human resources form the productive capacity (and their mix determines the level
of automation), what must be scheduled and managed are the following:
1. The single items of materials (Material Requirements Planning – MRP)
2. The capacity of each work centre (Capacity Requirements Planning – CRP)
From a logical viewpoint, materials and capacity can be considered on the same
level, although it makes no sense to schedule the capacity in the absence of materi-
als. Hence, material requirements are scheduled beforehand, and once their
presence is ensured, productive capacity can be planned. In the event of overload,
workloads can be levelled or material is allowed to queue.
The requirement of materials is planned through the MRP technique, except in the
case of materials of less value or importance for which the Re-Order Point (ROP)
technique is used: in this case, the stock level displays a typical sawtooth pattern.
MRP calculates the net requirement of materials from the gross needs, subtracting the
available stock and the amounts that are being processed and are not yet assigned
(materials exceeding previous requirements, and released for reasons of minimum lot
size); production, assembly or purchase orders are released with an as late as possible
logic, taking into account the lead times (relative to production or supply).
Figure 2.6 shows an example of MRP calculation. The typical MRP record (one
for each item) consists of four rows of information, periodically updated: (1) gross
requirements, (2) scheduled receipts (existing replenishment orders not yet assigned),
(3) stock availability (from a certain time zero on), (4) planned order release (this last
row is the result of a calculation). To calculate this value, certain parameters must be
specified, such as lot size (if operating by lots), production lead times (the production
order of the 40 units in Fig. 2.6 is released two periods before reaching the minimum
stock threshold level, as the lead time is of 2 periods), safety stock, etc.
It must be noted that when receding from a future date (i.e. the time of release)
towards the present, time availability may be insufficient (i.e. production should
have already started): in this case, the MPS must be revised (the dotted feedback
lines in Fig. 2.5). If there are parent items, their release primes a gross requirement
for child items. These parent–child relationships between the various items are
described in the Bill of Materials (BOM).
In the case of single contract orders, it is possible to plan material requirements
through an MRP focusing on critical items, and in that of repetitive contract orders,
through an MRP with planning bills; these facilities are already present in the more
updated MRP modules, which are valid for cone and mushroom/sandglass-shaped
bills of materials.
In cone-shaped BOMs, a limited number of end products, if not only one, are
obtained by combining a large number of items (typical examples can be found in
the aeronautical industry, shipyards, industrial plants, etc.). Items may be consid-
ered critical for a variety of reasons: technological (difficulties in their production),
managerial (long production or procurement lead times), economical (high unit
value) or structural (linked to other critical items). The MRP focusing on critical
items gives them priority, and tries to limit subsequent variations (depicted by the
dotted, feedback lines in Fig. 2.5) as much as possible.
2.5 From Engineering to Manufacturing 25