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Lean Manufacturing, Basics, Principles, Techniques

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Best of Gaw Lean Management Articles

Scheduling

An important task of execution is developing schedules for processing materials. These must be valid so people can be accountable for ex­ecuting them. There are computer programs for several techniques to suit different types of processing. The simplest is shown in figure 6. These rules reveal how little processing time is used actually working on materials. Usually this requires less than 5 percent of scheduled time; 95 percent, therefore, is used on non-value-added activities in support and service work. Accurate data on each required time element are necessary for valid schedules.

 

Another picture of lead time is shown in figure 7; this involves a series of process steps taken to produce a finished product from raw materials. Clearly, a few hours, at most, of working (setup and run) time are spread over several weeks of planning horizon for this product.

 

To generate more valid schedules, accurate time standards are needed for the myriad elements of time in figure 8. This shows on the left Operation 1 on some lot of material being completed in Work Cen­ter 1. After waiting there for a time, it leaves, is moved in some short time, and arrives in Work Center 2. These two elements are called tran­sit time. Awaiting its turn, the lot sits in queue; the two elements, tran­sit and queue times, are called interoperation time. Work then starts, the operation is set up and the lot is processed in the operation time. Finished in Work Center 2, the lot moves on and another is started there. For valid schedules, accurate measured standards or estimated times are needed for each element. Getting such data and keeping them accurate in any plant of reasonable size and complexity is an enormous task. It is rarely done well and is the reason actions to improve accu­racy must precede expensive systems.

Flow Control

Since about 1950, a technique called flow control has been applied successfully in plants where families of similar parts follow the same sequence of operations. This technique greatly minimizes execution work by eliminating detailed scheduling. The requirements to make it effective are

      semi-process flow of a majority of items in similar families

      planned production rates for running these in critical operations

      low and tightly controlled levels of work-in-process

      well-defined In and Out stations so work moves promptly

      clearly visible identification and dating of materials in
work-in-process

      periodic reports on items delayed beyond one or two days

      good housekeeping for easy visual control

      prompt action to overcome delays

If these requirements are met, flow control will speed work through a plant without scheduling.

Loading

Operation time standards are used to develop work center loads in ca­pacity planning. Figure 9 shows a work center with eight weeks of load from released orders scheduled in each week. The vertical scale is hours of total setup and run times. Present capacity is 80 hours per week. The total numbers of hours each week are shown below the week numbers. This a typical load profile; it shows late work, it has weeks of excess load, it is erratic, and it drops off in future weeks.

 

If underqualified people viewed this, they would conclude that the center is overloaded this week and next but will be underloaded later, having only six weeks total work. They may believe that costly actions are required in weeks 1 and 2, not thinking that work late here can be brought back on schedule in a lightly loaded downstream work center. They would think that other costly actions will be needed in future weeks, not seeing that more work might arrive in those eight weeks from planned orders not yet released to the plant. The truth is that no one can tell from just this load picture what the real situation is. Plans made to handle this situation would be very wrong.

Adding planned hours, shown as open bars in figure 10, gives a far different profile. The total work load has increased to 10 weeks' worth. Underqualified people would now make much different plans—and could again be very wrong. Qualified people would recognize that the erratic load requires some cushion of work hours to avoid down time. This cushion is included in the total hours shown. It must be consid­ered in corrective actions.

Infinite and Finite Loading

Figures 9 and 10 resulted from loading the work center as if it had infinite capacity. This is fine for capacity requirements planning, which uses averages over long horizons. For detailed schedules in execution, loading to finite capacity is used. Comparing the steps for both:

•    Steps in loading and scheduling to infinite capacity:

-      Schedule all work order operations.

-      Calculate load hours for all operations.

-      Load operations into proper work centers in scheduled time
periods.

-      To update, remove completed work hours and load new order
hours.

•    Steps in loading and scheduling to finite capacity:

-      Schedule all work order operations.

-      Calculate load hours for all operations.

-      Define work center capacity.

-      Identify alternatives.

-      Set order priorities.

-      Load operations into proper work centers in scheduled time pe­
riods in priority sequence.

-      For overloads, select alternatives, reschedule earlier, or resched­
ule later.

-      To update, start over.

This shows clearly the simplicity of the former and the complexity of the latter. To be effective, finite loading and scheduling depend on the completeness, accuracy, and validity of the data involved, useful only if limited to critical work centers over very short horizons. Only a small number of companies can depend on it beyond the time covered by firm customer orders.

To Be Continued

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

Lean Manufacturing Articles and go to Series 10


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