The Saturn Site in Spring Hill, TN, consists of three plants -- Powertrain, Body and Assembly. The site covers 4.3 million sq. ft. and has a total of approximately 10,000 operations such as stamping, molding, fabrication, casting, machining, assembly, fluid fill, and so on. Saturn brings these operations together in an integrated manufacturing system designed around just-in-time principles.
The Saturn manufacturing site, in essence, is a network of processes and buffers. Processes represent the operations required to build a car. An operation, in turn, can involve actions (such as the casting, machining, and welding of car parts) or assemblies (such as for transmissions, engines, and the final car). Each process has associated measurements that indicate its productivity. These measurements—critical information in making business decisions—include entities such as cycle time, production count, work in process, and downtime.
Buffers (or banks) lie between processes. A buffer holds parts and/or subassemblies produced by an upstream process for consumption by a downstream process. In different sections of the plant, buffers take on different physical forms: They may be parts contained in kanbans or on conveyors, for example. Despite their different physical manifestations, buffers have common measurements that are pertinent to production:
- bank count (the number of parts or subassemblies in the buffer) and
- the minimum and maximum buffer capacities.
The interconnectivity of processes and buffers captures the sequence of operations required to produce a car; it also defines the interdependence of processes on each other and on buffer capacities. A process may have to remain idle if an upstream process is not producing enough parts (an obstruction condition called starving). On the other hand, a process may be forced to stop producing if a downstream process is not consuming enough parts (blocking). Starving and blocking conditions may arise from mismatches between process cycle times, downtime, the capacities of buffers between processes, and so on.
Production flow concerns with the flow of material (raw materials, parts, subassemblies, and so on) through processes and buffers, and encompasses all the production-related entities of processes and buffers (such as production count, bank count, starving, and blocking).
The figure above depicts the business process Saturn employs to continuously improve throughput. The process consists of
- gathering relevant plant data,
- visualizing plant operations and analyzing each operations’ capacity,
- identifying bottlenecks, and
- formulating engineering and business plans to improve throughput.
To better support that business process, Saturn designed and implemented a new production flow concept in 1993. As part of that effort, they selected key measurements from the available data for all processes and buffers. Saturn team members identified these key indicators through experience and observation. Over the years, buffer indicators had proven extremely useful, and they received more attention than in the past.
Beginning in late 1994, Saturn initiated a manual evaluation process that involved calculating and posting stand-alone capacity: Stand-alone capacity is what a process’ production rate would be without obstructions (without starving or blocking). This became a key measurement in identifying bottlenecks for potential investment opportunities.
Saturn’s traditional information system and engineering process could only support the manual updating and posting of the stand-alone capacities once a month. In addition, the system could not make postings available to all team members and leaders who participated in business decisions.