The most prevalent mistake made on assembly drawings is to put the parts list on the body of the drawing. Prior to general use of computers, it was accepted practice was to place the parts list on the body of assembly drawings. When computer use became common, they brought a powerful capability to produce detached parts lists for materials ordering and control. It was also too easy to leave them on the body of the drawing. The parts list should normally be on a separate detached list. The find or balloon number relates the picture to the list. The detached list can be obtained from your software system.
- Rule: Do not put parts lists on the body of an assembly drawing unless they are put there by a singular BOM database/data entry. If you have them on the body of the pictorial drawing, start a planned program to detach them.
- Reason: The pictorial parts list is redundant to a parts list in a database. This redundancy requires wasted effort to input more than once and to reconcile list differences. It is not just confusing, wasteful and error prone, it is dangerous as it allows possible diverging designs.
An exception would be wherein the company has one software system which feeds the parts list to the drawing as well as to other data processing systems. In this case, the existence of one data entry is the desired result. Unless your systems are automatically connected, the conclusion should normally be to detach the parts list and not to develop a separate one for the pictorial.
It is often stated that the production people need the parts list on the face of the pictorial drawing. This is usually said in companies who do not have an Industrial Engineer responsible for writing assembly processes – a much better tool for production than engineering pictorials.
In best industry practices, the part number of both assembly pictorial and parts list documents is identical. The find number allows easy cross-reference between the two documents. The revision level of both documents is maintained identically. Keep them that way to avoid confusion, even though all changes do not affect both documents. This avoids requiring customers to look-up the correct rev level of the matching “set” – all customers, forever.
Some companies have chosen to make the two documents different item numbers and to cross reference by listing the pictorial number on the parts list. Although less desirable, this is a workable scheme. It allows CM to only change the affected document and to allow their revision levels to be different. This scheme favors the CM department, however, not the customers of the documentation. Better from the customers’ viewpoint to spend the extra CM effort to make them the same part number and keep them at the same revision level.
Start up companies or companies changing their documentation system should seriously consider this issue. Established companies with a workable two number system should not change to a same-number system unless they are changing their part numbering system for some other reason.
CM Tips furnished by:
Frank Watts BSME, CCDM.