The simple, powerful mechanism is the core of the Theory of Constraints Application to Production
The name Drum-Buffer-Rope originated with an analogy in a Theory of Constraints book called The Race (there are far better TOC books today … although The Race has a simplicity that’s still appealing).
There are several principles behind Drum-Buffer-Rope; some key ones are:
1. In any set of resources, some will be more heavily loaded than others (on average). These are often termed the Capacity Constraints, or CCR (capacity constrained resource). In some plants there is really only the one; in others, there might be several; sometimes, it’s not easy to identify them without training in Theory of Constraints.
2. The most capacity constrained resource will disctate the rate the whole plant can flow work from materials through to finished goods.
3. There is no value in any resource that feeds the constraint, producing at a faster rate than the constraint. Common measurements might encourage this but the only outcomes are increased WIP, increased lead times, probably more expediting, probably damage to on-time performance, and probably damage to productivity as people waste time trying to sort out priorities and sort through the WIP to find what they are supposed to be working on next. Plus floor space gets consumed, cash is tied up, there’s more chance of damage, etc.
4. The rate of production of resources that are fed by the constraint is dictated by the rate of output of the constraint.
This lays the foundation for Drum-Buffer-Rope.
The DRUM
First, a schedule (a finite schedule) is developed for the work that has to pass through the capacity constraint. The nature of this can be as simple as,
1. Make 100 of part X
2. Then, make the 25 of part Y.
3. Then, … etc.
Often the schedule also includes timing.
This schedule is called the DRUM because it’s essentially the drum beat to which the whole plant will be marching. It sets the production pace for the whole plant. (The focus and leverage benefits of this are tremendous, of course).
Next: The Rope
The Rope concept is extremely simple; it is that the only parts and materials that should be released into the “gating” processes in the plant should be those specifically needed to support the Drum schedule. If this seems obvious … it isn’t. In many plants, work is released into the gating operations simply because people are running out of work and there’s a threat of resources being idled. And in others, work is released in quantities that are completely disconnected from what’s actually needed … there is often some “batching,” justified for various reasons. This might still occur with drum0-buffer-rope but it is less common, and more rigorously controlled.
The name “Rope” can be explained by the visualization of “lassoing” just exactly those parts needed for each job on the Drum. In a drum-buffer-rope implementation it is common to introduce “codes” such that if someone issues more material or parts into the system than the Drum calls for, then an explanation has to be provided or else there will be an inquiry. This is because over-issuing materials is often a symptom of a policy constraint i.e. a policy or procedure or measurement that encourages “keeping busy.”
The Buffer
The buffers in drum-buffer-rope can be viewed in two dimensions – time, and stock.
When the constraint resource is close to being a bottleneck, every minute wasted is a waste of the whole plant’s production capability. To get the most out of the plant, the employees have to get the most out of the constraint.
One thing that threatens this is, starving the constraint – preventing it from being fully activated because there aren’t any parts or materials ready for the CCR to work on … which can happen for any of several possible reasons. A logical solution to this is to make sure that there’s always a judiciously sized buffer of work sitting right in front of the constraint so that it is NEVER starved of work. This is a stock buffer.
However, it is created as a result of managing TIME, not STOCK.
Let’s say, for example, that when parts and materials are released into the gating operations of the plant, they take (on average) 20 hours to flow through the different processes until they reach the CCR. This would mean that if we released the work 20 hours before the Drum schedule called for it to be at the CCR … often it would not get there in time, because of the many types of delays and disruptions that can happen — set-ups take longer than planned, or a machine breaks down, or someone is absent, or else there’s a quality issue to be resolved, or else there’s some data missing from a drawing, etc.
So, what we do is, rel;ease the parts and materials (just for purposes of illustration) 28 hours ahead oif when they are needed at the CCR.
This way, if everything flows smoothly the work will arrive at the CCR 8 hours earlier than needed … creating an 8-hour stock buffer. The real advantage of this becomes obvious when you consider what happens when work DOESN’T flow smoothly … when there are sone disruptions and delays. provided those delays don’t accumulate to exceed 8 hours, the work will still arrive early at the CCR and the stock buffer will still exist, and the CCR can stay highly productive and on scheedule (i.e. the Drum is on-schedule).
So … Drum Buffer Rope
So to summarize:
1. Identify the CCRs in the system (this corresponds to the first of the 5 Theory of Constraints Focusing Steps, “Identify the constraints.”
2. By examining the orders in the system, and considering the finite capacity of the CCR resources, schedule the work in detail through the CCR. This is the DRUM. (This corresponds to the second of the 5 Theory of Constraints Focusing Steps, “Decide how to exploit the constraints.”)
3. Choose a Buffer Time to add to the set-up and run time associated with flowing work through the resources to the CCR. This of course is the BUFFER. This is also in line with TOC Focusing Step 2.
4. Only release materials and parts into the gating processes of the plant in sync with the needs of the Drum and the timing of the Buffer … i.e. release only the needed quantity of only the needed parts and materials, and do so only when the Buffer time offset from the Drum calls for the release. This is the ROPE mechanism.
There are many variations and refinements on this basic technique but as described above, it’s the key to generating a huge boost in productivity and exellent on-time delivery performance while shrinking lead times and work in progress inventories.