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Identifying alternative (ALT) parts for your bill of materials

Each item in PDXpert PLM software has a Source list, which may contain any number of approved supplier items that can be sourced for that item. Each approved item can be ranked relative to its peers and include its own rich data, including part number, data record revision, cost, and datasheet file attachments. This application note describes how to take full advantage of PDXpert software's Source list to identify parts used both company-wide and in application-specific bills of materials.

Basics of supplier source identification

Purchased parts often represent the bulk of a unique product's components. Screws, resistors, paint, and other off-the-shelf items are acquired to fulfill standard functions. Often these parts are available at little cost from several different mass producers, any one of which would be equally suitable for supplying a useful product.

By definition, every purchased part in your warehouse should have at least one, and preferably several, approved supplier parts. Multiple supplier parts are equivalent (or interchangeable) with each other when their relevant functional and physical properties are equivalent. Part interchangeability - that is: form, fit, or function equivalence - implies that any approved source can be used for the application. (For a complete discussion, see Applying principles of interchangeability to your bill of materials.)

Some approved parts may have attributes that are irrelevant when placed into your intended application. Occasionally, those irrelevant properties will actually increase the cost of the item, such as when a 100 Ω, 0.1% tolerance resistor is used in place of 100 Ω, 2% resistor. In this case, you may want to assign a preference rank that indicates to your Purchasing team how you judge the relative desirability of the interchangeable sources.

After you've identified your purchased part's sources and validated that their specifications meet your purchased part's requirements, they're approved on an engineering change form and can be used in production.

Application-specific alternative parts

The interchangeability of sources consider how the item is used in a specific application. In general, it's best to consider the "specific application" as broadly as possible for all expected uses: you don't want to have to carefully examine the original requirements for a 10 µF 10WVDC aluminum electrolytic capacitor each time it's applied to a new design.

However, in some cases, a specific application can accept an part that would not be equivalent in all possible situations. For example, an specific assembly may be able to accept a 12 mm screw length even when an 8 mm screw is preferred. Clearly, a 12 mm screw is not completely interchangeable in all applications with an 8 mm screw, but it may work perfectly well in one particular instance. Moreover, if the production line is stopped, it may be essential to provide a workable substitute for a preferred item. This substitute is commonly called an alternative ("ALT") part.

Problems with traditional attempts to identify ALT parts

An ALT part has always been a specific solution applied to a specific bill of materials. Intuitively, most design engineers believe that the easiest representation of the part alternatives would be to simply add them to the place they're needed, on the affected BOM.

The goal is great. However, the reality is that adding an ALT to the bill of materials is often meaningless, and often risky.

Consider how the manufacturing system works: parts listed on the BOM get translated into a demand based on their quantity, which is then multiplied by the number of assemblies to be produced. How does the MRP system translate an alternative part, possibly listed on the BOM with quantity of 0 or a unit of measure "ALT" or "A/R", into a purchase quantity? In most cases, it doesn't. What happens if the quantity and units are useful (like "7 ml") or the "ALT" unit is converted to "each"? Possibly both the preferred and ALT will get purchased for the same purpose, with unhappy effects on your inventory.

The manufacturing team may have a standard method for dealing with these problems. They may simply filter out all of engineering's alternative parts, or accept them at zero quantity. Neither of these is likely to support actual purchasing activity, so Purchasing may also get the engineering BOM with the hope that it'll be remembered months or years in the future. More likely, Engineering will get a request for help, or production will simply be delayed waiting for the preferred part. Engineering's efforts are, at best, occasionally used but often wasted.

Phantom parts: How to identify BOM-specific ALTs with non-inventoried parts

Returning to first principles, we know that a part number is assigned for each unique part. If the application of a part is new, then a new part number should be created.

Alternative parts are simply a set of items approved for a more specific application than standard parts. Creating a "phantom part", with a distinct part number and list of approved sources, solves the problem of transferring the engineering list into the manufacturing system. The preferred part can be ranked as 1, and non-preferred alternate parts ranked as 2; a more detailed preference ranking can also be applied (e.g., 9mm is ranked as 2, while 10mm is ranked as 3). A phantom part can be naturally represented in both PLM and MRP systems, the only effect being that it is never inventoried under its own number, but sourced from other inventory locations.

Unlike some product lifecycle management software that draws firm lines between internal "home" parts and supply chain "partner" parts, PDXpert PLM software can represent internal ("home") parts as sources for other internal parts. Therefore, any interchangeable home or partner item can be a source for another item. This allows you to easily identify both internal and normal third-party parts as sources for your non-inventoried phantoms.

PDXpert software can, if you wish, allow you to specially designate phantom parts with a unique part type (category) and even have a special part numbering prefix (such as "ALTnnnnn" or "99nnnn") and part title (like "Alt list, [description]").

Alternative approaches for representing your ALT parts

Consider these possibilities for identifying your alternate parts. Assume that your internal part numbers 1001 and 1002 are both interchangeable on a particular structure:

If there’s no special reason to identify ALT parts separately from normal parts, simply create a new part 1003 with parts 1001 and 1002 as sources. You can also add supplier parts to 1003 that work only in this application but aren’t qualified as sources for 1001 or 1002.
Perhaps it's desireable to signal to the MRP system that it's importing an ALT part. To uniquely identify ALT parts, create a new Part Type like "Alternate". Create the new "Alternate" part with a unique part number format like ALT-1003 or with a normal part number 1003. Add both parts 1001 and 1002 as sources.
Finally, it's possible to create 1003 to directly list all of the sources for 1001 and 1002, without explicitly showing 1001 or 1002 as a source. While very simple to describe, this method implies that there is a separate inventory bin for 1003 and, without also listing 1001/1002 as sources you may end up with unbalanced inventory in separate locations.

It may take a bit of analysis - and perhaps some discussion with your manufacturing staff and IT team - to determine which approach is preferable based on your inventory management practices and MRP capabilities.

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Managing part interchangeability is essential to quality products