21.1 OVERVIEW

•A design must be converted to a process plan before it may be produced.

•But, if we have thousands of process plans, and hundreds of customer orders, with dozens of parts in each, which machines do we use when to make the products? What parts do we need?

•Traditionally jobs have been scheduled on a first come, first served basis. This resulted in a lineup of various jobs waiting to be done at each work center.

•When jobs are not scheduled efficiently, we often will get jobs sitting half completed, while we wait for simple parts to be processed. This costs money, wastes time, takes up floor space, makes the customer unhappy, etc.

•Eventually computers were used to figure out how to schedule jobs so that parts were made before they were needed, and so that work was done on time.

•As computers were used more it also became obvious that strict schedules were a nice idea, but they don’t work. A schedule is only valid until the first breakdown.

•Newer control programs called Production Planning and Control (PPC) systems were used to generate schedules, and fix problems that came up.

•Most systems, manual, and automatic either push, or pull the work through the factory. If the work is pushed, then customer orders tend to drive the production. If the work is pulled, the factory often tries to satisfy some continuous demand, and when things are about to run out, more is produced.