Page 73 - Design for Six Sigma a Roadmap for Product Development

50 Chapter Two

Manual Electronic Signal Load leveling Sequenced-
information information Kanban box pull ball
flow flow

Weekly
schedule
Withdrawal Production Kanban Schedule
Kanban Kanban post
Figure 2.20 Icons used in information flows in value stream maps.

1. C/T (cycle time): It is the time required to produce one piece of prod-
uct by a machine, station, and/or operator. It is the time required to
repeat a given sequence of operations or events.
2. C/O (changover time): It is the time required to switch from one
product type to another product type, for example, how long it takes
for a pizza maker to switch from making one type of pizza to
another type of pizza.
3. Uptime: It is the proportion of time a process step is operational.
4. EPE (production batch size).
5. Number of operators.
6. Number of product variations.
7. Scrap rate.

Figure 2.21 shows a complete value stream map for a manufactur-
ing process.
In Fig. 2.21, we can see that below each process box, there is a data
box. For example, in the leftmost process box stamping, the cycle time
is 1 s, the changeover time is 1 h, uptime 85 percent, and the pro-
duction batch size (EPE) is 2 weeks of supply; that is, the stamping
press produces a big batch of parts (enough to supply for two weeks) in
one shot. In Fig. 2.21, between the first process box (stamping) and the
second process box (S. weld #1) there are in process inventory of semi-
finished parts, the average inventory holding time is 7.6 days. In the
first process box, the value-added time is 1 seconds, which is equal to
the stamping cycle time. From the lean operation point of view, in-process
inventory holding is a nonvalue-added activity. If we add all value-added
time for the whole process, it is equal to 184 seconds, which is recorded
at the low right corner of the Fig. 2.21. The production lead time for
the whole process is 23.5 days. Clearly, in the whole product lead time,