Page 113 - Orlicky's Material Requirements Planning

92 PART 2 Concepts

metal, coils of wire, etc.) do not meet this expectation cleanly and therefore require that
standard planning procedures be modified and the system adapted to handle such inven-
tory items properly. This is also a challenge in a process manufacturing environment,
where materials can be in silos or pipelines. Parts that are too small and too numerous to
count also can be a challenge to this assumption. These parts may be quantified by
weight rather than by counts.
An assumption implied under MRP is process independence. This means that a
manufacturing order for any given inven tory item can be started and completed on its
own and not be contingent on the existence or progress of some other order for purpos-
es of complet ing the process. Thus so-called mating-part relationships (item A at op -
eration 30 must meet item B at operation 50 for the machining of a com mon surface) and
setup dependencies (order for item Y should be set up only when a setup for item X pre-
cedes it) do not fit the scheme of things under MRP. This does not mean that MRP is inap-
plicable, only that it is inapplicable in its traditional form. See Chapter 18 for a complete
discussion on the application of MRP to a tightly integrated environment.
To recap, the principal prerequisites and assumptions implied by a stan dard MRP
system are as follows:

■ An MPS exists and can be stated in BOM terms.
■ All inventory items are uniquely identified.
■ A BOM exists at planning time.
■ Inventory records containing data on the status of every item are avail able.
■ There is integrity of file data.
■ Individual-item lead times are known.
■ Every inventory item goes into and out of stock.
■ All the components of an assembly are needed at the time of the assembly order
release.
■ There is discrete disbursement and use of component materials.
■ Process independence of manufactured items exists.

APPLICABILITY OF MRP METHODS
The preceding discussion of prerequisites and assumptions raises the ques tion of the
applicability of MRP to a given type of manu facturing business. Actually, all the prereq-
uisites and assumptions listed, as such, do not represent good criteria for applicability
because even where some of the required conditions do not exist, management general-
ly can create them in order to be able to use MRP methods. Inventory items can be
uniquely identified, a BOM can be created, integrity of file data can be maintained, and
so forth. Whether most of the preconditions for MRP do or do not exist in a given case is
usually a matter of management practice rather than an attribute of the type of business
in question. The application of the MRP tool differs by the segment of manufacturing, as
described in the volume/variety matrix in Figure 1-2. This application is described in
Chapters 15 through 19.