Page 139 - Orlicky's Material Requirements Planning

118 PART 2 Concepts

The three principal functions of MRP are to

1. Plan and control inventories
2. Plan and replan released-order priorities
3. Provide data for capacity requirements planning
Design of MRP programs should meet these objectives:
1. Planning inventories:
■ Order the right part
■ Order in the right quantity
■ Order at the right time
2. Planning priorities:
■ Order with the right due date
■ Keep the due date valid
3. Planning capacity:
■ A complete load
■ An accurate (valid) load
■ An adequate time span for visibility of future load

TECHNIQUES OF TIME PHASING

One of the oldest inventions of human civilization is the division of time’s continuous
flow into increments suitable for measuring its passage and the construction of calendars
to provide a frame of reference. Our Gregorian calendar serves satisfactorily for most
purposes, but when inventory-plan ning and production-scheduling procedures are to be
automated (i.e., their execution transferred from a human being to a machine), certain
charac teristics of this calendar present difficulties. The Gregorian calendar does not
employ a decimal base, months have an uneven number of days, and the pattern of hol-
idays is irregular.

Scheduling Calendars
Since these factors would complicate time-related computing procedures unnecessarily,
it is common to devise special decimal calendars that are used for this purpose. There are
a variety of so-called scheduling calendars or shop calendars, but what all have in com-
mon is the principle of consecu tively numbering weeks and/or days. This functionality
is now embedded in all commercially available MRP systems and is invisible to the user.
Commonly, there is one calendar for the company’s own work and another for supplier
lead times. Recommended action dates are adjusted by the declaration of working versus
nonworking days in the calendar.
In the use of this calendar, one week is normally equated with five working days.
Thus, if an item had a five-week manufacturing lead time, 25 would be sub tracted from