incremental leveling phase will require a reduction in buffer quantities through-
out the flow stream, thus driving ever-smaller problems to the surface, where
they demand attention. This will create new instability, and the cycle spirals
toward a tighter level of performance.
Key Criteria for Achieving Flow
As we discussed in the last chapter, foundational elements are necessary for
achieving smooth flow. These key criteria are generally met during the stability
phase, but bear repeating here.
◆ Ensure consistent capability, which is the primary intent of the stability
phase. At the very least, the level of capability should be on a daily basis.
During each day the operation must be capable of fulfilling the require-
ments of the customer.
◆ Consistent capability requires consistent application and availability of
resources—people, materials, and equipment. The inconsistent availability
of these resources is the primary reason that flow is unsuccessful.Methods
must be put in place to ensure availability of resources (not by simply
adding resources, which is added cost).
◆ Reliability of processes and equipment is imperative. Initially this would
encompass the larger issues such as downtime, or changeover, but as the
process is refined it would include lesser issues such as ease and simplicity
of use.
◆ Operation cycle times must be balanced (equal) to the takt time. Uneven
work times will create waiting time and overproduction.Chapter 5. Create Connected Process Flow 91TRAP
The Risk of One-Piece Flow Before Its Time
We have seen companies coming back from training classes excit-
ed about one-piece flow immediately create a cell, discover the cell
is shut down most of the time, and conclude that lean does not
work in the real world. They are suffering from a problem known
as “rolled throughput yield.” Take the case where five machines
are linked together in a one-piece flow and each machine inde-
pendently breaks down 10 percent of the time—that is 90 percent
uptime. In this case the uptime of the cell will be:. 95 . 9 . 9 . 9 . 9 . 9 59 percent uptime of the cell!
The solution: Keeping a few pieces of WIP between operations
in carefully selected locations can increase this to 90 percent.