The basic problem with MRP II is that it uses standardized lead times
instead of evaluating the impact of each order on the production
environment and adjusting material releases accordingly. Furthermore,
it assumes that you have to have all of the necessary raw materials to
make the finished product in order to start production. While you
ultimately need to have all the raw materials in order to produce the
finished item, certain raw materials are likely to be more essential
than others.
The concept that reconciles MRP II with lean manufacturing is known as
the Theory of Constraints. An excellent, highly readable book on the
subject Is "The Goal: A Process of Ongoing Improvement" by Eliyahu M.
Goldratt and Jeff Cox available at Amazon.com
http://www.amazon.com/exec/obidos/tg/detail/-/0884270610/002-4725186-4452838?v=glance.
For an overview of the basic concept, I refer you to
"Operations/Production Management-Speed , Reliability, and Capacity"
Focused Performance http://www.focusedperformance.com/prod1.html .
The basis of the Theory of Constraints is that production schedules
have to acknowledge the production process's constraints and
accommodate them. To make this as easy as possible, the theory
recommends specifically selecting a particular production step and
making it the constraint so that it is predictable and measurable. A
sensible way to this is to choose the step that requires the most
expensive piece of equipment per unit of production capacity. Then,
the object is to keep the constraint fully occupied as long as demand
permits.
By having excess capacity before and after the constraint, the
constraint is never starved or blocked, and inventory will only
accumulate in front of the constraint. This allows inventory to be
kept at a minimal level and easily managed. Furthermore, as long as
the raw materials used by the constraint are available, production can
be continued provided that the other necessary materials can catch up
to the constraint materials prior to final assembly. The scheduling
concept used is known as Drum-Buffer-Rope.
By making the MRP II release schedule much more realistic by
incorporating the schedule's effects on the manufacturing process's
constraint or constraints instead of simply assuming everything will
precede normally and releasing material regardless of what is already
on the shop floor, a much leaner and predictable manufacturing process
results. Inventories are reduced, quality is improved because smaller
batches are made and do not sit for long periods of time before they
are used by the next process, where their quality is evaluated, and
lead times are much more predictable and shorter.
Sincerely,
Wonko |
