I don't suppose that many people consider this to be an important topic, but it is.
Part numbering or symboling systems are really subspecies of cataloging systems generally. They can range from the entirely arbitrary (such as just numbering the parts sequentially) to the elaborately systematic. The advantage of an arbitrary system is that it always works; its disadvantage is that the part number tells you nothing about the role of the part in the machine. The advantage of a systematic approach is that the part's symbol conveys information about its role in the machine; its disadvantage is that ultimately all taxonomic systems break down. At some point, in some revision, you'll have a new part that won't fit the system. As Wilfred Bancroft says in his article on the Monotype symboling system:
"Probably no completed machine has ever been built, for improvements in both design and shop methods are as much in the day's work as the booking of orders and the boxing of machines." (531)
The choice of part numbering/symboling systems to be surveyed here isn't in any way comprehensive, or even characteristic of any field. It's just a survey of the more important systems that I've come across in my own research (primarily in typecasting machinery).
Bancroft / Monotype
The elaborately systematic parts scheme developed by the Lanston Monotype Machine Company in Philadelphia (and used, as well, by the Monotype Corporation Limited in England).
Mergenthaler
The system of parts identification used by the Mergenthaler Linotype Company (and to some degree by other companies for Linotype or Linotype-compatible parts) evolved into a two-part system which consisted of a part symboling system (usually called a "numbering" system even though the "numbers were alphanumeric) together with a part naming system. The symboling system was essentially numeric, with a single (or sometimes double) letter prefix which originally designed the drawing sheet on which the part appeared in the parts book.
A system of the same form was adopted by the Intertype Corporation.
Both the Mergenthaler system and the Intertype variation on it were used in an irregular fashion by the Linotype Parts Company / Star Parts (see below).
Thompson, Pre-Monotype
The Thompson Type-Caster employed, successively, two different parts symboling systems before their acquisition by Lanston Monotype (who changed everything over to a Bancroft-style system). The first was a purely numeric system; little is known about it. The second was an alphanumeric system using single-letter mostly-mnemonic prefixes.
Ludlow Typograph and Elrod
The system of parts symboling used by Ludlow was basically numeric, with both prefixed and suffixed letters (and fractional numbers used to fill in new parts when necessary). [TO DO: Analyze enough Ludlow parts books to figure out the interpretation of the prefixed and suffixed letters.]
Aside: In my opinion, the Ludlow demonstrates the unsuitability of a numeric scheme for machine with hundreds of parts. It can take a long time to spot a part number on one of the more complex pages of the manual, even when you know it's there.
Hunslet (Locomotives)
The system used by the Hunslet Engine Co. Ltd. (UK) for their "Austerity" locomotive is known because the parts book for it has been reprinted by Camden Miniature Steam Services. It is a pure numeric scheme which allocates numbers in ranges associated with particular sections of the machine, but with "holes" in the numbering to accomodate future changes and additions.
In some cases I have had to create parts numbering systems myself. I've had to do this either because none existed previously for the machine in question (e.g., the pivotal type caster) or because I did not have access to the "real" systems (e.g., the Chinese standard watch movement).
I tend to follow the Bancroft/Monotype system, primarily becuase I have found it to be very useful in my work with the Thompson Type Caster.
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