Page 465 - Design for Six Sigma for Service (Six SIGMA Operational Methods)

Theory of Constraints 423

• Develop alternate routings, processing procedures, or subcontractors.
• Move inspections and tests to a position just before the constraint in
order not to add additional strain to the bottleneck.

In the constraint or bottleneck, every hour of wasted time, whether it is
downtime or idle time, is a lost hour of the whole production system. So
every technique should be used to reduce every form of wasted time.
Employing well-trained people to run the bottleneck is very essential.
According to a story by Woepple (2001), in one of his consulting projects,
he found a bottleneck process step. Unfortunately, this most important step
in the whole production system was run by the lowest-paid people and the
employee turnover in this step was really high, which made the problem
even worse. When the pay structure was changed and the people in this
position became the highest paid, the employee turnover was greatly
reduced and the throughput was greatly increased.

12.3.3 Subordinate Everything Else in the Process to the Constraint

If you cannot increase the capacity of the bottleneck, you have to live with it.
Your throughput will not be greater than the capacity of the bottleneck. The
best you can hope for is that the throughput is exactly equal to the bottleneck
capacity. In this case, the plan will be to achieve the production pace equal to,
and not less than, the bottleneck capacity. An important precondition to achieve
this is the smooth production flow from the beginning of the process to the end,
with exactly the rate of the bottleneck capacity. Any fluctuation of the flow rate
will cause more blockage and starving, thus reducing the throughput.
The following are some basic rules in the theory of constraints (Goldratt and
Cox 1986, Goldratt 1990):
1. An hour lost at a bottleneck is an hour lost for the entire system.
2. An hour saved at a nonbottleneck is a mirage.
3. Do not balance the capacity; balance the flow.
4. The level of resource utilization of a nonbottleneck is not determined
by its own potential but by some other constraint in the system.
5. Bottlenecks govern both throughput and inventory in the system.
6. Priorities can be set only by examining the system’s constraints. Lead
times are the result of a schedule.

Goldratt also proposed a production control strategy based on the theory of
constraints. He called it the drum-buffer-rope (DBR) strategy (Goldratt and
Cox 1986, Goldratt 1990, Mabin and Balderstone 2000). This strategy is
based on the following ideas: