Theory of Constraints Drum Buffer Rope
Drum Buffer Rope (DBR) was developed by Eliyahu M. Goldratt (Eli Goldratt), the father of Theory of Constraints. His book The Goal tells a story of a plant manager applying the drum buffer rope concepts.
The drum is the constraint.
The resource that is limiting your output. Most shops typically only have one constraint at any point in time, but the issue is that for many custom job shops and machine shops the constraint can move as the mix changes. A constraint is any resources that had demand greater than it's available capacity. Any time lost on the constraint is output lost by the entire system. Increasing output at the constraint, increases overall output. Improving non-constraints has no effect on overall output. This is why measuring utilization everywhere makes no sense. Trying to 100% utilize non-constraints just results in too much WIP and the many issue associated with that.The buffer is measured in time.
It's the amount of work expressed in time (like days worth) prior to the constraint. We control this amount of work with the rope. By having a buffer of work in front of the constraint, we can ensure the constraint does not run out of work. The constraint is the only place where 100% utilization is a good thing! The buffer's job is to absorb variability. In traditional Drum Buffer Rope there are 2 buffers - one for the constraint and a shipping buffer. The one before the constraints is there to protect the constraint and the shipping buffer protects the due date. Simplified Drum Buffer Rope just has a shipping buffer. Drum is the same. Any buffers are divided into 3 zones - red, yellow, green. If the buffer is red we find out why. The buffer is sized so that it turns red about 5% of the time. For continuous improvement we track the whys so that we can reduce or eliminate our biggest disruptions to flow. These improvements to flow lead to less variability and needing less buffer!The rope is how we control the release of new work.
The idea is that if the constraint sets the pace, the drum beat, for the entire operation, then we should only release work at the rate that the constraint can consume it. If we release work faster than the constraint can consume it, then WIP (work in process) piles up and bad things begin to happen. (See Little's Law.) That's a brief Theory of Constraints summary of Drum Buffer Rope. The real challenge is figuring out how to make it work in YOUR shop. The plant in The Goal is a machine shop. It's not the most complex of cases, but it is a machine shop.Theory of Constraints Goldratt
Velocity Scheduling System was developed for the tough cases:- For the custom job shops and machine shops that run high mix and low volume work or prototypes.
- For the shops where the constraint or bottleneck moves based on the mix of work.
- For the shops that may not run many jobs twice but can also have a production job to fit in with everything else.
- For the shops that do repairs (in house or on-site) or emergency work.
- For the shops that don't have perfect employee or vendor performance.
- For the shops who don't have perfectly cross trained employees, but have jobs that require a particular machine.
- For the shops that produce a wide range of custom parts or products that have a wide range of lead-times, set-up times and outside processes.
- For the shops that may not have 100% quality performance and or yields that are less than 100%.
- And, for the shops who have customers call and change quantities, dates or both.
This article is copyrighted by Science of Business, Inc.
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