As alluded to in my prior post: With this entry, I’ll start a series of seven articles covering topics of interest to manufacturers. In particular, we’ll delve into constraints… Eli Goldratt’s The Goal… the use of Drum-Buffer-Rope and Theory of Constraints thinking to solve those constraints… and scheduling. Along the way we’ll point out a few worthwhile web resources, and mostly, hopefully, get you to thinking… about your constraints.
At the heart of the line of thinking that has been named “drum-buffer-rope” is the basic need to hurdle the obstacle –or relieve the ‘constraint’ — that is found in any process. It’s often about scheduling, as alluded to in our prior post. It is a basic construct of Eli Goldratt’s “Theory of Constraints” as first outlined in his book The Goal (where it first appears in the context of a weekend outing of scouts on a hike, and the overall effect on the troop of the slowest boy’s struggle to keep up). It’s beyond the scope of this blog to detail all the many elements of the The Goal; however, for a great synopsis, you can go here. Typically applied to production problems, the basic construct is as described below.
It’s called Drum-Buffer-Rope as a metaphor for each component. The Drum sets “the beat” or the rhythm by which production occurs, or should occur. In fact, the Drum is the constraint or bottleneck inherent in a production system. Nothing can exceed its beat, naturally, and hence running at full speed, it becomes the constraint.
The Buffer defines a solution to the situation around or outside the constraint, where a process upstream cannot produce as much as the bottleneck or constraint requires. So, we create Buffer, or excess, to ensure the constraint is not… constrained. It’s meant to ensure that the Constraint (or Drum) never has to wait, since waiting is pure waste. Buffer is inventory. Having it in excess leads, theoretically at least, to reduced lead time, and thus, better throughput. But of course, excess inventory is costly, and thus wasteful in itself.
Hence, the Rope. The rope serves to signal a non-bottleneck process upstream when to speed up or slow down, in an effort to maintain a constant but acceptable flow, to and through any constraint. It’s the throttle mechanism on a perfect world of production. In Lean, this is sometimes called “Pull” or Pull Scheduling. In software, we can use data about demand to know when to push or pull the schedule.
Taken together, it’s a construct built on common sense to solve a complex problem that at one time or another affects production almost everywhere in manufacturing.
How do we maximize throughput, smooth out the bumps, reduce costs and eliminate waste… all at the same time? In other words, how do we achieve maximum capacity at the bottlenecks with minimal waste, defects and expense?
We’ll head there next post…