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Master Scheduling
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Lean Manufacturing Articles


Good planning at the master schedule level means ad­herence to time fence policies. Time fences are among our most misunderstood concepts. They are supposed to represent the time it takes to produce a given product, including the purchase of raw materials and components, the manufacture of components and subassemblies, and final production or assembly. These time fences can be reduced by shrinking the lead times of the items on their critical paths or by stocking them in significant quanti­ties. Edicts or wishful thinking cannot shorten them.

The purpose of maintaining time fences is to give warning when scheduling decisions are needed that will affect manufacturing or purchase orders already in motion. They set the boundaries between orders that need schedulers' judgements and those that the com­puter can automatically plan. The rules that are gener­ally applied are that master schedules inside the time fences are firmed and manually planned based on ex­ception messages. Orders outside the time fences can be left to the computer based on the planning policies that have been loaded. This allows the master schedulers to concentrate on the most important orders that have the greatest impact on manufacturing and the outside suppliers.


Many companies ignore time fences, artificially shorten them, or take shortcuts in calculating them. It is com­mon for companies to understate their time fences be­cause long fences imply inflexibility for sales and customers. This does not fix the problem. It simply re­moves the warning and creates a false sense of capabil­ity and loss of control of the outcome.

Some of the software has rules that do not respect the boundaries. It is not unusual to see automatic sched­uling inside the time fences or system-planned orders inside of firm planned orders.


These problems lead to master schedules that are not valid because the computer logic is very literal. It will schedule the impossible, drive delinquent component orders, or expedite orders to replenish small quantities of safety stock. Master schedulers spend inordinate amounts of time undoing what the system has created. Customer ser­vice and bottom line results are not dependable.


Lead times should be measured for manufacturing and purchasing. Time fences should be calculated from these lead times and adjusted based on whether actions have been taken to make long-lead-time items readily available. Each master-scheduled item should have its own, independent calculated time fence. Orders must be firmed to cross into the period inside the fences. The system must not automatically plan orders inside the time fences or earlier than orders that have been firmed or released.

To Be Continued

For balance of this article, click on the below link:

Lean Manufacturing Articles and click on Series 14

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