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Manufacturing Capacity Planning

 

PART III. 

 

Let's look first at what is often called the "disconnected flow" environment. (Also known as "batch" or "job shop".) This environment is characterized by operation times (set-up and run) that are in the order of 20% of the total throughput time, with the remaining 80% being in wait, move, transit and queue time.

In this environment, production typically involves several indi­vidual operations, movement of material between operations is non-linear, and relatively long set-up times are the norm. Capacity Requirements Planning (CRP) involves breaking production orders down into their individual operations and calculating the capacity required for each operation from routing data. The timing of the capacity requirements are determined by scheduling back­ward from the order due date (Figure 2). The capacity require­ments are then collected for each individual work center and can be displayed in a time-phased fashion as in Figure 3. Since in the typical manufacturing plant there are likely to be hundreds of orders scheduled to move through a given work center in a week, this report is likely to be very lengthy. The report shown in Figure 4 shows the same information in summarized form.

Note that the summary report (Figure 4) shows periods of both overload and underload on both the equipment and the work force. It is obvious that one or more of the above noted changes must be employed to bring this situation into balance.

There are two ways to go about this—finite scheduling and capacity planning. The former technique is based upon the concept of automatically scheduling work within a stated resource capac­ity. The latter technique is based upon the "infinite load" concept of initially scheduling work regardless of resource capacity and then selecting the most appropriate solution via human analysis and decision.

The debate between these two techniques rages from time to time. Let it be said that neither is perfect. The problem with infinite loading is merely that capacity is not infinite, and to arrange a schedule based solely on this technique will most certainly lead to trouble due to both overloaded and under-loaded resources. On the other hand, the finite scheduling technique assumes that the available capacity cannot be changed and immediately leaps to the "reschedule" solution, which in turns violates the validity of the overall plant schedules since lower level schedules have been altered without reconciling them back to MRP and the Master Schedule.

What is useful is a combination of the two techniques. In the very short term, adjustments to available capacity are at best costly (i.e. overtime) if not altogether impossible (i.e. additional equip­ment). Thus, finite scheduling is reasonable in this part of the scheduling horizon. On the other hand, adjustments to available capacity are quite practical in the mid to long term. Utilizing infinite loading techniques in these areas can be very effective provided the human participation element in the capacity planning process is properly employed.

Use of finite loading in the mid to long term violates one of the objectives of capacity planning identified above: to solve capacity problems in a timely manner to meet customer needs. Presum­ably, the projected overloads and underloads are dictated by changes in the expected specifications, mix, or the volume of business. Looking again at Figure 4, the planned orders that are causing the overloads in the future would be scheduled out by the finite scheduling process, thus rendering it impossible to meet the Master Schedule that drove the material requirements that in turn created the capacity requirements. On the other hand, using Capacity Requirements Planning to address this situation would likely result in action to increase the available capacity in the appropriate future time frame so that the overload condition will be resolved by the time the planned work is released for actual production. This would minimize—if not eliminate altogether— the need for finite scheduling in that particular period.

Rough-Cut Capacity Planning (RCCP) is a simplification of the CRP process utilizing a "resource profile" for a product rather than a detailed routing for every individual part. Because of limitations of the process such as lack of inventory netting, lead time offsetting, and lot size and set-up assumptions, RCCP should only be used to estimate capacity requirements for the mid to long term. Not to be interpreted as a substitute for CRP, RCCP should be used as a process for validating the aggregate Production Plan or the Master Schedule prior to releasing those plans for full MRP and CRP calculations.

Input/Output Control (I/O) is a process of monitoring the flow of work to and from each work center against what was planned, and is an excellent tool for controlling the amount of queue in a work center.

To be Continued


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