Page 289 - Orlicky's Material Requirements Planning

268 PART 3 Managing with the MRP System

FIGURE 14-5
Week
Open orders do
not coincide with 61 62 63 64 65 66
dates of need.
Gross Requirements 30 5 10 10 10
Scheduled Receipts 20 20
On Hand 27 –3 12 12 22 12 2
Planned-Order Releases

may no longer coincide with the order-completion times (due dates). Such a condition is
illustrated in Figure 14-5. Note that in this example only the timing, not the quantity, of
the total requirements has changed. The coverage therefore is still adequate—no addi-
tional orders are required—but the timing of the open orders is now wrong.

Rescheduling an Open Order
A net-change MRP system detects this condition immediately on processing the transac-
tion that caused the gross requirements to change. A regenerative MRP system detects
this during the requirements planning run. A changed gross requirements schedule
necessitates a recomputation of the projected on-hand schedule, and the new schedule
contains the clues to the action required. In Figure 14-5, there is a net requirement (for
three) in the first period, followed by open orders in subsequent periods. The system is
programmed not to generate a new planned order to cover the net requirement (regular
logic) but to request rescheduling of the closest open order.
In the third period (week 63 in the example), there are 12 on hand, and the gross
requirement in the subsequent period is only 10. This requirement can be covered in full by
the quantity on hand, and there is clearly no need for the open order to arrive in week 64,
as originally scheduled. This order should be rescheduled (its due date changed) to week
65, when it is actually needed. The two tests for open-order misalignment are as follows:
1. Are there any open orders scheduled for periods following the period in which
a net requirement appears?
2. Is there an open order scheduled for a period in which the gross requirement
equals or is less than the on-hand quantity at the end of the preceding period?
The MRP program carries out these two simple tests whenever the projected on-
hand/net requirements schedule is being recomputed. If a test is positive, the system
generates the appropriate rescheduling message. Note that extension of the second test
to subsequent periods will indicate that an open order should be canceled when the on-
hand quantity in the period preceding the scheduled receipt of the order is sufficient to
cover all remaining gross requirements. This is equivalent to rescheduling the order
beyond the far edge of the planning horizon. Note also that the system recomputes the