MRP software can be adequately programmed to handle the tasks DBR requires. But there are definite advantages to computer application packages specifically designed to support DBR operations (i.e., not MRP software).
Most of these software applications are compatible with existing MRP/ERP packages. They replace the primary MRP data run (sometimes referred to as
“mrp”), but the MRP system is still used to maintain the database, data entry, and some of the tools such as rough-cut capacity planning and capacity requirement planning.
The first advantage of DBR-specific software applications is that most of them can support finite-loading constraint schedules. Second, they can pro-vide a feasibility check by showing (in simulated runs) whether all the non-constraints can really do what they must to support the constraint in the required time.
Third, they can provide the capability to selectively shorten buffer times for better lead times. By using DBR-specific software to analyze actual results, buffer management can be improved. The software can show, for instance, that Zone 2 or Zone 1 were never penetrated. In such a case, the buffer is probably too large and might be safely shortened. Alternatively, this same kind of analysis might show Zone-1 penetrations occurring too frequently, revealing a need to enlarge the buffer.
More buffer time might be advisable when a non-constraint can’t cope with constraint support demands, such as periodic seasonal peaks or
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unexpected demand spikes. Larger buffers may also be required when the variability in quality is high, or if there are many process steps.
The term “dynamic buffering” is used to describe the capability of DBR software to identify temporary load changes on non-constraints. In effect, this segregates internal resource variability from external demand uncer-tainty, so that the buffers for each can be managed separately. This feature can be extremely useful when internal variation is small enough not to require a very large buffer, but changes in customer demand (external uncertainty) fluctuate much more widely.
In software packages that have a dynamic buffering capability, the user can usually reduce the normal buffer and still protect against “true Murphy,”
or internal variability. Load fluctuations due to changes in customer demand are noted by the software. When the DBR automated scheduling system identifies a peak demand building on a certain non-constraint, it automati-cally enlarges the appropriate buffer. In other words, it releases material earlier than usual to allow non-constraints enough time to do their jobs and still allow the work-in-process to reach the protected resource on time. This capability can take such arbitrary peak loads out of the realm of uncertainty and make them predictable at the planning stage, so that appropriate man-agement action can be taken earlier. In other words, the normal buffer can be relatively short, yet be quickly expanded — automatically, by the DBR software — when the need arises.
After a transitory load on a non-CCR is gone, the software will adjust the buffer back to the original level. Other factors (variability in quality, number of process steps) require human intervention to reduce their impacts and further shrink the buffer.
Summary
Let’s review what we’ve covered in the last four chapters (Part II). Most production managers would agree that a robust plan beats expediting any day. The closer to full load the manufacturing process gets, the more it destabilizes in the face of variation and uncertainty. TOC production policies can liberate hidden capacity in the process. We obtain this extra capacity by:
Focusing on the capacity constraint (CCR).
—Exploiting it and subordinating non-constraints so the CCR won’t be starved for work, and
—Doing only orders that are needed now.
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146 Manufacturing at Warp Speed
Making-to-order whenever possible.
Completing work-in-process in smaller batches.
Transferring work between stations in smaller batches.
Preventing early release of materials for orders due much later.
Additional manufacturing cycle time reduction can be realized by sched-uling daily instead of weekly. A daily schedule should add new firm orders to the MPS, while preserving the plan for existing firm orders. And it’s better to defer work that isn’t immediately needed. In other words, complete firm orders before making-to-stock, even if it means breaking set-ups.
Drum-Buffer-Rope is a way to obtain the best overall system efficiency, safeguard against variation and uncertainty, deliver on time (in the shortest possible time), speed the flow of work-in-process, and attain greater control over the whole manufacturing system.
The “drum” is the schedule for the Capacity-Constrained Resource (CCR). The “rope” is the material release schedule, preventing premature introduction of work-in-process. “Buffers” are the time allowed for work orders to arrive on time at the shipping dock, the CCR, and assembly points, despite “Murphy.” Time buffers protect the delivery schedule so that less expediting is needed, and managing the buffers can help in identifying an emerging constraint. But even the best buffers can’t provide complete pro-tection against the unexpected. Consequently, we need some kind of control method to warn us of an impending problem before it’s too late to do anything about it.
Knowing what’s important and what isn’t helps control information over-load. Constraint theory helps distinguish what’s important from what isn’t.
Due-date reliability depends on control of the system. And finally, buffer management is the key to the success of DBR.
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