Part IV: " . . . scheduling production according to the capacity of a specific resource."

(excerpted from The Pipeline)

And — homebuilding production is what type of process?”

It’s a build-to-order process”, said the CEO.

Very good”, said the intrepid, results-based consultant.  “That’s the refrain in this little hymn.  Or, maybe it’s a responsive reading.  I’m not sure.  Anyway, whenever you hear the question, that’s the answer. 

Let’s give it a try”, she said, as she wrote on the board.

DRUM-BUFFER-ROPE

Theory of Constraints uses what it calls Drum-Buffer-Rope (or DBR) to simply convey the concept of scheduling production according to the capacity of a specific resource”, she said.  “Whatever TOC calls its scheduling method is not important.  Drum-Buffer-Rope is simply the terminology that TOC uses to describe a way of scheduling a production process.  Depending on the type of flow that it wants to create, Lean Production uses a combination of a pacemaker, takt time, pull, kanbans, FIFO, and inventory to achieve essentially the same idea. 

In either case, that same idea involves a process that (1) schedules its production according to a selected pace, (2) agrees on the resource the system will use to maintain that pace, (3) ties the work of the remainder of the resources to the pace-setting resource, and then (4) protects the entire system from variation and uncertainty. 

Lean schedules its pacemaker to meet customer demand or forecast, while TOC schedules more to maximize Throughput, by fully-utilizing the capacity on the most constrained resource — on the bottleneck or the constraint.  Other than that, TOC and Lean essentially agree on this point.   

There is one significant difference”, she said.  “TOC purposely unbalances the capacity of the system, in favor of managing a planned or imposed constraint, and subordinates all of the other non-constraint activities/resources to it.  Lean Production purposely levels, or balances, the capacity of the system by means of heijunka (production leveling), demand leveling, and continuous flow – unless, of course, it’s a build-to-order process, in which case, TOC and Lean at least partially agree, because Lean recommends selecting the bottleneck (or the constraint) as the pacemaker. 

And — homebuilding is what type of process?”

“It’s a build-to-order process.”

Under DBR production scheduling, the “drum” is the resource that sets the pace of production, and every other resource is subordinated to the requirements and needs of the drum resource.  Under TOC, the drum resource is always the constraint, which means that it is the resource that determines the throughput of the entire system.   Previously, we talked about a system being like a chain;  the constraint resource is the chain’s — the system’s — weakest link.  It is the activity/resource that either has the least capacity, has the greatest number of dependencies, or has the most work. 

The pace-setting resource in Lean Production is called the pacemaker”, she explained.  “For the most part, Lean doesn’t much like constraints, and views them as something to be prevented, fixed,or avoided, not something to be managed.  Lean prefers to level production and match it to takt-time. 

So, in Lean, the pacemaker is usually not the constraint — unless, of course, it’s a build-to-order process.  In which case, TOC and Lean essentially agree, because Lean recommends selecting the bottleneck resource — its constraint — as the pacemaker.   

And — homebuilding is what type of process?”

It’s a build-to-order process.”

That’s right”, said the intrepid, results-based consultant.  “Under either TOC or Lean, the entire production schedule is tied to the pace set by this single activity/resource — to the pace set by this pacemaker/drum — whether that resource is the constraint or not. 

Under Theory of Constraints DBR, the mechanism that ties the schedule to the drum is called the “rope”.  The rope is the synchronizing mechanism, and the outcome is synchronized flow.  In Lean, the schedule depends on the type of flow — on whether we have continuous flow, pull or FIFO.  If we have continuous flow, release is not an issue and we don’t need a release mechanism;  if we have pull, the mechanism is something that acts like a kanban. 

As you recall, Lean would prefer continuous flow — “flow where you can, pull where you must”.  If Lean cannot create flow, it will resort to pull;  it just has to have something to pull with.”

Like a rope?”, quipped a superintendent. 

Lean and TOC are both known as pull systems, in part, because of the actions of the rope and kanbans”, she continued.  “For a number of reasons, RB Builders very much intends its production system to be a pull system.”