Page 280 - DMGT206_PRODUCTION_AND_OPERATIONS_MANAGEMENT

Unit 14: Operations Scheduling

subtracting the times associated with producing that order, including material delivery, Notes
production, and shipping. These calculations resulted in a start date for that customer order. This
process, called backward scheduling, is an approach based on averages, it doesn’t consider the
daily fluctuations and operating factors and conditions on the factory floor.

14.1 Scheduling

Scheduling is the problem of assigning a set of tasks to a set of resources subject to a set of
constraints.

Example: scheduling constraints include deadlines (e.g., job ‘i’ must be completed by
time ‘t’), resource capacities (e.g., there are only four drills), precedence constraints on the order
of tasks (e.g., a piece must be sanded before it is painted), and priorities on tasks (e.g., finish job
‘j’ as soon as possible while meeting the other deadlines).

Example: scheduling domains include classical job-shop, manufacturing, and
transportation scheduling.
The early 1960s saw the emergence of the concept of MRP. It used backward scheduling to
highlight material shortages and then generate production and purchase orders to avoid those
shortages. With the emergence of IT, the computerization of MRP automated the process and
work associated with material requisition. These new tools rekindled the interest in plant
scheduling problem and a new system, MRP II, emerged.
MRP II introduced the concept called the Master Production Schedule (MPS). MPS was a layer
added on top of MRP. It also marked the end of ordering inventory based on past usage. Instead,
MPS focused on sales and marketing’s best guess of the future need for products. This best guess
was then passed to the next planning function, namely, the next MRP run. Both MRP and MPS
assume certain ideal characteristics about the imperfect world of production and the plant floor:

 Infinite resources (machine capacity and labour) are always available and do not change.
 Material resources will arrive as scheduled in the right quantities. Any variances, or
missed incoming shipments, were expedited manually until the next MRP run.

 Customer orders and products have the same priority. MRP aggregates demand (customer
orders) into lots and outputs.

 Lead times (production and material delivery) are fixed or proportional to lot size.
 Scheduling on a weekly basis will meet planning requirements.
Scheduling starts with the Master Production Schedule (MPS), which defines current and future
(forecasted) resource requirements based on current and forecasted customer orders. Completing
these orders is the goal—the MPS provides production targets toward reaching that goal. In
doing so, it takes into account the technical requirements of the task and available capacity and
matches it with the forecasted demand. Everything else in the planning system works from the
MPS. The result is a set of purchasing and manufacturing orders with start and due dates, and a
list of the minimum quantities of inventory to satisfy the MPS.

The planning system also initiates operations scheduling through capacity requirements
planning.

 It starts by determining whether the enterprise has the production capacity available to
build what’s listed in the MPS.

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