Finite Capacity Scheduling (PCS)
The steps to implement this powerful technique are listed under the
earlier subheading, Infinite and Finite Loading. Work standard
accuracy for setup and run times will determine the validity of
Figure 8 shows that transit time between operations can be
this can be avoided by using cells where materials move immediately
between machines or work stations. Queue times should not be included;
these will be calculated by PCS programs as orders arrive in
Use only demonstrated capacity data discussed under the heading
The Planning Phase; theoretical figures are nice to know but rarely
achieved, and increasing effective capacity is the primary goal of
Before running wildly ahead, learn to walk by
using only basic modules of software programs; don't turn on the
bells and whistles
applying PCS first to critical, bottleneck centers where capacity
changes are limited
limiting horizons to include firm customer orders plus a very few
released plant orders
avoiding priority criteria that will introduce too much
Much hype and humbug has characterized the claims of proponents
of PCS. Citing deficiencies in MRP programs that PCS overcomes,
these gems have appeared recently:
They run too long. Fact—not if net change and weekly time peri
ods are used.
Replanning causes earthquakes. Fact—attack the causes—unnecessary
They're just order launching. Fact—they only recommend release
person should select work input using input/output control.
They assume adequate capacity. Fact—their purpose is to permit
determining average capacity of resources needed to support the
They use fixed order quantities. Fact—options include period order
quantity (POQ), lot-for-lot (L4L), least total cost (LTC), and
least unit cost (LUC) techniques.
They can't synchronize orders. Fact—"Run with codes" can link
items to be run together.
They use only backward scheduling. Fact—they tie the provision
of components to the parents "need" dates to show when to start
parent production or procurement.
They react only to past-due orders. Fact—this is the task of execu
tion, not planning.
• Successive runs yield no better schedules. Fact—again, this is
Those who published
these comments obviously do not understand
the different roles of planning
and execution. They confuse precision
of calculations with accuracy of
data. They don't realize production operations cannot react
to reschedules as fast as computers can generate
them. Used properly, PCS systems can be very useful in execution.
recognize that future plans are unlikely to be accurate, firm
orders are better bases for schedules, capacity is finite, and
realistic schedules are necessary for plants to be run right.
have always been a wild environment. Murphy's Law was written in a
factory. We who worked there years ago knew that if
nothing bad had happened today,
tomorrow would certainly be worse than normal. Our advice to
peers then was, "If you don't like the heat,
get out of the kitchen." In the
interim, manufacturing planning and control became a profession with
a unique body of knowledge, language, laws, and principles.
Understanding of execution improved,
causes of upsets were attacked,
and most common problems were eliminated.
Today, many plants produce in hours what once took months,
new product designs are produced
as fast as marketers can promote
them, and direct labor
productivity rises steadily. The plant floor has been tamed.