Page 86 - How To Implement Lean Manufacturing

Lean Manufacturing Simplified 67

Pull Pull systems are production systems that are designed to minimize overproduction,
the most grievous of the wastes. Pull systems have two characteristics.
• They have a maximum inventory volume—for example, when using a kanban
system.
• Production is initiated only by a signal from the customer, and that only occurs
when some inventory has been consumed.
A pull system is one in which the customer, the next step in the process, removes
some product that then is the signal for the upstream step to produce. For example, for
some reason the finished goods inventory of our customer is full and the entire comple-
ment of kanban cards are attached to the finished goods in the storehouse. Since kanban
cards are the signal to produce, and they are all attached to the finished goods, produc-
tion has stopped. Hence, our overproduction is limited to whatever we have in finished
goods for the cycle stock, buffer stock, and safety stock. However, when our customer
arrives and withdraws product, then the kanban cards are removed and circulated back
to the production cell, signaling that production is authorized to begin. Once it starts
production, the cell will produce only that volume dictated by the kanban, and these
finished goods will then be placed in inventory. This process of replacing the inventory
that was withdrawn is specifically named replenishment.
A manufacturing system with a limit on the maximum inventory, and production
based on replenishment, is the essence of a pull system. The opposite of a pull system
is a push system. In a push system, there is no maximum inventory, the downstream
process produces until it is told not to, usually by the scheduler. It then pushes that
product onto the next step whether the next step needs the production or not. Hence,
on the production floor, there is no maximum control on the WIP, so WIP can grow
uncontrollably. With this uncontrolled growth of WIP, lead time will grow, with resul-
tant quality, delivery, and cost problems escalating.
Minimum Lot Size Minimum lot size is a means to reduce lead times. By reducing pro-
duction lot size and transfer lot sizes, the process proceeds much faster. Two benefits
are achieved. First, we reduce the lead time for the first piece through the process. This
benefit is usually felt in quality responsiveness. If the first piece lead time is reduced,
and there is a problem with the product, this information is fed back to the problematic
station more quickly. The problem can be resolved more quickly, and if rework is
required, fewer items will need to be reworked. (For a dramatic example of this effect,
see Chap. 15’s discussion of the Bravo Line.) The second benefit is that the overall prod-
uct will be completed more quickly, reducing production lead time for the lot. Mini-
mum lot size, with the ultimate being “one-piece flow,” is the key to plant flexibility and
product supply responsiveness.
Flow Flow is the concept that parts and subassemblies do not stop except to be pro-
cessed, and then only for value-added work. It is more of a concept to be attained than
a reality. It is the primary tool used to reduce production lead time. The typical tech-
nique is to design the process so that as little inventory as possible exists at each work
station, and the work stations are synchronized as close as practical. The design ideal is
a multistation cell with no inventory between work stations. The ideal state we seek is
one-piece flow with 100 percent value-added work only. This ideal state is frequently
not possible, at least initially, because there are obstacles to flow. (The Seven Obstacles