Page 218 - How To Implement Lean Manufacturing

196 Cha pte r T h i r tee n

The primary purpose of a cell, is to reduce wastes in the manufacturing system. The
seven wastes again are
• Transportation
• Waiting
• Overproduction
• Defects
• Inventory
• Movement
• Excess processing
It is easy to see that cells reduce transportation due to the close coupled nature of the
workstations. They do nothing directly to reduce waiting since that is a function of work
balancing and variation. However, cells make the balancing much easier to manage.
Cells, in and of themselves, also do nothing directly to prevent overproduction or
defects. They do minimize the inventory when one-piece flow is achieved, which is their
basic design. As for movement, they actually promote movement, more efficient move-
ment, depending upon the cell design and do nothing to reduce excess processing.
Thus, cellular production is designed to reduce the wastes of transportation and
inventory. Consequently, it is also designed to speed up the process and make it flow.
Cells almost always do this, resulting in production advantages such as:
• The reduction of first-piece lead time
• The reduction of lot lead time

So with the reduced lead times, we have greater flexibility and responsiveness.
However, both of these benefits can be achieved with a flow line. A flow line is a lin-
ear, rather than a U- or C-shaped, closed arrangement. To achieve these benefits in a flow
line, there needs to be the same low inventory approach as well as making sure the sta-
tions are close coupled as in a cell. So why are cells so popular? There are other benefits
to cells over flow lines that may be a bit harder to quantify, but are possibly even more
powerful reasons to choose cells over flow lines in many manufacturing circumstances.
Cells or Flow Lines?
Just what are these other benefits in choosing cells over flow lines?
The first and often the largest reason to select cells over flow lines is the production
rate flexibility possible with cells. For example, let’s say we have a typical balanced cell
staffed with six work stations and six workers. If we use three workers instead of six,
we can have each worker do the work of two stations and this will double the cycle time
or halve the production rate. If the workers are designed to move from station to station,
it is possible to use either one, two, three, four, five, or six workers and get 16 percent,
33 percent, 50 percent, 67 percent, 83 percent, or 100 percent of capacity with no increase
in labor unit costs. This allows the cell to operate at different rates as the customer’s
demand changes. This rate modulation is much more effective than starting up, run-
ning the cell at full rate, and then shutting down when the month’s demand has been
met. The cycling up and shutting down is nothing more than creating batches. The TPS
is a batch destruction system, not a batch creation system. This rate modulation by
modifying staffing is not practical to do with a flow line.