable for a given application, create alternate parts for that application.
Our book discusses this and related topics in more detail. How one uses revisions and alternates can really simplify change implementation. We call it the Uniform Implementation Model, which allows most all changes to be implemented immediately regardless of stock levels. Yes, it can be done, and in most cases with far greater control than with other methods.
Any comments, questions, or topics for future articles? E-mail us.
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