Page 357 - Design for Six Sigma for Service (Six SIGMA Operational Methods)

Design and Improvement of Service Processes—Process Management 317

Raw material 1 Product 1

Product 2
Raw material 2

Raw material 3
Product 3
Figure 10.4 A Typical Job Shop

that 75 percent of all manufacturing is done in production batch sizes of
50 items or less. Job shops may be either a “one-of” type in which only one
of something is produced (tooling or prototype shop) or a repeating type in
which quantities are usually greater than one and similar jobs are produced
again in the future. In the case of job shops, the general range of processes
anticipated are considered in the selection of equipment. General-purpose
equipment is used that is capable of providing processes for a broad range
of products. The flowchart for a typical job shop is illustrated in Fig. 10.4.
The boxes in the figure are machines or workstations.

Job shops tend to be very inefficient with long lead times and high work-in-
process inventories. These are some of the reasons for the inefficiencies of
job shops:

• Manual material movement
• Manual operations
• Long setup times
• Low equipment utilization

Examples of job shops include
• Metalworking
• Fabrication or machining operations
• Maintenance facilities for the aerospace industry

During operation, the general job shop problem is to schedule the production
of N jobs on M machines. For each job, the sequence of machines is known
as well as the processing time on each machine. Due dates may also be
known. In scheduling, four principal goals or objectives are to be achieved:
1. Minimize job lateness or tardiness
2. Minimize the flow time or time jobs spend in production